Q Angle Measurement and Significance

Q angle is the angle formed by a line drawn from the antosuprior iliac spine to central patella and a second line drawn from central patella to tibial tubercle

Patellofemoral joint biomechanics is influenced by the direction and magnitude of force exerted by quadriceps muscle.  In normal knee, the line of force exerted by the quadriceps is lateral to the joint line, probably due to larger force of vastus lateralis.

Normally angle is 14 deg for males and 17 deg for females.

Increase in Q angle is associated with increased risk of lateral subluxation of patella.

Therefore assessment of this angle is measure of pull of the quadriceps relative to the patella.

Q angle was described by Brattstrom.

Measurement of Q Angle

The angle is traditionally  be measured in supine position with knee in full extension. [Standing position is considered more suitable as it mimics the joint biomechanics during daily activity.]

It is measured with the knee at or near full extension.

Normal value is 13.5 ± 4.5°.

Women have greater value greater than that for men. This is due to wider  pelvis, increased femoral anteversion, and a relative knee valgus angle.

Method

• Patient supine with knee extended with hip in neutral position and foot also in neutral position. Ensure that the lower extremity is at a right angle to the line joining both anterosuperior iliac spine.
• Draw a line from anerosuperior iliac spine to the midpoint of patella.
• Draw another line from the midpoint of the patella to the tibial tubercle.
• The resultant angle formed by the crossing of these two lines is called the Q angle.

Factors Affecting Value

Q angle is only an estimate of the line of pull of the quadriceps and can be affected by

• Significant imbalance between the vastus medialis and vastus lateralis muscles
• An abnormally sitting patella
• Q angle may not be accurate 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

Q angle is increased by

• Genu valgum More obliquity of the femur and concomitantly, the obliquity of the pull of the quadriceps
• Increased femoral anteversion
• External tibial torsion
• Laterally positioned tibial tuberosity
• Tight lateral retinaculum