Abstract
INTRODUCTION
Femoral head diameter has a major influence on stability and dislocation resistance after Total Hip Arthroplasty (THA). Although routine use of large heads is common, several recent studies have shown that contemporary large head prostheses can directly impinge against native soft tissues, particularly the iliopsoas which wraps around the femoral head, leading to refractory anterior hip pain. To address this, we developed a novel Anatomically Contoured large diameter femoral Head (ACH). We hypothesized that anatomical contouring of the ACH implant for soft tissue relief would not compromise dislocation resistance, and the ACH implant would provide increased stability compared to small heads.
METHODS
In this study the dislocation resistance of a 36 mm ACH was compared to that of 28 mm and 36 mm contemporary heads. The ACH implant was based on a 36 mm sphere with smaller radii used to contour the peripheral region below the equator of the head. 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. 1). Impingement-free motion (motion without neck impingement against the acetabular liner) and jump distance (head separation from acetabulum prior to dislocation) were measured to evaluate the dislocation risk of 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 36 mm head and a conventional 36 mm head for both dislocation modes. The 36 mm ACH and conventional head showed greater impingement-free motion compared to the 28 mm conventional head, with an increase of 7° for dislocation mode A, and 4° for mode B. Relative to the 28 mm head, the jump distance for the 36 mm ACH and the 36 mm conventional head increased by 1.5 mm for dislocation mode A, and 2 mm for mode B (Fig. 2 and Fig. 3).
CONCLUSION
The novel Anatomically Contoured large diameter femoral Head (ACH) showed increased dislocation resistance compared to a conventional small diameter head and matched the stability of a conventional large head of the same size. This confirmed the hypothesis that large femoral heads can be anatomically shaped to alleviate the risk of soft-tissue impingement, as in the ACH implant, without jeopardizing the desired stability.
