Q Angle of the Knee

 

A well-functioning knee is important for mobility. The knee must be able to support the weight of the body during activities such as walking or running. A normal alignment of the knee is important for its function. The Q-angle of the knee provides useful information about the alignment of the knee joint. The Q-angle is the angle between the quadriceps tendon and the patellar tendon. An increased Q-angle is a risk factor for patellar subluxation. The Q-angle (quadriceps angle) is formed in the frontal plane by two line segments. The first line drawn is 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 is formed by the two lines is called the Q-angle. Find the patella and its border; find the center of the patella; find the tibial tubercle; 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 normal Q-angle is variable. In extension, the normal Q-angle for males is usually 14 degrees, and in females it is approximately 17 degrees. The normal Q-angle in flexion is approximately 8 degrees. A wider pelvis and increased Q-angle in females is linked to knee pain, patellofemoral pain, and ACL injury. The alignment of the patellofemoral joint is affected 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. 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, and tight lateral retinaculum. CT scan study of the patellofemoral articulation is found to be very helpful. An increased Q-angle in the knee will lead to an increase in lateral subluxation forces on the patella which may lead to pain, wear of the implant, and mechanical symptoms. When doing a total knee replacement, avoid the techniques that will cause increased Q-angle, such as internal rotation of the femoral component of the tibial component. Avoid medialization of the femoral component. Avoid lateral placement of the patellar component. The patellar prosthesis should be placed either in the center or slightly medial. You will put the patella slightly medial, but you will put the femoral component opposite to that; it will be slightly lateral. Terms used to describe a triad of anatomic features or findings which will increase the Q-angle include excessive femoral anteversion, genu valgum, and external tibial torsion or pronated feet. Symptoms for miserable malalignment syndrome include anterior knee pain, pain under the patella, and stiffness of the knee joint. When examining a patient for patellofemoral pain, alignment is important, including rotational alignment.

Author
Nabil Ebraheim, MD

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