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  • Can be classified into the following
    • 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 lateral structures - ITB and vastus lateralis)
  • 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
  • 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
  • Passive stability
    • medial patellofemoral ligament (MPFL)
      • femoral insertion origin is between medial epicondyle and adductor tubercle   
        • is usual site of avulsion of MPFL 
      •  is primary restraint in first 20 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)
  • 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 post
      • excessive lateral translation in extension which "pops" into groove as the patella engages the trochlea early in flexion
      • associated with patella alta
  • 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
      • 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 )  
      • congruence angle (normal is -6 degrees)
  • 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 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)  
    • 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
        • 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
      • indications
        • isolated release no longer indicated for 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) 
  • Recurrent dislocation
    • redislocation rates with nonoperative treatment may be high (15-50%) at 2-5 years
  • Medial patellar dislocation and medial patellofemoral arthritis
    • almost exclusively iatrogenic as a result of prior patellar stabilization surgery 

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