JBJS | Hydroxyapatite-Coated Femoral Stems. A Matched-Pair Analysis of Coated and Uncoated Implants*

The Journal of Bone & Joint Surgery, Volume 78, Issue 3

Articles | March 01, 1996

Hydroxyapatite-Coated Femoral Stems. A Matched-Pair Analysis of Coated and Uncoated Implants*

RICHARD H. ROTHMAN, M.D., PH.D.†; WILLIAM J. HOZACK, M.D.†; AMAR RANAWAT, B.A.†; LISA MORIARTY, R.N., M.S.N.†, PHILADELPHIA, PENNSYLVANIA

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Investigation performed at The Rothman Institute, Philadelphia

The Journal of Bone & Joint Surgery. 1996; 78:319-24

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Abstract

Fifty-two pairs of patients who had had a total hip arthroplasty with a porous-coated femoral implant were studied in a retrospective, matched-pair analysis. Half of the patients had received a femoral component coated with hydroxyapatite and the other half (the controls), an identical component but without hydroxyapatite. The patients were matched for age, sex, weight, diagnosis, Charnley class, operative approach, and duration of follow-up. Identical uncoated hemispherical acetabular implants were used in both groups.At the time of follow-up, at a mean of 2.2 years (range, two to 3.4 years) after the operation, the mean Charnley scores for pain, function, and motion were 5.6, 5.5, and 5.6 points, respectively, in the group that had received a hydroxyapatite-coated femoral component and 5.6, 5.6, and 5.6 points, respectively, in the group that had received a non-coated component; none of these differences were significant (p = 0.86, 0.89, and 0.80, respectively). There were no revisions in either group. Radiographs indicated stable fixation in both groups and no differences in the radiographic parameters of loosening between the two groups.Within the relatively short time-frame of this study, there appeared to be no clinical or radiographic advantage to the use of hydroxyapatite in primary total hip arthroplasties. However, these results should be considered as preliminary. Longer follow-up may reveal unrecognized advantages or disadvantages.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

The fundamental principle involved in the coating of a hip prosthesis with hydroxyapatite for use in an arthroplasty without cement is that doing so accelerates the adherence of bone to the surface of the implant and thus, in theory, improves the clinical and radiographic results. Geesink et al., who were early and enthusiastic investigators of this modality, stated that "hydroxyl-apatite coatings permit an implant fixation far superior to current methods using either cemented or cementless techniques."¹³ A preponderance of both laboratory and clinical studies evaluating hydroxyapatite have supported this enthusiasm; however, there has been a paucity of controlled clinical studies^7,18-21,23.

The specific biological advantages that are sought with the use of hydroxyapatite include faster and more uniform bone ingrowth and a stronger bond with bone^7,18,23. Additionally, recent evidence has indicated that hydroxyapatite helps to compensate for an imperfect fit of the prosthesis and perhaps improves the ability to achieve fixation even in the presence of micro-motion^19-21. The theoretical disadvantages of a hydroxyapatite coating include occlusion of the porous surface, resorption of the coating, and late delamination of the coating with formation of particulate debris^1,6. The particulate debris of hydroxyapatite might well accelerate polyethylene wear because of the hardness of the hydroxyapatite.

An early, uncontrolled clinical study by Geesink indicated good clinical results and radiographic findings suggestive of good osseous fixation of the prosthesis¹². Kroon and Freeman, in a series of twenty-six hydroxyapatite-coated press-fit non-porous femoral components and forty-three identical but uncoated implants, demonstrated reduced subsidence of the coated devices after one year of follow-up. Friedman et al., in a prospective, randomized, controlled study of 345 patients, noted a significant improvement (p < 0.05) in both clinical and radiographic parameters after two years of follow-up in 288 patients who had a hydroxyapatite-coated femoral implant compared with the results in fifty-seven patients who had an identical but uncoated press-fit non-porous implant. Using roentgen stereophotogrammetric analysis in a series of fifteen patients, Søballe et al. found less migration and improved clinical scores (p < 0.05) one year postoperatively in the seven patients who had a hydroxyapatite-coated plasma-sprayed titanium femoral component compared with the findings in the eight patients who had an uncoated femoral component²². In a preliminary, randomized study from our institution, roentgen stereophotogrammetric analysis and use of the clinical scoring system of Charnley³ showed that a hydroxyapatite-coated porous titanium implant had no advantage compared with an identical implant without a hydroxyapatite coating⁵. Pain scores that were determined according to a visual-analog scale were slightly improved at three and six months for the hips that had an implant with a hydroxyapatite coating, but no advantage was noted at the most recent, one-year evaluation⁵.

