Implant malposition remains one of the common causes of total knee replacement (TKR) failure and increased wear. Recent advances in computer technology have made available navigation systems for TKR and other orthopaedic procedures. The purpose of our study was:
to develop a method to assess the accuracy of an image-free TKR navigation system; to assess its accuracy in a leg with normal or near-normal mechanical axis; to assess its accuracy in a leg with abnormal mechanical axis. The system chosen was an image-free system based on electromagnetic technology, the MedTronic AxiEM TKR navigation system. To facilitate measurements, an artificial leg (phantom) was constructed from machined Plexiglas with simulated hip and knee joints. Additional joints located at the midshaft of the tibia and femur allowed deformation in the flexion/extension (y), varus/valgus (x) and rotational (z) planes. Using a highly accurate digital calliper unit (FaroARM Technologies, USA) to precisely measure co-ordinates with pre-machined points on the phantom, a software program was developed to convert these local co-ordinates into a determination of actual leg alignment. This technique was verified using repeated measurement with variable coordinates, giving accuracy to within 0.05 of a degree. Simulated procedures were then performed with both normal and abnormal leg mechanical axis. At specific points in the procedure, information was compared between the FaroARM digital measurements and the CAS system. Repeated serial measurements were undertaken. In the setting of normal alignment, accuracy to within one degree was demonstrated. In the setting of abnormal x, y and z plane alignment in both femur and tibia, accuracy to within two degrees was demonstrated. Several clinical studies have been performed to assess the precision of computer navigation in TKR. This study was designed to assess the accuracy of a clinically validated navigation system. The study demonstrates the high level of in-vitro accuracy of the MedTronic AxiEM navigation system in both normal and abnormal mechanical leg alignment settings.
The correct positioning of implant components in total knee replacement (TKR) is important for a successful long-term outcome. In order to address the problems inherent with conventional alignment methods, several computer-assisted navigation systems (CAS) have been developed. Despite numerous reports of clinical outcomes and system reliability, there is a lack of studies independently evaluating the precision and accuracy of such systems. We report on the design and development of a method and device to evaluate the accuracy of such a computer-assisted navigation system in two situations; 1) Normal or near-normal lower limb mechanical axis, and 2)Simulated femoral and/or tibial extra-articular deformity in either varus/valgus (x), internal/external rotation (y) or flexion/extension (z) planes. The system assessed was the Ci Knee-CAS navigation system (BrainLab/De Puy). This image-free system requires the registration of specific anatomical points to identify the mechanical axis of the lower limb and therefore provide information on resection level and alignment. In order to precisely measure and accurately reproduce these points we constructed a phantom device along anatomical guidelines, with lockable joints located at the mid-shaft of both femur and tibia. We then identified geometric CAS data; 1) Tibial resection height, and 2) Tibial resection plane, and using specially written software compared this against validated co-ordinate measurements independently obtained by a FaroArm co-ordinate measurement system (FARO Technologies, USA). This enabled data from the navigation system to be directly compared against highly accurate reference measurements. Accuracy of the system was then assessed with both normal mechanical alignment of the lower limbs and simulated extra-articular deformity.
