SACROILIAC MOTION FOR EXTREME HIP POSITIONS - A FRESH CADAVER STUDY

Study Design, This study placed fresh cadavers in different hip positi ons and obtained sacroiliac kinematics. The magnitudes and directions of angular and linear sacroiliac motion are reported. Objectives. To d etermine the three-dimensional motion at the left and right sacroiliac joints and to explore the mechanical structure of the sacroiliac join t. Summary of Background Data, Previous in vivo and in vitro studies h ave reported various amounts of motion at the sacroiliac joints, rangi ng from no motion to 18 degrees. 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 h ave not used external hip positions to elucidate the full range of sac roiliac motion. Methods. Radiopaque markers were placed in the sacrum end each innominate bone before standardized performance of computed t omography scans at 3-mm intervals, with the fresh cadaver stabilized i n 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 trac ed 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 va lidated. The largest amount of sacroiliac motion occurred in the sagit tal plane (7 degrees on left and 8 degrees on the right, with a range of 3 degrees to 17 degrees), Definite trends in the direction of angul ar sacroiliac motion occurred with respect to both bilateral and recip rocal hip joint positions, The translation or linear motion of the pos terior superior iliac spines with respect to the sacrum ranged from 4 to 8 mm. This motion tended to occur in all directions, with no detect able trends. The average intersubchondral thickness of the sacroiliac joint was 1.2 mm and the shape of the joint resembled that of an airpl ane propeller. Conclusions. Even though the subjects in this study wer e 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 direc tion of demonstrated sacroiliac motion appears to be sufficient to com plement hip joint motion and influence motion at the lumbosacral junct ion and, thus, low back pain in both the direct and indirect sense.