Abstract
Although an acetabular component with an elevated rim is thought to improve the postoperative stability of a total hip prosthesis, the actual clinical value has not yet been demonstrated. To address this question, we reviewed the results of 5167 total hip arthroplasties that had been performed at our institution from April 1, 1985, through December 31, 1991. The prostheses included 2469 acetabular components with an elevated-rim liner (10 degrees of elevation) and 2698 with a standard liner. The cumulative probability of dislocation was estimated as a function of time since the operation with use of the Kaplan-Meier survivorship method. Forty-eight of the 2469 hips that had the elevated-rim acetabular liner dislocated within two years, compared with 101 of the 2698 hips that had the standard acetabular liner. The two-year probability of dislocation was 2.19 per cent for the hips with the elevated-rim liner and 3.85 per cent for those with the standard liner (p = 0.001). A similar trend was seen at five years; however, because of a smaller sample the difference was not significant. Increased stability at two years was also demonstrated for the hips with the elevated-rim liner when the hips were analyzed according to the operative approach, the mode of fixation, the sex of the patient, and the type of total hip arthroplasty (primary or revision). Although these data demonstrate improved stability after total hip arthroplasty when an elevated liner is used, particularly in hips that are at greater risk for dislocation of the prosthesis, the long-term effect of this elevated liner on wear and loosening remains unknown but is of considerable concern. The elevated liner deserves additional study to clarify its effect on wear and loosening.
An elevated-rim acetabular liner is used as a potential means of improving stability after total hip arthroplasty. An elevated rim on a high-density-polyethylene acetabular liner is currently available from most manufacturers. An implant with this design was first used by Charnley in the early 1970's to decrease the tendency for posterior dislocation of the femoral head5. The asymmetrical build-up of these components is thought to provide additional support in regions of compromised stability12. The orientation of the augmented rim can be individualized depending on the unique anatomy of each patient, with the built-up region placed where it is most needed (usually posteriorly and superiorly).
Although the theoretical attractions of the elevated rim are obvious and have been widely accepted, the clinical advantages have not been demonstrated, to our knowledge. To address this issue, we retrospectively reviewed the cumulative probability of dislocation in 5167 total hip replacements inserted at the Mayo Clinic from 1985 through 1991. Our purpose was to determine the effect of an augmented acetabular component on the cumulative probability of dislocation after total hip arthroplasty.
*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.
†Mayo Clinic, 200 First Street S.W., Rochester, Minnesota 55905.
The results of all total hip arthroplasties performed at the Mayo Clinic from April 1, 1985, through December 31, 1991, were reviewed. Of the 7105 hip procedures, those that involved use of a bipolar prosthesis, fixed-head endoprosthesis, custom-design prosthesis, or acetabular liner with a rim elevated 15 or 20 degrees, as well as those performed for reconstruction after resection of a tumor, were excluded from the review. With use of these selection criteria, 5167 hips were enrolled in the study: 2469 with an elevated-rim acetabular component and 2698 with a standard (neutral) acetabular component. The relative percentage of elevated-rim acetabular liners used each year increased from 10 per cent in 1985 to more than 80 per cent in 1991. The critical variables relating to stability of the hip have been prospectively gathered and recorded in the Total Joint Registry. Dislocation treated with reduction by a physician was used as the discrete failure end point. The group that had a dislocation was analyzed according to the operative approach, the type of arthroplasty (primary or revision), the sex of the patient, and the type of fixation. The risk of dislocation was assessed on the basis of the design of the acetabular cup. Follow-up data were obtained by physical examination for 2739 hips (53 per cent), questionnaire for 1808 (35 per cent), and a telephone survey for 620 (12 per cent). Dislocation can be adequately assessed by any of these modes of evaluation4. Only the first dislocation after each index arthroplasty was recorded. When a patient had a revision total hip arthroplasty after a dislocation and then had a second dislocation after the revision, the dislocations were included separately in the study.
Because acetabular liners manufactured by different companies vary with regard to the degree of elevation of the rim, and because of a greater likelihood that a greater degree of elevation will be selected for patients thought to have a more unstable hip, only components with the smallest degree of elevation of the rim (10 degrees) provided by any manufacturer were assessed in this study.
Operative reports and radiographs were reviewed to determine the direction of dislocation in each hip. The direction was determined by the position of the femoral head relative to the acetabular cup on radiographs made before the reduction. This variable was analyzed for the standard and elevated-rim designs.
