Biomechanical Tolerance of Whole Lumbar Spines in Straightened Posture Subjected to Axial Acceleration

Quantification of biomechanical tolerance is necessary for injury prediction and protection of vehicular occupants. This study experimentally quantified lumbar spine axial tolerance during accelerative environments simulating a variety of military and civilian scenarios. Intact human lumbar spines (T12‐L5) were dynamically loaded using a custom‐built drop tower. Twenty‐three specimens were tested at sub‐failure and failure levels consisting of peak axial forces between 2.6 and 7.9 kN and corresponding peak accelerations between 7 and 57 g. Military aircraft ejection and helicopter crashes fall within these high axial acceleration ranges. Testing was stopped following injury detection. Both peak force and acceleration were significant (p \u3c 0.0001) injury predictors. Injury probability curves using parametric survival analysis were created for peak acceleration and peak force. Fifty‐percent probability of injury (95%CI) for force and acceleration were 4.5 (3.9–5.2 kN), and 16 (13–19 g). A majority of injuries affected the L1 spinal level. Peak axial forces and accelerations were greater for specimens that sustained multiple injuries or injuries at L2–L5 spinal levels. In general, force‐based tolerance was consistent with previous shorter‐segment lumbar spine testing (3–5 vertebrae), although studies incorporating isolated vertebral bodies reported higher tolerance attributable to a different injury mechanism involving structural failure of the cortical shell. This study identified novel outcomes with regard to injury patterns, wherein more violent exposures produced more injuries in the caudal lumbar spine. This caudal migration was likely attributable to increased injury tolerance at lower lumbar spinal levels and a faster inertial mass recruitment process for high rate load application. Published 2017. This article is a U.S. Government work and is in the public domain in the USA

Influence of bone microstructure on the mechanical properties of skull cortical bone – A combined experimental and computational approach

The strength and compliance of the dense cortical layers of the human skull have been examined since the beginning of the 20th century with the wide range in the observed mechanical properties attributed to natural biological variance. Since this variance may be explained by the difference in structural arrangement of bone tissue, micro-computed tomography (μCT) was used in conjunction with mechanical testing to study the relationship between the microstructure of human skull cortical coupons and their mechanical response. Ninety-seven bone samples were machined from the cortical tables of the calvaria of ten fresh post mortem human surrogates and tested in dynamic tension until failure. A linear response between stress and strain was observed until close to failure, which occurred at 0.6% strain on average. The effective modulus of elasticity for the coupons was 12.01 ± 3.28 GPa. Porosity of the test specimens, determined from μCT, could explain only 51% of the variation of their effective elastic modulus. Finite element (FE) models of the tested specimens built from μCT images indicated that modeling the microstructural arrangement of the bone, in addition to the porosity, led to a marginal improvement of the coefficient of determination to 54%. Modulus for skull cortical bone for an element size of 50 μm was estimated to be 19 GPa at an average. Unlike the load bearing bones of the body, almost half of the variance in the mechanical properties of cortical bone from the skull may be attributed to differences at the sub-osteon ( < 50 μm) level. ANOVA tests indicated that effective failure stress and strain varied significantly between the frontal and parietal bones, while the bone phase modulus was different for the superior and inferior aspects of the calvarium. The micro FE models did not indicate any anisotropy attributable to the pores observable under μCT.This research was sponsored by contract no. N00421-11-C-0004 from the U.S. Naval Air Warfare Center, Aircraft Division, Patuxent River, MD

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

Upright magnetic resonance imaging measurement of prevertebral soft tissue in the cervical spine of normal volunteers

