Prof. N. Reha Tandoğan, M.D. - Asım Kayaalp, M.D.
The patella (kneecap) is an important part of your knee joint. Patella dislocation is a condition in which your kneecap pops out of its normal groove on the femur (called the “trochlea”), or may go in or out of its anatomical position. Although patella dislocations are rare (6 cases per 100.000/population), there is an increased risk in females and patients under 17 years-of age. Fifteen percent of the patients have a family history of patella dislocation, and a very high risk of dislocation in the other knee.

How does the patella dislocate?

Most of the patients with patella dislocation have an underlying cause that increases the risk of dislocation. Hyper-elastic joints (joint hyperlaxity), insufficiency of the soft tissue structures that guide the patella during knee motion, malalignment of the leg (X-bowed legs), shallow boney groove (trochlea) of the femur are the most important ones. The patella usually dislocates with minimal trauma, such as dancing or during rotation of the body over a fixed foot. Following the first dislocation, the soft tissues heal with elongation, and subsequent dislocations occur with even less trauma. Very rarely, the patella is dislocated at birth; this is called a “congenital” dislocation. It is impossible to reduce the patella to its normal position without extensive surgery in congenital dislocations. In some others, the patella is not fully dislocated but travels slightly out of its anatomical groove, this is called a “subluxation” and is a risk factor for dislocation and cartilage damage.

What are the symptoms of patella dislocation?

Acute patella dislocation is a painful event when the kneecap pops out of its place and the patient can feel the patella sliding out. This is followed shortly by swelling of the knee due to bleeding and limitation of knee motion. The patella usually goes relocates by itself when the knee is extended, although some patients describe that they needed to gently push the patella back to its position. Patella dislocation should be ruled out in any adolescent presenting to the emergency ward with swelling and painful knee after trauma. Once the acute phase has passes, the patient may feel the patella sliding out during twisting motions and activities of daily living. Pain and swelling might be a problem in patients with cartilage or bone injuries that occur during the dislocation. A loose bone fragment may cause symptoms of catching and locking in the knee.

What is the emergency management of acute patella dislocation?

Figure 1: Knee X-ray depicting a subluxed patella
Figure 1: Knee X-ray depicting a subluxed patella
The principles of rest, ice, bandaging and elevation are applicable for all severe knee injuries. If the patella is still dislocated in the emergency ward, your physician might gently put it back. A soft knee brace might be helpful to reduce swelling and pain. Casting should be avoided since this causes discomfort and severe muscle weakness. Emergency management is followed by X-rays, MRI or CT scans to define the extent of injury and underlying risk factors. (Figure 1).

Is there a risk for recurrence after the first dislocation?

The risk of a second patella dislocation after the initial event is 17-71%. This risk increases dramatically after the second and third dislocations. Patients with underlying anatomical risk factors are more prone to recurrent dislocations. Repeated dislocations may cause irreversible cartilage injuries on the patella and femur. These cartilage injuries may cause early osteoarthritis of the knee, which is a disabling condition for young and active patients. The main goals of treatment in patella dislocations are to prevent repeated dislocations and cartilage damage, and return the patient to his/her active lifestyle or sports.

Who does not need surgery after a patella dislocation?

Patients without underlying anatomical risk factors, severe cartilage or bone damage or loose fragments in the joint, and patients with normal patella tracking after the initial may be treated without surgery. A brace and anti-inflammatory medication are used for a few weeks, followed by a structured rehabilitation program focusing on regaining knee motion and muscle strength once the pain subsides. However, only about 15% of the patients fulfill these criteria. The risk of recurrence should be kept in mind for return to sports.

Who needs surgery after a patella dislocation?

Patients with a large bone fragment floating freely in the joint, patients whose patella are still subluxed, who have underlying anatomical risk factors need to be treated surgically after the first dislocation. (Figure 2). Surgery is the only option in patients with congenital or recurrent dislocations.

Figure 2 : X-ray of a large fragment fixed with screws after a patella dislocation.

What is personalized surgery for patella dislocations?

The underlying, risk factors, bone morphology, soft tissue constraints, rotational alignment of the leg and the extent of cartilage damage are different for every patient with a patella dislocation. A variety of other factors such as the age, gender, previous surgeries, number of episodes of dislocation, and remaining growth also affect the decision making process. Your surgeon evaluates all these factors with a clinical examination, magnetic resonance or computed tomography imaging and decides on the best treatment plan adapted to your specific needs (Figure 3). A standard soft tissue procedure, followed by a bony procedure was routinely used on all patients without taking these risk factors into account in previous years. This led to high rates of failure and in some cases, irreversible cartilage damage. Current treatment of patella dislocations addresses all the risk factors with a combination of soft tissue and boney procedures and cartilage regeneration/repair techniques. This requires a personalized surgical strategy and careful planning. Increased interest and knowledge on the subject have led to several developments in the treatment of patellar instability. Some historical techniques have been abandoned, being replaced by modern, less invasive surgical procedures. The rehabilitation has improved, with focus on early return to activities of daily living and sports.

Figure 3: Planning of surgery using highly accurate MRI and CT scans for patella dislocation.
Figure 4 : X-ray of a patient who underwent a combined surgery for patellar instability. MPFL reconstruction has been performed with tunnels in the patella and femur. The pull of the quadriceps muscle has been modified using a tibial tubercule osteotomy and fixed with 2 metal screws.
Figure 4 : X-ray of a patient who underwent a combined surgery for patellar instability. MPFL reconstruction has been performed with tunnels in the patella and femur. The pull of the quadriceps muscle has been modified using a tibial tubercule osteotomy and fixed with 2 metal screws.
The surgical treatment of patella dislocations starts with an arthroscopic examination of the knee joint. Small cartilage flakes can be removed, large bone fragment are fixed with screws. Large and deep cartilage defects need advanced cartilage restoration/repair techniques, and these may be performed simultaneously or in a two surgical settings. The rest of surgery is performed from small incisions around the knee joint. A tendon graft harvested from the hamstring muscles is usually needed to reconstruct the soft tissues that keep the kneecap in place during knee motion. This is called an MPFL (Medial patello-femoral ligament) reconstruction, different techniques are used in children and adults. Children with remaining growth need specialized growth preserving techniques to prevent deformity and leg length discrepancy. Bone procedures that change the rotational alignment of the leg called “derotation osteotomies” may be necessary in children. The pull of the quadriceps muscle may need to be changed with procedures called “tibial tubercule osteotomies”. The boney groove in which the patella moves may need to be deepened to increase stability, this is called a “trochleoplasty”. Children with congenital dislocation may require more extensive soft tissue procedures that require larger incisions in the leg.

What are the outcomes of personalized surgery for patellar instability?

The outcomes of modern personalized surgeries for patella instability are excellent. The risks of recurring dislocation or stiffness are less than 5%. Most patients return to activities of daily living and sports without limitations. The most important factor that influences the outcome is the presence and severity of cartilage damage. Your surgeon should have the skills & resources available to treat the cartilage injury in addition to the surgery for correcting the patellar instability. Your surgeon will decide on the best personalized treatment for your condition after a thorough physical examination, imaging studies and a detailed discussion on the risks and benefits of each procedure.

© Prof. N. Reha Tandoğan, M.D. - Asım Kayaalp, M.D.