Unicompartmental knee resurfacing: enlarged tibio-femoral contact area and reduced contact stress using novel patient-derived geometries

Abstract

Advances in imaging technology and computer-assisted design (CAD) have recently enabled the introduction of patient-specific knee implant designs that hold the potential to improve functional performance on the basis of patient-specific geometries, namely a patient-specific sagittal and coronal curvature, as well as enhanced bone preservation. The objective of this study was to investigate the use of a novel implant design utilizing a patient specific sagittal J-curve on the femoral component combined with a novel constant, patient-derived femoral coronal curvature and to assess tibio-femoral contact area and contact stress on a femur matched curved tibial polyethylene insert. Mean contact area and standard deviations were 81+/-5, 96+/-5 and 74+/-4 mm(2) for the heel strike, toe off and mid-stance positions, respectively. Mean contact stress and standard deviations were 23.83+/-1.39, 23.27+/-1.14 and 20.78+/-0.54 MPa for the heel strike, toe off and mid-stance positions, respectively. Standard deviations of the measurements were small, not exceeding 6-7% confirming the consistency of loading conditions across different flexion angles. The results were comparable to those reported for standard, off-the-shelf fixed-bearing implants with paired femoral and tibial geometries. These data show that a constant coronal curvature can be applied to a patient-specific implant by measuring coronal curvatures across the femoral condyle in each patient and by deriving an average curvature. This novel approach combines unique benefits of patient-specific geometry with proven design concepts for minimizing polyethylene wear.

Keywords: Unicompartmental; contact stress; knee replacement.; patient specific; prosthesis design.

Fig. (1)

CAD image of the patient-specific iUni femoral component on the distal femur.

Fig. (2)

Lateral view of the patient-specific iUni femoral component. Sagittal radius varies along the anteroposterior dimension of the femoral component reflecting the patient’s sagittal anatomy.

Fig. (3)

Frontal view of the patient-specific iUni femoral component. The red lines indicate the location where the patient’s coronal curvature is measured. The measurements are used to derive a mean coronal curvature for each patient which is then used to apply a constant coronal curvature along the bearing surface of the femoral component.

Fig. (4)

CAD image of the patient-specific tibial implant. The perimeter of the implant is matched to the cortical edge and the poly curvature is matched to the coronal radius of the femur at a specific ratio.

Fig. (5)

Test set-up for assessing contact area and contact stress.

Fig. (6)

Contact stress images acquired with Tekscan K-Scan sensors. The load distribution is homogenous for the different positions and polyethylene inserts.

Fig. (7)

Peak contact stress in mid-stance position measured for patient specific unicompartmental implant with patient derived, constant coronal curvature (mean 20.78, SD 0.54) as compared to standard, off-the-shelf devices.