Background context: Anteroposterior width of prevertebral soft tissues (PVSTs) in the cervical spine has long been considered a valuable radiographic measurement for evaluation of occult cervical spine pathology. These measurements, generally obtained from lateral radiographs of the cervical spine, have been used clinically as references for the evaluation of patients with traumatic, neoplastic, or other cervical spine disorders. Magnetic resonance imaging (MRI) offers a subtle delineation of the soft-tissue structures anterior to the vertebral column, with the potential for more accurate and sensitive determination of PVST width. Upright magnetic resonance images permit comparison with and validation of previously reported upright lateral radiographic measurements of PVST width. To our knowledge, evaluation of cervical spine PVST width using upright MRI has not been previously published in the English literature. Purpose: The purposes of this study were to validate lateral radiographic measurements of PVST width using upright weight-bearing MRI in healthy subjects and quantify effects of spinal level and gender. Study design: Clinical study in asymptomatic volunteers. Methods: Eleven male and eight female volunteers consented and were enrolled in the study. All volunteers were asymptomatic and had no history of cervical spine injury or degenerative disease. Prevertebral soft-tissue width was measured at each cervical level from C2 to C7 using upright weight-bearing MRI. Statistically significant differences in PVST width based on spinal level and gender were determined using two-factor analysis of variance. Results: Width magnitudes were significantly dependent on gender (p\u3c.0001) and spinal level (p\u3c.0001). All C3 and C6 measurements were below the traditionally accepted values of 7 and 20 mm, respectively, that would be considered abnormal. Prevertebral soft-tissue width was greater in men at upper and lower extents of the cervical spine. Prevertebral soft-tissue widths reported in the present study were similar in magnitude and level-by-level trends to measurements of asymptomatic volunteers obtained using lateral radiography. Conclusion: The present study validated the use of lateral radiography to measure PVST width, presented level-by-level and gender-specific normative data, and provided a weighted statistical analysis of differences between normal volunteers and injured patients. © 2011 Elsevier Inc. All rights reserved

Quantitative anatomy of subaxial cervical lateral mass: An analysis of safe screw lengths for Roy-Camille and Magerl techniques

STUDY DESIGN. Determination of lateral mass screw lengths with Roy-Camille and Magerl techniques of screw insertion using computerized tomography in 98 young, asymptomatic North American volunteers. OBJECTIVE. To provide reliable and normative data on safe screw lengths using the Roy-Camille and Magerl techniques of lateral mass fixation in the subaxial cervical spine. SUMMARY OF BACKGROUND DATA. Lateral mass screw lengths have been studied in the past using differing subject and measurement characteristics and small sample sizes. Results demonstrated considerable variation in screw length and influencing factors. Inappropriate screw lengths can result in neurovascular injury during screw insertion, facet joint damage, or inadequate fixation. METHODS. Bicortical screw lengths were bilaterally measured at each spinal level from C3-C7 in 98 young volunteers using computed tomography reconstructions through the lateral masses obtained in the plane of the screw in Roy-Camille and Magerl techniques. RESULTS. With both techniques, trajectories were longest at C4-C6, shorter at C3, and shortest at C7. Screw lengths were greater in males when compared with females at all levels. Average Magerl screw lengths were approximately 2.6 mm longer at C3-C6 levels, and approximately 1.3 mm longer at the C7 level when compared with Roy-Camille technique. There was minimal correlation between screw lengths and anthropometric measurements including stature, body weight, and neck length. CONCLUSION. Significant variations exist at each subaxial level with either technique. We recommend the surgeon determine screw lengths for fixation at each level using preoperative sagittal oblique computed tomography scans, which provide the most accurate technique of preoperative templating for screw length. © 2008 Lippincott Williams & Wilkins, Inc

Computerized tomographic morphometric analysis of subaxial cervical spine pedicles in young asymptomatic volunteers

