Clinically, spinal cord injuries (SCIs) are radiographically evaluated and diagnosed from plain radiographs, computed tomography (CT), and magnetic resonance imaging. However, it is difficult to conclude that radiographic evaluation of SCI can directly explain the fundamental mechanism of spinal cord damage. The von-Mises stress and maximum principal strain are directly associated with neurological damage in the spinal cord from a biomechanical viewpoint. In this study, the von-Mises stress and maximum principal strain in the spinal cord as well as the cord cross-sectional area (CSA) were analyzed under various magnitudes for contusion, dislocation, and distraction SCI mechanisms, using a finite-element (FE) model of the cervical spine with spinal cord including white matter, gray matter, dura mater with nerve roots, and cerebrospinal fluid (CSF). A regression analysis was performed to find correlation between peak von-Mises stress/peak maximum principal strain at the cross section of the highest reduction in CSA and corresponding reduction in CSA of the cord. Dislocation and contusion showed greater peak stress and strain values in the cord than distraction. The substantial increases in von-Mises stress as well as CSA reduction similar to or more than 30% were produced at a 60% contusion and a 60% dislocation, while the maximum principal strain was gradually increased as injury severity elevated. In addition, the CSA reduction had a strong correlation with peak von-Mises stress/peak maximum principal strain for the three injury mechanisms, which might be fundamental information in elucidating the relationship between radiographic and mechanical parameters related to SCI.

Recently, experimental results have demonstrated that the load carrying capacity of the human spine substantially increases under the follower load condition. Thus, it is essential to prove that a follower load can be generated in vivo by activating the appropriate muscles in order to demonstrate the possibility that the stability of the spinal column could be maintained through a follower load mechanism. The aim of this study was to analyze the coordination of the trunk muscles in order to understand the role of the muscles in generating the follower load. A three-dimensional finite element model of the lumbar spine was developed from T12 to S1 and 117 pairs of trunk muscles (58 pairs of superficial muscles and 59 pairs of deep muscles) were considered. The follower load concept was mathematically represented as an optimization problem. The muscle forces required to generate the follower load were predicted by solving the optimization problem. The corresponding displacements and rotations at all nodes were estimated along with the follower forces, shear forces, and joint moments acting on those nodes. In addition, the muscle forces and the corresponding responses were investigated when the activations of the deep muscles or the superficial muscles were restricted to 75% of the maximum activation, respectively. Significantly larger numbers of deep muscles were involved in the generation of the follower load than the number of superficial muscles, regardless of the restriction on muscle activation. The shear force and the resultant joint moment are more influenced by the change in muscle activation in the superficial muscles. A larger number of deep trunk muscles were activated in order to maintain the spinal posture in the lumbar spine. In addition, the deep muscles have a larger capability to reduce the shear force and the resultant joint moment with respect to the perturbation of the external load or muscle fatigue compared to the superficial muscles.

Computational musculoskeletal models have been developed to predict mechanical joint loads on the human spine, such as the forces and moments applied to vertebral and facet joints and the forces that act on ligaments and muscles because of difficulties in the direct measurement of joint loads. However, many whole-spine models lack certain elements. For example, the detailed facet joints in the cervical region or the whole spine region may not be implemented. In this study, a detailed cervico-thoraco-lumbar multibody musculoskeletal model with all major ligaments, separated structures of facet contact and intervertebral disk joints, and the rib cage was developed. The model was validated by comparing the intersegmental rotations, ligament tensile forces, facet joint contact forces, compressive and shear forces on disks, and muscle forces were to those reported in previous experimental and computational studies both by region (cervical, thoracic, or lumbar regions) and for the whole model. The comparisons demonstrated that our whole spine model is consistent with in vitro and in vivo experimental studies and with computational studies. The model developed in this study can be used in further studies to better understand spine structures and injury mechanisms of spinal disorders.

