A patella dislocation occurs when the kneecap leaves its groove, usually moving towards the outside of the knee. It can be painful and frightening, but what does it mean for the knee in the years ahead?
The short answer is that a patella dislocation can injure the structures that stabilise the kneecap and damage joint cartilage. Some people recover well after one episode, while others develop recurrent patella instability, ongoing pain or patellofemoral arthritis. Individual risk depends on the injury and the shape, alignment and stability of the knee.
What happens when the patella dislocates?
The patella normally glides within a V-shaped groove at the front of the femur bone, called the trochlea. It improves the leverage of the quadriceps muscles and helps the knee straighten.
During a lateral dislocation, the patella moves over the outer edge of this groove. This commonly stretches or tears the medial patellofemoral ligament (MPFL), an important restraint on the inner side of the kneecap. The joint surfaces may strike each other as the patella dislocates or returns to position.
A systematic review of 42 studies involving 2,254 patients found signs of MPFL rupture on MRI in 98% after a first dislocation. Patellar cartilage damage was reported in 85%, trochlear cartilage injury in 47%, and loose fragments within the joint in 11.5% (PubMed systematic review). These figures combine different studies and imaging methods, so they should not be treated as an individual prediction.
Cartilage injury ranges from bruising or a shallow defect to an osteochondral fracture, where a piece of cartilage and underlying bone breaks free. A loose fragment may cause catching or locking and can sometimes require early surgery.
Will it keep dislocating?
Not everyone experiences another episode, but recurrence is common. One review combining previous meta-analyses reported an overall repeat dislocation rate of 29.4% and recurrent instability symptoms in 32.8% of patients (PubMed review). The underlying studies and treatments varied, so these are broad averages.
Risk is generally higher in younger patients and in knees with anatomical features that make the patella less stable. These include a shallow or unusually shaped trochlear groove, a high-riding patella, increased sideways pull through the patellar tendon, rotational or knock-knee alignment, and looseness of the ligaments. A previous dislocation and problems in both knees can also matter.
Patella instability does not always mean a complete repeat dislocation. Some patients experience partial slipping, apprehension during turning, or a sense that the kneecap cannot be trusted. This can restrict sport, stairs, squatting and everyday confidence even when the patella has not fully dislocated again.
Can a patella dislocation cause arthritis later?
Evidence suggests that it can increase the risk of arthritis in the joint between the kneecap and trochlear groove. This is called patellofemoral arthritis. Possible reasons include cartilage damage during the original injury, repeated impact from further instability and abnormal loading related to the underlying anatomy.
A population-based cohort study compared 609 people after a first lateral patellar dislocation with matched individuals who had not dislocated their patella. At an average follow-up of 12.3 years, clinically significant patellofemoral arthritis had been diagnosed in 9.5% of the dislocation group and 1.3% of controls. The estimated risk was 7.8 times higher after dislocation (PubMed cohort study).
The researchers reported that “nearly half of patients have symptoms and radiographic changes consistent with arthritis at 25 years.” This was a statistical cumulative estimate rather than a finding that half of every patient group will inevitably develop arthritis. The study also found stronger associations with later arthritis when there had been recurrent dislocations, osteochondral injury or trochlear dysplasia.
A smaller MRI follow-up study provides another perspective. Eight years after a conservatively treated first-time traumatic dislocation, cartilage deterioration was visible in all 20 participants, and half had high-grade patellofemoral cartilage lesions (PubMed MRI follow-up study). Its small size and case-series design limit how broadly those percentages can be applied, but the findings support taking the initial injury seriously.
Importantly, an increased arthritis risk does not mean that every patient will develop severe pain or need a knee replacement. In the population study, knee arthroplasty was uncommon and the difference between groups was not statistically significant.
Does every first dislocation need surgery?
No. Many first-time dislocations are treated without an operation when the kneecap has returned to position and there is no major loose fragment or other urgent injury. Early care may include short-term protection, swelling management and progressive physiotherapy.
Rehabilitation aims to restore movement, quadriceps and hip strength, balance, coordination and confidence. It should also prepare the patient for the demands of work or sport. A brace may be used in selected situations, but it does not correct every anatomical risk factor.
Surgery may be discussed when imaging shows a repairable osteochondral fragment, the patella remains displaced, or instability repeatedly continues despite suitable rehabilitation. The operation should address the individual causes. Options may include MPFL reconstruction and, in selected patients, procedures to correct patellar height, alignment or a severely abnormal trochlear groove.
Research comparing surgery with non-operative treatment after a first dislocation remains mixed. Some studies report fewer redislocations after surgery, but functional advantages are less consistent and operations carry risks. A systematic review concluded that surgery may reduce recurrence without improving functional scores (PubMed review). This does not establish one treatment as best for every knee.
When should the knee be assessed?
An acutely dislocated patella needs prompt medical assessment, even if it moves back by itself. Seek urgent care if the kneecap remains out, the knee is locked, weight-bearing is impossible, swelling is rapid, the foot is numb or cold, or there is an open wound.
After the initial episode, assessment should consider the injury mechanism, examination findings and appropriate imaging. X-rays can identify alignment and fractures; MRI may be useful when cartilage injury, a loose fragment or ligament damage is suspected.
Patients with repeated slipping, persistent swelling, catching, loss of movement or difficulty returning to activity may benefit from review by a physiotherapist, sport and exercise physician or orthopaedic knee surgeon. Seeing a surgeon does not mean surgery will be recommended.
A dislocating patella is therefore not harmless, but neither does one episode guarantee a damaged knee later in life. Recognising cartilage injury, understanding recurrence risk and matching treatment to the individual knee can support informed decisions and an appropriate return to activity.
This article provides general educational information and does not replace individual medical advice. Treatment depends on age, examination findings, imaging, activity needs and the anatomy of the knee.
