PERCUTANEOUS HIP REPLACEMENT

Abstract

This paper describes a development project for a minimally invasive percutaneous hip arthroplasty. The prosthesis provides a percutaneous replacement of the hip, employing the transtrochanteric approach and imaging control. (Currently Xray, but eventually real time MRI.) It is envisaged that conventional anaesthesia and operating theatre facilities will eventually be redundant. An optimal access entry is obtained using a percutaneous guide wire up the femoral neck, over which a drill is passed to accommodate a working post with a blunt end to avoid penetration of the medial acetabulum. On the post, a lateral cortical protection hoop is accurately fitted and screwed with four splayed screws. A 19 mm access tunnel is milled to receive a 1mm wall thickness H.A. coated tube.to the proper depth for neck resection. The neck is resected using an expanding cutter, and the head removed piecemeal. Using a milling end cutter on a flexible drive, located on the working tube, a spherical concave bed of decorticated bone is produced in the acetabulum.

The acetabular implant bed is formed as a “flower bud”, from titanium alloy with hydroxyapatite coating on its outer surface. This device is introduced retrograde via the tube, and opened in its definitive location under direct vision. The acetabular articular surface is metal, with profiling of the rim margins to accommodate required movements, as in the normal anatomy. The device is installed in an acrylic cement bed whose function is to retain apposition and contact of the flower “Petals” against bone, and to support the acetabular bearing surface. The femoral component comprises a 16.5 mm head and 11mm neck, and 17 mm shaft machined to fit the working tube. The device has been tested to an ultimate lateral bending strength of 1 ton, and 400,000 cyclical loading to 10 hundredweight. Optimal length of femoral component is selected to reproduce precise anatomy, and final adjustments are achieved using intratubal thrust shims. The cortical protecting hoop is replaced with a matching profile H.A. coated thrust plate reinserting the same four splayed screws. The wound is closed and the patient mobilised immediately. The approach avoids interference with muscles, and will permit accurate restoration of the anatomy.

Raised expectations and increased population of retirement age impose impossible pressures. Logistics and resources cannot to keep abreast of demand for hip arthroplasty with existing technologies. Radical solutions are required to match demand .