Q angle is the angle formed by a line drawn from the anterosuperior iliac spine to the 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 a normal knee, the line of force exerted by the quadriceps is lateral to the joint line, probably due to a 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 the patella.
Therefore assessment of this angle is a measure of the 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 the supine position with the 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.
The normal value is 13.5 ± 4.5°.
Women have a greater value greater than that for men. This is due to the wider pelvis, increased femoral anteversion, and a relative knee valgus angle.
Method
- Patient supine with knee extended with the 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 anterosuperior iliac spine to the midpoint of the 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
- The 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 a 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
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