Biomechanical Stability of the Sacroiliac Joint with Differing Implant Configurations in a Synthetic Model

The sacroiliac joint (SIJ) is responsible for 15%-30% of chronic low back pain and fusion is increasingly used to alleviate chronic SIJ pain in adults. However, questions remain as to the most effective implant patterns to stabilize the joint. The objective of this biomechanical study was to evaluate how different implant spacing, configuration and quantity effect range of motion (ROM) of a synthetic foam SIJ model.

Triangular SIJ fusion implants were tested in six patterns using three implants, and two patterns with two implants (n = 5/pattern). Linear, triangular, and angled (10° or 20°) implant patterns were used with spacing of 13 or 22 mm between implants. Implants were placed through a denser polyurethane foam block (0.32 g/cm³) representing the ilium and into a less dense block representing the sacrum (0.16 g/cm³) to a depth 30 mm with a 2-mm gap between blocks. Cyclic torsion and shear testing were conducted for 10,000 cycles and ROM was recorded. Pullout testing was conducted on non-cycled (n = 10) implants and individually on all implants after construct cycling.

ROM was significantly lower for all 22-mm implant patterns compared to the 13 mm linear pattern after cyclic loading in both torsion and shear. The use of three implants provided 60% and 86% greater stability, respectively, than two implants with spacing of 22 and 13 mm. Pullout resistance followed similar trends with the lowest forces occurring in closely spaced patterns that used two implants.

This study demonstrated that the use of three implants and maximizing the spacing between implants might provide greater stability to the SIJ. If implants must be placed closely, then nonlinear patterns may improve construct stability.