SIJ dysfunction interferes greatly with people’s work and life. Manipulation has the advantages of minimal invasiveness and good curative effects, and it is widely used in clinical practice. However, the biomechanical mechanism of manipulation remains unclear. Clarifying its mechanism helps to improve the effect of manipulation, which is of great significance to the treatment of SIJ disorders.
Miller et al. studied the correlation between the loading force and displacement of the SIJ using eight specimens. By fixing the bilateral ilium, they applied 294 N of force from the upper, lower, anterior, posterior, and middle sides and 42 Nm torque of flexion, extension, lateral flexion, and rotation to the sacrum. They found that the average SIJ displacements were 0.28 mm, 0.26 mm, 0.48 mm, 0.53 mm and 0.01 mm, and the average rotation angles were 1.31°, 1.94°, 0.37° and 0.80°, respectively . Lindsey and colleagues performed a cadaveric study to investigate the motion of the SIJ and found that under the loading conditions of flexion-extension, rotation and lateral flexion, the ROMs of the SIJ were 1.94° and 2.65°, 1.20° and 1.77°, and 0.36° and 1.16° in the single- and double-leg stance states, respectively . In this experiment, nine hemi-pelvic specimens were used to study the effects of MOPs in different directions on the SIJ. The results showed that under a 100 N loading force, the total rotation angles of F1 and F2 were 0.84° and 1.52°, and the displacements were 0.61 mm and 0.98 mm, respectively. The ROM of the SIJ was consistent with previous studies [24, 25]. MOP-F2 could produce a greater rotation angle and displacement of the SIJ, which might be related to the fact that the force direction of MOP-F2 was more perpendicular to the SIJ surface, and the torque was greater. The rotation angles on the vertical axis produced by MOP-F1 and F2 were the largest among the three directions. MOPs were applied to the anterior superior iliac spine. The loading force applied from the ventral to the dorsal side caused the ilium to rotate outwardly with respect to the sacrum, so the rotation angle was at its maximum on the vertical axis. In the three directions, the displacements on the coronal axis caused by MOP-F1 and F2 were the largest, while the displacements on the vertical axis were the smallest. The results suggested that the main effect of MOP was to produce coronal axis movement of the SIJ. In brief, MOP can cause movement of the SIJ, but the ROM is small. MOP-F2 is a more effective manipulation.
The ligaments perform a vital role in holding the different components of the structure together against loads, which otherwise would cause separation at the pubis and SIJs. Sichting et al. considered that ligaments served as the mechanical stabilization device of the pelvis . Pool-Goudzwaard et al. found that when rotational torque was applied to the SIJ in the sagittal plane, the iliolumbar ligament could obviously constrain the SIJ motion, and the ventral band had the greatest effect on SIJ motion . Eichenseer et al. indicated that the ligaments around the SIJ could restrict its movement and decrease its stress through a finite element study . Abdelfattah and colleagues found that the pubic symphysis and the ASL played a greater role in maintaining the stability of the pelvis when the pelvis suffered an open book injury . In this study, it was found that after complete resection of the ASL, the displacement of the SIJ increased by 243%, and the rotation angle increased by 171%. It also proved that the ASL played an important role in maintaining the stability of the SIJ.
The sacrum is wedge-shaped, tilted from top to bottom and has a concave surface that is closely inserted into the convex surface of the ilium . Since humans are upright, the lower part of the SIJ surface fits more tightly. In addition, a previous CT imaging study showed that the SIJ space width gradually narrows from top to bottom . Theoretically, the SIJ gap is wider, the ROM of the SIJ is larger, and the ligaments maintaining SIJ stability bear greater strains. The results showed that, compared with the intact state of ASL, resecting the upper part increased the rotation angle and displacement of the SIJ by 70 and 85%, while resecting the lower part increased the rotation angle and displacement of the SIJ by 69 and 63%, respectively. After resecting the upper part, the rotation angle and displacement of the SIJ increased by 22 and 80%, respectively, by resecting the lower part. After resecting the lower part, the rotation angle and displacement of the SIJ increased by 118 and 133%, respectively, by resecting the upper part. The data indicated that the upper part of the ASL played a more important role in maintaining the stability of the SIJ than the lower part.
There are some limitations of this study. First, the pelvic specimens were hemi-pelvises, and the symphysis pubes had been dissected, which affected the stability of the SIJ. To this end, we took the following measures: (1) A smaller loading force of 100 N was applied; (2) A steel cable was placed at the symphysis pubis to restrict the movement of the symphysis pubis, and the tension was monitored. The forces caused by MOP-F1 and F2 were 7.62 N and 13.85 N, approximately 10% of the loading force. The results indirectly reflected that the movement of the symphysis pubis was small. Thus, the hemi-pelvis specimens had little effect on the experiment. Second, only six specimens were included in the ASL experiments, so the number of specimens was small. Finally, fresh pelvis specimens were studied, and the intact surrounding ligaments of the SIJ were preserved. However, the simulative MOP in this study could not fully reflect the characteristics of clinical MOP in vivo.