Spinal Geometry Research Articles

Degenerative scoliosis and stenosis—Surgical management

Adult degenerative scoliosis affects the growing elderly population and is an expanding challenge for spine surgeons. The etiology and natural history of this disease are imperfectly understood; however, it is accepted that degenerative spinal curvature is likely to progress. Severe deformity, particularly in the sagittal plane, is poorly tolerated. Treatment is complicated in this population where comorbid or mimicking diseases are the norm. Where appropriate, surgery is undertaken only after a thorough assessment of each patient’s spinal geometry. Guidelines aid in selecting among or combining surgical procedures. Increased potential for correction generally involves increased risk. Complications are common after surgery for adult degenerative scoliosis and their management should be anticipated.

Analysis of risk factors of subsequent fractures after vertebroplasty

The purpose of this study is to identify risk factors related to the development of subsequent fractures after vertebroplasty. A retrospective study was conducted to review 175 patients with a 1-year follow-up who underwent vertebroplasty for first-time and single-level osteoporotic vertebral fractures. Subsequent fractures were diagnosed as recurrent intractable back pain, post-operatively correlated with MR image. Clinical parameters, such as age, gender, baseline VAS-score, lumbar bone mineral density (BMD) T-score, history of use of steroids, bisphosphonate therapy, symptom-free interval, the amount of bone cement injected, vacuum clefts, leakage of cement into the disk space, treated level and the changes of spinal geometry were recorded. During the follow-up period, subsequent fractures developed in 37 (21.1 %) of 175 patients. Significant differences (P < 0.05) were found between the patients with subsequent fractures and the patients without subsequent fractures in regard to their BMD T-score, and treated vertebrae location. Average BMD T-score was -3.4 ± 1.5 in patients with subsequent fractures and -2.9 ± 1.6 in patients without subsequent fractures. The percentage of subsequent fractures was 13.9 % (10 of 72) for treated vertebrae located in non-thoracolumbar junction, and 26.2 % (27 of 103) in the thoracolumbar junction. The most important risk factors affecting subsequent fractures after vertebroplasty were osteoporosis and treated level at the thoracolumbar junction.

Computational fluid dynamics modelling of cerebrospinal fluid pressure in Chiari malformation and syringomyelia

The pathogenesis of syringomyelia in association with Chiari malformation (CM) is unclear. Studies of patients with CM have shown alterations in the CSF velocity profile and these could contribute to syrinx development or enlargement. Few studies have considered the fluid mechanics of CM patients with and without syringomyelia separately. Three subject-specific CFD models were developed for a normal participant, a CM patient with syringomyelia and a CM patient without syringomyelia. Model geometries, CSF flow rate data and CSF velocity validation data were collected from MRI scans of the 3 subjects. The predicted peak CSF pressure was compared for the 3 models. An extension of the study performed geometry and flow substitution to investigate the relative effects of anatomy and CSF flow profile on resulting spinal CSF pressure. Based on 50 monitoring locations for each of the models, the CM models had significantly higher magnitude (p<0.01) peak CSF pressure compared with normal. When using the same CSF input flow waveform, changing the upper spinal geometry changed the magnitude of the CSF pressure gradient, and when using the same upper spinal geometry, changing the input flow waveform changed the timing of the peak pressure. This study may assist in understanding syringomyelia mechanisms and relative effects of CSF velocity profile and spinal geometry on CSF pressure.

3-D Reconstruction of the Spine From Biplanar Radiographs Based on Contour Matching Using the Hough Transform

The purpose of this study was to develop and evaluate a method for three-dimensional (3-D) reconstruction of the spine from biplanar radiographs. The approach was based on vertebral contour matching for estimating vertebral orientations and locations. Vertebral primitives were initially positioned under constraint of the 3-D spine midline, which was estimated from manually identified control points. Vertebral orientations and locations were automatically adjusted by matching projections of 3-D primitives with vertebral edges on biplanar radiographs based on the generalized Hough transform technique with a deformation tolerant matching strategy. We used graphics processing unit to accelerate reconstruction. Accuracy and precision were evaluated using radiographs from 15 scoliotic patients and a spine model in 24 poses. On in vivo radiographs, accuracy was within 2.8° for orientation and 2.4 mm for location; precision was within 2.3° for orientation and 2.1 mm for location. results were slightly better on model radiographs than on in vivo radiographs but without significance (p>0.05). The duration for user intervention was less than 2 min, and the computation time was within 3 min. Results indicated the method's reliability. It is a promising tool to determine 3-D spinal geometry with acceptable user interaction.

