Patellofemoral instability refers to an unstable relationship between the patella (kneecap) and femur (thigh bone). It is commonly encountered in sportspersons, especially in women, and may disrupt normal movement, causing pain and inflammation.
Knee joint anatomy:
Four bones join together to form the knee joint. The thigh bone (femur) joins at its lower end to the lower leg bone (tibia), forming the femorotibial joint, while at the front it articulates with the kneecap or patella, forming the patellofemoral joint. The two lower leg bones, tibia and fibula, are also joined together forming the tibiofibular joint.
Bones in a movable joint are not attached surface-to-surface but are bound together by cord-like fibrous extensions called ligaments which extend between the bone ends that hold the bones together while allowing movement at the same time. Ligaments are tough, fibrous bands that ensure a stable relationship between the articulating bone surfaces in all positions and movements. Any ligament abnormality or trauma leading to loose or torn ligaments affects joint stability. An unstable joint leads to pain and disability.
The articulating end of the femur has two rounded heads, separated by a small groove. When the knee is extended or flexed, the patella slides vertically up and down in this groove. In the case of patellofemoral instability, the patella moves in an uncontrolled way, leaving its normal path of motion, giving the impression that it is sliding away.
Patellofemoral instability may vary in intensity. There may only be lateral maltracking of the patella in the femoral groove. As it moves, the patella slides outwards but remains inside the groove.
There may be recurrent partial dislocations referred to as subluxation, which correct automatically with the patella coming to normal alignment without external help. In severe cases, complete dislodgment of the patella may occur. The patella dislocates usually to the lateral side completely out of the groove and is locked there. When dislocated, the patella needs to be placed back into position.
Patellofemoral instability may be caused by a variety of factors:
- Excess laxity of conjoining ligaments, which fail to limit the patellar movement, the resulting excessive mobility damages the joint function and structure
- Anatomical variations such as
– An abnormally high position of the patella on the knee (patella alta), which fails to constrain the patella inside the groove
– A shallow femoral groove
– Tibial tuberosity positioned too far outwards. This is the projection of bone that we can feel just below and towards the outside of the kneecap; it is at this point that the tendon of the patella is attached.
- A previous patellar subluxation or dislocation, as this harms the medial patellofemoral ligament, which is the main stabiliser of gliding the patella over the femur. The younger the patient and the greater the degree of dislocation, the higher will be the risk of future dislocations and consequent joint instability.
Patellofemoral instability may cause pain and disability. There is a risk of damage to the articulating cartilage as a result of aberrant joint movement. Mainly there is a feeling of loosening or giving away of the joint during movement.
The patella is assessed for its stability by gliding and tilting in lateral and medial directions.
X-rays help to exclude arthritis and other pathology. MRI and CT scan are excellent tools for diagnosing and forming an elaborate treatment plan, especially if surgery is being considered.
Conservatively, the joint is supported and stabilised until the painful symptoms are over. A knee brace may be used to immobilise the joint for a brief period.
Correct taping techniques help prevent maltracking of the patella. Patella-supporting knee braces are also used to provide stability during routine activities.
Physiotherapy aims to strengthen the surrounding muscles, specifically the quadriceps (the anterior thigh muscle) as the patella is directly attached to it. A strong, well-conditioned muscle acts to support and stabilise the kneecap and may take over the function of the ligaments (if they are loose) in reducing joint instability.
Surgical options include:
Repair of torn or loose ligaments. Loose ligaments on the inner side of the knee are mostly the cause of instability; these are shortened or repaired, as the case may be. There is also an option of loosening the tight lateral ligaments to balance the forces stabilising the patella in place.
Shifting the tibial tubercle (the prominence of the tibia just below the kneecap, where the patellar tendon is attached. The tibial tubercle may be positioned in different ways to correct the instability. For example, it may be transferred downwards (distally) when the problem is caused by a high patella (patella alta) or inwards (medially) when the problem is excessive lateral patellar tracking.
The femoral groove, known as the trochlea may be deepened surgically to keep the patella on the track; a procedure called trochleoplasty.
The surgical plan depends on the patient’s condition and the cause of the problem. Several techniques may be used in combination. Surgical treatment does improve the stability of the joint but does not guarantee prevention of degenerative changes appearing later in life.