Beyond jump distance: modular dual mobility versus single mobility cups in robotic total hip arthroplasty - simulation of hip kinematics and impingement.

backgroundModular dual mobility (MDM) cups are widely used in primary total hip arthroplasty (THA) to reduce dislocation, but their impact on range of motion (ROM) to impingement versus single-mobility (SM) cups remains uncertain.

methodsIn this paired robotic-simulation study, 108 primary robotic-assisted THAs were virtually planned twice with CT-based software, once with an SM cup and once with an MDM cup. Cup and stem orientation were optimised to restore intra-articular length and global offset within 5 mm of the contralateral hip and to avoid impingement. ROM to first impingement was measured in extension/external rotation (ER), flexion/internal rotation (IR) with physiological pelvic tilt, and flexion/IR with 20° adduction. The mode of first impingement (intra-articular, extra-articular, or mixed) was recorded.

resultsCompared with SM, MDM cups increased ROM to impingement in extension and ER but slightly reduced ROM in flexion and IR at several test positions. Maximal extension to impingement and ER in extension were higher with MDM, whereas flexion with pelvic anteversion and IR at 100° flexion were lower. ROM to impingement was not uniformly greater with MDM, and first contact often shifted from intra-articular implant-on-implant impingement toward extra-articular impingement.

conclusionIn robotic THA, MDM cups provide selective gains in ROM to impingement and shift the impingement towards extra-articular structures rather than providing a global increase in safe ROM. Clinically, MDM may suit patients needing extension/ER reserve or with anterior instability risk, and planning should consider osteophytes and bony constraints. Further prospective clinical validation is warranted.