chapter 10: Patellar Dislocation
ACUTE PATELLAR DISLOCATIONPatellar malalignment, particularly subluxation, will increase the chance that a complete patellar dislocation will occur. Patellar dislocation indicates that the patella has been completely displaced from the femoral trochlea so that there is no longer any articular surface apposition. Dislocation frequently occurs as a sudden event, either related to trauma or to pre‑existing malalignment and torsional stress on the extensor mechanism, which causes complete displacement of the patella out of the trochlea. Hawkins et al (27) have pointed out that 30 to 50% of patients who have dislocated a patella will continue to have problems with pain and instability. The clinician should also be aware that a "locked" patellar dislocation can occur that can preclude closed reduction, even under general anesthesia (28).
The patient who has experienced a complete acute dislocation of the patella is usually aware that the patella has gone out of place. Sometimes the patient has reduced the patella with or without someone else's help. Occasionally, a patient will say he/she has "dislocated his/her knee" when, in fact, only the patella has dislocated and then subsequently relocated. The examiner should be wary of such self diagnosis. It is apparent, nonetheless, that the experience of dislocating a patella is quite impressive to the patient, and usually the patient is aware that something very significant has occurred. The dislocation may occur as a result of vigorous activity, direct trauma, or some trivial movement. The typical patient is a short, young, obese woman with patellar malalignment and ligamentous laxity, but patellar dislocation also occurs fairly frequently in vigorous young athletes, male or female. The patient may have had some pre‑existing anterior knee pain, but this is not always the case.
When the patient is initially seen, the knee is generally swollen, and examination can be difficult. In fact, depending on the history, there may be some confusion as to whether there is other damage to the knee such as a torn cruciate ligament or meniscus. Limited range of motion is common because of pain, swelling, and loose articular fragments.
The careful clinician will examine the knee, first in extension, and assess the retinacular structures as well as ligament deficiency. The examiner should test for cruciate ligament insufficiency and joint line tenderness. At this point, it will often become apparent that there is tenderness in the medial retinaculum and apprehension on any attempt to displace the patella laterally. There may be swelling in the region of the vastus medialis obliquus. Flexion and extension of the knee may be limited because of pain, but if it is possible to flex the knee somewhat, there may be a sense of lateral displacement of the patella, as compared with the contralateral, unaffected knee. The lateral retinaculum may or may not be tender. The examining physician should also palpate and observe the entire extensor mechanism. In older patients, quadriceps muscle rupture can occur immediately above the patella, resulting in extreme extensor mechanism weakness and a defect that may be difficult to find in the presence of a hematoma. The patient should be encouraged to do a straight leg raise, and if this is not possible, the examiner should question again whether the extensor mechanism is intact. McMurray's test is usually not possible in these patients, but the examiner must be certain to examine the joint for localizable tenderness or instability. If possible, the patient should be examined standing also to observe the alignment of both lower extremities to assess rotation and valgus. Also, the examining physician should evaluate the patient's overall connective‑tissue structure to determine if there is excessive laxity. In particular, the finger extension, elbow extension, thumb‑to‑wrist flexion, and knee recurvatum should be checked.
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