Long-term outcomes of uncemented or cemented arthroplasty revision following metal-on-metal total hip arthroplasty failure: A retrospective observational study with a mean follow-up of 7 years

A high rate of metal-on-metal total hip arthroplasty (MoM-THA) has been well-known. The aim of this study was to compare the long-term outcomes of patients who had undergone uncemented or cemented THA(UTHA or CTHA) following initial MoM-THA failure. Methods Data from 234 patients (234 hips) who were treated with UTHA or CTHA following initial MoM-THA failure during 2007 - 2018 were retrospectively compared. Follow-up occurred 3 months, 6 months, 1 year, 2 years, and then every 1 year after conversion. The mean follow-up was 84.15 months (67 - 101 months). The primary endpoint was the Harris Hip Scores (HHS); secondary endpoint was the incidence of major orthopaedic complications.


Abstract
Background A high rate of metal-on-metal total hip arthroplasty (MoM-THA) has been well-known. The aim of this study was to compare the long-term outcomes of patients who had undergone uncemented or cemented THA(UTHA or CTHA) following initial MoM-THA failure.

Methods
Data from 234 patients (234 hips) who were treated with UTHA or CTHA following initial

Results
The HHS demonstrated statistically greater differences in Group CTHA than in Group UTHA 12 months after conversion. From the 12th month after conversion to the final follow-up, CTHA yielded superior functional outcomes than UTHA. Between-group noteworthy differences were observed regarding the rates of re-revision, aseptic loosening, and periprosthetic fracture (10.3% for UTHA vs 2.5% for CTHA, p = 0.015; 16.3% for UTHA vs 5.9% for CTHA, p = 0.011; and 12.0% for UTHA vs 4.2% for CTHA, p = 0.045, respectively).

Conclusion
In the setting of revision following initial MoM-THA failure, we found definite evidence of the superiority of CTHA over UTHA in regard to improving functional outcomes and decreasing the incidence of major orthopaedic complications.

Background
Bearing surfaces provided by metal-on-metal total hip arthroplasty (MoM THA) became increasingly prevalent, particularly in the context of an ageing population, in earlier decades [1,2]. In recent years, however, the use of MoM THA has sharply declined due to the reported low 10-year survivorship and high failure rates that are associated with a host of issues (i.e., adverse reactions to metal debris [ARMDs], aseptic loosening, and infection) [3,4]. Failure after MoM THA is well recognised, is due to various reasons, and frequently requires revision intervention [5]. Among predominantly active individuals, failure that occurs secondary to MoM wear tends to be a concern [6]. Although MoM bearings have fallen out of favour as a result, orthopaedists continue to struggle with this issue of revision burden [7]. Poor bone stock may be attributed to the substantial bone and soft tissue destruction caused by ARMD that is powerfully implicated in the pathophysiology of MoM THA failure, contributing to the substantially high revision rate as well as the rapid time-to-failure [8].
This escalated MoM THA-related failure rate may also contribute to the increase in the use of uncemented or cemented total hip arthroplasty (UTHA or CTHA) [8,9]. A limited number of studies have assessed complications due to the conversion of MoM THA to the use of uncemented or cemented femoral components (UTHA or CTHA) [3]. Interest in CTHA has increased over the last decade, with several studies showing higher Harris hip scores (HHS) with fewer orthopaedic complications for CTHA than for UTHA, while others have demonstrated no significant differences between the two [5]. Furthermore, there remain concerns that longer-term outcomes of UTHA may not be as robust as those of CTHA regarding reduced revision rates [8,10]. Additionally, the highly selected features of the studied patient data are common in the published literature [11,12]. Consequently, the findings in those studies could not be referred to as valid.
To date, no definitive consensus exists on the long-term outcomes of conversion from primary MoM THA revision to UTHA or CTHA due to any cause [11]. Furthermore, given the lack of literature and in order to gain a better understanding of these types of conversion, we performed a retrospective study to assess the long-term outcomes of conversion from primary MoM THA to UTHA or CTHA.

Study population
A retrospective study was performed with an initial study cohort that included 326 patients (326 hips) identified from our joint registration database who underwent UTHA or via a direct anterior approach, as reported [13]. Manufacturer details of stems and cups employed in UTHA or CTHA were shown in Table 1. The main exclusion criteria included patients without an MoM-bearing surface at the time of conversion; patients with inadequate clinical data, active infection, dyskinesia, or bone-related diseases; patients unable to follow instructions; and patients with malignant tumours, injury severity scores (ISSs) ≥10, brain dysfunction due to any cause within 6 months, pulmonary complications (i.e., reintubation), cardiovascular complications (i.e., cardiac arrest, myocardial infarction), renal complications (i.e., insufficiency or failure), vascular cognitive impairment, a history of alcohol and/or drug abuse, a body mass index (BMI) > 40 kg/m 2 , and an American Society of Anesthesiologists (ASA) score of IV or V.
A standard protocol was utilised to obtain clinical and radiographic data. Follow-up occurred 3 months, 6 months, 1 year, 2 years, and then every 1 year after conversion. The primary endpoint was the modified HHS, which has scores ranging from 0 to 100 points, with higher scores representing better function. The secondary endpoints were the major orthopaedic complication rates. All the patients included underwent conversion to UTHA or CTHA at our medical centre and were assessed individually with the modified HHS at each follow-up. Image data were acquired at these same time points: anteroposterior (AP) radiographs of the pelvis and AP and lateral radiographs of the hip as well as computed tomography (CT) or magnetic resonance imaging (MRI) if necessary. The occurrence of major orthopaedic complications was recorded for each patient during the follow-up.

Statistical analysis
Between-group differences in terms of the baseline data and the main follow-up data were compared. Follow-up was calculated in days from the date of revision/conversion to the date of death or final follow-up, whichever occurred first. Revision was defined as the complete removal of the endoprosthesis. Re-revision was defined as the removal or exchange of any component. Prosthesis loosening and endoprosthesis failure were judged based on prior descriptions [8]. Heterotopic ossification was judged using the Brooker classification system [14]. Continuous data are presented as the means and standard deviations. T-tests were utilised to assess between-group differences if the data were consistent with assumptions regarding the normal distribution and homogeneity of variance. If not, Wilcoxon rank-sum tests were utilised. Categorical data are presented as frequencies and percentages and were compared between groups using Chi-squared tests or Fisher's exact tests, as appropriate. All statistical analyses were performed using SPSS,  Figure 1, and the baseline data are shown in Table 2.

Primary endpoint
The mean HHS after conversion are shown in Table 3 Differences in HHS were not significant between the groups 3 months or 6 months after conversion.

Discussion
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 rerevision 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 rerevision 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.

Conclusions
The long-term results reported in this study support a growing body of evidence that conversion to CTHA due to primary MoM THA failure is associated with more significant improvements in modified HHS as well as lower major orthopaedic complication rates than conversion to UTHA.

Acknowledgements
The authors would like to thank Hang Yu and Junxing Ye for help with retrieval of patients' data.

Funding
Funding for this research was received from the National Natural Science Foundation of China (Grant No. 81971315).

Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

Ethics approval and consent to participate
This study was approved by the Medical Ethics Committee (The First Affiliated Hospital, Sun Yat-sen University), and an exemption from informed consent was obtained from our responsible Investigational Ethics Review Board.

Consent for publication
Not applicable.