Abstract
The Articular Surface Replacement (ASR) hip system displayed unexpectedly high failure rates. All patients who underwent revision arthroplasty at our institution were identified. Indications for revision were classified as objectively identifiable or performed for unexplained pain. Clinical outcomes, post ASR revision, were superior in cases where the aetiology for failure was identifiable. Pain and function improved reliably when a targeted objective indication was identified pre-revision, but failed to do so in cases of unexplained pain. Surgeons familiar with the results of this study will be empowered to discuss expectations and realistic outcomes with their patients undergoing complex revision arthroplasty cases.
Keywords: Total hip replacement, Revision arthroplasty, Articular surface replacement, Metal on metal, Clinical outcomes, Patient reported outcome measures
1. Introduction
The Articular Surface Replacement (ASR) metal-on-metal hip system (hip resurfacing arthroplasty and XL total hip arthroplasty, DePuy Orthopaedics, Warsaw, IN) was introduced to the Irish market in 2004. Unexpectedly high failure rates were reported shortly thereafter.1, 2, 3 It is estimated that 3500 ASR prostheses were implanted in Ireland, and 93,000 worldwide, before their recall in 2010. 4 Public awareness, heightened concern and increased surveillance led to a surge in revisions in the years that followed.
Curtin et al. assessed the medium term results of the ASR implant system between 2004 and 2010 in the Galway University Hospitals group. A revision rate of 11% was identified, and was found to be associated with female gender, an elevated Western Ontario and McMaster Universities Index (WOMAC score), and elevated serum metal ion levels. The most common indication for further surgery was persistent hip pain. 4
Whilst there is literature describing the various correlations with failure and the need for revision arthroplasty post ASR, little is written about the clinical outcomes of patients who undergo such complex major surgery.5, 6, 7 The primary aim of this study was to further analyse those previously studied revision cases, so as to explore outcomes post ASR revision. The secondary outcomes of interest included the effects of demographics, revision indication and the primary prosthesis on patient reported outcome measures.
2. Methods
The institution's prospective database, described previously by our group,4 was accessed to identify all patients who underwent revision arthroplasty post primary ASR implantation. Both the ASR resurfacing and the ASR XL arthroplasty systems were identified as index procedures.
Under the terms formulated by the Health Service Executive and DePuy, all patients were evaluated clinically, radiologically and with serial cobalt and chromium ion analysis to evaluate prosthetic wear. 8 Clinical evaluation was performed by means of the Western Ontario and McMaster Universities Index (WOMAC score) - a questionnaire containing 24 items in three domains: pain, joint stiffness, physical functioning, utilising a 5-point Likert scale (score 0–4). The total score is 96 points if a patient has major problems on all items. 4,9 The compiled database recorded demographics, the date of the index procedure, WOMAC score post primary ASR implantation and follow-up serum metal ion levels (cobalt and chromium) thereafter. The indication for revision surgery was identified and grouped in to one of two categories based on whether or not an objective aetiology of hip pain could be determined on plain film radiograph or higher order imaging. Objective indications were those of aseptic loosening, aseptic lymphocytic vasculitis-associated lesion (ALVAL), significantly elevated serum cobalt and/or chromium ions, periprosthetic fracture and peri-prosthetic infection. Revision arthroplasty performed for unexplained hip pain was categorised as a subjective indication, whereby pre-revision ion levels were normal and all radiological investigations showed no definitive pathology. Patients underwent repeated post-revision WOMAC and serum metal ion testing, so as to allow for direct comparison, as well as modified Harris Hip Score (mHHS) assessment, the most frequently used outcome measure in both primary and revision arthroplasty. 10
