Jordy van Sambeeck

Chapter 1 14 on the patella 6 . The mediolateral position (i.e. the position in the coronal plane) of the midpoint of the tibial tubercle relative to the midpoint of the insertion of the patellar tendon at the inferior pole of the patella defines the line of action of the patellar tendon on the patella. The angle between the line of action of the patellar tendon and the direction of the resultant force of the quadriceps is a representation of the direction of force on the patella when contracting the quadriceps muscle (figure 4). The resultant of the forces acting on the patella in the coronal plane includes a lateral component when the quadriceps is activated, which increases with increased lateralization of the tibial tubercle. A lateral directed force may be generated when the quadriceps eccentrically activates in early flexion; this may subsequently contribute to patellar instability and even provoke a patellar dislocation. As mentioned before, after a primary patellar dislocation, the MPFL is elongated and most often ruptured 7 . Being the most important static soft tissue restraining force against lateral translation, it is an important structure contributing to patellofemoral stability. Therefore, injury of the MPFL after a primary patellar dislocation should always be taken into account as risk factor for recurrent instability. F VM F RF VI F VL R Valgus Q angle F pat. lig. Figure 4. Q-angle. The Q-angle is the angle between the quadriceps’ line of pull and the patellar tendon. The quadriceps’ line of pull is determined by direction of force of the vastus medialis, vastus intermedius, vastus lateralis and rectus femoris. The direction of the patellar tendon partially depends on rotation of the tibia. While extending the knee, the tibia rotates externally, thereby further increasing lateralization of the tubercle and the Q-angle. This figure was published in Insall & Scott Surgery of the Knee, Sixth Edition, 2018, Chapter 18 In Vivo Kinematics of the Patellofemoral Joint, Figure 18.2, Page 314, Copyright Elsevier (2018).