Patellar Dislocation

Acute patellar dislocation is displacement of the patella out of the trochlear groove, almost always laterally. Annual incidence is 5 to 43 per 100,000, peaking at 10 to 20 years, with a female predominance. The mechanism is a twisting injury with the knee in valgus and the foot planted, or direct trauma.

Anatomic risk factors:

• trochlear dysplasia (about 85% of recurrent dislocators)
• patella alta (Caton-Deschamps ratio over 1.2)
• tibial tubercle to trochlear groove distance over 20 mm
• genu valgum
• femoral anteversion
• generalized ligament laxity

The medial patellofemoral ligament tears in over 90% (usually at the femoral origin), and osteochondral fracture occurs in 25 to 40%.

After a first dislocation the redislocation rate is 15 to 50%, and as high as 70% with open physes or several risk factors. Chronic instability leads to chondral damage and patellofemoral arthritis.

• Sudden pain and deformity, usually reduced spontaneously on knee extension
• Swelling and hemarthrosis, medial retinacular tenderness and positive apprehension test
• J-sign (lateral tracking in early flexion) and a patella that translates over half its width laterally
• Red flags: an irreducible dislocation, an open injury, a large osteochondral loose body, and vascular compromise

• Radiographs: anteroposterior, lateral and skyline views for fracture, loose bodies, patella alta and trochlear dysplasia.
• MRI: medial patellofemoral ligament tear, bone bruises (lateral femoral condyle and medial patella), osteochondral injury.
• CT or MRI measurement of tibial tubercle to trochlear groove distance and femoral rotation when planning surgery.

Differentials: anterior cruciate tear, medial meniscal tear, patellar fracture, tibial plateau fracture, quadriceps tendon rupture.

Reduction: extend the knee gradually while pushing the patella medially. Analgesia with intra articular lidocaine 1% 10 mL or intravenous morphine 0.05 to 0.1 mg/kg titrated, and a post-reduction radiograph.

First time dislocation without osteochondral fracture:

• Non-operative: brace or patellar stabilizing sleeve for 2 to 3 weeks, early range of motion, and quadriceps and hip strengthening for 6 to 12 weeks.
• Ibuprofen 400 mg orally every 8 hours with food for 5 to 7 days (children 10 mg/kg/dose).
• Return to sport at 3 to 6 months.

Surgery:

• Osteochondral fracture or loose body: fixation or excision.
• Recurrent dislocation or high risk of redislocation: medial patellofemoral ligament reconstruction with hamstring autograft (femoral attachment at the radiographic Schottle point, tensioned at 30 degrees of flexion, avoiding overtightening).
• Add tibial tubercle osteotomy (medialization if tibial tubercle to trochlear groove distance exceeds 20 mm, distalization if patella alta) and trochleoplasty for severe dysplasia.
• In skeletally immature patients avoid the femoral physis when positioning the tunnel.
• Cefazolin 2 g IV before incision.
• Complications: stiffness, residual instability, patellar fracture, graft overtightening causing pain and arthritis.

Follow up at 2 weeks, 6 weeks, 3 months and 6 months.