Patient Information Sheet: Patellar Instabilities (dislocation or subluxation of the kneecap)

By Dr Chris Milne, Sports Physician

What is it?
It occurs when the patella moves out of its usual groove in front of the knee.

What causes it?
It is caused by a variety of factors including muscle imbalance, a small or high riding patella, a shallow groove for it to track in, plus excess joint mobility and flat feet.

Symptoms – what you notice

  • Pain – usually along the inner border of the patella as the muscle attachments are torn away from the bone. The pain may be very severe.
  • Deformity – the patella may appear as a lump on the outside of the knee. It usually pops back into its usual groove when you straighten your knee.
  • Instability – you may lack confidence in the knee, and feel it is about to give way.
  • Swelling – this usually comes on virtually immediately and is related to bleeding into the knee joint.
  • Signs – what the doctor finds

    • Effusion – usually there is a tense swelling in the knee that will last for a week or two after the injury.
    • Tenderness – usually down the inner border of the patella, related to tearing of muscle attachments as above.
    • A positive apprehension sign – this occurs when the doctor tries to push your patella towards the outside of your knee.
    • There may be associated flat feet, or generalised hypermobility of many joints in the body.
    • Investigations
      X-rays can show an associated fracture of the inner border of the patella. A ‘skyline’ view is most useful for this. X-rays can also show a small or high riding patella, plus a shallow groove for the patella to track in. MRI scans are rarely needed but may show bone bruising, which is an additional clue in tricky cases; patellar instability is often quite subtle and may be overlooked by less experienced doctors. CT scans are usually only ordered if surgical stabilisation is being considered.


      • First aid – try to push the knee out straight, as this will often be enough for the patella to clunk back into its proper place. Apply an ice pack.
      • Whether or not your patella is back in place, you need to go to an A & M Clinic to get the knee immobilised in a brace in full extension (out straight). Leave the brace on for 6 weeks.
      • You should take painkillers or anti-inflammatory tablets regularly for the first few days after injury.
      • If the blood in the joint is causing lots of pain, this can be sucked out of the joint by a doctor.
        Perform quadriceps exercises to strengthen the muscles in front of the thigh.
      • If you have flat feet, orthotics can help correct the faulty alignment in your legs.
        In severe cases of recurrent patellar instability, surgery to stabilise the patella may be required.
      • Recovery time
        Average recovery time is 4-6 weeks, but recurrences are common.

        Recovery sequence
        Step 1 Get the knee out straight and the patella back in position. Ice packs, pain relief.
        Step 2 Immobilise the knee in a brace for six weeks.
        Step 3 Perform quadriceps exercises every day.
        Step 4 Physiotherapy supervised exercises when you come out of the brace.
        Step 5 Cycling and swimming.
        Step 6 Restart gentle running and progress to stop-start then multidirectional activity.
        Step 7 Team training and skill sessions.
        Step 8 Resume playing, half game at first.

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