Kimio Nibu

Mechanical Properties of the Human Cervical Spine as Shown by Three-dimensional Load–displacement Curves

Study Design. The mechanical properties of multilevel human cervical spines were investigated by applying pure rotational moments to each specimen and measuring multidirectional intervertebral motions. Objectives. To document intervertebral main and coupled motions of the cervical spine in the form of load–displacement curves. Summary of Background Data. Although a number of in vivo and in vitro studies have attempted to delineate normal movement patterns of the cervical spine, none has explored the complexity of the whole cervical spine as a three-dimensional structure. Methods. Sixteen human cadaveric specimens (C0–C7) were used for this study. Pure rotational moments of flexion–extension, bilateral axial torque, and bilateral lateral bending were applied using a specially designed loading fixture. The resulting intervertebral motions were recorded using stereophotogrammetry and depicted as a series of load–displacement curves. Results. The resulting load–displacement curves were found to be nonlinear, and both rotation and translation motions were coupled with main motions. With flexion–extension moment loading, the greatest degree of flexion occurred at C1–C2 (12.3°), whereas the greatest degree of extension was observed at C0–C1 (20.2°). With axial moment loading, rotation at C1–C2 was the largest recorded (56.7°). With lateral bending moments, the average range of motion for all vertebral levels was 7.9°. Conclusions. The findings of the present study are relevant to the clinical practice of examining motions of the cervical spine in three dimensions and to the understanding of spinal trauma and degenerative diseases.

Whiplash produces an S-shaped curvature of the neck with hyperextension at lower levels

Study Design. A bench‐top trauma sled was used to apply four intensities of whiplash trauma to human cadaveric cervical spine specimens and to measure resulting intervertebral rotations using high‐speed cinematography. Objectives. To determine the cervical spine levels most prone to injury from whiplash trauma and to hypothesize a mechanism for such injury. Summary of Background Data. Whiplash injuries traditionally have been ascribed to hyperextension of the head, but other mechanisms such as hypertranslation also have been suggested. Methods. Six occiput to T1 (or C7) fresh cadaveric human spines were studied. Physiologic flexion and extension motions were recorded with an Optotrak motion analysis system by loading up to 1.0 Nm. Specimens then were secured in a trauma sled, and a surrogate head was attached. Flags fixed to the head and individual vertebrae were monitored with high‐speed cinematography (500 frames/sec). Data were collected for 12 traumas in four classes defined by the maximum sled acceleration. The trauma classes were 2.5g, 4.5g, 6.5g, and 8.5g. Significance was defined at P < 0.01. Results. In the whiplash traumas, the peak intervertebral rotations of C6‐C7 and C7‐T1 significantly exceeded the maximum physiologic extension for all trauma classes studied. The maximum extension of these lower levels occurred significantly before full neck extension. In fact, the upper cervical levels were consistently in flexion at the time of maximum lower level extension. Conclusions. In whiplash, the neck forms an S‐shaped curvature, with lower level hyperextension and upper level flexion. This was identified as the injury stage for the lower cervical levels. A subsequent C‐shaped curvature with extension of the entire cervical spine produced less lower level extension.

Critical load of the human cervical spine: an in vitro experimental study.

