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Patellar Instability

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Other Names

  • Unstable Patella




  • Incidence
    • 29/100,000 per capita risk of dislocation among adolescents (need citation)
  • Prevalence
    • 2-3% of presentations involving the knee joint include a history of patellar dislocation (need citation)
    • 50-60% of first time lateral patellar dislocations occur secondary to a sport related injury (need citation)
    • Up to 40% of skeletally immature patients may develop recurrent instability[1]
    • Up to 10% of patients may develop contralateral patellar instability[2]


  • General
    • Defined as recurrent dislocations or sensation that the patella is going to dislocation
    • Majority of first time dislocations occur with foot planted, tibia externally rotated
    • Most patellar dislocations occur in teen years with patellar instability in 20s-30s


  • Acute traumatic
  • Episodic/ recurrent instability/ chronic patholaxity
    • Most common type, which occurs after an initial dislocation
    • Seen in adolescent athletes, more commonly in women
  • Alternative patterns described by Chotel in pediatrics[3]
    • Congenital dislocation
    • Permanent dislocation
    • Habitual dislocation in knee flexion
    • Habitual dislocation in knee extension
  • Syndromic instability

Associated Conditions


  • Patella
  • Medial Patellofemoral Ligament
    • Primary static restraint to lateral instability during first 30° of flexion
    • Prevents excessive lateral movement of the patella
  • Medial Retinaculum
  • Vastus Medialis Obliquus (VMO)
    • Most distal portion of the medial quadriceps muscle
    • Exerts a medially directed force that helps keep the patella in position
  • Trochlear Groove of the Femur
    • Variants in trochlear morphology can predispose the patella to maltracking[4]
    • Gross subluxation/dislocation
    • Can influence recurrent patellar instability
  • Tibial Tubercle
    • Arises from the lateral aspect of the proximal tibia
    • Excessive lateralization increases tibial external rotation, severe genu valgum, or even increased femoral anteversion[5]
    • All affect patellar tracking

Risk Factors

Differential Diagnosis

Clinical Features

  • History
    • Important to review history of previous patellar dislocations, episodes of patellar instability
    • Patient will complain of anterior knee pain
  • Physical Exam: Physical Exam Knee
    • In acute setting, hemarthrosis is often present
    • Tenderness over MPFL, tenderness over medial femoral condyle
  • Special Tests


  • Standard Radiographs Knee
    • Ideally, standard AP and lateral weight bearing views, as well as sunrise view
    • May not be possible in setting of acute dislocation
  • Plain radiography
    • Help identify fractures of the patella, avulsion fractures, loose bodies and sometimes large cartilage defects
    • PA radiographs at 45 degrees flexion may aid in assessment of the coronal alignment of the tibiofemoral joint
  • Lateral views and Sunrise or Merchant views
    • Provide information to trochlear morphology, patellar height and patellar tilt
  • Lateral patellar Tilt ((Laurin’s angle))
    • Assessed by the lateral patellofemoral angle on sunrise or merchant view
    • Angle is measured between a line along the subchondral bone of the lateral trochlear facet and posterior femoral condyles
    • Normal: angle greater than 11° that opens laterally
    • Abnormal angles: parallel or open medially
  • Patellar height
    • Can be measured by both direct and indirect methods
    • The Insall-Salvati Ratio: ratio measuring the length of the patella ligament, patellar length
      • A normal ratio is 1.0; a ratio of 1.2 suggests patella alta and 0.8 patella baja
    • Caton-Deschamps Index (CDI): distance between the distal point of the patellar articular surface and the anterior superior margin of the tibia, divided by the patellar articular surface length
      • A normal ratio is 1.0; a ratio of less than 0.6 suggests patella baja and a ratio of 1.3 suggests patella alta
    • Blackburne-Peel method (BP): ratio of the height of the lower pole of the articular surface above a tibial plateau line to the articular surface length of the patella
      • Normal between 0.54- 1.06; A ratio of less than 0.54 is considered to be patella alta
    • Technique described by Blumensaat uses the roof of the intercondylar notch as a reference line and is one of the most commonly used direct methods for the assessment of patellar height
  • True lateral radiographs and sunrise views can help identify other risk factors
    • The trochlear findings were elucidated by Dejour and Le Coultre and were subsequently revised to create the trochlear dysplasia classification system [8]
    • Crossing sign: occurs when the trochlear groove lies in the same plane as the anterior border of the lateral condyle, which represents a flattened trochlear groove
    • Double contour sign: occurs when the anterior border of the lateral condyle lies anterior to the anterior border of the medial condyle, which represents a convex trochlear groove or hypoplastic medial condyle
    • Supratrochlear spur can arise from the proximal aspect of the trochlea and can also indicate a risk factor


  • Computed tomographic (CT)
    • Can more accurately characterize the morphology of the trochlea
    • Assess femoral and tibial torsion
  • Tibial tubercle to trochlear groove (TT-TG) distance
    • Assesses relative rotation of femur to tibia
    • The TT-GG distance is between two perpendicular lines; one from the posterior cortex to the tibial tubercle and one from the posterior cortex to the trochlear groove
    • Average 8-10 mm in pediatric and adult patients; a TT-TG distance of greater than 20 is highly associated with patellar instability.


