JBJS | The Long-Term Results of Charnley Low-Friction Arthroplasty in Young Patients Who Have Congenital Dislocation, Degenerative Osteoarthrosis, or Rheumatoid Arthritis*

The Journal of Bone & Joint Surgery, Volume 79, Issue 11

Articles | November 01, 1997

The Long-Term Results of Charnley Low-Friction Arthroplasty in Young Patients Who Have Congenital Dislocation, Degenerative Osteoarthrosis, or Rheumatoid Arthritis*

DAVID H. SOCHART, F.R.C.S.†; MARTYN L. PORTER, F.R.C.S.(ORTH)‡, WIGAN, LANCASHIRE, UNITED KINGDOM

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Investigation performed at The Centre for Hip Surgery, Wrightington Hospital, Wigan

The Journal of Bone & Joint Surgery. 1997; 79:1599-1617

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Abstract

We present the long-term results of 226 Charnley low-friction arthroplasties that were performed with use of cement in 161 patients between 1966 and 1978. Forty-four patients (sixty hips) had congenital dislocation of the hip, fifty-four patients (sixty-six hips) had degenerative osteoarthrosis, and sixty-three patients (100 hips) had rheumatoid arthritis. There were 114 female patients and forty-seven male patients. The average age of the patients at the time of the operation was 31.7 years (range, seventeen to thirty-nine years). Sixty-five patients (40 per cent) had a bilateral hip replacement. Thirty-eight patients (24 per cent; fifty-five hips), twenty-seven of whom had juvenile-onset chronic rheumatoid arthritis, died during the follow-up period. The average duration of follow-up for the entire series until the time of death, revision of both components, or the latest evaluation was 236 months (19.7 years; range, twenty-four to 361 months).Survivorship analysis was performed with the Kaplan-Meier method. At twenty-five years, the survival of the femoral component (with 95 per cent confidence intervals) was 89 per cent (80 to 98 per cent) in the patients who had congenital dislocation of the hip, 85 per cent (77 to 93 per cent) in the patients who had rheumatoid arthritis, and 74 per cent (61 to 87 per cent) in the patients who had degenerative osteoarthrosis. The rate of survival of the acetabular component was lower: at twenty-five years, it was 58 per cent (42 to 74 per cent) in the patients who had congenital dislocation, 79 per cent (70 to 88 per cent) in the patients who had rheumatoid arthritis, and 59 per cent (41 to 77 per cent) in the patients who had degenerative osteoarthrosis.The forty-four patients (sixty hips) who had congenital dislocation had the highest rates of aseptic loosening (twenty-two hips; 37 per cent), migration (seventeen hips; 28 per cent), and revision (twenty-two hips; 37 per cent) of the acetabular component. The fifty-four patients (sixty-six hips) who had degenerative osteoarthrosis had the highest rates of aseptic loosening (seventeen hips; 26 per cent) and revision (eighteen hips; 27 per cent) of the femoral component as well as the highest rate of femoral endosteal lysis (thirteen hips; 20 per cent). The sixty-three patients (100 hips) who had rheumatoid arthritis had the lowest prevalences of loosening and revision of the acetabular component but the highest rates of trochanteric non-union (fifteen hips; 15 per cent) and mortality (twenty-seven patients; 43 per cent). The average rate of wear of the acetabular component for the entire series was 0.11 millimeter per year; the average rate for the revised components (0.19 millimeter per year) was higher than that for the surviving components (0.09 millimeter per year). This was a consistent finding in each of the three diagnostic groups, and an increased annual rate of wear was found to be significantly associated with increased rates of migration and revision of the acetabular component (p < 0.01 for both).The femoral component proved to be durable: the twenty-five-year rate of survival for the entire series was 81 per cent (95 per cent confidence interval, 76 to 87 per cent). The major factors that limited the longevity of the cemented total hip implants in the present study were wear, loosening, and revision of the acetabular component, for which the twenty-five-year probability of survival was 68 per cent (95 per cent confidence interval, 61 to 75 per cent).

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Total hip arthroplasty with use of cement, one of the most successful major operative procedures currently being performed, has been shown to provide reliable relief of pain^9,51,56,64 as well as increased functional ability and an increased range of motion of the hip^10,16,59,63 for patients of all ages^4,31,44,64. Charnley, in 1961, predicted that this new operation was unlikely to be suitable for the management of robust middle-aged patients but that it would find a permanent place in the management of older patients and those who had rheumatoid arthritis⁸. During the first decade after its introduction, total hip arthroplasty was reserved mainly for the treatment of degenerative osteoarthrosis or rheumatoid arthritis in patients who were more than sixty years old, as the longevity of the implant was expected to be compatible with the life expectancy and level of activity of older individuals.

Relatively few patients who are less than forty years old need a total hip replacement, and operations on large numbers of such patients have been performed only at specialized centers. The published results of early series, however, were poor^7,19,53,59, and high rates of loosening^6,36,37,60 and revision^47,48,52,61 were reported even for patients who had rheumatoid arthritis. The worst results of which we are aware have been those for the youngest patients^{3,17,23,31,33} and for patients who had had an operation on the hip before the arthroplasty^7,34,41,57. Many surgeons have remained reluctant to perform total hip replacement in younger patients because of the belief that high rates of wear and mechanical failure would inevitably result in the need for technically demanding revision operations while the patients were still young. Some authors have advocated the use of osteotomy or arthrodesis as an alternative to total hip arthroplasty^19,46,55,57.

Many of these early studies had limitations, such as the inclusion of only small numbers of patients^6,7,26,37, a short duration of follow-up^4,14,15,60, and the use of various designs of implants in a single series^2,13,36,49. Patients who had different underlying conditions often were combined in an attempt to increase the total number^4,7,13,27,28, but the results for small diagnostic subgroups rarely were reported separately. As a result, it is difficult to draw any definite conclusions regarding the outcome of total hip arthroplasty in young patients purely on the basis of these studies.

Young patients tend to be highly motivated and often are willing to accept the risk of failure of the implant in exchange for the relief of pain and the independence that the operation may provide at what many patients consider to be the most crucial time in their personal and professional lives. The treatment of young patients is particularly important not only for socioeconomic and humanitarian reasons but also because the greater life expectancy of such patients provides an opportunity for prolonged assessment. Furthermore, young patients impose the most stringent tests of the durability and performance of the components because of their greater level of activity. We report the results of 226 total hip arthroplasties that were performed by a small group of surgeons with use of a single design of prosthesis and a standardized operative technique. The results are presented for each diagnostic group as well as for the series as a whole. As far as we know, this is the largest study, with the longest duration of follow-up, that has been performed to assess the results of total hip arthroplasty in patients who were less than forty years old at the time of the operation.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†7 Woodlea, Walkden Road, Worsley, Manchester M28 2QJ, United Kingdom.

‡The Centre for Hip Surgery, Wrightington Hospital, Hall Lane, Appley Bridge, Wigan, Lancashire WN6 9EP, United Kingdom.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†7 Woodlea, Walkden Road, Worsley, Manchester M28 2QJ, United Kingdom.

‡The Centre for Hip Surgery, Wrightington Hospital, Hall Lane, Appley Bridge, Wigan, Lancashire WN6 9EP, United Kingdom.

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TABLE I DEMOGRAPHIC DATA AND POSTOPERATIVE PAIN SCORES

*The difference was significant compared with the other two groups (p < 0.04).

	Diagnostic Group
	Congenital	Degenerative	Rheumatoid
	Dislocation	Osteoarthrosis	Arthritis	Entire Series
No. of patients	44	54	63	161
No. of hips	60	66	100	226
No. of patients managed with bilateral arthroplasty	16 (36%)	12 (22%)	37 (59%)	65 (40%)
Gender (male/female) (no. of patients)	3/41	29/25	15/48	47/114
Average age (yrs.)	32.4	33.9	30.5	31.7
Deaths (no. of patients)	3 (7%)	8 (15%)	27 (43%)	38 (24%)
Average duration of follow-up (mos.)	244	234	231	236
Charnley category⁹(no. of patients)
A	13 (30%)	31 (57%)	2 (3%)	46 (29%)
B	26 (59%)	13 (24%)	8 (13%)	47 (29%)
C	5 (11%)	10 (19%)	53 (84%)	68 (42%)
Postop. pain scores⁹(no. of patients)
No or only slight pain (grade 5 or 6)	44 (100%)	52 (96%)	63 (100%)	159 (99%)
Pain-free (grade 6)	41 (93%)	46 (85%)*	59 (94%)	146 (91%)

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TABLE I DEMOGRAPHIC DATA AND POSTOPERATIVE PAIN SCORES

*The difference was significant compared with the other two groups (p < 0.04).

	Diagnostic Group
	Congenital	Degenerative	Rheumatoid
	Dislocation	Osteoarthrosis	Arthritis	Entire Series
No. of patients	44	54	63	161
No. of hips	60	66	100	226
No. of patients managed with bilateral arthroplasty	16 (36%)	12 (22%)	37 (59%)	65 (40%)
Gender (male/female) (no. of patients)	3/41	29/25	15/48	47/114
Average age (yrs.)	32.4	33.9	30.5	31.7
Deaths (no. of patients)	3 (7%)	8 (15%)	27 (43%)	38 (24%)
Average duration of follow-up (mos.)	244	234	231	236
Charnley category⁹(no. of patients)
A	13 (30%)	31 (57%)	2 (3%)	46 (29%)
B	26 (59%)	13 (24%)	8 (13%)	47 (29%)
C	5 (11%)	10 (19%)	53 (84%)	68 (42%)
Postop. pain scores⁹(no. of patients)
No or only slight pain (grade 5 or 6)	44 (100%)	52 (96%)	63 (100%)	159 (99%)
Pain-free (grade 6)	41 (93%)	46 (85%)*	59 (94%)	146 (91%)

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TABLE II RATES OF LOOSENING, WEAR, AND REVISION

*N = number of hips.†The difference was significant compared with the two other groups (p < 0.05).

