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. 

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. 

• 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.

• 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

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. 

• 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.

• 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.

• 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.