In-vitro 3D Analysis of Sacroiliac Joint Kinematics: Primary and Coupled Motions

Study design: An in-vitro biomechanical study of human cadaver sacroiliac joints.

Objective: 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.

Summary of background data: Increasing attention has been given to the sacroiliac joint (SIJ) as a source of low back pain, despite its limited range of motion. We sought to characterize the rotational and translational motion in each axis utilizing standard pure moment flexion-extension (FE), lateral bending (LB), and axial rotation (AR) testing.

Methods: Sixteen sacroiliac joints were evaluated from eight lumbosacral cadaver specimens (six females, two males) from subjects aged 28 to 57 years (mean age 46.8) with body mass index (BMI) 22 to 36 (mean BMI 30). 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. Relative motions were collected with infrared markers.

Results: 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). In-plane translation was significantly higher (P = 0.005) in FE loading than in LB loading.

Conclusion: A nontrivial amount of rotation and translation occurred out of the expected axis of motion. The largest amount of off-axis rotation was observed in lateral bending. Relative to resultant translation, in-plane translation was lowest in lateral bending. 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.Level of Evidence: 5.