This paper has been one of the bases for the mechanical establishment of the tibial tuberosity advancement (TTA) procedure. The third paragraph opens with the statement, “In the human knee, the femorotibial forces that act during the weight-bearing phase of the gait are almost parallel to the patellar ligament.”
The author cannot find where this statement is made or deduced from in the referenced manuscript. Fig. 6b shows that the direction of the patellar tendon force relative to the tibial plateau changed, during knee flexion, from anterior to posterior at a mean joint angle of 100°. However, the direction of the shear force, and hence of the joint reaction force, changed direction relative to the tibial plateau between angles 50° and 90° depending on the subject's sex and the weight attached to the distal tibia (Fig. 8c). Let's assume that at some point during the gait, the forces are parallel (same or opposite direction). Their direction changes almost linearly as a function of the joint angle (Figs. 6b and 8c). For the forces to remain parallel, the rate of change in their direction needs to remain the same throughout flexion. This means that their direction would change signs at the same joint angle. But this happens at significantly different angles. So, their direction is also different during the various joint angles. Moreover, the results above were from isometric knee extension against a resistance attached to the anterior side of the distal tibia and not during weight-bearing as the authors state.[1] During weight-bearing the ground reaction force is one of the main determinants of the direction of the joint reaction force.
