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Dennis J. Maiman, Narayan Yoganandan and Frank A. Pintar

Object. The authors tested the hypothesis that initial alignment of the head—neck complex affects cervical spine injury mechanism, trauma rating, injury classification based on stability, and fracture pattern.

Methods. Thirty intact human cadaveric head—neck complexes were prepared by fixing the thoracic end in polymethylmethacrylate. The cranium was unconstrained. The initial spinal alignment was described in terms of eccentricity, defined as the anteroposterior position of the occipital condyles with respect to the T-1 vertebral body. The specimens were subjected to impact loading delivered using an electrohydraulic testing device. Outcomes after injury were identified using radiography and computerized tomography. The mechanisms of injury were classified according to fracture pattern into compression—extension, compression—flexion, hyperflexion, and vertical compression. Trauma was graded according to the Abbreviated Injury Scale rating system. Based on clinical assessment, injuries were classified as stable or unstable. Injuries were also classified into bone fracture or nonfracture groups. Analysis of variance tests were used to determine the influence of eccentricity on spinal injury outcomes. Eccentricity significantly influenced the mechanism of injury (p < 0.0001), trauma rating (p < 0.005), and fracture (p < 0.0001) classification. Statistically significant differences, however, were not apparent when the classification of injury was based on stability considerations.

Conclusions. Spinal alignment is a strong determinant of the biomechanics of impact-induced cervical spine injury.

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Cheryl A. Muszynski, Narayan Yoganandan, Frank A. Pintar and Thomas A. Gennarelli

Object. Injury to the brain as a result of motor vehicle accidents (MVAs) represents a frequent cause of pediatric disability. The authors analyze the correlation between the relative risk of pediatric brain injury and the use of child safety seats (CSSs).

Methods. A national database of MVAs was examined to provide data for the analysis of four age categories (infant, toddler, young child, and adolescent) and four restraint categories (unrestrained, properly restrained, improperly restrained, and other). The Abbreviated Injury Scale (AIS) was used to assess the severity of head injury; children with no injuries and children with moderate-to-maximum head injuries were evaluated.

The data confirm that proper use of a CSS substantially increases the likelihood of not sustaining head injury in an MVA. The data are most dramatic for infants (the likelihood of sustaining no head injury was 15.2% for unrestrained infants compared with 92.8% for properly restrained infants) but the protective effect is seen in all age categories, with the least difference observed in the adolescent category. For children who sustain a moderate-to-maximum head injury, proper use of a CSS reduces the incidence of injury, again most dramatically for the infant category (unrestrained infants had a 7% risk of moderate-to-maximum head injury compared with only 0.5% for properly restrained infants).

Conclusions. Improvements in CSSs have reduced the risk of moderate-to-maximum head injuries in children of all age categories. Overall, a CSS is most protective for the infant and toddler categories. The improperly restrained child still has substantial protection, although the properly restrained child has more. Detailed parental education regarding appropriate restraint system installation and use should be required.

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Joseph F. Cusick, Narayan Yoganandan, Frank A. Pintar and John M. Reinartz

✓ Compromise of the functional integrity of the posterior lumbar ligaments and facet joints is a common occurrence after repeated lumbar operative procedures. To evaluate the biomechanical effects of sequential surgical alterations, this investigation analyzed bilateral facetectomies (medial, total, and total with posterior ligament section) in three segments of human cadaveric lumbar spines under increasing compression-flexion. These iatrogenic alterations, designed to replicate common methods of surgical exposure, were created at the lower intervertebral joint (L4–5) while the upper joint (L3–4) remained intact. Overall strength characteristics in the physiological range of 400 N and 600 N demonstrated significant differences (p < 0.05) in applied compressions for all preparations compared to the intact specimen. Comparison of sequential surgeries, however, did not demonstrate this tendency. Significant changes in the movement of the spinous processes at the upper (unaltered) level occurred only after posterior ligament section, whereas the lower (altered) level showed markedly increasing distraction of both the facets and the spinous processes with sequential operations. Sectioning of the supraspinous/interspinous ligament and associated fascial attachments resulted in a marked transfer of motion to the altered level. This was manifested by the increased anterior displacement of the centrode at the lower level associated with probable posterior migration of the centrode at the upper level. These data suggest that the effects of progressive surgical alterations of the lumbar facet joints are controllable in a preparation undergoing acute compression-flexion loads until the supraspinous/interspinous ligaments, with associated residual tendinous, midline muscle, and fascial attachments, are violated.

