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Comparison of metal ion levels in patients with hip resurfacing versus total hip arthroplasty
⁎Corresponding author: Mark L. Barba. markb@orthoillinois.com
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
Mechanical wear at the articular surface and corrosive processes at modular junctions, such as the trunnion, are responsible for metal ions production. We retrospectively reviewed 64 patients who underwent THA with a metal on metal bearing surface and 34 patients with hip Resurfacing. Metal ion measurements, six-week post-op radiographs, and functional scores were included in the analysis. Cobalt ion levels were significantly elevated in THA patients, 2.95μg/L as compared to resurfacing patients, (2.95 versus 1.30μg/L, p<0.0005). Chromium levels were not significantly different between THA patients and resurfacing patients (1.05 versus 1.00μg/L, p=0.529).
Keywords
THA
Metal-on-metal
Hip resurfacing
Trunnion corrosion
Metal ions
1 Introduction
While metal on metal hip replacement had been attempted in the past, this bearing surface enjoyed another wave of popularity relatively recently which has since faded. There are notable advantages to this bearing construct that include allowing for a larger head size and potentially more durable bearing surface. With larger heads there are reduced dislocation rates, a greater jump distance of the femoral head, and a greater head-to-neck ratio that reduces impingement and increases range of motion.1–4
However, these devices have also been associated with increased local and systemic metal ion levels,5 in some cases resulting in adverse local tissue reactions and pseudotumor formation.6–8 Metal ion generation with these implants has been attributed to multiple sources, with wear at the articulation initially believed to be the predominate source. However, the trunnion of modular THA implants (Fig. 1) is increasingly being recognized as a source of metal ions (Fig. 2), seen even in more conventional metal on polyethylene bearing surfaces.9–13 Corrosion mechanisms have been proposed, but there are still no definitive answers. It is generally accepted that a combination of mechanical forces strip away the protective passive oxide layer on the metal surface, which exposes the underlying susceptible alloy to electrochemical reactions with the acidic synovial fluid.14,15 It is theorized that the mechanical forces are due to micromotion from an imperfect union at the metal-to-metal junction of the taper, which is complemented with torqueing forces by the femoral head.15 This concept explains the metal ion generation in some modular THA implants and the potential effect of head size on these ion levels.16
The purpose of this study was to compare metal ion levels, revision rates, and functional outcomes in patients with either conventional metal on metal THA or hip resurfacing, in constructs utilizing identical acetabular components, and similar femoral implants. We hypothesized that metal on metal hip total hip replacements would have higher ions levels than hip resurfacings as a result of metal ion generation at the trunion.
2 Materials and methods
A consecutive series of patients undergoing THA or hip resurfacing between October 2006 through April 2012 were retrospectively reviewed following IRB approval. Patients lacking metal ion measurements, follow-up less than one year post-operatively, THA implants other than DePuy Corail, or a diagnosis of renal disease were excluded. Patients with bilateral hip implants of any design were also excluded. A minimum of 1-year follow up was required to avoid the run-in period found in hip implants, which is characterized by elevated metal ion levels during the 1st year, which stabilize thereafter.17,18 There were 273 arthroplasty patients with 306 hip implants and of these, 98 patients met the inclusion criteria. Of the patients that were excluded, 63 were due to lack of ion measurements, 7 were due to presence of renal disease, 11 for only having ion measurements within one year of the index procedure, 39 had THA implants other than DePuy Corail, and 55 had bilateral hip implants.
This consisted 64 total hip arthroplasty patients, of which 39 patients were male and 25 were female. The resurfacing group contained 34 patients, which included 29 males and 5 females. The mean age of patients in the THA group was 70 years (range, 38–93 years old) and the resurfacing group was 60 years (range, 45–76 years old). Median BMI for the THA group was 32.1kg/m2 (range, 20.3–60.1kg/m2) and the resurfacing group was 30.5kg/m2 (range, 17.6–44.0kg/m2). The median follow-up interval for metal ion measurements was 3.12 years (range, 1.00–6.27 years) in the THA group, 3.12 years and 4.36 years (range, 1.10–8.27 years) in the BHR group, (p=0.017). Demographic data for each group is summarized in Table 1.
