Abstract
Introduction:
Dual Mobility (DM) hip implants have gained popularity for the treatment and preventions of instability. In DM implants a large diameter mobile insert matches the native femoral head size. However, studies have shown that the peripheral regions of such large diameter implants overhang beyond the native anatomy and can directly impinge against nearby soft tissues, especially the iliopsoas, leading to groin pain (Fig. 1). Soft-tissue impingement can also trap the mobile DM insert, leading to damage of its peripheral rim, which secures the small diameter inner head (Fig. 2). The goal of this research was to develop an anatomically contoured soft-tissue friendly DM insert.
Methods:
Various Anatomically Contoured Dual Mobility (ACDM) insert designs were constructed, wherein the outer articular surface extending from the pole to a theta (θ) angle, matched that of contemporary implants (Fig. 3). However, the articular surface in the peripheral region was moved inward towards the center, thereby reducing implant volume that could impinge on the soft tissue (Fig. 1 and Fig. 3). Finite element analyses were used to determine the insert-acetabular contact area under peak in vivo loads during different activities. Finite element analysis was also used to determine resistance to extraction of the inner head. Published data was used to compare the implant articular geometry to native anatomy. These analyses were used optimize the soft-tissue relief, while matching the load bearing contact area and the resistance to extraction of the inner head in contemporary implants.
Results:
The resultant ACDM insert had the outer profile of contemporary implants over approximately a hemispherical portion (Fig. 3). Beyond this, the peripheral articular surface was composed of smaller convex radii. The coverage of the small diameter inner head by the insert was increased slightly (<4 deg) to match the extraction resistance of the inner head in contemporary implants. The outer insert-acetabular contact area of the ACDM insert remained adequate. Additionally, while contemporary prosthesis extended beyond the native articular surface in the distal-medial and proximal-lateral regions, the ACDM insert remained with the margins of the native anatomy.
Conclusion:
A novel anatomically contoured dual mobility insert was developed to mitigate the risk of soft-tissue impingement present with contemporary prosthesis. The ACDM insert retains the outer profile of contemporary implants over approximately a hemispherical portion. However, in the peripheral region, exposed to the soft tissue, the ACDM insert has a smaller profile to reduce soft-tissue impingement.
