DISCUSSION
According to the Center for Disease Control and Prevention, 310,800 total hip replacements were performed among inpatients aged 45 years and older in the US in 2010.7 Specifically, in the 55- to 64-year-old age group, the number of procedures performed tripled from 2000 through 2010. As younger and more active patients opt for hip replacements, a growing need for prosthesis with enhanced durability is observed.
Despite the early proposed advantages of large head MoM bearings, our retrospective study of the DePuy Synthes ASR™ XL Acetabular hip system yielded 15.6% and 34.4% failure rates at 3 and 5 years, respectively. These higher-than-expected rates of failure are consistent with published data. The British Hip Society reported a 21% to 35% revision rate at 4 years and 49% at 6 years for the ASR XL prosthesis.8 In comparison, other MoM prosthesis, on average, report a 12% to 15% rate of failure at 5 years.
Considerable controversy surrounds the causes of adverse wear failure in MoM bearings.9,10 The non-modular design of the ASR prostheses is frequently implicated as a cause of early failure. The lack of a central hole in the 1-piece component compromises the tactile feel of insertion, thereby reducing the surgeon’s ability to assess complete seating.11 This condition may potentially increase the abduction angle at the time of insertion. Screw fixation of the non-modular device is not possible. The ASR XL device (148° to 160°) is less than a hemisphere (180°) in size and hence features a diminished functional articular surface, further compromising implant fixation.11 The functional articular surface is defined as the optimal surface area (10 mm) needed for a MoM implant.12 Griffin and colleagues13 reported a 48 mm ASR XL component, when implanted at 45° of abduction, to function similar to an implant at 59° of abduction, leading to diminished lubrication, metallosis, and edge loading. The version of the acetabular component may similarly and adversely affect implant wear characteristics. Furthermore, the variable thickness of the implant, which is thicker at the dome and thinner at the rim, may further promote edge loading by shifting the center of rotation of the femoral head out from the center of the acetabular prosthesis.11 Studies have also shown that increased wear of the MoM articulation is associated with an acetabular component inclination angle in excess of 55°10,14 and a failure of fixation at time of implantation.15 This study, however, found no correlation between the abduction angle and risk of early implant failure for the ASR acetabular component. No correlation was also detected between the acetabulum size and revision surgery.
The AOANJRR reported loosening (44%), infection (20%), metal sensitivity (12%), fracture (9%), and dislocation of prosthesis (7%) as the indications for revision surgery for the ASR prosthesis.6 Furthermore, a single-center retrospective review of 70 consecutive MoM THAs with ultra-large diameter femoral head and monoblock acetabular components showed that 17.1% required revision within 3 years for loosening, pain, and squeaking.1 Overall, 28.6% of patients reported implant dysfunction. In this study, we observed a similar rate of failure at 3 years (15.6%) for pain (11) and infection (1). The revision surgery successfully relieved all of these symptoms. One patient presented with heterotopic ossification and anterior hip pain after the original revision and required additional surgery with prosthesis retention. No patient in this series required repeat component revisions at a mean of 2.9 years after surgery. In all but 1 case, primary acetabular components were used in the revision, and in all cases except that with infection, the femoral component was retained. Replacement shells were 2 to 4 mm larger in diameter than the original ASR component.
Recently, concerns have arisen regarding the long-term effects of serum cobalt and chromium metal ions levels. Studies have shown increased serum metal ion levels,15 groin pain,16 pseudotumor formation,17 and metallosis18 after the implantation of MoM bearings. In a case study by Mao and colleagues,19 1 patient reported headaches, anorexia, continuous metallic taste in her mouth, and weight loss. A cerebrospinal fluid analysis revealed cobalt and chromium levels at 9 and 13 nmol/L, respectively, indicating that these metal ions can cross the blood-brain barrier. Another patient reported painful muscle fatigue, night cramps, fainting spells, cognitive decline, and an inability to climb stairs. His serum cobalt level reached 258 nmol/L (reference range, 0-20 nmol/L), and chromium level totaled 88 nmol/L (reference range, 0-100 nmol/L). At 8-week follow-up after revision surgery, the symptoms of the patient had resolved, with serum cobalt levels dropping to 42 nmol/L.19 None of the patients in this study presented with any signs or symptoms of metal toxicity. The upper limits of blood cobalt and chromium levels in our study population reached 18.9 and 15.9 µg/L for the revised group and 16.8 and 5.4 µg/L for the non-revised group, respectively. However, we noted a similar drop in post-revision blood cobalt (91% decrease) and chromium (78% decrease) levels.
In summary, our data showed a high revision rate of the DePuy Synthes ASR™ XL Acetabular hip system. Our findings are consistent with internationally published data. In the absence of reliable predictors of early failure, continued close clinical surveillance and laboratory monitoring of these patients are warranted.
CONCLUSION
This study demonstrates the high failure rate of the DePuy Synthes ASR™ XL Acetabular hip system used in THA at a minimum of 5 years of follow-up. No variable that was predictive of failure could be identified in this series. Close clinical surveillance of these patients is therefore required. Metal levels dropped quickly after revision, and the revision surgery can generally be performed with slightly larger primary components. Symptomatic patients with ASR hip replacements, regardless of blood metal-ion levels, were candidates for the revision surgery. Not all failed hips exhibited substantially elevated metal levels. Asymptomatic patients with high blood metal-ion levels should be closely followed-up and revision surgery should be strongly considered, consistent with recently published guidelines.20