Abstract
Metal-on-metal hip resurfacing arthroplasty is a conservative procedure that is becoming an increasingly popular option for young arthritic patients most likely to undergo a secondary procedure in their lifetime. The stability of the acetabular component is of particular concern in these patients who show an increased risk of failure of the cemented acetabular cups in conventional total hip replacements. The purpose of this study was to examine the initial stability of a cementless interference press-fit acetabular cup used in hip resurfacing arthroplasty and implanted into ‘normal’ versus poor quality bone. Also examined was the effect of the press-fit procedure on the contact mechanics at the cup-bone interface and between the cup and femoral head.
A finite element (FE) model of the DUROM resurfacing (Zimmer GmbH) was created and implanted anatomically into the hip joint, which was loaded physiologically through muscle and subtrochanteric forces.
The FE models included: a line-to-line, 1mm and 2mm interference press-fit cup. Also considered were two FE models based on the 1mm press-fit cups, in which the material properties of the cancellous and cortical bone tissues were reduced by 2 and 4 times, to represent a reduction in bone quality as seen with age or disease.
Increasing the cup-bone interference resulted in a sig-nificant reduction in implant micromotion. All the pressfit models showed predicted cup-bone micromotion below 50 micrometers. This would ensure adequate initial stability and encourage secondary fixation through bone in-growth. The predicted acetabular stresses were found to increase with the amount of press-fit, however, there was no suggestion of a fracture. These stresses would further contribute to securing the cup.
Reducing the bone quality showed an increase in the predicted micromotion and increased bone strain. Micromotion was below 50 micrometers, but the predicted compressive bone stresses, necessary for additional implant fixation, was reduced. This implied that poor quality bone would provide unsuitable support medium for the implant. The bearing surface contact mechanics were little affected by the amount of pressfitting.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
