Patellar Instability

Definition

Patellar instability is a cluster of conditions affecting movement of the patella or knee cap. It includes patellar dislocation and patellar subluxation which can be of various types.

The patella is a very important part of the knee joint forming part of the extensor mechanism. This is the chain of structures that provide the ability for the knee to straighten. They include the quadriceps muscle and tendon which is attached to the upper end of the patella and the patellar tendon that links the lower end of the patella to the shin bone. The quadriceps tendon and patellar tendons are confluent over the front of the patella so that it’s superficial surface is heavily enclosed in tendon, a tough leathery tissue.

Normally the patella rests in a groove at the lower end of the femur called the trochlear groove or trochlea. The keel shaped under-surface of the patella engages in this groove which keeps it central during knee flexion and extension. Ligaments on either side of the patella add further support in keeping it central. The various parts of the quadriceps muscle also impart added stability.

Causes of Instability

Malformations of the trochlea tend to occur in some families resulting in the groove being shallow in some, flat in others, and in severe cases domed so that the patella has no intrinsic stability.

Weakness of the vastus medialis muscle, part of the quadriceps group, is another contributory factor in patellar instability.

Tight lateral ligaments are a further possible cause and can pull the patella to the side leading to a greater tendency to dislocation or a chronically subluxed position.

Valgus (knock knee) deformities predispose to dislocation by altering the line of pull of the quadriceps so that it tends to cause the patella to deviate towards the outer side.

Finally there is an important ligament that supports the medial side of the patella called the medial patello-femoral ligament ad this can be ruptured during trauma contributing to dislocation or poor tracking of the patella.

Symptoms of Dislocation

Patellar instability may present suddenly and out of the blue with complete dislocation of the patella giving rise to sudden, intense pain over the front and side of the knee and a sensation of something ‘giving way’ or ‘popping out’. There may be a visible deformity of the knee owing to the patella moving out of position usually to the lateral side of the knee. There may also be a rapid onset of swelling within the first couple of hours after injury.

 

Some patients have recurrent episodes of dislocation in which the patella moves out of position rather more easily and can go back into its original position with certain knee movements, usually on straightening the knee. In this type of case cases, pain and swelling may be relatively minor.

 

After reduction of a dislocation patients usually experience ongoing discomfort that may increase on activity. Pain is also frequently encountered during knee flexion activities including going up and down stairs or hills, squatting or lunging.

 

There is often tenderness over the lateral side of the patella and clicking or crunching within the joint during flexion and extension due to articular damage sustained during the dislocation. There may be further episodes of giving way or collapse.


Diagnosis

In the acute stage x-rays are often performed to assess any bony damage and to check the position of the patella.

Further investigation generally involves MRI or CT scanning to evaluate the medial patello-femoral ligament and obtain a detailed understanding of the anatomy and the degree of articular cartilage damage.

Treatment

Treatment depends on the underlying structural abnormalities but frequently includes physiotherapy to strengthen the vastus medialis obliquus (VMO).

Arthroscopic surgery is of help in evaluating the amount of articular damage and the extent of instability can be assessed under general anaesthesia. Arthroscopic release of the lateral retinaculum may prove of value in some cases.

Where the dislocation has been due to injury which has caused damage to the medial patello-femoral ligament without much evidence of intrinsic morphological abnormality then a medial patello-femoral ligament repair may prove effective.

Certain cases benefit from patellar realignment surgery either to realign the patellar tendon a little more medially or in the case of patella alta, by distal transfer of the tubercle.

There is a group of patients in whom severe trochlea dysplasia merits the operation of trochleoplasty in which the trochlear groove is deepened or reconstituted.

In severe or recurrent cases where there has been significant damage to the articular cartilage at the back of the patella a partial knee replacement may be recommended.

Copyright London Knee Clinic 2014