Summary

The only step in implantation of Oxford unicompartmental knee arthroplasty which is not dependent on the standard instrumentation is the sagittal tibia cut, which determines the tibial component rotation and the hanging leg position improves accuracy of this step compared to the supine leg position.

Abstract

Introduction

The designer surgeons of the mobile bearing Oxford unicompartmental knee arthroplasty (OUKA) recommend that the surgery be performed with the leg in hanging position, with the thigh placed in a stirrup. This allows for the knee to be flexed to 120° so as to improve exposure and assist balancing of the knee. Most knee arthroplasty surgeons are trained and accustomed to operate with the limb in supine position and it is not known if operating in an unusual position would actually improve visuo-spatial orientation or lead to more errors. To determine if the limb position impacts surgical outcomes, a comparative cadaveric study was undertaken.

Materials And Methods

Sixteen fresh-frozen knees in eight human cadavera (four male, four female), without macroscopic anatomic defects were used in this study. The knees from each cadaver were randomized into two groups to have the Oxford UKA implanted. Group 1 knees were placed in the hanging leg position and Group 2 knees were positioned in supine position with thigh side support and foot stop at 900 of knee flexion. All procedures were performed by a single surgeon using 3rd generation surgical instrumentation using the described technique. Infrared trackers were placed on femur and tibia and a surgical navigation system was used for pre and post procedure recording of kinematic data including knee alignment, range of motion and implant alignment. Navigation was not used for aiding the surgery and implantation. Radiographs were taken to assess accuracy of implantation.

Results

There was no statistically significant difference between the 2 groups for implantation characteristics like femoral component flexion, tibial coronal alignment and tibial slope. The tibial base plate rotation was also not statistically different but group 2 had higher number of outliers (more than 50 variation) for this parameter. There was also no difference with respect to the thickness of bone resected from proximal tibia or posterior femur. Multivariate analysis of navigation data found no difference between the two groups based on kinematic values for varus/valgus as well as internal/external rotations observed at 0, 30, 45, 60, 90 and 135 degrees during pre and post-surgery range of motion test. Area under curve analysis showed that group 1 had much smaller differences between the pre and post-surgery conditions for kinematics mean values across the entire range of motion.

Conclusion

The only step in implantation of OUKA which is not dependent on the standard instrumentation is the sagittal tibia cut, which determines the tibial component rotation. The hanging leg position improves accuracy of this step compared to the supine leg position and this could be because it provides better visuo-spatial orientation of the hip and knee to the surgeon. However, it does not differ from supine leg operated knees for other implantation parameters which are executed with jigs or guides. Although kinematics of implanted knees in both groups does not differ, hanging leg position is probably closer in replicating native kinematics of the knee as shown by the area under curve analysis.