The measurement and correction of sacral obliquity

The primary objective of the study involved the identification and quantification of systematic errors associated with the measurement of sacral obliquity on radiographic images. A secondary objective was to provide an estimate of true sacral obliquity derived from measurements made on radiographic images of the spine that culd be used in a clinical setting to correct or reduce frontal plane obliquity. Methods used in the study included graphical, radiographic and mathematical models. They were used to evaluate the measurement of dihedral angles such as structural asymmetry of the sacral base and vertebral endplates in the frontal plane. A radiographic phantom was imaged using plain film, CT, and slot scan (scout) views. The variation of measurements using different views was compared to the known sacral obliquity value of the phantom. Results obtained from the studies demonstrate that different radiographic and analytical methods produced significant differences in the measurements of sacral obliquity for each of the imaging modalities and analytical methods. The study developed, tested and validated algorithms to calculate true dihedral angles from angles measured on radiographic images. The algorithms were designed to be employed when the protocols developed to ensure reliable measurement of angular relationships were not met or were unable to be adhered to. The measurement of sacral obliquity on radiographic images was used in the development of algorithms to calculate the amount of correction needed to level the sacral base in upright standing and sitting postures. Trigonometric techniques were developed and applied to measurements of sacral obliquity obtained from radiographic images of the erect spine to provide accurate dimensions of ischial or foot orthotics that would be required to level the sacral base in sitting or standing posture respectively.