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 tuberosity, 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 and vertical co-ordinate data describing the position of the anterior superior illiac spine, the centre of the patella and the centre of the tibial tuberosity, and the effect on the quadriceps angle determined. Results. Errors between 1 and 5 mm in the medial/lateral location of the centre of the patella produced changes in the quadriceps angle between 1.13° and 5.53°. Errors between 1 and 5 mm in the medial/lateral location of the tibial tuberosity produced changes in the quadriceps angle between 1.02° and 5.18°. Conclusions. The quadriceps angle is highly sensitive to error in the definition of the centre of the patella and tibial tuberosity. As an approximation, these centres need to be defined with an accuracy of less than 2 mm if the error in the quadriceps angle is to remain below 5°.