2017 ISAKOS Biennial Congress ePoster #1231

 

Geometry of the Femoral Components in Fixed Bearing UKRs

Pieter J. Erasmus, MBChB, MMed, Stellenbosch, Western Cape SOUTH AFRICA
Kyung-Jin Cho, MSc, Stellenbosch, Western Cape SOUTH AFRICA

Knee Clinic, Stellenbosch, Western Cape, SOUTH AFRICA

FDA Status Not Applicable

Summary

The sagittal, coronal and axial curves of six different commercially available femoral components of UKR were analyzed against each other and natural knee models.

Abstract

Introduction

The principle in uni-compartmental knee replacement (UKR) is to restore natural or constitutional alignment and normal kinematics. The geometry of the femoral condyles play a major role in knee kinematics and ligament tension during knee movement. If the spatial relationship between the surface geometry of the femur, the capsule and the ligaments are changed, it is impossible to restore normal kinematics. In a previous paper from our institute we published a paper on the radii of the femoral condyles. There was a difference between the medial and lateral femoral condyles and only the posterior one third of the femoral condyles have a single radius. (1)
To restore and achieve natural kinematics the geometry of the femoral component in UKRs should closely mimic that of the normal femoral condyle especially in the sagittal plane.

Method

We measured the radii in the sagittal planes of five different commercially available fixed bearing UKR femoral components and compared that to one another. The measurements were done virtually on 3D reconstructed models. The sagittal curves were represented by multiple circular arcs. Least squares method was used to approximate the single radius from the posterior proximal end of the curve within the tolerance of 0.3 mm.

Results

The prostheses showed different sagittal curve characteristics to one another. Depending on the number of the circular arcs, the prosthesis could be categorized into bi-circular or tri-circular. Two prostheses were tri-circular and three prostheses were bi-circular. Amongst the bi-circular prostheses, one prosthesis showed a single radius from the distal point to the most posterior point of the curve, while the other prostheses had transition point of one arc to the other within this region of the curve.

Conclusion

This variation from normal in the radii of fixed bearing femoral components create doubts about the ability of these prosthesis to restore normal knee kinematics and ligament tension. It might be necessary to redesign the femoral components to better mimic the natural radii of the femoral condyles if one wish to restore normal knee kinematics. In some less than average knees it might be necessary to design a patient specific femoral component.

1. Anatomic study of the radius and centre of curvature of the distal femoral condyle Kosel et al Journal of Biomechanical Engineering Vol 132 Sept 2010