The inter-lamellar connectivity of the annulus fibrosus in the intervertebral disc has been shown to affect the prediction of the overall disc behaviour in computational models. Using a combined experimental and computational approach, the inter-lamellar mechanical behaviour of the disc annulus was investigated under conditions of radial loading.Twenty-seven specimens of anterior annulus fibrosus were dissected from 12 discs taken from four frozen ovine thoracolumbar spines. Specimens were grouped depending on their radial provenance within the annulus fibrosus. Standard tensile tests were performed. In addition, micro-tensile tests under microscopy were used to observe the displacement of the lamellae and inter-lamellar connections. Finite elements models matching the experimental protocols were developed with specimen-specific geometries and boundary conditions assuming a known lamellar behaviour. An optimisation process was used to derive the interface stiffness values for each group. The assumption of a linear cohesive interface was used to model the behaviour of the inter-lamellar connectivity.The interface stiffness values derived from the optimisation process were consistently higher than the corresponding lamellar values. The interface stiffness values of the outer annulus were from 43% to 75% higher than those of the inner annulus. Tangential stiffness values for the interface were from 6% to 39% higher than normal stiffness values within each group and similar to values reported by other investigators. These results reflect the intricate fibrous nature of the inter-lamellar connectivity and provide values for the representation of the inter-lamellar behaviour at a continuum level.

There is an increased interest in studying the biomechanics of the facet joints. For in silico studies, it is therefore important to understand the level of reliability of models for outputs of interest related to the facet joints. In this work, a systematic review of finite element models of multi-level spinal section with facet joints output of interest was performed. The review focused on the methodology used to model the facet joints and its associated validation. From the 110 papers analysed, 18 presented some validation of the facet joints outputs. Validation was done by comparing outputs to literature data, either computational or experimental values; with the major drawback that, when comparing to computational values, the baseline data was rarely validated. Analysis of the modelling methodology showed that there seems to be a compromise made between accuracy of the geometry and nonlinearity of the cartilage behaviour in compression. Most models either used a soft contact representation of the cartilage layer at the joint or included a cartilage layer which was linear elastic. Most concerning, soft contact models usually did not contain much information on the pressure-overclosure law. This review shows that to increase the reliability of in silico model of the spine for facet joints outputs, more needs to be done regarding the description of the methods used to model the facet joints, and the validation for specific outputs of interest needs to be more thorough, with recommendation to systematically share input and output data of validation studies.

Subject-specific finite element (FE) models could improve decision making in canine long bone fracture repair. However, it preliminary requires that FE models predicting the mechanical response of canine long bone are proposed and validated. We present here a combined experimental-numerical approach to test the ability of subjectspecific FE models to predict the bending response of seven pairs of canine humeri directly from medical images. Our results show that bending stiffness and yield load are predicted with a mean absolute error of 10.1% (±5.2%) for the fourteen samples.This study constitutes a basis for the forthcoming optimization of canine long bone fracture repair.

The complex motion and geometry of the spine in the cervical region makes it difficult to determine how loads are distributed through adjacent vertebrae or between the zygapophysial (facet) joints and the intervertebral disc. Validated finite element modes can give insight on this distribution. The aim of this contribution was to produce direct validation of subject-specific finite element models of Functional Spinal Units (FSU׳s) of the cervical spine and to evaluate the importance of including fibre directionality in the mechanical description of the annulus fibrosus. Eight specimens of cervical FSU׳s were prepared from five ovine spines and mechanically tested in axial compression monitoring overall load and displacements as well as local facet joints pressure and displacement. Subject-specific finite element models were produced from microCT image data reproducing the experimental setup and measuring global axial force and displacement as well as local facet joints displacement and contact forces. Material models and parameters were taken from the literature, testing isotropic and anisotropic materials for the annulus fibrosus. The validated models showed that adding the direction of the fibres to their non-linear behaviour in the description of the annulus fibrosus improves the predictions at large strain values but not at low strain values. The load transferred through the facet joints was always accurate, irrespective of the annulus material model, while the predicted facet displacement was larger than the measured one but not significantly. This is, to the authors׳ knowledge, the first subject-specific direct validation study on a group of specimens, accounting for inter-subject variability.