Review Question - QID 219384

This patient presents with a congenital patellar dislocation. In this setting, the extensor mechanism subluxes laterally, converting the quadriceps from an extensor of the knee to a flexor (Answer 2).

Congenital patellar dislocation is a rare condition presenting with an irreducible, laterally dislocated patella. This condition typically occurs immediately after birth, occurring either in isolation or in association with specific syndromes, including nail-patella syndrome and Down syndrome. The patella may be hypoplastic at birth and irreducibly fixed in a laterally dislocated position. There is a subset of patients who present with a located patella at birth but develop progressive patellar subluxation with age, eventually resulting in a fixed dislocation by early childhood. Diagnosis is typically made clinically, with children presenting with genu valgum, a knee flexion contracture, external tibial torsion, and a visibly or palpably dislocated patella. Physical examination of the infantile knee is difficult, and palpation of even a normal patella can often be uncertain. Children with congenital dislocation of the patella tend to start walking later than normal, often without radiographic evidence of dislocation until after the patella begins to ossify at around 3 years of age. Additionally, it is important to distinguish congenital dislocation, which occurs in much younger children, from recurrent patellar instability, which usually presents in older children who dislocate as a result of physical activity and are otherwise relatively asymptomatic between episodes. Treatment of congenital patellar dislocations is usually surgical reduction, as nonoperative management can lead to long-term functional deficits, progressive joint malalignment, and increased pain.

Gordon and Schoenecker describe in their 1999 JPO article their experience with surgical treatment of congenital patellar dislocations over 15 years. Despite several previous reports describing patients with congenital dislocations as pain-free, the authors found that all patients 7 years and older complained of pain. All patients were treated with open reduction, extensive lateral release, and transfer of either the patellar tendon or tibial tubercle (depending on skeletal maturity). The authors note that the principal stability of the procedure depends on the proper placement of the patellar tendon insertion and proper tensioning of the VMO.

Wada et al. described their experience with surgical treatment of congenital patellar dislocation in their 2008 retrospective review. The seven knees in six patients over the 25-year study period were treated with a combination of lateral release, medial plication, V-Y lengthening of the quadriceps, medial transfer of the lateral patellar tendon, and posterior release of the knee. The authors state that serial casting and a brace are effective in reducing flexion contracture of the affected knee, but genu valgum, external tibial torsion, and subluxation of the tibia inevitably develop without surgical intervention. They recommend that surgery be performed early, as soon as the diagnosis is confirmed, and preferably before 1 year of age to allow for trochlear remodeling.

Figure A is a radiograph demonstrating external rotation of the tibia compared to the unaffected contralateral side. Note the absence of an ossified patella. Figure B is an ultrasound image demonstrating a dislocated left patella. Illustration A is an MRI image of the same patient at 6 months of age demonstrating a persistently dislocated left patella. Illustration B is a postoperative MRI demonstrating a reduced patella sitting in the trochlear groove.

Incorrect Answers:
Answer 1: The laterally subluxed extensor mechanism would act as a flexor, not an extensor, of the knee.
Answer 3: The laterally subluxed extensor mechanism would provide an external, not internal, rotation moment about the tibia.
Answer 4: The laterally subluxed extensor mechanism would provide a valgus, not varus, rotation moment.
Answer 5: The tibia would not be significantly translated anteriorly or posteriorly in congenital patellar dislocations.