The present study was undertaken to evaluate the clinical and radiographic results for patients who had a hydroxyapatite-coated porous titanium implant compared with those who had an identical implant without a hydroxyapatite coating. The study was performed with use of a carefully constructed matched-pair analysis, and all patients had been followed for a minimum of two years. The findings should help hip surgeons to reach an informed decision in terms of the additional cost entailed with use of a hydroxyapatite coating for prostheses inserted without cement.

*Although none of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received but are directed solely to a research fund, foundation, educational institution, or other non-profit organization with which one or more of the authors are associated. No funds were received in support of this study.

†The Rothman Institute, 800 Spruce Street, Philadelphia, Pennsylvania 19107. Please address requests for reprints to Dr. Rothman.

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TABLE I DEMOGRAPHIC DATA ON THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The values are given as the mean and the standard deviation, with the range in parentheses.†The values are given as the number of patients.

	Hydroxyapatite-Coated-Implant Group	Non-Hydroxyapatite-Coated-Implant Group
No. of patients	52	52
Age* (yrs.)	67 ± 6.27 (53—79)	66 ± 5.96 (51—78)
Weight* (kg)	79 ± 28.4 (53—113)	78 ± 27.1 (52—91)
Male:female ratio	30:22	30:22
Diagnosis	Osteoarth.	Osteoarth.
	(all patients)	(all patients)

Charnley class⁴†
A	23	23
B	16	16
C	13	13

Operative approach†
Modified Hardinge	4	4
Transtrochanteric	48	48

Duration of follow-up* (yrs.)	2.2 ± 0.31 (2—3.2)	2.2 ± 0.59 (2—3.4)

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TABLE I DEMOGRAPHIC DATA ON THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The values are given as the mean and the standard deviation, with the range in parentheses.†The values are given as the number of patients.

	Hydroxyapatite-Coated-Implant Group	Non-Hydroxyapatite-Coated-Implant Group
No. of patients	52	52
Age* (yrs.)	67 ± 6.27 (53—79)	66 ± 5.96 (51—78)
Weight* (kg)	79 ± 28.4 (53—113)	78 ± 27.1 (52—91)
Male:female ratio	30:22	30:22
Diagnosis	Osteoarth.	Osteoarth.
	(all patients)	(all patients)

Charnley class⁴†
A	23	23
B	16	16
C	13	13

Operative approach†
Modified Hardinge	4	4
Transtrochanteric	48	48

Duration of follow-up* (yrs.)	2.2 ± 0.31 (2—3.2)	2.2 ± 0.59 (2—3.4)

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TABLE II DEMOGRAPHIC DATA FOR THE OVER-ALL GROUP OF PATIENTS WHO HAD A HYDROXYAPATITE-COATED IMPLANT AND THE COMBINED GROUP OF MATCHED PATIENTS

*The values are given as the mean and the standard deviation, with the range in parentheses.†According to the Student t test.‡According to the chi-square test with Yates' correction.§The values are given as the number of hips.

	Over-All Group That Had a Hydroxyapatite-Coated Implant	Combined Matched-Pair Group	P Value
No. of patients	98	104

Age* (yrs.)	64 ± 11.01	67 ± 6.05	0.07†
	(31.2—86.1)	(51.3—79.18)

Weight* (kg)	75 ± 15.1	78 ± 12.5	0.05†
	(41—113)	(52—113)

Duration of	2.1 ± 0.34	2.2 ± 0.48	0.04†
follow-up *(yrs.)	(2—3.1)	(2—4.87)

Male:female ratio	49:49	60:44	0.3‡

Diagnosis§
Osteoarthrosis	91	104	0.001‡
Other	9	0

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TABLE II DEMOGRAPHIC DATA FOR THE OVER-ALL GROUP OF PATIENTS WHO HAD A HYDROXYAPATITE-COATED IMPLANT AND THE COMBINED GROUP OF MATCHED PATIENTS

	Over-All Group That Had a Hydroxyapatite-Coated Implant	Combined Matched-Pair Group	P Value
No. of patients	98	104

Age* (yrs.)	64 ± 11.01	67 ± 6.05	0.07†
	(31.2—86.1)	(51.3—79.18)

Weight* (kg)	75 ± 15.1	78 ± 12.5	0.05†
	(41—113)	(52—113)

Duration of	2.1 ± 0.34	2.2 ± 0.48	0.04†
follow-up *(yrs.)	(2—3.1)	(2—4.87)

Male:female ratio	49:49	60:44	0.3‡

Diagnosis§
Osteoarthrosis	91	104	0.001‡
Other	9	0

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TABLE III RESULTS FOR THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The power was 97 per cent.†The values are given as the number of patients, with the percentage in parentheses.