Additional Analysis
Anteversion of the acetabular component, determined with the technique described by Woo and Morrey, was measured on lateral radiographs as the angle formed by the intersection of a line drawn across the face of the acetabular component and a line perpendicular to the horizontal plane. Anteversion was measured for all of the cups with the standard liner that were complicated by dislocation. It was also measured, for the purpose of comparison, in a random sample of fifty acetabular cups with the standard liner that were not associated with dislocation. Similarly, anteversion was measured for all of the cups with the elevated-rim liner that were associated with dislocation and in a random sample of fifty cups with the elevated-rim liner that were not associated with dislocation. The mean anteversion in the entire group of standard and elevated-rim acetabular liners was estimated by calculating a weighted average of the mean of the dislocated and non-dislocated prostheses with use of the formula:
where nND is the sample size of the non-dislocated prostheses, nD is the sample size of the dislocated prostheses, ?xND is the estimate of the mean anteversion in the sample of fifty non-dislocated prostheses, and xD is the mean of the entire group of dislocated prostheses.
Statistical Methods
The cumulative probability of dislocation was estimated as a function of time since the operation with use of the Kaplan-Meier survivorship method. As the risk of dislocation is highest early on (less than two years postoperatively)4, all hips should be included in the denominator of the Kaplan-Meier survival estimate. For this reason, we included fifteen hips that had been followed for less than two years. A minimum duration of follow-up of two years is most appropriate for studies of events that are expected to occur later in the postoperative period, such as loosening of an implant. As most dislocations of total hip prostheses occur in the early postoperative period, we are confident that the inclusion of these fifteen hips does not compromise the validity of our results. Comparisons of the survivorship curves were made with the log-rank test16. The Cox proportional-hazards general linear model was used to assess the multivariate risk factors affecting the probability of dislocation. The variables that were assessed included insertion of the components with cement, insertion of the components without cement, the operative approach, primary total hip arthroplasty, and revision total hip arthroplasty. Survivorship curves were censored at two and five years after the operation. All significance tests were two-sided, and p values of less than 0.05 were considered significant.
Surveillance
The patients who were alive, had had no episode of dislocation, and had the implant in situ were followed for a mean of 3.04 years (range, forty-four days to 7.7 years). Of the 3204 patients who were operated on before 1990, 3089 (96 per cent) were followed for at least two years. Only fifteen patients who were alive and had an intact implant that had not dislocated were followed for less than two years. Of the 1385 hips that had been operated on at least five years before our analysis, 1324 (96 per cent) were followed for at least five years.
Instability
One hundred and forty-nine of the over-all group of 5167 hips included in this study were complicated by dislocation within two years after the total hip arthroplasty. Forty-eight of the 2469 hips with the elevated-rim acetabular liner had a dislocation of the femoral prosthesis within two years, compared with 101 of the 2698 hips with the standard acetabular liner. The two-year Kaplan-Meier probability of dislocation was 2.19 per cent for the hips with the elevated-rim liner and 3.85 per cent for those with the standard liner (Fig. 1-A). This difference was significant (log-rank test; p = 0.001).
A similar trend was seen at five years; however, the difference was not significant. The subgroup analyzed at five years consisted of 1385 hips. Of the 173 with the elevated-rim acetabular liner, five were complicated by dislocation. Of the 1212 with the standard acetabular liner, fifty-three were complicated by dislocation. The five-year Kaplan-Meier probability of dislocation was 2.97 per cent in the group with the elevated-rim acetabular liner and 4.46 per cent in the group with the standard liner (p = 0.34) (Fig. 1-B).
Primary Total Hip Arthroplasty
Of the 1949 primary total hip arthroplasties in which the elevated-rim acetabular liner was used, twenty-five were complicated by a dislocation, compared with fifty of the 2168 primary total hip arthroplasties in which the standard liner was used (Fig. 2-A). The cumulative probability of dislocation was 2.35 per cent for the hips with the standard liner and 1.43 per cent for those with the elevated-rim liner (p = 0.04).
Revision Total Hip Arthroplasty
There were twenty-three dislocations, within two years after the arthroplasty, in the 520 hips that had had a revision with the elevated-rim acetabular component after a previous total hip arthroplasty and fifty-one dislocations in the 530 hips that had had a revision with the standard liner (Fig. 2-B). The two-year cumulative probability of dislocation was significantly lower (p = 0.005) in the hips with the elevated-rim liner (5.02 per cent) compared with that in the hips with the standard liner (10.03 per cent).
Operative Approach
Three operative approaches (anterior, posterior, and transtrochanteric), which have been previously described18, were used at our institution during this study. Regardless of the approach that had been employed, the hips with the elevated-rim acetabular liner were more frequently stable at two years than those with the standard liner (Table I). This difference was significant in the group treated with the anterior approach (p = 0.01) and that treated with the transtrochanteric approach (p = 0.03) but, because of the small sample size, it was not significant in the group treated with the posterior approach (p = 0.23).