OBJECTIVE: To determine the critical load of the osteoligamentous cervical spine in frontal plane. DESIGN: Whole human cervical spine specimens were loaded in axial compression with increasing force until the point of buckling. BACKGROUND: The osteoligamentous cervical spine and the surrounding muscles support the weight of the head and the external loads applied to it. Critical load is the maximum compressive force that the spinal column can sustain before buckling. Critical loads have been obtained for the osteoligamentous thoracolumbar spine (without the rib cage) and the lumbar spine. Critical load of the cervical spine has not yet been determined. METHODS: When a compressive force is applied to the cervical spine, it bends in the sagittal plane producing greater lordosis. The determination of critical load in Eulers sense requires blocking of this sagittal plane bending. A special apparatus was developed that constrained such bending in the sagittal plane, but allowed complete freedom of the spine motion in the frontal plane. Experiments were conducted to determine the axial force-lateral bending curves of whole cervical spine specimens. Critical load values were obtained from these curves. As an alternative to this method, bending stiffness in the frontal plane was experimentally determined and the critical load was computed using Eulers theory of columns. RESULTS: Based upon the study of seven spine specimens (CO-T1), the critical load for the human cervical spine was found to be 10.5 (3.8) N obtained by direct experimentation. The average critical load calculated with the Euler theory using bending stiffness data, was 11.9 (2.0), but there were large individual differences when compared with the experimental results. CONCLUSIONS: The critical load of the osteoligamentous human cervical spine is about one-fifth to one-quarter the weight of the average head.

Simulation of whiplash trauma using whole cervical spine specimens

Study Design. Whiplash injuries were studied in an experiment using whole cervical spine specimen. Objectives. To develop a whiplash trauma model that uses a whole cervical spine specimen, and to show the feasibility and unique features of such a model. Summary of Background Data. Whiplash trauma has been simulated in biomechanical experiments using volunteers, whole body cadavers, animals, anthropometric dummies, and mathematic models. These experiments require large facilities, are expensive, and provide limited information about cervical spine injuries. Methods. An alternate approach, in which a benchtop sled accelerating apparatus is used to produce whiplash trauma, has been developed to study such trauma in whole cervical spine specimens. Several transducers were developed to monitor soft tissue injuries during the trauma. The model also provides quantification of injuries to the cervical spine. Results. To assess the feasibility and usefulness of the model, a specimen was traumatized, and the following parameters were monitored during the trauma: linear acceleration of the sled, linear and angular acceleration of the head surrogate, displacements of the head surrogate, loads at T1 and C1 vertebrae, and linear deformations of capsular ligaments and vertebral artery. Conclusions. This model, which incorporates a fresh cadaveric whole human cervical spine specimen, can simulate whiplash trauma effectively and is useful in providing a comprehensive set of clinically relevant information during the trauma. This model gives insight into the complex events and interactions that cause the injuries that occur during whiplash trauma.

Mechanism of whiplash injury

OBJECTIVE: To propose a different hypothesis of whiplash injury mechanism based on a series of experimental studies summarized in this communication. DESIGN: A series of biomechanical studies simulating whiplash trauma using isolated human cadaveric spine specimens. BACKGROUND: Whiplash injuries are on the rise as reported in several recent studies, due primarily to the increased traffic density. Although the symptoms associated with whiplash have been described, our understanding of the injury mechanism remains poor. The prevailing view of neck hyper-extension causing the injury has not been supported by recent experimental studies. METHODS: Eight fresh human cadaveric cervical spine specimens were prepared and traumatized to varying degrees under controlled conditions using a bench-top model of whiplash trauma. Before and after each trauma, the specimen was studied by functional radiography and flexibility test to document changes in the anatomic alignment and biomechanical properties at each level indicating injuries sustained. At the end of all testing, CT-scans, MRI and cryomicrotome images were obtained. During each trauma, relative motions of all intervertebral joints were recorded with a high speed movie camera. Elongations of the vertebral artery and several capsular ligaments were also monitored during the trauma using specially designed transducers. RESULTS: The hyper-extension hypothesis of injury mechanism was not supported by these studies. We found a distinct bi-phasic kinematic response of the cervical spine to whiplash trauma. In the first phase, the spine formed an S-shaped curve with flexion at the upper levels and hyper-extension at the lower levels. In the second phase, all levels of the cervical spine were extended, and the head reached its maximum extension. The occurrence of anterior injuries in the lower levels in the first phase was confirmed by functional radiography, flexibility tests and imaging modalities. The largest dynamic elongation of the capsular ligaments was observed at C6-C7 level during the initial S-shaped phase of whiplash. Similarly, the maximum elongation of the vertebral artery occurred during the S-shape phase of whiplash. CONCLUSION: We propose, based upon our experimental findings, that the lower cervical spine is injured in hyperextension when the spine forms an S-shaped curve. Further, this occurs in the first whiplash phase before the neck is fully extended. At higher trauma accelerations, there is a tendency for the injuries to occur at the upper levels of the cervical spine. Our findings provide truer understanding of whiplash trauma and may help in improving the diagnosis, treatment, and prevention of these injuries.

