A well-functioning knee joint is important for mobility. The knee must be able to support the weight of the body during activities such as walking or running.
What is a Q-angle? 
The Q Angle, or quadriceps angle, is the angle between the quadriceps tendon and the patellar tendon. An increased Q-angle is a risk factor for patellar subluxation.
How do you measure the Q-angle?
First, you will need to find the patella and its border. Then, you will need to find the center of the patella. You will then need to find the tibial tubercle and draw a line from the ASIS to the center of the patella and a second line from the tibial tubercle through the center of the patella.
The Q-angle is formed in the frontal plane by the two line segments. It is the angle formed by a line drawn from the Anterior Superior Iliac Spine (ASIS) to the center of the patella. A second line is drawn from the center of the patella to the tibial tubercle. The angle formed by the two lines is called the Q-angle. The normal Q-angle is variable. In males, the angle is usually 14° and 17° in females. A wider pelvis and an increased Q-angle in females is linked to knee pain, patellofemoral pain, and ACL injury. The alignment of the patellofemoral joint is effected by the patellar tendon length and the Q-angle. It is best to measure the Q-angle with the knee in extension as well as flexion.
A larger Q-angle plus a strong quadriceps contraction can dislocate the patella. The Q-angle is increased by:
- Genu valgum
- External tibial torsion
- Femoral anteversion
- Lateral positioned tibial tuberosity
- Tight lateral retinaculum
A CT scan study of the patellofemoral articulation is found to be very helpful.
Dr. Nabil Ebraheim's Blog 