Patellar Instability

Topic updated on 08/21/16 5:48pm
  • 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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Qbank (9 Questions)

(OBQ12.72) Which of the following structures attaches between the medial epicondyle and adductor tubercle of the femur? Topic Review Topic

1. Medial head of gastrocnemius
2. Medial collateral ligament
3. Semimembranosus
4. Adductor magnus
5. Medial patellofemoral ligament

(OBQ11.188) Which of the following best describes the radiographic landmarks on a lateral radiograph for locating the femoral attachment of the medial patellofemoral ligament (MPFL) during reconstruction? Topic Review Topic

1. The intersection of a line extended from the middle of the shaft and Blumensaat's line
2. Anterior to a line extended from the middle of the shaft and Blumensaat's line
3. Posterior to a line extended from the posterior cortex of the shaft and distal to Blumensaat's line
4. Anterior to a line extended from the posterior cortex of the shaft and distal to Blumensaat's line
5. Anterior to a line extended from the posterior cortex of the shaft and proximal to Blumensaat's line

(OBQ10.188) A 27-year-old football player sustains an acute lateral patellar dislocation. Which of the following is the most likely site of injury seen on MRI? Topic Review Topic

1. Midsubstance oblique retinacular ligament rupture
2. Soft-tissue avulsion of medial patellofemoral ligament
3. Midsubstance medial patellofemoral ligament rupture
4. Partial quadriceps tendon rupture
5. Bony avulsion of medial patellofemoral ligament

(OBQ08.49) A high school softball player has chronic activity-related anterior knee pain without a history of instability. Which radiographic measurement is used to indicate when a lateral retinacular release may be helpful? Topic Review Topic

1. Congruence angle
2. Q angle
3. Sulcus angle
4. Lateral patello-femoral angle
5. Patellar height index

(OBQ07.30) An athlete sustains a traumatic patellar dislocation. The MRI shows a hemarthrosis with a floating osteochondral fragment. Which of the following is the most likely site of origin for the loose fragment? Topic Review Topic

1. The lateral patellar facet
2. The medial patellar facet
3. The odd patellar facet
4. The medial trochlea
5. The central trochlea

(OBQ07.112) You see a patient in the emergency room with an acute lateral patellar dislocation. Which of the following factors is associated with the highest risk of persistent patellar instability? Topic Review Topic

1. Younger age
2. Increased Q-angle
3. Male gender
4. Previous patellar instability event
5. Amount of lateral patellar tilt

(OBQ04.46) All of the following are predisposing factors for lateral patellar dislocation in a native knee EXCEPT? Topic Review Topic

1. Excess femoral internal rotation
2. Excess external tibial rotation
3. Lateral femoral condylar hypoplasia
4. Increased Q-angle
5. Insufficiency of the vastus lateralis

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