	Diagnostic Group*
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Acetabular component
Aseptic loosening (no. of hips)	22 (37%)	19 (29%)	15 (15%)†	56 (25%)
Migration (no. of hips)	17 (28%)	14 (21%)	13 (13%)†	44 (19%)
Revision (no. of hips)	22 (37%)	22 (33%)	17 (17%)†	61 (27%)
Average time from index op. to revision (mos.)	193	156	166	167
Average annual rate of wear (mm/yr.)
All components	0.13	0.11	0.09	0.11
Unrevised components	0.1	0.08	0.08	0.09
Revised components	0.17	0.24	0.17	0.19
Femoral component
Aseptic loosening (no. of hips)	6 (10%)	17 (26%)†	10 (10%)	33 (15%)
Revision (no. of hips)	6 (10%)	18 (27%)†	12 (12%)	36 (16%)
Average time from index op. to revision (mos.)	156	170	164	167

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TABLE II RATES OF LOOSENING, WEAR, AND REVISION

*N = number of hips.†The difference was significant compared with the two other groups (p < 0.05).

	Diagnostic Group*
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Acetabular component
Aseptic loosening (no. of hips)	22 (37%)	19 (29%)	15 (15%)†	56 (25%)
Migration (no. of hips)	17 (28%)	14 (21%)	13 (13%)†	44 (19%)
Revision (no. of hips)	22 (37%)	22 (33%)	17 (17%)†	61 (27%)
Average time from index op. to revision (mos.)	193	156	166	167
Average annual rate of wear (mm/yr.)
All components	0.13	0.11	0.09	0.11
Unrevised components	0.1	0.08	0.08	0.09
Revised components	0.17	0.24	0.17	0.19
Femoral component
Aseptic loosening (no. of hips)	6 (10%)	17 (26%)†	10 (10%)	33 (15%)
Revision (no. of hips)	6 (10%)	18 (27%)†	12 (12%)	36 (16%)
Average time from index op. to revision (mos.)	156	170	164	167

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TABLE III COMPLICATIONS AND RADIOGRAPHIC FEATURES*

*N = number of hips.†The difference was significant compared with the other two groups (p < 0.05).

	Diagnostic Group
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Fracture of femoral component	2 (3%)	2 (3%)	4 (4%)	8 (4%)
Deep infection	0	0	2 (2%)	2 (1%)
Early dislocation	0	0	1 (1%)	1 (<1%)
Footdrop	2 (3%)	2 (3%)	0	4 (2%)
Heterotopic ossification	20 (33%)	40 (61%)†	29 (29%)	89 (39%)
Breakage of wire	9 (15%)	11 (17%)	19 (19%)	39 (17%)
Trochanteric displacement	2 (3%)	3 (5%)	15 (15%)†	20 (9%)
Trochanteric reoperation	2 (3%)	5 (8%)	4 (4%)	11 (5%)
Subsidence of femoral component
<5 mm	9 (15%)	9 (14%)	20 (20%)	38 (17%)
>5 mm	0	5 (8%)	2 (2%)	7 (3%)
Fracture of cement around tip of femoral component	3 (5%)	7 (11%)	6 (6%)	16 (7%)
Femoral endosteal lysis	8 (13%)	13 (20%)†	9 (9%)	30 (13%)
Changes in calcar	22 (37%)	17 (26%)	39 (39%)	78 (35%)
Cortical hypertrophy around tip of femoral component	27 (45%)	35 (53%)	19 (19%)	81 (36%)

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TABLE III COMPLICATIONS AND RADIOGRAPHIC FEATURES*

*N = number of hips.†The difference was significant compared with the other two groups (p < 0.05).

	Diagnostic Group
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Fracture of femoral component	2 (3%)	2 (3%)	4 (4%)	8 (4%)
Deep infection	0	0	2 (2%)	2 (1%)
Early dislocation	0	0	1 (1%)	1 (<1%)
Footdrop	2 (3%)	2 (3%)	0	4 (2%)
Heterotopic ossification	20 (33%)	40 (61%)†	29 (29%)	89 (39%)
Breakage of wire	9 (15%)	11 (17%)	19 (19%)	39 (17%)
Trochanteric displacement	2 (3%)	3 (5%)	15 (15%)†	20 (9%)
Trochanteric reoperation	2 (3%)	5 (8%)	4 (4%)	11 (5%)
Subsidence of femoral component
<5 mm	9 (15%)	9 (14%)	20 (20%)	38 (17%)
>5 mm	0	5 (8%)	2 (2%)	7 (3%)
Fracture of cement around tip of femoral component	3 (5%)	7 (11%)	6 (6%)	16 (7%)
Femoral endosteal lysis	8 (13%)	13 (20%)†	9 (9%)	30 (13%)
Changes in calcar	22 (37%)	17 (26%)	39 (39%)	78 (35%)
Cortical hypertrophy around tip of femoral component	27 (45%)	35 (53%)	19 (19%)	81 (36%)

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TABLE IV SURVIVAL OF THE ACETABULAR AND FEMORAL COMPONENTS*

*The values are given as percentages, with the 95 per cent confidence intervals in parentheses.†The difference was significant at p < 0.05.

	Diagnostic Group
	Congenital Dislocation	Degenerative Osteoarthrosis	Rheumatoid Arthritis	Entire Series
Acetabular component
10 yrs.	97 (92—100)	89 (81—97)	98 (94—100)	93 (90—96)
20 yrs.	70 (57—83)	64 (51—78)†	79 (70—88)	71 (65—77)
25 yrs.	58 (42—74)	59 (41—77)	79 (70—88)†	68 (61—75)
Femoral component
10 yrs.	97 (92—100)	94 (88—100)	97 (93—100)	95 (93—98)
20 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	82 (77—88)
25 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	81 (76—87)
Both components
10 yrs.	97 (92—100)	86 (77—95)	96 (91—100)	91 (88—95)
20 yrs.	64 (50—77)	63 (49—77)	77 (67—87)	67 (61—74)
25 yrs.	54 (39—69)	52 (34—71)	77 (67—87)†	65 (58—72)

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TABLE IV SURVIVAL OF THE ACETABULAR AND FEMORAL COMPONENTS*

*The values are given as percentages, with the 95 per cent confidence intervals in parentheses.†The difference was significant at p < 0.05.

	Diagnostic Group
	Congenital Dislocation	Degenerative Osteoarthrosis	Rheumatoid Arthritis	Entire Series
Acetabular component
10 yrs.	97 (92—100)	89 (81—97)	98 (94—100)	93 (90—96)
20 yrs.	70 (57—83)	64 (51—78)†	79 (70—88)	71 (65—77)
25 yrs.	58 (42—74)	59 (41—77)	79 (70—88)†	68 (61—75)
Femoral component
10 yrs.	97 (92—100)	94 (88—100)	97 (93—100)	95 (93—98)
20 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	82 (77—88)
25 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	81 (76—87)
Both components
10 yrs.	97 (92—100)	86 (77—95)	96 (91—100)	91 (88—95)
20 yrs.	64 (50—77)	63 (49—77)	77 (67—87)	67 (61—74)
25 yrs.	54 (39—69)	52 (34—71)	77 (67—87)†	65 (58—72)

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Fig. 1 Bar graphs comparing the preoperative and postoperative scores for pain, function, and range of motion for the hips that had congenital dislocation (C.D.H.).

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Fig. 2 Individual survivorship curves for the hips that had congenital dislocation (C.D.H.), showing the acetabular and femoral components separately and in combination, with 95 per cent confidence intervals.

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Fig. 3 Bar graphs comparing the preoperative and postoperative scores for pain, function, and range of motion for the hips that had degenerative osteoarthrosis.

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Fig. 4 Individual survivorship curves for the hips that had degenerative osteoarthrosis, showing the acetabular and femoral components separately and in combination, with 95 per cent confidence intervals.

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Fig. 5 Bar graphs comparing the preoperative and postoperative scores for pain, function, and range of motion for the hips that had rheumatoid arthritis.

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Fig. 6 Individual survivorship curves for the hips that had rheumatoid arthritis, showing the acetabular and femoral components separately and in combination, with 95 per cent confidence intervals.

