The patella is a sesamoid bone formed and enclosed within the tendon of the quadriceps femoris. This tendon extends above and on both sides of the patella and inserts onto the tuberosity of the tibia. At this level it is called the patellar tendon. The patella is suspended between the femoral condyles. It has been seen that in patients suffering from recurrent subluxation of the patella, the direction of traction of the quadriceps above the patella does not correspond to that of the underlying patellar tendon. This direction descends medially, while the ligament is located on the vertical line. This leads to a tendency towards subluxation (lateral exit) of the patella, particularly among women, and among those men with slight valgus of the knees. The patella is represented by a thick cartilage, and its central area is poorly nourished.
In many mild cases the patella may slip momentarily over the lateral condyle, giving a feeling of unsteadiness or give in the knee, followed by recurrent synovial effusion (fluid within the joint).
Numerous causes of recurrent dislocation have been listed in the orthopedic literature. Goldthwait in 1904 said that a recurrent dislocation is often associated with flat feet hence many studies have been done on this topic. Valgus knees, both congenital and acquired, represent one of the most convincing causes.
Other reported causes are:
hyperextended knees
weakness of the vastus medialis
internal rotation of the thigh with compensatory tibial torsion
external rotation of the tibia at the knee joint
It may be noted that a faulty alignment of the leg, with the exception of a congenitally high or very small patella, is one of the most frequent factors in recurrent knee subluxation.
Lewin indicated that patellar chondromalacia is almost always accompanied by subluxation of the patella.
Other pathologies regarding the mechanics of the knee due to subluxation are:
lesions of the articular cartilage of the patella and femur, leading to traumatic osteoarthritis
compression of the fat pads in the internal structure of the knee
damage to the cartilage of the knee itself
Severe osteoarthritis may develop in chronic recurrent subluxations.
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