The scapulo-humeral rhythm: effects of 2-D roentgen projection

Objective. The objective of this study is to illustrate the low accuracy of two-dimensional (2-D) X-ray projection methods for the quantification of t he three-dimensional (3-D) shoulder motions. Background. The traditional method for the quantification of the gleno-hume ral motion is by means of 2-D X-ray recording. The motion was characterized by the scapulo-humeral rhythm: the ratio of the nett humeral elevation ove r nett scapular rotation. The method was based on the quantification of the planar projection of the spatial positions of X-ray dense structures of th e scapula. The deformations introduced by the central projection method, a feature of X-ray projection, cannot be compensated for by calibration: the position of the scapula with respect to the camera setting is unknown, and skeletal landmarks of the scapula cannot uniquely be identified. The transf ormation from 3-D orientations to 2-D angles will, therefore, be inaccurate . Methods. A 2-D X-ray projection of the scapula during a typical arm abducti on was simulated. The 3-D motion was obtained by means of palpation and sub sequent digitization of skeletal landmarks of the scapula. The 3-D position s of the recorded landmarks were projected on a plane by a simulation based on the parameters of the X-ray equipment. The scapulo-humeral rhythm was c alculated for the different scapular landmarks, and for the orientation of the subject with respect to the projection axis. The results were compared with previous published scapulo-humeral rhythms. Results. The scapulo-humeral rhythm depends both on the choice of the skele tal landmarks, used to quantify the scapular rotations, and on the orientat ion of the subject in the X-ray setting. The full range of results obtained from earlier published experiments could be obtained from a simulation bas ed on a single 3-D arm abduction. Conclusions. The 2-D scapulo-humeral rhythm, obtained from planar X-ray pro jection, is an inaccurate parameter to define the scapular motions.