chapter 10: Patellar Dislocation
The older the child, the more likely there is to be some secondary deformity such as genu valgum, external rotation of the tibia, flattening of the lateral condyle of the femur, and even shortening of the femur. The small size of the patella along with flattening of the articular surface also seem to be secondary deformities. There is, therefore, considerable interest in both early diagnosis and surgical correction before these early deformities become irreversible.
Although progressive extension contracture of the knee due to quadriceps fibrosis and the acquired form of permanent dislocation appear to have a common etiology, by and large, they tend to run separate clinical courses. No cases have been reported of a progressive extension contracture progressing to permanent dislocation. However, Williams (64) observed a case of permanent dislocation on one side and extension contracture on the other. As with the congenital form of permanent dislocation, the acquired form is also more likely to be associated with secondary deformities if allowed to continue untreated for significant periods of time.
The adult form presents with the signs and symptoms of patellofemoral arthrosis. The symptoms are generally mild in comparison to the radiologic and physical findings. Rapid clinical deterioration is not the usual pattern.
It is unusual for the congenital form to be detected at birth. The patella is palpable in the infant only with the knee in full extension. It is not ossified until 4 to 5 years of age, so radiographs are of no help. The presence of fixed flexion contracture at birth is characteristic of both arthrogryposis and congenital dislocation of the patella. If the former can be excluded, the latter is likely. Even though the patella is dislocated throughout the full range of knee movement, extension is often quite good. Even though extension may not be complete, the patient is able to bear weight on the extremity. Flexion and lateral seating of the patella external to the lateral condyle is not accompanied by pain, but the children have difficulty arising from the squatting or sitting position.
The knee, particularly in the flexed position, lacks the normal contour. The condyles are uncovered anteriorly, giving an apparent predominance of the medial femoral condyle. Genu valgum is a frequent concomitant deformity (70‑72) and is likely due to the lateral positioning of the entire extensor apparatus. The patella may be absent from its normal anterior position in extension as well as in flexion, or it may be nearly in the midline anteriorly in full extension, springing to the lateral side with the initiation of flexion. Because of the lateral position of the patella, the flexed knee presents a broader than usual appearance.
The range of movement of the knee may often be normal, although extensor power is diminished. Some authors have reported extensor lag (70), whereas others have reported fixed flexion contracture of the knee (64, 66, 72, 73). The quadriceps inevitably shows considerable atrophy, most notably in the vastus medialis. In addition, a fibrous band from either the fascia lata or the vastus lateralis can often be palpated inserting into the superolateral border of the patella. An attempt to prevent the lateral dislocation of the patella, while flexing the knee passively, will bring out this band of fibrous tissue, which plays a determining role in the dislocation.
The usual age of presentation is between 5 and 7 years. The patient reports that the patella has been moving laterally on flexion for only a short time (months), sometimes as a result of injury. Often, this is not accompanied by functional disability or acute pain, giving way, and swelling, as are commonly associated with recurrent dislocation in the adolescent or adult.
Because age of presentation for the acquired form is older, the patella is easily palpable and visualized. It is usually more nearly in the midline with the leg fully extended. Neither fixed flexion contracture nor quadriceps lag is present. The remainder of the physical examination is nearly the same as for the congenital form.
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