	Hydroxyapatite-Coated-Implant Group (N = 52)	Non-Hydroxyapatite-Coated-Implant Group (N = 52)	P Value*
Mean Charnley score³(points)
Preop.
Pain	2.3	2.6	0.15
Function	3.3	3.1	0.43
Motion	4.2	3.7	0.002
Postop.
Pain	5.6	5.6	0.86
Function	5.5	5.6	0.89
Motion	5.6	5.6	0.80
No. of revisions	0	0

Radiographic data†
Subsidence	1 (2)	0
Distal pedestal formation, zone I	2 (4)	2 (4)
Radiolucent line >50% of zone-I interface	0	0
Atrophy of medial part of femoral neck	43 (83)	32 (62)
Spot welds	35 (67)	41 (79)
Heterotopic ossification²
Grade 4	1 (2)	1 (2)
Grade 3	3 (6)	3 (6)

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TABLE III RESULTS FOR THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The power was 97 per cent.†The values are given as the number of patients, with the percentage in parentheses.

	Hydroxyapatite-Coated-Implant Group (N = 52)	Non-Hydroxyapatite-Coated-Implant Group (N = 52)	P Value*
Mean Charnley score³(points)
Preop.
Pain	2.3	2.6	0.15
Function	3.3	3.1	0.43
Motion	4.2	3.7	0.002
Postop.
Pain	5.6	5.6	0.86
Function	5.5	5.6	0.89
Motion	5.6	5.6	0.80
No. of revisions	0	0

Radiographic data†
Subsidence	1 (2)	0
Distal pedestal formation, zone I	2 (4)	2 (4)
Radiolucent line >50% of zone-I interface	0	0
Atrophy of medial part of femoral neck	43 (83)	32 (62)
Spot welds	35 (67)	41 (79)
Heterotopic ossification²
Grade 4	1 (2)	1 (2)
Grade 3	3 (6)	3 (6)

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The Taperloc femoral prosthesis.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

One hundred consecutive total hip arthroplasties with use of a hydroxyapatite-coated porous Taperloc stem (Biomet, Warsaw, Indiana) were performed without cement in ninety-eight patients from May 1989 through June 1991 at our institution (Fig. 1). The patients were followed for a mean of 2.1 years (range, two to 3.1 years). The mean age of the patients was sixty-four years (range, 31.2 to 86.1 years), and the mean weight was seventy-five kilograms (range, forty-one to 113 kilograms). There were forty-nine men and forty-nine women. The preoperative diagnosis was osteoarthrosis in ninety-one hips, rheumatoid arthritis in four, avascular necrosis in three, and congenital dysplasia in two. Thirty-six hips were classified as having Charnley class-A disease (unilateral involvement); thirty-eight, Charnley class-B (involvement of the contralateral hip); and twenty-six, Charnley class-C (involvement of other joints or systemic problems that limited activity)⁴.

During the same time period, we performed 299 total hip arthroplasties with insertion of a non-hydroxyapatite-coated porous Taperloc stem without cement. There were no selection criteria with regard to the use of a coated as opposed to an uncoated implant; however, true statistical randomization was not performed. A matching process was undertaken to compare the results in the patients who had been managed with the hydroxyapatite-coated implant with those in a similar group of patients who had an identical femoral component but without a hydroxyapatite coating. In order to eliminate any confounding influences, the two groups were matched for age, sex, weight, diagnosis, Charnley class⁴, operative approach, and duration of follow-up (Table I). This strategy of matched-pair analysis, while inferior to a prospective, randomized trial, creates a control group that lessens the biases inherent in a retrospective study such as this. Fifty-two matched pairs were created. The combined group of matched pairs had a mean age of sixty-seven years and a mean weight of seventy-eight kilograms, each group comprised thirty men and twenty-two women, all patients had an underlying diagnosis of osteoarthrosis, and the mean duration of follow-up was 2.2 years. There were no significant differences between these matched pairs. The characteristics of the combined group of matched pairs did differ from those of the first ninety-eight patients who had a hydroxyapatite-coated implant (Table II). If any bias was introduced by this selection process, it would tend to favor the results in the hydroxyapatite-coated-implant group as a whole rather than the group selected for the matched-pair analysis, as the patients weighed more and there were more men in the matched group that had a hydroxyapatite-coated implant than in the over-all group that had such an implant.

