Load-displacement behavior of sacroiliac joints

We measured the load-displacement behavior of both single and paired sacroiliac (SI) joints in fresh cadaver specimens obtained from eight adults between the ages of 59 and 74 years. With both ilia fixed, static test loads were applied to the center of the sacrum along and about axes parallel and normal to the superior SI endplate. Test forces up to 294 N were applied in the superior, inferior, anterior, posterior, and lateral directions. Moments up to 42 N-m were applied in flexion, extension, lateral bending, and axial torsion. Displacements of the center of the sacrum were measured 60 s after each load increment was applied, using dial gauges and an optical lever system. The tests were then repeated with only one ilium fixed. Finally, the three-dimensional location and overall geometry of each SI joint were measured. For an isolated left joint at the maximum test loads, the mean (SD) sacral displacements in the direction of the force ranged from 0.76 mm (1.41) in the medial to 2.74 mm (1.07) in the anterior direction. The mean rotations in the directions of the moments ranged from 1.40 degrees (0.71) in right lateral bending to 6.21 degrees (3.29) in clockwise axial torsion viewed from above. We also examined load-displacement behavior under larger loads. Single sacroiliac joints resisted loads from 500 to 1440 N, and from 42 to 160 N-m without overt failure.