Abstract
Introduction
Dual mobility (DM) implants provide increased stability and range-of-motion through the use of a large diameter mobile liner articulating against an acetabular shell. However, recent studies have reported that such contemporary large head prostheses can directly impinge against the local soft tissues leading to anterior hip pain. To address this drawback, a novel Anatomically Contoured Dual Mobility (ACDM) liner was developed that maintains the outer spherical geometry over an approximately hemispherical portion and then contours inward the distal profile of the DM liner for soft-tissue relief. The extent of the inner profile encapsulating the small diameter head is increased to provide more coverage of the head and maintain the inner head pullout force. We hypothesized that the ACDM liner for soft-tissue relief would not affect retention of the small diameter inner head or liner-acetabular load-bearing contact area.
Methods
A finite element model to evaluate head retention and contact mechanics was created with a rigid acetabular shell, a plastically deformable UHMWPE DM liner, a rigid femoral head and a rigid femoral stem. For the head retention analysis, the extent of head coverage (Fig. 1) was optimized to match the inner head pullout force of a conventional DM liner. Contact mechanics of a conventional DM and ACDM liner were analyzed at the maximum joint load of three activities: gait, deep-knee bend and chair sit. One set of simulations was completed with the mobile liner and head axes aligned and another with the axes mal-aligned so that the mobile liner rim was adjacent to the femoral stem neck and the potential area of contact was away from the mobile liner apex. This allowed a broader range of potential contact to be assessed including what was determined to be a worst-case alignment.
Results
The head extraction force of the conventional mobile liner with 224° of coverage was 909 N (Fig. 2). The ACDM liner with 232° of head coverage and an 8 mm reduced radius had an inner head extraction force of 901 N.
The contact simulation results were practically identical for the ACDM liner and the conventional DM liner. Contact stresses between the two designs differed by less than 3.6%. In most cases, contact area (Fig. 3) was virtually equal with a slightly higher contact area in the ACDM (∼6.3%). In two of three worst-case liner orientations, the contact area between the shell and liner was found to be slightly lower for the ACDM liner (∼15%). The contact area and its distribution in all cases were found to be sufficient.
Conclusion
This study showed that the novel anatomically contoured dual mobility liner maintains adequate inner head retention and articular contact area. The ACDM liner matched the head retention capacity of the conventional DM by slightly increasing the coverage of the inner head. The ACDM articular contact area was comparable to that of a conventional DM and satisfactory in all cases. These results demonstrate that a soft-tissue friendly design can be achieved while providing adequate head retention and load-bearing contact area.
