Abstract
The primary stability of an uncemented femoral total knee replacement component is provided by press-fit forces at the bone-implant interface. This press-fit is achieved by resecting the bone slightly larger than the inner dimensions of the implant, resulting in a so-called interference fit. Previous animal studies have shown that an adequate primary stability is required to minimize micromotions at the bone-implant interface to achieve bone-ingrowth, which provides the secondary (long-term) fixation. It is assumed that during implantation a combination of elastic and plastic deformation and abrasion of the bone will occur, but little is known about what happens at the bone-implant interface and how much interference fit eventually is achieved. Purpose of this study was therefore to assess the actual and effective interference fit and the amount of bone damage during implantation of an uncemented femoral knee component.
In this study, five cadaveric distal femora were prepared and femoral knee components were implanted by an experienced surgeon. Micro-CT scans and conventional CT-scans were obtained pre- and post-implantation for geometrical measurements and to measure bone mineral density. In addition, the position of the implant with respect to the bone was determined by optical scanning of the reconstructions (Figure.1). By measuring the differences in surface geometry, assessments were made of the cutting error, the actual interference fit, the amount of bone damage, and the effective interference fit.
Our analysis showed an average cutting error of 0.67± 0.17 mm, which pointed mostly towards bone under-resections. We found an average actual AP interference fit of 1.48± 0.27 mm, which was close to the nominal value of 1.5 mm. We observed combinations of bone damage and elastic deformation in all bone specimens (Figure. 2), which showed a trend to be related with bone density. Higher bone density tended to lead to lower bone damage and higher elastic deformation (Figure. 3). The results of the current study indicate different factors that interact while implanting an uncemented femoral knee component. This knowledge can be used to fine-tune design criteria of femoral components and obtain adequate primary stability for all patients in a more predictable way.
