When the kneecap won't stay in place

Patellar instability means the kneecap (patella) slips partly or fully out of its groove at the front of the thighbone. A first dislocation is painful and frightening. For many people it does not stay a one-off: once the soft tissues that hold the kneecap in place have been stretched or torn, it can slip out again and again.

The good news is that patellar instability is very treatable. Many people do well with targeted physiotherapy. Those who keep dislocating can have their knee stabilised with modern surgery tailored to the exact reason their kneecap is unstable.

This guide explains why the kneecap dislocates, how instability is diagnosed, and how it is treated, from rehab and bracing to MPFL reconstruction and bony realignment procedures. It also sets out recovery timelines and what to ask your surgeon.

How the kneecap stays on track

As the knee bends and straightens, the kneecap glides up and down in a groove at the end of the thighbone called the trochlea. Three things keep it on track:

Bony shape: a deep trochlear groove cradles the kneecap, especially once the knee bends past about 30°.
Soft-tissue restraints: the most important is the medial patellofemoral ligament (MPFL), a band on the inner side of the knee that stops the kneecap sliding outwards. On the outer side, the lateral retinaculum pulls the other way.
Muscle control: the quadriceps, especially the inner part (VMO), along with the hip and core muscles, steer the kneecap and control knee alignment.

Almost all dislocations happen towards the outside of the knee. In nearly every first dislocation, the MPFL is torn or stretched. That is why repairing or rebuilding this ligament is central to modern treatment.

Why some kneecaps keep dislocating

A dislocation usually happens during a twist on a planted foot with the knee slightly bent, as in football, basketball, kabaddi or dance, or from a direct blow. Whether it happens again depends largely on the knee's built-in risk factors:

Risk factor	What it means
Trochlear dysplasia	A shallow or flat groove that gives the kneecap little to sit in; the most important bony factor
Patella alta	A high-riding kneecap that engages the groove late during bending
Lateralised tibial tubercle (high TT–TG distance)	The patellar tendon attaches too far to the outside, pulling the kneecap outwards
Generalised ligament laxity	Naturally loose, flexible joints
Limb alignment	Knock knees, inward thigh rotation or outward tibial rotation
Weak hip and quadriceps muscles	Poor dynamic control of the kneecap and knee
Young age at first dislocation	Teenagers, especially with open growth plates, have the highest recurrence risk
Previous dislocation	Each dislocation makes the next more likely

After a first dislocation, recurrence risk is relatively low in someone with normal anatomy. It is much higher in a young person with several of the factors above. These factors also determine which treatment is right.

Symptoms and diagnosis

Signs of patellar instability

The kneecap visibly shifting to the outside of the knee, sometimes popping back on its own as the leg straightens
Sudden pain and swelling after a twist
A feeling that the kneecap is "slipping" or about to go, especially when turning or on stairs
Apprehension: avoiding certain movements for fear the kneecap will dislocate
Pain around or behind the kneecap, sometimes with catching or locking if a fragment of cartilage has broken off

How it is diagnosed

History and examination: the patellar apprehension test, assessment of kneecap tracking (including the "J-sign"), ligament laxity, limb alignment and muscle strength.
X-rays: check kneecap height, the groove's shape, and any bone fragments.
MRI: shows the MPFL tear, bone bruising, cartilage injury and loose bodies. It also measures the groove depth and the TT–TG distance.
CT scan: sometimes used to measure alignment and rotation in detail before surgery.

After a first dislocation, an MRI is especially important. A loose fragment of cartilage or bone needs prompt surgery whatever else is decided.

Non-surgical treatment

For most first-time dislocations without a loose fragment, treatment starts without surgery:

Short-term protection: ice, elevation, pain relief and a brace or splint for comfort, with early gentle movement
Physiotherapy: the cornerstone of treatment. It focuses on quadriceps strength (especially the VMO), hip abductor and external rotator strength, core stability, balance, and safe landing and cutting technique
Patellar bracing or taping: a stabilising brace or taping during sport can add support and confidence
Gradual return to activity: usually over about 3 to 6 months, once strength and control are restored

Non-surgical care works well for many people with normal anatomy. It is less effective for those who have already dislocated more than once or who have significant risk factors such as trochlear dysplasia or patella alta.

