The present study documented the long-term follow-up of an RCT comparing LDH THA and HR, both with identical MoM bearing. Assessing PROMS for all cases (including revised ones), LDH THAs provided better WOMAC scores, compared to HR, and patients more frequently reported no limitation with their artificial joints. We found higher revision rates in the LDH THA group and different reasons for revision in each group. The main reasons for revision were femoral head loosening for HRs and ARMD secondary to trunnionosis in LDH THAs. At the last follow-up, Co levels were significantly higher in LDH THA, whereas radiographic outcomes were similar.
This study has some limitations. First, when the study was designed, gait analysis was the primary outcome and power analysis was calculated accordingly. Interestingly, while this may have limited the power of the study, we could still find statistically significant results, and our data represent the longest findings published from an RCT comparing LDH THA to HR using the same bearing. Second, patients were kept blind to the type of prosthesis implanted for only 1 year after surgery. The impact of their knowledge of implanted components on PROMs after that time point is unknown. Third, patients were not systematically assessed by imaging techniques (ultrasound, MRI) to identify ARMD. It is likely that asymptomatic ARMDs were missed.
In our study, all PROMs were higher in the LDH THA group, even though only WOMAC and the perception of no limitation on the PJP question reached a statistical significance (Table 2). In the LDH THA group, 1/19 patients had a WOMAC score < 80, compared to 6/20 in HR. When comparing WOMAC scores over time for all cases, we found a deterioration for both study cohorts (Table 2, Fig. 2). This may be explained by patients aging, or by the impact of surgical revision on the clinical scores. Interestingly, when we compared PROMs of patients with well-functioning implants, LDH THA was found to have better overall PJP results, but no significant differences in other PROMs (WOMAC was nearly significant with p = 0.05). However, there are some limitations to most validated PROMs and patients may feel their hip as “natural” without necessarily having better WOMAC scores . The data from our current study do not provide a robust explanation for the differences in PROMs between groups in the long term. On the other hand, in the senior author’s practice, when revising a HR into a LDH THA, most patients show preference to the revised hip. Similar subjective feedback is given by patients with a HR on one side and subsequently operated on the other side with a LDH THA. They all prefer the LDH THA side, mainly for its increase flexibility and greater range of motion. At a shorter follow up of 9 years, a similar RCT by Konan et al. , comparing 104 Durom HR or LDH THA, noted no difference in PROMs. Using a CoC LDH THA in 276 patients, our group, using the same surgical technique as in the current study, observed mean WOMAC score of 92.3, UCLA activity score of 6.6, and FJS of 88.5 after a mean follow-up of 67 months . These results are very similar to the well-functioning MoM LDH THA data of the current study (Table 2).
Revision rates and other adverse events
In our study, the reasons for revision were implant specific. All HR’s revisions and failed implant on radiographic analysis were due to femoral component loosening (3/24). The failed femoral head diameters were all ≤48 mm, which is considered an important risk factor for loosening [2, 12, 24]. Indeed, Durom HR was reported to have a higher rate of femoral head loosening compared to other HR implants and the US acetabular cup version (not used in the present study) was recalled by the manufacturer in 2008. In the LDH THA group, all revisions except 1 deep infection were performed for ARMD. In addition, there are 2 more patients with suspected ARMD: one of which is scheduled for surgery, while the other has mild symptoms and refuses further treatment, leading to a total ARMD rate of 25%. LDHA THA sharing the same bearing as HR, adding the modular head introduced a problematic modular junction responsible for trunnionosis, and thus ARMD. Our macroscopic findings of blackened corroded debris at the head-neck junction at time of revision confirmed the failure mechanism. Higher failure rates were also reported with Durom LDH THA (28.9%) in comparison to Durom HR (2.3%) by Ridon et al.  after 10-year follow-up. Similarly, Konan et al.  after a mean follow-up of 9 years, reported a 2.1% revision rate in HR versus 12.5% in LDH THA. From these results we understand that the MoM LDH bearing was not the source of the failures. With an appropriate modular junction, it would provide low failure rate and then its stability benefits may become appealing.
Using a CoC LDH THA in 276 patients, we reported a revision rate of 1.4% after a mean follow-up of 67 months (min 48, max 79) . The 4 revisions were unrelated to the modular junction or the bearing. Furthermore, we did not observe radiographic or clinical signs of ARMD. To indirectly assess the performance of the modular junction of this specific CoC LDH THA, we measured the Ti level in 57 unilateral cases after a mean follow up of 79 months . We observed low mean Ti levels (mean 1.9, SD 0.53), suggesting good modular junction performance. If the encouraging results of these studies are confirmed in the long term then the use of CoC LDH THA, would be a favorable alternative to HR in the young active patient. In recent years, CoC or MoP HR were also introduced but femoral head necrosis and loosening would stay unresolved [8, 27, 28].
In our study no unrevised implant suffered a dislocation, highlighting the benefit of LDH THA and HR implants to reduce the dislocation and maximize range of motion. In a series of 1748 LDH THAs at a mean follow-up of 31 months, the dislocation rate was 0.05% . In in a retrospective study comparing 559 conventional THAs and 248 LDH THAs at a mean follow-up of 5 years, the dislocation rate was 1.8% versus 0% respectively . Similarly, in an RCT, we reported no dislocation rate in HR versus 3% in 28 mm THA . Similarly, Pollard et al. in retrospective study, reported a dislocation rate of 7.4% among 54 THA compared to none in 54 HRs .
Metal ion levels
Systemic Co and Cr ions levels are an indirect way to assess the in vivo bearing and/or modular junction performance. Although both implants in our study had the same bearing, Co ion levels were significantly higher in LDH THA, whereas no statistical difference in Cr could be found (Table 3). Moreover, the Co/Cr ratio was much higher in LDH THAs (1.9 vs 1.0). These differences can be explained by the poor performance of the modular junction of the Durom LDH THA. The wear and corrosion of the modular junction releases additional Co and Cr . Part of the Cr wear particles precipitate locally, forming a black tartrate around the femoral neck, reducing the amount of Cr released systemically, explaining a higher whole blood Co/Cr ratio and the lack of measured difference in systemic Cr between groups . Ridon et al.  reported 6-fold higher Co (p < 0.0001) and increased Cr (p < 0.0001) whole blood ion concentrations in patients with LDH THA compared to patients with HR, when comparing the results of the same acetabular component (Durom) after more than 10 years’ follow-up. Similarly, in a RCT evaluating the Durom system at 1 year postoperatively, patients with a LDH THA had 10-fold higher serum Co levels (p = 0.000) compared to patients with HR, and no difference for Cr levels . Finally, metal ion levels were not statistically different over time in both groups, indicating that the unrevised LDH THA and HR prostheses continue to perform well in the long-term follow-up.
On radiographic evaluation at the last follow-up, all components were considered to be stable, except for 1 asymptomatic HR with a loose femoral component. Lucent lines on the femoral side were not seen in other HR, whereas proximal non-progressive femoral radiolucent lines observed in the LDH THA cases are known to be linked to normal long-term bone remodeling pattern of the CLS stem . These results are reassuring as the remaining unrevised cases seem to be producing limited wear particles and should continue to do so in the future.
HR has been proposed as a great treatment option for the high expectations’ young patients. On the other hand, its indications may be limited in presence of secondary osteoarthritis associated with anatomical challenges, such as acetabular retroversion, hip dysplasia, femoral retroversion, Perthes, and pistol-grip deformity. LDH THA replacing the femoral head-neck pathological anatomy and offering a supraphysiologic head-neck ratio has the advantage of simplifying these complex cases .