  • Common Findings[9]
    • Bruising pattern of lateral femoral condyle, medial patella
    • Disruption of the MPFL (at the medial femoral epicondyle insertion)
    • Articular cartilage injuries if present


Parikh Classification of Patellar Instability

  • Type I: first patellofemoral dislocation with (A) or without (B) osteochondral fracture[10]
  • Type II: recurrent subluxation (A) or dislocation (B)
  • Type III: dislocatable patella by the examiner or patient which is either passive (A) or habitual in flexion/extension (B).
  • Type IV: dislocated patella that is either reducible (A) or irreducible (B).





  • Indications
    • Failure of conservative management
    • Associated osteochondral fragment (≥ 5 mm)
    • Associated osseous avulsion of the MPFL
    • Associated meniscus tear
  • Techniques
    • Medial patellofemoral ligament reconstruction
    • MPFL reconstruction with autograft vs allograft
    • Trochleoplasty
    • Tibial tubercle osteotomy
    • Arthroscopic debridement (removal of loose body) vs Repair
    • Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)
    • Tibial tubercle distalization
    • Lateral release

Rehab and Return to Play


  • Needs to be updated

Return to Play

  • Full recovery time to return to sport ranges from 3-8 months
    • Depends on any procedures performed
  • Return to sport following Menetrey guidelines[11]
    • Full recovery of knee motion
    • Recovery of strength
    • Absence of a knee effusion, pain
    • Competence with sport specific exercises
  • Strongly consider


  • Patellofemoral Osteoarthritis
    • Up to 20% of cases at 20 years following initial dislocation[2]
  • Patella Fracture
    • Can occur as a surgical complication
  • Recurrent instability
  • Failure of surgical fixation
  • Inability to return to sport
  • Quadriceps weakness
  • Pediatric specific complications
    • Physeal arrest

See Also


  1. Lewallen LW, McIntosh AL, Dahm DL. Predictors of recurrent instability after acute patellofemoral dislocation in pediatric and adolescent patients. Am J Sports Med. 2013;41(3):575–81.
  2. 2.0 2.1 Sanders TL, Pareek A, Hewett TE, Stuart MJ, Dahm DL, Krych AJ. High rate of recurrent patellar dislocation in skeletally immature patients: a long-term population-based study. Knee Surg Sports Traumatol Arthrosc. 2017
  3. Chotel F, Berard J, Raux S. Patellar instability in children and adolescents. Orthop Traumatol Surg Res. 2014;100(1 Suppl):S125–37.
  4. Weber-Spickschen TS, Spang J, Kohn L, Imhoff AB, Schottle PB. The relationship between trochlear dysplasia and medial patellofemoral ligament rupture location after patellar dislocation: An MRI evaluation. Knee. 2011;18:185–8
  5. Steensen RN, Bentley JC, Trinh TQ, Backes JR, Wiltfong RE. The prevalence and combined prevalences of anatomic factors associated with recurrent patellar dislocation: A magnetic resonance imaging study. Am J Sports Med. 2015;43:921–7.
  6. Christensen TC, Sanders TL, Pareek A, Mohan R, Dahm DL, Krych AJ. Risk factors and time to recurrent ipsilateral and contralateral patellar dislocations. Am J Sports Med. 2017;45(9):2105–10.
  7. Beighton P, Solomon L, Soskolne CL. Articular mobility in an African population. Ann Rheum Dis. 1973 Sep;32(5):413-8
  8. Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc. 2007 Mar;15(1):39-46
  9. Elias DA, White LM, Fithian DC. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella. Radiology. 2002 Dec;225(3):736-43.
  10. Parikh SN, Lykissas MG. Classification of lateral patellar instability in children and adolescents. Orthop Clin North Am. 2016;47(1):145–52.
  11. Menetrey J, Putman S, Gard S. Return to sport after patellar dislocation or following surgery for patellofemoral instability. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2320–6.
Created by:
John Kiel on 15 March 2021 18:40:10
Last edited:
4 October 2022 15:55:42