| BHR (n=34) | THA (n=64) | p-value | |
| Mean Age (years) | 60 (45–76) | 70 (38–93) | <0.0005a |
| Median BMI (kg/m2) | 30.5 (17.6–44.0) | 32.1 (20.3–60.1) | 0.550b |
| Sex (males) | 85.30% (29/34) | 60.90% (39/64) | 0.013c |
| Reason for arthroplasty | 0.258d | ||
| Arthrosis | 33 | 53 | |
| Hip Dysplasia | 1 | 2 | |
| Avascular Necrosis | 0 | 6 | |
| Acetabular Fracture | 0 | 1 | |
| Femoral Neck Fracture | 0 | 2 |
2.1 Surgical procedure
All procedures were performed using the posterior approach by the senior author. Total hip arthroplasty constructs consisted of a Corail stem (Depuy, Warsaw, IN), with a Birmingham femoral head and cup (Smith & Nephew, London, UK) All hip resurfacings were the Birmingham Hip Resurfacing (Smith & Nephew, London, UK).
2.2 Metal ion analysis
Blood samples were drawn from the antecubital fossa and collected in three EDTA containing royal blue-top tubes; the first draw discarded to rinse possible contaminants from the needle. Chromium samples were stored in EDTA royal-blue top tubes and sent to Quest Diagnostics (Valencia, CA) for analysis. Cobalt samples were centrifuged after collection then transferred to a metal free plastic vial and sent to Quest Diagnostics (Chantilly, VA) for analysis. All samples were analyzed using inductively coupled plasma mass spectrometry (ICP/MS). Reference ranges were reported as 0.1–0.4μg/L for cobalt and ≤1.2μg/L for chromium. Ion measurements that were below the limit of detection were arbitrarily set to half of the limit of detection. This allowed for inclusion of patients with undetectably levels in the statistical analysis. This method did not affect the findings since these values were reported as medians. Ion levels from the most recent blood draw were included for the statistical analysis with all previous metal ion measurements being excluded.
2.3 Radiographic methods
Anteroposterior radiographs were taken six weeks post-operatively. Acetabular inclination angles were measured using the inter-teardrop line technique; the angle measured between a horizontal line connecting the inferior tip of the teardrops and a line that paralleled the acetabular cup on AP radiographs.19 Harris Hip Scores (HHS) and Western Ontario & McMaster Universities Index (WOMAC) scores were obtained on routine follow up appointments. Most recent scores were included for analysis.
2.4 Statistics
All statistics were calculated using SPSS 23 (IBM; North Castle, NY). Independent-samples t-test was used to compare means of normally distributed data. Medians were compared using the Mann-Whitney U test and the Kruskall Wallis H test non-normal data. Normality of data was tested using Q–Q plots and Shapiro-Wilk test. Group associations were analyzed using the Χ2 test. Statistical significance was determined with a p-value<0.05.
3 Results
Metal ion levels from most recent blood draw measurements and implant placement data are summarized in Table 2. Median whole blood cobalt levels were significantly higher in the THA group, 2.95μg/L (range, 0.60–31.90μg/L), than the BHR group, 1.30μg/L (range, 0.80–19.60μg/L), (p<0.0005). There was no significant difference in the median whole blood chromium levels between the THA group, 1.05μg/L (range, 0.10–9.10μg/L), and the BHR group, 1.00μg/L (range, 0.10–10.20μg/L), (p=0.529). Mean femoral head size was slightly larger in the BHR group, 49.4mm (range, 42–58mm), than the THA group, 47.2mm (range, 40–56mm), (p=0.002). The mean inclination angle was not significantly different between the BHR group, 40.32° (range, 26.5–51°), compared to the THA group, 38.41° (range, 25–48°), (p=0.145).
| BHR (n=34) | THA (n=64) | p-value | |
| Median Cobalt (μg/L) | 1.30 (0.80–19.60) | 2.95 (0.60–31.90) | <0.0005a |
| Median Chromium (μg/L) | 1.00 (0.10–10.20) | 1.05 (0.10–9.10) | 0.529a |
| Metal Follow-up (years) | 4.36 (1.10–8.27) | 3.12 (1.00–6.27) | 0.017a |
| Median HHS | 93 (66–100) | 86 (31–100) | 0.002a |
| Median WOMAC | 98 (8–100) | 94 (20–100) | 0.014a |
| Follow-up (years) | 3.02 (1.02–6.26) | 2.36 (1.13–4.90) | 0.132a |
| Mean Head Size (mm) | 49.4 (42–58) | 47.2 (40–56) | 0.002b |
| Inclination Angle (degrees) | 40.32 (26.5–51) | 38.41 (25–48) | 0.145b |
| Revision Rate | 0% (0/34) | 14.1% (9/64) | 0.022c |
| Metal Related Revision Rate | 0% (0/34) | 6.3% (4/64) | 0.137c |
Functional scores and revision rates are summarized in Table 2. The median Harris Hip Score (HHS) was significantly higher in the BHR group, 93 (range, 66–100), than the THA group, 86 (range, 31–100), (p=0.002). The median WOMAC score was significantly higher in the BHR group, 98 (range, 8–100), than the THA group, 94 (range, 20–100), (p=0.014). Median functional score follow-up interval was significantly longer in the BHR group, 3.02 years (range, 1.02–6.26 years), than the THA group, 2.36 years (range, 1.13–4.90 years), (p=0.132). Revision rates were significantly higher between the THA group, 14.1% (9 patients), than the BHR group, 0% (0 patients), (p=0.022). Of the 9 patients who underwent revision arthroplasty, 4 were due to pain, 4 were due to trunionosis, and 1 was due to infection. There was a trend towards increased metal ion specific revision rates between the THA group, 3.9% (4 patients), and the BHR group, 0%, (p=0.137), however this finding did not reach statistical significance with the numbers available for the study. The mean time interval to revision for all implants was 2.67 years (range, 1.88–3.54 years).
