The revision of initial MoM-THA using CTHA yielded superior long-term clinical outcomes compared to the use of UTHA. For all we know, this is the largest report regarding outcomes of conversion after MoM-THA.
The MOC of MoM-THA related to ARMD could be the occasion of increased metal ion levels, potentially increasing the risk of endo-prothesis failure[8, 5]. These ions reduce the number and activity of osteoblasts by inhibiting the expression of osteoblast genes, eventually leading to intraosseous growth disorder[3, 7, 14]. The result of the metal ion is a challenge to UTHA or CTHA. In response to this challenge, the force on UTHA may be transmitted to one part of the bone, while it acts as a stress shielding to other bones[15-17]. On account of the limitation of stress buffering or conduction[4], UTHA can lead to further bone destruction, which will form a vicious cycle, and eventually lead to the failure of the prosthesis[11, 18, 19].
The risk of converting from MoM-THA to UTHA or CTHA remains an increasing concern[14, 8, 3]. Nonetheless, the reported literature on the outcomes of these conversions is underprovided[20, 4, 21]. Several studies have revealed noteworthy distinctions in clinical outcomes, although they are constrained by small sample sizes[3, 13, 8]. Undeniably, revision MoM-THA is related to a high rate of MOC[8]. Even so, we failed to detect noteworthy distinctions regarding the rates of MOC 1 year after conversion. There has been increasing concern as to whether there are substantial differences in the long-term results of these two types of conversions[3, 8, 17]. In 2009, Eswaramoorthy et al.[23] reviewed 76 cases who undertook conversion from MoM-THA to UTHA. Consistent with the current findings, a high rate of MOC (24%) was observed involving prosthesis loosening and periprosthetic fracture, predominantly attributable to a high rate of revision (7%).
The reason for the conversion has a significant effect on the result of the conversion[4, 7]. With the continuous updating of arthroplasty equipment and further development of surgical technology, the total re-revision rate due to MoM wear is low, regardless of whether long- or short-stem UTHA is used for THA[11, 15]. Periprosthetic fractures frequently result in high re-revision rates for UTHA[16, 19]. Consistent with our results. A possible explanation for these phenomena is that the stress transmission is severely disrupted by the periprosthetic fractures[20, 21]. It may be difficult to reconstruct the supporting structure by using long- or short-stem of UTHA[15, 18]. In this way, it is unlikely that long- or short-stem of UTHA will resist stress under the weight-bearing[2, 8]. However, for CTHA, the situation may be different, because cemented fixation itself is an antagonistic relationship to fracture or stress[6-8]. It can enhance the friction coefficient of the bone-cement interface, and also promote the tension of the entire femoral part, which is crucial for the hip stability after revision[13, 18].
Several limitations should be acknowledged in this study. Firstly, this is a retrospective study, and bias is inevitable. However, the results are basically quantitative because clinical outcomes are the focus of our attention. In addition, because there were no significant differences in baseline data in this comparative study, the effect of data on retrospective collection is limited. Secondly, subjective factors may play a key role in assessing baseline comorbidities in the absence of objective diagnostic data, which may result in unexplained baseline variables, which reduces the ability to draw reliable conclusions. Thirdly, the subjects we included have a large time span (more than 11 years), which may have limited the reliability of our conclusions. Because the clinical experience of surgeons is not fixed. With the accumulation of surgical volume, the surgeon's experience may be a potential variable, which plays an important role in postoperative outcomes.