Patellar Malalignment Information

The knee is the largest joint in the body and consists of the lower end of femur (thigh bone) bone, which rotates on the upper end of the tibia (shin bone), and the patella (knee cap), which slides in a groove on the end of the femur. Large ligaments attach to the femur and tibia to provide stability. 

In a healthy knee the patella sits in the femur trochlear groove on the end of the femur.

The way these two bones move against each other is affected by a number of factors. Any injury or deviations from normality in any one of these factors can lead to problems with the way the patella articulates with the femoral trochlea and can result in pain and instability of the knee.

Patellofemoral malalignment can cause pain at the front of the knee (anterior knee pain), which can lead to patellofemoral arthritis. In its more severe forms it  can cause the patella to dislocate.

The medial patellofemoral ligament (MPFL) attaches to the inner side of the patella and the inner side of the end of the femur. It is the primary medial stabiliser of the patella. The role of this rope like ligament is to prevent the knee from lateral dislocation (dislocating to the outer side of the knee) and subluxation, which is a partial dislocation of the joint. With any lateral movement of the patella the MPFL can be injured or torn. After a patella has dislocated once, the MPFL is often ruptured or stretched and is less reliable in preventing the patella dislocating in the future. 


Patella Dislocation

Patella dislocation is typically caused by a direct blow to the knee or a sudden twist of the leg. It occurs when the patella slips out of its normal position in the patellofemoral groove and generally causes intense pain when it occurs along with swelling of the knee.

Patella dislocation is a common injury mainly in young females with joint hyper flexibility, as well as young athletes. 

It used to be a difficult problem to manage, however, there have been many surgical and non-surgical advancements in techniques to treat this condition.


Classification of patellar dislocation:

  • Traumatic: This is where dislocation is a result of trauma, such as direct blow to the knee, a sharp twist of the leg or rapid change of direction. Traumatic dislocations are often the result of sporting injuries.
  • Recurrent: After the patella has dislocated once before there is often damage to the ligaments stabilising the knee causing them to rupture and stretch. Once this has occurred they become less reliable and dislocations are likely to reoccur.
  • Congenital: This is very rare and usually occurs bilateral (both knees) and is associated with other major anomalies (for example, Down’s Syndrome and arthrogryposis). Patients present at birth with a fixed knee flexion, externally rotated tibia and a high patella which is difficult to palpate. Treatment of this condition is difficult and usually needs surgical correction.
  • Habitual: This is a rare condition where the patella dislocates during flexion and relocates during extension without pain and swelling unlike recurrent patellar dislocation. This condition is very difficult to treat and surgery should be avoided where possible.


Traumatic and recurrent patellar dislocations

These two conditions are related and represent the majority of dislocations. A traumatic dislocation will become recurrent in 15-45% of cases. This occurs more in females with a ratio of 2:1. 


Common Causes of Patella Dislocation:

  • Direct blow to the knee
  • Twisting or pivoting injury to the lower leg, such as with rapidly changing direction
  • Powerful muscle contraction and increased quadriceps angle. The quadriceps angle is the angle between the line of pull of the quadriceps and the line of the pull of the patella ligament. The wider this angle, the harder the quadriceps muscle will try to dislocate the patella when it contracts
  • Congential abnormality, such as shallow or malformed joint surfaces
  • Patella Alta, where the patella sits too high in relation to the femoral trochlea


Signs and Symptoms of Patella Dislocation:

  • Patella dislocation to the outer side of the knee causing an obvious deformity. The patella often relocates on its own once the knee is straightened, reversing the deformity. However, damage to the ligament is still sustained
  • Tenderness, swelling and bruising of the knee
  • Severe pain when attempting to move the knee
  • Difficulty moving the knee
  • The feeling the knee is unstable and might give way



It is common that there is intra-articular damage resulting from patella dislocation. A physical examination can reveal instability of the knee but the best current method of identifying patella malalignment and dislocation is by using both plain radiograph and MRI scans in order to reach the appropriate diagnosis. 

MRI scan is very useful especially in the presence of recent dislocation. Many pathological findings can be diagnosed via MRI scan such as:

  • Rupture of the MPFL
  • Joint effusion (fluid such as blood in the joint)
  • Bruising to the medial patellar surface, bruising to the lateral femoral condyle where the patellar dislocated
  • The presence of loose bodies and other soft tissue, such as cartilage damage