Abstract
Background:
Excessive early migration of femoral stems following total hip arthroplasty and tibial components following total knee arthroplasty is associated with their long-term survival and allows reliable early evaluation of implant performance. However, a similar relationship involving acetabular components following hip arthroplasty has not been evaluated. This prospective, long-term study with clinical and Roentgen stereophotogrammetric analysis (RSA) follow-up establishes the existence of this relationship and its associated diagnostic performance.
Methods:
Thirty-nine consecutive patients (forty-one hips) who underwent total hip arthroplasty with a cemented Exeter stem and a cemented Exeter all-polyethylene cup had prospective clinical and RSA follow-up. Patients were evaluated postoperatively at six weeks, at three, six, and twelve months, and annually thereafter. Conventional anteroposterior and lateral radiographs were made at six weeks and at two, five, and ten years postoperatively as well as when indicated. The mean duration of follow-up (and standard deviation) was 9.4 ± 3.2 years. No patients were lost to follow-up; fifteen patients died during the follow-up period.
Results:
Eleven acetabular components were observed to be loose on conventional radiographs after a mean of seventy-six months (range, twelve to 140 months). During the first two postoperative years, the failed acetabular components showed markedly greater and more rapid cranial translation and sagittal rotation. Both cranial translation (hazard ratio = 19.9 [95% confidence interval, 4.94 to 80.0], p < 0.001) and sagittal rotation (hazard ratio = 11.1 [95% confidence interval, 2.83 to 43.9], p = 0.001) were strong risk factors for late aseptic loosening. Eight of the eleven failed components showed a distinctive pattern of excessive cranial translation combined with excessive sagittal rotation. The associated diagnostic performance of two-year cranial translation and/or sagittal rotation for predicting late aseptic loosening of the acetabular component was good (area under the receiver operating characteristic curve, 0.88 [95% confidence interval, 0.74 to 1.00; p < 0.001] and 0.84 [95% confidence interval, 0.68 to 1.00; p = 0.001], respectively).
Conclusions:
Early migration, as measured by RSA at two years postoperatively, has good diagnostic capabilities for the detection of acetabular components at risk for future aseptic loosening, and this method appears to be an appropriate means of assessing the performance of new implants or implant-related changes.
Level of Evidence:
Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.
National registries or large studies are usually used to evaluate the survival of hip and knee prostheses after five to ten years of follow-up and to gather information regarding the effect of influential variables such as prosthesis type, type of fixation, and type of cement1. Such data show that aseptic loosening is still the most common cause of implant failure1,2.
The ability of standard clinical and radiographic assessment methods to assess aseptic loosening is relatively crude, and definitive results regarding aseptic loosening are only available after a lengthy follow-up. Before these results are available, widespread implantation of the prosthesis may have already occurred. Not every new implant development results in improved or even comparable results3. Recent introductions that proved less than optimal, such as Boneloc cement (Polymers Reconstructive, Farum, Denmark)4,5, the Capital Hip System (3M Health Care, Loughborough, United Kingdom)6,7, and polymethylmethacrylate (PMMA)-precoated femoral stems8,9, illustrate the need for early evaluation of new implants.
One method to reliably assess suboptimal performance of prostheses as early as one to two years postoperatively is analysis of implant migration with use of Roentgen stereophotogrammetric analysis (RSA)10-12. Because of the high accuracy of RSA, small patient groups are generally sufficient to assess fixation and expected performance. The predictive value of early RSA-measured migration for long-term implant survival (at seven to eleven years postoperatively) has been explicitly evaluated in several studies of femoral stems in total hip arthroplasty and of tibial components in total knee arthoplasty10-12. However, to our knowledge, no published studies have assessed the predictive value of early RSA-measured migration of acetabular components in total hip arthroplasty. The present study evaluated the ten to twelve-year survival of the acetabular components in a previously published, short-term clinical and RSA study13 to assess the relationship between early implant migration and long-term aseptic loosening of the acetabular components and to determine the associated diagnostic performance of this method.
The original prospective, randomized, double-blinded study13 compared a new high-viscosity cement (Simplex AF; Stryker-Howmedica, Kalamazoo, Michigan) with a widely used, conventional, low-viscosity bone cement (Simplex P; Stryker-Howmedica) before potentially widespread introduction of the new cement. Migration of the femoral and acetabular components during the first two postoperative years did not differ according to the fixation method13.
The complete study cohort continued to receive clinical and RSA follow-up to assess the relationship between short-term acetabular cup migration and long-term outcome. Thirty-nine patients (six male, thirty-three female) who underwent forty-one consecutive primary total hip arthroplasties with cement between February 1997 and October 1998 were included in the study. The mean patient age (and standard deviation) at the time of the index arthroplasty was 70.0 ± 5.7 years. Nineteen hips were treated for primary osteoarthritis, twenty for osteoarthritis secondary to rheumatoid arthritis, and two for ankylosing spondylitis. All patients received Exeter total hip arthroplasty components (Stryker-Howmedica) consisting of a collarless, double-tapered, polished femoral stem and an ultra-high molecular weight all-polyethylene acetabular cup.
