chapter 10: Patellar Dislocation
PERMANENT PATELLAR DISLOCATION
As will be seen in other areas of pathology of the patellofemoral joint, there are some problems with semantics. The literature contains reference to permanent, congenital, habitual, and recurrent dislocations of the patella without, in many instances, defining what the author means by the term chosen. By permanent dislocation of the patella, we choose to mean that the patella is no longer in contact with the articular cartilage of the distal femur throughout the full range of flexion. On extension, the patella may return toward, or nearly to, the midline, but on flexion, the patella always returns to the lateral surface of the lateral condyle. There are two clinical forms of permanent dislocation of the patella: congenital and acquired. These forms share certain aspects of their etiology, clinical and radiologic features, and treatment but remain distinct enough to be considered separately.
Green and Waugh (58) believed that congenital dislocation of the patella occurs in utero or at birth. However, the diagnosis is seldom made until several years of age. Green and Waugh also presented a family tree suggesting hereditary predisposition.
We select this as a descriptive term to avoid the confusing term "habitual." Habitual has been used as a synonym for "recurrent" (59, 60). It has been defined by Benoist and Ramadier (61) and Rutt (62) to describe a form of dislocation that spontaneously reduces at 90 degrees of flexion. The term habitual dislocation should be restricted to the form of permanent dislocation for which we prefer the term "acquired." On extension, the patella returns to near the midline while at each flexion it rolls over the lateral condyle to lie lateral to it. This form of dislocation is apparently not present at birth.
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