Reviewed by Dr Vishal Pai
Patellofemoral instability is when your kneecap (patella) slips partly or fully out of its groove at the front of your knee. This page explains why it happens, how it is diagnosed and treated, and when surgery is considered.
At a Glance
- Your kneecap normally glides in a groove at the end of your thigh bone. In patellofemoral instability it slips out of that groove, almost always towards the outside of the knee.
- It is most common in teenagers and young adults, often after a twisting injury during sport.
- After a first dislocation, most people are treated without surgery, with physiotherapy to strengthen the thigh and hip muscles.
- About 1 in 3 people have another dislocation. The risk is higher if you are young, if your groove is shallow or if your kneecap sits high.[1]
What is patellofemoral instability?
Your kneecap sits inside the tendon of your quadriceps, the large muscle at the front of your thigh. When you bend and straighten your knee, the kneecap glides up and down in a V-shaped groove at the end of your thigh bone (femur). This is called the trochlear groove, or trochlea.
Two things keep the kneecap in place. The sides of the groove hold it in. On the inner side of the knee, a band of tissue called the medial patellofemoral ligament (MPFL) acts like a tether that stops the kneecap sliding outwards. When the kneecap dislocates, the MPFL is usually torn or stretched.
Dislocation and subluxation
A dislocation is when the kneecap comes right out of the groove. It often slips back in by itself when you straighten your leg. If it does not, paramedics or emergency staff put it back. A subluxation is when the kneecap slips partly out and back again, often with a feeling that the knee is about to give way. Repeated episodes of either are called recurrent instability.
What causes patellofemoral instability?
Most first dislocations happen when you twist on a bent knee with your foot planted, for example when changing direction in netball, football or basketball. A blow to the inner side of the kneecap, or a fall, can also push it out.
Some knees are shaped in a way that lets the kneecap slip out more easily. The main risk factors are:[1]
- A shallow or flat groove (trochlear dysplasia)
- A kneecap that sits higher than usual (patella alta)
- A bony bump at the top of the shin bone, where the kneecap tendon attaches (the tibial tuberosity), that sits further to the outside than usual and pulls the kneecap outwards
- Being young, especially if your growth plates are still open.
Loose ligaments throughout the body (hypermobility) and a twist in the thigh or shin bones can also play a part.
What are the symptoms of patellofemoral instability?
When the kneecap dislocates, you usually feel sudden pain and your knee gives way. You may see the kneecap sitting on the outside of the knee. The knee often swells within a few hours.
After the first episode, or with subluxation, you may notice:
- The kneecap shifting or slipping when you twist or turn
- The knee giving way
- Pain at the front or inner side of the knee
- Feeling nervous about certain movements, such as pivoting or kneeling (apprehension)
- Swelling after activity.
Pain around the kneecap without any slipping is more often patellofemoral pain syndrome or chondromalacia patella, which are treated differently.
When should you see a doctor?
See a doctor after any first dislocation, even if the kneecap went back in by itself. Seek urgent care if:
- The kneecap stays out of place. Keep the knee still and do not try to force it back
- You cannot put weight on your leg
- Your knee swells quickly and a lot, which can mean bleeding inside the joint or a broken piece of cartilage or bone
- Your knee locks or catches, which can mean a loose fragment.
Also see your GP or physiotherapist if your kneecap keeps slipping or your knee keeps giving way.
How is patellofemoral instability diagnosed?
Your doctor will ask how the injury happened and examine your knee. They will check for swelling, tenderness on the inner side, how the kneecap moves, and whether you feel apprehensive when it is pushed outwards.
X-rays show the bones and can show a broken fragment. An MRI scan shows the cartilage, the MPFL and the shape of the groove. In a review of 40 studies, about 4 in 10 people with a first dislocation had a piece of cartilage or bone broken off (an osteochondral fracture), and MRI found these more often than plain X-rays.[2] If your kneecap keeps dislocating, you may also need an X-ray of your whole leg to check its alignment, and a CT scan to measure any twist in the bones.
How is patellofemoral instability treated without surgery?
After a first dislocation, treatment is usually non-surgical, as long as there is no loose fragment in the joint. Dr Vishal Pai’s practice is to arrange an MRI scan first, to check for a loose fragment and for features that raise the risk of another dislocation. You can read about the first few days in what to do after a kneecap dislocation.
Early care
Rest, ice and crutches help in the first days. A brace may be used for a short time for comfort. Gentle movement starts as the pain settles, guided by your physiotherapist. Studies have not shown that one way of protecting the knee early on works better than another.[3]
Physiotherapy
Physiotherapy is the main treatment. It aims to strengthen your thigh (quadriceps) and buttock muscles, which help hold the kneecap in its groove, and to improve your balance and control of the leg. In a review of 25 studies of 1,066 people, knee function improved after non-surgical treatment, although it did not always return to normal.[3] Trials have not found a specific exercise program that works better than general leg strengthening.[4]
Your physiotherapist will help you return to sport in stages, once your strength, movement and confidence have come back. Drills such as cutting, pivoting and landing come last.
Braces and taping
A kneecap brace or tape can help you feel more secure when you go back to sport. Braces are widely used, but there is little good evidence that any one brace or support prevents another dislocation.[3]
When is surgery considered?
