Patellar Instability - Knee & Sports (2024)

• 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

• 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

      See Also

      Measurement of the quadriceps (Q) angle with respect to various body parameters in young Arab population

• 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

• provided by vastus medialis (attaches to MPFL)

• complaints of instability

• anterior knee pain

• 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

• 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 +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

• 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

• 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

• 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

• 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 positioned too proximally results in graft that is too tight ("high and tight")

      • in pediatric patients, 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 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

• guided growth (temporaryhemiepiphysiodesis)

  • 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

• must preserve the physis

  • do not do tibial tubercle osteotomy (will harm growth plate of proximal tibia)

• 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

• almost exclusively iatrogenic as a result of prior patellar stabilization surgery