4 Discussion
This study demonstrated that cobalt ion levels were significantly higher in patients with MoM THA than with hip resurfacing, while chromium ion levels did not significantly differ between the two groups. Both groups in this study had identical bearing surfaces; therefore, a possible explanation for the difference in ion levels is the presence of a trunnion in the THA group.
Studies with a similar design to ours, comparing metal on metal total hips to resurfacing, have yielded similar results. A retrospective study by Johnson et al. that included 110 resurfacing patients and 22 MoM THA patients demonstrated that both chromium (2.94μg/L vs. 1.49μg/L) and cobalt (2.86μg/L vs. 1.11μg/L) levels were elevated in THA patients.20 The inconsistency of their chromium ion findings with ours could be explained by the relatively small THA group and the potential of variation of manufacturers included in their study since that information was not disclosed. A prospective analysis by Beaule et al. compared 26 patients with Profemur MoM THA implants to 26 patients with Conserve Plus resurfacing implants at 6, 12, and 24 months follow up and demonstrated that cobalt levels were elevated in the THA group (3.77μg/L vs. 1.22μg/L, respectively), but there was no difference in chromium levels (2.58μg/L vs. 2.40μg/L, respectively).21 While these studies demonstrate similar results to our study, the Profemur is a modular neck design, which represents another source of metal ions.
Other published studies have also demonstrated a larger proportion of cobalt than chromium ions when the trunion appears to be the source. A large retrieval analysis by Hothi et al. examined corrosion damage and pre-revision ion levels in 395 failed MoM THA and 529 retrieved resurfacing implants and demonstrated a ratio of cobalt to chromium of 1.58 in the THA group while the ratio was 1.08 ratio in the HRA group.22 The disproportionately elevated cobalt levels were attributed to the presence of the trunnion due to a correlation between the Co to Cr ratio and the level of corrosive damage scored at the taper. Another retrieval study by Matthies et al. analyzed the volumetric loss in different aspects of 110 MoM THA implants undergoing revision and found a positive correlation between Co levels and the volume of material lost at the female component of the taper.23
The major limitation to our study is that our total hip constructs were used in an off label manner as the Birgmingham femoral head was not specifically designed for the Corail stem taper. While both components require a 12/14 taper and it seems to have a congruent fit, there are subtle differences in taper geometry that may lead to micromotion. These constructs were used in a time period before there was as widespread understanding of trunionosis and the negative effects of corrosion with an imperfect taper match. We acknowledge that given today’s understanding of failure mechanisms due to trunion corrosion, this is not an ideal construct. However, this did give us the unique opportunity to compare two cohorts of patients with identical bearing surfaces and similar head size, with only one group having the presence of taper. We also acknowledge that there are biomechanical and demographic differences between the groups that could also account for differing metal ion levels. There were differences between groups regarding age, gender, and femoral head size, which have been known to affect metal ion levels and outcomes, specifically with bias from BHR patients being young and active male patients.24–26
This study demonstrated the trunnion as a likely source of metal ions, with more abundant generation of cobalt ions as compared to chromium ions. Metal ion generation from the trunion and its resultant effects is increasingly being recognized as an additional failure mechanism in arthroplasty constructs metal on metal bearing surfaces. It also has implications as an additional failure mechanism for conventional metal on polyethylene total hips, especially in those with larger head sizes. Further study is needed to better understand the trunnion as a source of complications in arthroplasty procedures.
Conflict of interest
None.
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