Our findings provide evidence that the revision of initial MoM-THA failure using CTHA yielded superior long-term clinical outcomes compared to 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 could result in noteworthy bone and soft tissue destruction as well as increased metal ion levels, potentially increasing the risk of endoprothesis failure[8, 5]. These ions can downregulate osteoblast gene expression and have a negative impact on osteoblast cell number and activity, which could ultimately lead to bone ingrowth failure in the uncemented components[3, 7, 14]. The results of MoM-THA revision have revealed high rates of orthopaedic complications due to aseptic loosening[3]. In a recent report[15], the 14-year cumulative probability of revision was 22.2% for uncemented MoM-THA; the 15-year cumulative probability of revision was 29.6%. Whether increases in hip stability exist after MoM-THA revision has become one of the key indicators[8, 16, 17]. 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]. Macroscopic damage or bone defects were frequently observed at the time of UTHA [14]. The trigger of bone defects has been reported to be associated with malposition and a flawed design for the acetabular component, resulting in abnormally elevated wear triggered by edge loading[18, 1]. Failure due to aseptic loosening occurs more frequently with UTHA than with CTHA[19].
The risks associated with conversion from MoM-THA to UTHA or CTHA remain a substantial concern[14, 8, 3].Nonetheless, the obtainable literature on the outcomes of this type of conversion is deficient[20, 4, 21]. Several studies have revealed noteworthy differences in clinical outcomes, although these studies are limited by small sample sizes and/or short-term follow-up[3, 13, 8]. Undeniably, invasive revision procedures are associated with a high incidence of orthopaedic complications[8]. However, we failed to detect noteworthy distinctions regarding the incidences 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 incidences[3, 8, 17]. In 2009, Eswaramoorthy et al[23] reported on 76 patients who underwent conversion from MoM-THA to UTHA. Similar to the findings observed in the current study, both aseptic loosening and periprosthetic fracture were the primary orthopaedic complications. They also described a high incidence of major orthopaedic complications (24%), mostly attributable to a high rate of aseptic loosening (20%). Then, Stryker et al.[24] reported on 114 cases of conversion from MoM-THA to CTHA and demonstrated a major orthopaedic complication rate of 18%, with a re-revision rate of 7%, primarily attributable to aseptic loosening (14%) and deep infection (6%).
This present study 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.
Several limitations should be acknowledged in this study. Firstly, selection bias was inevitable due to the exclusion of a number of cases. Secondly, this retrospective observational study was susceptible to errors in recording differences in comorbidities and orthopaedic complications, which may have produced unaccounted confounding variables and may have led to a diminished power to draw convincing conclusions. Thirdly, we failed to involve metal ion concentrations as well as information about high- and low-volume orthopaedists.