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Brian D. Stemper, Narayan Yoganandan, Thomas A. Gennarelli and Frank A. Pintar

Object. Although facet joints have been implicated in the whiplash injury mechanism, no investigators have determined the degree to which joint motions in whiplash are nonphysiological. The purpose of this investigation was to quantify the correlation between facet joint and segmental motions under physiological and whiplash loading.

Methods. Human cadaveric cervical spine specimens were exercise tested under physiological extension loading, and intact human head-neck complexes were exercise tested under whiplash loading to correlate the localized component motions of the C4–5 facet joint with segmental extension. Facet joint shear and distraction kinematics demonstrated a linear correlation with segmental extension under both loading modes. Facet joints responded differently to whiplash and physiological loading, with significantly increased kinematics for the same-segmental angulation. The limitations of this study include removal of superficial musculature and the limited sample size for physiological testing.

Conclusions. The presence of increased facet joint motions indicated that synovial joint soft-tissue components (that is, synovial membrane and capsular ligament) sustain increased distortion that may subject these tissues to a greater likelihood of injury. This finding is supported by clinical investigations in which lower cervical facet joint injury resulted in similar pain patterns due to the most commonly reported whiplash symptoms.

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Type II odontoid fracture from frontal impact

Case report and biomechanical mechanism of injury

Narayan Yoganandan, Jamie L. Baisden, Dennis J. Maiman and Frank A. Pintar

✓ The authors report a case of Type II odontoid fracture from a frontal impact sustained in the crash of a late-model motor vehicle. They discuss the biomechanical mechanisms of injury after considering patient demographic data, type and use of restraint systems including seatbelt and airbags, crash characteristics, and laboratory-based experimental studies. Multiple factors contributed to the Type II odontoid fracture: the patient's tall stature and intoxicated state; lack of manual three-point seat belt use; obliqueness of the frontal impact; and the most likely preflexed position of the head—neck complex at the time of impact, which led to contact of the parietal region with the A-pillar roof-rail area of the vehicle and resulted in the transfer of the dynamic compressive force associated with lateral bending. Odontoid fractures still occur in individuals involved in late-model motor vehicle frontal crashes, and because this injury occurs secondary to head impact, airbags may not play a major role in mitigating this type of trauma to an unrestrained occupant. It may be more important to use seat belts than to depend on the airbag alone for protection from injury.

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Marjorie C. Wang, Frank Pintar, Narayan Yoganandan and Dennis J. Maiman

Object

Spine fractures are a significant cause of morbidity and mortality after motor vehicle crashes (MVCs). Public health interventions, such as the National Highway Traffic Safety Administration's Federal Motor Vehicle Safety Standards, have led to an increase in automobiles with air bags and the increased use of seat belts to lessen injuries sustained from MVCs. The purpose of this study was to evaluate secular trends in the occurrence of spine fractures associated with MVCs and evaluate the association between air bag and seat belt use with spine fractures.

Methods

Using the Crash Outcome Data Evaluation System, a database of the police reports of all MVCs in Wisconsin linked to hospital records, the authors studied the occurrence of spine fractures and seat belt and air bag use from 1994 to 2002. Demographic information and crash characteristics were obtained from the police reports. Injury characteristics were determined using International Classification of Disease, 9th Revision, Clinical Modification (ICD-9-CM) hospital discharge codes.

Results

From 1994 to 2002, there were 29,860 hospital admissions associated with automobile or truck crashes. There were 20,276 drivers or front-seat passengers 16 years of age and older who were not missing ICD-9-CM discharge codes, seat belt or air bag data, and who had not been ejected from the vehicle. Of these, 2530 (12.5%) sustained a spine fracture. The occurrence of spine fractures increased over the study period, and the use of a seat belt plus air bag, and of air bags alone also increased during this period. However, the occurrence of severe spine fractures (Abbreviated Injury Scale Score ≥3) did not significantly increase over the study period. The use of both seat belt and air bag was associated with decreased odds of a spine fracture. Use of an air bag alone was associated with increased odds of a severe thoracic, but not cervical spine fracture.