Surgery may be advised soon after a first dislocation if the MRI shows a loose piece of cartilage or bone that needs to be fixed back or removed. It may also be discussed after a first dislocation if you have several risk factors for another one (see the outlook section below). Most often, surgery is considered when your kneecap keeps dislocating or giving way despite a good course of physiotherapy. For these people, stabilisation surgery is effective: in a review of 19 studies of 948 people, about 3 in 100 dislocated again after MPFL reconstruction.[5]
The two main operations are MPFL reconstruction, which rebuilds the torn tether on the inner side of the kneecap, and tibial tuberosity osteotomy, which moves the bony bump where the kneecap tendon attaches so the kneecap sits in better line. They are sometimes done together, depending on the shape of your knee. You can read how they work, the recovery and the risks on the patella stabilisation surgery page.
Next step
Have your knee assessed
After a first dislocation, most people are treated without surgery. Dr Vishal Pai can examine your knee, review your scans and explain your options, starting with physiotherapy.
What is the outlook for patellofemoral instability?
Many people never have another dislocation. In one study of 291 people treated without surgery after a first dislocation, about 7 in 10 had no further instability.[6] Across studies, about 1 in 3 people dislocate again. In a long-term study of 584 people, the figure was 36% after 20 years.[1][7]
What raises the risk of another dislocation?
Your own risk depends mostly on the shape of your knee and your age at the first dislocation. The factors most strongly linked with another dislocation are:
- Significant trochlear dysplasia. A very shallow or flat groove is the strongest single risk factor in most studies. In one long-term study of 584 people, trochlear dysplasia made another dislocation about 18 times more likely.[1][7]
- Young age at the first dislocation. The risk is higher if you were 16 or younger, or your growth plates were still open, when the kneecap first came out.[1][8]
- A high kneecap (patella alta).[1][7]
- A tibial tuberosity that sits further to the outside, measured on a scan as the TT-TG distance.[1][7]
- A previous dislocation of your other kneecap, in some studies.[8]
These risks add up. In the studies that looked at this, about 1 in 10 people with no risk factors dislocated again, compared with about 3 in 4 of those with three risk factors.[1] Dr Vishal Pai uses your examination, X-rays and MRI to look for these factors, and they help decide whether physiotherapy alone or surgery is likely to suit you better.
A kneecap dislocation also raises the chance of arthritis behind the kneecap later in life, especially after repeated dislocations or damage to the cartilage.[9]
References
Show all 9 referencesHide references
- Huntington LS et al. 2019. Factors associated with an increased risk of recurrence after a first-time patellar dislocation: a systematic review and meta-analysis. American Journal of Sports Medicine.
- Qiu RY et al. 2022. MRI as the optimal imaging modality for assessment and management of osteochondral fractures and loose bodies following traumatic patellar dislocation: a systematic review. Knee Surgery, Sports Traumatology, Arthroscopy.
- Moiz M et al. 2018. Clinical outcomes after the nonoperative management of lateral patellar dislocations: a systematic review. Orthopaedic Journal of Sports Medicine.
- Flores GW et al. 2023. Conservative management following patellar dislocation: a level I systematic review. Journal of Orthopaedic Surgery and Research.
- Vivekanantha P et al. 2022. Isolated medial patellofemoral ligament reconstruction results in similar postoperative outcomes as medial patellofemoral ligament reconstruction and tibial-tubercle osteotomy: a systematic review and meta-analysis. Knee Surgery, Sports Traumatology, Arthroscopy.
- Lewallen LW et al. 2015. First-time patellofemoral dislocation: risk factors for recurrent instability. Journal of Knee Surgery.
- Christensen TC et al. 2017. Risk factors and time to recurrent ipsilateral and contralateral patellar dislocations. American Journal of Sports Medicine.
- Wierer G et al. 2022. The patellar instability probability calculator: a multivariate-based model to predict the individual risk of recurrent lateral patellar dislocation. American Journal of Sports Medicine.
- Sanders TL et al. 2017. Patellofemoral arthritis after lateral patellar dislocation: a matched population-based analysis. American Journal of Sports Medicine.
Frequently asked questions
What is the trochlear groove?
The trochlear groove (trochlea) is the V-shaped channel at the lower end of your thigh bone (femur). Your kneecap glides up and down in it as you bend and straighten your knee. If the groove is shallow or flat, called trochlear dysplasia, the kneecap is more likely to slip out.[1]
What is patella alta, and can it be fixed without surgery?
Patella alta means your kneecap sits higher than usual. A high kneecap engages with the groove later as you bend your knee, so it is less well held and more likely to dislocate.[1] If your kneecap is stable, patella alta on its own does not need treatment. Exercise cannot change the height of the kneecap, but physiotherapy can improve your control of it. If the kneecap keeps dislocating, a tibial tuberosity osteotomy can move it lower.
Can patellar instability be fixed without surgery?
Often, yes, after a first dislocation. In one study, about 7 in 10 people treated without surgery had no further instability.[6] If your kneecap keeps slipping despite good rehabilitation, surgery is often recommended, because repeated dislocations are linked with damage to the cartilage and later arthritis.[9]
Is a kneecap that feels out of line the same as instability?
Not always. Instability means the kneecap actually slips out of its groove. Many people whose kneecap looks or feels slightly out of line have pain around the kneecap without it slipping, which is more often patellofemoral pain syndrome. An examination, and sometimes a scan, can tell the two apart.