Summary Patellar instability defines a spectrum from subluxation to dislocation that results from injury, ligamentous laxity, or increased Q angle of the knee. Diagnosis is made clinically in the acute setting of a patellar dislocation with a traumatic knee effusion and in the chronic setting with passive patellar translation and a positive J sign. Treatment is nonoperative with bracing for first time dislocation without bony avulsion or presence of articular loose bodies. Operative management is indicated for chronic and recurrent patellar instability. Epidemiology Incidence 2-3% of all knee injuries annual risk of first-time patellar dislocation is 5.8 per 100,000 Demographic most commonly occurs in the 2nd-3rd decades of life Risk factors general factors login to view 10 more bullets anatomical factors login to view 12 more bullets Etiology Pathophysiology mechanism login to view 6 more bullets Associated conditions medial patellofemoral ligament (MPFL) rupture login to view 1 more bullet articular cartilage damage login to view 1 more bullet Anatomy Passive stability MPFL login to view 13 more bullets patellar-femoral bony structures account for stability in deeper knee flexion login to view 1 more bullet Dynamic stability provided by vastus medialis (attaches to MPFL) Classification Can be classified into the following Patellar instability classification Acute traumatic Occurs equally by gender May occur from a direct blow (ex. helmet to knee collision in football) Chronic patholaxity Recurrent subluxation episodes Occurs more in women Associated with malalignment Habitual Usually painless Occurs during each flexion movement Pathology is usually proximal (e.g. tight ITB and vastus lateralis) Trochlear dysplasia can be described by the Dejour classification types B and D more amenable to trochleoplasty Dejour Classification Dejour Type Lateral radiograph findings Axial image findings Type A Crossing sign Shallow or concave trochlea Type B Crossing sign and supratrochlear spur Flat or convex trochlea Type C Crossing sign and double contour Convex lateral facet with hypoplastic medial facet Type D Crossing sign, supratrochlear spur, and double contour Asymmetry of trochlear facets with a vertical slope/cliff pattern Presentation Symptoms complaints of instability anterior knee pain painful "pop" or "clunk" felt with patellar dislocation login to view 1 more bullet Physical exam acute dislocation is usually associated with a large hemarthrosis login to view 2 more bullets medial sided tenderness (over MPFL) increase in passive patellar translation login to view 3 more bullets patellar apprehension login to view 1 more bullet increased Q angle J sign login to view 2 more bullets Imaging Radiographs rule out a fracture or loose body login to view 2 more bullets AP views login to view 1 more bullet lateral views login to view 20 more bullets Sunrise/Merchant views login to view 8 more bullets CT scan TT-TG distance login to view 3 more bullets MRI help further rule out/characterize suspected loose bodies login to view 3 more bullets evaluate MPFL and medial retinaculum login to view 1 more bullet Adult Treatment Nonoperative NSAIDS, activity modification, and physical therapy login to view 13 more bullets Operative Arthroscopic debridement (removal of loose body) vs Repair with or without stabilization login to view 6 more bullets MPFL repair login to view 5 more bullets MPFL reconstruction with autograft or allograft login to view 12 more bullets Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer) login to view 7 more bullets tibial tubercle distalization login to view 4 more bullets lateral release/lengthening login to view 7 more bullets trochleoplasty login to view 8 more bullets guided growth (temporary hemiepiphysiodesis) login to view 6 more bullets Pediatric Treatment Same principles as adults in general but must preserve the physis login to view 1 more bullet Complications Recurrent dislocation redislocation rates with nonoperative treatment may be high (15-60%) at 2-5 years recurrence rate is highest in those patients who sustain a primary dislocation under the age of 20 Medial patellar dislocation and medial patellofemoral arthritis almost exclusively iatrogenic as a result of prior patellar stabilization surgery Inferolateral anterior knee numbness damage to the infrapatellar branch of the saphenous nerve during the midline skin approach to TTO