Study Design. This study placed fresh cadavers in different hip positions and obtained sacroiliac kinematics. The magnitudes and directions of angular and linear sacroiliac motion are reported.
Objectives. To determine the three‐dimensional motion at the left and right sacroiliac joints and to explore the mechanical structure of the sacroiliac joint.
Summary of Background Data. Previous in vivo and in vitro studies have reported various amounts of motion at the sacroiliac joints, ranging from no motion to 18°. The results of previous studies have typically not been reported as data in classic engineering terms, and three‐dimensional analyses are sparse. Importantly, previous studies have not used external hip positions to elucidate the full range of sacroiliac motion.
Methods. Radiopaque markers were placed in the sacrum and each innominate bone before standardized performance of computed tomography scans at 3‐mm intervals, with the fresh cadaver stabilized in the side‐lying position on a specially constructed pallet. Computed tomography images were stored on magnetic tape, from which coordinates for centroids of the markers were obtained and subsequent mechanical analyses performed. The intersubchondral lines on the images were traced and reconstructed using computer technology to obtain the thickness of the intersubchondral joint space and the general configuration of the sacroiliac joints.
Results. The methods used in this study were validated. The largest amount of sacroiliac motion occurred in the sagittal plane (7° on left and 8° on the right, with a range of 3° to 17°). Definite trends in the direction of angular sacroiliac motion occurred with respect to both bilateral and reciprocal hip joint positions. The translation or linear motion of the posterior superior iliac spines with respect to the sacrum ranged from 4 to 8 mm. This motion tended to occur in all directions, with no detectable trends. The average intersubchondral thickness of the sacroiliac joint was 1.2 mm and the shape of the joint resembled that of an airplane propeller.
Conclusions. Even though the subjects in this study were elderly, considerable angular and linear motion was in evidence. As such, it appears that extreme hip positions are necessary to elucidate full range of motion at the sacroiliac joint. The magnitude and direction of demonstrated sacroiliac motion appears to be sufficient to complement hip joint motion and influence motion at the lumbosacral junction and, thus, low back pain in both the direct and indirect sense.