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Fig. 7 Individual twenty-five-year survivorship curves for the entire series, showing the acetabular and femoral components separately and in combination, with 95 per cent confidence intervals.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

From 1966 to 1978, 10,469 total hip replacements were performed at The Centre for Hip Surgery; only 280 (2.7 per cent) of these procedures were in patients who were less than forty years old. Forty-three primary total hip arthroplasties were performed in twenty-four patients who had ankylosing spondylitis⁵⁴, and six revision operations were performed in six patients who had had a previous hemiarthroplasty with a long-stem component. The remaining 231 arthroplasties (2.2 per cent) were performed in 164 patients; only three patients (five hips) could not be located and were excluded from additional analysis. Thus, the current study comprised 226 arthroplasties (161 patients). The underlying diagnosis was congenital dislocation of the hip in forty-four patients (sixty hips), degenerative osteoarthrosis in fifty-four patients (sixty-six hips), and rheumatoid arthritis in sixty-three patients (100 hips) (Table I). There were 114 female patients and forty-seven male patients. Sixty-five patients (40 per cent) had a bilateral arthroplasty, which was performed as a staged procedure with a typical interval of twelve weeks between the operations. The average age at the time of the index arthroplasty was 31.7 years (range, seventeen to thirty-nine years). The average duration of follow-up for the entire series until the time of death, revision of both components, or the latest clinical evaluation was 236 months (19.7 years; range, twenty-four to 361 months).

Most of the surviving patients continued to have regular biennial clinical and radiographic follow-up examinations, and those who had been lost to follow-up were located through the National Health Service Central Register at the Office for National Statistics. Thirty-eight patients (24 per cent; fifty-five hips) were known to have died; in each case, the date of death was confirmed and the outcome of the arthroplasty was established by means of a review of the hospital records and an interview of the family practitioner. All 123 patients (171 hips) who were still alive were contacted and evaluated, initially with use of a written questionnaire and then with a telephone interview. Most patients were seen at specific research clinics for clinical and radiographic assessment; eleven patients (fifteen hips) were seen at their local hospital and had their radiographs forwarded to us for assessment.

Preoperative and postoperative pain, function, and range of motion were graded with use of the 6-point scale of Merle d'Aubigné and Postel⁴³ as modified by Charnley⁹. This modification of the scoring system includes an assessment of preoperative disability with the addition of an alphabetical prefix (A, B, or C) to the numerical classification. Category A included forty-six patients (29 per cent) who had involvement of only one hip; category B, forty-seven patients (29 per cent) who had bilateral involvement; and category C, sixty-eight patients (42 per cent) who had some other factor, such as polyarticular arthritis or neurological dysfunction, that may have contributed to their inability to walk normally. The score for function was originally designed for the evaluation of patients who had monarticular disease and, although many of the patients were not in this category, it was useful as an initial indication of the success of the operation when the postoperative score was compared with the preoperative score. The score for motion is cumulative and is based on the sum of the ranges of motion in the three standard directions (flexion-extension, abduction-adduction, and rotation).

Preoperative radiographs were available for forty-one of the forty-four patients (fifty-six of the sixty hips) who had congenital dislocation, and the hips were classified according to the percentage of subluxation as described by Crowe et al.¹⁵. The sixteen hips that were subluxated less than 50 per cent were classified as grade 1; the sixteen that were subluxated 50 to 75 per cent, as grade 2; the thirteen that were subluxated 75 to 100 per cent, as grade 3; and the eleven that were completely dislocated, as grade 4. At the time of the operation, attempts were made to position the acetabular component at the level of the true acetabulum or to move the component medially. Superolateral autologous bone-grafting was performed to augment acetabular deficiencies in three hips.

Of the sixty-six hips in the group of patients who had degenerative osteoarthrosis, twenty were in patients for whom the preoperative diagnosis was primary osteoarthrosis or avascular necrosis; thirty-three were in patients who had another condition involving the hip, such as a previous fracture (seventeen hips), a childhood infection (seven hips), or congenital dysplasia, Perthes disease, or slipped capital femoral epiphysis (three hips each); and thirteen were in patients who had a generalized disorder, such as Morquio disease (four hips), osteopetrosis (three hips), multiple epiphyseal dysplasia (three hips), Gaucher disease (two hips), or psoriasis (one hip). Three hips had acetabular protrusion, which necessitated the use of an autologous bone graft from the femoral head.

Thirty-five hips (15 per cent) had been treated with a total of thirty-eight operations before the index total hip replacement. These procedures had included nineteen osteotomies, ten internal fixations, three hemiarthroplasties (two Judet femoral head replacements and one Smith-Petersen femoral head resurfacing arthroplasty), three adductor releases, two Girdlestone excisional arthroplasties, and one removal of a loose body.

All of the operations were performed in a Charnley-Howorth clean-air enclosure that was equipped with laminar airflow (Howorth Engineering; Brinscall Blackburn, United Kingdom), first introduced in 1966; the surgeons wore body-exhaust suits after the suits were introduced in 1970. A lateral approach was used with a planar trochanteric osteotomy and standard Charnley reattachment with stainless-steel wires. All of the hips were treated with a Charnley prosthesis (Thackray, Leeds, United Kingdom). This implant consists of a tapered, monoblock, stainless-steel femoral stem with a 22.25-millimeter-diameter femoral head that forms a low-frictional-torque articulation with an unflanged ultra-high molecular weight polyethylene acetabular component with an integral wire marker. Both components were fixed with radiopaque self-curing polymethylmethacrylate cement, which was inserted and pressurized digitally without previous occlusion of the distal part of the femoral intramedullary canal. One hundred and sixty-seven arthroplasties (74 per cent) were performed by one of two consultant orthopaedic surgeons, and the remaining fifty-nine procedures (26 per cent) were performed by senior trainees who were directly supervised by these two surgeons. All procedures were performed with a standard operative exposure and technique.

A radiograph of the involved hip was made immediately after the operation, and additional radiographs were made before the patient was discharged from the hospital and at each subsequent visit to the clinic. Patients returned annually for the first ten years and then were seen in alternate years. The anteroposterior radiographs of the pelvis were made with use of the standard technique of centering the x-ray beam over the pubic symphysis to show both hips, which were positioned in extension and neutral rotation with the patellae pointing upward. Demarcation of the acetabular components was recorded on the radiographs according to the zones described by DeLee and Charnley¹⁸, and loosening was assessed with use of the criteria of Hodgkinson et al.³⁰. An acetabular component was considered to be loose if a continuous radiolucent line of any thickness was evident in all three zones (type-3 demarcation) or if the cup had changed position or had migrated (type-4 demarcation). Migration of the cup was assessed on sequential radiographs by comparing the vertical distance from the center of the cup, measured with use of the integral wire marker, to a horizontal line joining the two anatomical teardrops. Wear of the acetabular component was measured with a graded circular gauge⁶² that uses the standard 22.25-millimeter-diameter femoral head as a point of reference in order to account for the difference in magnification between radiographs^11,24,62. In order to avoid interobserver variation, all measurements of the total amount of wear were made on the most recent radiograph by one of us (D. H. S.). The average annual rate of wear was calculated by dividing the total amount of wear by the duration that the component had been in situ.

The position of the femoral stem within the medullary canal was considered to be neutral when the long axis of the stem of the prosthesis was aligned to within 5 degrees of the neutral axis of the shaft of the femur. If the axis of the stem was at an angle to that of the femoral shaft, with the distal tip of the implant pointing medially, then the prosthesis was considered to be in valgus alignment; if the axis of the stem was directed laterally, then the implant was considered to be in varus alignment. Demarcation of the femoral components and femoral endosteal lysis were assessed with use of the zonal system of Gruen et al.²⁵ and by identification of any of the features associated with loosening as described by Loudon and Charnley³⁸ as well as Loudon and Older³⁹. Definite loosening was defined as subsidence of more than five millimeters or continuous demarcation around the stem. Probable loosening was defined as subsidence of two to five millimeters or a radiolucent line surrounding 50 per cent of the stem (three zones²⁵) or more. Heterotopic ossification was classified with use of the system of Brooker et al.⁵, but no specific prophylactic measures were used to prevent it.

The Kaplan-Meier method³⁵ was used to calculate the probability of retention of the original prosthesis from the time of the initial operation until one of three separate end points: revision of either component for any reason, revision of the femoral stem, and revision of the acetabular component. Ninety-five per cent confidence intervals were calculated, and the chi-square and log-rank tests were used to compare the rates of survival of the components, revision, radiographic findings, and wear.

Results

Introduction | Materials and Methods | Results | Discussion | References

Patients Who Had Congenital Dislocation of the Hip

Sixty arthroplasties were performed in forty-four patients who had congenital dislocation of the hip (Table I). The average age at the time of the operation was 32.4 years (range, seventeen to thirty-nine years). Thirteen patients (30 per cent) had unilateral disease; twenty-six (59 per cent) had bilateral disease; and only five (11 per cent) were in Charnley⁹ category C, meaning that another factor, such as polyarticular arthropathy, neurological impairment, or cardiorespiratory disease, may have limited their ability to walk. Three patients (7 per cent; four hips) died at an average of 212 months (17.7 years; range, 158 to 281 months) after the operation; one of these patients had had revision of the femoral component at eleven years. All forty-one surviving patients (fifty-six hips) were located; these patients had been followed for an average of 258 months (21.5 years; range, 216 to 361 months). The average duration of follow-up for the entire group, including the patients who had died, was 244 months (20.3 years; range, eighty-eight to 361 months). At the time of death or at the latest follow-up evaluation, twenty-two acetabular components (37 per cent) and six femoral components (10 per cent) had been revised and thirty-five hips (58 per cent) still had both of the original components in place (Table II).

