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Patellar Dislocation

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Other Names

  • Kneecap Dislocation
  • Dislocated Kneecap
  • Patellar Subluxation

Background

  • 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

History

Epidemiology

  • 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]

Pathophysiology

  • 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

Etiology

  • 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

Pathoanatomy

  • 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

Evaluation

  • AP Knee xray demonstrating laterally dislocated patella in a 12 year old.

  • AP Knee xray with osteochondral defect following patella dislocation.

  • Knee MRI axial view with marrow edema of the medial patellar facet and two small osteochondral fragments[8]

  • Knee MRI coronal view with marrow edema of the lateral femoral condyle[9]

Radiographs

  • 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

CT

  • 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.

MRI

  • 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

Classification

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

Management

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

Nonoperative

  • 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

Operative

  • 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

Rehabilitation

  • 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

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

Complications

See Also

References

  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 October 2022 18:51:42
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