A Mathematical Evaluation of the Effects of the Head and Neck Diameter on the Arc of Motion and the Implications in Total Hip Arthroplasty.

Background: Instability following total hip arthroplasty (THA) is a leading cause of revisions. Our objective was to evaluate the options that the surgeon has, to gain inherent stability with the use of conventional large femoral and dual mobility systems, and how the arc of motion (AOM) and jump distances (JDs) vary between them. Methods: The head sizes examined spanned from 22 mm-54 mm, and neck sizes spanned from 10 mm-14 mm. Autodesk Fusion 360 is full-scale computer-aided designsoftware that can run simulations to validate a design. It was employed to calculate the AOM for each model. The JD was calculated with varying head sizes and 10- and 20-degree highwall liners. Results: Increasing head sizes from 22, 28, 32-36 mm showed a considerable increase in the AOM for every neck size; however, there was substantially less of an increase with head sizes larger than 36 mm. As neck sizes increased from 10-14 mm, the AOM decreased. The JD increased substantially with the addition of 10- and 20-degree highwall liners. Conclusions: Both classical THA and the dual mobility systems achieve greater AOM than an anatomical normal hip. As the head diameter increases, the AOM and the JD increases. Increases in head size improve range of motion; however, head sizes over 36-40 mm gain little regarding component stability. The more durable and thinner modern polyethylene liners allow for larger conventional femoral heads. When considering between a classical THA or a dual mobility system, the minimal gains with increasing the head size in using a dual mobility system may be far outweighed by the increase in directional stability offered by the large head THA with highwall liners.