The kneecap (patella) connects the muscles in the front of the thigh to the shinbone (tibia). As you bend or straighten your leg, the kneecap is pulled up or down. The thighbone (femur) has a V-shaped notch (femoral groove) at one end to accommodate the moving kneecap. In a normal knee, the kneecap fits nicely in the groove.

But if the groove is uneven or too shallow, the kneecap could slide off, resulting in a partial or complete dislocation. A sharp blow to the kneecap, as in a fall, could also pop the kneecap out of place.

Patellar instability can lead to a dislocated patella.

• Complete dislocation: The ligaments that hold the kneecap in place slide to the outside of the knee, taking the kneecap with them. The ligaments may tear or stretch. The kneecap is entirely out of place.
• Partial dislocation (subluxation): The kneecap slips partially out of the groove.

What are the symptoms of patella instability?

• Knee buckles and can no longer support your weight
• Kneecap slips off to the side
• Knee catches during movement
• Pain in the front of the knee that increases with activity
• Pain when sitting
• Stiffness
• Creaking or cracking sounds during movement
• Swelling

Do I need imaging for patella instability?

X-rays may be recommended to see how the kneecap fits in its groove. Your doctor will also want to eliminate other possible reasons for the pain, such as a tear in the cartilage or ligaments of the knee.

MRI is utilized to assess the surrounding soft tissues, ligaments and the cartilage. A TT-TG measurement is also performed to assess the relationship between the tibial tubercle and the trochlear groove, which impacts the determination of non operative versus surgical management.

Do I need surgery for patella instability?

Non-operative management includes physical therapy and bracing. Success with non-operative management is influenced by your anatomy (TT-TG and trochlear dysplasia), age, activity level and history of dislocations. If you have chronic patellar instability or a complete patella dislocation, surgery may be recommended. 

If an injury causes patellar instability, you may have loose cartilage or bone fragments in the knee that need to be removed. The damage to the surrounding cartilage may need to be addressed (cartilage restoration).

Surgical options include:

• Medial patellofemoral ligament (MPFL) repair to strengthen and repair the ligaments that hold the patella in place.
• MPFL reconstruction to replace a damaged ligament with a hamstring tendon.
• A Tibial tubercle transfer (TTO) to realign the tibia, femur, patella and surrounding soft tissues. This open surgical procedure requires a larger incision and longer recovery than arthroscopic surgery.
• Patella arthroplasty can treat severe arthritis secondary to recurrent dislocations.