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Anatomy of Patella Bone
Traumatic patellar dislocation can be caused by an impact or direct blow, avulsing or shearing forces. The patella dislocates laterally and medical side of patella may hit the anterior aspect a lateral femoral condyle, as shown below in Figure (b) and (c) (Batt et al., 2011). In the first mechanism, dislocation of the patella occurs at knee flexion, while force from opposing side occurs at the vasti muscles (Hughston et al. 1984). The patella rests upon the patellofemoral groove, which is the anterior, inferior side of the femur. The superior part of the patella is held by the quadriceps tendon, which is attached to the quadriceps muscle.

Risks
A risk that causes patellar symptoms can be known as the lateral patellar compression syndrome, which can be caused from lack of balance or inflammation in the joints (Ficat 1997). The pathophysiology of the kneecap is complex and it deals with the osseous soft tissue or abnormalities within the patellofemoral groove. The patellar symptoms cause knee extensor dysplasia and sensitive small variations affect the muscular mechanism that controls the joint movements (Zaffagnini et. al, 2010).

Forces
When there is too much tension on the patella, the ligaments will weaken and be susceptible to tearing ligaments or tendons due to shear force or torsion force, which then displaces the kneecap from its origination. Another cause that patellar dislocation can occur is when the trochlear groove that has been completely flattened is defined as trochlear dysplasia (Dejour, 1994). Not having a groove because the trochlear bone has flattened out can cause the patella to slide because nothing is holding the patella in place.