Revision surgery was performed via the posterior approach to the hip. Following fascial incision, fluid was collected and sent for microbiological analysis. The revision surgeon noted the intraoperative findings, and periarticular tissue was sent for histological analysis. Both femoral and acetabular components were revised to uncemented collared Corail femoral prostheses and uncemented Pinnacle Gription acetabular shells, with or without screw fixation, depending on the stability of the press fit implantation (DePuy Orthopaedics, Warsaw, IN). An extended trochanteric osteotomy was performed in cases where an ASR XL total hip arthroplasty system was found to well fixed prior to retrieval, and repaired in situ thereafter with either cables or a greater trochanteric plate.11
Statistical analysis was performed using IBM SPSS Version 24 (IBM Corp. Released 2013; IBM SPSS Statistics for Windows, Version 22.0; Armonk, NY: IBM Corp.). The database was explored and Shapiro-Wilk testing of continuous data points identified non-parametric testing as appropriate. WOMAC scores and serum ion levels were compared pre and post revision using Wilcoxon signed ranked tests. WOMAC and Harris Hip scores were compared post revision between categories using MANN Whitney U tests. The effects of age at primary ASR, age at ASR revision and time to revision (in years) on clinical outcomes were assessed using Spearman correlation testing. A binary data point was created for serum ion levels (normal or high) and McNemar testing was used to analyse the effect of revision arthroplasty thereafter. Regression analysis was performed to explore the effect of revision indication on clinical outcomes whilst accounting for confounding factors of age, gender, pre revision WOMAC score and femoral prosthesis. A p value of <0.05 determining statistical significance throughout the analysis.
3. Results
3.1. Demographics
As shown in Table 1, out of 274 ASR prostheses implanted, 36 were revised (13.1%) in 35 patients. With regards to the indication for revision, 28 were performed for pain, 3 for high ions, 9 for aseptic loosening, 7 for MRI confirmed ALVAL, 2 for confirmed periprosthetic infection and 1 for periprosthetic fracture. 18 patients had an objective revision indication identified, and 18 patients underwent revision for unexplained pain.
Table 1.
Descriptive statistics.
| N | % | |
|---|---|---|
| Total number of revisions | 36 | – |
| Male/Female | 21/15 | 58%/42% |
| Mean age at primary ASR, yrs (range) | 54.5 (16–77) | – |
| Mean age at revision, yrs (range) | 60.3 (23–82) | – |
| Mean time to revision, yrs (range) | 5.8 (2–11) | – |
| Mean follow up, yrs (range) | 6.6 (3–10) | – |
| Mean age at follow up, yrs (range) |
67.3 (31–90) |
– |
| Prosthesis Revised | ||
| Hip resurfacing arthroplasty | 16 | 44% |
| XL total hip arthroplasty |
20 |
56% |
| Revision Indications (all findings) | ||
| Pain | 28 | 77.8% |
| Loosening | 9 | 25% |
| ALVAL | 7 | 19.4% |
| Elevated ions | 3 | 8.3% |
| Infection | 2 | 5.6% |
| Fracture |
1 |
2.8% |
| Objective revision indication | ||
| No – revision for unexplained pain | 18 | 50% |
| Yes | 18 | 50% |
ALVAL; aseptic lymphocytic vasculitis associated lesion.
3.2. Clinical outcomes
The mean WOMAC score pre-revision was 47.4 (SD, 23.0), and the median was 53 (range, 96). The mean WOMAC score post-revision was 34.7 (SD, 24.8) and the median was 29 (range, 77). The mean Harris Hip score post-revision was 68.9 (SD, 24.8), and the median was 67 (range, 81). Wilcoxon signed ranked testing revealed a significant improvement in WOMAC scores following revision arthroplasty (Z −1.974, p = 0.048).
Clinical outcome comparisons post revision can be seen in Table 2. There was no significant difference in clinical outcomes between male and female patients. Assessment of the mHHS showed a mean for men of 76.4 vs that of 59.9 for women (p = 0.058). With regards to revision indication, revision for confirmed ALVAL did result in a significantly improved WOMAC score out of all other revision indications, although this was not identified on mHHS analysis.