Capsular ligament stretches during in vitro whiplash simulations

Clinical symptoms of whiplash are presently not well understood. Injuries to capsular and other spinal ligaments of the cervical spine during trauma are a possible pathomechanism that could explain some aspects of the whiplash symptom complex. This study quantified the elongations of capsular ligaments (CLs) at all cervical spinal levels during whiplash simulation using an in vitro model. Seven fresh human cadaveric specimens (occiput-C7 or T1) were carefully dissected, preserving the osteoligamentous structures. Spinal ligament transducers were attached across the CLs from C2-C3 to C6-C7 in each specimen, alternating the two sides. Physiological elongations of the CLs were measured with a standard flexibility test using 1 Nm of pure moments. Next, the specimen was fitted with a surrogate head representing 50th percentile human head. The specimen was mounted on a sled designed to simulate whiplash and subjected to 2.5, 4.5, 6.5, 8.5, and 10.5 g (1 g = 9.81 m/s2) horizontal accelerations sequentially. The dynamic elongations of the CLs were continuously recorded during the entire trauma and were later converted to strains. There were modest increases in capsular ligament strains during the trauma over the maximum physiological values. The two largest peak strains of 29.5 and 35.4% were seen at C6-C7 during the 6.5- and 10.5-g accelerations. We did not find strong correlation between the strain during the trauma and the trauma sled acceleration.

Whiplash injuries and the potential for mechanical instability

Abstract Whiplash injury to the cervical spine is poorly understood. Symptoms often do not correlate to the clinical findings. It has been hypothesized that the long-term clinical symptoms associated with whiplash have their basis in mechanical derangement of the cervical spine caused at the time of trauma. Before such a hypothesis can be proven, one needs to document and quantify the soft tissue injuries of the cervical spine in whiplash. The purpose of the study was to quantify the mechanical changes that occur in the cervical spine specimen as a result of experimental whiplash trauma. Utilizing a whiplash trauma model, injuries to human cadaveric cervical spine specimens (C0 – T1 or C0 – C7) were produced by increasingly severe traumas. The flexibility tests determined the motion changes at each intervertebral level in response to 1.0 Nm pure flexion-extension moment. Parameters of range of motion (ROM) and neutral zone (NZ) were determined before and after each trauma. Significant flexibility increases first occurred in the lower cervical spine after 4.5–g rear-end (anteriorly directed) acceleration of the T1 vertebra. At this acceleration magnitude, extension ROM and NZ at C5 – C6 increased (P < 0.05) by 98% and 160% respectively. There was also a tendency (P < 0.1) for the extension NZ at C0 – C1 and C6 – C7 levels to increase after the 6.5-g acceleration by 52% and 241% respectively. There were no such tendencies for the ROM parameter. We have identified the threshold and sites of whiplash injury to the cervical spine. This information should help the clinician make more precise diagnoses in the case of whiplash trauma patients.

Head kinematics during in vitro whiplash simulation.

