Have you ever felt like your knee cap keeps wanting to pop out of your knee whenever you play certain sports or get out of your car?
It could be that your knee cap (patella) is unstable. An unstable patella may sublux as it tries to dislocate, or it may even dislocate when you make certain movements in your knee.
A: Normal knee anatomy where the patella sits well within the femoral groove
B: Patella subluxation where the patella sits partially out of the femoral groove
C: Patella dislocation where there the patella is completely out of the femoral groove
There are several factors which contribute to patella instability:
- Depth of patella groove (femoral dysplasia_
- Position of the patella (normal, high or low)
- Alignment of the knee itself (valgus)
- Torn stabilising ligament (MPFL)
Knee dysplasia
A normal trochlea is generally deep and provides stability to the patella as the knee moves from flexion to extension. In cases of dysplasia, the depth of the groove is shallow or even flat. This makes it difficult for the patella to be contained, so it subluxes or even dislocates laterally.
Abnormal patella height and position
Patellas which are low in the knee may cause symptoms such as weakness of knee extension, impingement and reduced range of motion. Most patients are born with this, while a minority may develop this as a result of previous trauma, knee infection or surgery. High riding patellas (patella alta) are not well positioned in the femoral groove. As such they may be unstable during flexion and extension movements and are at increased risk of subluxation and dislocations.
These deformities may be corrected by relocating the tibial tuberosity of the knee with the patella tendon attaches to the tibia.
Lower limb malalignment (Valgus)
As humans, we are all made quite different from person to person. Some of us have fairly straight legs, while others or either bow legged (varus knees) or have knocked knees (valgus knees). Patients with valgus knees experience more lateral or outward translation of their patella and this increases the risk of subluxation or dislocation. The greater their bowleggedness, the greater the force to sublux or dislocate the knee (shown in yellow arrows).
Patients with severe valgus knees may require a knee straightening procedure to correct their leg alignment. This may be done to prevent further patella dislocations from happening in future.
Medial patella-femoral ligament (MPFL) rupture
The MPFL is a ligament on the inside of the knee that helps to keep the patella within the femoral groove. It helps prevent the patella from dislocating laterally. In patients who frequently sublux or have experienced a patella dislocation before, this particular ligament may have ruptured. Once ruptured, there is no ligament to prevent the patella from dislocating further.
The MPFL is a restraint on the medial or inner aspect of the knee
Image C shows the torn MPFL when the patella dislocates
Orthopaedic surgeons regularly reconstruct the torn MPFL in patients who have had a patella dislocation. This surgery helps to stabilise the patella and prevent further injuries to the cartilage in the patella-femoral joint.
Summary
There are many reasons why patients experience instability of their patella. Some have experienced subluxation while others have dislocated their patella altogether. Soft tissue, bony and alignment factors all contribute to these symptoms. It is important for your orthopaedic surgeon to determine the underlying cause for it before the appropriate treatment can be administered.
