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Patellofemoral (kneecap) instability

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  • Patellofemoral instability refers to a condition where the kneecap (patella) moves out of its normal position in the groove (trochlea) at the end of the thigh bone (femur). This can cause the kneecap to partially shift (sublux) sideways, or fully dislocate. It often leads to pain, a feeling of the knee giving way, and reduced confidence in the knee’s stability. It can be extremely debilitating, that severely limits activities and quality of life.

  • The patella sits in a groove on the front of the femur, called the trochlea. It moves up and down this groove as the knee bends and straightens. Stability of this joint is maintained by:

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    • Bony anatomy of the trochlea - the shape and depth of the groove

    • Ligaments and soft tissues, especially the medial patellofemoral ligament (MPFL), which runs on the inner side of the knee, between the patella and the femur

    • Muscle control, particularly the quadriceps

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    When any of these supports are weak, injured, or abnormally shaped, the patella may become unstable.

  • Patellofemoral instability may occur due to:

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    Traumatic injury

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    A sudden impact or twisting injury can dislocate the patella. This can happen in contact sports with a direct blow to the side of kneecap, knocking it out of place, A first-time dislocation usually requires a large amount energy, and is a significant injury to the knee. Additional cartilage and bone injuries should be excluded at this stage.

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    Anatomical factors

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    The anatomy of the leg, and the knee itself, can predispose to patellofemoral instability:

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    • Rotational abnormalities of the femur and tibia can result in altered forces upon the knee cap, and the way in which it ‘tracks’ within the groove.

    • Leg malalignment - if the knees are particularly ‘knock-kneed’ (valgus) or ‘bow-legged’ (varus), the pull of the muscles may not be inline with the groove, causing instability.

    • ‘High’ riding kneecaps (patella alta), means that the knee needs to be in a more flexed (bent) position before the kneecap sits within the groove and is stable. It is therefore more unstable for longer when the knee is straight, putting it at risk of dislocation.

    • Trochlea dysplasia - The shape of the groove affects how well it can capture the patella and prevent dislocations. Some people are born with a shallow groove (mild dysplasia), a flat groove (moderate dysplasia), or even a bump instead of a groove (severe dysplasia).

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    Generalised ligament laxity

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    • People with naturally looser ligaments (hypermobility) may be more prone to dislocations. On occasions, this may be as part of specific connective tissue disorders (e.g. Ehlers Danlos, Marfan’s syndrome).

    • Feeling of the kneecap "slipping out" or giving way

    • Pain at the front of the knee, especially during activity or going down stairs or slopes

    • Swelling after dislocation

    • Clicking, catching or grinding sensation

    • Apprehension with certain movements (e.g. twisting, squatting)

  • Patellofemoral instability is diagnosed predominantly based on your symptoms. The physical examination is used to assess the anatomical contributing factors , and this is confirmed with  X-rays and an MRI scan. A limited CT scan may also be indicated in some cases.

  • Non-Surgical Treatment

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    This is  recommended for first-time dislocation or mild cases (relatively infrequent dislocations).

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    • Physiotherapy helps to strengthen core and quadriceps muscles, and improve control of patella tracking 

    • Bracing or taping can sometimes help stabilise the patella

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    Surgical Treatment

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    Surgury is considered for first time dislocation where there is associated damage to the cartilage or bone, in cases of recurrent instability,  and for unsuccessful  non-operative management. The anatomical abnormality driving the instability can be corrected using a range of surgical procedures:

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    • Medial Patellofemoral Ligament reconstruction –  this is an arthroscopically (keyhole) assisted operation that involves reconstructing the ruptured or stretched ligament between the patella and the thigh bone. This is done by using one of your hamstring tendons as a donor ‘graft’.

    • Tibial Tubercle Osteotomy – this is an open procedure that changes the position of the patella within the groove of the knee, and usually involves bringing the kneecap down when it is too high (alta). This is achieved by taking the patella tendon off the tibia with a block of bone, moving it, and fixing it with a metal plate and screws.

    • Trochleoplasty – this is a sub-specialist operation indicated for severe trochlea dysplasia (bump rather than groove). It is an open procedure that involves carefully raising the cartilage from the front of the knee, reshaping the bone, then replacing and fixing  the cartilage back.

    • Some people recover well with physiotherapy and activity modification.

    • Surgery has good success rates in restoring stability and reducing dislocations.

    • Long-term complications may include patellofemoral arthritis, particularly in untreated or late presenting cases.

  • You should seek assistance if you experience frequent dislocations,  persistent knee pain, swelling, or if your symptoms interfere with daily life.

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