Unstable Kneecap Injury / Subluxing Patella

Reduced range of motion, pain, swelling, and stiffness of the affected knee

What is an unstable kneecap / subluxing patella?

An unstable kneecap injury, also known as patella instability or subluxing patella, is a condition in which the patella (kneecap) slides partially or entirely out of its usual position in the trochlear groove of the femur (thighbone), producing discomfort and instability in the knee joint. Patella instability can range from minor to severe, temporary or permanent, depending on the underlying aetiology and degree of the injury.

The term subluxation refers to the occurrence in which kneecaps only move partially out of position and then return to their usual position. This is unlike its counterpart, a knee dislocation where the kneecap comes out of its position completely. 

knee bones
The patella or kneecap sits in the trochlear groove of the femur.

Subluxing patella is often classified with factors affecting instability which are:

  • Trochlear dysplasia: trochlear dysplasia occurs when the trochlear groove is shallow or abnormal.
  • Abnormal patella height: abnormal positioning of the patella.
  • Pathological tibial tubercle-trochlear groove (TT-TG) distance: indicates the abnormal (large or small) distance between the tibial tubercle and where the patella sits in the grove. 

Kneecap instability is classified into three levels:

  • Objective patellar instability: patients who have experienced at least one event of subluxation with one principle factor of instability.
  • Potential patellar instability: patients who have never experienced subluxation but deal with symptoms of pain and present one or more principal factors of instability.
  • Patellofemoral pain: pain is the main symptom presented with no factors of instability. 

What causes unstable kneecap / subluxing patella injury?

Unstable kneecap, also known as patella instability, can be caused by a myriad of factors, including:

  • Injury: a direct blow to the knee or a twisting injury from contact sports or falls can result in unstable kneecaps or subluxing patella injury. 
  • Structural abnormalities: such as trochlear dysplasia, hypermobility or loose/flexible joints, and connective tissue disorders can affect kneecap stability.

What are the symptoms of an unstable kneecap injury?

Some of the signs that you have an unstable kneecap  injury or subluxing patella include:

  • Obvious knee deformity/kneecap might be visibly out of place (severe cases)
  • Pain 
  • Stiffness and swelling
  • Difficulty conducting any form of movement (walking, running)
  • Generally feeling the kneecap “slipping”, “popping”, or “dislocating” out of place
  • Grinding or cracking sensation in the kneecap when moving
  • Weakness and instability
knee pain
Unstable kneecap/subluxing patella is characterised by knee pain and discomfort.
If you notice any of the symptoms, make an appointment with an orthopaedic specialist to get an accurate diagnosis and treatment.

Dr Puah KL is our Senior Consultant Orthopaedic Surgeon at Artisan Sports & Orthopaedic Surgery. He used to serve the sports service of Singapore General Hospital - the highest volume trauma centre for orthopaedics in Singapore.

Is unstable kneecap / subluxing patella injury painful?

Yes, an unstable kneecap injury can be painful, especially during activities that involve bending or straightening the knee. The severity of the pain varies depending on the cause and level of instability. 

Some people may experience mild discomfort or aching, while others may experience sharp or intense pain that limits their ability to move or bear weight on the affected leg. The pain is usually located around the kneecap. 

Who is at risk of unstable kneecap / subluxing patella injury in Singapore?

There are several factors that increase your risk of an unstable kneecap/subluxing patella injury, these are:

  • Athletes: unstable kneecap injuries may occur among athletes of contact sports such as football and basketball.  
  • Age: subluxing patella is more common among people from 1-20 years of age. Children are more prone to the condition due to softer tissues. 
  • Medical history: individuals with pre-existing knee conditions such as previous patella subluxation or hypermobility, have a higher risk of being susceptible to unstable kneecaps.
joint hypermobility syndrome
 Individuals with joint hypermobility syndrome are at a higher risk of developing unstable kneecaps / subluxing patella.

How is unstable kneecap / subluxing patella injury diagnosed?

Unstable kneecap / subluxing patella injury is diagnosed in the following ways:

  • Physical examination and medical history: inform your orthopaedist of the potential activities that may have led to an injury such as trauma, exercise, or sports. Physically the area of injury will be examined and several motion tests such as bending and straightening the injured knee would be conducted. 
  • Imaging tests: x-rays will be conducted to identify the position of the kneecap in relevance to the groove. Other potential bone injuries would also be investigated.  Magnetic resonance imaging (MRI) would be required to examine the surrounding areas to assess the extent of the injury and check for any associated damage to other structures like the ACL
  • Diagnostic arthroscopy: a minimally invasive procedure known as an arthroscopy would be conducted where a small camera is inserted into the knee joint to visualise the structures inside.

What are the treatment options for unstable kneecap injury / subluxing patella in Singapore?

Typically, the treatment approach for unstable kneecap/subluxing patella would include: 

  • Rest: you would be advised to observe complete rest, while icing the affected area, to relieve pain and reduce swelling.
  • Medications: non-steroidal anti-inflammatory drugs (NSAIDs) and painkillers are commonly prescribed as the first course of treatment to reduce inflammation and manage pain.
  • Brace: to reduce movement of the joint and allow the ligament to heal.
  • Crutches: to limit placing weight on the affected knee.
  • Physiotherapy: upon discontinuation of the use of the brace, an active range of motion exercise would be administered via physiotherapy to regain mobility and strengthen muscle.
  • Surgery: knee arthroscopy surgery would be administered if deemed necessary to restore the damaged structures in the knee. Moreover, realignment surgery would be conducted, if the patient’s kneecap needs to be repositioned in order to improve stability such as the MPFL reconstruction. Lastly, ligament reconstruction may be required, should the patient’s ligaments that support the kneecap, be damaged.
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Feeling aches and pains?

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For emergency appointment, please call or Whatsapp us at (65) 8909 8877. We will arrange for a limousine to send you to our clinic.
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