When surgery becomes necessary

Surgery is usually recommended when:

The kneecap has dislocated two or more times despite good rehab
A first dislocation has broken off a loose piece of cartilage or bone that needs fixing or removing
Instability or apprehension keeps limiting daily life, work or sport
Imaging shows major risk factors that make further dislocations very likely, particularly in a young, active person

Each dislocation can damage the cartilage under the kneecap. Stabilising a knee that keeps dislocating helps protect that cartilage from wear over the long term.

Surgical options: matching the operation to the cause

There is no single "kneecap operation". The surgeon chooses one procedure or a combination, based on which risk factors are present.

Procedure	What it does	Best suited to
MPFL reconstruction	Rebuilds the torn inner ligament with a tendon graft, usually the patient's own hamstring	Most recurrent dislocations; the core of most stabilisation surgery
Tibial tubercle osteotomy (TTO)	Cuts and shifts the bony attachment of the patellar tendon inwards and/or downwards, then fixes it with screws	A high TT–TG distance or patella alta
Trochleoplasty	Deepens a flat or shallow groove	Severe trochlear dysplasia; a specialised procedure
Cartilage repair or fragment fixation	Reattaches or removes loose fragments and treats cartilage damage	Fragments broken off during a dislocation
Derotation osteotomy	Corrects twisted thigh or shin bones	Significant rotational malalignment
Lateral retinacular lengthening or release	Loosens a tight outer retinaculum	Only as an add-on when the outer tissues are genuinely tight

MPFL reconstruction

This is the most common stabilising operation. Through small incisions, the surgeon secures a tendon graft between the inner edge of the kneecap and the inner side of the thighbone, recreating the ligament that stops the kneecap sliding outwards. An arthroscopy is usually done first to examine the cartilage and remove loose fragments. In children with open growth plates, the surgeon positions fixation to avoid the growth plate.

Bony procedures

When the cause is bony, a ligament reconstruction alone may not be enough. A tibial tubercle osteotomy realigns the pull of the patellar tendon. A trochleoplasty reshapes the groove. These are bigger operations with longer recoveries, and are generally combined with MPFL reconstruction.

Where arthroscopic lateral release fits in

Arthroscopic lateral release is a keyhole procedure in which the tight lateral retinaculum on the outer side of the kneecap is cut. It was once widely used for kneecap problems, but its role is now limited:

It is not recommended on its own for patellar instability or recurrent dislocation. Cutting the outer restraint can make the kneecap less stable, and in some cases causes it to slip towards the inside (medial instability).
Its main use is for lateral patellar compression syndrome, pain from a tight outer retinaculum that tilts the kneecap and overloads its outer edge, when months of physiotherapy have failed.
In instability surgery, the outer tissues may be lengthened (preferred over a full release) alongside an MPFL reconstruction or osteotomy, but only when they are clearly tight.

If you are offered a lateral release alone for a kneecap that dislocates, ask your surgeon why, and consider a second opinion.

What to expect on the day of surgery

Understanding the process can ease anxiety. A typical MPFL reconstruction runs like this:

Anaesthesia: spinal or general anaesthesia, often with a nerve block for pain relief.
Arthroscopy: a camera inserted through small incisions to inspect the cartilage and remove or fix any loose fragments.
Graft preparation: a tendon, usually a hamstring tendon, is harvested and prepared.
Graft fixation: the graft is attached to the inner edge of the kneecap and to the thighbone, with its tension set so the kneecap tracks centrally through the full range of movement.
Additional procedures: an osteotomy or retinacular lengthening, if planned.
Closure: the incisions are stitched and a dressing and brace are applied.

MPFL reconstruction alone typically takes about 1 to 1.5 hours; combined procedures take longer. Many patients go home the same day or the next.

Recovery and rehabilitation

Recovery depends on which procedures were done. A bony procedure such as a TTO adds bone-healing time, and weight-bearing is often restricted for longer.

