Modelling skin pelvic landmark coordinates into corresponding internal bone for wheelchair users.

The purpose of this study was to investigate the relationships, by linear regression, between internal and external pelvic landmarks identified by two techniques: manual digitization or skin markers. It was hypothesized that the body mass index or the skinfold thickness are significant variables in these relationships. The internal pelvic landmarks were obtained with a stereoradiographic method. Results showed that the external coordinates are generally statistically different from the internal ones; manual digitization of the landmark reduces the soft tissue artifacts compared to the use of skin markers. Different regression models were obtained according to the external acquisition method. Body mass index or skinfold thickness was generally included as a significant variable in models along the direction of the soft tissue thickness: postero-anterior direction for the anterior-superior iliac spine, medio-lateral direction for the apex of the iliac crests. With the use of skin markers, models obtained for a specific internal landmark coordinate include generally many variables, such as the other two coordinates of the landmark, body mass index, or skinfold measurements. This study presented preliminary results on the relationships between internal and external pelvic landmark coordinates. More research is needed before the full relationships are understood and adequate models are developed.

Accessing the influence of repositioning on the pelvis' 3-D orientation in wheelchair users.

This study aimed at evaluating the effects of mechanical repositioning, obtained by the increase in seat-to-back (STB) and system tilt angles, on the position of the pelvis with spinal-cord injured subjects seated in a wheelchair. The noninvasive method used combined magnetic resonance imaging (MRI) images of the whole pelvis obtained in a supine posture and ultrasound images of the pelvic iliac crests obtained in four seating positions. The matching of the two image data sets enabled the location of fourteen pelvic landmarks in the seated positions. From these landmarks, the pelvic tilt, obliquity, and transverse rotation, and the three-dimensional (3-D) motion of the pelvis were calculated. Results showed that the increase in STB angle is not equal to the calculated increase in pelvic tilt and that the pelvis rotated posteriorly, moved forward and downwards. An increase in the system tilt moved the pelvis rearwards and downwards, which counter-balanced the movement seen with the increase in STB. At the return to the first position, no significant changes were observed in the pelvis' position and orientation compared to the initial posture. Results also demonstrated the importance in calculating the total 3-D rotations and translations to characterize adequately the pelvic movement.

Differences between pelvic skin and bone landmark identification in different seated positions on spinal-cord injured subjects.

The purpose of this paper was to determine the differences between internal and external pelvic landmark locations in different seating positions. A computer tool developed for the registration of two series of images was used to obtain the internal geometry. First, images of the pelvis were acquired by magnetic resonance imaging (MRI) for each subject, in a supine position; internal landmarks were then identified on the images. Second, ultrasound images of the iliac crests were acquired in four seated positions. A registration algorithm was applied to obtain the transformation matrix between the two image reference systems. The MRI anatomical landmarks were, therefore, transferred into the ultrasound referential, to obtain their three-dimensional (3-D) location in the different seating positions. The external landmarks in those seated positions were identified with a 3-D digitizer. The results revealed that generally the internal and external coordinates of corresponding landmarks are statistically different. The differences are not only due to soft tissue thickness but also to different interpretations of the landmarks' locations between the supine and the seated postures. However, these differences generally did not affect significantly the accuracy with which orientation indexes can be estimated (pelvic tilt, obliquity, transverse rotation). Correlations were found between the internal and external coordinates, implying that linear regressions can be established.

Method to geometrically personalize a detailed finite-element model of the spine.

To date, developing geometrically personalized and detailed solid finite-element models (FEMs) of the spine remains a challenge, notably due to multiple articulations and complex geometries. To answer this problem, a methodology based on a free-form deformation technique (kriging) was developed to deform a detailed reference finite-element mesh of the spine (including discs and ligaments) to the patient-specific geometry of 10- and 82-year-old asymptomatic spines. Different kriging configurations were tested: with or without smoothing, and control points on or surrounding the entire mesh. Based on the results, it is recommended to use surrounding control points and smoothing. The mean node to surface distance between the deformed and target geometries was 0.3±1.1 mm. Most elements met the mesh quality criteria (95%) after deformation, without interference at the articular facets. The method's novelty lies in the deformation of the entire spine at once, as opposed to deforming each vertebra separately, with surrounding control points and smoothing. This enables the transformation of reference vertebrae and soft tissues to obtain complete and personalized FEMs of the spine with minimal postprocessing to optimize the mesh.