Level- and Region-Specific Properties of Young Human Lumbar Annulus

ABSTRACT The objective of this study was to determine the material properties of the human lumbar intervertebral disc annulus as a function of anatomical region and spinal level. Samples from minimally or nondegenerated spines were extracted from young post mortem human subjects and tested in tension. Statistically significant differences were found based on anatomical region. Trends appear to indicate spinal level dependency, although additional samples are required to attain statistical significance. It is possible to use finite element models incorporating these region-and level-specific properties to quantify internal load-sharing and delineate the mechanism of disorders such as herniation

Regional variations in C1–C2 bone density on quantitated computed tomography and clinical implications

Background: Our elderly population is growing and the number of spine fractures in the elderly is also growing. The elderly population in general may be considered as poor surgical candidates experience a high rate of fractures at C1 and C2 compared with the general population. Nonoperative management of upper cervical fractures is not benign as there is a high nonunion rate for both C1 and C2 fractures in the elderly, and orthosis compliance is often suboptimal, or complicated by skin breakdown. The optimal technique for upper cervical stabilization in the elderly may be different than in younger populations as the bone quality is inferior in the elderly. The objective of this basic science study is to determine whether the bone mineral density (BMD) of C1 and C2 vary by region, and if this is a gender difference in this elderly age group. Methods: Twenty cadaveric spines from 45 to 83 years of age were used to obtain BMD using quantitated computed tomography (QCT). BMD was measured using a QCT. For C1, 8 regions were determined: anterior tubercle, bilateral anterior and medial lateral masses, bilateral posterior arches, and posterior tubercle. For C2, 7 regional BMDs were determined: top of odontoid, base of odontoid-body interface, mid body, bilateral lateral masses, anterior inferior body near the discs space, and the C2 spinous process. Results: The BMD was greatest at the C1 anterior tubercle (564.4±175.8 mg/cm3) and C1 posterior ring (420.8±110.2 mg/cm3), and least at the anterior and medial lateral masses (262.8±59.5 mg/cm3, 316.9±72.6 mg/cm3). At C2 QCT BMD was greatest at the top of the dens (400.6±107.9 mg/cm3) decreasing down through the odontoid-C2 body junction (267.8±103.5 mg/cm3) and least in the mid C2 body 249.1±68.8 mg/cm3). The posterior arch of C1 and the spinous process of C2 had higher BMD's 420.8±110.2 mg/cm3 and 284.1±93.0 mg/cm3, respectively. A high correlation was observed between the BMD at the interface of the dens-vertebral body with the vertebral body with a Pearson correlation coefficient of 0.86. The BMD of the top of dens was significantly higher (p<.05) than all the regions in C2. Conclusions: Regional and segmental BMD variations at C1 and C2 have clinical implications for surgical constructs in the elderly population. Given the higher BMDs of the C1 and C2 spinous process and posterior arches, consideration should be given to incorporate these areas using various C1–C2 wiring techniques. In the elderly, lateral masses particularly at C1 with lower BMD may result in potential screw loosening and nonunion in this age group. Old-school wiring techniques have a track record of efficacy and safety with less blood loss, reduced operative time, reduced X-ray exposure, and should be considered in the elderly as a primary stabilization technique or a belt-over suspenders approach based on regional variations in BMD in the elderly

Finite element modeling of the human cervical spinal cord and its applications: A systematic review

Background Context: Finite element modeling (FEM) is an established tool to analyze the biomechanics of complex systems. Advances in computational techniques have led to the increasing use of spinal cord FEMs to study cervical spinal cord pathology. There is considerable variability in the creation of cervical spinal cord FEMs and to date there has been no systematic review of the technique. The aim of this study was to review the uses, techniques, limitations, and applications of FEMs of the human cervical spinal cord. Methods: A literature search was performed through PubMed and Scopus using the words finite element analysis, spinal cord, and biomechanics. Studies were selected based on the following inclusion criteria: (1) use of human spinal cord modeling at the cervical level; (2) model the cervical spinal cord with or without the osteoligamentous spine; and (3) the study should describe an application of the spinal cord FEM. Results: Our search resulted in 369 total publications, 49 underwent reviews of the abstract and full text, and 23 were included in the study. Spinal cord FEMs are used to study spinal cord injury and trauma, pathologic processes, and spine surgery. Considerable variation exists in the derivation of spinal cord geometries, mathematical models, and material properties. Less than 50% of the FEMs incorporate the dura mater, cerebrospinal fluid, nerve roots, and denticulate ligaments. Von Mises stress, and strain of the spinal cord are the most common outputs studied. FEM offers the opportunity for dynamic simulation, but this has been used in only four studies. Conclusions: Spinal cord FEM provides unique insight into the stress and strain of the cervical spinal cord in various pathological conditions and allows for the simulation of surgical procedures. Standardization of modeling parameters, anatomical structures and inclusion of patient-specific data are necessary to improve the clinical translation