The Knee Q angle (also known as Quadriceps Angle) is defined as the angle between the quadriceps muscle (primarily the rectus femoris) and the patellar tendon.
It corresponds to the physiologic valgus angle of the femoral shaft, this creates a lateral pull on the patella.
This tendency is a factor in habitual patellar dislocation and in the patellofemoral pain syndrome (anterior knee pain). It also creates problems in total knee arthroplasty.
How to Measure Knee Q Angle?
A line is drawn from the anterior superior iliac spine to the midpoint of the patella, corresponding to the quadriceps’ tensile direction.
Another line is drawn from the tibial tubercle to the midpoint of the patella, corresponding to the patellar tendon.
The angle formed by the crossing of these two lines is called the Q angle.
The hip and the foot should be placed in a neutral position, because significant internal rotation of the leg and pronation of the foot alter the Q angle.
Quadriceps Angle Assessment
Normally the Knee Q angle is 13° for men and 18° for women when the knee is straight.
Any angle less than 13° may be associated with patellofemoral dysfunction or patella alta.
A Q angle greater than 18° is often associated with subluxing patella, increased femoral anteversion, genu valgum , or increased lateral tibial torsion.
The Q angle of the knee can be influenced distally through motions of the tibia. For example, external rotation of the tibia moves the tibial tuberosity laterally, thereby increasing the Q angle, whereas tibial internal rotation decreases the Q angle by moving the tibial tuberosity medially.
As with tibial rotation, the Q angle can be influenced through motions of the femur. For example increased femoral internal rotation results in a larger Q angle, whereas femoral external rotation minimizes the Q angle.
The Q angle increased as the foot moved from external to internal rotation. While it’s decreased as the foot shifted from pronation to supination.
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Biedert RM, Warnke K. Correlation between the Q angle and the patella position: a clinical and axial computed tomography evaluation. Arch Orthop Traum a Surg 2001;121(6):346–349.