Human Cadaveric Cervical Spine Specimens Research Articles

Total Disk Replacement Adjacent to a Multilevel Fusion in the Cervical Spine: A Biomechanical Motion Analysis

Cervical total disk replacement (TDR) has emerged as a motion-preserving alternative to anterior cervical diskectomy fusion (ACDF). Biomechanical studies have demonstrated that the TDR preserves motion at the diseased segment and minimizes motion and stress at adjacent segments compared with fusion. There has been growing interest in performing a TDR adjacent to a cervical fusion. The purpose of this study was to investigate the kinematics of a TDR after sequentially fusing adjacent segments. Seven fresh-frozen human cadaveric cervical spine specimens from C1-T1 were used (average age, 56.2 ± 7.3 years). The effect on cervical flexion-extension motion, by instrumenting a TDR above or below a 1-, 2-, or 3-level fusion, was measured. The protocol consisted of taking fluoroscopic images of each cervical specimen obtained at maximal angular displacement in flexion and extension during force application. Cobb angles were measured on digital radiographs to determine flexion-extension range of motion (ROM). Segmental ROM of the C6-7 TDR in the unfused spine was 11.3° ± 1.9°. After performing a 3-level fusion at C3-6, the motion of the C6-7 TDR increased to 12.9° ± 1.3° (P= 0.33). ROM of the C2-3 TDR in the unfused spine was 5.0° ± 1.1°. After performing a 3-level fusion of C3-6, the C2-3 TDR segmental motion was 6.1° ± 1.3° (P=0.09). Biomechanically performing a cervical TDR adjacent to a long-segment fusion did not subject the implant to significantly greater motion than when the TDR was instrumented alone.

Thursday, September 27, 2018 3:35 PM–5:05 PM Section on Motion Technology Abstract Presentations: 139. Artificial disc replacement adjacent to a multilevel fusion in the cervical spine: a biomechanical study

BACKGROUND CONTEXT Cervical total disc replacement (TDR) has emerged as a motion-preserving alternative to anterior cervical discectomy fusion. Biomechanical studies have demonstrated that the TDR preserves motion at the diseased segment and minimizes motion and stress at adjacent segments compared to fusion. There has been growing interest in performing a TDR adjacent to a cervical fusion. The kinematics of a cervical TDR adjacent to a multilevel fusion construct (>2 levels) has not been investigated. PURPOSE The purpose of this study was to evaluate the motion of a cervical total disc replacement after fusing adjacent cervical segments both caudal and cranial to the implant. OUTCOME MEASURES The primary outcome measure was the segmental flexion-extension range of motion at the level of a cervical total disc repalcement. The secondary outcome measures were the overall cervical range of motion and the segemental motion of the remaining cervical segments. METHODS Seven fresh-frozen human cadaveric cervical spine specimens from C1-T1 were used (average age 56.2±7.3 years). A C6-7 TDR was tested adjacent to a C5-6, C4-6 and C3-6 fusion. A C2-3 TDR was tested adjacent to a C3-4, C3-5 and C3-6 fusion. The protocol consisted of taking lateral radiographs of each construct at maximal angular displacement in flexion and extension. Cobb angles were measured on the digital radiographs to determine flexion-extension range of motion. RESULTS The segmental ROM of the C6-7 TDR in the unfused spine was 11.3±1.9°. After performing a three-level fusion (C3-6) the segmental motion of the C6-7 TDR increased to 12.9±1.3°(p=.33). The difference between the motion of the C6-7 TDR and the C6-7 native segment caudal to a three-level fusion was not statistically significant; 12.9±1.3° versus 11.3±1.9°, respectively (p=.30).The segmental ROM of the C2-3 TDR in the unfused spine was 5.0 ±1.1. After performing a three-level fusion (C3-6), the C2-3 TDR segmental motion was 6.1±1.3° (p=.09). The difference in motion of the TDR cranial to a three-level fusion and the native disc cranial to a three-level fusion was not statistically significant; 6.1± 1.3° versus 6.4± 0.8°, respectively (p=.15). CONCLUSIONS The TDR had similar flexion extension motion to the native segment when instrumented either caudal or cranial to the single, two and three level fusions. The TDR had a trend toward increased motion as additional fusion levels were added adjacent to the implant. Biomechanically, performing a TDR adjacent to a long fusion construct did not subject the implant to greater motion than when instrumented alone. This result suggests that a TDR adjacent to a long segment fusion may not necessarily be subjected to greater mechanical stress. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Upper Cervical Spine Loading Simulating a Dynamic Low-Speed Collision Significantly Increases the Risk of Pain Compared to Quasi-Static Loading With Equivalent Neck Kinematics.

