Patellar Dislocation (Acute)

Key Points:

  • Acute patellar dislocations are common in adolescents and most occur secondary to noncontact mechanisms.
  • Acute patellar dislocations can occur secondary to a traumatic event in patients without anatomic risk factors or secondary to indirect trauma in patients with anatomic factors predisposing them to patellar instability
  • Following a patellar dislocation most patients have a knee effusion and a positive patellar apprehension test.
  • Radiographs are the standard initial imaging in the evaluation of a confirmed or suspected patellar dislocation.
  • Nonoperative treatment is standard for most acute patellar dislocations
  • MRI is utilized to evaluate for osteochondral fractures/loose bodies
  • Operative treatment is considered in cases of recurrent instability or those with critical osteochondral injuries

Description:

Acute patellar dislocations are common in the pediatric and adolescent populations.  Adolescents and young adults, 14 to 20 years of age, account for the greatest percentage of acute traumatic patellar dislocations (Khormaee, 2015; Willis, 2012).  Most acute dislocations are the result of noncontact mechanisms in which the knee is subject to a valgus force with internal rotation of the femur and a planted foot (Khormaee, 2015). This indirect mechanism of injury is responsible for 66-82% of acute patellar dislocations.  A direct blow to the knee may also result in a traumatic patellar dislocation (Khormaee, 2015; Steiner, 2010).  Sporting injuries account for 61-72% of acute patellar dislocations (Steiner, 2010). Gallie first recognized a distinction between acute patellar dislocations in individuals with normal anatomy and those with predisposing anatomic risk factors for dislocation (Steiner, 2010).  Risk factors for acute patellar dislocation are listed in Table 1 (Khormaee, 2015; Willis, 2012; Steiner, 2010).

Epidemiology:

Table 1. Risk Factors for Patellar Dislocation
  • Female gender
  • Femoral Anteversion
  • Patella Alta
  • Trochlear Dysplasia
  • Increased Q angle
  • Lateralized tibial tubercle (Increased TT-TG distance)
  • Genu Valgum
  • External tibial torsion
  • Prior patellar dislocation
  • Ligamentous laxity
  • Collagen disorders (i.e. Ehlers-Danlos)
  • Vastus medialis obliqus (VMO) and core weakness

Clinical Findings:

In most cases of acute patellar dislocation, the patella spontaneously reduces (Willis, 2012).  In lateral patellar dislocations, this can occur as the knee is extended.  In cases where spontaneous reduction does not occur, the medial femoral condyle appears prominent as the patella is displaced over the lateral femoral condyle (Steiner, 2010).  

Examination of the knee following a dislocation will demonstrate a moderate to large effusion, particularly in cases of osteochondral fracture.  Patellar dislocations are one of the most common causes of hemarthrosis secondary to injury (Steiner, 2010).   A complete ligamentous exam is required to rule out concomitant cruciate or collateral ligament injury.  Tenderness over the medial retinaculum is typical.  Though tenderness over the medial patellar facet and/or lateral femoral condyle may be the result of contusion alone, it may also signify osteochrondral injury.  Apprehension to lateral displacement of the patella with the knee in 20°-30° of flexion, the apprehension test, is a typical finding (Khormaee, 2015).

Imaging Studies:

Evaluation of a knee after an acute patellar dislocation should begin with plain radiographs including standing AP, standing 45° flexion weight bearing, a 30° lateral, and patellar Merchant views of the involved knee (Khormaee, 2015).  Radiographs can show osteochrondral fracture or persistent subluxation of the patella.  Persistent subluxation of the patella can be found in up to 97% of acute patellar dislocations (Steiner, 2010). Additionally, anatomic risk factors for patellar dislocation, particularly patella alta and trochlear dysplasia, can be assessed.

More advanced imaging techniques, including ultrasound and magnetic resonance imaging (MRI), are used to further evaluate for osteochondral injury, loose bodies, medial retinacular injury, or bony avulsion injuries (Steiner, 2010).  O’Reilly evaluated the ability to identify injury using ultrasound by comparing ultrasound findings to operative findings (O’Reilly, 2003).  The medial retinacular findings identified on ultrasound were confirmed intra-operatively in all 10 patients.  Bony avulsion injuries were accurately identified using ultrasound but a patellar osteochondral injury in one patient was missed (O’Reilly, 2003).    

MRI is the advanced imaging modality of choice for evaluating acute patellar dislocations because of the ability to evaluate for osteochondral injury, loose bodies and soft tissue injury (Seeley, 2013; Steiner, 2010). Quinn described the MRI findings following acute patellar dislocation as contusion or impaction of the medial patellar facet and lateral femoral condyle, along with injury of the medial retinaculum and/or medial patellofemoral ligament (MPFL) (Quinn, 1993).   MRI has been found to be 85%-92% sensitive for diagnosing MPFL injury (Seeley, 2013).  In children and adolescents, the majority of MPFL tears occur at the femoral insertion of the ligament.  In 113 cases of surgical intervention for patellar instability in patients 5 to 17 years of age, Putney found that 73% of MPLF tears were at the adductor tubercle (Putney, 2012).   Osteochondral injury as a result of patellar dislocation ranges from 5% to 50%, although Seeley reported an incidence as high as 73% (Seeley, 2013).

