Effect of errors in the identification of anatomical landmarks on the accuracy of Q angle values

Objective. To determine the effect of error in the location of the anterior superior illiac spine, and the centres of the patella and tibial tuberosit y, on the measured value of the quadriceps angle. Background. The quadriceps angle is said to be relevant in the etiology and management of patello-femoral pain. However, the issues around measurement accuracy have not been reported. Methods. Errors between 1 and 5 mm were introduced to the medial/lateral an d vertical co-ordinate data describing the position of the anterior superio r illiac spine, the centre of the patella and the centre or the tibial tube rosity. and the effect on the quadriceps angle determined. Results. Errors between 1 and 5 mm in the medial/lateral location of the ce ntre of the patella produced changes in the quadriceps angle between 1.13 d egrees and 5.53 degrees. Errors between 1 and 5 mm in the medial/lateral lo cation of the tibial tuberosity produced changes in the quadriceps angle be tween 1.02 degrees and 5.18 degrees. Conclusions. The quadriceps angle is highly sensitive to error in the defin ition of the centre of the patella and tibial tuberosity. As an approximati on, these centres need to be defined with an accuracy of less than 2 min if the error in the quadriceps angle is to remain below 5 degrees.