All patients had marked clinical improvement in terms of relief of pain, function, and range of motion (Fig. 1). The average score for pain was 3.5 points (range, 2 to 6 points) preoperatively and 5.9 points (range, 5 or 6 points) postoperatively. After the operation, forty-one patients (93 per cent [fifty-six hips]) were entirely free of pain and the remainder had only occasional discomfort. The score for function averaged 3.0 points (range, 2 to 5 points) preoperatively and 5.5 points (range, 4 to 6 points) postoperatively, reflecting the fact that most of the patients were in Charnley⁹ category A or B. The score for the cumulative range of motion of the hip averaged 2.9 points (range, 1 to 6 points) preoperatively and 5.0 points (range, 4 to 6 points) postoperatively. After the operation, forty-nine hips (82 per cent) had a cumulative range of motion of more than 210 degrees (grade 5 or 6, according to the scale of Merle d'Aubigné and Postel⁴³), and no hip had a total range of motion of less than 160 degrees.

Two patients (two hips; 3 per cent) had a postoperative peroneal nerve palsy with footdrop, one had deep venous thrombosis, and one had a non-fatal pulmonary embolism. There was one superficial wound infection, but there were no deep infections. There were no early dislocations, and there was only one late dislocation. The dislocation occurred nineteen years after the operation and was associated with marked wear of the acetabular component; the patient was managed successfully with a revision. Two patients (two hips; 3 per cent) had a trochanteric non-union in association with breakage of a wire, and one subsequently had removal of the wire. An additional seven hips had late breakage of a wire, and the wire was removed from one of them because it was associated with persistent discomfort. Twenty hips (33 per cent) had heterotopic ossification: sixteen were rated as class 1, two were rated as class 2, and two were rated as class 3, according to the system of Brooker et al.⁵. No hip had complete ankylosis (a class-4 rating), and only two (those that had a class-3 rating) had changes that were considered to be clinically important. The remaining forty hips (67 per cent) had no evidence of heterotopic ossification (Table III).

Twenty-two acetabular components (37 per cent) had been revised by the time of the latest follow-up. Sixteen components (27 per cent) had been revised because of aseptic loosening and three (5 per cent), because of excessive wear (which had resulted in a late dislocation in one hip); the remaining three acetabular components had been removed at the time of revision of a broken (two hips) or loose (one hip) femoral component because they had been in place for more than ten years and had a marked degree of wear. Before the revision, two hips had no evidence of radiolucent demarcation of the cup, three had type-1 demarcation, one had type-2 demarcation, five had type-3 demarcation, and eleven had type-4 demarcation³⁰. The average time to revision of the acetabular component was 193 months (sixteen years; range, eighty-six to 346 months) (Table II).

Thirty-eight (63 per cent) of the original acetabular components had not been revised by the time of death (four hips) or the latest follow-up evaluation (thirty-four hips). Of the thirty-four hips in the surviving patients, nineteen (56 per cent) had no evidence of radiolucent demarcation, eight had type-1 demarcation, and one had type-2 demarcation³⁰. No hip had type-3 demarcation. Six cups (18 per cent) in four patients had migrated and were definitely loose (type-4 demarcation³⁰), although three patients (four hips) remained asymptomatic. Over-all, twenty-two (37 per cent) of the sixty acetabular components had loosened aseptically and seventeen (28 per cent) had also migrated (Table II).

The total amount of wear of the acetabular components averaged 2.7 millimeters (range, 0.12 to 5.15 millimeters), yielding an average rate of wear of 0.13 millimeter per year (range, 0.01 to 0.29 millimeter per year) for the entire group. The average rate of wear for the twenty-two acetabular components that had been revised (0.17 millimeter per year; range, 0.05 to 0.29 millimeter per year) was higher than that for the thirty-eight acetabular components that had not been revised (0.1 millimeter per year; range, 0.01 to 0.23 millimeter per year). This difference was found to be significant (p = 0.004) (Table II).

Forty-one femoral components (68 per cent) were in neutral alignment, fifteen (25 per cent) were in valgus, and four (7 per cent) were in slight varus. Fifty-four (90 per cent) of the original femoral components were still in place at the time of death or the most recent follow-up evaluation. One of the three patients who died had had revision of the femoral component compared with five of the forty-one patients who were still alive. Thus, a total of six femoral components (10 per cent) had been revised. Only four femoral components (7 per cent) were revised because they had failed mechanically: two were revised after fracture of the implant, and two (one of which was a small component) were revised because of aseptic loosening. The other two components were removed at the time of revision of a loose acetabular component because they had been in place for more than ten years and were likely to have a damaged bearing surface. The average time to revision of the femoral component was 156 months (thirteen years; range, eighty-six to 246 months) (Table II).

Nine femoral components (15 per cent) had subsided, but no component had subsided more than five millimeters (Table III). The subsidence was associated with fracture of the cement around the tip of the prosthesis in three hips and with an incomplete cement mantle in two. (The remaining four hips had a complete cement mantle.) Two of these nine components had been revised, and the remaining seven had no other features associated with loosening at the latest follow-up evaluation. Thirty-eight patients (fifty-one original femoral components; 85 per cent) were available at the latest follow-up evaluation. Forty-two (82 per cent) of the fifty-one hips had no evidence of radiolucent demarcation, four had demarcation in only one zone, and one had demarcation in two zones; loosening was apparent in the remaining four hips, three of which had demarcation in three zones and one of which had demarcation in four. Three of the four loose components had subsided, and one of them was in a hip that had an incomplete cement mantle because of the presence of voids in the cement. Over-all, only six femoral components (10 per cent) had loosened aseptically (Table II).

Femoral endosteal lysis was noted in eight hips (13 per cent), three of which had a revision of the femoral component and three of which had radiographic loosening of that component at the time of the latest follow-up. The other two hips had non-progressive osteolysis in one zone only and had no other evidence of loosening of the femoral component. Loss of bone in the medial portion of the femoral neck (the calcar) was seen in twenty-two hips (37 per cent): eight of these hips had marked osteopenia, and the other fourteen had resorption causing loss of height of the calcar. Hypertrophy of the femoral cortex around the tip of the stem (zones²⁵ 3, 4, and 5) was seen in twenty-seven hips (45 per cent) (Table III).

Because of the generally small caliber of the bones and dysplasia of the acetabulum, a small or extra-small acetabular component was used in forty-three hips (72 per cent), and a miniature stem or a stem specifically designed for use in patients with congenital dislocation was used in twelve hips (20 per cent). The outer diameter of the small acetabular components was thirty-eight millimeters, and the all-polyethylene design allowed use of the maximum thickness of ultra-high molecular weight polyethylene (the minimum thickness was 7.6 millimeters). Seventeen (40 per cent) of these forty-three small acetabular components had been revised compared with five of the seventeen standard ones, but we could detect no significant difference with the numbers available. The original degree of subluxation or dislocation of the hip did not affect the rate of revision of the acetabular component: five of the sixteen grade-1¹⁵, seven of the sixteen grade-2, six of the thirteen grade-3, and four of the eleven grade-4 hips were revised.

The probability of survival of both of the original components at ten and twenty-five years was 97 per cent (95 per cent confidence interval, 92 to 100 per cent) and 54 per cent (95 per cent confidence interval, 39 to 69 per cent), respectively. The ten and twenty-five-year survival rates for the acetabular component were 97 per cent (95 per cent confidence interval, 92 to 100 per cent) and 58 per cent (95 per cent confidence interval, 42 to 74 per cent); the corresponding rates for the femoral component were 97 per cent (95 per cent confidence interval, 92 to 100 per cent) and 89 per cent (95 per cent confidence interval, 80 to 98 per cent) (Fig. 2 and Table IV).

Patients Who Had Degenerative Osteoarthrosis

Sixty-six arthroplasties were performed in fifty-four patients who had degenerative osteoarthrosis (Table I). The average age at the time of the operation was 33.9 years (range, twenty-one to thirty-nine years). Thirty-one patients (57 per cent) had monarticular disease, thirteen (24 per cent) had bilateral disease, and ten (19 per cent) had another factor that impaired the ability to walk (Charnley⁹ category C). Eight patients (ten hips) died at an average of 173 months (14.4 years; range, eighty-eight to 304 months) after the operation; none of these patients had had a revision procedure. All forty-six surviving patients were located; they had been followed for an average of 244 months (range, 216 to 350 months). The average duration of follow-up for the entire group, including those who had died, was 234 months (19.5 years; range, eighty-eight to 350 months).

Three patients (three hips) had a superficial postoperative infection, but none had a deep infection, deep venous thrombosis, or pulmonary embolism (Table III). Two patients (two hips; 3 per cent) had a postoperative peroneal nerve palsy with footdrop, and two patients (two hips; 3 per cent) had a late post-traumatic fracture of the femoral component. Breakage of a trochanteric wire occurred in eleven hips (17 per cent); three hips (5 per cent) had a non-union, and five (8 per cent) had an operative procedure to remove the broken wire because of persistent symptoms. Forty hips (61 per cent) had evidence of heterotopic ossification, but only four (6 per cent) had a clinically important amount that restricted the range of motion. Three of the latter four hips were rated as class 3, according to the system of Brooker et al.⁵; the fourth hip was completely ankylosed and therefore was rated as class 4.

