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
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
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
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
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.
In the acute
stage x-rays are often performed to assess any bony damage and to check the
position of the patella.
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.
depends on the underlying structural abnormalities but frequently includes
physiotherapy to strengthen the vastus medialis obliquus (VMO).
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.
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.
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
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
Copyright London Knee Clinic 2014