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Q angle of the Knee


- Discussion:
    - Q angle is the angle formed by a line drawn from the ASIS to central patella and a second line drawn from central patella to tibial tubercle;
    - an increased Q angle is a risk factor for patellar subluxation;
    - normally Q angle is 14 deg for males and 17 deg for females;
           - Agliettis et. al. Clin. Ortho 1983:
           - 75 normal males:    Q angle = 14 deg (+/- 3)
           - 75 normal females: Q angle = 17 deg (+/- 3)
    - biomechanics of patellofemoral joint are effected by patellar tendon length & the Q angle;
    - q angle is increased by:
           - genu valgum
           - increased femoral anteversion
           - external tibial torsion
           - laterally positioned tibial tuberosity
           - tight lateral retinaculum

- Clincal Determination: (see examination of the patellofemoral joint)
    - deficiency of vastus medialis oblique is best assessed while leg is suspended in 15-20 deg of flexion;
    - large convexity at superomedial corner of patella indicates vastus medialis deficiency;
    - this is best measured both w/ the knee in extension as well as flexion;
    - Q angle may not be accurrate in extension, since a laterally dislocated patella may give false impression that the Q angle is normal;
            - in flexion, this is not a problem since the patella is well seated in the trochlear groove;
            - a sitting Q angle of more than 8 deg is abnormal


A new method using computed tomographic scan to measure the rectus femoris-patellar tendon Q-angle comparison with conventional method.

Patellar tracking in primary total knee arthroplasty.