All patients had marked clinical improvement in terms of relief of pain, function, and range of motion (Fig. 3). The average score for pain was 3.4 points (range, 3 to 5 points) preoperatively and 5.8 points (range, 3 to 6 points) postoperatively. After the operation, fifty-two patients (96 per cent [sixty-three hips]) had little or no pain and forty-six patients (85 per cent [fifty-six hips]) were entirely pain-free. The average score for function was 3 points (range, 2 to 5 points) preoperatively and 5.2 points (range, 2 to 6 points) postoperatively. The average score for the cumulative range of motion was 2.6 points (range, 1 to 5 points) preoperatively and 5.2 points (range, 0 to 6 points) postoperatively; this represented an improvement from an average range of 100 degrees to one of 210 degrees.

A total of twenty-six hips (39 per cent) had a revision: fourteen hips had a revision of both components; eight, a revision of the acetabular component only; and four, a revision of the femoral component only. The twenty-two acetabular components (33 per cent) were revised at an average of 156 months (thirteen years; range, ninety-eight to 247 months) after the operation: seventeen (25 per cent) were revised because of aseptic loosening (twelve components had also migrated) and five, because of wear (Table II). Twenty-four (55 per cent) of the forty-four hips in which the acetabular component had not been revised had no evidence of demarcation of the cup, seventeen (39 per cent) had type-1 demarcation, and one had type-2 demarcation³⁰. The remaining two surviving components had migrated and were loose (type-4 demarcation), but both hips remained asymptomatic. The total rate of aseptic loosening of the acetabular components at an average of 19.5 years was therefore 29 per cent (nineteen of sixty-six hips), with fourteen components (21 per cent) having migrated. The average total amount of wear of the acetabular components was 2.16 millimeters (range, 0.19 to 7.21 millimeters), yielding an average rate of wear of 0.11 millimeter per year (range, 0.01 to 0.55 millimeter per year) for the entire group. The average rate of wear for the hips that had had a revision (0.24 millimeter per year; range, 0.1 to 0.55 millimeter per year) was three times higher than that for the hips that had not had a revision (0.08 millimeter per year; range, 0.01 to 0.18 millimeter per year). This difference was found to be significant (p = 0.004) (Table II).

Thirty-three femoral components (50 per cent) were in neutral alignment, twenty-three (35 per cent) were in valgus, and ten (15 per cent) were in varus. Eighteen femoral components (27 per cent) had been revised after an average of 170 months (14.2 years; range, sixty-two to 247 months) (Table II). Twelve were revised because of aseptic loosening and two, after fracture of the implant (at eight and eighteen years); the other four femoral components were revised at the time of revision of a loose acetabular component because they had been in place for more than ten years and were likely to have a suboptimum bearing surface. Sixteen (33 per cent) of the forty-eight hips in which the femoral component had not been revised had evidence of demarcation, but in ten (21 per cent) of these hips only one zone was affected. Five hips had loosening of the femoral component at the latest follow-up evaluation, with demarcation in more than three zones. The over-all rate of aseptic loosening of the femoral component was therefore 26 per cent (seventeen hips). Bending of the femoral component, with impending fracture of the component, was noted in two hips; both femoral components were loose, and one subsequently was revised. The other hip remained pain-free twenty-three years after the operation, and the patient was awaiting an additional operation at the time of the most recent follow-up.

Femoral endosteal lysis was seen in thirteen hips (20 per cent), six of which subsequently had a revision because of loosening. Of the remaining seven hips, three were associated with definite loosening and four had demarcation in only one zone²⁵. The relationship between endosteal lysis and loosening was significant (p = 0.002). Femoral cortical hypertrophy was observed around the tip of the implant in thirty-five hips (53 per cent), but it was not associated with an increased rate of loosening (Table III).

Subsidence of the femoral component occurred in fourteen hips (21 per cent). The subsidence was associated with a fracture of the cement around the tip of the implant in seven hips (11 per cent) and with an incomplete cement mantle (due to the presence of voids in the cement) in two (3 per cent). The five components (8 per cent) that subsided more than five millimeters were definitely loose because of a fracture of the cement around the tip of the prosthesis or an incomplete cement mantle. The other nine components subsided less than five millimeters; seven were loose, and six subsequently were revised. Radiographic changes in the calcar were seen in seventeen hips (26 per cent); four hips (6 per cent) had marked osteopenia, and thirteen (20 per cent) had loss of height of the calcar (Table III).

The probability of survival of both of the original components at ten and twenty-five years was 86 per cent (95 per cent confidence interval, 77 to 95 per cent) and 52 per cent (95 per cent confidence interval, 34 to 71 per cent), respectively. The ten and twenty-five-year survival rates for the acetabular component were 89 per cent (95 per cent confidence interval, 81 to 97 per cent) and 59 per cent (95 per cent confidence interval, 41 to 77 per cent); the corresponding rates for the femoral component were 94 per cent (95 per cent confidence interval, 88 to 100 per cent) and 74 per cent (95 per cent confidence interval, 61 to 87 per cent) (Fig. 4 and Table IV).

Patients Who Had Rheumatoid Arthritis

One hundred arthroplasties were performed in sixty-three patients who had rheumatoid arthritis (Table I). The average age of the patients at the time of the operation was 30.5 years (range, nineteen to thirty-nine years). Fifty-three patients (84 per cent) had polyarticular disease (Charnley⁹ category C), eight (13 per cent) had bilateral disease, and only two (3 per cent) had monarticular disease. Twenty-seven patients (43 per cent; forty-one hips) died at an average of 169 months (14.1 years; range, twenty-four to 339 months) after the index arthroplasty; none had needed a revision. The average duration of follow-up was 231 months (19.3 years; range, twenty-four to 360 months) for the entire group and 271 months (22.6 years; range, twenty-four to 360 months) for the hips in which the original implant survived to the time of death or the latest follow-up evaluation.

Six hips had a superficial infection that responded to oral administration of antibiotics. Only two hips (2 per cent) had a deep infection; both eventually were revised (Table III). One patient had a non-fatal pulmonary embolism; no other patient had a documented thromboembolism. Only one hip (1 per cent) had an early dislocation, which was treated successfully non-operatively. Three hips had a late dislocation in association with marked wear; all three hips subsequently had a revision of the acetabular component. Breakage of a trochanteric wire occurred in nineteen hips (19 per cent), with displacement of the trochanter in fifteen (15 per cent) and non-union in ten (10 per cent). Four hips (4 per cent) had an additional operation: three hips had removal of a wire, and one was treated with trochanteric reattachment. Twenty-nine hips (29 per cent) had heterotopic ossification, but only two (both of which were rated as class 3 according to the system of Brooker et al.⁵) had changes that were considered to be clinically important (as indicated by a decreased range of motion).

All patients reported marked improvement in terms of relief of pain and range of motion. Improvement in function also was reported, but it was less pronounced because most patients had polyarticular disease (Fig. 5). The average score for pain was 3.2 points (range, 1 to 6 points) preoperatively and 5.9 points (range, 5 or 6 points) postoperatively, with all patients having little or no pain in the hip and fifty-nine patients (94 per cent [ninety-five hips]) being entirely pain-free at the time of the most recent follow-up. The average score for the cumulative range of motion was 2.4 points (range, 1 to 5 points) preoperatively and 5.2 points (range, 0 to 6 points) postoperatively; this represented an improvement from an average range of 100 degrees to one of 210 degrees. The average score for function improved from 2.1 points (range, 1 to 4 points) preoperatively to 4.5 points (range, 0 to 6 points) postoperatively; this represented a valuable improvement but reflected the global disability of these patients resulting from polyarticular arthropathy.

A total of nineteen hips (19 per cent) had a revision procedure: ten had a revision of both components; seven, a revision of the acetabular component only; and two, a revision of the femoral component only. The seventeen acetabular components (17 per cent) were revised at an average of 166 months (13.8 years; range, sixty to 240 months) after the index operation: ten were revised because of aseptic loosening; two, because of deep infection; and five (three of which were associated with late dislocation), because of excessive wear. Of the eighty-three hips in which the original acetabular component had survived, fifty (60 per cent) had no evidence of demarcation of the cup, twenty-eight had type-1 demarcation, and five had type-2 demarcation³⁰. Five acetabular components were radiographically loose at the latest follow-up evaluation; three of these components had migrated (type-4 demarcation), but all five hips remained asymptomatic. Aseptic loosening of the acetabular component occurred in fifteen hips (15 per cent), and thirteen components (13 per cent) had migrated an average of five millimeters. The average rate of wear of the acetabular component was 0.09 millimeter per year (range, 0.01 to 0.3 millimeter per year) for the entire group. The average rate for the components that had been revised (0.17 millimeter per year; range, 0.01 to 0.26 millimeter per year) was approximately twice as high as that for the surviving components (0.08 millimeter per year; range, 0.01 to 0.3 millimeter per year). This difference was found to be significant (p = 0.02) (Table II).

Sixty-two femoral components (62 per cent) were in neutral alignment, twenty-seven (27 per cent) were in valgus, and eleven (11 per cent) were in varus. Twelve femoral components (12 per cent) had been revised at an average of 164 months (13.8 years; range, sixty to 240 months). Five components were revised because of aseptic loosening; two, because of deep infection; and four (two of which were also definitely loose), because of breakage of the implant. The remaining component was revised during a procedure to replace a loose acetabular component; although the femoral component was not loose, it had been in place for more than ten years and had a suboptimum bearing surface because of scratches on the femoral head. Eighty-eight original femoral components were in situ at the time of death or at the latest follow-up evaluation. Seventy had no evidence of radiolucent demarcation, and fifteen were in hips that had only one or two affected zones. The remaining three femoral components were radiographically loose at the latest follow-up evaluation, resulting in an over-all rate of aseptic loosening of 10 per cent (ten hips) (Table II).

