What is it?
Kneecap, or patellofemoral, instability, is when the kneecap becomes displaced from the groove (trochlea) either during normal day-to-day activity, or as a result of trauma.
Why does it occur?
There are a number of reasons why kneecap instability develops, but generally, it can be due to an increase in the forces trying to pull the kneecap out of place, or a reduction in the forces trying to keep the kneecap in place.
Problems that lead to increased ‘pull out’ are:
- A muscle imbalance involving weakness of the medial quadriceps in comparison to the lateral muscles can cause the kneecap to be dragged laterally out of the groove.
- Poor core strength, poor hip strength, flat feet and abnormalities of the bones in the lower leg can cause lead to similar problems with the kneecap.
Problems that lead to increased ‘pull in’ are:
- An abnormal development of the groove in which the kneecap runs, meaning it becomes too shallow, or flat, or domed, making it difficult for the kneecap to remain centred and can result in dislocaiton.
- Rupture or stretching of the medial patellofemoral ligament, which is a structure on the side of the knee in place to prevent displacement.
- On occasion the kneecap can be sitting too high, which means, when used, other parts of the joint take longer to engage and make it more prone to dislocation.
What are the symptoms?
Kneecap, or patella, instability can manifest as either an acute or partial dislocation. Acute dislocations have to be manually corrected, where as partial dislocations often spontaneously self-correct, leaving patients with a feeling of weakness or lack of trust in their knee.
How is it diagnosed?
Instability of the kneecap can be diagnosed by discussing your history and examining your knee. Certain signs will indicate potential causes, but in most cases an MRI scan will be required to obtain the best overall view of what is happening in your knee.
How is it treated?
Treating kneecap instability is a complex and specialised process. There is no one operation to fit all patients, and the combination of procedures and non-operative options we might prescribe will depend entirely on what is causing the instability.
If a biomechanical abnormality has been identified as the cause of kneecap instability, meaning the joint’s ability to cope with internal and external forces is compromised, physiotherapy can often provide adequate and effective treatment.
Medial patellofemoral ligament reconstruction
If frequent dislocations are having a significant impact on your quality of life, but the general anatomy of the knee joint is otherwise normal, then MPFL reconstructive surgery can dramatically improve the stability of your knee.
Tibial tubercle transfer
Sometimes it may be necessary to change the insertion point, patellar tendon, which runs from the kneecap to the shin bone, to enable the knee hinge to work more effectively.
In cases of severe trochlear dysplasia, whereby the groove for the kneecap is no longer deep enough for the bone that sits on top of it to do so comfortably, an operation known as a trocheoplasty is required to prevent dislocations. This operation can be hugely successful, but as a very complex procedure, must be carried out in a specialised patellofemoral clinic, such as ours.
The One Orthopaedics team specialists
Consultant Orthopaedic Surgeon FRCS (Tr&Orth), Knee and Hip
Consultant Orthopaedic Surgeon FRCS (Tr&Orth), Knee
Consultant Orthopaedic Surgeon FRCS (Tr&Orth), Hip and Knee