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The anterior part of the sacroiliac joint (SIJ) is a synovial joint, with little gliding and rotary movement between the contact surfaces of SIJ during locomotion. Due to its complex structure, especially when considering the surrounding ligaments, it is difficult to construct an accurate three-dimensional (3-D) finite element model for the human pelvis. Most of the pelvic models in the previous studies were simplified with either SIJ fusing together or without the sacral bone. However, the influence of those simplifications on the load transmission in human pelvis has not been studied, so the reliability of those studies remains unclear. In this study, two 3-D pelvic models were constructed: an SIJ fusing model and an SIJ contacting model. In the SIJ fusing model, the SIJ interfaces were fused together. In the SIJ contacting model, the SIJ interfaces were just in contact with each other without fusion. Compared with the SIJ contacting model, the SIJ fusing model have smaller movements in the SIJ. The stress distribution area in the SIJ fusing model on sacroiliac cartilages was also different. Those differences contributed to the decline of tensile force in the SIJ surrounding ligaments and the re-distribution of stress in the pelvic bones. In addition, the SIJ fusing model was far less sensitive to the increase in modulus of the sacroiliac cartilages, and decrease in stiffness of the ligaments surrounding the SIJ. The presence of synovia in the SIJ had greater influence on the load transmission in the human pelvic system. Therefore, the effect of the presence of synovia should not be neglected when the biomechanical behavior of human pelvis is being studied, especially for those studies related to clinical applications., (Crown Copyright © 2014. Published by Elsevier Ltd. All rights reserved.)

Previous finite element studies of the pelvis, including subject-specific studies have made extensive simplifications with regards to the boundary conditions used during analysis. Fixed boundary conditions are generally utilised at the pubis and superior part of the ilium. While it can be demonstrated that these models provide a close match for certain in vitro experiments that use similar boundary conditions, the resulting stress-strain fields in the cortex in particular are unlikely to be those found in vivo. This study presents a finite element analysis in which the pelvis is supported by muscular and ligamentous boundary conditions, applied using spring elements distributed over realistic attachment sites. The analysis is compared to an analysis in which the pelvis is restrained by fixed boundary conditions applied at the sacro-iliac joints. Striking differences in the stress-strain fields observed in cortical bone in particular, are found between the two analyses. The inclusion of muscular and ligamentous boundary conditions is found to lower the occurrence of stress concentrations within the cortex.

A new experimental technique for the measurement of three-dimensional (3-D), six degrees of freedom motion of a human joint is described. The apparatus consists of a solid cube and an orthogonal frame. Six spring-loaded linear variable differential transformers (LVDTs) are held by the frame in contact with three adjacent orthogonal surfaces of the cube. Three of the six LVDTs are positioned non-colinearly in contact with the first surface of the cube, two with the second surface, and one with the third surface. With the cube mounted on one of the moving bodies of a joint and the frame on the other one relative 3-D motion of the joint can be obtained from the six LVDT measurements based on the rigid body kinematics. An algorithm was developed to compute three Euler rotation angles and the 3-D translational components of a reference point on the body segment. The system was calibrated and applied to an in vitro study of sacroiliac joint motion. It has a rotational accuracy of 0.04 degree (with a S.D. of 0.07 degree) and a translational accuracy of 0.01 mm (with a S.D. of 0.01 mm). The possibility of adding a redundant seventh LVDT has also been explored and improvements in accuracy are reported. The main advantages of this technique are its compact size, accuracy, and low overall cost.