Effect of liner geometry on jumping distance: as relevant as head size

Dislocation is a common complication after total hip arthroplasty (THA). Previously, we found that different liner models provided varying degrees of articulating head coverage. Jumping distance (JD) is a key indicator of joint stability. Therefore, we aimed to investigate the differences in JD values among various liner models. JD values were determined using previously published articulating head coverage data for 25 modular polyethylene liners used in 6 uncemented cup designs from 4 major manufacturers using a mathematical model. For each liner, the average values corresponding to each femoral head size within each cup size were used in the analysis. Within the safe zone (5–25° anteversion and 30–50° inclination), average JD values ranged from 10.79 to 17.29 mm, depending on cup positioning, liner design, and femoral head size. On average, enlarging the femoral head size from 28 to 36 mm resulted in a 3.53 mm increase in JD within the safe zone. The JD variation between different liner designs was of the same magnitude as the effect of increasing the femoral head size from 28 to 36 mm: 3.16 mm with 32 mm and 3.32 mm with 36 mm femoral heads. The JD of the liner with the lowest coverage combined with a 36 mm femoral head was lower than that of the liner providing the greatest coverage, even when coupled with a 28 mm femoral head. Liner design can influence JD to a degree comparable to increasing femoral head size from 28 to 36 mm. Among the evaluated designs, liners providing greater coverage, such as the X3, yielded higher JD values for the same femoral head size. As optimal joint stability cannot be ensured solely by selecting a larger head size, liner geometry should be carefully considered alongside head size in THA component selection.