Patellar instability
Lateral dislocation or subluxation of the kneecap (patella) out of its groove (trochlea), causing pain and giving way. Most commonly affects adolescent females and is associated with trochlear dysplasia and other anatomical risk factors.
📊 Acute patellar dislocation has an incidence of approximately 29 per 100,000 per year, with the highest rates in adolescent females. Recurrence after a first dislocation is approximately 30-50% overall and higher in those with anatomical risk factors.
What is patellar instability?
Patellar instability occurs when the patella dislocates or subluxes laterally out of the trochlear groove. The medial patellofemoral ligament (MPFL) is the primary soft tissue restraint preventing lateral patellar dislocation. It is torn in virtually every acute patellar dislocation. The medial retinaculum may also be stretched or torn, and osteochondral fragments are dislodged from the medial patellar facet or lateral femoral condyle in up to 20% of acute dislocations.
Multiple anatomical risk factors predispose to patellar instability: trochlear dysplasia (a flat or convex trochlear groove), patella alta (a high-riding patella), increased tibial tubercle-trochlear groove (TT-TG) distance (indicating lateral displacement of the patellar tendon attachment), and femoral anteversion or external tibial torsion. Assessment of these factors using CT and MRI guides the choice of surgical procedure.
The Dejour classification of trochlear dysplasia (Types A-D) grades the severity of trochlear abnormality and influences surgical decision-making. In patients without significant anatomical risk factors, MPFL reconstruction alone achieves good results. In those with high-grade trochlear dysplasia or elevated TT-TG distance, additional procedures (trochleoplasty, tibial tubercle transfer) may be required to achieve reliable stability.
Common causes
- Trochlear dysplasia (the most important anatomical risk factor)
- Patella alta (high-riding patella)
- Increased TT-TG distance (lateral displacement of tibial tubercle)
- Ligamentous hyperlaxity
- Direct blow to the medial patella causing lateral dislocation
- Indirect mechanism (twisting with the knee slightly bent)
Who is at risk? Adolescent females, trochlear dysplasia, patella alta, generalised ligamentous hyperlaxity, and previous dislocation are the main risk factors for recurrence.
Symptoms
Symptoms vary depending on the severity and duration of the condition. Common symptoms include:
- A sensation of the kneecap slipping out and returning to position (subluxation)
- Complete dislocation requiring manipulation to reduce the kneecap
- Pain and swelling (haemarthrosis) after dislocation
- Giving way episodes during sport or walking
- Anterior knee pain between episodes
- Apprehension when the patella is pushed laterally on examination
When to seek help: Attend A&E after an acute patellar dislocation. Imaging is required to exclude osteochondral fracture. Referral to an orthopaedic knee surgeon is appropriate if there is recurrent instability.
How is it diagnosed?
Your surgeon will take a detailed history and examine the joint. The following investigations may be arranged to confirm the diagnosis:
- X-ray - AP, lateral, and skyline views; lateral view assesses patellar height (Caton-Deschamps index)
- MRI - assesses MPFL tear, osteochondral injury, trochlear morphology, and TT-TG distance
- CT scan - accurate measurement of TT-TG distance, trochlear depth, and torsional alignment
- Clinical examination - patellar apprehension test, J-sign
CT measurement of the TT-TG distance is essential before surgery. A TT-TG over 20mm is an indication for tibial tubercle medialization. Trochlear dysplasia classification requires careful CT and MRI assessment and influences the surgical procedure selected.
Treatment pathway
Treatment is tailored to the severity of the condition, your age, activity level, and overall health. Most conditions are treated in a stepwise fashion, starting with the least invasive options.
Physiotherapy and bracing
After a first acute dislocation without significant osteochondral injury, a structured VMO and hip strengthening physiotherapy programme gives reasonable results. Patellar stabilising brace for return to sport. Surgery after a first dislocation is considered when there is a large osteochondral fragment requiring fixation.
MPFL reconstruction
Reconstruction of the medial patellofemoral ligament using a tendon graft (gracilis or semitendinosus). The primary procedure for recurrent patellar instability without significant trochlear dysplasia or elevated TT-TG. Achieves excellent stability in appropriately selected patients.
Tibial tubercle transfer or trochleoplasty
Tibial tubercle transfer (Fulkerson osteotomy) mediates the patellar tendon attachment to reduce the TT-TG distance. Trochleoplasty deepens a dysplastic trochlear groove. May be combined with MPFL reconstruction for highest-risk anatomical configurations.
