Patella dislocation / Instability
Patella (‘Knee-cap’) instability is a complex condition. The term ‘Instability’ refers to a spectrum of conditions ranging from subtle mal-alignment of the patella, to a chronic patella dislocation. The most common form of ‘Instability’ is recurrent subuxations/ dislocations of the patella.
Whats causes a Patella dislocation / Patella instability?
The patella is a large bone over the front of the knee that has connections to both the quadriceps tendon and the patella tendon. This arrangement forms the ‘Extensor mechanism’ that is responsible for being able to extend your knee out straight. As the knee bends, the patella is designed to glide smoothly in a ‘V’ shaped groove in the femur (‘Thigh bone’) called the ‘Trochlea’. Any disruption to this smooth gliding design can result in ‘patella instability’
There are many factors that may disrupt this process and cause instability of the patella.
Traumatic patella dislocation
The patella can be acutely displaced as the result of a specific acute injury.
Atraumatic patella dislocation
In this situation, the patella loses contact with the trochlea without an acute injury. This can be seen in patients that have experiences multiple traumatic dislocations in the past, or in patients with bony abnormalities (see below) that predispose to this ‘Instability’. While there are many factors that can lead to this condition, it is often a combination of several of these that result in the patella losing its normal contact with the thigh bone.
Abnormal leg alignment / Rotation
Severe Knock-knees (‘Genu Valgum’) or excessive rotation of the thigh bone (‘Femoral anteversion’)
High riding patella (‘Patella alta’).
An abnormally shallow trochlea (groove) has difficulty containing the patella.
Soft Tissue Abnormalities
Generalised Ligamentous Laxity
Patients with extreme joint mobility and flexibility
Tight lateral (outer) structures
Tight soft tissues (‘Lateral Retinacular’) can pull the patella out of its groove
Quadricep (VMO) weakness
The vastus medialis (inner quadricep muscle) has attachments to the patella that help position the patella appropriately in its groove. In patients with other predisposing anatomy, this weakness will contribute to the ‘instability’
Patella Instability – Symptoms
Symptoms may vary from a vague pain at the front of the knee, through to daily dislocations of the kneecap. Patients with subtle mal-alignment may never experience a frank dislocation. Symptoms for these patients are likely to be exacerbated by knee bending exercises and it is not unusual to feel a ‘cracking’ sound whilst bending the knee.
Patients who experience a frank dislocation will notice that the knee cap sits on abnormally on the outer (‘lateral’) side of the knee. An ambulance or medical officer may be required to reposition the knee cap by straightening the leg. In patients who experience multiple dislocations, the patella can often be repositioned by the patient themselves. Fluid often accumulates inside the knee joint and pain is experienced on the inner (‘medial’) side of the knee owing to a ruptured ligament (‘Medial patellofemoral Ligament’)
Patella Instability is a complex spectrum that certainly warrants a very thorough evaluation. A detailed history is taken to determine the patients symptoms and frequency of dislocations. A detailed examination is performed to assess for underlying bony and soft tissue factors that may be contributing.
Plain xrays are routinely ordered. Frequently a CT scan and a MRI scan are required in the setting of patient that is experiencing multiple episodes or ongoing pain.
An individualised management plan is needed. Young patients who have experienced only 1-2 of these episodes are often best managed with a tailored physiotherapy strengthening program designed to ‘balance’ the muscles around the knee joint.
Who needs Surgery?
Surgical intervention is considered in the following situations
Occasionally the patella will ‘knock off’ a fragment of bone from either the patella itself, or the end of the thigh bone during a dislocation episode. This requires an arthroscopy to remove or repair the loose fragment depending on its size.
Patients who have ongoing episodes, despite initial conservative treatment, are candidates for patella realignment surgery. This involves restoring the normal gliding of the patella in its groove of the thigh bone.
Patients who have had ‘Instability’ for many years, often have cartilage damage underneath the knee cap. Surgical treatment is aimed at reducing pain.