Peer-review started: December 6, 2017
First decision: December 18, 2017
Revised: December 22, 2017
Accepted: February 4, 2018
Article in press: February 5, 2018
Published online: March 18, 2018
Minimally invasive fusion of the sacroiliac (SI) joint is a potential treatment for patients suffering with symptoms related to the SI joint. The use of a lateral procedure for SI joint fusion has been shown to be an effective method for reducing SI joint pain. Previous anatomical studies have demonstrated significant variability in sacral anatomy and the resultant location and size of safe zones for implant placement.
A surgeon has options regarding the number of implants, length of implants, and their orientation; the optimal placement parameters for SI joint fixation are currently unknown. Quantification of the changes in SI joint motion as a result of varying the potential implant placement variables will provide a surgeon input when performing an SI joint fusion procedure.
The objective of this study was to investigate and quantify the effect of implant orientation, superior implant length, and implant number on SI joint range of motion.
This study used a previously validated finite element analysis to investigate how implant orientation, superior implant length, and implant number affect SI joint range of motion. Implant orientation was simulated using either an inline or a transarticular placement. The length of the superior implant was varied to end either in the middle of the ala or at the sacral midline. The number of implants was 1, 2, or 3 implants. The SI joint range of motion was calculated using a constant moment of 10 N-m with a follower load of 400 N in flexion-extension, lateral bending, and axial rotation. A total of 23 model configurations were tested and the difference in SI joint range of motion compared.
The use of a transarticular placement with a mid-sacrum length superior implant resulted in the greatest reduction in SI joint ROM. The use of transarticular placement resulted in median reductions in motion of 16%, 9%, and 18%, in flexion-extension, lateral bending, and axial rotation, respectively. Extending the superior implant to the sacral midline resulted in median reductions in motion of 14%, 8%, and 9%, in flexion-extension, lateral bending, and axial rotation, respectively. Reducing the number of implants (i.e., 1 or 2 implants) resulted in increased motions in all directions. Implant configurations with 2 implants placed farthest apart had the smallest increases.
This study demonstrates that the treated SI joint range of motion is affected by implant orientation, superior implant length, and implant number. These results show that the optimal placement investigated was 3 implants placed using a transarticular placement with a superior implant that reaches the sacral midline. This study suggests that a surgeon can optimize implant placement in 3 ways: (1) Longer superior implants; (2) transarticular placement; and (3) using 3 implants (and/or increasing implant separation).
The use of a finite element model to simulate the SI joint and treatment effects allows for investigation of many variables and provides valuable insight regarding how each variable effects SI joint stability. These results allow for more detailed investigation using either in vitro or in vivo studies.