The sagittal geometry of the articular surfaces of the femoral condyles, also called J-Curves because of the letter J-shaped profiles, is one of the main factors affecting knee kinematics in the normal knee[1] as well as artificial knee [2]. For example, Clary et al. [2] showed that large changes in the J-curves’ radii cause abrupt changes in the center of rotation, leading to decreased anterior-posterior stability. In literature, the sagittal profile has been described mathematically by different geometric figures, such as arcs, circles, involutes of a circle, and Archimedean and logarithmic spirals [3]. The circular approximation has been often followed in the different concepts of knee implant designs, such as single- radius-, dual-radius-, or multiple-radius-designs. Single-radius-designs have a fixed flexion-extension axis. Dual-radius-designs consist of a larger distal and smaller posterior radius aiming a higher congruence during low flexion (high loading) and lower congruence at high flexion angles (high mobility). Multi-radius-designs try to mimic a physiological roll-glide ratio. However, the description of these circles is usually not standardized. A summary of different measurement methods was given by Nuno and Ahmed [4].Thereby, the radii are very sensitive regarding the length of the fitting arc [5] and position of the sagittal plane [3]. Nuno and Ahmed [3] found that medial and lateral condyles can be adequately described by two-circular arcs and proposed a quantitative description. However, the posterior limits of their arcs were not considered individually, the anterior limits were defined based on soft-tissue measurements (anterior margins of the menisci), and the sagittal plane was positioned at the posterior extreme points, which might be inadequate in arthritic knees.The goal of this study was to automatically analyse the medial and lateral sagittal profiles of the femoral condyles mathematically by two-circular arcs in a standardized and robust fashion.

The areas of the most frequent cartilage loss in mild–moderate medial osteoarthritis (OA) were reviewed from previous studies. Implant components were designed to resurface these areas. The surface geometries of the components were based on an average femur and tibia produced from 20 magnetic resonance imaging (MRI) models of normal knees. Accuracy of fit of the components was determined on these 20 individual knees. The femoral surface was toroidal, covering a band on the distal end of the femur, angled inward anteriorly. For a five-size system, the average deviations between the implant surfaces and the intact cartilage surfaces of 20 femurs were only 0.3 mm. For the tibia, the deviations were 0.5–0.7 mm, but the errors were mainly around the tibial spine, with smaller deviations in the central bearing region. Hence, these small implant components would accurately restore the original bearing surfaces and allow for preservation of all the knee structures. Using a thin metal component for the tibia would preserve the strong cancellous bone near the surface, an advantage for fixation. In this case, the femoral component would have a plastic bearing surface, but still be less than 10 mm thickness. Such a design could have a useful place in the early treatment of medial OA of the knee.

No abstract