Patellar Instability

What is patellar instability?

The kneecap sits in a groove at the end of the thighbone called the trochlea. It glides up and down as the knee bends. Several structures keep it on track:

• the shape and depth of the trochlear groove
• the medial patellofemoral ligament (MPFL), a band on the inner side that acts as the main restraint against outward slipping
• the quadriceps muscles, particularly the inner part (vastus medialis obliquus)
• the alignment of the leg, hips and feet

When one or more of these is weak or abnormal, the kneecap can slip out sideways. A first dislocation almost always tears the MPFL, which then heals loose, making further episodes more likely.

Subluxation vs dislocation

The dislocated kneecap page covers what to do during and immediately after a dislocation.

Symptoms

• the knee giving way, buckling or feeling unreliable, especially when twisting, cutting, landing or going downstairs
• a feeling that the kneecap moves or slides out of place
• pain at the front or inner side of the knee
• swelling after an episode
• catching or locking if a cartilage fragment is loose
• apprehension: discomfort or fear when someone pushes the kneecap outward

A knee that gives way can also be caused by an ACL tear, which usually follows a twisting injury with a pop and fast swelling. Front-of-knee pain without any slipping is more often patellofemoral pain syndrome.

Causes and risk factors

Anatomy

• Trochlear dysplasia: a shallow or flat groove, the single strongest risk factor.
• Patella alta: a high-riding kneecap that engages the groove late during bending.
• Increased TT-TG distance: the attachment of the kneecap tendon on the shinbone is too far to the outside, pulling the kneecap outward.
• Leg alignment: knock knees, inward-turned thighbones (femoral anteversion), outward-twisted shins and flat feet.

Laxity

Joint hypermobility and conditions such as Ehlers-Danlos syndrome make the ligaments looser. Instability often affects both knees.

Activity and injury

Pivoting sports such as soccer, basketball, gymnastics and dance, and a direct blow to the inner side of the knee, can trigger a first dislocation.

Risk of recurrence

After a first dislocation, roughly 1 in 3 people have another. The risk is much higher in those under 16, with open growth plates, trochlear dysplasia, patella alta or a history of dislocation in the other knee.

How patellar instability is diagnosed

History and exam. The doctor checks for swelling, tenderness along the inner side of the kneecap, how far the kneecap moves sideways, the apprehension test, and the "J-sign" (the kneecap jumping sideways as the knee straightens). They also look at hip rotation, foot posture and general joint laxity.

X-rays. These show fractures, patella height and the groove shape.

MRI. After a first dislocation, MRI shows MPFL tears, bone bruising, and cartilage or bone fragments (osteochondral fractures) that may need surgery. It also measures TT-TG distance and trochlear shape.

CT scan is sometimes used for measuring rotation of the leg bones before surgery.

Treatment

After a first dislocation

Most people are treated without surgery unless there is a loose fragment:

• a brief period in a brace or splint for comfort, with early movement
• ice, elevation and pain relief
• crutches until walking is comfortable
• physical therapy from early on

Surgery is advised early if MRI shows a significant loose cartilage or bone fragment.

Physical therapy and exercises

Rehabilitation usually runs 6 to 12 weeks or more and focuses on:

• Quadriceps strengthening: straight-leg raises, mini squats, leg press in a pain-free range, step-ups and later single-leg work
• Hip strengthening: side-lying leg raises, clamshells and hip abduction exercises, which reduce inward knee collapse
• Core and balance training: single-leg balance and landing control
• Flexibility: stretching tight hamstrings, calves and the outer thigh (iliotibial band)
• Sport-specific drills before returning to cutting and jumping

Stabilizing braces or taping can help some people during sport.

Surgery for recurrent instability

Surgery is considered after two or more dislocations, or after one in a high-risk knee:

• MPFL reconstruction: the torn ligament is rebuilt with a tendon graft. It is the most common operation and has high success rates, with recurrence in under 10% in most series.
• Tibial tubercle osteotomy: the bony attachment of the kneecap tendon is moved inward or downward to correct alignment or a high kneecap.
• Trochleoplasty: deepening a severely flat groove, done in specialist centers.
• Derotation osteotomy: correcting severe twisting of the thighbone or shinbone.
• In children with open growth plates, techniques are adapted to avoid growth damage.

Lateral release by itself is no longer recommended for instability, because it can make the kneecap less stable.

Recovery

After physical therapy alone, many people return to sport in 6 to 12 weeks. After MPFL reconstruction, crutches are usually needed for a few weeks, with a return to running around 3 to 4 months and to pivoting sports around 6 to 9 months, once strength and confidence have returned.

Outlook and complications

Repeated dislocations damage the cartilage under the kneecap and raise the long-term risk of patellofemoral arthritis. Stabilizing the knee early in people at high risk of recurrence protects the cartilage. Most people return to normal activities after appropriate treatment.

Frequently asked questions

What does patellar instability feel like?

It feels like the kneecap is shifting or sliding sideways, often with the knee suddenly giving way when you twist, land or go downstairs. You may have pain at the front or inner side of the knee and swelling after an episode. Some people become afraid to trust the knee.

Can patellar instability heal without surgery?

Many people with a first dislocation or occasional subluxations do well with physical therapy focused on quadriceps, hip and core strength. Surgery is usually recommended when dislocations keep happening, when there is a loose fragment, or when anatomy puts the knee at high risk.

What exercises help an unstable kneecap?

Straight-leg raises, mini squats, step-ups, leg press in a comfortable range, side-lying leg raises, clamshells and single-leg balance work help. Avoid deep squats and lunges that cause pain early on. A physical therapist can progress exercises safely.

What is MPFL reconstruction?

It is surgery to rebuild the medial patellofemoral ligament, the main band that stops the kneecap from slipping outward. A tendon graft is attached to the kneecap and thighbone. It is the most common operation for recurrent dislocation, with low recurrence rates afterward.

Will my kneecap keep dislocating?

About 1 in 3 people have another dislocation after the first. The risk is higher in teenagers, people with loose joints, a shallow groove or a high-riding kneecap. Strengthening helps, and surgery greatly lowers the risk for those who keep dislocating.