Patellar Dislocation and MPFL Tear: Causes, Symptoms, and Treatment

What Is Patellar Dislocation?

A patellar dislocation occurs when the kneecap (patella) slides completely out of the trochlear groove on the front of the femur. In nearly every case, the patella displaces laterally — toward the outside of the knee — and then either spontaneously reduces (pops back in) or requires manual reduction.

The patella sits inside the quadriceps mechanism and glides up and down in a shallow bony channel called the trochlea. Several structures keep it centered in the groove, but the most important soft-tissue restraint against lateral dislocation is the medial patellofemoral ligament (MPFL).

What Is the MPFL?

The medial patellofemoral ligament is a thin but critical ligament that runs from the inside edge of the patella to a point on the medial femoral condyle (the inside of the thigh bone just above the knee). It provides approximately 50–60% of the total restraining force preventing the patella from shifting laterally.

When the kneecap dislocates laterally, the MPFL is torn in virtually every case — studies using MRI confirm MPFL injury in over 90% of first-time patellar dislocations.

How Patellar Dislocations Happen

Patellar dislocations are most common in adolescents and young adults, particularly females. Peak incidence is in patients aged 15–19.

Typical mechanisms:

Pivoting or twisting on a planted foot — the quadriceps contracts forcefully while the knee is slightly flexed and the tibia externally rotated
Direct blow to the inside of the kneecap, forcing it laterally
Non-contact deceleration and cutting — similar to ACL injury mechanisms

Anatomic risk factors make some individuals predisposed to dislocation:

Trochlear dysplasia: A shallow or flat trochlear groove provides less bony constraint
Patella alta: A kneecap that sits abnormally high, reducing the distance it engages the groove during flexion
Increased Q-angle / valgus alignment: Wider hips or knock-knee alignment increase the lateral pull on the patella
Ligamentous laxity: Generalized hypermobility (common in young females)
Lateral patellar tilt or subluxation: Chronic maltracking

Symptoms

At the time of dislocation:

Visible deformity — the kneecap is visibly displaced to the outside of the knee (may reduce before the patient reaches the emergency department)
Severe pain, particularly along the medial (inside) aspect of the knee where the MPFL tears
Immediate, significant swelling (hemarthrosis — blood in the joint from the torn MPFL and possible osteochondral injury)
Inability to bear weight or bend the knee
A sensation of the knee "giving out"

After reduction:

Tenderness along the medial border of the patella and medial femoral condyle (the MPFL attachment sites)
Apprehension — anxiety or guarding when the kneecap is pushed laterally during examination
Difficulty regaining quadriceps control
Persistent swelling for several weeks

Diagnosis

Physical examination:

Patellar apprehension test: The examiner pushes the patella laterally with the knee slightly flexed — a positive test produces visible anxiety or guarding and is highly specific for patellar instability
J-sign: The patella visibly jumps laterally as the knee moves from flexion to full extension
Assessment of patellar tracking, tilt, and mobility
Evaluation of overall limb alignment and ligamentous laxity

Imaging:

X-ray: Rules out fracture and may show a laterally displaced or tilted patella; look for osteochondral loose bodies (a piece of cartilage and bone can shear off during dislocation in up to 25% of cases)
MRI: The essential imaging study — confirms MPFL tear location (patellar attachment, midsubstance, or femoral attachment), identifies osteochondral injury, evaluates trochlear morphology, and assesses cartilage damage
CT scan: Sometimes ordered to measure tibial tubercle–trochlear groove (TT-TG) distance and assess trochlear dysplasia for surgical planning

Recurrence Risk

Patellar dislocation recurrence rates are among the highest in orthopedics:

First-time dislocators: 15–45% recurrence rate with non-surgical treatment
After a second dislocation: Recurrence rate rises to 50% or higher
Patients with anatomic risk factors (trochlear dysplasia, patella alta, increased TT-TG distance): Even higher recurrence
Young patients (<25 years): Higher recurrence than older patients

Each subsequent dislocation causes cumulative cartilage damage and increases the risk of early-onset patellofemoral arthritis.

Treatment Options

Non-Surgical Treatment (First-Time Dislocation Without Loose Bodies)

Initial management of a first-time patellar dislocation is usually conservative:

Immobilization: Knee brace locked in extension or a patellar stabilizing brace for 2–4 weeks
Protected weight bearing: Crutches as needed
Physical therapy: Emphasis on quadriceps strengthening (especially vastus medialis obliquus — VMO), hip strengthening, and neuromuscular control
Graduated return to activity: Typically 6–12 weeks

Surgical Treatment

Surgery is recommended for:

Osteochondral loose body requiring fixation or removal
Recurrent dislocations (two or more episodes)
Significant anatomic risk factors predisposing to further dislocation
High-demand athletes after first-time dislocation (increasingly supported by evidence, though still debated)

The primary surgical procedure is MPFL reconstruction, which restores the torn ligament. Additional procedures may be performed to correct bony anatomy when needed.