David J. Nuckley

Neural space integrity of the lower cervical spine: effect of normal range of motion.

Study Design. An experimental investigation of intervertebral foramen and spinal canal neural space integrity was performed throughout physiologic range of motion of the lower cervical spine in intact human cadaver specimens. Objective. To investigate cervical positions that might place the neural tissues of the spine in heightened risk of injury. To meet this objective the following hypotheses were tested: 1) spinal canal integrity varies with specific normal range of motion positions of the lower cervical spine, and 2) intervertebral foramen integrity is dependent on and unique for different physiologic positions of the lower cervical spine. Summary of Background Data. Cervical spine injuries are frequently associated with compressive damage to neurologic tissues and consequently poor clinical outcomes. Neurologic injury typically occurs from disc, ligamentous, or bony occlusion of the spinal canal and intervertebral foraminal spaces dynamically during an injury event or with abnormal alignment and position after the injury event. Prior studies have shown pressure and geometric changes in cervical spine neural spaces in certain cervical spine positions. However, to the authors’ knowledge, this is the first research effort aimed at elucidating the integrity of the cervical spine neural spaces throughout the normal physiologic range of motion. Methods. The authors instrumented 17 fresh-frozen unembalmed cadaveric human cervical spines (C3–C7) with specially designed intervertebral foramen occlusion transducers and a spinal canal occlusion transducer. The specimens were loaded with pure bending moments to produce simulated physiologic motions of the lower cervical spine. The resulting occlusion profiles for the intervertebral foramen and spinal canal were recorded along with the 6-degree of freedom position of the cervical spine. Because these occlusion measurements describe the ability of the spine to preserve the space for the neural structures, the authors define this neuroprotective role of the vertebral column as neural space integrity. Results. The range of motion developed experimentally in this study compared well with published reports of normal cervical motion. Thus, subsequent changes in neural space integrity may be regarded as resulting from normal human cervical spine motion. No significant change in the spinal canal space was detected for any physiologic motion; however, intervertebral foramen integrity was significantly altered in extension, ipsilateral bending, combined ipsilateral bending and extension, and combined contralateral bending with extension when compared with intact upright neutral position. Conclusions. This study defines the range of neural space integrity associated with simulated physiologic motion of the lower cervical spine in an experimental setting. This information may be useful in comparing neural space changes in pathologic conditions and may enhance refinement of neurologic injury prevention strategies.

Effect of Loading Rate on the Compressive Mechanics of the Immature Baboon Cervical Spine

Thirty-four cervical spine segments were harvested from 12 juvenile male baboons and compressed to failure at displacement rates of 5, 50, 500, or 5000 mm/s. Compressive stiffness, failure load, and failure displacement were measured for comparison across loading rate groups. Stiffness showed a significant concomitant increase with loading rate, increasing by 62% between rates of 5 and 5000 mm/s. Failure load also demonstrated an increasing relationship with loading rate, while displacement at failure showed no rate dependence. These data may help in the development of improved pediatric automotive safety standards and more biofidelic physical and computational models.

Intervertebral disc degeneration in a naturally occurring primate model: radiographic and biomechanical evidence.

Classic degenerative disc disease is a serious health problem worldwide, whose etiological basis—mechanical stimulus, biochemical changes, or natural aging—is poorly understood. Animal models are critical to the study of degenerative disc disease initiation and progression and for attempts to regulate, ameliorate, or eliminate it. The macaque represents a primate model with natural disc degeneration that might serve to advance the field; we aimed to provide radiographic (morphologic) and biomechanical evidence of natural disc degeneration in this model. A factorial study design was used to examine the relationship between the radiographic appearance of disc degeneration and its biomechanical consequences. Eighteen macaques of advanced age (22.3u2009±u20090.9 years) had radiographs taken to assess the degree of thoracolumbar intervertebral disc degeneration using a standard atlas method. Each spine was harvested and dynamic biomechanical tests were performed. Advancing disc degeneration (degree of disc space narrowing and osteophytosis) was associated with increased stiffness, decreased energy absorption, and increased natural frequency of the intervertebral disc. These associations linking the dynamics of the intervertebral disc and its degree of degeneration are similar to those found in humans. Our results indicate the macaque model with morphologic and biomechanical efficacy could aid in understanding the progression of disc degeneration and in developing therapeutic strategies to prevent or inhibit its course.

Neural space integrity of the lower cervical spine: Effect of anterior lesions

Study Design. A repeated measures study design was used to evaluate intervertebral foramen and spinal canal neural space integrity subsequent to sequential surgical anterior lesions of the lower cervical spine in a human cadaver model. Objective. To investigate the degree to which sequential ablation of anterior vertebral elements places the neural structures at risk of injury. Summary of Background Data. Classic instability management utilizing functional-structural criteria has been widely examined associating specific lesions or pathologies to a degree of mechanical instability. Unfortunately, these studies have not assessed the neuroprotective role of the vertebral column. Methods. Eight human cadaveric lower cervical spines were instrumented with transducers to measure geometrical changes in the intervertebral foramen and spinal canal. Sequential lesions were performed anteriorly on the anterior and middle column structures (C4–C5 disc and C5 vertebra), and their effects on neural space integrity and range of motion were measured under physiologic loading. Results. Range of motion significantly increased with successively more destructive lesions, whereas the spinal canal exhibited few changes. Intervertebral foramen integrity was statistically reduced for corpectomy (66% intact), hemivertebrectomy (62% intact) and full vertebrectomy (57% intact) lesions when loaded in concomitant extension and ipsilateral bending (4 Nm). Conclusions. Lesions more extensive than a surgical discectomy have significant effects on the cervical neural foramens specifically when the spine is placed in extension, ipsilateral bending, and coupled ipsilateral bending and extension. Our study establishes a quantitative relationship between the risk of neural structure compression and anterior lesions of the spinal column under physiologic loading.