Impact of Prone Surgical Positioning on the Scoliotic Spine

To study the impact of patient-specific prone positioning on the sagittal and coronal curves of scoliotic spines, including the impact of various patient and surgical frame factors. Prone operative positioning has been shown to impact the geometry of various individual spinal segments. Its impact on global spinal geometry and influential factors remains unknown. Lateral and coronal radiographs were acquired of 6 scoliotic patients while standing, prone on a dynamically adjustable surgical frame and intraoperatively on the Relton-Hall frame. Standing lateral bending radiographs were also acquired. Lordosis, kyphosis, and Cobb angles were measured in each position. Personalized finite element models (FEMs), including the spine, ribcage, pelvis, and lower limbs were generated for each patient based on their standing radiographs. The FEM's ability to reproduce prone spinal geometry was evaluated by using different values of intervertebral disc elastic moduli: published, optimized based on lateral bending radiographs and optimized based on prone radiographs. The 6 FEMs were then exploited to study the impact of surgical frame cushion configuration, standing curve magnitudes, and patient weight on spinal geometry changes due to prone positioning. All coronal and sagittal curves decreased in the prone position; averaging 12% in lordosis, 19% in kyphosis, 7%, 14%, and 26%, respectively, for proximal thoracic, main thoracic, and thoracolumbar/lumbar Cobb angles. FEM prone simulations yielded best results when optimized by using the prone position radiographs (Δ<5 degrees for all segmental curves). Lateral bending optimization yielded similar results by using published properties. Surgical frame cushion configuration, standing curve magnitudes, and patient weight all had an important impact on spinal geometries with the exception of thoracic cushion longitudinal position. A strong correlation (R=0.86) was found between standing kyphosis and its reduction in the prone position. Prone positioning results in a reduction of all spinal segmental curves which is dependent on a number of patient and surgical frame factors.

Geometry of the vertebral bodies and the intervertebral discs in lumbar segments adjacent to spondylolysis and spondylolisthesis: pilot study

The objective is to evaluate the geometric parameters of vertebral bodies and intervertebral discs in spinal segments adjacent to spondylolysis and spondylolisthesis. This pilot cross-sectional study was an ancillary project to the Framingham Heart Study. The presence of spondylolysis and spondylolisthesis as well as measurements of spinal geometry were identified on CT imaging of 188 individuals. Spinal geometry measurements included lordosis angle, wedging of each lumbar vertebra and intervertebral disc. Last measurements were used to calculate ΣB, the sum of the lumbar L1-L5 body wedge angles; and ΣD, the sum of the lumbar L1-L5 intervertebral disc angles. Using Wilcoxon-Mann-Whitney test we compared the geometric parameters between individuals with no pathology and ones with spondylolysis (with no listhesis) at L5 vertebra, ones with isthmic spondylolisthesis at L5-S1 level, and ones with degenerative spondylolisthesis at L5-S1 level. Spinal geometry in individuals with spondylolysis or listhesis at L5 shows three major patterns: In spondylolysis without listhesis, spinal morphology is similar to that of healthy individuals; In isthmic spondylolisthesis there is high lordosis angle, high L5 vertebral body wedging and very high L4-5 disc wedging; In degenerative spondylolisthesis, spinal morphology shows more lordotic wedging of the L5 vertebral body, and less lordotic wedging of intervertebral discs. In conclusion, there are unique geometrical features of the vertebrae and discs in spondylolysis or listhesis. These findings need to be reproduced in larger scale study.

