Patellar dislocation

A 32 male presents to the ED following a hefty collision with another player during a football game. During the altercation he sustained a direct blow to the medial aspect of his left knee.

On examination the patient is lying on a trolley in considerable pain, you look at the left kneecap and noticed it’s displaced laterally. The nurse starts administering Nitrous Oxide to the patient which helps the pain considerably.

Patella-dislocation-AP
Patella-dislocation-Lat

Patella Dislocations

  • Majority of patella dislocations occur in females, especially targeting teenage girls.
  • Literature shows, depending on the study that 30-72% of patellar dislocations are sports related and 28-39% will involve osteochrondral fractures
  • The most common location of patella dislocations is lateral dislocation.
  • Patients complain of the knee suddenly giving way, and inability to weight-bear or extend the knee and are often in considerable pain.
Patellar-dislocation
Patell-Dislocation-AP

Anatomy

  • The Patellar is the largest sesamoid bone in the body, and it resides within the complex of the quadriceps and patellar tendons.
  • It functions as both a lever and a pulley.
  • As a lever, the patella magnifies the force exerted by the quadriceps or knee extension.
  • As a pulley, the patella redirects the quadriceps force as it undergoes normal lateral tracking during flexion.
Knee_diagram

Mechanism of Injury:

  • Generally there is two types of mechanisms that result in patella dislocations.
  • Type 1: results when there is a powerful contraction of the quadriceps in combination with sudden flexion and external rotation of the tibia on the femur. This is the most common reason for the patellar to dislocate.
  • Type 2: results from direct trauma to the patella with the knee in flexion, can cause dislocation, however this is seen uncommonly.

Patellar Dislocations are common in patients with the following:

  • Genu valgum
  • Genu recurvatum
  • Excessive femoral neck anteversion or internal femoral torsion
  • External tibial torsion
  • Lateral insertion of patella ligament on the tibia
  • Contracture of the lateral patellar retinaculum
  • Relaxation of attenuation of medial patellar retinaculum
  • Hypoplasia or dysplasia of the patellar
  • Hypoplasia or flattening of the trochlear grove
  • Patellar alta or high riding patellar
  • Atrophy of the vastus medialis muscle
  • Pes planus
  • Generalised joint laxity

Assessment of Patellar Dislocation:

  • Assess neurovascular status distal to the patellar
  • The apprehension test can be used to assess for patellar dislocation and subluxation. Apprehension sign occurs when the knee is placed at 30° flexion, and lateral pressure is applied. Medial instability results in apprehension by the patient.
  • Examine for anterior defect, the laterally deviated patellar, and swelling and medial joint line tenderness of the partially flexed knee.
  • Order X-rays of the knee AP and lateral generally provide adequate views.
  • If the patellar is already relocated, still order X-rays to rule out fracture

Management of Patellar Dislocation

  • Provide analgesia, Nitrous Oxide with oxygen is generally effective and can also be used during reduction of the dislocation
  • Reduction is achieved by pushing the patellar medially with firm pressure, whilst extending the knee.
  • First time dislocations should be splinted in above knee plaster cast and referred to the next available fracture/orthopaedic clinic
  • Patients presenting with recurrent dislocations post reduction,  can be splinted in a pressure bandage or Richards splint, until follow up can be arranged.
  • Patients should be discharge home on crutches with simple analgesia for pain control.
  • Generally conservative management is all that is required for patellar dislocations, surgical intervention is considered when recurrent dislocations occur, and those with anatomical abnormality.

Reference
  • Fiebel, R. Dehghan, N. Cwinn, A. (2007). Irreducable lateral pattelar dislocation: The importance of impaction fracture recognition. The Journal of Emergency Medicine. 33(1), 11-15. [Reference]
  • Patellar Dislocations. (2007). In Buttaravoli, P (Ed), Minor Injuries Splinter to Fractures (p.509-512). Philadelphia: Mosby Elsevier.
  • Lu, D. Wang, E. Self, W. & Kharasch, M. (2010). Patellar Dislocation Reduction. Academic Emergency Medicine. 226.
  • Malanga, G. White, B. Lee, W. & Angeson, T. (2009). Patellar Injury and Dislocation. Accessed 30/04/2010 @ http://emedicine.medscape.com/article/90068-overview
  • Simon, R. Sherman, S. Koenigsknecht, S. (2007). Patellar Dislocations: Emergency Orthopaedics The Extremities. pp.430-432. New York: McGraw Hill

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