Femoral endosteal lysis was present in nine hips (9 per cent), seven of which subsequently were revised. The remaining two hips had only one affected zone²⁵ and no other evidence of loosening. Cortical hypertrophy at the level of the tip of the femoral component was present in nineteen hips (19 per cent), but it was not associated with loosening in any hip. Subsidence of the femoral component occurred in twenty-two hips (22 per cent) and was associated with a fracture of the cement mantle in six hips (6 per cent). Only two components (2 per cent) had subsided more than five millimeters; both were associated with a fracture of the cement at the tip of the implant and both were loose. The other four components that were associated with a fracture of the cement at the tip of the implant had subsided less than five millimeters but also were loose; a fracture of the cement mantle therefore was associated with all six instances of loosening in this group. Changes in the calcar were seen in thirty-nine hips (39 per cent): eleven had marked osteopenia, and twenty-eight had a decrease in the height of the calcar (Table III).

The probability of survival of both of the original components at ten and twenty-five years was 96 per cent (95 per cent confidence interval, 91 to 100 per cent) and 77 per cent (95 per cent confidence interval, 67 to 87 per cent), respectively. The ten and twenty-five-year survival rates for the acetabular component were 98 per cent (95 per cent confidence interval, 94 to 100 per cent) and 79 per cent (95 per cent confidence interval, 70 to 88 per cent); the corresponding rates for the femoral component were 97 per cent (95 per cent confidence interval, 93 to 100 per cent) and 85 per cent (95 per cent confidence interval, 77 to 93 per cent) (Fig. 6 and Table IV).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

In the present study, total hip arthroplasty was associated with a general improvement in the range of motion and function of the hip as well as with dramatic relief of pain. Postoperatively, 159 (99 per cent) of the 161 patients had little or no pain (grade⁹ 5 or 6) and 146 (91 per cent) were entirely pain-free (grade 6). Only forty-six (85 per cent) of the fifty-four patients who had degenerative osteoarthrosis were completely free of pain compared with 100 (93 per cent) of the 107 patients in the other two groups; this difference was found to be significant (p < 0.04). Fifty-three patients (33 per cent) still needed walking aids, but seventy-two (45 per cent) were able to walk normally and thirty-six (22 per cent) were able to walk without a cane although they had a limp. Postoperatively, 189 (84 per cent) of the 226 hips had a cumulative range of motion of more than 210 degrees and ninety-two (41 per cent) had a range of more than 260 degrees. These results are comparable with those of previous series ranging from sixty-seven to 218 arthroplasties, in which good or excellent results were reported in association with more than 90 per cent of the procedures^{4,10,27,34,64}.

Deep infection was noted in only two hips (1 per cent); this is consistent with the rates of 0.5 to 2 per cent that have been reported for series ranging from 240 to 1041 arthroplasties^31,44,51, but it is in contrast to the higher rates of 3 to 7 per cent that have been reported for young patients in series ranging from seventy-one to eighty-three arthroplasties^21,45,52,58. In the present study, only one patient (one hip; less than 1 per cent) had a documented deep venous thrombosis, two patients (two hips; 1 per cent) had a non-fatal pulmonary embolism, and no patient had a fatal pulmonary embolism. These complications were diagnosed on the basis of the clinical symptoms alone as routine screening procedures were not performed. Although the use of various prophylactic modalities in the present study made it impossible for us to draw any definitive conclusions, thromboembolism did not appear to be a problem in this population of young patients.

Four patients (four hips; 2 per cent) had a postoperative peroneal-nerve palsy with footdrop; in three, this complication resolved spontaneously. The prevalence of nerve palsy has been reported to be 1 to 2 per cent in series ranging from sixty-seven to 413 arthroplasties^{4,10,27,42,49}, but higher rates (3 to 9 per cent) have been reported in series ranging from twenty-three to sixty-six arthroplasties in patients who had congenital dislocation^{15,17,20,23,41}. In the current series, only one hip (less than 1 per cent) dislocated in the early postoperative period (within three months after the operation); the patient was managed successfully with closed reduction. Rates of dislocation of 0.5 to 4 per cent have been reported in series ranging from seventy-five to 1041 arthroplasties^{31,42,51,53,56}, which have included many young patients; however, the highest rates (5 to 11 per cent) consistently have been reported among patients who had congenital dislocation of the hip, in series ranging from twenty-three to eighty arthroplasties^20-22,41.

Aseptic loosening of the acetabular component occurred in fifty-six (25 per cent) of the 226 hips in the present study after an average duration of follow-up of 19.7 years. This rate of loosening compares well with the rates of 11 to 31 per cent at 7.5 to ten years in series ranging from seventy-five to 1041 arthroplasties^16,31,33,53 and of 16 to 50 per cent at fifteen to twenty years in series ranging from ninety-nine to 322 arthroplasties^34,51,56,63. The rate of migration of the acetabular component (forty-four hips; 19 per cent) also compares favorably with the rates of 14 to 22 per cent that have been reported in series ranging from forty-nine to 1041 arthroplasties^28,31,60,63. The highest rate of loosening in the current series (twenty-two hips; 37 per cent) was seen in the group of patients who had congenital dislocation of the hip. This problem has been noted by other authors, who reported rates of loosening of 24 to 53 per cent at 7.5 to sixteen years in series ranging from twenty-three to eighty arthroplasties^{2,21,22,41,45}. High rates of loosening also have been reported for young patients who have degenerative osteoarthrosis (rates of 50 to 60 per cent have been reported in series ranging from thirty-three to eighty-nine arthroplasties^7,56,57) and patients who have rheumatoid arthritis (rates of 15 to 26 per cent have been reported at five to twelve years in series ranging from fifty-seven to eighty-three arthroplasties^12,36,58,60). In the current series, patients who had rheumatoid arthritis had the lowest rates of both migration (13 hips; 13 per cent) and aseptic loosening (fifteen hips; 15 per cent); these findings are in contrast to those described in previous reports, in which poor bone quality and rheumatoid disease were implicated as being the major factors that contributed to failure of the implant^37,53,60. In the present series, there was an increased risk of loosening and migration of the acetabular component (p < 0.01 for both) when there was any evidence of radiolucent demarcation on the radiograph made one year postoperatively.

The rate of migration mirrored the rate of loosening in all three groups of patients, and more than half of the patients who had either a loose or a migrated socket remained completely asymptomatic; these findings are consistent with those described in many previous reports^{18,19,30,31,44}. Thus, strong justification remains for the policy of continuous long-term follow-up of young patients who have had an arthroplasty; the aim is to detect these silent changes before the loss of bone stock becomes so extensive that it precludes a successful revision operation.

Aseptic loosening of the femoral component was less common than that of the acetabular component, with an over-all rate of 15 per cent (thirty-three of 226 hips). This rate is consistent with previously reported rates of 4 to 10 per cent at six to ten years in series ranging from forty-four to 1041 arthroplasties^{16,28,31,42,59} and of 7 to 14 per cent at eighteen to twenty years in series ranging from sixty-three to 244 arthroplasties^34,56,57. The rate of loosening in the patients who had degenerative osteoarthrosis (26 per cent; seventeen of sixty-six hips) was significantly higher than those in the other two groups (p < 0.05); this rate is substantially higher than the rate of 14 per cent (thirty-one of 218 hips) reported at twenty years by Joshi et al.³⁴, but it is more consistent with the rate of 21 per cent (thirteen of sixty-three hips) reported at twelve years by Torchia et al.⁵⁷. In the present study, the rate of aseptic loosening of the femoral component was 10 per cent both in the group of patients who had congenital dislocation (six of sixty hips) and in the group of patients who had rheumatoid arthritis (ten of 100 hips). With the numbers available, we could detect no significant relationship between the alignment of the femoral component (neutral, valgus, or varus) and the rate of loosening or of revision (p > 0.1).

The femoral component subsided less than five millimeters in thirty-eight hips (17 per cent); this amount is widely considered to be compatible with a good long-term outcome as minor subsidence is to be expected in the early years after the operation^38,39,42. Subsidence of more than five millimeters has been accepted as indicative of loosening as it suggests that the proximal part of the cement mantle is unable to support the prosthesis, making additional subsidence and loosening more likely. In the present study, seven femoral components (3 per cent) subsided more than five millimeters; all of these components were loose. Five of the seven components were in patients who had degenerative osteoarthrosis, and two were in patients who had rheumatoid disease.

Hypertrophy of the femoral cortex at the level of the tip of the implant, which is thought to represent the physiological adaptation of the bone to an area of transition from the artificially stiffened proximal segment to the normal distal aspect of the femur, has been reported in 16 to 37 per cent of hips in series ranging from sixty-six to 664 arthroplasties^{12,16,34,39,63}. This finding was noted in eighty-one (36 per cent) of the 226 hips in the current series, most commonly in the group of patients who had degenerative osteoarthrosis (thirty-five [53 per cent] of sixty-six hips; p < 0.01). We could detect no significant relationship, with the numbers available, between distal femoral cortical hypertrophy and loosening of the femoral component (p > 0.1). Fracture of the femoral component occurred in eight (4 per cent) of the 226 hips in the present study and has been reported in association with 1 to 4 per cent of Charnley implants in series ranging from fifty-nine to 664 arthroplasties^{16,33,39,40,41,44}. However, the introduction of stronger implant materials and improved cementing techniques has decreased this problem^3,16.