The first days

Mild to moderate swelling, bruising and discomfort, managed with prescribed pain relief, ice and elevation
A hinged brace and crutches
Early exercises to switch on the quadriceps and regain full straightening

Typical rehab phases after MPFL reconstruction (your surgeon and physiotherapist will tailor these)

Phase	Timing	Focus
1. Protect	Weeks 0–2	Swelling control, full extension, quad activation, weight-bearing as tolerated in a brace
2. Restore motion	Weeks 2–6	Bending to about 90° and beyond, wean off crutches and brace, stationary bike
3. Build strength	Weeks 6–12	Quadriceps, hip and core strengthening, balance work, normal walking and stairs
4. Return to running	Months 3–4	Straight-line jogging, early plyometrics, agility drills
5. Return to sport	Months 4–6+	Sport-specific training, cutting and pivoting, functional testing

Typical milestones

Walking without crutches: about 2–4 weeks after MPFL reconstruction; often 6 weeks or more after an osteotomy
Return to desk work: about 1–2 weeks
Driving: about 2–6 weeks, depending on the leg and your surgeon's advice
Return to pivoting sports: usually about 4–6 months after MPFL reconstruction alone, and 6–9 months or more after bony procedures

Consistency is what makes the difference. Quadriceps and hip strengthening, done regularly, gives the new ligament the muscular support it needs.

Risks and complications

Stabilisation surgery is generally safe in experienced hands, but every procedure carries some risk:

Infection, blood clots and wound problems
Stiffness or loss of bending, sometimes from a graft set too tight
Recurrent instability if a contributing factor was not addressed
Pain at the front of the knee or kneecap arthritis, particularly if cartilage was already damaged
Fracture of the kneecap at the graft fixation site (rare)
After an osteotomy: delayed bone healing, or prominent screws that may need removal
After an isolated lateral release: medial instability, which is one reason this procedure is now used selectively

An experienced knee surgeon who assesses every risk factor before operating lowers the chance of complications and of further dislocations.

Outcomes, choosing a surgeon, and when to seek help

What results to expect

MPFL reconstruction, combined with bony correction where needed, prevents further dislocations in the large majority of patients. Most return to their previous sport. Results are best when every risk factor has been identified and addressed, and when rehab is completed in full.

Choosing a surgeon

A specialist in knee or patellofemoral surgery
One who reviews your X-rays, MRI and alignment measurements with you, and explains which risk factors you have
One who offers a tailored plan rather than the same operation for everyone

Questions to ask

Which risk factors do I have, and which will the surgery address?
Do I need an MPFL reconstruction alone, or a bony procedure too?
Is there any cartilage damage, and how will it be treated?
What is my expected recovery and return-to-sport timeline?

When to see a doctor

After any first kneecap dislocation, to rule out a loose fragment
If your kneecap keeps slipping or dislocating
If knee pain or instability does not improve, or gets worse, with physiotherapy
If fear of the knee giving way is stopping you from everyday tasks or sport

The earlier instability is assessed and treated, the better the chance of protecting the cartilage and getting a good long-term result.

Frequently asked questions

Why does my kneecap keep dislocating?

Usually because the MPFL was torn in the first dislocation and has healed loose. Often a built-in factor contributes too, such as a shallow groove, a high-riding kneecap or an outward pull from the patellar tendon.

Can patellar instability be treated without surgery?

Yes, often. After a first dislocation with no loose fragment, physiotherapy and bracing are usually the first step. Surgery is considered after repeated dislocations, or when a fragment needs fixing.

Is lateral release a good treatment for patellar dislocation?

Not on its own. An isolated lateral release does not stabilise a dislocating kneecap and can cause the kneecap to slip the other way. It is mainly used for lateral patellar compression pain, or as a small add-on during stabilisation surgery when the outer tissues are tight.

What is MPFL reconstruction?

An operation that rebuilds the ligament on the inner side of the knee that holds the kneecap in its groove, usually using a tendon graft from your own hamstring.

How long is recovery after MPFL reconstruction?

Most people walk without crutches within about 2–4 weeks and return to sport at around 4–6 months. Recovery takes longer if a bony procedure is added.

Can children have kneecap stabilisation surgery?

Yes. Techniques are adapted to protect the growth plates. Bony procedures such as a tibial tubercle osteotomy are generally delayed until growth has finished.

Will my kneecap dislocate again after surgery?

The risk is low when all risk factors are addressed and rehab is completed, but it is not zero. Following the rehab programme and keeping up strength work reduces it further.

This article is for general information and is not a substitute for advice from an orthopaedic surgeon who has examined your knee.

Kneecap Keeps Slipping? A Complete Guide to Treating Patellar Instability and Recurrent Dislocation