Abstract
INTRODUCTION
Femoral head diameter has a major influence on stability and dislocation resistance of the hip joint after Total Hip Arthroplasty (THA). Dual Mobility (DM) implants can also reduce the risk of dislocation due the large diameter mobile liner which forms the femoroacetbular articulation. However, recent studies have shown that large head prostheses can directly impinge against native soft tissues, particularly the iliopsoas, leading to anterior hip pain. Dual mobility systems have emerged as a revision option in the treatment of failed metal on metal devices because of the high incidence of post revision instability secondary to abductor loss and need for capsulectomy. We hypothesized that an Anatomically Contoured Dual Mobility (ACDM) liner could provide joint stability while better accommodating the soft tissues surrounding the hip joint.
METHODS
The dislocation resistance of a 44 mm ACDM implant was compared to that of a 44 mm conventional DM liner. Both implants consisted of a 28 mm inner small diameter head and the liner was abducted to be in the worst case position for dislocation (Fig. 1). The ACDM liner was based on a 44 mm sphere with smaller radii used to contour the peripheral region below the equator of the liner. MSC Adams was used for dynamic simulations based on two previously described dislocation modes: (A) Posterior dislocation (at 90° hip flexion) with internal rotation of the hip and a posterosuperior directed joint force; (B) Posterior dislocation (starting at 90° flexion) with combined hip flexion and adduction and a posteromedial force direction (Fig. 2). Impingement-free motion (motion without neck impingement against the acetabular cup) and jump distance (head separation from acetabulum at dislocation) were measured for each implant. The acetabular cup was placed at 42.5° abduction and 19.7° anteversion, while the femoral component was anteverted by 9.75° based on published data.
RESULTS
The results showed no differences between the novel anatomically contoured 44 mm liner (ACDM) and a conventional 44 mm DM implant for both dislocation modes. The 44 mm ACDM and conventional DM liner showed impingement-free motion of 47° for mode A and 29° for mode B which are much higher compared to the contemporary small heads. The jump distance between the 44 mm ACDM and the conventional liner was also identical (Fig. 3).
CONCLUSION
The novel Anatomically Contoured Dual Mobility (ACDM) liner matched the dislocation resistance of a conventional DM liner of the same size. This confirmed the hypothesis that dual mobility liners can be anatomically shaped to alleviate the risk of soft tissue impingement, without jeopardizing stability.