Mean subaxial space available for the cord index as a novel method of measuring cervical spine geometry to predict the chronic stinger syndrome in American football players

The chronic stinger syndrome is a distinct entity from acute stingers and has been shown to have its own pathophysiology that, unlike acute stingers, may reflect long-standing geometrical changes of the subaxial spinal canal and chronic irritation/degeneration of the exiting nerve root complex. There is no method available, however, to accurately predict these symptoms in athletes. The mean subaxial cervical space available for the cord (MSCSAC) is a novel alternative to the Torg ratio for predicting neurological symptoms caused by cervical spondylosis in elite athletes. It is the goal of this study to determine critical values for this measurement index and to retrospectively correlate those values to neurological symptoms. Magnetic resonance images obtained in 103 male athletes participating in the 2005 and 2006 National Football League Scouting Combine and a control group of 42 age-matched male nonathletes were retrospectively reviewed. The Torg ratio and SAC values were calculated in triplicate at each cervical level from C3-6 by using lateral radiographs and midsagittal T2-weighted MR images of the cervical spine, respectively. These values were then averaged for each individual to produce mean subaxial cervical Torg ratio (MSCTR) and MSCSAC values. Receiver operating characteristic curves were constructed for each measurement technique and were compared based on their respective area under the curves (AUCs). The MSCSAC difference between athletes with and without chronic stingers was statistically significant (p < 0.01). The difference between athletes with and without chronic stingers compared with controls was also statistically significant (p < 0.001 and p < 0.001, respectively). The AUC for the MSCSAC was 0.813, which was significantly greater than the AUC for both the MSCTR (p = 0.0475) and the individual Torg ratio (p = 0.0277). The MSCTR had the second largest AUC (0.676) and the conventional method of measuring individual Torg ratio values produced the lowest AUC (0.661). It was found that using the MSCSAC with a critical value of 5.0 mm produced a sensitivity of 80% and a negative likelihood ratio of 0.23 for predicting chronic stingers. Lowering the cutoff value to 4.3 mm for the MSCSAC resulted in a possible confirmatory test with a specificity of 96% and a positive likelihood ratio of 13.25. A critical value of 5.0 mm for the MSCSAC provides the clinician with a screening test for chronic stingers and anything < 4.3 mm adds additional confidence as a confirmatory test. These results are approximately 20% more accurate than the classic Torg ratio based on our AUC analysis. It was found that measuring the spinal geometry throughout the length of the subaxial cervical spine produced a more reliable method by which to predict neurological symptoms than the traditional approach of measuring individual levels. This shows that the underlying pathogenesis of the chronic stinger syndrome is best characterized as a process that involves the entire subaxial region uniformly.

The Pathogenesis of Syringomyelia: A Re-Evaluation of the Elastic-Jump Hypothesis

Syringomyelia is a disease in which fluid-filled cavities, called syrinxes, form in the spinal cord causing progressive loss of sensory and motor functions. Invasive monitoring of pressure waves in the spinal subarachnoid space implicates a hydrodynamic origin. Poor treatment outcomes have led to myriad hypotheses for its pathogenesis, which unfortunately are often based on small numbers of patients due to the relative rarity of the disease. However, only recently have models begun to appear based on the principles of mechanics. One such model is the mathematically rigorous work of Carpenter and colleagues (2003, "Pressure Wave Propagation in Fluid-Filled Co-Axial Elastic Tubes Part 1: Basic Theory," ASME J. Biomech. Eng., 125(6), pp. 852-856; 2003, "Pressure Wave Propagation in Fluid-Filled Co-Axial Elastic Tubes Part 2: Mechanisms for the Pathogenesis of Syringomyelia," ASME J. Biomech. Eng., 125(6), pp. 857-863). They suggested that a pressure wave due to a cough or sneeze could form a shocklike elastic jump, which when incident at a stenosis, such as a hindbrain tonsil, would generate a transient region of high pressure within the spinal cord and lead to fluid accumulation. The salient physiological parameters of this model were reviewed from the literature and the assumptions and predictions re-evaluated from a mechanical standpoint. It was found that, while the spinal geometry does allow for elastic jumps to occur, their effects are likely to be weak and subsumed by the small amount of viscous damping present in the subarachnoid space. Furthermore, the polarity of the pressure differential set up by cough-type impulses opposes the tenets of the elastic-jump hypothesis. The analysis presented here does not support the elastic-jump hypothesis or any theory reliant on cough-based pressure impulses as a mechanism for the pathogenesis of syringomyelia.

