Patellar Dislocation — MPFL and Patellar Instability

What happens

The Patella Has Dislocated — Why?

The patella (kneecap) normally glides within a groove in the femur called the trochlea. Dislocation occurs when the patella moves out of that groove — almost always laterally — through a flexion-rotation mechanism of the knee, direct trauma or a sudden muscle contraction.

The medial patellofemoral ligament (MPFL) is the main restraint to lateral translation of the patella. In a first dislocation, the MPFL tears in 94–100% of cases. This tear is the reason the risk of recurrence exists and why specialist assessment is essential.

Dislocation is more common in adolescents and young adults, particularly females (a wider pelvis creates a lateral pull vector). Anatomical factors such as trochlear dysplasia, patella alta or a lateral insertion of the patellar tendon increase the risk.

In a first dislocation, the MPFL tears in almost every case. Without adequate stabilisation — surgical or through muscle strengthening — the risk of a further dislocation is real and cumulative.

Who Has the Greatest Risk of Recurrence?

Stratifying the individual risk of recurrence is the most important factor in the treatment decision after a first dislocation.

Trochlear Dysplasia

A shallower trochlea does not "guide" the patella. The anatomical factor with the greatest impact on the risk of recurrence.

Non-modifiable

Patella Alta

A patella positioned above the trochlea — less engagement, greater instability. Measured by the Caton-Deschamps index.

Non-modifiable (without surgery)

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Increased TT-TG

Tibial tuberosity–trochlear groove distance >20mm. Indicates lateralisation of the patellar tendon insertion.

Correctable surgically

Young Age (<25 years)

Young people have a significantly higher risk of recurrence. Especially in the skeletally immature.

Non-modifiable

Female Sex

A wider pelvis creates a greater Q angle (valgus) — more lateral pull on the patella.

Non-modifiable

Muscle Weakness

A weak quadriceps and vastus medialis obliquus reduce dynamic control of the patella.

Modifiable — physiotherapy

First Dislocation — Conservative or Surgery?

The ESSKA 2024 Consensus for a first patellar dislocation recommends an individualised approach based on the risk of recurrence. Surgery is increasingly considered in young patients with anatomical risk factors.

✓ Conservative Treatment (first line)

First dislocation without significant osteochondral injury
Low risk of recurrence (no trochlear dysplasia, normal TT-TG)
Adult with low sporting demand
Immobilisation 2–4 weeks + physiotherapy
Intensive strengthening of the VMO and quadriceps
Patellar stabilisation brace

→ Surgery (consider)

Recurrent dislocation (2nd or more dislocations)
First dislocation with osteochondral injury ≥1 cm²
Young patient with multiple anatomical risk factors
Residual instability after correct physiotherapy
Skeletally immature with severe trochlear dysplasia
High-performance athlete needing a rapid return

ESSKA 2024: MPFL reconstruction is the surgical technique of choice for soft-tissue stabilisation — with superior results to simple repair. In cases with severe trochlear dysplasia or a markedly increased TT-TG, additional bony procedures may be required (tibial tuberosity osteotomy, trochleoplasty).

MPFL Reconstruction — The Standard Procedure

When surgery is indicated, reconstruction of the medial patellofemoral ligament (MPFL) is the reference procedure to stabilise the patella.

Pre-operative Assessment

X-ray (TT-TG, Caton-Deschamps), MRI (MPFL quality, chondral injuries), assessment of trochlear dysplasia. Plan whether additional bony procedures are required.

MPFL Reconstruction

A graft (autologous gracilis or allograft) fixed at the isometric femoral point and on the medial border of the patella. Precise tensioning — avoid over-constraint.

Postoperative Rehabilitation

Immediate weight-bearing with a brace. Progressive strengthening of the quadriceps/VMO. Return to sport ~4–6 months after confirming adequate stability and strength.

Questions about Patellar Dislocation

Not necessarily. Most first dislocations are treated conservatively: short immobilisation, intensive physiotherapy and a brace. Surgery is considered if there are significant chondral injuries, multiple anatomical risk factors, or if the instability persists after correct physiotherapy.

After a first dislocation, the risk of recurrence is 15–44% without surgical treatment. It is significantly higher in young people, patients with trochlear dysplasia, increased TT-TG or patella alta. Stratifying this risk is the most important step in the treatment decision.

The medial patellofemoral ligament (MPFL) is the main static restraint to lateral translation of the patella. In a dislocation, it tears in 94–100% of cases. Without an intact MPFL, the patella is more vulnerable to further dislocations. Its surgical reconstruction is the reference procedure when surgery is indicated.

Yes — return to sport is typically between 4 and 6 months, after confirming patellar stability, adequate muscle strength and the absence of subjective instability. The results of MPFL reconstruction are consistently good, with low recurrence rates when the technique and the indication are correct.

Patellar Dislocation — What to do?

The Patella Has Dislocated — Why?

Who Has the Greatest Risk of Recurrence?

Trochlear Dysplasia

Patella Alta

Increased TT-TG

Young Age (<25 years)

Female Sex

Muscle Weakness

First Dislocation — Conservative or Surgery?

✓ Conservative Treatment (first line)

→ Surgery (consider)

MPFL Reconstruction — The Standard Procedure

Pre-operative Assessment

MPFL Reconstruction

Postoperative Rehabilitation

Questions about Patellar Dislocation

Dr. Nuno Camelo Barbosa

Has your patella dislocated?