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Does the Q Angle Reflect the Force on the Patella in the Frontal Plane?

SS Schulthies, PhD, PT, ATC, is Assistant Professor, Department of Physical Education, Brigham Young University, 122 Richards Bldg, PO Box 22116, Provo, UT 84602-2116 (USA).
RS Francis, PhD, PT, ATC, is Professor, Department of Physical Education, Brigham Young University.
AG Fisher, PhD, is Professor, Department of Physical Education, Brigham Young University.
KM Van De Graaff, PhD, is Professor, Department of Biological Sciences, Brigham Young University.

Background and Purpose. The quadriceps femoris muscle angle (Q angle) is used to reflect the quadriceps femoris muscle's force on the patella in the frontal plane. We found no studies, however, that validate this assumption. The purpose of this study was to determine whether the Q angle can be used to represent the force on the patella in the frontal plane. Subjects. Seven lower extremities from four male cadavers were dissected and investigated. Methods. We devised a model in which the line of action of quadriceps femoris muscle's resultant force was calculated in the frontal plane on the seven lower-extremity specimens. We then compared these calculations with the Q angles from the same cadaver specimens. The differences between the measured and calculated Q angles were tested for significance using a paired t test. In addition, we calculated a simple linear regression to test the relationship between the calculated and measured Q angles. Results. Our data showed that the angle for the average resultant force of the quadriceps femoris muscle was 3.90 degrees greater (P=.0003) than the measured Q angles. A significant relationship (r=.919, P=.0035), however, was found between the measured and calculated Q angles. Conclusion and Discussion. The Q angle, as measured in clinical practice, appears to reflect the angle of the resultant quadriceps femoris muscle force. We believe, however, that this measurement is significantly less than the actual quadriceps femoris muscle force vector and underestimates the lateral force on the patella.

Key Words: Biomechanics • Knee • Patellofemoral • Q angle • Validity

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