Patellar instability results from one or more dislocations or partial dislocations, also called subluxations. This malalignment can damage the underlying soft structures such as muscles and ligaments that hold the knee in place. Once damaged, these soft structures are unable to keep the patella (knee cap) in position.
Signs and Symptoms
Signs and symptoms of patellar instability can include the following:
- Pain, especially when standing up from a sitting position
- Feeling of unsteadiness or tendency of the knee to "give way" or "buckle"
- Recurrent Subluxation: When the kneecap slips partially out of place repeatedly
- Recurrent Dislocation: When the kneecap slips all the way out of position repeatedly
- Severe pain, swelling and bruising of the knee immediately following subluxation or dislocation
- Visible deformity and loss of function of the knee often occurs after subluxation or dislocation
- Sensation changes such as numbness or even partial paralysis can occur below the dislocation as a result of pressure on nerves and blood vessels
Patellar instability can be caused by a number of factors that affect the way the patella moves along the groove of the femur (trochlear groove) when the leg is bent or straightened. The patella normally moves up and down with a slight tilt without touching the other knee bones. In patellar instability the patella does not maintain its normal path of movement and can slip out of the trochlear groove either partially (subluxation) or completely (dislocation).
A combination of factors can cause this abnormal tracking and include the following:
- Anatomical Defect: Flat feet or fallen arches and congenital abnormalities in the shape of the patella bone can cause misalignment of the knee joint
- Abnormal "Q" Angle: The "Q" angle is a medical term used to describe the angle between the hips and knees. The higher the "Q" angle, such as in patients with Knock Knees, the more the quadriceps pull on the patella causing malalignment
- Patellofemoral Arthritis: Patellar malalignment causes uneven wear and tear and can eventually lead to arthritic changes to the joint
- Improper Muscle Balance: Quadriceps, the anterior thigh muscle functions to help hold the kneecap in place during movement. Weak thigh muscles can lead to abnormal tracking of the patella, causing it subluxate or dislocate
The goal of conservative treatment for patellar instability is to restore full range of motion by restoring the normal tracking pathway of the patella during flexion and extension of the knee.
Treatment options include the following:
- Closed Reduction
- Pain Medications
- Physical Therapy
Surgical treatment of patellar instability is sometimes necessary to help return the patella to a normal tracking path when conservative treatment options are unsuccessful.
The goal of surgery is to re-align the patella and decrease the Q-angle. Surgical treatments can be categorized into two types:
- Proximal re-alignment procedures: Proximal re-alignment involves lengthening structures on the outside of the patella that are restraining movement and/or shortening ligaments on the inside of the patella. Usually done in young patients still growing, it is often done in combination with distal re-alignment
- Distal re-alignment procedures: Distal re-alignment decreases the Q-angle by dividing the tibial tubercle so that the bone and patellar tendon can be moved toward the inside of the knee. Screws are used to reattach the bone to the tibia
Your surgeon will decide which options are best for you depending on your specific circumstances.