Five cadaveric pelves and 40 dry bony specimens were used to assess the feasibility of the lumbosacral plate fixation extending to beyond the S1 region and quantitatively to evaluate the posterior sacroiliac region and the dimensions of S1 and S2 pedicle and lateral mass. Partial removal of the posterior ilium was undertaken to measure the surface area available for plate fixation on the dorsal aspect of the sacrum. The results showed that the average distances between the outer edges of S1 and S2 dorsal foramina and the medial edge of the posterior ilium increased from 11.3 mm before removal of a portion of the medial posterior ilium to 16.6 mm after removal at the S1 level, and from 8.4 mm before removal of partial medial posterior ilium to 13 mm after removal at the S2 level, respectively. The average depths of the S1 and S2 pedicles were 37.1 and 32.2 mm in the direction anteromedial to the sagittal plane, respectively. The average depths of the S1 and S2 lateral mass were 37.3 and 33.9 mm in the direction anterolateral to the sagittal plane, respectively. In cases of vertebral metastases or osteoporosis, plating extending to S2 may be needed if good bony purchase cannot be achieved by the S1 pedicle or lateral mass screw. This study suggested also that partial removal of the posterior ilium enhances the space on the posterior aspect of the sacrum without severe compromise of the sacroiliac joint.