Abstract
INTRODUCTION
Understanding the biomechanics of the anatomical knee is vital to innovations in implant design and surgical procedures. The anterior – posterior (AP) laxity is of particular importance in terms of functional outcomes. Most of the data on stability has been obtained on the unloaded knee, which does not relate to functional knee behavior. However, some studies have shown that AP laxity decreases under compression (1) (2). This implies that while the ligaments are the primary stabilizers under low loads, other mechanisms come into play in the loaded knee. It is hypothesized this decreased laxity with compressive loads is due to the following: the meniscus, which will restrain the femur in all directions; the cartilage, which will require energy as the femur displaces across the tibial surface in a plowing fashion; and the upwards slope of the anterior medial tibial plateau, which stabilizes the knee by a gravity mechanism. It is also hypothesized that the ACL will be the primary restraint for anterior tibial translation.
METHODS
A test rig was designed where shear and compressive forces could be applied and the AP and vertical displacements measured (Figure 1). The AP motion was controlled by the air bearings and motor, allowing for the accurate application of the shear force. Position and force data were measured using load cells, potentiometers, and a linear variable differential transducer.
Five knee specimens less than 60 years old and without osteoarthritis (OA), were evaluated at compressive loads of 0, 250, 500, 750 N, with the knee at 15° flexion. Three cycles of shear force at ±100 N constituted a test. The intact knee was tested, followed by testing after each of the following resections: LCL, MCL, PCL, ACL, medial meniscus, and lateral meniscus.
RESULTS
The average displacement of the tibia without load was 6.17 mm anterior and −4.92 mm posterior. Under load the posterior translation of the tibia was reduced essentially to zero. After ACL resection, the anterior tibial displacement increased substantially, with a further increase after medial meniscus resection. Cartilage deformation had a minimal effect.
DISCUSSION
The hypotheses that the ACL and the upwards tibial slope would provide stability under load were validated. The ACL was essential under all load conditions because the posterior tibial surface was flat (figure 2). The medial meniscus provided vertical stability, as a space buffer (figure 3), and in two specimens under load it provided the same restraint as the ACL (figure 2). The experiment was limited by lack of muscle action, the number of specimens, and a single flexion angle.
SIGNIFICANCE
The test rig and methodology had capabilities exceeding those of previous work in determining the mechanisms of AP knee stability under load due to its frictionless air bearings. The results have application ranging from sports medicine to total knee design. The stabilizing effect of the tibial slope seen here validates tibial osteotomies for improved stability. The importance of reproducing ACL function in total knee design is emphasized.
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