chapter 10: Patellar Dislocation
Recurrent patellar dislocation is often a disorder of the second decade. In Ficat's series (41), the youngest patient was 10 years old, and the oldest was 27, with a peak at 15 years. Baum and Bensahal (5) had similar findings, with a range from 10 to 16 years and a peak at 14 years. Both studies noted peak incidents during puberty that may reflect both changes in axial alignment and length of the lower extremity as well as change in level of activity. Most series have reported a female predominance varying from 1.5:1 to as high as 5:1 (7, 42‑44).
The initial dislocation most frequently occurs without any warning. It may occur simply as the result of twisting in one direction. In Baum and Bensahal's series (5), 38% experienced their initial dislocation during athletics in which external rotation and valgus stress were applied to the affected knee at the time of dislocation. The initial dislocation is acute and is generally associated with a fall and severe discomfort on the medial aspect of the knee. The patient is unable to arise alone, but this period of complete disability is generally short‑lived, because it is only necessary to extend the knee in order to achieve spontaneous relocation. It is, therefore, unusual for the patient to come to the emergency room with a completely dislocated patella.
While the anterior surface of the knee is uncovered, the prominence of the medial femoral condyle may attract the patient's attention. Particularly if the period of dislocation is short, therefore, the patient may not directly report that the patella has dislocated laterally. Once relocation has taken place by passive extension of the knee, the patient is generally able to walk unassisted. However, the knee is acutely tender, and each flexion causes not only discomfort but the apprehension of a recurrent episode. Therefore, gait is frequently characterized by holding the knee in fixed extension. The nature of the episode and symptoms are such that the patient frequently seeks immediate medical attention. Some, however, treat themselves with a compression bandage, believing that the knee has been sprained, and do not come for evaluation until the second or third episode.
In the unusual circumstance that the patient presents with an acute unreduced dislocation, the diagnosis is evident, as the deformity is characteristic. The knee is painfully fixed in flexion by hamstring spasm. The anterior surface of the distal femur is not covered by the patella, and the outline of the condyles can be easily seen. The patella can be visualized as a prominence against the lateral surface of the lateral condyle. Passive extension of the extremity generally results in spontaneous relocation.
More commonly, the patient presents several hours after the accident. The clinical history will not always lead one directly to the correct diagnosis, particularly if the patient was involved in sports at the time of dislocation. However, the physical examination is characteristic. The hemarthrosis, although frequently considerable, is rarely under tension, because the synovial membrane has been torn at the same time as the medial retinaculum. The patella is hypermobile laterally and could easily be redislocated if the patient would allow. The most pathognomonic sign is tenderness along the medial border of the patella, which testifies to the torn medial parapatellar structures.
Not infrequently, the patient presents for examination sometime after the initial or subsequent episodes during a period when the knee is relatively, but not completely, asymptomatic. The characteristic reason for seeking an opinion is that the knee continues to feel insecure. In contrast with the acute presentation, the knee at this point is generally not very painful. The most important physical finding is the apprehension sign (Fig. 10.3) described by Fairbank (45) and Apley (46) and popularized by Smillie (47). With the patient supine, the knee extended, and the quadriceps relaxed, the examiner applies firm pressure to the medial border of the patella, subluxing it laterally while passively initiating flexion. The patient experiences acute apprehension that the patella is about to dislocate, and contracts the quadriceps, which recenters the patella and prevents further flexion. This apprehension may or may not be associated with acute discomfort. The prominent finding is fear of dislocation. Although the sign is frequently present and very indicative of recurrent dislocation, it is not constant.
Under certain circumstances, when the diagnosis is in doubt, MRI may be helpful. Trillat and Dejour (48) have reported an associated meniscal tear 13 times in 91 operations for patella dislocation. Ficat (41) reported similar findings in only 1 of 18 operated cases. It is important to beware of a mistaken diagnosis in either direction.
Upon physical examination, it is important to look for a loose body. These are frequently cartilaginous and will not show up on routine radiography. Occasionally, loose bodies can be palpated in the suprapatellar pouch or along the medial or lateral recess (lateral is more common).
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