Lightning Podiums: Spinal Potpourri - 803B

Presented by: K. Odeh


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

(1) San Francisco Orthopaedic Residency Program, San Francisco, CA, United States
(2) The Taylor Collaboration, San Francisco, CA, United States
(3) 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. Coupled motion is described as the phenomenon of consistent association of one motion about an axis with another motion about a second axis. 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 translational motion in each axis utilizing the standard flexion-extension (FE), lateral bending (LB), and axial rotation (AR) testing. Our hypothesis was that current descriptions do not fully describe SIJ motion given the possibility of coupled motions. 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 the directions of FE, right/left AR, and right/left LB. Three preconditioning cycles were run, then a final cycle applied a moment from 0 to 7.5 Nm in 1.5 Nm increments; data was taken from the final cycle. Relative motions between the sacrum and iliac wings were collected with an Optotrak system and infrared markers. We defined on-axis rotation as the component rotation in the same plane as the loading moment, resultant translation as the Pythagorean sum of the three translation components, and in-plane translation as the Pythagorean sum of the two component translations in the same plane as the loading moment.

Results: In FE loading, on-axis rotation measured 2.65±1.71°, resultant translation was 1.87±1.43 mm, and in-plane translation was 1.80±1.43 mm. In AR loading, on-axis rotation measured 1.77±1.25°, resultant translation was 1.57±1.13 mm, and in-plane translation was 1.38±0.97 mm. In LB loading, on-axis rotation measured 1.16±1.16°, resultant translation was 1.50±1.23 mm, and in-plane translation was 1.20±1.13 mm. In-plane translation was significantly higher (p=0.005) in FE loading than in LB loading. The correlation between on-axis rotation and resultant translation is highest in AR loading (r2 = 0.813) and lower in FE and LB loading (r2 = 0.685 and r2 = 0.667, respectively).

Conclusion: A non-trivial amount of translation occurred out of the expected plane of motion in our in vitro study. Relative to resultant translation, in-plane translation was lowest in LB. Our results indicate that translation 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 translation measurements when describing SIJ kinematics.