Patella instability
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Anatomy
The kneecap, patella, sits in the front of the knee and articulates to the lowest part of the thighbone (femur), in a groove called trochlea. The deep surface of the patella has a V-shape and the trochlea has an inverted V-shape making them fit well together. This also creates stability to the patella when it slides up and down the trochlea every time the knee is bent or straightened.
To further assist with stability of the patella, there is a ligament connecting the medial side of the femur to the patella, called the medial patellofemoral ligament (MPFL). MPFL acts as the “seatbelt of the kneecap”, not active in normal range of motion, but keeps the patella from sliding out laterally when the knee is turning, twisting or even getting a knock or tackle.
Injury
Sometimes that knock or tackle comes with too much force and the ligament tears and the patella slides off the femur and dislocates laterally. If this happens in a more bent position, the patella can even get stuck on the outside of the knee, needing the knee to be straightened for the patella to fall back into its normal central position.
This is most common in young females, and there are multiple contributing factors, such as hypermobility, anatomical abnormalities, valgus malalignment and weakness.
If the patella is dislocated during sports, comes with significant pain or swelling or if it doesn’t fall back into place, the knee needs to be evaluated urgently, usually at the nearest hospital. The patella needs to be reduced back in its neutral position and possibly get a brace to keep it stable.
Evaluation
Examination and history can indicate instability and tell the story of a patella dislocation. The knee is usually swollen, painful and tender on the medial side. X-rays validate that the kneecap is back in position and can exclude fractures.
Recurrent dislocations require more evaluation, MRI for example, to visualize the MPFL, and other structures and anatomy to fully assess why the kneecap keeps dislocating.
Several anatomical abnormalities increase the risk of recurrent instability, and these need to be assessed.
MRI showing tear of medial patellofemoral ligament (MPFL)
Treatment
First of all, if the patella is not in the right position, it needs to be reduced back. This is usually done at the nearest hospital.
The first time it happens, the standard treatment is to start physiotherapy, get muscular control, reduce pain and swelling and re-evaluate the stability after about 2 months. There is a special brace to stabilize the patella that can be used and with proper physiotherapy most patients can return to their normal activities, including sports.
However, some patients continue to feel the instability and have recurrent subluxations or dislocations and need further treatment. This is when the full assessment of the anatomy and alignment becomes more important. Firstly, the MPFL can be torn, leaving the patella without a “seatbelt”. Secondly, valgus malalignment (knocked knees) puts the patella in a lateral position even before starting to bend the knee, increasing the risk of recurrent dislocations. Furthermore, a flat trochlea and a lateral attachment of the patellar tendon on the lower leg creates unfavourable conditions and further increases instability and risk of dislocations. All of these factors need to be assessed and weighed into the final picture to decide what surgery is needed.
Most commonly, a reconstruction of the MPFL is efficient. By just reconstructing the “seatbelt”, many patients can start to trust their knee again and get back to activities and sports.
Sometimes bigger surgeries are needed to increase the chance of a successful surgery with a stable patella going forward. This includes osteotomy of the tibia to realign the lower leg to centralize muscle forces, osteotomy of the femur to correct malalignment and get the patella to sit in the middle of the trochlea and sometimes, rarely, the V-shape is too flat or even inverted creating a highly unstable kneecap situation and a trochleoplasty (re-shaping the trochlea to a V) needs to be performed.
Tear of MPFL (left) and schematic reconstruction of the MPFL (right), to increase stability and have the “seatbelt of the kneecap” back.
To the right: Animated video of MPFL tear and reconstruction & tibial tuberosity osteotomy to increase patellar stability.
Rehabilitation
The difficulty and length of the rehabilitation depends on what procedures were done during surgery. If there was only need for an MPFL reconstruction, the patient is allowed full range of motion and full weightbearing from day 1 and will recover from the swelling and pain within 2-4 weeks, but the physiotherapy will be ongoing for 3 months until the patient has the strength to start jogging and continue until the patient is ready to go back to sports, generally around 6 months after the surgery.
If an osteotomy or trochleaplasty is needed, the first 6-8 weeks will be focusing on motion exercises, reducing swelling and pain and gradually get back to a normal walking pattern thereafter. It usually takes 6-12 months to get back to all activities after these types of procedures.