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Patellar Dislocation – Causes, Symptoms & Treatment

Patellar dislocation occurs when the kneecap slips out of its normal groove. It is common in sports and causes sudden, severe knee pain and instability.

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Patellar dislocation occurs when the kneecap slips out of its normal groove. It is common in sports and causes sudden, severe knee pain and instability.

What is a Patellar Dislocation?

A patellar dislocation occurs when the kneecap (patella) is displaced completely or partially out of its normal groove on the thighbone (femur), known as the trochlear groove. The patella is a small, triangular bone that plays a key role in knee extension. In most cases, it dislocates toward the outside of the knee (laterally). A partial displacement is called a subluxation. Patellar dislocation is especially common in young, physically active individuals and can lead to significant pain and joint instability.

Causes

Patellar dislocation can result from a combination of traumatic events and predisposing anatomical factors:

Traumatic causes: A direct blow to the kneecap or a sudden twisting movement of the knee, often during sports activities.
Anatomical risk factors: A shallow trochlear groove (trochlear dysplasia), a high-riding kneecap (patella alta), or an increased Q-angle (altered leg alignment) significantly increase the risk.
Muscle imbalances: Weakness of the inner thigh muscle (vastus medialis obliquus) reduces kneecap stability.
Hypermobility: Generalized ligamentous laxity or connective tissue disorders can predispose individuals to dislocation.
Genetic predisposition: Anatomical variants that increase risk can run in families.

Symptoms

Common signs and symptoms of patellar dislocation include:

Sudden, severe pain in the knee, typically during physical activity
Visible deformity of the knee, with the kneecap displaced to the outside
Inability to straighten the knee or bear weight
Swelling of the knee joint due to bleeding into the joint space (hemarthrosis)
Feeling of instability or giving way of the knee
Tenderness along the inner edge of the kneecap, where the medial patellofemoral ligament (MPFL) is located

Diagnosis

Diagnosis is typically made through a combination of clinical examination and imaging:

Physical Examination

The physician assesses kneecap mobility using the patellar apprehension test, evaluates leg alignment, and checks for tenderness along the MPFL. A thorough musculoskeletal assessment is performed to identify underlying risk factors.

Imaging

X-ray: Used to rule out associated bone fragments (osteochondral fractures) and to assess kneecap position.
MRI (Magnetic Resonance Imaging): Provides detailed information about ligament injuries (especially the MPFL), cartilage damage, and other soft tissue structures.
CT (Computed Tomography): Used to evaluate bony anatomy in detail, particularly in cases of suspected trochlear dysplasia or malalignment.

Treatment

Conservative Treatment

For first-time dislocations without significant associated injuries, conservative management is often the first choice:

Reduction: Repositioning the kneecap back into its groove, usually performed in the emergency department.
Immobilization: Short-term use of a brace or splint to protect injured structures.
Physiotherapy: Targeted strengthening of the inner quadriceps muscle (VMO) and general knee stabilization exercises.
Pain management: Anti-inflammatory medications (NSAIDs) and ice application to reduce swelling and pain.

Surgical Treatment

Surgery is recommended in cases of:

Associated cartilage or bone injuries (osteochondral fractures)
Recurrent dislocations (habitual or chronic patellar instability)
Significant anatomical abnormalities (trochlear dysplasia, patella alta)

Common surgical procedures include MPFL reconstruction (rebuilding the medial patellofemoral ligament), trochleoplasty (deepening the trochlear groove), and tibial tubercle osteotomy (realigning the attachment point of the patellar tendon).

Prognosis and Prevention

After a first-time dislocation, the risk of recurrence is up to 60% in children and adolescents. Regular strength training, the use of appropriate knee braces during sports, and early treatment of anatomical risk factors can significantly reduce the likelihood of re-dislocation. A structured rehabilitation program is essential for a full return to sport and everyday activities.

References

Petri M. et al. – Patellar Dislocation: Diagnosis and Treatment. Der Unfallchirurg, 2017.
Stefancin J.J., Parker R.D. – First-time traumatic patellar dislocation: a systematic review. Clinical Orthopaedics and Related Research, 2007.
Dejour H., Walch G. et al. – Factors of patellar instability: an anatomic radiographic study. Knee Surgery, Sports Traumatology, Arthroscopy, 1994.

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