# Ligament pre-tension determines outcome in sacroiliac joint in silicon modelling

## TL;DR

This study shows that ligament pre-tension has a bigger impact on sacroiliac joint stability and stress than body weight, and these effects vary between male and female joint geometries.

## Contribution

The study demonstrates that ligament pre-tension influences sacroiliac joint mechanics more than body weight and that these effects are sex-specific.

## Key findings

- Ligament pre-tension had a greater effect on stress and joint movement than body weight in finite element models of the sacroiliac joint.
- Stresses and relative movement were higher in typical female joint geometries compared to typical male geometries under the same pre-tension.
- Relative joint motion was more sensitive to parameter variations than stress in the models.

## Abstract

Biomechanical analyses of the sacroiliac joint (SIJ) are limited. We hypothesize that influence of ligament pre-tension on strain and relative joint movement is morphologically sex-specific and more pronounced than effects of body weight. Finite element models were developed from CTs of a larger cohort (N = 818) for typical male (TMJ) and typical female joint (TFJ) geometries. For different loading scenarios, stresses were higher in TFJ than TMJ for same pre-tension, only considering sex-specific morphology. Loading in antero-posterior direction caused highest stresses and relative movement. Ligament pre-tension was most sensitive with mean sensitivity factor (change output [%]/change input [%]): 71.04/33.64 for translation, 43.09/4.02 for rotation, 2.11/ − 8.97 for stress for TFJ/TMJ respectively. Mean sensitivity factor of ligament stiffness was − 1.14/ − 1.06 for translation, − 0.90/ − 0.89 for rotation and 0.17/0.13 for stress, while mean sensitivity of load intensity was 1.09/1.10 for translation, 0.91/0.88 for rotation and 0.54/0.58 for stress for TFJ/TMJ respectively. Relative motion was more sensitive to parameter variations than stress. The hypothesis was confirmed: influence of ligament pre-tension on stress but especially relative joint movement of SIJ is morphologically sex-specific and larger than body weight effects. As this may play a crucial role in pain development, ligament pre-tension must be verified in situ in the future.

Sacro-iliac joint models are determined by ligament laxity: stiffness & pretension. Our analyses show the relative effect of parameter assumptions on modeling results. A substantial preload or pretension changes joint stability / mobility and stress. The influence of ligament pre-tension on stress and relative joint mobility of the sacro-iliac joint was larger than load intensity (bodyweight of patient) in our modelling set-up. Evaluation of the specific ligament pretension is imperative for patient-specific finite element models.