Is Modular Dual Mobility Superior to Standard Bearings for Reducing Dislocation Risk after Primary Total Hip Arthroplasty? A Retrospective Comparative Multicenter Study

Abstract

Background: Dual mobility (DM) has been proven to reduce dislocation risk after total hip arthroplasty (THA). In the last decade modular DM (modDM) constructs have been introduced to allow the use of DM articulation with standard cementless acetabular shells. However, clinical evidence of modDM effectiveness is still low in primary THA and concerns about implant-related complications are increasing. This retrospective comparative multicenter study is aimed to investigate if the dislocation rate after primary THA could be reduced with modDM in comparison to standard bearing (SB).

Methods: 262 THAs were performed between 2017 and 2019, using SB (129 hips) or modDM (133) with the same cementless highly porous modular acetabular cup. Dislocations, complications and revisions were recorded and implant survival was analyzed.

Results: At 2.5-year mean follow-up, dislocation occurred in 4 hips (3.1%) within the SB group while intraprosthetic dislocation in 2 hips (1.5%) within the modDM group (p = 0.44). Implant survivals with revision due to dislocation were 95.2% and 95.9% at 4-year follow-up for SB and modDM, respectively (p = 0.50).

Conclusions: modDM used in primary THA might reduce dislocation rate in comparison to SB, even in high-risk patients, however, caution is advocated due to specific intraprosthetic dislocation.

Keywords: dislocation; intraprosthetic dislocation; modular dual mobility; standard bearing; total hip arthroplasty.

Figure 1

Image of the cementless acetabular cup implanted in all included patients, the Jump System Traser^® acetabular cup by Permedica Orthopaedics, Merate, Italy. Traser^® is a 3D-printed cancellous bone-like irregular titanium lattice, additively manufactured by selective laser melting in one-step process together with the cup. Traser^® has 70% fully interconnected porosity with a mean pore size of 520 microns. The Jump System Traser^® cup allows for seating of a standard fixed insert, as vitamin E blended cross-linked polyethylene (A), or a modular stainless steel DM insert, fully coated by titanium-niobium nitride (B).

Figure 2

Early IPD of a modular DM construct occurred 11 months after revision surgery of a THA with a standard fixed polyethylene insert and 32 mm femoral head which previously failed for dislocation at a 19-month follow-up. CT scan (images (A–C)) was taken before re-revision. (A): CT scan image shows the eccentric position of the small femoral head with respect to the acetabular shell. It is well recognized the modular adapter between the ceramic head and the femoral stem morse taper. (B): the modular CoCr liner appears to be malseating into the titanium acetabular shell: the superior-lateral margin of the metal liner (white arrow) is more prominent from the acetabular shell equatorial rim than its medial margin. Another radiographic sign of modular liner malseating is the eccentric void space between the liner and shell. (C): white arrow shows how the acetabular cup was placed with excessive anteversion and modular CoCr liner malseating. (D): wear signs on the retrieved ceramic head surface due to its articulation against the modular CoCr liner edge.

Figure 3

IPD case occurred 41 months after modular DM THA. (A): CT scan image showing the acetabular cup implanted with 55° of anteversion, leaving the anterior portion of the acetabular cavity uncovered (black arrow). (B,C): Massive wear and deformation of the mobile polyethylene liner were observed at revision along the liner edge. These signs on the retrieved component could represent the catastrophic results of a mobile liner impingement against the anterior portion of the acetabulum caused by cup malpositioning for excessive anteversion.

Figure 4

Early IPD case occurred 2 months after modular DM THA during unsuccessful hip closed reduction to treat a dislocation caused by a traumatic event. (A): Plain radiograph taken after dislocation, showing outside articulation the mobile polyethylene liner correctly assembled over the 28 mm ceramic head (white arrows). (B): Plain radiograph taken immediately after the hip closed reduction attempt, showing the IPD with the mobile polyethylene liner dissociated from the small head, the so-called “bubble sign” (white arrows). Polyethylene liner dissociation is likely due to the great traction forces during the closed reduction maneuver which are higher and overwhelm the push out force of the retentive mobile polyethylene liner.

Figure 5

Kaplan–Meier survival of standard bearing and modular DM groups with revision of any component due to dislocation as the end-point.

Figure 6

Stratified analysis (excluding hips with femoral neck fractures as index diagnosis) for Kaplan Meier survival of standard bearing and modular DM subgroups with revision of any component due to dislocation as the end-point.