Comparing Dual-mobility Cups Versus Unipolar Cups for Unselected Primary Total Hip Arthroplasty by Direct Anterior Approach

Monocentric Randomised Controlled Trial Comparing Dual-mobility Cups Versus Unipolar Cups for Unselected Primary Total Hip Arthroplasty by Direct Anterior Approach

THA comprises the replacement of the damaged bone and cartilage with prosthetic components. The proximal end of the femur is replaced with a stem and ball/head, while the acetabulum is most commonly replaced with a unipolar cup (UP) that consists of a liner inside of a metal shell. UP cups are associated with risks of dislocations, especially in elderly and female patients, as well as in patients with lumbar stiffness or operated for femoral neck fractures. In fact, one of the most common reasons for early revision of UP cups are dislocations.

Dual-mobility (DM) cups were introduced in the 1970s by the French surgeon Gilles Bousquet, as an alternative to UP cups. They are made up of two articulations, a femoral head moving inside a mobile polyethylene liner, which in turn moves inside a metal shell. This design allows for a greater jump distance and range of motion (ROM) compared to UP cups, thus resulting in reduced rates of dislocation.

A recent systematic review that evaluated 5 case-control studies (549 DM cups vs 649 UP cups) and 3 registry studies (5935 DM cups vs 217362 UP) found that in case-control studies, the dislocation rates were 0.2% for DM cups versus 7.1% for UP cups (p=0.009). Furthermore, in registry studies the revision rates due to dislocation were 0.2% for DM cups versus 0.5% for UP cups (p=0.050).

Nonetheless, there have been some concerns associated with the use of DM cups, including intra-prosthetic dislocations (IPD), polyethylene wear, and iliopsoas impingement. Since the early 2000s, new improvements in the design of DM cups have nearly eliminated the risk of IPD, and the use of ultra-high molecular weight polyethylene (UHMWPE) has greatly reduced wear rates, even in young and active patients. Lastly, although some studies have reported iliopsoas impingement with DM cups, the impingement is usually against the optional fixations, and not against the mobile polyethylene insert.

The hypothesis of the present interventional randomised controlled trial (RCT) is that THA using DM cups will not provide inferior outcomes compared to THA using UP cups, in terms of hip awareness measured using the forgotten joint score (FJS).

Inclusion Criteria:

• men and women aged ≥18 years old
• set to undergo elective primary THA via direct anterior approach (DAA)
• affiliated with a social security scheme
• able to answer questionnaires and to communicate freely in French
• who have been informed and have provided free, informed, and written consent (no later than the day of inclusion and before any examination required by the study)

Exclusion Criteria:

• with prior ipsilateral hip surgery
• with prior contralateral hip surgery
• that require acetabular cup diameters ≤50mm (because DM cups are not available from the same manufacturer for diameters ≤50mm, and UP cups require head diameters of 36mm and thus minimum cup diameters of 52mm)
• undergoing emergency or trauma surgery (e.g. femoral neck fractures)
• with any concomitant disease or disorder which, in the opinion of the surgeon, may put the patient at risk, notably of hip instability or dislocation (hip dysplasia, neuromuscular deficit, fracture sequelae…) or may influence the result of the study
• that are pregnant or breastfeeding
• under legal protection (guardianship or curatorship or deprived of liberty).