Assessment of Damage on a Dual Mobility Acetabular System

Abstract

Background: Dual mobility designs were introduced to increase stability and reduce the risk of dislocation, both being common reasons for surgical revision after total hip arthroplasty. The in vivo behavior of dual mobility constructs remains unclear, and to our knowledge, no data have been published describing in vivo surface damage to the polyethylene bearing surfaces.

Methods: We used surface damage assessed on the inner and outer polyethylene bearing surfaces in 33 short-term retrieved dual mobility liners as evidence of relative motion at the 2 bearings. A lever out test was performed to determine the force required for dislocation of the cobalt-chromium femoral head from the polyethylene liner.

Results: Both bearings showed damage; however, the inner polyethylene bearings had higher damage scores, lower prevalence of remaining machining marks, and higher incidence of concentric wear, all consistent with more motion at the inner polyethylene bearing. The inner polyethylene bearings also had a higher occurrence of embedded titanium debris. The damage sustained in vivo was insufficient to lead to intraprosthetic dislocation in any of the retrieved components. Lever out tests of 12 retrievals had a mean dislocation load of 261 ± 52 N, which was unrelated to the length of implantation.

Conclusion: Our short-term retrieval data of 33 highly cross-linked polyethylene dual mobility components suggest that although motion occurs at both bearing articulations, the motion of the femoral head against the inner polyethylene bearing dominates. Although damage was not severe enough to lead to intraprosthetic dislocation, failure may occur long term and should be assessed in future studies.

Keywords: bearing surfaces; dual mobility; polyethylene wear; retrieval analysis; total hip arthroplasty.

Conflict of interest statement

One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2016.01.039.

Copyright © 2016 Elsevier Inc. All rights reserved.

Figures

Fig. 1.

(A) A cobalt-chrome alloy or ceramic femoral head is snap fit into a highly cross-linked polyethylene liner, which is retained by (B) the mouth of the liner that is slightly smaller in diameter than the femoral head.

Fig. 2.

The setup for the lever out testing used a cantilevered force (F) applied near the end of the femoral stem causing impingement between the neck of the femoral component and the rim of the acetabular component, leading to a levering out of the head from the acetabulum. ADM, anatomic dual mobility.

Fig. 3.

Outer bearing (A) and inner bearing (B) zones were visually assessed for damage.

Fig. 4.

Eight helical CMM scans were taken of 16 inner and 25 outer polyethylene bearing surfaces. The raw scans ranged in size from 60,000 to 105,000 points per surface. CMM, coordinate measuring machine.

Fig. 5.

(A) Images of retrieved polyethylene outer bearings showed pitting and scratching (left), embedded debris (middle), and machining lines (right). (B) Images of inner bearings showed furrowing (left), scratching (middle), and embedded debris (right).

Fig. 6.

(A) A particle of embedded debris is shown in this scanning electron photomicrograph of a polyethylene bearing surface. (B) The corresponding spectrum from energy dispersive x-ray analysis of the particle included a titanium peak.

Fig. 7.

Deviation maps of outer bearing CMM data showed bearings with no wear, edge loading, concentric wear, and edge loading and concentric wear (left to right).

Fig. 8.

Deviation maps of inner bearing CMM data showed 2 bearings with no wear (left 2 images) and 2 with concentric wear (right 2 images).

Fig. 9.

Three-dimensional analysis of the inner polyethylene bearing of a specimen after lever out testing showed a negative deviation at the point of impingement (arrow) and a ring of positive deviation where the locking mechanism was so severely deviated that the femoral head was able to escape from the liner.