Posterior Cervical Spine Crisscross Fixation: Biomechanical Evaluation

Background Biomechanical/anatomic limitations may limit the successful implantation, maintenance, and risk acceptance of posterior cervical plate/rod fixation for one stage decompression-fusion. A method of posterior fixation (crisscross) that resolves biomechanical deficiencies of previous facet wiring techniques and not reliant upon screw implantation has been devised. The biomechanical performance of the new method of facet fixation was compared to the traditional lateral mass plate/screw fixation method. Methods Thirteen human cadaver spine segments (C2-T1) were tested under flexion-compression loading and four were evaluated additionally under pure-moment load. Preparations were evaluated in a sequence of surgical alterations with intact, laminectomy, lateral mass plate/screw fixation, and crisscross facet fixation using forces, displacements and kinematics. Findings Combined loading demonstrated significantly lower bending stiffness (p \u3c 0.05) between laminectomy compared to crisscross and lateral mass plate/screw preparations. Crisscross fixation showed a comparative tendency for increased stiffness. The increased overall motion induced by laminectomy was resolved by both fixation techniques, with crisscross fixation demonstrating a comparatively more uniform change in segmental motions. Interpretation The crisscross technique of facet fixation offers immediate mechanical stability with resolution of increased flexural rotations induced by multi-level laminectomy. Many of the anatomic limitations and potentially deleterious variables that may be associated with multi-level screw fixation are not associated with facet wire passage, and the subsequent fixation using a pattern of wire connection crossing each facet joint exhibits a comparatively more uniform load distribution. Crisscross wire fixation is a valuable addition to the surgical armamentarium for extensive posterior cervical single-stage decompression-fixation

Mechanisms of Cervical Spine Disc Injury under Cyclic Loading

Study Design Determination of human cervical spine disc response under cyclic loading. Purpose To explain the potential mechanisms of intervertebral disc injury caused by cyclic loading. Overview of Literature Certain occupational environments in civilian and military populations may affect the cervical spine of individuals by cyclic loading. Research on this mechanism is scarce. Methods Here, we developed a finite element model of the human C4–C5 disc. It comprised endplates, five layers of fibers, a nucleus, and an annulus ground substance. The endplates, ground substance, and annular fibers were modeled with elastic, hyperviscoelastic, and hyper-elastic materials, respectively. We subjected the disc to compressive loading (150 N) for 10,000 cycles at frequencies of 2 Hz (low) and 4 Hz (high). We measured disc displacements over the entire loading period. We obtained maximum and minimum principal stress and strain and von Mises stress distributions at both frequencies for all components. Further, we used contours to infer potential mechanisms of internal load transfer within the disc components. Results The points of the model disc displacement versus the loading cycles were within the experimental corridors for both frequencies. The principal stresses were higher in the ground matrix, maximum stress was higher in the anterior and posterior annular regions, and minimum stress was higher along the superior and inferior peripheries. The maximum principal strains were radially directed, whereas the minimum principal strains were axially/obliquely directed. The stresses in the fibers were greater and concentrated in the posterolateral regions in the innermost layer. Conclusions Disc displacement was lower at high frequency, thus exhibiting strain rate stiffening and explaining stress accumulation at superior and interior peripheries. Greater stresses and strains at the boundaries explain disc injuries, such as delamination. The greater development of stresses in the innermost annular fiber layer (migrating toward the posterolateral regions) explains disc prolapse

Effect of Head Restraint Position on Neck Injury Criteria in Rear Impact

ABSTRACT The objective of this study was to determine the forces and bending moments at the top of the Hybrid III dummy neck secondary to rear impact acceleration and evaluate the various proposed injury criteria. Rear impact sled tests were conducted by applying the Federal Motor Vehicle Safety Standards FMVSS 202 acceleration pulse. Differing positions of the head restraint in terms of height (750 and 800 mm) and backset (zero, 50, and 100 mm) were used to determine the axial and shear forces, bending moments, and injury criteria (NIC, N ij , and N km ). The time sequence of attainment of these parameters was determined along with peak values