All components were implanted through a lateral surgical approach with the patient in the lateral decubitus position. RSA measurements were made possible by insertion of six to eight 1-mm tantalum balls (Industrial Tectonics, Dexter, Michigan) into the acetabular region during surgery and insertion of markers into the cup by the implant manufacturer. Patients remained non-weight-bearing until the first RSA radiographs were made (on the first or second postoperative day) and were allowed full weight-bearing thereafter.
Patients were evaluated postoperatively at six weeks, at three, six, and twelve months, and annually thereafter. The Harris hip score14 was determined and RSA radiographs were made at each evaluation. Conventional anteroposterior and lateral radiographs were made at six weeks and at two, five, and ten years postoperatively as well as if the patient had pain or suspected implant failure.
The fate of all forty-one prostheses was known and no patients were lost to follow-up. The mean duration of follow-up was 9.4 ± 3.2 years (range, 3.1 to 12.0 years). Follow-up of at least ten years was available for twenty-six prostheses (63%) in twenty-four patients; the other fifteen patients died during follow-up from causes unrelated to the total hip arthroplasty.
Acetabular components that had progressive radiolucency of ≥2 mm in all three acetabular zones described by DeLee and Charnley or that had a change of ≥5° in inclination angle were considered to be loose15-17.
Blinded RSA measurements were performed (Model Based RSA 3.21; Medis specials, Leiden, The Netherlands)18. The position of the prosthesis relative to the bone at the first RSA examination served as the reference baseline for all subsequent examinations. Migration was expressed as displacement along and rotation around three orthogonal axes: longitudinal, transverse, and sagittal. The precision of the RSA measurements was determined by performing duplicate examinations19 at the one-year follow-up visit (Table I).
The mean condition number was 32.2 ± 8.28 for the markers in the cup and 52.3 ± 28.3 for those in the acetabulum. The condition number is a measure of the spatial distribution of the markers, with a low number (≤150) indicating appropriate distribution15. The mean error of rigid-body fitting was 0.16 ± 0.07 mm for the analysis of the markers in the cup and 0.21 ± 0.08 mm for those in the acetabulum. The mean error of rigid-body fitting is a measure of the stability of the markers, and the threshold set in the analysis should be 0.35 mm19, as was the case in the present study. The values of both measures in the present study satisfied the guidelines given by Valstar et al.19. In total, seventeen pairs of RSA radiographs (4%) in five patients had to be excluded because of relative displacement of the acetabular reference markers resulting from gross and evident migration of the loose cup.
The standard anteroposterior and lateral radiographs made at six weeks postoperatively were used to determine the cup orientation (inclination angle), the thickness of the cement mantle in each of the three acetabular zones defined by DeLee and Charnley15, and the quality of cement penetration13,20. The mean inclination angle was 47.0° ± 7.7°. On average, the minimum cement mantle thickness was 4.2 ± 1.7 mm, the maximum was 14.6 ± 4.2 mm, and the mean was 8.8 ± 1.8 mm. The cement penetration was type A in twenty cups, type B in twenty-one, and type C in none.
Statistical Analysis
Measured values are reported as the mean accompanied by the standard deviation and/or range. Migration was assessed with use of linear mixed-model analysis to take into account the longitudinal nature of the data and the repeated measurements within patients. Early migration (from zero to two years) and steady-state migration (from two to twelve years) were modeled separately. Model estimates are reported as the mean and the associated 95% confidence interval (CI). Migration rates were estimated with use of the regression coefficients calculated in the mixed-model analysis. The effects of the cement type, the maximum, minimum, and mean cement mantle thickness, the cement penetration grade, the presence of degenerative compared with inflammatory disease, the acetabular cup size, and the orientation of the acetabular component on migration were successive model factors.
The significance of migration at two years postoperatively as a predictor of the risk of late loosening was assessed with use of Cox regression landmark analysis (at two years). Since migration at two years served as a proxy variable for all baseline factors and we were only interested in the discriminatory power of migration at two years, no other factors were included in the Cox regression model.
The diagnostic performance of migration at two years postoperatively as a predictor of late aseptic loosening was evaluated with use of receiver operating characteristic (ROC) curve analysis. A p value of <0.05 was considered significant, and a Bonferroni correction was applied when evaluating the significance of individual model factors (resulting in a p value threshold of <0.01).
Source of Funding
This study received funding from the European Information and Communication Technologies Community Seventh Framework Programme (FP7/2007-2013). The funding source had no role in the collection, analysis, and interpretation of the data or in any other aspect of the study.