All operations were performed in an ultra-clean air enclosure with use of body-isolator systems. Prophylactic antibiotics were given intravenously at the time of the operation and were continued for forty-eight hours. Ten milligrams of low-dose warfarin was given on the night of the operation, and the dose was then adjusted daily to maintain the prothrombin time between fourteen and sixteen seconds²⁵. All patients wore knee-high elastic compression stockings; were allowed to walk on the day after the operation; were encouraged to perform exercises of the calf, thigh, and buttock; and were instructed to bear only 10 per cent of the body weight on the affected limb for six weeks, at which time they progressed to use of a cane.

A hemispherical titanium-alloy acetabular component (Universal cup; Biomet) with a plasma-spray coating was used in all patients. This component incorporates a peripheral flange to enhance fixation after reaming of the acetabular bed with hemispherical reamers that correspond to the outer diameter of the cup without its porous coating. Routinely, at least two titanium screws were inserted through holes in the cup for supplemental fixation. A modular polyethylene liner was then impacted into position within the metal shell of the cup. This liner has a cylindrical outer shape with varying thicknesses of the rim. None of the cups were coated with hydroxyapatite.

The Taperloc femoral component is composed of titanium alloy with a plasma-spray coating on its proximal one-third. It is tapered, collarless, and modular, and it is available in both standard and lateral (plus five-millimeter) offset variations. Morse-taper modular twenty-two and twenty-eight-millimeter head-neck attachments are available. The femoral head was made of titanium alloy at the start of the study, but later it was composed of cobalt-chromium alloy. Of the hydroxyapatite-coated implants, thirty-five had a titanium femoral head and seventeen, a cobalt-chromium head. Of the non-hydroxyapatite-coated implants, forty-nine had a titanium head and three, a cobalt-chromium head. Insertion of the Taperloc femoral component requires no endosteal reaming. After sequential broaching to achieve a firm, snug fit, the broach is used as a trial to check for stability and limb length. The final prosthesis is then inserted with firm impaction.

According to the manufacturer, the hydroxyapatite coating was at least 95 per cent pure hydroxyapatite, with a mean crystallinity of 62 per cent and a density that was 99 per cent that of the theoretical density of a solid block of hydroxyapatite. The hydroxyapatite coating was applied to the porous titanium surface of the implant with use of an air-plasma process. The thickness of the coating was between fifty and seventy-five micrometers.

An objective follow-up evaluation was performed by a specially trained physical therapist or a research fellow. Neither the surgeon nor the patient was blinded as to the presence of the hydroxyapatite coating, but the reviewer of the radiographs was blinded. Clinical evaluations were performed preoperatively, six weeks postoperatively, and then yearly with use of the scoring system of Charnley³. The patients were questioned specifically regarding the levels of pain, function, and motion with use of a grading system in which a score of 6 points represents an excellent result; 5 points, a good result; and 1 point, a poor result. The presence of a limp was also recorded.

