Activity-dependence of the "safe zone" for impingement versus dislocation avoidance

Presently, the basis for optimal cup positioning to minimize the likelihood of dislocation rests upon subjective clinical impressions. To help elucidate optimal cup positioning more objectively, and to clarify the distinction between impingement avoidance and dislocation avoidance, kinematic and kinetic inputs for seven dislocation-prone activities were applied to finite element models of a contemporary 22-mm modular total hip reconstruction. Twenty-five cup placement positions (combinations of 30, 40, 50, 60, and 70 degrees of abduction with 0, 10, 20, 30, and 40 degrees of anteversion) were chosen to include the conventional 'safe zone' of 30-50 degrees of tilt and 5-25 degrees of anteversion. Activities studied were: rising from a low seat (toilet) and from a normal height chair, leg-crossing while seated, tying a shoe from a seated position, bending at the hip from an erect stance to retrieve an object on the floor (stooping), a standing pivot maneuver, and rolling over in bed. Neck-on-cup impingement occurred during one or more of the dislocation-prone activities at all 25 cup positions. Of the 175 combinations of cup position and kinetic challenge, dislocation and impingement events both occurred for 51 situations, while impingement occurred in 45 instances without dislocation, and dislocation occurred in 10 instances without impingement. Neither dislocation nor impingement was observed in the 69 other combinations of cup position and loading challenge. Kappa statistics showed dislocation and impingement, as outcome measures of activity-dependent challenges, exhibit little more than chance agreement. Therefore, the use of impingement range of motion (ROM) as a predictor of a given cup position's vulnerability to dislocation should be viewed cautiously.