A Prospective, Masked 18-Month Minimum Follow-up On Neurophysiologic Changes In Persons with Spinal Stenosis, Low Back Pain, and No Symptoms

To describe neurophysiologic changes over time in persons with and without spinal complaints and to assess whether paraspinal denervation predicts change in stenosis on magnetic resonance imaging (MRI) and clinical course. Prospective, controlled, masked trial. University spine program. Persons aged 55 to 80 years, screened for polyneuropathy and determined on clinical examination to have spinal stenosis, mechanical low back pain, or no spinal symptoms. A comprehensive codified history was obtained and subjects underwent physical examination, ambulation testing, masked electrodiagnostic testing including paraspinal mapping, and MRI, repeated at greater than 18 months. This study presents detailed technical information and additional analyses not reported previously. Change in electrodiagnostic findings. Among persons with clinical stenosis, relationship of change in paraspinal mapping scores to MRI findings and clinical changes. Of 149 initial subjects, 83 (79.3% of eligible subjects) repeated testing at 20 (+/-2 SDs) months. No significant change in limb muscle spontaneous activity or motor unit pathology was noted in any group. In 23 persons with initial diagnosis of stenosis, paraspinal mapping electromyography related to change in diagnosis over time (analysis of variance F = 3.77, P = .037), but not to most initial magnetic resonance imaging measurements or to change in spinal canal diameter. Clinical spinal stenosis is neurophysiologically stable in most persons. Paraspinal electromyographic changes reflect large changes in clinical course, but neither neurophysiologic nor clinical changes relate to change in spinal geometry over 20 months.

Follow-up problems in scoliosis patients

With reference to 4 different methods - theoretical derivation, computed model, recent radiological analysis based on 120 scoliosis patients and measurement of the posterior median line in 40 scoliosis patients - the unreliability of follow-up in scoliosis patients is illustrated. The results are compared with previous publications by other authors and discussed. A method of determining spinal geometry three-dimensionally by means of topometric methods (5) is described. The validity of this measuring method is demonstrated in the patient by geometrical measurement of the posterior median line without treatment, under halo gravity extension and in movement.

Anatomical Gender Differences in Cervical Vertebrae of Size-Matched Volunteers

Clinical literature consistently identifies women as more susceptible to trauma-related neck pain, commonly resulting from soft tissue cervical spine injury. Structural gender differences may explain altered response to dynamic loading in women leading to increased soft tissue distortion and greater injury susceptibility. Identify anatomic gender differences in cervical spinal geometry that contribute to decreased column stability in women. Previous studies investigating male and female vertebral and vertebral body geometry demonstrated female vertebral dimensions were smaller. However, populations were not size matched and parameters related to biomechanical stability were not reported. Computed tomography scans of the cervical spine were obtained from size-matched young healthy volunteers. Geometrical dimensions were obtained at the C4 level and analysis of variance determined significant gender differences. Two volunteer subsets were size matched based on sitting height and head circumference. All geometrical measures were greater in men for both subsets. Vertebral width and disc-facet depth were significantly greater in men. Additionally, segmental support area, combining interfacet width and disc-facet depth, was greater in men, indicating more stable intervertebral coupling. Present results of decreased linear and areal cervical dimensions leading to decreased column stability may partially explain increased traumatic injury rates in women.

Analysis and simulation of progressive adolescent scoliosis by biomechanical growth modulation