Knowledge of precise head kinematics during whiplash trauma is important for identifying possible injury mechanisms and their prevention. This study reports a comprehensive data set describing head kinematic response to horizontal accelerations simulating whiplash. Seven isolated fresh human cervical spine specimens (C0 to T1 or C7), each carrying a surrogate head designed to represent a 50th percentile human head, were mounted on the sled and subjected to incremental trauma by horizontal sled accelerations of 2.5, 4.5, 6.5, 8.5, and 10.5 g. Sled and head kinematics were measured with potentiometers and accelerometers. The incremental sled accelerations resulted in average (standard deviations) sled velocity changes (delta V) ranging from 5.8 (0.2) to 15.8 (0.2) km/h. Generally, all the peak head kinematic parameters increased with increasing sled acceleration, except for the peak head angular displacement, which decreased. In the initial phase of a whiplash trauma, the head translated posteriorly with respect to T1, without rotation. In the later phase, the head rotated backwards, but much less than its physiological limit. Maximum head rotation of 31.5 (23.9) degrees occurred in a 2.5 g trauma class, and this was less than the maximum physiological head extension of 55.1 (13.3) degrees. Head kinematics expressed in the T1 or shoulder coordinate system is better suited to study potential neck injury in whiplash.

Transforaminal and posterior decompressions of the lumbar spine. A comparative study of stability and intervertebral foramen area.

Study Design. Ten fresh, cadaveric, two‐vertebrae, functional spinal units were used to study the pathoanatomy, intervertebral foraminal area, and flexibility changes after posterior and transforaminal decompression. Objectives. To determine the feasibility of an endoscopic transforaminal approach as an alternative to conventional approaches, to establish the adequacy of transforaminal decompression without destabilizing the spine, and to study the structural changes in the spine after decompressions. Summary of the Background Data. Posterior decompression entails major dissection and excision of bone and ligaments to access the spinal canal. Posterior decompression may be complicated by acute or chronic spinal instability, and the adequacy of lateral decompression is highly subjective. Methods. The functional spinal units were mounted in quick‐setting epoxy blocks. Pre‐ and postoperative computed tomography scans were taken to study changes in the foraminal area. Pre‐ and postoperative flexibility and anatomic studies were performed to compare the results. Results. A 45.5% increase in the intervertebral foraminal area was possible, there was no flexibility change, and minimal anatomic damage to the spine was noted after transforaminal decompression. A 34.2% increase in the intervertebral foraminal area and a significant increase in extension and axial rotation flexibility were noted after the posterior decompression. Conclusion. Transforaminal decompression produced a significantly larger increase in the intervertebral foraminal area than posterior decompression, without increasing the range of motion or neutral zone in any direction. Because there was no violation of the anatomic integrity of the spine in the transforaminal approach, the risk of surgically induced instability was minimized. Endoscopic transforaminal decompression is a feasible alternative to current approaches.

Dynamic elongation of the vertebral artery during an in vitro whiplash simulation.

Clinical signs of whiplash are presently not well understood. Vertebral artery (VA) stretch during trauma is a possible pathomechanism that could explain some aspects of the whiplash symptom complex. This study quantified the VA elongation during whiplash simulation using an in vitro model. Seven fresh human cadaveric specimens (occiput to C7 or TI) were carefully dissected, preserving the osteoligamentous structures. The right VA was replaced with a thin nylon-coated flexible cable. This cable was fixed at one end to the occipital bone and at the other end to a specially designed VA transducer. Physiological motion of the occiput and physiological elongation of the VA were measured with a standard flexibility test. Next the specimen was mounted on a specially designed sled and subjected to 2.5, 4.5, 6.5, and 8.5 g (1 g = 9.81 m/s2) horizontal accelerations. Elongation of the VA was continuously recorded from the start of the trauma. The average (standard deviation) physiological VA elongation was 5.8 (1.6) mm in left lateral bending and 4.7 (1.8) mm in left axial rotation. Flexion and extension did not result in any appreciable elongation of the VA. The maximum VA elongation during the whiplash trauma significantly correlated with the horizontal acceleration of the sled (R2 = 0.7,P < 0.05). The VA exceeded its physiological range by 1.0 (2.1), 3.1 (2.6), 8.9 (1.6), and 9.0 (5.9) mm in the 2.5-, 4.5-, 6.5-, and 8.5-g trauma classes respectively.