Constrained cups appear incapable of meeting the demands of revision THA

Abstract

Background: Failure rates of constrained cups for treating recurrent dislocation in revision THA range from 40% to 100%. Although constrained liners are intended to stabilize the hip by mechanically preventing dislocation, the resulting loss of range of motion may lead to impingement and, ultimately, implant failure.

Questions/purposes: We therefore documented the mechanisms of failure of constrained acetabular cups in revision THA and determined the type and severity of damage (wear, fracture, and impingement) that occurs in situ.

Methods: We retrieved 57 constrained components of four different designs at revision THA and examined for the presence of rim impingement, oxidation, cracks within the liner, backside wear, pitting, scratching, abrasion, burnishing, and the presence of embedded particles. Articular wear was calculated from the volume of the concave articular bearing surface, which was measured using the fluid displacement method.

Results: Failure of the locking ring was responsible for 51% of failures, whereas 28% of revisions were the result of acetabular cup loosening, 6% backside wear, and 22% infection. Impingement damage of the rim of the polyethylene liner was seen in all retrievals with moderate or severe damage in 54%. The average volumetric wear rate of the articular surface was 95 mm(3)/year.

Conclusions: Failure of the locking liner ring and loosening of the acetabular cup are the primary causes of mechanical failure with constrained liners; polyethylene is an inadequate material for restricting motion of the hip to prevent instability. The durability of these devices is unlikely to improve unless the mechanical demands are modified through increased range of motion leading to less frequent rim impingement.

Fig. 1

The four different designs of constrained cups that were examined in this study are shown.

Fig. 2

A retrieved S-ROM constrained liner is shown with the titanium locking ring in place. The arrow points to one of the vertical slits, which, before snapping the metal ring, expand to allow reduction of the metal head. The liner shown has a flat rim.

Fig. 3A–B

Measurement of volumetric wear using the fluid displacement method is shown. The specimen with macroscopically open slits in the articular surface that have been occluded (arrows) to prevent fluid leakage (A). The same liner with the corresponding head (B).

Fig. 4A–B

Macroscopic view is shown of a 32-mm S-ROM constrained liner with numerous embedded metal particles (A). Appearance is shown under the microscope of isolated embedded particles and abrasion of the adjacent surface of a different S-ROM specimen (B).

Fig. 5A–B

Macroscopic view of damaged locking rings from S-ROM cups is shown. (A) Dramatic material loss (circle) caused by impingement against the femoral neck; damage was classified as severe. (B) Ring breakage (arrow) with material loss and gross deformation.