The current findings provide evidence that the revision of primary MoM THA failure using CTHA results in superior long-term clinical outcomes compared with the use of UTHA. To our knowledge, this is the largest study regarding outcomes due to conversion after MoM THA failure.
Complications of MoM THA related to ARMD can result in significant bone and soft tissue destruction as well as increased metal ion levels, especially cobalt and chromium, potentially increasing the risk of implant failure and posing a challenge for future revision[8, 5]. Metal ions can inhibit osteoblast gene expression, and they have a negative impact on osteoblast cell number and activity[3, 7, 15]. Hence, this could ultimately result in bone ingrowth failure in the uncemented components utilised during the conversion to UTHA after MoM THA failure[15]. The results of MoM THA revision have revealed high rates of orthopaedic complications due to aseptic loosening, deep infection, and dislocation[3]. In the 2018 National Joint Registry Annual Report, the 14-year cumulative probability of revision was 22.2% for uncemented stemmed MoM THA[16]. Additionally, in the 2018 Australian Orthopaedic Association’s Annual Report, the 15-year cumulative probability of revision was 29.6%[16]. Whether increases in hip stability exist following MoM THA revision has become one of the key indicators[8, 17, 18]. Hip stability following conversion to CTHA is superior to that following conversion to UTHA owing to the instability of the bone and uncemented components[8]. There is often macroscopic damage or bone defects at the time of UTHA re-revision[15]. The cause of bone defects has been shown to be associated with malposition and a design that is too shallow for the acetabular component, resulting in atypically elevated wear triggered by edge loading[19, 1]. A prior study showed a high rate of aseptic loosening of the femur component after UTHA re-revision due to MoM THA failure[5]. Failure due to aseptic loosening occurs more frequently with UTHA re-revision than with CTHA re-revision[20]. The most appropriate re-revision intervention to decrease the high rate of aseptic loosening tends to be a matter of ongoing debate[8]. Perhaps there is an effective solution when both the femoral and acetabular components are well ingrown.
While there have been continued advancements in prosthetic materials, the risks associated with conversion from MoM THA to UTHA or CTHA remain a substantial concern[15, 8, 3]. However, the obtainable literature on the outcomes of this type of conversion is lacking and contradictory[21, 4, 22]. A growing but still extremely limited body of literature has described the role of UTHA or CTHA revision in the setting of prior MoM THA failure and has demonstrated significant differences in clinical outcomes, although all the studies are limited by small sample sizes and/or short-term follow-up periods[3, 13, 8]. Undeniably, invasive revision procedures are associated with a high rate of orthopaedic complications[8]. However, we failed to detect noteworthy distinctions regarding the rates of major orthopaedic complications 12 months after conversion. Concerns have existed regarding whether these two types of conversion have substantial differences in long-term outcomes, including orthopaedic complication rates[8, 18, 3]. In 2002, Sierra et al.[23] identified 132 cases of conversion from MoM arthroplasties to UTHA and reported a high rate of major orthopaedic complications (45%), including a 13% aseptic loosening rate and a 9% dislocation rate, which were all higher than the rates found in this current study. In 2009, Eswaramoorthy et al[24] reported on 76 patients who underwent conversion from MoM arthroplasties to UTHA. Similar to the findings observed in the current study of failed MoM THA treatment, both aseptic loosening and periprosthetic fracture were the primary orthopaedic complications due to conversion. They also described a high rate of major orthopaedic complications (24%), mainly attributable to a high rate of aseptic loosening (20%). Then, Taheriazam et al.[25] assessed outcomes in 138 patients who were treated via conversion from MoM THA to CTHA and showed a major orthopaedic complication rate of 16% at the 2-year follow-up, including a 16% aseptic loosening rate. Based on a similar premise, Stryker et al.[26] reported on 114 cases of conversion from MoM THA to CTHA and showed a major orthopaedic complication rate of 18%, with a re-revision rate of 7%, primarily attributable to aseptic loosening (14%) and deep infection (6%).
Femoral aseptic loosening, especially in young, active patients, was a common factor for re-revision after conversion in the current study, which was also found by others assessing UTHA or CTHA revisions. CTHA has been developed in an effort to improve the fusion of cement and bone tissue and has become conventional for MoM THA revision failure[3, 13]. Short-term or midterm results of CTHA conversion following MoM THA failure have revealed extremely low rates of major orthopaedic complications, especially aseptic loosening[20, 5, 27]. Rahman et al.[5] reported on 20 patients who experienced MoM THA failure and who underwent conversion using CTHA; few patients were found to have aseptic loosening.
This current analysis also reveals that the reason for conversion has a prevailing impact on the outcomes of conversion. With modern THA and surgical techniques, conversion due to an indication of MoM wear has low rates of re-revision, regardless of the use of UTHA or CTHA for conversion, whereas conversion due to conventional periprosthetic fracture tends to be associated with a higher rate of re-revision. It is imperative that these facts are understood by orthopaedists and patients prior to conversion.
It should be acknowledged that there are limitations in this study. First, selection bias appears to be unavoidable due to the exclusion of a number of patients. Second, this retrospective observational study was susceptible to errors in recording differences in comorbidities and orthopaedic complications, which may have created unaccounted confounding variables and may have resulted in a diminished power to draw convincing conclusions. Attempts were made to allow for more than a few confounding variables; nevertheless, we believe that this analysis is inadequate. Third, we failed to include data on metal ion concentrations as well as information about high- and low-volume orthopaedists. Despite these limitations, we believe that the margin of error is tolerable in the current setting because of the relatively large sample size.