Scoliosis is thought to progress during growth because spinal deformity produces asymmetrical spinal loading, generating asymmetrical growth, etc. in a ‘vicious cycle.’ The aim of this study was to test quantitatively whether calculated loading asymmetry of a spine with scoliosis, together with measured bone growth sensitivity to altered compression, can explain the observed rate of scoliosis progression in the coronal plane during adolescent growth. The simulated spinal geometry represented a lumbar scoliosis of different initial magnitudes, averaged and scaled from measurements of 15 patients’ radiographs. Level-specific stresses acting on the vertebrae were estimated for each of 11 external loading directions (‘efforts’) from published values of spinal loading asymmetry. These calculations assumed a physiologically plausible muscle activation strategy. The rate of vertebral growth was obtained from published reports of growth of the spine. The distribution of growth across vertebrae was modulated according to published values of growth sensitivity to stress. Mechanically modulated growth of a spine having an initial 13° Cobb scoliosis at age 11 with the spine subjected to an unweighted combination of eleven loading conditions (different effort direction and magnitude) was predicted to progress during growth. The overall shape of the curve was retained. The averaged final lumbar spinal curve magnitude was 32° Cobb at age 16 years for the lower magnitude of effort (that produced compressive stress averaging 0.48 MPa at the curve apex) and it was 38° Cobb when the higher magnitudes of efforts (that produced compressive stress averaging 0.81 MPa at the apex). An initial curve of 26° progressed to 46° and 56°, respectively. The calculated stresses on growth plates were within the range of those measured by intradiscal pressures in typical daily activities. These analyses predicted that a substantial component of scoliosis progression during growth is biomechanically mediated. The rationale for conservative management of scoliosis during skeletal growth assumes a biomechanical mode of deformity progression (Hueter-Volkmann principle). The present study provides a quantitative basis for this previously qualitative hypothesis. The findings suggest that an important difference between progressive and non-progressive scoliosis might lie in the differing muscle activation strategies adopted by individuals, leading to the possibility of improved prognosis and conservative or less invasive interventions.

Analysis and simulation of progressive adolescent scoliosis by biomechanical growth modulation

Scoliosis is thought to progress during growth because spinal deformity produces asymmetrical spinal loading, generating asymmetrical growth, etc. in a 'vicious cycle.' The aim of this study was to test quantitatively whether calculated loading asymmetry of a spine with scoliosis, together with measured bone growth sensitivity to altered compression, can explain the observed rate of scoliosis progression in the coronal plane during adolescent growth. The simulated spinal geometry represented a lumbar scoliosis of different initial magnitudes, averaged and scaled from measurements of 15 patients' radiographs. Level-specific stresses acting on the vertebrae were estimated for each of 11 external loading directions ('efforts') from published values of spinal loading asymmetry. These calculations assumed a physiologically plausible muscle activation strategy. The rate of vertebral growth was obtained from published reports of growth of the spine. The distribution of growth across vertebrae was modulated according to published values of growth sensitivity to stress. Mechanically modulated growth of a spine having an initial 13 degrees Cobb scoliosis at age 11 with the spine subjected to an unweighted combination of eleven loading conditions (different effort direction and magnitude) was predicted to progress during growth. The overall shape of the curve was retained. The averaged final lumbar spinal curve magnitude was 32 degrees Cobb at age 16 years for the lower magnitude of effort (that produced compressive stress averaging 0.48 MPa at the curve apex) and it was 38 degrees Cobb when the higher magnitudes of efforts (that produced compressive stress averaging 0.81 MPa at the apex). An initial curve of 26 degrees progressed to 46 degrees and 56 degrees, respectively. The calculated stresses on growth plates were within the range of those measured by intradiscal pressures in typical daily activities. These analyses predicted that a substantial component of scoliosis progression during growth is biomechanically mediated. The rationale for conservative management of scoliosis during skeletal growth assumes a biomechanical mode of deformity progression (Hueter-Volkmann principle). The present study provides a quantitative basis for this previously qualitative hypothesis. The findings suggest that an important difference between progressive and non-progressive scoliosis might lie in the differing muscle activation strategies adopted by individuals, leading to the possibility of improved prognosis and conservative or less invasive interventions.

CERVICAL LAMINECTOMY

Spinal canal decompression via cervical laminectomy with or without foraminotomy is a mainstay of treatment of cervical spondylotic myelopathy and myeloradiculopathy. The goal of this surgery is to expand the cervical canal dorsally by removing the spinous processes, laminae, ligamentum flavum, and bony hypertrophy that are contributing to the canal stenosis. In selecting this particular approach to decompression, the surgeon must take into account the spinal geometry and the primary pathology of the patient: an "effective" cervical kyphosis is a contraindication to a dorsal approach, and spinal canal compromise secondary to ventral compression is best addressed through a ventral or a combined ventral and dorsal approach. This technique is technically facile and versatile in application. Complications with this procedure are relatively rare, with postoperative spinal instability being the primary concern. With proper patient selection and attention to surgical technique, laminectomy is a safe and effective approach to the management of cervical myelopathy in selected patients.

