Patellar dislocation usually refers to lateral displacement of the patella out of its normal alignment in the trochlear groove of the femur.
The patella is held stable by ligamentous forces, muscular forces, and bony anatomy.
Disruption of any of these 3 components can lead to recurrent patellar instability.
Subluxation: Patella sits on the edge of the femoral groove, but not out of the track.
Dislocation: Patella is completely displaced out of the patellofemoral groove, usually laterally.
- Seen primarily in young patients
- Occurs more often in females than in males
- The incidence is difficult to quantitate because many knees relocate spontaneously and are misdiagnosed.
- An incidence of a positive family history is noted in some cases
- Positive family history
- Participation in football, basketball, baseball, gymnastics, or dancing
- Age 10 to18 years
- High level of activity or competition in a youth
- Mechanism other than a direct blow
- Hypermobility of the patella
- Previous dislocations
- Patella alta
- Shallow patellofemoral groove
- Excessive Q angle
- Ligamentous laxity
- Excessive femoral ante version
- Vastus medialis dysplasia
- Excessive genu valgus
A congenital predisposition to knee malalignment and propensity to patellar dislocation is thought to exist.
- Patellar dislocation can disrupt the medial patellofemoral ligament.
- Chondral injury or fracture can occur from the impact of the patella on the trochlea
- A direct blow to the medial aspect of the patella
- Severe valgus injury to the knee
- Twisting injury or other minor trauma, usually associated with congenital deficiencies
- Connective tissue disease with ligamentous laxity, such as Ehlers-Danlos and Marfan syndromes
- Femoral anteversion and pes planus
Signs and Symptoms
- Patients with acute dislocation may present with the knee held in a flexed position as a result of hamstring spasms.
- The femoral condyles may be prominent medially.
- Often, the patella has spontaneously reduced, with the following findings:
o Diffuse parapatellar tenderness
o Positive apprehension test
o Palpable defect at the insertion of the vastus medialis muscle
- Direct blow to the knee
- Twisting injury to the leg
- After the acute symptoms subside, examine the knee for the following:
Ø Apprehension, with patellar translation both medially and laterally
Ø Lateral tracking of the patella (in the shape of a â€œJâ€) with the knee extended from a flexed position (Fig. 1)
Ø Injury to the medial, collateral, or cruciate knee ligaments
Ø Lateral tilt
- Postreduction plain radiographs are obtained for evidence of osteochondral fragments.
- Axial views of the bilateral patella may show substantial lateral tracking.
- MRI or CT can reveal osteochondral injury and rupture of the medial patellofemoral ligament
- Abnormalities in the patellofemoral articulation allow the pull of the vastus lateralis and lateral retinaculum muscles to overcome that of the vastus medialis, even during minor trauma.
- This unbalanced pulling often tears the medial retinaculum and vastus medialis insertion.
- Cruciate ligament injury
- Patellar fracture
- Patellofemoral pain syndrome
- Osteochondral fracture
- Reduce acute dislocations and then immobilize the knee.
- Reduce acute dislocation by gentle, steady extension of the knee, facilitated with the patient prone and the patient's hip extended to relax the hamstrings.
- Avoid forceful manipulation.
- Once the knee is reduced, immobilize it in extension with a compression dressing.
- Evaluate the medial retinacular structure for tenderness every 2 weeks for up to 6 weeks.
- When the patient is comfortable, apply a Neoprene sleeve with a laterally based felt pad.
- Surgical stabilization is recommended for the following:
- Recurrent dislocations
- Dislocations in carefully selected, highly active, competitive athletes
- Acute dislocations with avulsive detachment of the vastus medialis muscle by bony fragment, seen on radiographs
- Patients may bear weight after relocation.
- Crutches should be supplied to facilitate weight bearing as tolerated.
- Twisting motions should be avoided.
- The main goal of therapy is to strengthen the injured extensor mechanism and to improve patellofemoral tracking.
- Straight-leg raises may begin immediately with appropriate support.
- Patients with recurrent dislocations often advance more quickly with physical therapy for quadriceps strengthening
Treatment may comprise:
- soft tissue massage
- the use of crutches
- the use of a knee immobilisation brace
- patella taping or bracing to correct patella position
- dry needling
- ice or heat treatment
- progressive exercises to improve flexibility, balance and strength (especially the VMO muscle)
- activity modification advice
- biomechanical correction
- anti-inflammatory advice
- the use of Real-Time Ultrasound to assess and retrain the VMO muscle
- a gradual return to activity program