Jordy van Sambeeck

General introduction 13 1 anatomic risk factors are generally accounted for in the standard workup, but the amount of contribution of a risk factor to patellofemoral instability is still a matter of debate. There is not a simple relationship between one anatomic risk factor and patellar instability. A normal trochlea has a sufficient depth, length, sulcus angle and lateral trochlear inclination angle to contain the patella during the whole range of motion. The sulcus angle and lateral trochlear inclination angle represent measurements of the depth of the trochlear groove relative to the medial and lateral facet. A decreased angle, means an increased depth of the groove. In trochlear dysplasia the development of the trochlea has resulted in an abnormal geometry, with a too shallow, a flat or even a convex shape of the sulcus 4 . This results in insufficient bony restraint to lateralization of the patella and/or to a bump that the patella needs to override to enter the shallow groove and with that to a higher risk of patellar instability and dislocation. Figure 3. On the left the bony alignment of the patella and trochlea in a normal knee. On the right in a knee with trochlear dysplasia. The left part of each knee is the medial side. Patella alta (‘alta’ is the Latinword for high) means that there is a too high position of the patella relative to the trochlea. Patella alta results in a later engagement of the patella into the trochlear groove when flexing the knee from the neutral position. This means that there is a longer trajectory of the patella without engagement in the trochlea leading to later restraint to lateral translation by the trochlea. This can contribute to instability and/or dislocation of the patella in early flexion. Lateralization of the tibial tubercle relative to the trochlear groove (or any other reference point that could be used) results in an increased lateral directed force