Anteroposterior radiographs of the pelvis and hip and frog-leg lateral radiographs of the hip were evaluated at the same intervals at which the clinical examinations were performed. A limb-holder was used to position the limb consistently for sequential radiographic studies. Correction factors for magnification were calculated for each radiograph on the basis of the ratio of the measured diameter of the prosthetic head to the actual known diameter¹⁴. Postoperative radiographs were evaluated for ectopic bone according to the classification of Brooker et al.², and the greater trochanter was evaluated for union, non-union, and, if appropriate, the degree of separation. The position of the femoral component was assessed with use of a fixed point of reference on the prosthesis and with use of the lesser trochanter as the fixed point of reference on the femur. The component was considered to have subsided if it had settled five millimeters or more. The orientation of the component was considered to be neutral if the center lines of the component and femur were within 3 degrees of each other; otherwise, the component was designated as being in either varus or valgus alignment. All changes around the femoral component were documented on the basis of a system modified from that described by Engh et al.^8-10. The femoral interface was divided into two zones. Zone I was defined as the area around the porous surface of the femoral component and zone II, as the area around the smooth, distal part of the stem¹⁵. Changes in the medial portion of the femoral neck, such as atrophy or hypertrophy, radiolucent lines, endosteal new-bone formation near the prosthesis (so-called spot welds), so-called distal pedestals, and changes in the cancellous bone, were evaluated according to definitions proposed by Engh et al.¹⁰. Proximity of the component to within one millimeter of endosteal bone was identified in each zone, and anteroposterior fill of the medullary canal by the component was measured at the junction of zones I and II and expressed as a percentage, calculated with use of the anteroposterior diameter of the component as the numerator and the anteroposterior diameter of the medullary canal as the denominator. The primary sign of instability of the component was subsidence of five millimeters or more; conversely, the absence of subsidence was considered a sign of probable stability. Other signs considered suggestive of instability were hypertrophy of the medial portion of the femoral neck, distal pedestal formation, and progressively widening radiolucent lines around the component²⁴. A complete radiolucent line in the porous-coated area (zone I) was considered a sign of failure of bone ingrowth (fibrous fixation). A radiolucent line encompassing more than 50 per cent of the interface in zone I was considered a sign of probable failure of bone ingrowth (probable fibrous fixation). Spot welds in zone I implied fixation of the component by bone ingrowth¹⁰.

Statistical analysis of the parametric data was performed with use of a Mann-Whitney U test. Non-parametric data were analyzed with chi-square analysis.

Results

Introduction | Materials and Methods | Results | Discussion | References

At a minimum of two years, the scores for pain, function, and motion were not significantly better after use of the hydroxyapatite coating than after use of the identical but non-hydroxyapatite-coated implant (Table III). The statistical analysis of power (97 per cent power to detect differences with regard to postoperative pain, function, and motion between the two groups, given the sample size) indicated adequate sample sizes. No revisions had been performed in either the group managed with the hydroxyapatite-coated implant or that managed with the non-hydroxyapatite-coated implant. Radiographic analysis also revealed no evidence of improvement with use of the hydroxyapatite coating.

The mean postoperative Charnley score³ for pain was 5.6 points in both the group treated with the hydroxyapatite-coated implant and that treated with the non-hydroxyapatite-coated implant, and the mean scores for function were 5.5 and 5.6 points, respectively. These scores were not significantly different (p = 0.86 and 0.89, respectively). The mean score for motion was identical (5.6 points) in the two groups. Radiographic analysis comparing the two groups in terms of subsidence, distal pedestal formation, radiolucency, spot welds, and atrophy of the medial portion of the femoral neck showed no difference in the degree of fixation between the two groups. No wear or lysis was noted; therefore, the change from the use of titanium to the use of cobalt-chromium femoral heads during this time-frame had no apparent effect.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

This study was performed to evaluate the short-term value (after a minimum of two years of follow-up), if any, of hydroxyapatite coating of a porous femoral implant. The Taperloc design has been used with great success during the past decade; thus, an effective baseline of performance was available¹⁶. A highly effective implant, however, makes it more difficult to demonstrate the incremental value of modifications such as a hydroxyapatite coating.

Controlled studies that have been reported to date have indicated that, for the most part, the differences between hydroxyapatite-coated and non-hydroxyapatite-coated implants become less dramatic with the passage of time⁵. It would therefore seem reasonable to conclude that, if no clinical or radiographic advantages are evident at two years, it is unlikely that substantial advantages are forthcoming. Disadvantages, however, may become evident over time. Nonetheless, it might be speculated that a tighter bond, enhanced by hydroxyapatite, diminishes the likelihood of lysis due to wear debris. On the basis of the available clinical data from the current study, this hypothesis can be neither supported nor refuted. Since a wide disparity of results has been reported with different designs of porous-coated femoral implants, it is particularly important to construct clinical studies of hydroxyapatite with use of either randomized or matched-pair controls.