Table 2.
Post revision clinical outcomes – univariate analysis.
| Post Revision WOMAC | p-value | Post Revision mHHS | p-value | |
|---|---|---|---|---|
| Gender | ||||
| Male | 31.3 | 0.357 | 76.4 | 0.054 |
| Female | 39.2 | 59.9 | ||
| Prosthesis Revised | ||||
| Hip resurfacing arthroplasty | 40.2 | 0.286 | 61.9 | 0.168 |
| XL total hip arthroplasty | 30.7 | 74.1 | ||
| Revision for Aseptic Loosening | ||||
| Yes | 25.1 | 0.213 | 78.0 | 0.239 |
| No | 37.8 | 66.0 | ||
| Revision for ALVAL | ||||
| Yes | 16.0 | 0.022* | 84.9 | 0.053 |
| No | 39.8 | 64.6 | ||
| Revision for Elevated Ions | ||||
| Yes | 40.2 | 0.469 | 84.3 | 0.264 |
| No | 30.7 | 67.4 | ||
| Objective Revision Indication | ||||
| Yes | 22.0 | 0.003* | 80.5 | 0.007* |
| No – Revision For Unexplained Pain | 46.7 | 58.0 | ||
ALVAL; aseptic lymphocytic vasculitis associated lesion mHHS; modified Harris hip score.
* denotes statistical significance.
An objective revision indication was found to result in a statistically significant clinical improvement, as seen in WOMAC and mHHS testing, when compared to those revised for unexplained pain. Table 3 displays how patients who underwent revision for unexplained pain were found to have a mean pre-revision WOMAC score of 45.9 (SD 23.4), and a post-revision WOMAC score of 46.8, which was not a statistically significant difference (p = 0.856). Patients who underwent revision with an identifiable objective indication were found to have a mean pre-revision WOMAC score of 49.1 (SD 23.2) and a post-revision WOMAC score of 22.0 (SD 20.0), which showed statistical significance (p = 0.016). Patients who underwent revision for elevated ion levels, without radiological evidence of a pseudotumour, were found to have a mean pre-revision WOMAC score of 39.0 (SD 35.4), and a post-revision WOMAC score of 24.7 (SD 29.7), which did not show statistical significance (p = 0.593). Patients who underwent revision for aseptic loosening were found to have a mean pre-revision WOMAC score of 51.9 (SD 12.2) and a post-revision WOMAC score of 25.1 (SD 19.4), which showed statistical significance (p = 0.036). Patients who underwent revision due to a confirmed ALVAL were found to have a mean pre-revision WOMAC score of 57.4 (SD 20.0) and a post-revision WOMAC score of 16.0 (SD 16.6), which showed statistical significance (p = 0.018).
Table 3.
Comparison of pre and post WOMAC scores based on revision indication.
| Revision Indication | Pre Revision WOMAC | Post Revision WOMAC | p-value |
|---|---|---|---|
| Unexplained pain | 45.9 | 46.8 | 0.856 |
| Loosening | 51.9 | 25.1 | 0.036* |
| ALVAL | 57.4 | 16.0 | 0.018* |
| Elevated ions | 39.0 | 24.7 | 0.593 |
| Objective Identifiable Indication | 49.1 | 22.0 | 0.016* |
ALVAL; aseptic lymphocytic vasculitis associated lesion.
* denotes statistical significance.
Age at initial ASR, age at revision and time (in years) to revision were not shown to have an effect on clinical outcome (correlation coefficient −0.189, p = 0.293; correlation coefficient −0.167, p = 0.352 and correlation coefficient −0.07, p = 0.70 respectively).