Etiology:

Treatment:

Most acute patellar dislocations can be managed nonoperatively.  In a study of 266 first time patellar dislocations with an average age of 13.7 years, 83.5% were treated nonoperatively (Khormaee, 2015; Jaquith, 2015).  Nonoperative treatment generally consists of a period of immobilization followed by rehabilitation.   In cases in which there is a large knee effusion, aspiration may be used to improve comfort and motion recovery.  Traditionally, the knee was immobilized for a period of 3-6 weeks to allow for soft tissue healing, although earlier motion has been more recently advocated.  No randomized trials have been done to compare the outcomes of these two forms of treatment.  Based on the current literature, it remains unclear whether or not patellar stabilizing braces are effective for instability.

Acute surgical treatment is performed for loose body removal or for fixation of osteochondral fractures. Surgical treatment is indicated in the above setting and in cases of recurrent instability. Treatment for recurrent instability includes medial patello-femoral ligament (MPFL) and medial retinacular repair or reconstruction, or proximal or distal realignment. Ligament reconstruction is felt to be more reliable than repair (due to the common occurrence of multiple zones of ligament injury) and treatments may be performed in combination when multiple anatomic abnormalities exist. Distal realignment in the form of tibial tubercle osteotomy is reserved for those who are skeletally mature (Khormaee, 2015) while a Roux-Goldwaithe procedure can be performed on a skeletally immature patient.   A lateral release may be included as part of the surgical plan although an isolated lateral release is not advised. In rare instances where true ‘miserable malalignment’ (excessive femoral anteversion with excessive external tibial torsion) contributes to instability, femoral or tibial derotation osteotomy may be performed if other treatments fail.  Guided growth if the physes are open, or corrective osteotomies such as a distal femoral osteotomy if the patient is skeletally mature are important considerations for treatment of coronal plane deformities (usually genu valgum).

Complications:

Re-dislocation is a complication of both nonoperative and operative management of acute patellar dislocations.   In the retrospective review by Jaquith and Parikh, 34.7% of first time dislocations had recurrent patellar instability episodes (Jaquith, 2015).  A consequence of recurrent patellar instability may be patellofemoral osteoarthritis (Seeley, 2013).  In one series of 27 surgically treated knees, the rate of re-dislocation was just 7% in 3 years (Luhmann, 2011).  Overall however, recurrent instability rates are similar for nonoperative and operative management (consisting of medial repair of torn structures with or without lateral release) in the case of patellar dislocations without osteochondral fracture (Willis, 2012; Palmu, 2008).

References:

  1. Jaquith BP, Parikh SN. Predictors of Recurrent Patellar Instability in Children and Adolescents After First-time Dislocation. J Pediatr Orthop. 2015; [Epub ahead of print].
  2. Khormaee S, Kramer DE, Yen Y-M, Heyworth BE. Evaluation and Management of Patellar Instability in Pediatric and Adolescent Athletes. Sport Heal A Multidiscip Approach. 2015;7(2):115-123.
  3. Luhmann SJ, O’Donnell JC, Fuhrhop S. Outcomes after patellar realignment surgery for recurrent patellar instability dislocations: a minimum 3-year follow-up study of children and adolescents. J Pediatr Orthop. 2011;31(1):65-71.
  4. O’Reilly, MA; O’Reilly, PM; Bell J. Sonographic appearances of medial retinacular complex injury in transient patellar dislocation. Clin Radiol. 2003;58:636-641.
  5. Palmu S, Kallio PE, Donell ST, Helenius I, Nietosvaara Y. Acute patellar dislocation in children and adolescents: a randomized clinical trial. J Bone Joint Surg Am. 2008;90(3):463-470.
  6. Putney S a, Smith CS, Neal KM. The location of medial patellofemoral ligament injury in adolescents and children. J Pediatr Orthop. 2012;32(3):241-244.
  7. Quinn, SF; Brown, TR; Demlow T. MR imaging of patellar retinacular ligament injuries. J Magn Reson Imaging. 1993;3(6):843-847.
  8. Seeley M a, Knesek M, Vanderhave KL. Osteochondral injury after acute patellar dislocation in children and adolescents. J Pediatr Orthop. 2013;33(5):511-518.
  9. Seeley M, Bowman KF, Walsh C, Sabb BJ, Vanderhave KL. Magnetic Resonance Imaging of Acute Patellar Dislocation in Children. J Pediatr Orthop. 2012;32(2):145-155.
  10. Steiner, Timothy; Parker RD. Patellofemoral Instability: Acute Dislocation of the Patella. In: DeLee, Jesse C; Drez, David Jr.; Miller MD, ed. DeLee & Drez’s Orthopaedic Sports Medicine. 3rd ed. Philadelphia: Saunders Elsevier; 2010:1534-1547.
  11. Willis RB, Firth G. Traumatic patellar dislocation: loose bodies and the MPFL. J Pediatr Orthop. 2012;32 Suppl 1(1):S47-S51

Top Contributors:

Pamela Lang MD
Jennifer Beck MD