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 recurrence rate of 15-60% following first-time dislocation risk of recurrence is 3.8 per 100,000 younger age, patella alta, and trochlear dysplasia increase the risk of recurrence "miserable malalignment syndrome" a term named for the 3 anatomic characteristics that lead to an increased Q angle femoral anteversion genu valgum external tibial torsion / pronated feet anatomical factors osseous patella alta causes patella to not articulate with sulcus, losing its constraint effects trochlear dysplasia excessive lateral patellar tilt (measured in extension) lateral femoral condyle hypoplasia muscle muscle imbalance vastus medialis oblique (VMO) muscle weakness overpull of lateral structures iliotibial band vastus lateralis Etiology Pathophysiology mechanism noncontact twisting injury with the knee extended and foot externally rotated patient will usually reflexively contract quadriceps thereby reducing the patella osteochondral fractures occur most often as the patella relocates direct blow to the medial knee less common ex. knee-to-knee collision in basketball, or football helmet to side of the knee 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 4.5-6.4 cm long x 1.9 cm wide tensile strength of 208 N femoral origin between the medial epicondyle and adductor tubercle proximal to the superficial MCL attachment proximal and posterior to the medial epicondyle anterior and distal to the adductor tubercle patellar insertion junction of the proximal and middle thirds on the medial border of the patella undersurface of the VMO primary restraint at 0-30 degrees of knee flexion primary passive restraint to lateral patellar translation 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 lateral translation of the medial border of the patella to the lateral edge of the trochlear groove is considered "2" quadrants and is considered an abnormal amount of translation 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 crossing sign trochlear groove lies in the same plane as the anterior border of the lateral condyle represents flattened trochlear groove associated with patellar instability and found in 96% of patients with true patellar dislocation double contour sign anterior border of the lateral condyle lies anterior to the anterior border of the medial condyle represents convex trochlear groove/hypoplastic medial condyle supratrochlear spur arises in proximal aspect of trochlea evaluate for patellar height (patella alta vs. baja) Blumensaat's line should extend to inferior pole of the patella at 30 degrees of knee flexion Insall-Salvati method normal between 0.8 and 1.2 Blackburne-Peel method normal between 0.5 and 1.0 Caton Deschamps method normal between 0.6 and 1.3 Plateau-patella angle normal between 20 and 30 degrees Sunrise/Merchant views best to assess for lateral patellar tilt lateral patellofemoral angle (normal is an angle that opens laterally) angle between line along subchondral bone of lateral trochlear facet + most prominent aspects of anterior portion of the trochlea normal > 11° congruence angle (normal is -6 degrees) sulcus angle evaluate for trochlear dysplasia values > 140 degrees indicate flattening of the trochlea concerning for dysplasia 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 >20mm is highly associated with patellar instability MRI help further rule out/characterize suspected loose bodies osteochondral lesion and/or bone bruising medial patellar facet (most common) lateral femoral condyle evaluate MPFL and medial retinaculum tear frequently at the medial patellar insertion Adult Treatment Nonoperative NSAIDS, activity modification, and physical therapy indications mainstay of treatment for first time patellar dislocator without any loose bodies or intraarticular damage habitual dislocator techniques short-term immobilization for comfort followed by 6 weeks of controlled motion emphasis on strengthening closed chain short arc quadriceps exercises quad strengthening core and hip strengthening to improve limb positioning and balance (hip abductors, gluteals, and abdominals) patellar stabilizing sleeve or "J" brace consider knee aspiration for tense effusion positive fat globules indicate fracture Operative Arthroscopic debridement (removal of loose body) vs Repair with or without stabilization indications displaced osteochondral fractures or loose bodies can be an indication for operative treatment in a first-time dislocator techniques arthroscopic vs open removal versus repair of the osteochondral fragment primary repair with screws or pins if sufficient bone available for fixation MPFL repair indications acute first-time dislocation with bony fragment techniques direct repair when surgery can be done within first few days no clinical studies support this over nonoperative treatment MPFL reconstruction with autograft or allograft indications recurrent instability no significant underlying malalignment techniques gracilis or semitendinosus commonly used (stronger than native MPFL) femoral origin can be reliably found radiographically (Schottle point) 1 mm anterior to the posterior femoral cortex, 2.5 mm distal to the posterior origin of the medial femoral condyle, and proximal to Blumensaat's line a femoral tunnel positioned too proximally results in graft that is too tight ("high and tight") in pediatric patients, the femoral side should be secured more anterior/distal to Schottle's point outcomes severe trochlear dysplasia is the most important predictor of residual patellofemoral instability after isolated MPFL reconstruction rate of recurrent instability does not differ with regard to graft choice (allograft vs. autograft vs. synthetic graft) Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer) indications may be used with or without MPFL reconstruction for significant malalignment TT-TG >20mm on CT techniques anteromedialized displacement of osteotomy and fixation patellofemoral contact pressures increased proximally and medially correct TT-TG to 10-15mm (never less than 10mm) tibial tubercle distalization indications patella alta techniques distal displacement of osteotomy and fixation lateral release/lengthening indications isolated release no longer indicated for patellainstability may lead to iatrogenic medial instability lateral lengthening has shown better outcomes, less quadriceps atrophy, and lower incidence of medial patellar instability only indicated if there is excessive lateral tilt or tightness after medialization technique arthroscopic trochleoplasty indications rarely addressed (in the USA) even if trochlear dysplasia present severe dysplasia recent literature reports that Dejour types B and D are most amenable to trochleoplasty revision cases with residual patellar instability techniques arthroscopic or open sulcus deepening procedure open recession wedge trochleoplasty guided growth (temporary hemiepiphysiodesis) indications in those with genu valgum greater than 10° and patellar instability and at least six months of growth remaining techniques tension band (8-plate) staples believed to be more rigid, providing faster correction 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