Summary Patellar instability is a condition characterized by patellar subluxation or dislocation episodes as a result of injury, ligamentous laxity or increased Q angle of the knee. Diagnosis is made clinically in the acute setting with a patellar dislocation with a traumatic knee effusion and in chronic settings 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 Demographics most commonly occurs in 2nd-3rd decades of life Risk factors general factors ligamentous laxity (Ehlers-Danlos syndrome) previous patellar instability event "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 dysplastic vastus medialis oblique (VMO) muscle overpull of lateral structures iliotibial band vastus lateralis Etiology Pathophysiology mechanism usually on 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 less common ex. knee to knee collision in basketball, or football helmet to side of knee Anatomy Passive stability medial patellofemoral ligament (MPFL) femoral origin-insertion is between medial epicondyle and adductor tubercle is usual site of avulsion of MPFL is primary restraint in first 20-30 degrees of knee flexion 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) Presentation Symptoms complaints of instability anterior knee pain Physical exam acute dislocation usually associated with a large 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 patella is considered "0"), and also should be compared to contralateral side normal motion is <2 quadrants of patellar translation lateral translation of medial border of patella to lateral edge of trochlear groove is considered "2" quadrants and is considered 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 fracture or loose body medial patellar facet (most common) lateral femoral condyle AP views best to evaluate overall lower extremity alignment and version lateral views best to assess for trochlear dysplasia crossing sign trochlear groove lies in same plane as anterior border of lateral condyle represents flattened trochlear groove double contour sign anterior border of lateral condyle lies anterior to anterior border of 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 + posterior femoral condyles 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 >20mm usually considered abnormal MRI help further rule out/characterize suspected loose bodies osteochondral lesion and/or bone bruising medial patellar facet (most common) lateral femoral condyle tear of MPFL tear usually at medial femoral epicondyle 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 indicates fracture Operative Arthroscopic debridement (removal of loose body) vs Repair with or without stabilization indications displaced osteochondral fractures or loose bodies may 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 vs 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) a femoral tunnel positoined too proximally results in graft that is too tight ("high and tight") 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 in addition to MPFL or in isolation 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 instability 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 may consider in severe or revision cases techniques arthroscopic or open trochlear deepening procedure Pediatric Treatment Same principles as adults in general but must preserve the physis do not do tibial tubercle osteotomy (will harm growth plate of proximal tibia) Complications Recurrent dislocation redislocation rates with nonoperative treatment may be high (15-50%) 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