3.3. Serum ion levels
The mean pre-revision serum cobalt ion level was 298.4, which showed a statistically significant improvement to 97.2 post-revision (p = 0.028). The mean pre-revision serum chromium ion level was 180.3, which showed a statistically significant improvement to 55.5 post-revision (p = 0.004). Serum chromium levels were shown to have normalised at one year post-revision in 75% of cases, which was shown to be statistically significant (p = 0.031). Cobalt levels normalised in 50% of cases, which was not shown to be statistically significant (p = 0.375). There was no correlation identified between pre-revision cobalt or chromium ion levels and post-revision clinical outcomes. Revisions performed with evidence of elevated serum ion levels, without the presence of an ALVAL, did not result in significantly improved clinical outcomes post-revision.
3.4. Multivariate regression analysis
Following adjustment for age, gender, pre-revision WOMAC score and primary femoral prosthesis, on regression analysis, both the WOMAC score and modified Harris hip score were both found to be significantly better post ASR revision when surgery was performed for an identifiable objective indication, as opposed for unexplained pain (see Table 4, Table 5).
Table 4.
Linear regression of the effect of revision indication on post revision WOMAC, adjusting for confounding factors.
| Variable | B | Confidence Interval | p-value |
|---|---|---|---|
| Objective Revision Indication | −23.74 | −40.4 to −7.03 | p = 0.007* |
| Gender | 5.88 | −11.3 to 23.1 | p = 0.489 |
| Age | −0.324 | −1.14 to 0.495 | p = 0.424 |
| Femoral Component | −3.492 | −20.97 to 13.98 | p = 0.685 |
| Pre Revision WOMAC | −0.062 | −0.438 to 0.314 | p = 0.739 |
Categorical variable reference values: Objective revision indication = no.
Gender = male; Femoral component = resurfacing.
* denotes statistical significance.
Table 5.
Linear regression of the effect of revision indication on post revision modified Harris Hip Score, adjusting for confounding factors.
| Variable | B | Confidence Interval | p-value |
|---|---|---|---|
| Objective Revision Indication | 21.29 | 5.04 to 37.54 | p = 0.012* |
| Gender | −13.80 | −30.54 to 2.95 | p = 0.102 |
| Age | 0.224 | −0.573 to 1.020 | p = 0.569 |
| Femoral Component | 4.253 | −12.75 to 21.25 | p = 0.612 |
| Pre Revision WOMAC | −0.026 | −0.391 to 0.340 | p = 0.887 |
Categorical variable reference values: Objective revision indication = no.
Gender = male; Femoral component = resurfacing.
* denotes statistical significance.
4. Discussion
A revision rate of 11% was identified across all primary ASR's implanted within our department between 2004 and 2010. The majority of ASR's were revised after the recall date. Activity levels within our department were similar to what Lainiala et al. described in Finland.12 Overall, ASR revision showed an improvement in clinical outcomes on both WOMAC and mHHS testing. Contrary to findings of our group for the mid-term review of the ASR prosthesis, for revisions that occurred following the recall date, we found no effect of gender, elevated pre-revision WOMAC score or elevated serum metal ions on clinical outcomes post revision.
Revising an ASR prosthesis was shown to improve the patient reported outcomes. The mean improvement of 12.7 (SD 34.4, range 127) in WOMAC score was shown to be statistically significant. The mean clinical important difference (MCID) of a scoring system is described as the change on the scale that equates to a measurable clinical difference experienced by a patient. The MCID for the WOMAC score has been identified as a change of greater than 16–21% in prior studies.13,14 The overall mean improvement of 12.7 does correlate with a 26.8% change in WOMAC score, signifying that an important clinical difference in pain and function levels was experienced by patients post ASR revision.