The total amount of wear of the acetabular component for the entire series averaged 2.3 millimeters (range, 0.12 to 7.21 millimeters). The rate of wear of the acetabular component for the entire series averaged 0.11 millimeter per year (range, 0.01 to 0.55 millimeter per year); this rate is similar to those of 0.07 and 0.14 millimeter per year reported in earlier studies^24,51,56,63. In the current series, the average rate of wear for the components that were revised was more than twice the rate for the components that survived (0.19 compared with 0.09 millimeter per year). This was a consistent finding in all of the diagnostic groups, with high rates of wear being associated with increased rates of migration and revision of the acetabular component (p < 0.01 for both). Rates of 0.19 to 0.21 millimeter per year have previously been reported by direct measurement of explanted cups^32,62.

Osteolysis surrounding the femoral stem has been reported in 10 to 23 per cent of hips at twelve to sixteen years in series ranging from sixty-six to 218 arthroplasties^12,34,45,63. Osteolysis was noted in thirty (13 per cent) of the 226 hips in the current series and was most common in the group of patients who had degenerative osteoarthrosis (thirteen [20 per cent] of sixty-six hips; p < 0.01). Osteolysis was strongly associated with an increased rate of wear of the acetabular component as well as with loosening of the femoral component (p < 0.01 for both). Changes in the calcar have been reported in 24 to 66 per cent of hips in series ranging from sixty-six to 664 arthroplasties^{12,16,34,57,63}; such changes were noted in seventy-eight (35 per cent) of the 226 hips in the current series. Changes in the calcar were associated with increased wear of the acetabular component (p = 0.03), but we did not detect a significant relationship, with the numbers available, with loosening or revision of the femoral component (p > 0.1).

Heterotopic ossification was noted in eighty-nine (39 per cent) of the 226 hips in the entire series, but it usually was minor; only eight hips (4 per cent) had a clinically important amount that was likely to restrict the range of motion (class 3 or 4 of Brooker et al.⁵), and only one hip in the entire series became ankylosed. Heterotopic ossification was most common in the group of patients who had degenerative osteoarthrosis (forty [61 per cent] of sixty-six hips; p < 0.05). The reported rates of heterotopic ossification have ranged widely, with 3 to 10 per cent of hips in series ranging from sixty-six to 664 arthroplasties having clinically important amounts that were likely to interfere with the range of motion^{16,33,51,56,57}.

Non-union or displacement at the site of the trochanteric osteotomy has been reported in 2 to 14 per cent of hips in series ranging from seventy-three to 664 arthroplasties^{10,16,33,53,56}, with the highest rates (10 to 29 per cent, in series ranging from twenty-one to eighty arthroplasties^2,17,20,21) having been reported among patients who had congenital dislocation of the hip. The rate of non-union and displacement in the current series was 9 per cent (twenty of 226 hips), with the highest rate (15 per cent; fifteen of 100 hips) being seen in the group of patients who had rheumatoid arthritis (p = 0.04). Many surgeons have abandoned the routine use of trochanteric osteotomy because of its attendant complications, but it is still widely recommended for complex or revision procedures when optimum exposure of both the acetabulum and the proximal aspect of the femur is essential^15,17,20,29.

The average duration of follow-up and the long-term survival of both the acetabular and the femoral components in the current study (Fig. 7 and Table IV) exceeded those in previous studies of patients who have had an arthroplasty, including studies of young patients. Joshi et al. reported an 86 per cent probability of survival of the femoral component and an 84 per cent probability of survival of the acetabular component at sixteen years in a series of 218 arthroplasties³⁴, but Torchia et al. reported only a 55 per cent probability of survival of both components at fifteen years in a series of sixty-three arthroplasties⁵⁷. In the present study, as in most studies^12,34,51,56, the probability of survival of the acetabular component was consistently lower than that of the femoral component. Chmell et al. reported a 70 per cent probability of survival of the acetabular component and an 85 per cent probability of survival of the femoral component at fifteen years in a series of sixty-six arthroplasties in patients who had juvenile-onset chronic rheumatoid arthritis¹². Garvin et al. reported a 59 per cent probability of survival, also at fifteen years, in a series of twenty-three arthroplasties in patients who had congenital dislocation of the hip²².

There was no difference between the female and the male patients with regard to survival of the acetabular component, but the rate of revision of the femoral component was significantly higher in the male patients (p < 0.01). Patients who had had an operation on the hip before the index arthroplasty had a significantly higher rate of revision (p < 0.05); with the numbers available, however, we detected no significant difference between the patients who were less than thirty years old and those who were thirty to forty years old.

The current report documents the long-term results of 226 total hip arthroplasties that were performed by a small group of experienced surgeons with use of a relatively unsophisticated implant and a standardized operative approach. We observed a low prevalence of dislocation, trochanteric non-union, and general complications despite the complex osseous and soft-tissue abnormalities that were noted in many patients. The central concepts of the Charnley low-friction arthroplasty—use of a small (22.25-millimeter-diameter) femoral head, use of an all-polyethylene acetabular component, and use of acrylic cement for immediate stable fixation—have remained unaltered. The performance of the femoral component was superior to that of the acetabular component. The major problems that remain to be solved with regard to arthroplasty in young patients are wear, loosening, and the need for revision of the acetabular component.

Only through prolonged and consistent clinical and radiographic follow-up can the true long-term outcomes and modes of failure of any given design of total hip replacement be established. Loss of patients, which compromises the completeness of follow-up, is inevitable in the older age-group of patients who typically have an arthroplasty; the rates of mortality for these patients are greater than 50 per cent at fifteen years⁴⁰ and greater than 60 per cent at twenty years^44,51. The rate of mortality in the current series of younger patients was only 24 per cent (thirty-eight of 161 patients) at twenty years, with most of the deaths occurring in patients who had juvenile-onset rheumatoid arthritis. These patients are known to have a higher rate of mortality, which previously has been reported to be 18 to 20 per cent at ten to twelve years^12,37,47. When one considers the high level of activity of the young patients in our series, the long-term survival of both components was satisfactory; the probability of survival of the acetabular and femoral components at twenty-five years was 68 per cent (95 per cent confidence interval, 61 to 75 per cent) and 81 per cent (95 per cent confidence interval, 76 to 87 per cent), respectively. Improvements in cementing techniques and metallurgy have decreased the problem of breakage of the femoral component^3,16,45, and the greater emphasis on placement of the acetabular component at its original level as well as the more widespread use of bone-grafting^15,23,29,50 is likely to improve the performance of the acetabular component. Improvements in the bearing surfaces, including the use of ceramics and more durable polyethylene, are predicted to result in decreased levels of wear³². With the introduction of these advances, the future results of total hip arthroplasty with use of cement can be expected to surpass those that have been reported to date. At present, this procedure continues to offer the best long-term results in young, active patients who need a total hip replacement.

NOTE: The authors thank Mrs. E. Jenkins of the Clinical Outcomes Research Group at Wrightington Hospital and Dr. B. Farragher of the Department of Statistics at Manchester University for their assistance in compiling and analyzing the data. The authors also thank Mr. K. Hardinge, Consultant Orthopaedic Surgeon at the Centre for Hip Surgery, Wrightington Hospital, for his advice on the section of the original manuscript regarding the results of total hip arthroplasty in patients who had degenerative osteoarthrosis.

References

Introduction | Materials and Methods | Results | Discussion | References

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Fig. 1 Bar graphs comparing the preoperative and postoperative scores for pain, function, and range of motion for the hips that had congenital dislocation (C.D.H.).

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Fig. 3 Bar graphs comparing the preoperative and postoperative scores for pain, function, and range of motion for the hips that had degenerative osteoarthrosis.

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Fig. 5 Bar graphs comparing the preoperative and postoperative scores for pain, function, and range of motion for the hips that had rheumatoid arthritis.

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Fig. 6 Individual survivorship curves for the hips that had rheumatoid arthritis, showing the acetabular and femoral components separately and in combination, with 95 per cent confidence intervals.

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Fig. 7 Individual twenty-five-year survivorship curves for the entire series, showing the acetabular and femoral components separately and in combination, with 95 per cent confidence intervals.

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TABLE I DEMOGRAPHIC DATA AND POSTOPERATIVE PAIN SCORES

*The difference was significant compared with the other two groups (p < 0.04).

	Diagnostic Group
	Congenital	Degenerative	Rheumatoid
	Dislocation	Osteoarthrosis	Arthritis	Entire Series
No. of patients	44	54	63	161
No. of hips	60	66	100	226
No. of patients managed with bilateral arthroplasty	16 (36%)	12 (22%)	37 (59%)	65 (40%)
Gender (male/female) (no. of patients)	3/41	29/25	15/48	47/114
Average age (yrs.)	32.4	33.9	30.5	31.7
Deaths (no. of patients)	3 (7%)	8 (15%)	27 (43%)	38 (24%)
Average duration of follow-up (mos.)	244	234	231	236
Charnley category⁹(no. of patients)
A	13 (30%)	31 (57%)	2 (3%)	46 (29%)
B	26 (59%)	13 (24%)	8 (13%)	47 (29%)
C	5 (11%)	10 (19%)	53 (84%)	68 (42%)
Postop. pain scores⁹(no. of patients)
No or only slight pain (grade 5 or 6)	44 (100%)	52 (96%)	63 (100%)	159 (99%)
Pain-free (grade 6)	41 (93%)	46 (85%)*	59 (94%)	146 (91%)

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TABLE I DEMOGRAPHIC DATA AND POSTOPERATIVE PAIN SCORES

*The difference was significant compared with the other two groups (p < 0.04).

