The human cervical spine in tension: effects of frame and fixation compliance on structural responses.
ABSTRACT There is little data available on the responses of the human cervical spine to tensile loading. Such tests are mechanistically and technically challenging due to the variety of end conditions that need to be imposed and the difficulty of strong specimen fixation. As a result, spine specimens need to be tested using fairly complex, and potentially compliant, apparati in order to fully characterize the mechanical responses of each specimen. This, combined with the relatively high stiffness of human spine specimens, can result in errors in stiffness calculations. In this study, 18 specimen preparations were tested in tension. Tests were performed on whole cervical spines and on spine segments. On average, the linear stiffness of the segment preparations was 257 N/mm, and the stiffness of the whole cervical spine was 48 N/mm. The test frame was found to have a stiffness of 933 N/mm. Assembling a whole spine from a series combination of eight segments with a stiffness of 257 N/mm results in an estimated whole spine stiffness of 32.1 N/mm (32% error). The segment stiffnesses were corrected by assuming that the segment preparation stiffness is a series combination of the stiffnesses of the segment and the frame. This resulted in an average corrected segment stiffness of 356 N/mm. Taking the frame compliance into account, the whole spine stiffness is 51 N/mm. A series combination of eight segments using the corrected stiffnesses results in an estimated whole spine stiffness of 45.0 N/mm (12% error). We report both linear and nonlinear stiffness models for male spines and conclude that the compliance of the frame and the fixation must be quantified in all tension studies of spinal segments. Further, reported stiffness should be adjusted to account for frame and fixation compliance.
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ABSTRACT: Investigations of biomechanical properties of pediatric cadaver cervical spines subjected to tensile or bending modes of loading are generally limited by a lack of available tissue and limiting sample sizes, both per age and across age ranges. It is therefore important to develop fixation techniques capable of testing individual cadavers in multiple modes of loading to obtain more biomechanical data per subject. In this study, an experimental apparatus and fixation methodology was developed to accommodate cadaver osteoligamentous head-neck complexes from around birth (perinatal) to full maturation (adult) [cervical length: 2.5-12.5 cm; head breadth: 6-15 cm; head length: 6-19 cm] and sequentially test the whole cervical spine in tension, the upper cervical spine in bending and the upper cervical spine in tension. The experimental apparatus and the fixation methodology provided a rigid casting of the head during testing and did not compromise the skull. Further testing of the intact skull and sub-cranial material was made available due to the design of the apparatus and fixation techniques utilized during spinal testing. The stiffness of the experimental apparatus and fixation technique are reported to better characterize the cervical spine stiffness data obtained from the apparatus. The apparatus and fixation technique stiffness was 1986 N/mm. This experimental system provides a stiff and consistent platform for biomechanical testing across a broad age range and under multiple modes of loading.Journal of biomechanics 11/2011; 45(2):386-9. · 2.66 Impact Factor
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ABSTRACT: Study Design. Biomechanical tensile testing of perinatal, neonatal and pediatric cadaveric cervical spines to failure.Objective. To assess the tensile failure properties of the cervical spine from birth to adulthood.Summary of Background Data. Pediatric cervical spine biomechanical studies have been few owing to the limited availability of pediatric cadavers. Therefore, scaled data based on human adult and juvenile animal studies have been used to augment the limited pediatric cadaver data. Despite these efforts, substantial uncertainty remains in our understanding of pediatric cervical spine biomechanics.Methods. A total of 24 cadaveric osteoligamentous head-neck complexes, 20 weeks gestation to 18 years, were sectioned into segments (O-C2, C4-C5, and C6-C7) and tested in tension to determine axial stiffness, displacement at failure and load-to-failure.Results. Tensile stiffness-to-failure (N/mm) increased by age (O-C2: 23-fold, neonate - 22 ± 7, 18 years - 504; C4-C5: seven-fold, neonate - 71 ± 14, 18 years - 509; C6-C7: seven-fold, neonate - 64 ± 17, 18 years - 456). Load-to-failure (N) increased by age (O-C2: 13-fold, neonate - 228 ± 40, 18 years - 2888; C4-C5: nine-fold, neonate - 207 ± 63, 18 years - 1831; C6-C7: 10-fold, neonate - 174 ± 41, 18 years - 1720). Normalized displacement at failure (mm/mm) decreased by age (O-C2: six-fold, neonate - 0.34 ± 0.076, 18 years - 0.059; C4-C5: three-fold, neonate - 0.092 ± 0.015, 18 years - 0.035; C6-C7: two-fold, neonate - 0.088 ± 0.019, 18 years - 0.037).Conclusion. Cervical spine tensile stiffness-to-failure and load-to-failure increased non-linearly, while normalized displacement at failure decreased non-linearly, from birth to adulthood. Pronounced ligamentous laxity observed at younger ages in the O-C2 segment quantitatively supports the prevalence of spinal cord injury without radiographic abnormality (SCIWORA) in the pediatric population. This study provides important and previously unavailable data for validating pediatric cervical spine models, for evaluating current scaling techniques and animal surrogate models, and for the development of more biofidelic pediatric crash test dummies.Spine 10/2012; · 2.16 Impact Factor
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ABSTRACT: Head and neck injuries, the leading cause of death for children in the U.S., are difficult to diagnose, treat, and prevent because of a critical void in our understanding of the biomechanical response of the immature cervical spine. The objective of this study was to investigate the functional and failure biomechanics of the cervical spine across multiple axes of loading throughout maturation. A correlational study design was used to examine the relationships governing spinal maturation and biomechanical flexibility curves and tolerance data using a cadaver human in vitro model. Eleven human cadaver cervical spines from across the developmental spectrum (2-28 years) were dissected into segments (C1-C2, C3-C5, and C6-C7) for biomechanical testing. Non-destructive flexibility tests were performed in tension, compression, flexion, extension, lateral bending, and axial rotation. After measuring their intact biomechanical responses, each segment group was failed in different modes to measure the tissue tolerance in tension (C1-C2), compression (C3-C5), and extension (C5-C6). Classical injury patterns were observed in all of the specimens tested. Both the functional (p<0.014) and failure (p<0.0001) mechanics exhibited significant relationships with age. Nonlinear flexibility curves described the functional response of the cervical spine throughout maturation and elucidated age, spinal level, and mode of loading specificity. These data support our understanding of the child cervical spine from a developmental perspective and facilitate the generation of injury prevention or management schema for the mitigation of child spine injuries and their deleterious effects.Journal of biomechanics 02/2013; · 2.66 Impact Factor