Knee

Patellar Instability / Dislocation

Patellar instability, ranging from subluxation to complete dislocation, is one of the most common knee injuries in adolescents and young adults. The patella almost always dislocates laterally, and the medial patellofemoral ligament (MPFL) is torn in virtually every acute dislocation event. While most first-time dislocations are treated non-operatively, patients with underlying anatomical risk factors face redislocation rates exceeding 50%, making careful anatomical assessment essential. At Maryland Orthopedic Specialists, our sports medicine surgeons use advanced imaging to guide individualized, anatomy-based treatment: from structured rehabilitation programs to MPFL reconstruction and bony realignment procedures.

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What is patellar instability / dislocation?

The patella glides within the trochlear groove of the femur, maintained centrally by a balance of static restraints (ligaments) and dynamic restraints (quadriceps muscles). The medial patellofemoral ligament (MPFL) is the primary medial restraint, providing 50–60% of resistance to lateral patellar displacement.

The patella glides within the trochlear groove of the femur, maintained centrally by a balance of static restraints (ligaments) and dynamic restraints (quadriceps muscles). The MPFL is the primary medial restraint, providing 50–60% of resistance to lateral patellar displacement.

When the patella dislocates laterally, the MPFL tears (typically from its femoral attachment, the medial epicondyle region) and medial retinacular structures are injured. Osteochondral fractures from impaction of the lateral femoral condyle or medial patellar facet occur in up to 40% of first dislocations and must be identified.

Anatomical risk factors for instability:

Trochlear dysplasia: Shallow or flat trochlea fails to provide osseous constraint; the most powerful predictor of recurrence
Patella alta: High-riding patella exits the trochlea in early flexion, losing groove stability
TT-TG distance > 20 mm: Lateralized tibial tubercle increases lateral patellar pull vector
Femoral anteversion / genu valgum: Alters extensor mechanism alignment
Ligamentous laxity (generalized): Reduces soft-tissue restraint across the entire knee

The J-sign (a lateral J-shaped jump of the patella as the knee approaches full extension) reflects a high-riding or laterally maltracked patella exiting the trochlea.

Symptoms — do you recognize these?

**Acute Dislocation**

Sharp medial knee pain and visible lateral displacement of the patella at time of injury
Patella typically reduces spontaneously when the knee is straightened
Immediate swelling (hemarthrosis from MPFL tear and potential osteochondral fracture)
Profound apprehension and inability to weight-bear
Tenderness along the medial patellar border and femoral MPFL origin
Episodes of patella "slipping out" or "giving way" with pivoting, jumping, or ascending/descending stairs
Anterior knee pain between episodes from patellofemoral articular cartilage overload
Positive patellar apprehension test — patient reflexively resists lateral patellar translation as the examiner applies gentle lateral pressure with the knee at 0–30° flexion
J-sign on dynamic examination

Call us at (301) 515-0900. Same-day appointments available at our Bethesda and Germantown locations.

How we diagnose it

Physical Examination

Patellar apprehension test: lateral translation at 30° flexion
J-sign (dynamic tracking)
Glide and tilt tests
Assessment of TT-TG alignment clinically (Q-angle approximation)
MPFL tenderness at femoral origin (medial epicondyle) and patellar insertion

Imaging

X-ray: AP, lateral, and Merchant (axial) views; identifies trochlear dysplasia (crossing sign, supratrochlear spur), patella alta (Caton-Deschamps ratio > 1.2), osteochondral fractures, and prior bony injury
CT scan — gold standard for TT-TG distance measurement (tibial tubercle to trochlear groove; normal < 15 mm; > 20 mm indicates bony malalignment); also quantifies trochlear dysplasia morphology and patellar height
MRI — identifies MPFL tear (location and quality), osteochondral fractures, medial retinacular injury, lateral femoral condyle "kissing contusion" (bone bruise from patellar impaction), and meniscal or ligamentous co-injuries; MPFL tear is visible in > 90% of acute dislocations

Treatment options

Treatment is guided by whether this is a first-time dislocation or recurrent instability, and by the presence of anatomical risk factors that predict a high likelihood of redislocation.

Non-Operative Management

Most first-time patellar dislocations without large displaced osteochondral fragments are managed non-operatively, with redislocation rates of approximately 15–44% depending on patient age and the severity of underlying anatomical risk factors. Initial treatment uses a knee immobilizer for 3–6 weeks to control pain and allow the torn MPFL and medial retinacular structures to scar. Structured physical therapy then targets strengthening of the VMO (the most medially directed quadriceps head) along with hip abductor and gluteal strengthening to optimize proximal alignment of the extensor mechanism. Patellar taping and a patellofemoral stabilizing brace support the return to activity during the later phases of rehabilitation.

Chondral Injury Management

Osteochondral fractures occur in up to 40% of first-time dislocations from impaction of the lateral femoral condyle and medial patellar facet; when identified, these are addressed concurrently at the time of surgical stabilization through fragment fixation or removal depending on fragment size and viability.

Surgical Procedure

MPFL Reconstruction (Patellar Instability)

Reconstruction of the medial patellofemoral ligament (the primary restraint against lateral patellar dislocation) using an anatomically placed graft carefully tensioned to restore normal patellar tracking without over-constraining the joint.

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Tibial Tubercle Osteotomy (TTO)

Surgical repositioning of the tibial tubercle to correct abnormal patellofemoral alignment and reduce lateral patellar tilt. Performed as a standalone procedure or combined with MPFL reconstruction to address both the bony and soft-tissue causes of instability.

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Recovery & rehabilitation

Non-operative: 6–12 weeks for return to sport; VMO-focused PT throughout
MPFL Reconstruction: 3–4 months return to training; 6 months return to competitive sport; graft healing requires approximately 6 months
TTO + MPFL: Bone healing of the osteotomy requires 6–8 weeks of protected weight-bearing; full sport return at 6–9 months

Frequently Asked Questions

What is the TT-TG distance and why does it matter?

The TT-TG distance measures how far the tibial tubercle (patellar tendon insertion) is lateralized relative to the deepest point of the trochlear groove. A large TT-TG (> 20 mm) pulls the patella laterally, predisposing it to dislocation. When elevated, a tibial tubercle osteotomy to medialized the insertion point is often required alongside MPFL reconstruction.

Can a first-time dislocation be treated with surgery?

Surgery for a first-time dislocation is generally reserved for patients with displaced osteochondral fractures requiring fixation, or in high-risk patients (young athletes with severe anatomical risk factors) after shared decision-making. Most first-time dislocations are treated non-operatively.

What is the redislocation rate after MPFL reconstruction?

Published series report redislocation rates of 1–5% after isolated MPFL reconstruction in appropriate candidates, compared to 15–44% with non-operative treatment. Outcomes are best when anatomical risk factors (high TT-TG, patella alta) are concurrently addressed.

Does patellar instability cause arthritis?

Recurrent dislocation damages the medial patellar and lateral femoral condyle articular cartilage through repeated impaction, gradually increasing the risk of patellofemoral arthritis. Stabilizing the patella early reduces this long-term risk.

How long does recovery take after MPFL reconstruction, and when can I return to sport?

Recovery after medial patellofemoral ligament (MPFL) reconstruction typically takes six to nine months before return to competitive sport. The first six weeks focus on swelling control, restoring range of motion, and early quadriceps activation. Progressive strengthening of the quadriceps and hip stabilizers follows, with sport-specific training beginning around four to five months. Your MOS surgeon will use functional and strength benchmarks, not just time, to determine when it is safe to return to cutting, jumping, and pivoting activities.