Patellar tendon angle
The patellar tendon angle (PTA) is the angle in the sagittal plane between the tendon and the long axis of the tibia. Because of its central location in the knee, it is little affected by axial rotation and gives an indirect measure of sagittal plane kinematics. The tendon rotates posteriorly during flexion, moving steadily backwards about its insertion into the tibial tubercle. The effect is predicted by the sagittal plane model of the knee during passive motion (Fig. 3.14), and by the model of passive motion of the replaced knee (Fig. 3.17). The changes in PTA with flexion arise in part due to the cam-like shape of the distal femur, with the trochlea placed anterior to the flexion axis near extension (Fig. 3.14(a)), and the closer approach of the patella to the flexion axis in deeper flexion (Fig. 3.14(c)). The PTA changes are further increased by femoral roll-back during flexion.
It was shown in cadaver studies (Miller et al., 1998) that the normal pattern of PTA was restored throughout the range of flexion after medial OUKA with both cruciates preserved. After TKA with an unconstrained fixed-bearing prosthesis (implanted after division of the ACL), anterior subluxation of the femur caused an increase in the PTA in high flexion, i.e. loss of the normal rollback. When a posterior stabilised TKA was implanted after division of both cruciates, the PTA became normal in flexion as the cam of the prosthesis artificially restored natural rollback.
Price et al. (2004) used dynamic fluoroscopy to measure the PTA in the knees of five patients at 1 year after OUKA and five patients at 10 years during a step-up exercise. The measurements were compared with the knees of five patients who had undergone TKA and five normal volunteers (Fig. 3.27). The graphs show no significant difference in the pattern of tendon rotation between the control knees and those with OUKA. In contrast, the sagittal plane mechanics after TKA were significantly disturbed.
