Patellar dislocation

The patella is the largest sesamoid bone in the human body. It is located within the quadriceps femoris tendon and acts as a fulcrum to increase the force exerted on the tibia. In patellar dislocation, the patella slips out of the femoral trochlear groove, usually laterally. Patellar dislocation usually occurs following torsion of a semiflexed knee; less frequently, dislocation is the result of direct trauma to the patella. Recurrent dislocation may be associated with underlying biomechanical abnormalities such as medial patellofemoral ligament injury. In some patients, the condition is congenital. Patellar dislocation is primarily a clinical diagnosis, but x-rays should be obtained in all patients to rule out concomitant osteochondral fractures. Joint aspiration and/or further imaging (e.g., MRI or CT scan) may be indicated based on physical examination and/or x-ray findings. Many first episodes of patellar dislocation can be managed conservatively, but surgical treatment is often indicated for concurrent fractures or major soft tissue lesions, failure of conservative management, or recurrent dislocations.

• Patellar instability is a general term which can refer to either of the following. ^[1]
  • Patellar dislocation: complete displacement of the patella from the trochlear groove of the femur
  • Patellar subluxation: partial displacement of the patella from the trochlear groove of the femur
• Recurrent patellar dislocation: patellar dislocation that has occurred multiple times ^[2]
• Congenital (fixed) patellar dislocation: lateral displacement of the patella that cannot be reduced manually, often presenting at birth ^[3]

• Sex: ♀ > ♂
• The first episode of patellar dislocation typically happens before the age of 20 years.
• Up to 45% of patients will have recurrent patellar dislocations after the first episode.

Epidemiological data refers to the US, unless otherwise specified.

Traumatic patellar dislocations ^[4]

• Non-contact (most common): usually due to a twisting injury when the knee is simultaneously in flexion and under valgus strain
• Direct contact: usually due to a direct sideward impact on the patella (e.g., during contact sports or a car accident)

Risk factors for recurrent dislocations ^[4]

• Anatomical risk factors for patellar dislocation
  • Patellofemoral dysplasia (trochlear dysplasia and patellar dysplasia) ^[4][5]
  • A large Q angle; e.g., as seen in patients with genu valgum (“knock-knee” deformity) ^[5]
  • Injury to the medial patellofemoral ligament
  • Weakness of the vastus medialis
  • Patella alta (high-riding patella)
  • Ligament or joint hypermobility: e.g., due to connective tissue conditions (Ehlers-Danlos syndrome, Marfan syndrome)
  • Lower extremity malalignment
  • Patellar subluxation
• Younger age
• Skeletal immaturity
• Sports-related injury
• Family history of patellar dislocation
• History of contralateral patellar dislocation

Over 90% of patellar dislocations result from a twisting injury when the knee is simultaneously in flexion and under valgus strain, often during an activity such as sports or dance, rather than from direct traumatic contact. ^[4][6]

• Severe pain
• Knee joint effusion ^[7][8]
• Restricted range of motion and instability in the knee ^[5]
• Feeling of knee moving or giving way after injury ^[5]
• Dislocation of the patella, which is almost always lateral ^[4]
• Ongoing feeling of instability
• Positive lateral patellar apprehension test: indicates lateral patellar instability ^[9][10]
  • Relax the knee in 30 degrees of flexion.
  • Apply lateral force to the medial patella.
  • Positive test: the maneuver produces discomfort or apprehension (either expressed by the patient or evidenced by contraction of the quadriceps)
• Anatomical risk factors for patellar dislocation

Congenital (fixed) patellar dislocations ^[3]

• Epidemiology: very rare
• Clinical features
  • Patella is permanently displaced laterally and cannot be manually reduced.
  • Typically associated with:
    • Foot abnormalities
    • External rotation of the tibia
    • Flexion contracture of the knee
    • Genu valgum
  • May be associated with other congenital conditions (e.g., arthrogryposis)
• Diagnostics: imaging (typically, MRI)
• Treatment: surgical correction

For the approach to a painful swollen knee of unclear etiology, see “Acute internal knee derangement.”

Clinical evaluation ^[2][6]

• Complete knee examination to assess for:
  • Additional injuries (e.g., knee ligament injuries)
  • Anatomical risk factors for patellar dislocation
  • Neurovascular injury
• Aspiration of large effusions, which may reveal:
  • Hemarthrosis: suggests intraarticular soft tissue injury and/or osteochondral fracture
  • Lipohemarthrosis: suggests osteochondral fracture

Imaging ^[2][6][11]

Full knee x-ray series

• Indications
  • Positive Ottawa knee rules criteria
  • Effusion
  • Recurrent patellar instability
• Findings may include:
  • Osteochondral fractures
  • Location and degree of displacement
  • Evidence of bony anatomical factors for dislocation (e.g., a high-riding patella, patellar tilt, trochlear dysplasia). ^[2]

Lipohemarthrosis in the presence of normal knee x-rays suggests an underlying osteochondral fracture. ^[5][12]

Additional imaging

• Indications
  • Fracture on x-ray
  • Hemarthrosis on joint aspiration
  • Plan for operative management
  • Recurrent dislocation
• Modalities: CT and/or MRI

There are currently no consensus guidelines for the management of patellar dislocation. The decision to proceed with conservative versus surgical management may vary based on patient and provider preferences. ^[2][4]

Conservative management ^[1][2][4]

The patella often relocates spontaneously, making manual reduction unnecessary. ^[1]

Indications

• Initial dislocation without associated osteochondral fractures or significant soft tissue injuries
• Recurrent dislocation if patient prefers nonsurgical treatment ^[2]

Methods

• Manual reduction of the patella ^[5]
  • Provide adequate pain management; consider procedural sedation.
  • Fully extend the knee.
  • Push the patella medially to guide it back into the femoral trochlea.
• Pain management
• Immobilization for up to 6 weeks with weight-bearing as tolerated. ^[4][6]
• Physical therapy to improve knee range of motion and strengthen the quadriceps

Surgery ^[1][2][4]

• Indications ^[4]
  • Associated osteochondral fractures
  • Significant injury to medial stabilizing structures of the knee
  • Failure of conservative management
  • Recurrent dislocations
• Methods
  • Arthroscopy: in some patients, may be used to repair intra-articular loose bodies and some osteochondral fractures ^[1]
  • Surgery to stabilize the patella and correct anatomical risk factors for patellar dislocation, e.g.:
    • Repair or reconstruction of the medial patellofemoral ligament
    • Tibial tubercle transfer ^[2]
    • Trochleoplasty (in patients with trochlear dysplasia) ^[13]

• Additional injuries associated with acute patellar dislocation
  • Tear in the medial patellar retinaculum
  • A small avulsion fracture of the lateral femoral condyle or patella
• Recurrent patellar dislocation → osteoarthritis

We list the most important complications. The selection is not exhaustive.