Kneecap instability

What is it?

Kneecap or patellofemoral instability is a condition whereby during normal activity, or as a result of trauma, the kneecap is displaced out of the groove (trochlea) in the femur that it normally runs in.  Patellofemoral instability can be acute secondary to an injury or recurrent during normal day to day activities. 


Why does it occur?

There are a multitude of reasons that patients can develop patellofemoral instability but in general it is a result of either 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.  Patients can have one or a number of pathologies that can lead to either simple or complex kneecap instability. 


Problems that lead to increased ‘pull out’ are:

  1. Imbalance of muscle biomechanics.An increase in the lateral vector created by your quadriceps on your patella can cause it to be dragged laterally out of the groove.This is predominantly due to a muscle imbalance involving weakness of the medial quadriceps in comparison to the lateral muscles.
  2. Malalignment of the lower limb.Conditions that cause a malalignment of the lower limb into a valgus attitude such as poor core or hip strength, planovalgus (flat) feet or previous growth abnormalities of the femur or tibia can cause the leg to take on a knock kneed appearance.This in turn causes an increased lateral pull of the quadriceps on the patella.


Problems that result in reduced restraint of the patella can be one or a combination of:

  1. Dysplastic trochlea. On occasions the development of the groove in which the kneecap runs can be abnormal leading to a shallow or flat groove or, in some extreme circumstances, a dome rather than a groove.This means that it is very difficult for the kneecap to centralise within the trochlea as the knee bends leading to recurrent severe dislocations.
  2. Rupture or stretching of the medial patellofemoral ligament.The MPFL is a structure on the medial side of the knee that acts as a check rein to prevent lateral displacement and dislocation of the patella.Normally as a result of previous trauma this can be ruptured or stretched reducing its ability to prevent dislocation of the patella.
  3. Patella alta.On occasions the natural development of the knee can lead to a kneecap that is sitting too high in relation to the trochlear groove.This leads to a prolonged period of time prior to patella engagement meaning it is more likely to track abnormally and cause potential dislocations.


What are the symptoms?

Patella instability can manifest as either the severe acute dislocations where the kneecap is visualised in a displaced position requiring manual manipulation to reduce it back to its normal location or as recurrent subluxation where the kneecap partially  dislocates but then spontaneously relocates leading to a feeling of weakness or lack of trust in the knee.  In some of the more severe situations the kneecap can fail to ever engage in the trochlea leading to a permanently displaced kneecap.


How is it diagnosed?

Patellofemoral instability is often evident from taking a history and examining your knee.  Certain signs will indicate potential causes such as patella alta or trochlear dysplasia but in most cases an MRI scan is needed to further define the anatomy of the knee and decide which of the above causes is the most likely reason for the patellofemoral instability.


How is it treated?

The treatment of patellofemoral instability is a complex and specialised process.  There is no one operation to fit all patients and the combination of procedures and non-operative management is highly dependent upon the combination of causes for the patellofemoral instability.


  1. Non-operative management

    In the vast majority of cases, if a biomechanical abnormality has been identified as the cause of the patellofemoral instability, physiotherapy in order to focus on core strengthening, hip abduction training and medial quadriceps strengthening as well as posterior chain stretching to prevent early toe off in the gait cycle will adequately treat the patellofemoral instability.

  2. Medial patellofemoral ligament reconstruction

    If frequent dislocations are significantly impacting on your quality of life and the general anatomy of the knee joint and trochlear groove is normal then an MPFL reconstruction to re-function the medial check rein ligament can dramatically improve the stability of your knee. 

  3. Tibial tubercle transfer

    On occasions, normally in the setting of patella alta, it can be necessary to alter the insertion point of the patellar tendon (the structure that runs from your kneecap to your shin bone) to move it further down and/or towards the inside of the leg, this improves the engagement of your patella within the groove

  4. Trochleoplasty

In cases of severe trochlear dysplasia an operation known as a trocheoplasty to re-fashion a groove within your thigh bone and lay the articular cartilage back down to create a congruent patellofemoral joint is required to prevent dislocations.  In these dramatic settings this operation can be hugely successful but it is a complex procedure and carried out in specialised patellofemoral clinics.