MODULAR NECK IN HIP PROSTHESIS: MID-TERM RESULTS OF A VERSATILE SOLUTION.

Abstract

A modular neck allows to choose the offset of the femoral head and the degree of anti-retroversion, lateralization and varus-valgus intraoperatively. At the G. Pini Institute we have been using modular necks in custom prostheses since 1989. Excellent results in this application did open the way to a larger use in off-the-shelf prostheses. Modular necks can be now coupled with different stems, leaving the surgeon free to use the preferred prosthetic stem design. Modular necks have been implanted in more than 50,000 in the world. Medium term results in custom prosthesis and the experience in off-the-shelf non-cemented stems are presented, together with further improvements of this technology under study.

From 1989 to December 1999, 481 custom stems have been implanted. All patients but ten received modular necks. The prostheses were made of a titanium alloy and HA coated. 61 % of patients had dysplastic oxarthrosis. 372 implants performed between 1989 and 1996 were retrospectively evaluated. Data from off-the-shelf prosthesis, at a shorter follow-up, are also reported. Laboratory data showed that the use of an elliptical Morse cone of the neck reduced wear debris production to less than 1 mg/year. In custom implants, (mean follow-up: 7 years), we did not observe any thigh pain or radiological signs of osteolysis or fretting. Mean leg-length discrepancy was 2.8 cm pre-op and 0.3 cm post-operatively. Off-the-shelf implants also showed good clinical and radiological results. New design modular necks will increase the possible range of motion and provide more solutions for positioning the center of rotation. Modular neck is a safe and reliable solution to obtain the correct position of the center of rotation intra-operatively, without side effects. Applications in off-the-shelf prostheses allow to reduce costs while maintaining the advantages of this technology.