Conclusions

Among drivers and front-seat passengers admitted to the hospital after MVCs, the occurrence of spine fractures increased from 1994 to 2002 despite concomitant increases in seat belt and air bag use. However, the occurrence of severe spine fractures did not increase over the study period. The use of both seat belt and air bag is protective against spine fractures. Although the overall increased occurrence of spine fractures may appear contrary to the increased use of seat belts and air bags in general, it is possible that improved imaging technology may be associated with an increase in the diagnosis of relatively minor fractures. However, given the significant protective effects of both seat belt and air bag use against spine fractures, resources should continue to be dedicated toward increasing their use to mitigate the effects of MVCs.

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Jason Moore, Narayan Yoganandan, Frank A. Pintar, Jason Lifshutz and Dennis J. Maiman

Object

The aim of this study was to determine the in vitro biomechanical responses of lumbar spinal segments after implantation of tapered cages.

Methods

Range of motion (ROM)– and stiffness-related data were determined in 10 human cadaveric T12–S1 columns subjected to flexion, extension, and lateral bending modes before and after anterior lumbar interbody fusion in which stand-alone LT-CAGE devices were used. The overall column showed no significant changes in ROM or stiffness. At the instrumented level, stiffness increased significantly (p < 0.05) in flexion and lateral bending modes. Indications of instability in extension were present, but these values were not statistically significant. There was no evidence of adjacent-level instability at any level in any mode, except for the segment superior to the fixation level in flexion; here there was a significant increase in ROM (p < 0.05) and a decrease in stiffness.

Conclusions

The anatomical conformity and bilateral placement of cages provide ample stability and rigidity at the treated level, comparable to that of other cage systems. Because hypermobility is traditionally related to early degenerative changes, the present results appear to suggest that cages do not significantly contribute to such alterations.

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Harlan J. Bruner, Yabo Guan, Narayan Yoganandan, Frank A. Pintar, Dennis J. Maiman and Michael A. Slivka

Object

Interest is increasing in the development of polyaryletherketone (PAEK) implants for posterior lumbar fusion. Due to their inherent physical properties, including radiolucency and the ability to customize stiffness with carbon fiber reinforcement, they may be more advantageous than traditional instrumentation materials. Customization of these materials may allow for the development of a system that is stiff enough to promote fusion, yet flexible enough to avoid instrumentation failure. To understand the feasibility of using such materials in posterior lumbosacral instrumentation, biomechanical performances were compared in pure moment and combined loadings between two different PAEK composite rods and titanium rods.

Methods

Four human cadaver L3–S1 segments were subjected to pure moment and combined (compressionflexion and compression-extension) loadings as intact specimens, and after L-4 laminectomy with complete L4–5 facetectomy. Pedicle screw/rod fixation constructs were placed from L-4 to S-1, and retested with titanium, pure poly(aryl-ether-ether-ketone) (PEEK), and carbon fiber reinforced PEEK (CFRP) rods. Reflective markers were fixed to each spinal segment. The range of motion data for the L3–S1 column and L4–5 surgical level were obtained using a digital 6-camera system. Four prewired strain gauges were glued to each rod at the level of the L-4 screw and were placed 90° apart along the axial plane of the rod to record local strain data in the combined loading mode. Biomechanical data were analyzed using the ANOVA techniques.

Results

In pure moment, when compared with intact specimens, each rod material similarly restricted motion in each mode of bending, except axial rotation (p < 0.05). When compared with postfacetectomy specimens, each rod material similarly restricted motion (p < 0.05) in all bending modes. In combined loading, rod stiffness was similar for each material. Rod strain was the least in the titanium construct, intermediate in the CFRP construct, and maximal in the pure PEEK construct.

Conclusions

Pure PEEK and CFRP rods confer equal stiffness and resistance to motion in lumbosacral instrumentation when compared with titanium constructs in single-cycle loading. The carbon fiber reinforcement reduces strain when compared with pure PEEK in single-cycle loading. These biomechanical responses, combined with its radiolucency, suggest that the CFRP may have an advantage over both titanium and pure PEEK rods as a material for use in posterior lumbosacral instrumentation. Benchtop fatigue testing of the CFRP constructs is needed for further examination of their responses under multicycle loading.