Experimental characterization of motion resistance of the sacroiliac joint

BACKGROUND: The human sacroiliac joint (SIJ) in vivo is exposed to compressive and shearing stress environment, given the joint lines are almost parallel to the direction of gravity. The SIJ supports efficient bipedal walking. Unexpected or unphysiological, repeated impacts are believed to cause joint misalignment and result in SIJ pain. In the anterior compartment of the SIJ being synovial, the articular surface presents fine irregularities, potentially restricting the motion of the joints. OBJECTIVE: To clarify how the SIJ articular surface affects the resistance of the motion under physiological loading. METHODS: SIJ surface models were created based on computed tomography data of three patients and subsequently 3D printed. Shear resistance was measured in four directions and three combined positions using a customized setup. In addition, repositionability of SIJs was investigated by unloading a shear force. RESULTS: Shear resistance of the SIJ was the highest in the inferior direction. It changed depending on the direction of the shear and the alignment position of the articular surface. CONCLUSION: SIJ articular surface morphology is likely designed to accommodate upright bipedal walking. Joint misalignment may in consequence increase the risk of subluxation.