Q-Angle

woman running
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The Q-angle is often measured when examining the knee, especially when kneecap problems are being evaluated.

Three landmarks are needed to determine the Q-angle:

  • Anterior Superior Iliac Spine (ASIS)
    The ASIS is the front of the pelvic bone that is felt in front of the hip at the level of your waist.
  • Center of the Patella (Kneecap)
    The center of the kneecap is best identified by locating the top, bottom and each side of the kneecap, and then drawing intersecting lines.
  • Tibial Tubercle
    The tibial tubercle is the bump about 5 centimeters below the kneecap on the front of the shin bone (tibia).

The Q-angle is formed from a line drawn from the ASIS to the center of the kneecap, and from the center of the kneecap to the tibial tubercle. To find the Q-angle, measure that angle, and subtract from 180 degrees.

A normal Q-angle in men is 14 degrees and in women is 17 degrees. An increase in Q-angle can mean a higher risk of kneecap problems including patellar subluxation and patellar dislocation.

Dynamic Lower Extremity Stability

An extension of the Q-angle, is to measure the dynamic stability of the extremity, or how stable the knee is during movement.  While the normal Q-angle is a static measurement (with the knee extended and the patient lying down), a dynamic measurement is performed with the patient moving.  Dynamic measurements are thought to be more clinically relevant as they better predict how the knee functions during activity.

Dynamic lower extremity stability can be measured by having a patient perform a maneuver such as a single leg squat.  While facing an examiner, a patient does several single leg squats.  The examiner can follow the position of the kneecap as the extremity moves up and down.  Under ideal circumstances, the kneecap will remain in a stable position, not moving side-to-side much during the squat motion.

  In more concerning situations, the kneecap will wobble around, a sign of the condition called dynamic lower extremity instability.

Dynamic lower extremity instability has been linked to an increased risk of numerous acute and chronic lower extremity problems.  Everything from a higher likelihood of ACL tears to a higher chance of IT band tendonitis, dynamic lower extremity instability is often used to screen for an athlete's risk of injury.  Treatment of dynamic lower extremity instability can be best accomplished with a core and pelvic stabilization program.  By strengthening the muscles of the core, athletes control the extremity better, and are less likely to sustain a number of these injuries.

Sources:

Conley S, "The Female Knee: Anatomic Variations" J. Am. Acad. Ortho. Surg., September 2007; 15: S31 - S36.

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