Clinical Results
The mean Harris hip score increased from 31 ± 19 points preoperatively to 57 ± 18, 71 ± 20, 76 ± 13, and 73 ± 23 points at six weeks and one, five, and ten years, respectively (p = 0.002).
Two cups (5%) had revision scheduled by two years postoperatively because of aseptic loosening13, and loosening became apparent in nine additional cups during the remainder of the follow-up period. The loosening in these eleven acetabular components (27%) became evident on conventional radiographs after a mean of seventy-six months (range, twelve to 140 months). All eleven of these components showed evident displacement, as shown by an increase of >5° (mean, 10.4°; range, 6° to 17°) in the acetabular cup inclination, and progressive radiolucency in all three acetabular zones. These eleven cases were considered definitive failures of the implant (Table II).
Early RSA Results
During the first two postoperative years, both the eleven failed components and the thirty well-performing components showed significant translation only in the cranial-caudal direction (p < 0.001 for both) and significant rotation only around the sagittal axis (p < 0.001 and p = 0.012, respectively). The mean cranial translation of the failed components at two years postoperatively was 2.02 ± 0.96 mm (range, 0.58 to 3.58 mm), and the mean sagittal rotation was 4.33° ± 4.18° (range, 0.07° to 11.3°). The mean cranial translation of the thirty well-performing components was 0.55 ± 0.42 mm (range, –0.15 to 1.29 mm), and the mean sagittal rotation was 0.38° ± 1.19° (range, –1.80° to 3.17°).
The migration pattern of the failed acetabular components was markedly different from that of the well-performing components (Fig. 1). The cranial translation of the failed components was greater (by a mean of 1.37 mm [95% CI, 0.78 to 1.96 mm] at two years, p < 0.001) and more rapid (by a mean of 0.74 mm/yr [95% CI, 0.50 to 0.97 mm/yr] over the two-year period, p < 0.001). The rotation of the failed components around the sagittal axis (i.e., the increase in inclination) was also greater (by a mean of 3.62° [95% CI, 0.10° to 7.14°] at two years, p = 0.045) and more rapid (by a mean of 1.51°/yr [95% CI, 0.48 to 2.55°/yr] over the two-year period, p = 0.006).
Long-Term RSA Results
Progressive migration of the failed components continued during the interval from two to twelve years postoperatively and was most pronounced for cranial translation and sagittal rotation. Because of the gross displacement of the failed components and the revision of two of these components, reliable migration measurements were available for only eight of the failed components at five years postoperatively and seven of the components at ten years. The mean cranial translation of the failed components was 2.95 ± 1.40 mm (range, 0.59 to 4.33 mm) at five years and 5.54 ± 2.30 mm (range, 2.30 to 8.90 mm) at ten years. The mean sagittal rotation was 7.82° ± 6.35° (range 0.04° to 19.6°) at five years and 11.8° ± 4.68° (range 7.37° to 20.8°) at ten years.
The thirty well-performing cups underwent continuous cranial translation of 0.07 mm/yr (95% CI, 0.02 to 0.13 mm/yr; p = 0.015) during the interval from two to twelve years postoperatively. The mean cranial translation was 1.18 ± 1.26 mm (range, –0.35 to 3.57 mm) at ten years. No significant translation was measured in the anterior-posterior direction (0.04 mm/yr [95% CI, –0.03 to 0.11 mm/yr], p = 0.279) or the medial-lateral direction (–0.00 mm/yr [95% CI, –0.06 to 0.06 mm/yr], p = 0.987). No significant rotation was measured around any of the three orthogonal axes (transverse rotation, –0.08°/yr [95% CI, –0.24 to 0.08°/yr], p = 0.276; longitudinal rotation, 0.01°/yr [95% CI, –0.40 to 0.42°/yr], p = 0.971; sagittal rotation, –0.09°/yr [95% CI, –0.27 to 0.08°/yr], p = 0.288).
Migration Factors
On the basis of the number of patients available, the cement type, the maximum, minimum, and average cement mantle thicknesses, the cementing grade, the presence of degenerative or inflammatory disease, and the cup size and orientation had no significant influence on the migration pattern of the cup (p > 0.01 for all).
Value of Early Migration in Predicting Long-Term Component Failure
Examination of the migration patterns of the individual cups during the first two years revealed a distinctive pattern of continuous excessive cranial translation and sagittal rotation in several cups (Fig. 2).
Distinctive excessive cranial translation was measured at two years postoperatively in eight cups. The cranial translation was >1.76 mm (mean, 2.37 ± 0.59 mm; range, 1.77 to 3.58 mm) in these cups compared with <1.30 mm (mean, 0.58 ± 0.38 mm; range, –0.15 to 1.29 mm) in the remaining cups (Fig. 2). All eight of the cups with excessive cranial translation later failed. No excessive short-term cranial translation was measured in the other three failed cups. Thus, the sensitivity of cranial translation exceeding 1.29 to 1.76 mm for predicting future loosening was 73% and the specificity was 100%.
