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Patellar Dislocation

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Other Names

  • Kneecap Dislocation
  • Dislocated Kneecap
  • Patellar Subluxation


  • This page describes subluxations and dislocations of the Patella
    • Dislocation: defined as patella disengages completely from the trochlear or femoral groove
    • Subluxation: defined as partial disengagement and spontaneous reduction



  • Represents about 2-3% of knee injuries
  • Overall incidence is around 8 to 43 per 100,000[1][2]
    • Highest among adolescents aged 14 to 18 years
    • Males and females seem to be affected equally
  • Recurrence rate following first time dislocation is 15-60%[3]


  • General
    • It is important to take thorough history to determine if occurrence was first time dislocation
    • Patients may describe a popping sensation without obvious dislocation suggesting subluxation or dislocation with spontaneous reduction
  • Lateral Dislocation
    • Most common by far
    • Occurs with contact during contact or collision sport
    • Without contact during aggressive cutting or pivoting maneuver
  • Other Directions
    • Medial dislocations, intra-articular and superior dislocations are rare
    • Associated with congenital conditions, quadriceps atrophy or iatrogenic


  • Typically traumatic from:
    • Non-contact twisting injury to the knee
    • Direct blow to the medial aspect of the knee
    • Tibia is often externally rotated with foot planted, knee slightly flexed with a valgus position
  • Generalized ligamentous laxity
    • These patients may have more benign etiology
    • Often subluxation events rather than frank dislocation


  • Vastus Medialis Obliquus (VMO)
    • Most distal portion of the medial quadriceps muscle
    • Exerts a medially directed force that helps keep the patella in position
  • Medial retinaculum
  • Medial Patellofemoral Ligament (MPFL)
    • Primary static restraint to lateral instability during first 30° of flexion
    • Prevents excessive lateral movement of the patella
    • Almost universally torn during dislocations

Associated Conditions

Risk Factors

  • General
    • Female > male[4]
    • Younger age
  • Muscle related factors
  • Anatomic factors
    • Increased Q Angle
    • Patella Alta or high riding patella
    • Trochlear dysplasia
    • Excessive lateral patellar tilt
    • Lateral femoral condyle hypoplasia
    • Laterally located tibial tubercle
  • Generalized ligamentous laxity
  • Connective Tissue Disorder
    • Marfan Syndrome
    • Ehlers Danlos Syndrome
  • Down Syndrome
    • Due to congenitally small patella, hypoplastic condyle

Differential Diagnosis

Clinical Features

Clinical example of laterally dislocated patella[5]
  • History
    • Most describe pain and potentially a noticeable deformity of the knee
    • Usually preceded by direct trauma to the knee or a sudden change in direction
    • Many describe a sense of giving way or instability when the dislocation occurs
    • Feel a pop or multiple pops and there may be generalized pain in the anteromedial knee
  • Exam: Physical Exam Knee
    • Acutely dislocated patella is typically easy to see
    • May have joint effusion or hemarthrosis (#2 cause in pediatrics behind ACL[6])
    • Carefully identify the patellar poles, medial and lateral joint lines and retinaculum
    • Palpable tenderness along the medial retinaculum or MPFL may be appreciable
    • Patellar laxity, looseness, or increased motion may be noted
    • Important to stress other structural ligaments if possible
    • Range of motion is very limited in dislocation, may be intact in subluxation
    • Strength testing can be limpted in knee extension, hip abduction
    • Normal patellar glide medially and laterally is between 25-50% the width of the patella
    • Other findings include femoral anteversion, patella alta, tibial torsion, genu recurvatum, genu valgum or varum, pes planus, and general ligamentous laxity
  • Special tests
    • Special tests may need to be deferred in the acute setting depending on how symptomatic patient is
    • Consider evaluating for ligamentous laxity using the Beighton Score[7]
    • Patellar Apprehension Test: Apply medial and lateral pressure to the patella testing for apprehension
    • Patellar J Sign: Evaluate path of patella during flexion and extension



  • 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
    • 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: 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 [10]
    • 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.


  • Indicated if any evidence of loose body or Osteochondral Lesion on radiographs
  • Common Findings[11]
    • Bruising pattern of lateral femoral condyle, medial patella
    • Disruption of the MPFL (at the medial femoral epicondyle insertion)
    • Articular cartilage injuries if present


Dejour Classification of Trochlear Dysplasia

  • Type A: flatter than normal with a sulcus angle greater than 145°
  • Type B: which is flat
  • Type C: which is convex
  • Type D: which is convex with a supratrochlear spur


Patellar Dislocation Reduction

  • Acute dislocations require reduction
  • Consideration should be made to transfer the patient to the emergency department depending on the clinical context
  • Patient will require some degree of analgesia, possibly procedural sedation
  • Procedure
    • Dislocation should be obvious
    • Patient should have knee in a resting position of slight flexion
    • With distal hand gently extend knee
    • Simultaneously, proximal hand applies gentle medial pressure to the patella
    • As the knee extends, the patella should slide over the lateral femoral condyle and pop into the groove
    • Patients typically feel much better after reduction is complete