A limitation of the current study is the matched-pair analysis and, particularly, the imperfect match with regard to the sex of the patients. Even though equal numbers of women and men were managed with a hydroxyapatite-coated implant, the patients also had to be matched with regard to age, weight, diagnosis, Charnley class⁴, operative approach, and duration of follow-up. If there is a bias, it is inherent in the matching process. This problem indicates the difficulty of attempting to match multiple factors in a pool of modest size. Although the match is less than perfect, such is often the case in clinical studies of this type, and matching is a far stronger control than is use of a historical control group derived from the literature. Another limitation of our study may be the fact that the group managed with the non-hydroxyapatite-coated implant had a greater improvement in the range of motion. This might suggest that these patients had a better result; however, it should be noted that the most recent ranges of motion were similar in the two groups and were excellent in both, allowing little opportunity for additional improvement.

It is of interest to note that previous controlled studies demonstrated improvement in clinical and radiographic parameters in association with the hydroxyapatite coating^11,17,22. It is difficult to reconcile these data with the lack of clinical improvement noted in the present study. Of the three previously reported studies, two^11,17 involved the use of a femoral implant that did not have a porous coating and that depended on a press fit for fixation. The third study, by Søballe et al.²², involved an implant that was somewhat different in configuration than the Taperloc design used in the present study. It may be that the presence of a porous surface or the design of the implant is important enough to obscure the effect, if any, of a hydroxyapatite coating. These three controlled studies were limited to two years of follow-up or less.

It should also be noted that the presence of hydroxyapatite may be disadvantageous, possibly causing formation of additional particulate debris and creating a granulomatous response with bone lysis.

In summary, despite encouraging basic-science and preliminary clinical reports, we noted no clinical improvement with use of the hydroxyapatite coating within the relatively short time-frame of our study. Therefore, although our study had limitations and our findings are preliminary, there appears to be little justification for the additional expense incurred with the use of hydroxyapatite in primary total hip arthroplasty with a porous implant of the design used in this investigation.

References

Introduction | Materials and Methods | Results | Discussion | References

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Brooker, A. F.; Bowerman, J. W.; Robinson, R. A.; and |and |Riley, L. H., Jr.: Ectopic ossification following total hip replacement. Incidence and a method of classification. J. Bone and Joint Surg.,55-A: 1629-1632, Dec. 1973.55-A1629 1973

Charnley, J.: The long-term results of low-friction arthroplasty of the hip performed as a primary intervention. J. Bone and Joint Surg.,54-B(1): 61-76, 1972.54-B(1)61 1972

Charnley, J.: Low-Friction Arthroplasty of the Hip. Theory and Practice, pp. 66-90. New York, Springer, 1979.

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Collier, J. P.; Surprenant, V. A.; Mayor, M. B.; Wrona, M.; Jensen, R. E.; and |and |Surprenant, H. P.: Loss of hydroxyapatite coating on retrieved, total hip components. J. Arthroplasty,8: 389-393, 1993.8389 1993 [PubMed][CrossRef]

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Engh, C. A.; Massin, P.; and |and |Suthers, K. E.: Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components. Clin. Orthop.,257: 107-128, 1990.257107 1990 [PubMed]

Friedman, R. J.; Dorr, L. D.; Gustke, K. A.; Braunohler, W. M.; Savory, C. G.; Guyer, W. R.; and |and |DeAndrade, R. J.: Effects of hydroxyapatite on total hip arthroplasty. Trans. Orthop. Res. Soc.,16: 538, 1991.16538 1991

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Geesink, R. G. T.; De Groot, K.; and |and |Klein, C. P. A. T.: Chemical implant fixation using hydroxyl-apatite coatings. The development of human total hip prosthesis for chemical fixation to bone using hydroxyl-apatite coatings on titanium substrates. Clin. Orthop.,225: 147-170, 1987.225147 1987 [PubMed]

Griffith, M. J.; Seidenstein, M. K.; Williams, D.; and |and |Charnley, J.: Socket wear in Charnley low friction arthroplasty of the hip. Clin. Orthop.,137: 37-47, 1978.13737 1978 [PubMed]

Hozack, W. J.; Rothman, R. H.; Booth, R. E., Jr.; and |and |Balderston, R. A.: Cemented versus cementless total hip arthroplasty. A comparative study of equivalent patient populations. Clin. Orthop.,289: 161-165, 1993.289161 1993 [PubMed]

Hozack, W.; Gardiner, R.; Hearn, S.; Eng, K.; and |and |Rothman, R.: Taperloc femoral component. A 2-6 year study of the first 100 consecutive cases. J. Arthroplasty,9: 489-493, 1994.9489 1994 [PubMed][CrossRef]

