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 ligamentous laxity (Ehlers-Danlos syndrome) previous patellar instability event login to view 3 more bullets "miserable malalignment syndrome" login to view 4 more bullets anatomical factors osseous login to view 5 more bullets muscle login to view 5 more bullets Etiology Pathophysiology mechanism noncontact twisting injury with the knee extended and foot externally rotated login to view 2 more bullets direct blow to the medial knee login to view 2 more bullets Associated conditions medial patellofemoral ligament (MPFL) rupture most commonly disrupted at the patellar insertion in complete patellar dislocations articular cartilage damage most commonly at the medial patellar facet Anatomy Passive stability MPFL anatomy login to view 2 more bullets femoral origin login to view 4 more bullets patellar insertion login to view 2 more bullets primary restraint at 0-30 degrees of knee flexion login to view 1 more bullet patellar-femoral bony structures account for stability in deeper knee flexion trochlear groove morphology, patella height, patellar tracking 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 frequently reduces spontaneously Physical exam acute dislocation is usually associated with a large hemarthrosis 2nd most common cause of traumatic knee hemarthrosis absence of swelling supports ligamentous laxity and habitual dislocation mechanism medial sided tenderness (over MPFL) increase in passive patellar translation measured in quadrants of translation (midline of the patella is considered "0"), and also should be compared to the contralateral side normal motion is <2 quadrants of patellar translation login to view 1 more bullet patellar apprehension passive lateral translation results in guarding and a sense of apprehension increased Q angle J sign excessive lateral translation in extension which "pops" into groove as the patella engages the trochlea early in flexion associated with patella alta Imaging Radiographs rule out a fracture or loose body medial patellar facet (most common) lateral femoral condyle AP views best to evaluate for malalignment and osteoarthritis lateral views best to assess for trochlear dysplasia login to view 9 more bullets evaluate for patellar height (patella alta vs. baja) login to view 9 more bullets Sunrise/Merchant views best to assess for lateral patellar tilt lateral patellofemoral angle (normal is an angle that opens laterally) login to view 2 more bullets congruence angle (normal is -6 degrees) sulcus angle login to view 2 more bullets CT scan TT-TG distance measures the distance between 2 perpendicular lines from the posterior cortex to the tibial tubercle and the trochlear groove normal values between 9 and 13 mm login to view 1 more bullet MRI help further rule out/characterize suspected loose bodies osteochondral lesion and/or bone bruising login to view 2 more bullets evaluate MPFL and medial retinaculum tear frequently at the medial patellar insertion Adult Treatment Nonoperative NSAIDS, activity modification, and physical therapy indications login to view 3 more bullets techniques login to view 8 more bullets Operative Arthroscopic debridement (removal of loose body) vs Repair with or without stabilization indications login to view 2 more bullets techniques login to view 2 more bullets MPFL repair indications login to view 1 more bullet techniques login to view 2 more bullets MPFL reconstruction with autograft or allograft indications login to view 2 more bullets techniques login to view 5 more bullets outcomes login to view 2 more bullets Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer) indications login to view 2 more bullets techniques login to view 3 more bullets tibial tubercle distalization indications login to view 1 more bullet techniques login to view 1 more bullet lateral release/lengthening indications login to view 4 more bullets technique login to view 1 more bullet trochleoplasty indications login to view 4 more bullets techniques login to view 2 more bullets guided growth (temporary hemiepiphysiodesis) indications login to view 1 more bullet techniques login to view 3 more bullets Pediatric Treatment Same principles as adults in general but must preserve the physis tibial tubercle osteotomy contraindicated (will harm growth plate of proximal tibia) 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