  • Indications
    • First time dislocation
    • No evidence of osteochondral defect, intra-articular damage
  • Relative rest from offending activities
  • NSAIDS, Ice Therapy
  • Patellar J Brace
    • Often helpful initially to provide additional stability
  • Kinesiology Tape can be considered
    • Especially McConnell technique
  • Physical Therapy
    • Begin around 2-4 weeks


  • Indications
    • Presence of osteochondral defect or loose body
    • Subluxation of patella on sunrise view
    • Failure to improve with conservative management
    • Anatomic factors predisposing to dislocation
    • Recurrent Dislocation
  • Technique
    • Arthroscopy with or without open debridement
    • MPFL repair (re-attachment) or reconstruction (proximal realignment)
    • Lateral release (distal realignment)
    • Tibial Osteotomy (distal realignment)
    • Trochleoplasty

Rehab and Return to Play


  • Goals
    • Core stability
    • Strengthen extensor mechanism, especially VMO
    • Proprioception
  • Four phases of rehabilitation before full return to activity:
    • Protective phase (up to 6 weeks post-operation)
    • Moderate protection phase (weeks 7-12)
    • Minimum protection phase (weeks 13-16)
    • Return to activity phase (weeks 17-20+)

Return to Play

  • After first dislocation, majority of athletes can return to play at about 3-4 weeks[12]

Complications and Prognosis


  • Symptoms
    • Atkin et al. found that at 6 months, 58% of patients experience limitation in strenuous activities[13]
    • Maenpaa et al. found that over 50% of patients have complications following a first-time dislocation including re-dislocation, subluxation or patellofemoral pain[14]
  • Risk of redislocation
    • 20-40%, higher if second dislocation[15]
    • After second dislocation, subsequent risk is 50% or greater (need citation)
    • Risk increases if presence of trochlear dysplasia, patella alta, laterally placed tibial tubercle
  • Sillanpaa et al studied early stabilization surgery for primary dislocation[16]
    • Redislocation rate was significantly lower in surgical group compared to nonsurgical group
    • No benefit from surgery was seen at long-term follow up


See Also


  1. Jain NP, Khan N, Fithian DC. A treatment algorithm for primary patellar dislocations. Sports Health. 2011 Mar;3(2):170-4.
  2. Ries, Zachary, and Matthew Bollier. "Patellofemoral instability in active adolescents." The journal of knee surgery 28.04 (2015): 265-278.
  3. Fithian DC, Paxton EW, Stone ML, Silva P, Davis DK, Elias DA, White LM. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med. 2004 Jul-Aug;32(5):1114-21.
  4. DeFroda, Steven F., et al. "Diagnosis and management of traumatic patellar instability in the pediatric patient." Orthopedics 40.5 (2017): e749-e757.
  5. Sanchis-Alfonso, Vicente, et al. "Failed medial patellofemoral ligament reconstruction: causes and surgical strategies." World journal of orthopedics 8.2 (2017): 115.
  6. Pedowitz, Jason M., et al. "Recurrence of patellar instability in adolescents undergoing surgery for osteochondral defects without concomitant ligament reconstruction." The American journal of sports medicine 47.1 (2019): 66-70.
  7. Beighton P, Solomon L, Soskolne CL. Articular mobility in an African population. Ann Rheum Dis. 1973 Sep;32(5):413-8
  8. https://radiopaedia.org/cases/25852
  9. https://radiopaedia.org/cases/25852
  10. Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc. 2007 Mar;15(1):39-46
  11. 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.
  12. Hergenroeder, Albert C., and Richard G. Bachur. "Approach to acute knee pain and injury in children and skeletally immature adolescents."
  13. Atkin DM, Fithian DC, Marangi KS, Stone ML, Dobson BE, Mendelsohn C. Characteristics of patients with primary acute lateral patellar dislocation and their recovery within the first 6 months of injury. Am J Sports Med. 2000 Jul-Aug;28(4):472-9.
  14. Mäenpää H, Lehto MU. Patellar dislocation. The long-term results of nonoperative management in 100 patients. Am J Sports Med. 1997 Mar-Apr;25(2):213-7
  15. Mäenpää H, Huhtala H, Lehto MU. Recurrence after patellar dislocation. Redislocation in 37/75 patients followed for 6-24 years. Acta Orthop Scand. 1997 Oct;68(5):424-6.
  16. Sillanpää PJ, Mattila VM, Mäenpää H, Kiuru M, Visuri T, Pihlajamäki H. Treatment with and without initial stabilizing surgery for primary traumatic patellar dislocation. A prospective randomized study. J Bone Joint Surg Am. 2009 Feb;91(2):263-73.
Created by:
John Kiel on 25 June 2019 19:03:38
Last edited:
4 April 2024 14:04:42