Kroon, P.-O., and |and |Freeman, M. A. R.: Hydroxyapatite coating of hip prostheses. Effect on migration into the femur. J. Bone and Joint Surg.,74-B(4): 518-522, 1992.74-B(4)518 1992

Oonishi, H.; Yamamoto, M.; Ishimaru, H.; Tsuji, E.; Kushitani, S.; Aono, M.; and |and |Ukon, Y.: The effect of hydroxyapatite coating on bone growth into porous titanium alloy implants. J. Bone and Joint Surg.,71-B(2): 213-216, 1989.71-B(2)213 1989

Søballe, K.: Hydroxyapatite ceramic coating for bone implant fixation. Mechanical and histological studies in dogs. Acta Orthop. Scandinavica,Supplementum 255: 1993.Supplementum 255 1993

Søballe, K.; Hansen, E. S.; Brockstedt-Rasmussen, H.; and |and |Bünger, C.: Hydroxyapatite coating converts fibrous tissue to bone around loaded implants. J. Bone and Joint Surg.,75-B(2): 270-278, 1993.75-B(2)270 1993

Søballe, K.; Hansen, E. S.; Brockstedt-Rasmussen, H.; Hjortdal, V. E.; Juhl, G. I.; Pedersen, C. M.; Hvid, I.; and |and |Bünger, C.: Gap healing enhanced by hydroxyapatite coating in dogs. Clin. Orthop.,272: 300-307, 1991.272300 1991 [PubMed]

Søballe, K.; Toksvig-Larsen, S.; Gelineck, J.; Fruensgaard, S.; Hansen, E. S.; Ryd, L.; Lucht, U.; and |and |Bünger, C.: Migration of hydroxyapatite coated femoral prostheses. A roentgen stereophotogrammetric study. J. Bone and Joint Surg.,75-B(5): 681-687, 1993.75-B(5)681 1993

Stephenson, P. K.; Freeman, M. A.; Revell, P. A.; Germain, J.; Tuke, M.; and |and |Pirie, C. J.: The effect of hydroxyapatite coating on ingrowth of bone into cavities in an implant. J. ArthroplastY,6: 51-58, 1991.651 1991 [PubMed][CrossRef]

Vresilovic, E. J.; Hozack, W. J.; and |and |Rothman, R. H.: Radiographic assessment of cementless femoral components. Correlation with intraoperative mechanical stability. J. Arthroplasty,9: 137-141, 1994.9137 1994 [PubMed][CrossRef]

Wolf, L. D.; Hozack, W. J.; and |and |Rothman, R. H.: Pulmonary embolism in total joint arthroplasty. Clin. Orthop.,288: 219-233, 1993.288219 1993 [PubMed]

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The Taperloc femoral prosthesis.

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TABLE I DEMOGRAPHIC DATA ON THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The values are given as the mean and the standard deviation, with the range in parentheses.†The values are given as the number of patients.

	Hydroxyapatite-Coated-Implant Group	Non-Hydroxyapatite-Coated-Implant Group
No. of patients	52	52
Age* (yrs.)	67 ± 6.27 (53—79)	66 ± 5.96 (51—78)
Weight* (kg)	79 ± 28.4 (53—113)	78 ± 27.1 (52—91)
Male:female ratio	30:22	30:22
Diagnosis	Osteoarth.	Osteoarth.
	(all patients)	(all patients)

Charnley class⁴†
A	23	23
B	16	16
C	13	13

Operative approach†
Modified Hardinge	4	4
Transtrochanteric	48	48

Duration of follow-up* (yrs.)	2.2 ± 0.31 (2—3.2)	2.2 ± 0.59 (2—3.4)

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TABLE I DEMOGRAPHIC DATA ON THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The values are given as the mean and the standard deviation, with the range in parentheses.†The values are given as the number of patients.

