Q Angle

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Background

The direction and magnitude of force produced by the quadriceps muscle has great influence on patellofemoral joint biomechanics. The line of force exerted by the quadriceps is lateral to the joint line mainly due to large cross-sectional area and force potential of the vastus lateralis. Since there exists an association between patellofemoral patholgy and excessive lateral tracking of the patella, assessing the overall lateral line of pull of the quadriceps relative to the patella is a meaningful clinical measure. Such a measure is referred to as the Quadriceps angle or Q angle.

Measurement

The Q angle is acute angle formed between:

1. A line representing the resultant line of force of the quadriceps, made by connecting a point near the ASIS to the mid-point of the patella
2. A line representing a long axis of the patellar tendon, made by connecting a point on the tibial tuberosity with the mid-point of the patella.

  

The Q-angle is usually measured with the knee at or near full extension (but not hyperextension) because lateral forces on the patella may be more of a problem in these circumstances. With the knee flexed, the patella is set within the intercondylar notch, and even a very large lateral force on the patella is unlikely to result in dislocation. Furthermore, the Q-angle will reduce with knee flexion as the tibia rotates medially in relation to the femur.

Factors affecting 'Q' Angle

Increases in q-angle are associated with :

• Femoral anteversion
• External tibial torsion
• Laterally displaced tibial tubercle
• Genu valgus

Clinical Importance

An understanding of the normal anatomical and biomechanical features of the patellofemoral joint is essential to any evaluation of knee function. One important concept in patellofemoral joint function is the quadriceps femoris muscle angle,better known as the "Q angle." This angle, which has been defined by Hungerford and Barryas the acute angle formed by the vector for the combined pull of the quadriceps femoris muscle and the patellar tendon, is important because of the lateral pull it exerts on the patella. Any alteration in alignment that increases the Q-angle is thought to increase the lateral force on the patella. This can be harmful because an increase in this lateral force may increase the compression of the lateral patella on the lateral lip of the femoral sulcus. In the presence of a large enough lateral force, the patella may actually sublux or dislocate over the femoral sulcus when the quadriceps mus-cle is activated on an extended knee.