Dual mobility cemented cups have low dislocation rates in THA revisions

Abstract

THA revisions using standard cups are at risk of dislocation (5.1% to 14.4% incidence), especially in patients over 70 years of age. Constrained tripolar cups have reduced this risk (6% incidence) but are associated with substantial loosening rates (9%). The nonconstrained dual mobility cup was designed to improve prosthetic stability (polyethylene head >or= 40 mm diameter) without increasing loosening rates by reducing wear and limiting impingement (rotation range of 108 degrees). We implanted 88 cemented dual mobility cups for THA revisions in 82 patients at high risk of dislocation. Average patient age was 72 years (range, 65-86 years). Eighty-five of the 88 hips were reviewed at 2 to 5 years followup. One patient (1.1%) had a traumatic dislocation at 2 years postoperatively. Two patients (2.3%) had asymptomatic early loosening and three patients (3.5%) had localized radiographic lucencies. These results confirm those with press-fit dual mobility cups suggesting a low dislocation rate at 5 years and a cup survival of 94.6%. At middle term followup, cemented dual mobility cup achieved better results than constrained cups in cases at risk of dislocation and recurrent loosening.

Level of evidence: Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Figures

Fig. 1

The cemented Medial Cup® DMC consists of a steel inox polished sphericocylindrical cemented metal cup and a PE cup insert. Flexion occurs mainly in the small inner articulation. Abduction/adduction and rotation start in the small articulation (up to 25°) and continue up to 54° in the large peripheral articulation between the PE cup and the metallic socket. The PE cup (≥ 8-mm thickness) is not constrained in the metallic socket (no retentive rim), but the PE inner articulation is retentive. The femoral head, usually metallic (22 mm), is clipped inside the PE cup intraoperatively.

Fig. 2A–B

Revision of a loose THA with a cemented DMC is shown. (A) This 77-year-old patient had bilateral combined segmental and cavitary acetabular osteolysis with severe instability. (B) The patient underwent bipolar revision with a femoral Charnley prosthesis. Impaction grafting plus structural allografts, reinforced by a Kerboull cross-plate, were used. Three years postoperatively, a good clinical result (Merle d’Aubigné-Postel functional score, 5–6–4) was achieved with no loosening or osteolysis.

Fig. 3A–B

Revision of a loose THA with a cemented DMC is shown. (A) This 65-year-old patient had combined segmental and cavitary acetabular osteolysis after first revision. (B) Bipolar revision was performed using a femoral Charnley prosthesis. Impaction grafting and a Kerboull cross-plate reinforcement device were used. Two years postoperatively, the patient’s Merle d’Aubigné-Postel functional score was 6–6–5, and the minute demarcation between allografts and host bone was similar to the immediate postoperative aspect.

Fig. 4

The DMC design was improved by medialization of the PE cup. The center of rotation of the surface bearing of the metal socket is more medial than the center of the socket periphery. Therefore, resultant forces acting on the PE cup avert its tilting in varus (and thus the risk of upper PE cup rim wear, leading to subluxation). To reduce the wear of the entire rim, the head:neck ratio is 22 mm:10 mm, which allows more than 25° of abduction or rotation in the inner articulation. The femoral neck is polished, and its good congruity with the PE rim limits chipping during impingement.