	Hydroxyapatite-Coated-Implant Group	Non-Hydroxyapatite-Coated-Implant Group
No. of patients	52	52
Age* (yrs.)	67 ± 6.27 (53—79)	66 ± 5.96 (51—78)
Weight* (kg)	79 ± 28.4 (53—113)	78 ± 27.1 (52—91)
Male:female ratio	30:22	30:22
Diagnosis	Osteoarth.	Osteoarth.
	(all patients)	(all patients)

Charnley class⁴†
A	23	23
B	16	16
C	13	13

Operative approach†
Modified Hardinge	4	4
Transtrochanteric	48	48

Duration of follow-up* (yrs.)	2.2 ± 0.31 (2—3.2)	2.2 ± 0.59 (2—3.4)

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TABLE II DEMOGRAPHIC DATA FOR THE OVER-ALL GROUP OF PATIENTS WHO HAD A HYDROXYAPATITE-COATED IMPLANT AND THE COMBINED GROUP OF MATCHED PATIENTS

	Over-All Group That Had a Hydroxyapatite-Coated Implant	Combined Matched-Pair Group	P Value
No. of patients	98	104

Age* (yrs.)	64 ± 11.01	67 ± 6.05	0.07†
	(31.2—86.1)	(51.3—79.18)

Weight* (kg)	75 ± 15.1	78 ± 12.5	0.05†
	(41—113)	(52—113)

Duration of	2.1 ± 0.34	2.2 ± 0.48	0.04†
follow-up *(yrs.)	(2—3.1)	(2—4.87)

Male:female ratio	49:49	60:44	0.3‡

Diagnosis§
Osteoarthrosis	91	104	0.001‡
Other	9	0

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TABLE II DEMOGRAPHIC DATA FOR THE OVER-ALL GROUP OF PATIENTS WHO HAD A HYDROXYAPATITE-COATED IMPLANT AND THE COMBINED GROUP OF MATCHED PATIENTS

	Over-All Group That Had a Hydroxyapatite-Coated Implant	Combined Matched-Pair Group	P Value
No. of patients	98	104

Age* (yrs.)	64 ± 11.01	67 ± 6.05	0.07†
	(31.2—86.1)	(51.3—79.18)

Weight* (kg)	75 ± 15.1	78 ± 12.5	0.05†
	(41—113)	(52—113)

Duration of	2.1 ± 0.34	2.2 ± 0.48	0.04†
follow-up *(yrs.)	(2—3.1)	(2—4.87)

Male:female ratio	49:49	60:44	0.3‡

Diagnosis§
Osteoarthrosis	91	104	0.001‡
Other	9	0

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TABLE III RESULTS FOR THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The power was 97 per cent.†The values are given as the number of patients, with the percentage in parentheses.

	Hydroxyapatite-Coated-Implant Group (N = 52)	Non-Hydroxyapatite-Coated-Implant Group (N = 52)	P Value*
Mean Charnley score³(points)
Preop.
Pain	2.3	2.6	0.15
Function	3.3	3.1	0.43
Motion	4.2	3.7	0.002
Postop.
Pain	5.6	5.6	0.86
Function	5.5	5.6	0.89
Motion	5.6	5.6	0.80
No. of revisions	0	0

Radiographic data†
Subsidence	1 (2)	0
Distal pedestal formation, zone I	2 (4)	2 (4)
Radiolucent line >50% of zone-I interface	0	0
Atrophy of medial part of femoral neck	43 (83)	32 (62)
Spot welds	35 (67)	41 (79)
Heterotopic ossification²
Grade 4	1 (2)	1 (2)
Grade 3	3 (6)	3 (6)

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TABLE III RESULTS FOR THE HYDROXYAPATITE-COATED-IMPLANT AND NON-HYDROXYAPATITE-COATED-IMPLANT MATCHED GROUPS

*The power was 97 per cent.†The values are given as the number of patients, with the percentage in parentheses.

	Hydroxyapatite-Coated-Implant Group (N = 52)	Non-Hydroxyapatite-Coated-Implant Group (N = 52)	P Value*
Mean Charnley score³(points)
Preop.
Pain	2.3	2.6	0.15
Function	3.3	3.1	0.43
Motion	4.2	3.7	0.002
Postop.
Pain	5.6	5.6	0.86
Function	5.5	5.6	0.89
Motion	5.6	5.6	0.80
No. of revisions	0	0

Radiographic data†
Subsidence	1 (2)	0
Distal pedestal formation, zone I	2 (4)	2 (4)
Radiolucent line >50% of zone-I interface	0	0
Atrophy of medial part of femoral neck	43 (83)	32 (62)
Spot welds	35 (67)	41 (79)
Heterotopic ossification²
Grade 4	1 (2)	1 (2)
Grade 3	3 (6)	3 (6)