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Updated: Sep 1 2023

Patellar Instability


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Images patella.jpg patella angle.jpg
  • 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
        • 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)
  • 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
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