The patella (knee-cap) is an important link between the quadriceps muscle and the tibia (leg bone) and helps improve the mechanical advantage of the quadriceps muscle. It is the largest sesamoid bone (this is a bone that is found within a tendon only) in the body.
The Patella has the thickest layer of articular cartilage (the layer of cartilage that lines the bones) in the body.
- The Patella runs in a groove on the femur called the trochlea.
- The trochlea is shaped like an open-V and provides bony stability for the patella
- There are muscles and ligaments that also contribute to the stability of the patella as well
The x-rays show the patella (knee-cap) when looking from the front of the knee (first image below) and from the top of the knee down towards the foot (second image below). You can see that the knee-cap runs in a groove (trochlea groove), which is part of the femur (thigh) bone. This is also marked by the bottom red line.
Conditions That Can Affect the Patella:
Patella dislocation is where the knee-cap comes out of the groove (trochlea) that it normally runs in. Patella or knee-cap dislocation is not uncommon especially in teenage females between the ages of 13-17. The patella almost always dislocates laterally.
It occurs either when:
- There is a twisting injury to the knee
- Direct blow (contact) with the patella
There are a number of reasons why the patella may dislocate or why there may be an increased chance of dislocation:
- More common in females
- Sports participation
- People who have loose (lax) ligaments
- Family history of dislocation
- Previous dislocation
- Trochlea dysplasia (the groove the patella runs in has not developed normally)
- Mal-alignment of the patella and quadriceps (thigh muscle) mechanism
- High riding patella
- Lateralised tibial tubercle (the part of the bone where the patella tendon attaches to may be more lateral on the tibia (leg bone) than normal)
- Pain: Will be very painful initially, and also with repeat dislocations.
- Swelling: You will develop a large swelling of the knee and the swelling will develop rapidly.
- Deformity: Your knee-cap will be out to the side.
- Most of the time the knee-cap will reduce (go back into place) by itself as you straighten (extend) your knee
- In a small number of cases it will need to be reduced by ambulance staff, or in an emergency department
- Very rarely will it require reduction in an operating theatre
- After your patella has been relocated (reduced) there are a number of different ways that you may be told to manage your knee. This will depend on who has treated you and where you have been treated.
- Generally speaking you will be referred to an Orthopaedic Surgeon or Sports medicine physician for an opinion
- Surgery for a first time dislocation is not indicated except for a small number of cases
- Once you have dislocated your patella you do have a higher chance of further dislocation than someone who has not had a dislocation
- The rate of re-dislocation increases each time your patella dislocates
- The more dislocations that occur the greater the likelihood of damage to other structures in the knee
This is a generic term for those people whose patellas:
- Subluxate (partial dislocation)
- And Have either a:
- Bony abnormality
- Soft-tissue abnormality
- Genetic abnormality
- Developmental abnormality
- Islolated arthritis of the patello-femoral joint (PFJ, knee-cap joint) is rare
- Arthritis of the PFJ normally occurs with arthritis of one of the other parts of the knee, medial (inside) and/or outside (lateral)
- This is a difficult condition to treat in isolation
- There is no one operation that can used to be treat this condition in everyone and the operation has to be tailored to the individual patient
- Nigel will discuss which option will be appropriate for you
- This is a condition where the articular cartilage (lining of the joint) is abnormal
- This results from an abnormal “rubbing” of the cartilage on the articular cartilage of the trochlea groove
- This will lead to roughening and softening of the cartilage
- It can eventually lead to arthritis
Symptoms and Signs
Given the many different conditions that exist around the patella, Nigel will tailor the questions that he asks and his examination of your knee will depend on the condition that you have.
Nigel will commonly ask for x-rays of your knee but will also get a MRI scan of your knee. He may well arrange for a 4D CT scan of your knee depending on the condition you have. He does this to assess your:
- Articular cartilage
- Damage to other structures in the knee
- Bony abnormalities
- Alignment of your knee
Who Needs Surgery?
- Surgery is indicated in some cases of re-dislocation or for an initial first time dislocation
- Ruptured / torn MPFL (Medial Patellar Femoral Ligament)
- Trochlea dysplasia (under-developed groove that patella runs in)
- Chondromalacia patellae (changes to the joint on the undersurface of the patella)
- Osteochondral defect (damage to the joint either on the undersurface of the patella or on the femur)
- Bony malalignment
- Tilting of the Patella
- Tibial tubercle malalignment
- Patella Alta (this is a condition where the length of the patella tendon / ligament is longer than normal and the patella sits higher than normal)
There are approximately 130 operations described for patella instability. If surgery is required it will be tailor made to your condition or conditions.
There are a number of surgical procedures that can be done. It does depend on what your abnormality is.They can be divided into a two groups:
1. Proximal realignment - This typically involves soft-tissue procedures:
- MPFL reconstruction or repair
- Lateral Release
- Medial Imbrication / Capsulorrhaphy or Retinacular repair
- Vastus Medialis Obliquus (VMO) advancement
2. Distal realignment - This typically involve bony procedures
- Distalisation Osteotomy
- Medialisation Osteotomy
- Or a combination of these
Combinations of proximal and distal realignment procedures can be performed.
- MPFL reconstruction is a soft tissue procedure
- If the MPFL ligament is torn it may need to be reconstructed
- An arthroscopy of your knee will be performed first to assess the patello-femoral (knee-cap) joint and other structures in your knee
- To reconstruct the MPFL an hamstring tendon is harvested (gracilis)
Other Soft Tissue Procedures
- Lateral release
- Medial imbrication
- VMO advancement
- Distalisation osteotomy
- Medialisation osteotomy