Recovery
Recovery after MPFL reconstruction requires a structured rehabilitation programme. Sport-specific training begins at 3-4 months and return to competitive sport at 6-9 months, guided by functional testing.
- After MPFL reconstruction: crutches: 2-4 weeks
- Return to straight-line sport: 3-4 months
- Return to pivoting sport: 6-9 months
What results can I expect?
MPFL reconstruction achieves excellent stability in carefully selected patients with recurrence rates below 5-10%. Outcomes are less predictable in patients with high-grade trochlear dysplasia who require additional procedures.
Patellar instability - MPFL reconstruction explained
In numbers
Typical activity timelines for this condition. These are approximate and vary considerably between patients. Always follow the specific guidance given by your surgeon and physiotherapist.
| Activity | Typical timeline | Notes |
|---|---|---|
| Brace and rehab after first dislocation | 6-12 weeks | First-time dislocations are usually managed non-operatively with bracing and structured physiotherapy.[1] |
| Drive after MPFL reconstruction | 4-6 weeks | When off crutches and the brace permits safe control of the vehicle and an emergency stop. Sooner for left knee in an automatic.[2] |
| Return to work | 2-12 weeks | Desk work: 2-4 weeks. Standing or manual work: 6-12 weeks.[1] |
| Swim and cycle | 6-8 weeks after surgery | Once the brace is off and the wound has healed.[1] |
| Straight-line running | 3-4 months after surgery | After MPFL reconstruction, once strength and brace-free control are restored.[1] |
| Return to pivoting sport | 6-9 months | Guided by functional testing, hop tests, and quadriceps strength symmetry.[1] |
Common concerns from patients with this condition or recovering from treatment, and whether they are expected or worth mentioning to your team.
Your questions, answered
Plain-English answers to the things people most often ask, grounded in UK clinical guidance. Tap a question to open it.
About thisWhat is patellar instability?
Patellar instability is when the kneecap slips partly or fully out of its groove, usually towards the outer side of the knee. It can follow a one-off injury or happen repeatedly, and is more common in younger, active people and where the anatomy makes the kneecap less well held in place.
SymptomsWhat does a dislocation feel like?
People often feel the kneecap shift or pop out, with sudden pain and the knee giving way, followed by swelling. Even after it goes back, the knee can feel unstable and apprehensive, particularly when twisting or straightening from a bent position.
TreatmentWill I need an operation?
Not usually after a first dislocation, which is generally treated with physiotherapy, sometimes a brace, and a graded return to activity. Surgery, often to reconstruct the ligament that tethers the kneecap, is considered for recurrent dislocations or where there is a clear structural cause.
RecoveryHow long is recovery?
After a first dislocation, rehabilitation to restore strength and control usually takes several weeks to a few months. After stabilising surgery, recovery is more structured and return to sport typically takes several months, guided by regaining strength and confidence.
OutlookWill it keep happening?
The risk of a further dislocation is higher after the first one, especially in younger people and those with predisposing anatomy. Good rehabilitation reduces the risk, and surgery is effective at preventing recurrence when the instability keeps coming back.
When to worryWhat needs prompt review?
A kneecap that stays out of place, a knee that locks or cannot bear weight, or marked swelling after the injury should be assessed promptly, as there can be an associated cartilage injury.
- 1.British Association for Surgery of the Knee: Patient information
- 2.Royal College of Surgeons of England: Recovering from surgery, driving guidance
References are to UK clinical guidance and patient information from recognised organisations. This page is for general information and does not replace personalised advice from your own clinical team.
Preparing for surgery?
Read our step-by-step guide - what to expect before, during, and after your procedure.
How is it diagnosed?
- ✓X-ray - AP, lateral, and skyline views; lateral view assesses patellar height (Caton-Deschamps index)
- ✓MRI - assesses MPFL tear, osteochondral injury, trochlear morphology, and TT-TG distance
- ✓CT scan - accurate measurement of TT-TG distance, trochlear depth, and torsional alignment
- ✓Clinical examination - patellar apprehension test, J-sign
Recovery milestones
- ●After MPFL reconstruction: crutches: 2-4 weeks
- ●Return to straight-line sport: 3-4 months
- ●Return to pivoting sport: 6-9 months