Sex of the Patient
The hips with the elevated-rim acetabular liner were stable at two years significantly more frequently in both the men (p = 0.01) and the women (p = 0.04) (Table II). For the men, the cumulative probability of dislocation at two years was 3.60 per cent in the hips with the standard liner and 1.86 per cent in those with the elevated-rim liner. Similarly, for the women, the probability of dislocation was 4.05 per cent in the hips with the standard liner and 2.51 per cent in those with the elevated-rim liner.
Type of Fixation
The acetabular component was inserted with cement in 2585 arthroplasties and without cement in 2582 arthroplasties. The 865 elevated-rim cups that had been inserted with cement were associated with a lower two-year probability of dislocation (2.51 per cent) than the 1720 standard cups that had been inserted with cement (4.40 per cent), and this difference was significant (p = 0.03). When cement had not been used on the acetabular component, the two-year probability of dislocation for the 1604 cups with an elevated-rim liner was 2.02 per cent compared with 2.86 per cent for the 978 cups with the standard liner. However, this difference was not significant (p = 0.79).
Multivariate Analysis
A multivariate Cox-model analysis revealed that the use of the elevated-rim acetabular liner was associated with a lower probability of dislocation after adjusting for the effect of primary or revision total hip arthroplasty and for the effect of the type of fixation (p = 0.04).
Direction of Dislocation
Adequate lateral radiographs were available to determine the direction of 111 (74 per cent) of the 149 dislocations. Of these, seventy-four were associated with the standard acetabular liner and thirty-seven, with the elevated-rim liner. The direction of dislocation was posterior in fifty-six (76 per cent) of the hips with a standard liner, anterior in twelve (16 per cent), and lateral in six (8 per cent). In the group with the elevated-rim liner, the direction of dislocation was posterior in twenty-one hips (57 per cent), anterior in eleven (30 per cent), and lateral in five (14 per cent). Chi-square analysis demonstrated a significantly greater number of posterior dislocations in the group with the standard acetabular liner than in the group with the elevated-rim liner (p = 0.005). A greater percentage of the dislocations in the hips with the elevated-rim liner were anterior, but this was not significant (p = 0.12).
Anteversion of the Acetabular Component
In the random sample of fifty non-dislocated prostheses with a standard acetabular liner, the mean (and standard deviation) anteversion of the cup was 16.9 ± 9.3 degrees, compared with 16.2 ± 8.9 degrees for the random sample of fifty non-dislocated prostheses with an elevated-rim liner. The standard acetabular cups associated with dislocation had a mean anteversion of 12.5 ± 10.3 degrees, and the elevated-rim acetabular cups associated with dislocation had a mean of 13.7 ± 10.1 degrees. There was no significant difference in the mean anteversion between the cups with a standard rim and those with an elevated rim in either the non-dislocated group (p = 0.7) or the dislocated group (p = 0.52).
The calculated mean anteversion for the entire group of standard-rim cups was 16.67 degrees, compared with 16.10 degrees for the elevated-rim components. Thus, the difference was less than 1 degree and is a clinically unimportant risk factor for the development of dislocation.
The theoretical advantages of elevated-rim acetabular liners are obvious, and their use is common and accepted. It is therefore of interest that no studies have specifically addressed or confirmed the theoretical advantages in a clinical setting, to our knowledge. We reviewed the results of 5167 total hip arthroplasties to determine the effect of augmented acetabular liners on dislocation after total hip arthroplasty.
Augmentation of the acetabular component was introduced by Charnley5, who extended the posterior aspect of a high-density-polyethylene cup in an attempt to prevent posterior dislocation of the femoral head. There has been limited use of the concept of elevated-rim acetabular cups in the intraoperative setting. A wedge of high-density polyethylene has been fixed to the existing acetabular component in the direction of instability. Olerud and Karlström reported success with this method in six patients who had recurrent dislocation. Other reports have been of limited numbers of patients and have described variable success6,13,17,19.
More recently, various elevated liners have been incorporated into many systems of total hip arthroplasty (Figs. 3-A and 3-B). For example, of the implants used commonly at our institution, one has for many years allowed the surgeon the option of using or not using an elevated-rim liner as well as options regarding the degree of augmentation. One attractive feature of the modular design of the current implants is that the augmented segment can be rotated into the desired position, theoretically enhancing stability12.
There has been concern about the long-term effect of the enhanced stability derived from a more constrained articulation. Two worrisome complications could develop: increased wear debris from the high-density-polyethylene elevated rim, and loosening from the increased rotatory moment introduced by force being transmitted at the point of contact with the augmented rim (Fig. 4)13.