Distinctive excessive sagittal rotation was measured in ten acetabular cups at two years postoperatively. The sagittal rotation represented an increase in acetabular inclination of >2.53° (mean, 5.68° ± 3.68°; range, 2.54° to 11.7°) in these ten cups compared with <1.85° in either direction (mean, 0.32° ± 1.00°; range, –1.80° to 1.84°) in the other cups. Eight of the ten cups with excessive sagittal rotation later failed. The three failed components that showed no excessive early increase in inclination were also the three components that showed no excessive early cranial translation. The associated sensitivity for predicting future loosening was 73% and the specificity was 93%.
A Cox regression landmark analysis revealed that cranial translation exceeding 1.29 to 1.76 mm at two years postoperatively was a strong risk factor for later component failure (hazard ratio = 19.9 [95% CI, 4.94 to 80.0], p < 0.001), as was sagittal rotation of at least 1.85° to 2.53° (hazard ratio = 11.1 [95% CI, 2.83 to 43.9], p = 0.001).
ROC curves were constructed to further assess the diagnostic performance of early migration as a predictor of late aseptic loosening (Fig. 3). The diagnostic performance of cranial translation at two years postoperatively for predicting later aseptic loosening of the acetabular component was good (area under the curve, 0.88 [95% CI, 0.74 to 1.00], p < 0.001). Likewise, the diagnostic performance of sagittal rotation was good (area under the curve, 0.84 [95% CI, 0.68 to 1.00], p = 0.001). Sensitivity and specificity values for other possible thresholds were also calculated with use of the ROC curves (Table III).
Our results showed that excessive early migration of cemented acetabular components was related to their long-term survival. Both cranial translation of the acetabular component exceeding 1.76 mm and sagittal rotation exceeding 2.53° during the first two years of follow-up were strong risk factors for subsequent aseptic component failure. Notably, every acetabular component with cranial translation exceeding 1.29 to 1.76 mm failed. More importantly, the associated diagnostic performance of early migration as a predictor of late aseptic loosening was good in individual cases. Different threshold values for migration may be chosen depending on the purpose of the measurement(s), and diagnostic performance was therefore estimated for various thresholds.
This study confirms, updates, and extends the evidence for the feasibility of early RSA-measured migration as a means for early detection of implants at risk for late failure due to aseptic loosening. Long-term follow-up studies using RSA are scarce, but only these studies can establish a one-to-one relationship between short-term migration and long-term outcome since both are known for the same patient. The association between early RSA-measured migration and long-term implant failure due to aseptic loosening had been shown previously for femoral components in total hip arthroplasty10,21,22 and for tibial components in total knee arthroplasty11,12, but to our knowledge this is the first study to establish this association for acetabular components in total hip arthroplasty. It is both notable and intuitively plausible that excessive early migration of the cemented components in our study was most apparent in the directions corresponding to the eventual failure mode of the component. Since acetabular components are not designed to migrate after implantation, we believe that our results should also be applicable to other types and designs of acetabular cups.
Our migration results are in agreement with the findings of Aspenberg et al.23, who investigated the pattern of migration of 147 cemented acetabular cups of seven different types during the first two postoperative years. A dichotomous migration pattern was identified: the majority of the components appeared to be stable, but a subgroup including implants of a variety of types showed continuous migration and was considered at risk of loosening. Unfortunately, no long-term outcomes were available in that study to confirm that loosening of the at-risk components did subsequently occur. However, since a clear and implant-independent dichotomy was identified in that study, this suggests the feasibility and generalizability of using excessive initial migration as an early predictor of implants at risk for loosening.
The present study confirms the potential of RSA-measured migration as a means to detect potentially inferior new implant designs and cements before their widespread use19,22,24,25. However, use of the proportion of implants showing excessive short-term migration is very likely to produce a conservative estimate of the long-term failure rate, since only baseline factors whose effect on long-term behavior begins in the short term are accounted for. Later events that affect the prosthesis failure rate, such as late loosening caused by wear-induced osteolysis or the occurrence of fractures of the cement mantle, cannot be anticipated on the basis of the short-term migration rate.
Although forty-one total hip arthroplasties was an adequate sample size for the short-term RSA follow-up, it represents a limitation of the present study because of the large number of deaths during follow-up. Another limitation is the fact that the study group, which included a substantial number of patients with inflammatory disease, might not be representative of the general population. Finally, only one type of acetabular cup was used.
In conclusion, this study substantiates the relation between early implant migration and long-term implant survivorship, and it promotes RSA as a feasible tool for detecting possibly inferior new implants or cements after a short period of postoperative follow-up.
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Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.