General Session: Biomechanics - Hall F

Presented by: K. Odeh


K. Odeh(1), B. Taylor(2), C. Purviance(2), G. Gajudo(2), D. Lindsey(3), S. Yerby(3), D. Kondrashov(1,4), J. Leasure(1,2)

(1) San Francisco Orthopaedic Residency Program, San Francisco, CA, United States
(2) The Taylor Collaboration, San Francisco, CA, United States
(3) SI-BONE, Inc., San Jose, CA, United States
(4) St. Mary's Spine Center, San Francisco, CA, United States


Introduction: Increasing attention has been given to the sacroiliac joint (SIJ) as a source of low back pain, despite a limited range of motion. Fusion of the SIJ seeks to minimize this movement as a possible pain generator. Our study aimed to develop a more comprehensive understanding of the native motion of the SIJ within the context of spinal kinematics and spinal implant evaluation. We sought to characterize the rotational motion in each axis utilizing standard flexion-extension (FE), lateral bending (LB), and axial rotation (AR) testing. Our hypothesis was that current descriptions do not fully describe SIJ motion. Methods and materials: Eight human lumbosacral cadaver specimens (6 female, 2 male) were harvested from subjects aged 28-57 (mean age 46.8) with BMI 22-36 (mean BMI 30). Both ischia were potted in two separate blocks of epoxy resin, and the L4 endplate was secured in a cylindrical potting. Single leg stance was modeled by clamping the blocks on one ischium in a vise and letting the contralateral ischium hang freely. Pure moment loading was applied in FE, right/left AR, and right/left LB. Three preconditioning cycles were run and then a final cycle was applied a moment from 0 to 7.5 Nm in 1.5 Nm increments which data was collected from. Relative motions between the sacrum and iliac wings were collected with an Optotrak system and infrared markers. We defined resultant rotation as the Pythagorean sum of the three sub-component rotations, on-axis rotation as the component rotation in the same plane as the loading moment, on-axis rotation ratio as the on-axis rotation divided by the resultant rotation, and rotational deviation angle as the angle between the on-axis rotation plane and the plane of resultant rotation.

Results: All eight specimens completed bilateral ROM testing. In FE resultant rotation measured 2.76±1.77°, on-axis rotation measured 2.65±1.71°, and the mean on-axis ratio was 0.96±0.05. In AR resultant rotation measured 1.85±1.26°, on-axis rotation measured 1.77±1.25°, and the mean on-axis ratio was 0.95±0.06. In LB resultant rotation measured 1.90±1.93°, on-axis rotation measured 1.16±1.16°, and the mean on-axis ratio was 0.75±0.30. The on-axis ratio was significantly lower in LB than in FE (p=0.012) and in AR (p=0.017). The rotation deviation angle measured 13.9±9.1° in FE, 17.1±8.7° in AR, and 35.7±25.7° in LB. In LB the rotational deviation angle is significantly higher than both FE and AR (p=0.003 and p=0.011, respectively).

Conclusion: A non-trivial amount of rotation occurred out of the expected axis of motion in our in-vitro study. The largest amount of off-axis rotation was observed in LB. Our results indicate that rotation of the SIJ is not fully described with the in-plane metrics which are normally reported in evaluation of fusion devices. Future studies of the SIJ may need to consider including off-axis rotation measurements when describing SIJ kinematics.