PRECISION OF THE POSITIONING OF AN UNICOMPARTMENTAL KNEE PROSTHESIS BY A MINI-INVASIVE NAVIGATED TECHNIQUE

Abstract

Introduction: Unicompartmental knee replacement (UKR) is accepted as a valuable treatment for isolated medial knee osteoarthritis. Minimal invasive implantation might be associated with an earlier hospital discharge and a faster rehabilitation. However these techniques might decrease the accuracy of implantation, and it seems logical to combine minimal invasive techniques with navigation systems to address this issue.

Materials and methods: The authors are using a non image based navigation system (OrthoPilot TM, Aesculap, FRG) on a routine basis for UKR. The used version of the software helps the surgeon orienting the bone resections through a minimal invasive medial approach without splitting the quadriceps tendon or the vastus medialis muscle. The proximal tibial resection is performed with a conventional motorized saw blade guided by a free hand navigated orienting device. For the femoral resection, a bow is fixed by three percutaneous screws to the distal femur. The bow is navigated to be oriented along the knee flexion axis. A guide is fixed on the bow and oriented under navigation control to perform the distal femoral resection with a burr. Neither guides are fixed directly into the joint.

42 patients have been operated on in the 4 participating centers for an isolated medial osteoarthritis. There were 29 women and 13 men, with a mean age of 65 years. The post-operative coronal and sagittal orientation of both prosthetic components were measured, and the time to get 90° of knee flexion was recorded.

Results: The mean coronal angle between the femoral component and the femoral mechanical axis was 89° for an expected goal of 90°. The mean coronal obliquity of the femoral component was 91°, for an expected goal of 90°. The mean coronal angle between the tibial component and the tibial mechanical axis was 86° for an expected goal of 88°. The mean coronal obliquity of the tibial component was 88°, for an expected goal between 85 and 90°. The mean sagittal obliquity of the femoral component was 6°, for an expected goal of 10. The mean sagittal obliquity of the tibial component was 88°, with an expected goal of 87. The patients achieved 90° of knee flexion after a mean period of time of 9 days.

Discussion: The used navigation system is based on an anatomic and kinematic analysis of the knee joint during the implantation. The modification of the existing software for its use with a minimal invasive approach has been successful. It enhances the quality of implantation of the prosthetic components and avoids the inconvenients of a smaller incision with potentiel less optimal visuliazation of the intra-articular reference points. However, all centers observed a significant learning curve of the procedure, with a significant additional operative time during the first implantations. The postoperative rehabilitation was actually easier and faster, despite the additional percutaneous fixation of the navigation device.

Conclusion: This system has the potentiel to allow the combination of the high accuracy of a navigation system and the low invasiveness of a small skin incision and joint opening.