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

A Comparison Study of Four Cervical Disk Arthroplasty Devices Using Finite Element Models

Study Design The study examined and compared four artificial cervical disks using validated finite element models. Purpose To compare and contrast the biomechanical behavior of four artificial cervical disks by determining the external (range of motion) and internal (facet force and intradiscal pressure) responses following cervical disc arthroplasty (CDA) and to elucidate any device design effects on cervical biomechanics. Overview of Literature Despite CDA’s increasing popularity most studies compare the CDA procedure with anterior cervical discectomy and fusion. There is little comparative evaluation of different artificial disks and, therefore, little understanding of how varying disk designs may influence spinal biomechanics. Methods A validated C2–T1 finite element model was subjected to flexion-extension. CDAs were simulated at the C5–C6 level with the Secure-C, Mobi-C, Prestige LP, and Prodisc C prosthetic disks. We used a hybrid loading protocol to apply sagittal moments. Normalized motions at the index and adjacent levels, and intradiscal pressures and facet column loads were also obtained. Results The ranges of motion at the index level increased after CDA. The Mobi-C prosthesis demonstrated the highest amount of flexion, followed by the Secure-C, Prestige LP, and Prodisc C. The Secure-C demonstrated the highest amount of extension, followed by the Mobi-C, Prodisc C, and Prestige LP. The motion decreased at the rostral and caudal adjacent levels. Facet forces increased at the index level and decreased at the rostral and caudal adjacent levels following CDA. Intradiscal pressures decreased at the adjacent levels for the Mobi-C, Secure-C, and Prodisc C. Conversely, the use of the Prestige LP increased intradiscal pressure at both adjacent levels. Conclusions While all artificial disks were useful in restoring the index level motion, the Secure-C and Mobi-C translating abilities allowed for lower intradiscal pressures at the adjacent segments and may be the driving mechanism for minimizing adjacent segment degenerative arthritic changes. The facet joint integrity should also be considered in the clinical decision-making process for CDA selection

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

Mechanical Factors and the Threshold of Trauma of the Intervertebral Joint: A Finite Element Analysis with Experimental Validation

Experimental and analytical finite element analyses were conducted to determine the effects of external load resulting in spinal injury. Axial compressive loading on human thoracolumbar functional units exhibits initial ambient, physiologic, traumatic and post traumatic loading phases. Initiation of trauma was defined as the point where the instantaneous tangent stiffness of the structure begins to decrease for the first time. For both normal and degenerate segments this microfailure load was 80% of the conventional traumatic failure load. The more flexible degenerate spines resulted in a lower value for these loads. Hypotheses regarding these microfailures which were not obvious on radiographs, were confirmed by morphologic cryosections. The energy absorbed by normal discs between microfailure and traumatic failure was 43% of total energy required for traumatic failure. In degenerate discs this was 28%. This may be an important index in determining the reserve strength after the initiation of injury. A materially and geometrically non-linear axisymmetric finite element model was constructed using geometry from experimental records. The purpose was to estimate the load dependent material constants and to explain the mechanics of injury based on regional strain energy density distribution. Experimental load-deflection data was used for validation. A critical evaluation of the inverse finite element method typically used for such estimations revealed the importance of obtaining the deformation data from a large number of experimental measurements. Results indicated the Young\u27s modulus value of the annulus fibrosus to be the highest in the physiologic loading phase. It reduced in the traumatic phase after the initiation of trauma. Stress-deformation analysis included the strain energy, annulus and endplate stresses, and nucleus pressure distributions, from the ambient to traumatic phases. The structure exhibited a concentration of strain energy density at the annulus-endplate junction near the outer disc circumference. A gradual yielding of the junction results in a settlement of the supports of the endplate at its outer edges. Due to external load and internal nucleus pressure, this support yielding phenomenon adds to its instability producing microfailures. This study has provided experimental evidence and a theoretical basis for the initiation of injury to a functional unit. Microfailures occur at a lower physiological loading than the conventional traumatic failure of the structure