Pelvic angles: a mathematically rigorous definition which is consistent with a conventional clinical understanding of the terms.

The most common definition of pelvic angles in conventional gait analysis uses the sequence tilt, obliquity, rotation. This is used in most commercially available gait analysis software. This definition of angles, however, is not in agreement with the conventional clinical understanding of the terms when both tilt and rotation are large. This paper shows that by using the sequence rotation, obliquity, tilt it is possible to make a mathematically rigorous definition of pelvic angles which it is consistent with that conventional clinical usage. A model of the pelvis in which the hips are maintained level is developed. It is shown that as tilt and rotation are varied, in a clinically relevant range, that obliquity measured using the conventional sequence can be as much as 10 degrees. By definition it is 0 degrees for the new sequence. A case study shows that measures of obliquity correlate better with the relative height of the hips using the new sequence than the conventional one. It is proposed that use of the new sequence would lead to data which is easier to interpret clinically.