chapter 6: Dysplasias
SUPERIOR AND INFERIOR PATELLA MALPOSITION
Radiologic criteria for the diagnosis of patella alta have been considered in the preceding chapter. Patella alta has been associated with recurrent subluxation and dislocation (28), chondromalacia, Sinding‑Larsen‑Johanssen apophysitis, recurrent effusion (16), and cerebral palsy (29, 30), where walking is habitually in the knee‑flexed position. It appears that increased tension on the patellar ligament results in its greater length, although Micheli et al (31) have correlated femoral growth rate with proximal patella migration, which suggests that overgrowth during the adolescent "growth spurt" may cause patella alta.
I believe that patella alta as an isolated entity will not require surgical treatment unless there is symptomatic delayed entry of the patella into the femoral sulcus with knee flexion. It is most important to separate problems of patellar tilt or subluxation from specific symptoms related to delayed patellar engagement of the trochlea. The Bernageau index, as referenced in the chapter on imaging, is probably the most accurate ndicater of true patella alta. When adjustment of the patella level is necessary, the surgeon should proceed extremely cautiously as overdoing this adjustment can lead to problems with painful initial knee flexion. Most distalizations of the tibial tubercle require no more than 5-10 millimeters of tibial tubercle distalization. This is always preferable to patella tendon shortening.
Although this condition may rarely occur spontaneously in otherwise normal children, it is a common finding in achondroplastic dwarfs, where it does not appear to cause any functional problem (Fig. 6.14). In fact, achondroplastic dwarfs can comfortably squat for long periods of time. Patellofemoral complaints are infrequent in a large group of patients with achondroplasia (500) who were followed up for other orthopedic problems. Poliomyelitis in childhood may also be responsible for patella baja.
The most frequent reason for this malposition, however, is as a complication of previous knee surgery or injury. Tibial tubercle transposition performed in an underaged patient, where the repositioned tubercle continues to grow distally with the tibia, may cause patella baja. Noyes et al (32) and Paulos et al (33) have recognized the occurrence of patella infera as a postoperative complication of knee surgery. In such cases, open release of the captured infrapatellar fat pad and other infra‑articular scar, followed by mobilization, may be indicated. Fortunately, excision of the infrapatellar fat pad does not seem to compromise patellar vascularity. Poorly performed tibial tubercle transfer surgery can be disastrous, resulting in extreme patella infera, as shown in Figure 6.15. Accurate
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