Which femoral neck for a dual mobility cup? A biomechanical evaluation

PURPOSE: This study aimed to evaluate polyethylene (PE) damage and wear lesions to the chamfer of mobile components under mobile and fixed femoral neck impingement at the third articulation, and to determine which femoral neck characteristics should be considered with a dual mobility cup to limit those lesions. METHODS: Two femoral neck geometries (cylindrical and quadrangular) with two surface finishing roughness (rough and polished), and two head-to-neck ratios (28- and 22.2-mm diameter femoral heads) were evaluated in a hip simulator testing. For each characteristic, six femoral necks were tested with six dual mobility cups under fixed and mobile femoral neck impingement conditions. Chamfer PE damage and volumetric wear were evaluated and compared for each femoral neck characteristic and impingement condition. RESULTS: Under mobile impingement condition, femoral neck characteristics did not significantly affect PE damage and wear lesions to the chamfer (p = 0.283 to 0.810). However, under fixed impingement condition, significantly higher PE damage and wear lesions to the chamfer were produced by the quadrangular geometry compared to the cylindrical geometry (p = 0.004 to 0.025). In addition, with the quadrangular geometry, rough surface finishing was demonstrated to increase volumetric wear of the chamfer (p = 0.009). No significant influence of head-to-neck ratio was observed on PE damage and wear lesions to the chamfer (p = 0.244 to 0.714). DISCUSSION: This biomechanical study emphasized that femoral neck characteristics are critical with dual mobility cup and tend to favor a cylindrical geometry particularly whether fixed impingement at the third articulation occurs.