HOW SUPERIOR PLACEMENT OF THE JOINT CENTER IN HIP-ARTHROPLASTY AFFECTS THE ABDUCTOR MUSCLES

This study examines the effects of a superiorly placed hip center on t he strength of the abductor muscles, A 3-dimensional computer model of the hip and the surrounding musculature was used to calculate the mom ent arms, forces, and moments generated when the hip abductor muscles are maximally activated, A representation of a hip prosthesis was impl anted into the computer model with altered hip center positions and a range of prosthetic neck lengths, Analysis of these simulated hip repl acements demonstrated that superolateral placement of the hip center ( 2 cm superior and 2 cm lateral) decreases the moment arms of the hip a bductor muscles by an average of 28%, This decrease in moment arm cann ot be restored by increasing prosthetic neck length, resulting in an u nrecoverable loss of abduction strength with superolateral displacemen t, By contrast, a 2-cm superior displacement of the hip center changes the moment arms and force generating capacities of the abductors by L ess than 10% if prosthetic neck length is increased to compensate for decreased muscle length, The results of this study suggest that superi or positioning of the hip center, without lateral placement, does not have major, adverse effects on abduction moment arms or force generati ng capacities when the neck length is appropriately increased.