Geometric and Electromyographic Assessments in the Evaluation of Curve Progression in Idiopathic Scoliosis

The natural history of patients with idiopathic scoliosis was analyzed radiographically and electromyographically in a prospective longitudinal study. To identify changes in geometric variables and the sequence in which these changes occur during curve progression in the natural history of patients with idiopathic scoliosis. In addition, to study the relationship between several geometric variables and electromyographic (EMG) measurements to determine their predictive value as risk factors to curve progression of the scoliotic deformity. The main area of concern in treating children with adolescent idiopathic scoliosis is the unpredictability of curve progression during the early development of the deformity. The changes in radiographic geometric and EMG variables between the first presentation and consecutive 4-6-month follow-up periods were analyzed in 105 patients with idiopathic scoliosis. Statistical analyses were performed to elucidate in more detail how spinal geometry evolves during curve progression. Curve severity was associated with remaining growth potential expressed as an increasing spinal growth velocity (SGV). With increasing SGV, an enhanced EMG activity at the lower part on the convex side of the curve expressed as EMG ratio was found. High EMG ratio was associated with increased axial rotation and diminished kyphosis before the rapid increase in Cobb angle. Lateral deviation, wedge angle, and axial rotation all increased during periods of progression. Changes in tilt angle and lordosis were not associated with curve progression. In the natural history of idiopathic scoliosis, SGV and EMG ratio at the lower end vertebra are prominent risk factors of curve progression. The asymmetric muscle activity is associated with increased axial rotation, which in its turn is associated with increasing Cobb angle and diminishing kyphosis. The combination of these variables provides insight in the physiologic and 3-dimensional biomechanical evolution of the natural history of curve progression in idiopathic scoliosis.

Temperature and age effects on high rate viscoelasticity of cervical spinal components

This study examined the effect of loading (displacement) rate on the tensile mechanics of cervical spine functional spinal units. A total of 40 isolated functional spinal units (two vertebrae and the adjoining soft tissues) from juvenile male baboons (10±0.6—human equivalent years old) were subjected to tensile loading spanning four orders of magnitude from 0.5 to 5000 mm/s. The stiffness, ultimate failure load, and corresponding displacement at failure were measured for each specimen and normalized by spinal geometry to examine the material properties as well as the structural properties. The tensile stiffness, failure load, normalized stiffness, and normalized failure load significantly increased (ANOVA, p<0.001) with increasing displacement rate. From the slowest to fastest loading rate, a two-fold increase in stiffness and four-fold increase in failure load were observed. The tensile failure strains (1.07±0.31 mm/mm strain) were not significantly correlated with loading rate (ANOVA, p=0.146). Both the functional (non-destructive stiffness and normalized stiffness) and failure mechanics of isolated functional spinal units exhibited a power-law relationship with displacement rate. Modeling efforts utilizing these rate-dependent characteristics will enhance our understanding of the tensile viscoelastic response of the spine and enable improved dynamic injury prevention schemes.

To graft or not to graft: rationalizing choice in anterior cervical discectomy

Anterior cervical discectomy has been performed for almost 50 years. Initially, bone grafts were used routinely, but soon their necessity was questioned. It remains disputed to this day. The aim was to establish whether there are grounds for basing choice of technique on individual patient data. The cervical spine radiographs of 148 patients who had undergone grafted or ungrafted anterior cervical discectomy were reviewed and changes in geometry at the operated level were measured. These data were then examined for any correlation between preoperative geometry, choice of operative technique and adverse clinical outcome. Disturbances to spinal geometry after ungrafted discectomy are minimal at disc heights below 4 mm. At and above 4 mm significant settlement and angulation occur. Complications in our series were fewer where small disc spaces were left ungrafted and larger ones grafted. Disc spaces below 4 mm should not be grafted, but spaces of 4 mm or more should.

