Constrained liner in neurologic or cognitively impaired patients undergoing primary THA

Abstract

Background: THA performed in patients with cognitive deficits or neuromuscular diseases has been associated with a high postoperative dislocation rate. The constrained liner reportedly provides stability in patients with recurrent dislocation. However, achieving stability could be offset by early loosening when used in patients with neurologic diseases.

Questions/purposes: We therefore asked whether constrained liners had a higher risk of loosening when used in primary THA for patients with neurologic diseases.

Methods: We retrospectively reviewed a 144 patients (164 hips) with neuromuscular disease who had a constrained polyethylene insert from 1999 to 2004 and compared them to another 120 patients (132 hips) with neuromuscular disease operated on immediately before this period (from 1994 to 1998) who had a conventional polyethylene insert.

Results: Thirty-three (25%) of the 132 hips without a constrained liner were known to have had at least one dislocation and 21 had revision for recurrent dislocation. Ten other hips had revision for loosening of the cup. The survival rate was 82% at 5 years and 77% at 10 years with revision due to recurrent dislocation or loosening of the cup as the endpoint. With a constrained liner, at minimum 5-year followup (mean, 7 years; range, 5-10 years), the incidence of complications, particularly dislocation, was decreased (three dislocations among 164 hips; 2%), with one revision for recurrent dislocation and one revision for loosening.

Conclusions: This constrained acetabular component provides durable protection against dislocation without substantial increased loosening at midterm followup.

Fig. 1A–B

(A) A photograph and (B) diagram illustrate the retentive cup. The principle of retentiveness is obtained with a more enveloping cup than the hemispheric model. The flange in the equatorial plane permits insertion of a split PE ring reinforced by a stainless steel hoop.

Fig. 2A–C

(A) The cup without the head is shown. (B) On crossing the equator of the cup, the head opens the ring, (C) which then closes through simple elasticity.

Fig. 3

Once the head has crossed the ring, it falls below the equatorial plane. Therefore, in most situations, the cup is not really a constrained liner. Only in movements at risk for dislocation is the ring constrained.

Fig. 4A–C

(A) Disassembled head; (B) Assembled head. Less than 30 N (± 6 N) is required to successfully insert the head; (C) Dislocation requires 2153 N (± 32 N).

Fig. 5

Kaplan-Meier curves depicting failure as dislocation. The survival rate was lower (p = 0.006, log-rank test) in patients with unconstrained liners.

Fig. 6

Kaplan-Meier curves depicting failure as loosening. The survival rate was similar (p = 0.01, log-rank test) in patients with constrained and unconstrained liners.