Neural space and biomechanical integrity of the developing cervical spine in compression.

Study Design. A factorial study design was used to examine the biomechanical and neuroprotective integrity of the cervical spine throughout maturation using a postmortem baboon model. Objective. To investigate changes with spinal development that affect the neuroprotective ability of the cervical spine in compressive loading. Summary of Background Data. Child spinal cord injuries claim and debilitate thousands of children in the United States each year. Many of these injuries are diagnostically and mechanistically difficult to classify, treat, and prevent. Biomechanical studies on maturing spinal tissues have identified decreased stiffness and tolerance characteristics for children compared with adults. Unfortunately, while neurologic deficit typically dictates functional outcome, no previous studies have examined the neuroprotective role of the pediatric cervical spine. Methods. Twenty-two postmortem baboon cervical spines across the developmental age spectrum were tested. Two functional spinal unit segments (Oc–C2, C3–C5, and C6–T1) were instrumented with transducers to measure dynamic changes in the spinal canal. These tissues were compressed to 70% strain dynamically, and the resultant mechanics and spinal canal occlusions were recorded. Results. Classic injury patterns were observed in all of the specimens tested. The compressive mechanics exhibited a significant age relationship (P < 0.0001). Furthermore, while the peak-percent spinal canal occlusion was not age dependant, the percent occlusion just before failure did demonstrate a significant decrease with advancing age (P = 0.0001). Conclusions. The neuroprotective ability of the cervical spine preceding failure appears to be age dependent, where the young spine can produce greater spinal canal occlusions without failure than its adult counterpart. The overall percent of the spinal canal occluded during a compression injury was not age dependent; however, these data reveal the neuroprotective ability of the child spine to be more sensitive as an injury predictor than the biomechanical fracture data.

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 tissues 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.

Developmental biomechanics of neck musculature

Neck mechanics is central to head injury prevention since it is the musculoskeletal neck, which dictates the position and movement of the head. In the US, traumatic injury is the leading cause of death for children; however prevention is hampered by the lack of data concerning the mechanics of the immature head-and-neck. Thus, the objective of this study was to quantify neck muscle strength and endurance across the maturation spectrum and correlate these with head-and-neck anthropometry. A factorial study was performed on 91 human subjects measuring head-and-neck anthropometry and neck strength and endurance in three bending directions (flexion, extension, and lateral) as a function of age (6-23 years). Using a custom device, neck maximum voluntary contraction (MVC) force was measured in triplicate. Next, neck muscle endurance (sustained effort) was measured as the subjects ability to maintain 70% of peak force over 30s. Linear regression of peak force and endurance as a function of age revealed each direction to significantly (p<0.0001) increase with age. The MVC force, averaged across all directions and normalized to the adult values, exhibits the following maturation curve: %MVC Force=-0.0879(age)(2)+6.018(age)+8.120. Neck muscle strength, similar between young males and females, becomes disparate in adolescence and adulthood with males exhibiting greater strength. Bending direction differences were also found with extension strength being the greatest regardless of age and sex. Furthermore, neck circumference appears predictive of neck strength and endurance in children. Together, these relationships may facilitate improved design of injury prevention interventions.

Dynamic tensile failure mechanics of the musculoskeletal neck using a cadaver model.

Although the catapult phase of pilot ejections has been well characterized in terms of human response to compressive forces, the effect of the forces on the human body during the ensuing ejection phases (including windblast and parachute opening shock) has not been thoroughly investigated. Both windblast and parachute opening shock have been shown to induce dynamic tensile forces in the human cervical spine. However, the human tolerance to such loading is not well known. Therefore, the main objective of this research project was to measure human tensile neck failure mechanics to provide data for computational modeling, anthropometric test device development, and improved tensile injury criteria. Twelve human cadaver specimens, including four females and eight males with a mean age of 50.1+/-9 years, were subjected to dynamic tensile loading through the musculoskeletal neck until failure occurred. Failure load, failure strain, and tensile stiffness were measured and correlated with injury type and location. The mean failure load for the 12 specimens was 3100+/-645 N, mean failure strain was 16.7+/-5.4%, and mean tensile stiffness was 172+/-54.5 N/mm. The majority of injuries (8) occurred in the upper cervical spine (Oc-C3), and none took place in the midcervical region (C3-C5). The results of this study assist in filling the existing void in dynamic tensile injury data and will aid in developing improved neck injury prevention strategies.