Effect of displacement rate on the tensile mechanics of pediatric cervical functional spinal units

This study examined the effect of loading (displacement) rate on the tensile mechanics of cervical spine functional spinal units. A total of 40 isolated functional spinal units (two vertebrae and the adjoining soft tissues) from juvenile male baboons (10±0.6—human equivalent years old) were subjected to tensile loading spanning four orders of magnitude from 0.5 to 5000 mm/s. The stiffness, ultimate failure load, and corresponding displacement at failure were measured for each specimen and normalized by spinal geometry to examine the material properties as well as the structural properties. The tensile stiffness, failure load, normalized stiffness, and normalized failure load significantly increased (ANOVA, p < 0.001 ) with increasing displacement rate. From the slowest to fastest loading rate, a two-fold increase in stiffness and four-fold increase in failure load were observed. The tensile failure strains (1.07±0.31 mm/mm strain) were not significantly correlated with loading rate (ANOVA, p = 0.146 ). Both the functional (non-destructive stiffness and normalized stiffness) and failure mechanics of isolated functional spinal units exhibited a power-law relationship with displacement rate. Modeling efforts utilizing these rate-dependent characteristics will enhance our understanding of the tensile viscoelastic response of the spine and enable improved dynamic injury prevention schemes.

Biomechanical Modeling of Posterior Instrumentation of the Scoliotic Spine

Scoliosis is a three-dimensional deformation of the spine that can be treated by vertebral fusion using surgical instrumentation. However, the optimal configuration of instrumentation remains controversial. Simulating the surgical maneuvers with personalized biomechanical models may provide an analytical tool to determine instrumentation configuration during the pre-operative planning. Finite element models used in surgical simulations display convergence difficulties as a result of discontinuities and stiffness differences between elements. A kinetic model using flexible mechanisms has been developed to address this problem, and this study presents its use in the simulation of Cotrel-Dubousset Horizon surgical maneuvers. The model of the spine is composed of rigid bodies corresponding to the thoracic and lumbar vertebrae, and flexible elements representing the intervertebral structures. The model was personalized to the geometry of three scoliotic patients (with a thoracic Cobb angle of 45°, 49° and 39°). Binary joints and kinematic constraints were used to represent the rod-implant-vertebra joints. The correction procedure was simulated using three steps: (1) Translation of hooks and screws on the first rod; (2) 90° rod rotation; (3) Hooks and screws look-up on the rod. After the simulation, slight differences of 0-6° were found for the thoracic spine scoliosis and the kyphosis, and of 1-8° for the axial rotation of the apical vertebra and for the orientation of the plane of maximum deformity, compared to the real post-operative shape of the patient. Reaction loads at the vertebra-implant link were mostly below 1000 N, while reaction loads at the boundary conditions (representing the overall action of the surgeon) were in the range 7-470 N and maximum torque applied to the rod was 1.8 Nm. This kinetic modeling approach using flexible mechanisms provided a realistic representation of the surgical maneuvers. It may offer a tool to predict spinal geometry correction and assist in the pre-operative planning of surgical instrumentation of the scoliotic spine.

Examining the relationship between whiplash kinematics and a direct neurologic injury mechanism

Despite the prevalence of whiplash-related injuries, a connection between clinical symptoms and injury mechanism has been elusive. Previous studies have attempted to correlate the whiplash kinematic response to injury mechanisms; however, none has specifically examined the potential for neurologic involvement due to foraminal occlusion. This biomechanical study measured cadaver cervical spine whiplash kinematics and compared these with changes in the neural space geometry of the cervical spine, providing a measure of the direct neurologic injury potential. Extension and shear displacements of each cervical level were measured and found to be similar to that reported in the literature and within the tissue's physiologic limits. Further, changes to the spinal canal and intervertebral foraminal geometry were recorded during whiplash and cross-sectional area changes were documented (up to 15.3%). Because these foraminal occlusions were smaller in magnitude than those resulting from normal cervical motion, our findings do not support direct neurologic injury resulting from segmental vertebral kinematics as a whiplash injury mechanism.