An additional limitation of the elevated-rim design is that several biomechanical studies have shown that it is associated with a decreased arc of motion (Fig. 5)8,14. Krushell et al. demonstrated that the stable arc of motion was not increased, but rather reoriented, with the use of an elevated rim. When the elevated rim was placed posteriorly, stability was increased with the hip in flexion and in flexion with internal rotation with some designs and only in internal rotation in flexion with other designs. Extension and external rotation in extension were decreased by elevated-rim liners. Therefore, the range of motion was increased in some directions and decreased in complementary directions. The results of their study are supported by our clinical findings. A greater percentage of dislocations occurred anteriorly in the hips that had the elevated liner (eleven of thirty-seven; 30 per cent) compared with the percentage in the hips that had the standard liner (twelve of seventy-four; 16 per cent). This most likely resulted from impingement of the femoral neck on the elevated portion of the liner during extension of the hip.
Malorientation of one or both components has been shown to be the cause of approximately one-half of dislocations after total hip arthroplasty4. The instability caused by malorientation is theoretically overcome by proper placement of an augmented acetabular liner. However, our analysis demonstrated less than a 1-degree difference between the anteversion of the standard acetabular cups and that of the elevated-rim cups in the dislocated and non-dislocated groups, which demonstrates that anteversion did not appear to play a role in the difference in stability observed between these two designs of component.
Charnley reported erosion of his elevated-rim components. This worrisome complication was confirmed in a recent study by Murray, who found severe erosion of four of ten elevated-rim acetabular liners obtained at the time of revision. Similar problems with wear have been observed by the senior one of us (B. F. M.). Deformation and wear of the polyethylene liner is not only of concern from the standpoint of loss of mechanical support; it is also a problem from a biological standpoint. Wear debris of polyethylene total joint components has stimulated considerable research in recent years. A relationship between wear of polyethylene acetabular components and osteolysis after total hip arthroplasty has recently been demonstrated9. A recent case report has implicated osteolysis induced by particulate polyethylene secondary to wear debris from an elevated acetabular liner as a cause of loosening of a femoral stem1. This finding raises yet another concern of a possible complication associated with augmented-rim components. Patients who have these cups might theoretically be considered to be at an increased risk of osteolysis and distention of the pseudocapsule, which are known to occur in some patients with excessive wear debris. The possibility of higher rates of osteolysis with this design is currently under investigation.
Hence, the amount of build-up of the acetabular component should be limited by concerns of wear and restriction of motion as well as of excessive force transmission from impingement of the neck of the femoral component on the rim of the acetabular cup. A practical concern is that excessive augmentation will cause the neck of the prosthesis to impinge and lever on the acetabular rim, forcing the head out of the cup anteriorly (Fig. 6)6,10. This concern is underscored by the poor results observed by some6 who have used the wedge-revision technique described by Olerud and Karlström.
We recognize that our study has several limitations. Although an association between the use of the elevated-rim liner and dislocation was shown, a cause-and-effect relationship was not proved. Numerous variables could not be controlled, including the fact that the procedures were performed by multiple surgeons who used various prostheses. The position of the elevated portion of a liner cannot be determined adequately in a retrospective study. Likewise, the indication for the use of the elevated liner was based on the preference of the surgeon and could not be fully documented for this study. However, selective factors of surgeon preference tend to favor the use of an elevated liner in a patient who is at greater risk for dislocation. Thus, the selection factor should, if anything, increase the rate of dislocation in the group that had the elevated-rim liner. However, an informal poll of the orthopaedic staff at the Mayo Clinic revealed that most of those who used this design did so on a routine basis, without regard for the position or the perceived risk of instability of the implant.
Although the enhanced stability provided by elevated acetabular liners has been shown to be significant, one could argue that the difference is not clinically important when weighed against the additional expense of these components and the potentially adverse effects with regard to wear and loosening. Indeed, in primary total hip arthroplasties, the difference in the probability of dislocation was less than 1 per cent. However, in revision total hip arthroplasties, the difference was 5 per cent and, in revisions with cement, it was 6.3 per cent. A previous study from our institution showed that an operation was performed for one-third of all dislocations18. Therefore, an estimated two patients per 100 revision total hip arthroplasties may be spared a subsequent operation by using an elevated-rim liner.
However, it must again be emphasized that additional studies are warranted regarding the possibility of excessive polyethylene wear or increased torque causing loosening of the acetabular component. Because of the lack of data concerning these theoretical disadvantages, the senior one of us (B. F. M.) does not advocate the routine insertion of an acetabular component with an elevated-rim liner at this time.
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