ACHIEVING REQUIRED MEDIAL OFFSET AND LIMB LENGTH IN TOTAL HIP REPLACEMENT

Abstract

Background The magnitude of the medial offset and the limb length discrepancy after a total hip replacement (THR) significantly alters the biomechanics of the hip. If both these components are not properly restored, the rate of dislocation may increase. Further decreased offset may result in impingement at the extremes of movement, and also results in soft-tissue laxity, while increased offset increases stress within the stem that may lead to stem fracture or loosening. In addition to affecting the clinical outcome, limb length discrepancy may also cause legal problems.

Aim To find out whether intraoperative assessment and restoration of desired offset, and correction of limb length discrepancy actually corrects these two components as assessed by postoperative radiographs.

Material and Methods We evaluated 39 consecutive THRs in 37 patients who had the surgery performed via the posterior approach. Intraoperatively the medial offset was measured using a ruler from the tubercle in the trochanteric fossa to the centre of rotation of the head, and then check again after the seating of the femoral prosthesis. The size of the head was then accordingly altered. The limb length was measured using the ruler parallel from the lesser trochanter, and taking it upto the tip of the greater trochanter. The preoperative and the postoperative radiographs were evaluated for the medial offset and limb length discrepancy. The medial offset was calculated as a ratio in reference to the opposite side.

Results The median medial offset was 93.9 (85–100) preoperatively and 94.2 (85–110) postoperatively. The median limb length discrepancy was improved from a preoperative −4.84mm (0 to −30mm) to a postoperative −0.06mm (−9 to +16mm).

Discussion Preoperative templating may be a way of obtaining the correct medial offset and limb length in THRs. However, varus or valgus placement, and sinking or protrusion of the prosthesis may alter both these components significantly. Hence, intraoperative measurement and thus changing the components and the position of the stem accordingly may be the best method in addition to preoperative templating, in achieving the required offset and minimising limb length discrepancy in THRs.