	Diagnostic Group
	Congenital	Degenerative	Rheumatoid
	Dislocation	Osteoarthrosis	Arthritis	Entire Series
No. of patients	44	54	63	161
No. of hips	60	66	100	226
No. of patients managed with bilateral arthroplasty	16 (36%)	12 (22%)	37 (59%)	65 (40%)
Gender (male/female) (no. of patients)	3/41	29/25	15/48	47/114
Average age (yrs.)	32.4	33.9	30.5	31.7
Deaths (no. of patients)	3 (7%)	8 (15%)	27 (43%)	38 (24%)
Average duration of follow-up (mos.)	244	234	231	236
Charnley category⁹(no. of patients)
A	13 (30%)	31 (57%)	2 (3%)	46 (29%)
B	26 (59%)	13 (24%)	8 (13%)	47 (29%)
C	5 (11%)	10 (19%)	53 (84%)	68 (42%)
Postop. pain scores⁹(no. of patients)
No or only slight pain (grade 5 or 6)	44 (100%)	52 (96%)	63 (100%)	159 (99%)
Pain-free (grade 6)	41 (93%)	46 (85%)*	59 (94%)	146 (91%)

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TABLE II RATES OF LOOSENING, WEAR, AND REVISION

*N = number of hips.†The difference was significant compared with the two other groups (p < 0.05).

	Diagnostic Group*
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Acetabular component
Aseptic loosening (no. of hips)	22 (37%)	19 (29%)	15 (15%)†	56 (25%)
Migration (no. of hips)	17 (28%)	14 (21%)	13 (13%)†	44 (19%)
Revision (no. of hips)	22 (37%)	22 (33%)	17 (17%)†	61 (27%)
Average time from index op. to revision (mos.)	193	156	166	167
Average annual rate of wear (mm/yr.)
All components	0.13	0.11	0.09	0.11
Unrevised components	0.1	0.08	0.08	0.09
Revised components	0.17	0.24	0.17	0.19
Femoral component
Aseptic loosening (no. of hips)	6 (10%)	17 (26%)†	10 (10%)	33 (15%)
Revision (no. of hips)	6 (10%)	18 (27%)†	12 (12%)	36 (16%)
Average time from index op. to revision (mos.)	156	170	164	167

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TABLE II RATES OF LOOSENING, WEAR, AND REVISION

*N = number of hips.†The difference was significant compared with the two other groups (p < 0.05).

	Diagnostic Group*
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Acetabular component
Aseptic loosening (no. of hips)	22 (37%)	19 (29%)	15 (15%)†	56 (25%)
Migration (no. of hips)	17 (28%)	14 (21%)	13 (13%)†	44 (19%)
Revision (no. of hips)	22 (37%)	22 (33%)	17 (17%)†	61 (27%)
Average time from index op. to revision (mos.)	193	156	166	167
Average annual rate of wear (mm/yr.)
All components	0.13	0.11	0.09	0.11
Unrevised components	0.1	0.08	0.08	0.09
Revised components	0.17	0.24	0.17	0.19
Femoral component
Aseptic loosening (no. of hips)	6 (10%)	17 (26%)†	10 (10%)	33 (15%)
Revision (no. of hips)	6 (10%)	18 (27%)†	12 (12%)	36 (16%)
Average time from index op. to revision (mos.)	156	170	164	167

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TABLE III COMPLICATIONS AND RADIOGRAPHIC FEATURES*

*N = number of hips.†The difference was significant compared with the other two groups (p < 0.05).

	Diagnostic Group
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Fracture of femoral component	2 (3%)	2 (3%)	4 (4%)	8 (4%)
Deep infection	0	0	2 (2%)	2 (1%)
Early dislocation	0	0	1 (1%)	1 (<1%)
Footdrop	2 (3%)	2 (3%)	0	4 (2%)
Heterotopic ossification	20 (33%)	40 (61%)†	29 (29%)	89 (39%)
Breakage of wire	9 (15%)	11 (17%)	19 (19%)	39 (17%)
Trochanteric displacement	2 (3%)	3 (5%)	15 (15%)†	20 (9%)
Trochanteric reoperation	2 (3%)	5 (8%)	4 (4%)	11 (5%)
Subsidence of femoral component
<5 mm	9 (15%)	9 (14%)	20 (20%)	38 (17%)
>5 mm	0	5 (8%)	2 (2%)	7 (3%)
Fracture of cement around tip of femoral component	3 (5%)	7 (11%)	6 (6%)	16 (7%)
Femoral endosteal lysis	8 (13%)	13 (20%)†	9 (9%)	30 (13%)
Changes in calcar	22 (37%)	17 (26%)	39 (39%)	78 (35%)
Cortical hypertrophy around tip of femoral component	27 (45%)	35 (53%)	19 (19%)	81 (36%)

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TABLE III COMPLICATIONS AND RADIOGRAPHIC FEATURES*

*N = number of hips.†The difference was significant compared with the other two groups (p < 0.05).

	Diagnostic Group
	Congenital Dislocation (N = 60)	Degenerative Osteoarthrosis (N = 66)	Rheumatoid Arthritis (N = 100)	Entire Series (N = 226)
Fracture of femoral component	2 (3%)	2 (3%)	4 (4%)	8 (4%)
Deep infection	0	0	2 (2%)	2 (1%)
Early dislocation	0	0	1 (1%)	1 (<1%)
Footdrop	2 (3%)	2 (3%)	0	4 (2%)
Heterotopic ossification	20 (33%)	40 (61%)†	29 (29%)	89 (39%)
Breakage of wire	9 (15%)	11 (17%)	19 (19%)	39 (17%)
Trochanteric displacement	2 (3%)	3 (5%)	15 (15%)†	20 (9%)
Trochanteric reoperation	2 (3%)	5 (8%)	4 (4%)	11 (5%)
Subsidence of femoral component
<5 mm	9 (15%)	9 (14%)	20 (20%)	38 (17%)
>5 mm	0	5 (8%)	2 (2%)	7 (3%)
Fracture of cement around tip of femoral component	3 (5%)	7 (11%)	6 (6%)	16 (7%)
Femoral endosteal lysis	8 (13%)	13 (20%)†	9 (9%)	30 (13%)
Changes in calcar	22 (37%)	17 (26%)	39 (39%)	78 (35%)
Cortical hypertrophy around tip of femoral component	27 (45%)	35 (53%)	19 (19%)	81 (36%)

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TABLE IV SURVIVAL OF THE ACETABULAR AND FEMORAL COMPONENTS*

*The values are given as percentages, with the 95 per cent confidence intervals in parentheses.†The difference was significant at p < 0.05.

	Diagnostic Group
	Congenital Dislocation	Degenerative Osteoarthrosis	Rheumatoid Arthritis	Entire Series
Acetabular component
10 yrs.	97 (92—100)	89 (81—97)	98 (94—100)	93 (90—96)
20 yrs.	70 (57—83)	64 (51—78)†	79 (70—88)	71 (65—77)
25 yrs.	58 (42—74)	59 (41—77)	79 (70—88)†	68 (61—75)
Femoral component
10 yrs.	97 (92—100)	94 (88—100)	97 (93—100)	95 (93—98)
20 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	82 (77—88)
25 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	81 (76—87)
Both components
10 yrs.	97 (92—100)	86 (77—95)	96 (91—100)	91 (88—95)
20 yrs.	64 (50—77)	63 (49—77)	77 (67—87)	67 (61—74)
25 yrs.	54 (39—69)	52 (34—71)	77 (67—87)†	65 (58—72)

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TABLE IV SURVIVAL OF THE ACETABULAR AND FEMORAL COMPONENTS*

*The values are given as percentages, with the 95 per cent confidence intervals in parentheses.†The difference was significant at p < 0.05.

	Diagnostic Group
	Congenital Dislocation	Degenerative Osteoarthrosis	Rheumatoid Arthritis	Entire Series
Acetabular component
10 yrs.	97 (92—100)	89 (81—97)	98 (94—100)	93 (90—96)
20 yrs.	70 (57—83)	64 (51—78)†	79 (70—88)	71 (65—77)
25 yrs.	58 (42—74)	59 (41—77)	79 (70—88)†	68 (61—75)
Femoral component
10 yrs.	97 (92—100)	94 (88—100)	97 (93—100)	95 (93—98)
20 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	82 (77—88)
25 yrs.	89 (80—98)	74 (61—87)†	85 (77—93)	81 (76—87)
Both components
10 yrs.	97 (92—100)	86 (77—95)	96 (91—100)	91 (88—95)
20 yrs.	64 (50—77)	63 (49—77)	77 (67—87)	67 (61—74)
25 yrs.	54 (39—69)	52 (34—71)	77 (67—87)†	65 (58—72)