Three-dimensional in vivo displacements of the shoulder complex from biplanar radiography

The goal of this study was to adapt roentgen photogrammetry to in vivo studies of shoulder skeletal motion during arm elevation in the scapular plane. Numerous in vitro and in vivo studies have been published describing shoulder bone movements. They involve plain radiographic measurements and utilize a three-dimensional (3D) approach. Measurements are either direct using pins implanted in bones, or indirect recording points on medical images. Roentgen photogrammetry locates points in space from two projections obtained from two different radiographic incidences. The technique has been applied in vivo by implanting metallic balls in bones. However, to be used as a standard clinical procedure, the technique must be adapted to be less invasive. In vivo photogrammetric reconstruction of known points in 3D space requires that the subject is strictly motionless between the successive radiographic exposures or that the exposures are obtained simultaneously. Methods used in this study were developed to allow subsequent exposures to be used for analysis. Numerical tools have been developed to align the two projections of a point in 3D space which have moved slightly between two successive exposures. The standard photogrammetric technique is completed by geometric modeling of the shoulder complex and humerus, and by the control of their mutual proximity at the level of joints. Bones are modeled as a set of simple volumes linked together using geometric shapes described by shape parameters. The coincidence between real bone contours and radiographic projections of the modeled bone gives the values of the shape parameters and the accurate location in space. Results focus on two different topics: errors related to the use of roentgen photogrammetry with successive exposures, and results obtained by applying roentgen photogrammetry to the in vivo shoulder complex. Results describing shoulder bone and joint displacements are presented for comparison with previously published results. The technique of roentgen photogrammetry can successfully be applied to patients. The radiographic protocol is simple, and data can be obtained easily and quickly from the digitized films. The data obtained from asymptomatic shoulders compared favorably with published values. Future research will focus on comparisons between kinematics of the symptomatic and asymptomatic contralateral limbs in volunteers.