Background: Although cervical spine pedicle screws have been shown to provide excellent fixation, widespread acceptance of their use is limited because of the risk of injury to the spinal cord, nerve roots, and vertebral arteries. The risks of pedicle screw insertion in the cervical spine can be mitigated by a three-dimensional appreciation of pedicle anatomy. Normative data on three-dimensional subaxial pedicle geometry from a large, young, and asymptomatic North American population are lacking. The purpose of the present study was to determine three-dimensional subaxial pedicle geometry in a large group of young volunteers and to determine level and sex-specific morphologic differences. Methods: Helical computerized tomography scans were made from the third cervical to the seventh cervical vertebra in ninety-eight volunteers (sixty-three men and thirty-five women) with an average age of twenty-five years. Pedicle width, height, length, and transverse and sagittal angulations were measured bilaterally. Pedicle screw insertion positions were quantified in terms of mediolateral and superoinferior offsets relative to readily identifiable landmarks. Results: The mean pedicle width and height at all subaxial levels were sufficient to accommodate 3.5-mm screws in 98% of the volunteers. Pedicle width and height dimensions of \u3c4.0 mm were rare (observed in association with only 1.7% of the pedicles), with 82% occurring in women and 72% occurring unilaterally. Screw insertion positions generally moved medially and superiorly at caudal levels. Transverse angulation was approximately 45° at the third to fifth cervical levels and was less at more caudal levels. Sagittal angulation changed from a cranial orientation at superior levels to a caudal orientation at inferior levels. Mediolateral and superoinferior insertion positions and sagittal angulations were significantly dependent (p \u3c 0.05) on sex and spinal level. Transverse angulation was significantly dependent (p \u3c 0.05) on spinal level. Conclusions: Pedicle screw insertion points and orientation are significantly different (p \u3c 0.05) at most subaxial cervical levels and between men and women. Preoperative imaging studies should be carefully templated for pedicle size in all patients on a level-specific basis. Although the prevalence was low, women were more likely to have pedicle width and height dimensions of \u3c4.0 mm. Clinical Relevance: The present study provides normative data on subaxial cervical pedicle geometry from a large sample of young, healthy men and women. The data may be useful for preoperative planning for pedicle screw fixation. Copyright © 2008 by the Journal of Bone and Joint Surgery, Incorporated

Location of the transverse foramen in the subaxial cervical spine in a young asymptomatic population

Study Design. Computerized tomography of the subaxial cervical spine in 98 young, asymptomatic North American volunteers. Objective. To provide normative data on subaxial transverse foramen dimensions and location in relation to surgical landmarks routinely used during operative intervention in the anterior cervical spine. Summary of Background Data. Vertebral artery injury during anterior cervical spinal surgery is a rare but potentially catastrophic injury. There have been no prior studies in a large group of young, asymptomatic subjects without pathology and where the age, weight, and gender are known. There are no published computerized tomography data evaluating distances between the tip of the uncovertebral joint and the medial margin of the uncovertebral joint, 2 commonly used surgical landmarks. Methods. Axial and reconstructed coronal computerized tomography images of cervical vertebrae from C3 to C7 in 98 asymptomatic young volunteers were analyzed to measure interforaminal distance, transverse foramen distance from anterior and posterior vertebral body margins, transverse foramen dimensions, and transverse foramen medial margin distance from the uncus tip and medial margin. Results. All measurements were significantly different between males and females, with smaller female dimensions. Interforaminal distance gradually increased from C3 to C7. Transverse foramen anterior margin in relation to the anterior vertebral body was significantly more posterior at C7 compared with the C3-C6 levels. Transverse foramen posterior margin in relation to the vertebral body posterior margin gradually moved anteriorly from C3 to C6 and then posterior again at C7. The vertebral uncus tip and medial margin in relation to the medial transverse foramen averaged 2.8 mm and 5.7 mm for males and 2.7 mm and 5.3 mm for females from C3 to C6. Conclusion. Useful morphometric data are provided that may assist the operating surgeon to avoid vertebral artery injury during anterior surgical approaches to the cervical spine. The medial margin of the uncovertebral joint may be the safest landmark to avoid vertebral artery injury during anterior cervical disc surgery. The vertebral artery is at increased risk of injury during neural decompression at more cephalad levels. © 2010, Lippincott Williams & Wilkins