The Rationale for CAD-CAM Uncemented Custom Hips: An Interim Assessment

Abstract

The purpose of this paper is to examine whether the rationale for CAD-CAM Custom Hips is realised in clinical practice. Previous studies demonstrated that custom uncemented stems, with a close fit proximally and a sliding fit distally, produced stresses closer to normal than for other stem types, which should result in the preservation of proximal bone. Custom stems showed less micromotion, especially in torsional loading, and hence should demonstrate interface osseointegration. The hips are designed and manufactured using specially written software. The standard design includes proximal macro-grooves with HA coating, an anterior flare, a lateral flare, a collar, and a smooth distal stem for a sliding fit. Elective features are added such as proximal stem twist and neck retroversion in CDH, increased stem length to bypass defects, curvatures in AP and ML views, and distal cutting flutes when extra torsional stability is required. From 1989 to 1994, 411 cases were carried out, approximately one-third in each of the categories of OA, JCA/CDH, and revision. Studies were made of the available radiographs at yearly intervals, while DEXA scans were taken of the RNOHT patients pre-operatively, at 6 months and then yearly. There were four failures requiring revision, three of the early primary design without HA coating, and one a revision design. The radiographs in primary hips showed complete proximal bone-implant apposition in 81% of all cases. The DEXA scans showed that the mean bone mass after two years for all seven Gruen zones was greater than 90%. It was concluded that the CAD-CAM HA-coated Custom Hips showed bone and interface stability up to this time. The hip has played a useful role in providing the ideal stem for each particular case, which may result in an improvement of long-term results, compared with the use of off-the-shelf implants.