Many clinical case series have reported the predisposing factors for C5 palsy and have presented comparisons of the two types of laminoplasty. However, there have been no biomechanical studies focusing on cervical spinal cord and nerve root following laminoplasty. The purpose of this study is to investigate biomechanical changes in the spinal cord and nerve roots following the two most common types of laminoplasty, open-door and double-door laminoplasty, for cervical ossification of the posterior longitudinal ligament (OPLL). A finite element (FE) model of the cervical spine and spinal cord with nerve root complex structures was developed. Stress changes in the spinal cord and nerve roots, posterior shift of the spinal cord, and displacement of the cervical nerve roots were analyzed with two types of cervical laminoplasty models for variations in the degree of canal occupying ratio and shape of the OPLL. The shape and degree of spinal cord compression caused by the OPLL had more influence on the changes in stress, posterior shift of the spinal cord, and displacement of the nerve root than the type of laminoplasty. The lateral-type OPLL resulted in imbalanced stress on the nerve roots and the highest nerve root displacement. Type of laminoplasty and shape and degree of spinal cord compression caused by OPLL were found to influence the changes in stress and posterior displacement of the cervical spinal cord and nerve roots. Lateral-type OPLL might contribute to the development of C5 palsy due to the imbalanced stress and tension on the nerve roots after laminoplasty.

Biomechanical studies have indicated that the conventional nonanatomic reconstruction techniques for lateral ankle sprain (LAS) tend to restrict subtalar joint motion compared to intact ankle joints. Excessive restriction in subtalar motion may lead to chronic pain, functional difficulties, and development of osteoarthritis (OA). Therefore, various anatomic surgical techniques to reconstruct both the anterior talofibular and calcaneofibular ligaments (CaFL) have been introduced. In this study, ankle joint stability was evaluated using multibody computational ankle joint model to assess two new anatomic reconstruction and three popular nonanatomic reconstruction techniques. An LAS injury, three popular nonanatomic reconstruction models (Watson-Jones, Evans, and Chrisman–Snook) and two common types of anatomic reconstruction models were developed based on the intact ankle model. The stability of ankle in both talocrural and subtalar joint were evaluated under anterior drawer test (150 N anterior force), inversion test (3 N·m inversion moment), internal rotational test (3 N·m internal rotation moment), and the combined loading test (9 N·m inversion and internal moment as well as 1800 N compressive force). Our overall results show that the two anatomic reconstruction techniques were superior to the nonanatomic reconstruction techniques in stabilizing both talocrural and subtalar joints. Restricted subtalar joint motion, which is mainly observed in Watson-Jones and Chrisman–Snook techniques, was not shown in the anatomical reconstructions. Evans technique was beneficial for subtalar joint as it does not restrict subtalar motion, though Evans technique was insufficient for restoring talocrural joint inversion. The anatomical reconstruction techniques best recovered ankle stability.

A cervical hemicontusion spinal cord injury (SCI) produces forelimb deficits on the ipsilateral side of the injury while sparing the function of the limbs on the contralateral side of the injury, allowing for the evaluation of experimental therapeutics for functional recovery. Although the effects of contusion force on the functional and behavioral outcomes were adequately described in previous experimental studies, the size of the impactor tip also had significant effects on the extent of the lesion on the contralateral side of the injury in the hemicontusion rat model. However, studies regarding the effects of impactor size on the spinal cord for the hemicontusion model are limited. In this study, a finite element (FE) model of the rat cervical spinal cord was developed to investigate the effects of impactor size in the hemicontusion SCI model on the stress, strain, and displacement of the spinal cord for the New York University (NYU) and Infinite Horizon (IH) impactors. The impactor tip diameters of 1.2 mm and 1.6 mm with high impact loading resulted in the highest stresses and strains in the right (ipsilateral) side of the spinal cord. Thus, impactor tip diameters between 1.2 mm and 1.6 mm would be convenient to use in the rat hemicontusion SCI models for the cervical region without damaging the left (contralateral) side of the spinal cord. Our findings provide an insight into SCI mechanisms in the rat cervical hemicontusion model.