Abstract
Total Knee Arthroplasty (TKA), has now become a reliable, successful, and widely used treatment for osteoarthritis. Numerous reports indicate that for the majority of patients, the TKA lasts a lifetime with pain relief and the ability to perform most everyday activities. However there are a number of ways in which the procedure can be further improved, the focus here being on function. One of the problems in evaluating function is that it depends upon the inherent ability, motivation, and expectation of the patients. There are several well-used questionnaire systems which capture functional ability objectively. In the effort to simplify evaluation, a ‘forgotten knee’ evaluation has been introduced, the concept being that ‘the ideal TKA design’ would feel and function like a normal knee. Such a measure would include factors such as surgical technique, alignment, and rehabilitation, as well as the TKA design itself. Another approach to evaluation is to measure biomechanical parameters, such as in gait analysis and fluoroscopy, which evaluate kinematic or kinematic parameters, using normal controls for comparison. Nevertheless, such evaluations still include factors other than the TKA design itself, and do not apply to new designs.
The approach taken here for the evaluation of a new TKA design independent of other factors, is to measure the neutral path of motion and the laxity boundaries of the loaded knee on the application of shear and torque over a full range of flexion. The benchmark is the same kinematic data from the normal intact knee. The rationale has some analogy to the ‘forgotten knee’ in that if the laxity response of a design of TKA is the same as that of the anatomic knee itself, the behavior of that implanted knee in any functional condition will be indistinguishable from that of the anatomic knee itself. Such a testing concept has some similarities to the constraint test described in the ASTM standard. In this paper, a novel design algorithm is proposed for creating different design concepts. First, a general morphological form is formulated for each design concept, a Cam-Post PS, a Saddle-Ramp, and a Converging Condyle, all with overall anatomic-like surfaces. Each femoral component is then designed, which is then moved through the normal neutral path and laxity paths, which creates the tibial surface. The concepts are evaluated using a Desktop Knee Machine configured to move the knee dynamically through full flexion while applying combinations of compression, shear and torque; kinematic data being captured optically and plotted using custom software. The normal benchmark was obtained from 10 normal knee specimens, which showed the restraint of the medial femoral condyle to anterior displacement and the overall rollback and laxity laterally. Compared with standard CR and PS designs, the Guided Motion designs were seen to more closely resemble normal. It is proposed that this approach can result in designs which will more likely reproduce a ‘forgotten knee’ and achieve the optimal function for a given patient.