An objective indication for revision was found to result in significantly improved clinical outcomes on WOMAC and mHHS testing when compared to those undergoing revision for unexplained pain, after adjusting for confounding factors. The mean WOMAC score for those who had an identifiable objective revision indication was 22.0, having improved from 49.1 pre-revision. Interestingly, those who underwent revision for unexplained pain displayed a slight deterioration in WOMAC scoring from 45.9 to 46.8, although this did not show statistical significance. Patients post-revision following an objective indication were found to have a mean mHHS of 80.5, which fell in to the “good outcome” category.9,15
Revision arthroplasty has been shown to improve clinical outcomes in patients with metal-on-metal resurfacings and hip replacements.6,16 Functional improvements have been shown to be similar to those of revision total hip arthroplasties, but can take up to 12 months to manifest.6,16,17 Female patients have been shown to have twice the revision rates of men, however, in line with the findings of this study, display similar functional outcomes thereafter.5 Hip arthroplasty constructs were shown to have higher revision rates than resurfacings,18 however the femoral component was not shown to influence clinical outcomes within our cohort. Throughout the literature, functional outcomes are guided by the indication for revision. Despite the findings of Grammatopolous19 and Liddle,20 the results of our study showed that revisions perform well in the setting of a confirmed ALVAL, likely due to policies including strict surveillance,8 appropriate use of radiological investigations21 and early intervention. As expected, unexplained pain, also labelled as metal sensitivity, has been shown to carry the worst prognosis for clinical scores post revision.17,20 The results of revision are superior in cases where the aetiology for failure is clear.
Both cobalt and chromium serum ion levels reduced significantly post revision. Lainiala et al. described that cobalt ion levels tended to fall to normal levels in all patients within one year, but chromium ion levels can remain elevated even at three years post revision.6 Ball et al. also described that chromium levels display a less predictable decline compared to cobalt,22 likely due to accumulation in the liver, spleen and soft tissues.23 The fastest rate of serum metal ion decline has been shown to take place within the first six weeks post-revision, but can remain high in patients with reduced renal clearance.6,22 Elevated serum metal ions have been shown to identify poorly performing metal-on-metal prostheses, and can identify patients at risk for pseudotumour formation if above a certain threshold.17,24 This study showed that clinical outcomes did not significantly improve in those who underwent revision for elevated ions without evidence of a pseudotumour. Thus, in line with Hart et al.,25 the findings of this study support the conclusion that elevated serum metal ions alone lack sufficient predictive power to mandate revision surgery.
Limitations of this study include its retrospective nature, albeit from a prospectively collated database within our tertiary revision arthroplasty referral centre. Cases were not matched to controls of either primary or revision total hip arthroplasty. The sample size of 36 was adequately powered to detect differences between larger groups within the dataset, but likely underpowered when comparing outcomes between the specific revision indications. The revision arthroplasties were performed by one of two surgeons within our tertiary revision arthroplasty department, however the primary procedures were performed by any of the heterogenous referring surgeon cohort. Ion levels were only performed in post revision cases where there was a concern surrounding ultra high pre-revision ion levels, or intra-operative findings of metallosis. Thus, whilst numerous revision cases displayed elevated ion levels pre-revision, routine serum checks did not form part of their post-revision follow-up criteria. The MCID for the WOMAC score has been identified as a change of greater than 16–21%, or an actual difference of 15–20 on the scale. Thus, whilst the difference identified did equate to a change of 26.8%, the actual differential was that of 12.7 points on the scale, less than that of 15.
This is the first study to focus on clinical outcomes specifically post ASR revision arthroplasty, in the post-recall setting, and demonstrates an interesting divergence in outcomes between those patients with clear radiographic changes and patients with isolated elevated metal ions, or worse still those patients with revision triggered by subjective symptoms.
5. Conclusion
Revision arthroplasty in the setting of the ASR resurfacing and XL Arthroplasty systems poses multiple peri-operative challenges. Clinical outcomes are superior in cases where the aetiology for failure is clear. Pain and function improve reliably when a targeted objective indication is identified pre-revision, but fail to do so in cases of unexplained pain. Surgeons familiar with the results of this study will be empowered to discuss expectations and realistic outcomes with their patients ahead of such complex revision arthroplasty cases.
Sources of financial support
None.
Declaration of competing interest
None.
References
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