Dynamic cervical spine loading can produce facet capsule injury. Despite a large proportion of neck pain being attributable to the C2/C3 facet capsule, potential mechanisms are not understood. This study replicated low-speed frontal and rear-end traffic collisions in occiput-C3 human cadaveric cervical spine specimens and used kinematic and full-field strain analyses to assess injury. Specimens were loaded quasi-statically in flexion and extension before and after dynamic rotation of C3 at 100 deg/s. Global kinematics in the sagittal plane were tracked at 1 kHz, and C2/C3 facet capsule full-field strains were measured. Dynamic loading did not alter the kinematics from those during quasi-static (QS) loading, but maximum principal strain (MPS) and shear strain (SS) were significantly higher (p = 0.028) in dynamic flexion than for the same quasi-static conditions. The full-field strain analysis demonstrated that capsule strain was inhomogeneous, and that the peak MPS generally occurred in the anterior aspect and along the line of the C2/C3 facet joint. The strain magnitude in dynamic flexion continued to rise after the rotation of C3 had stopped, with a peak MPS of 12.52 ± 4.59% and a maximum SS of 5.34 ± 1.60%. The peak MPS in loading representative of rear-end collisions approached magnitudes previously shown to induce pain in vivo, whereas strain analysis using linear approaches across the facet joint was lower and may underestimate injury risk compared to full-field analysis. The time at which peak MPS occurred suggests that the deceleration following a collision is critical in relation to the production of injurious strains within the facet capsule.

Biomechanical comparison of pedicle screws versus spinous process screws in C2 vertebra: A cadaveric study.

Background:Biomechanical studies have shown C2 pedicle screw to be the most robust in insertional torque and pullout strength. However, C2 pedicle screw placement is still technically challenging. Smaller C2 pedicles or medial localization of the vertebral artery may preclude safe C2 pedicle screw placement in some patients. The purpose of this study was to compare the pullout strength of spinous process screws with pedicle screws in the C2.Materials and Methods:Eight fresh human cadaveric cervical spine specimens (C2) were harvested and subsequently frozen to −20°C. After being thawed to room temperature, each specimen was debrided of remaining soft tissue and labeled. A customs jig as used to clamp each specimen for screw insertion firmly. Screws were inserted into the vertebral body pairs on each side using one of two methods. The pedicle screws were inserted in usual manner as in previous biomechanical studies. The starting point for spinous process screw insertion was located at the junction of the lamina and the spinous process and the direction of the screw was about 0° caudally in the sagittal plane and about 0° medially in the axial plane. Each vertebrae was held in a customs jig, which was attached to material testing machine (Material Testing System Inc., Changchun, China). A coupling device that fit around the head of the screw was used to pull out each screw at a loading rate of 2 mm/min. The uniaxial load to failure was recorded in Newton'st dependent test (for paired samples) was used to test for significance.Results:The mean load to failure was 387 N for the special protection scheme and 465 N for the protection scheme without significant difference (t = −0.862, P = 0.403). In all but three instances (38%), the spinous process pullout values exceeded the values for the pedicle screws. The working distances for the spinous process screws was little shorter than pedicle screws in each C2 specimen.Conclusion:Spinous process screws provide comparable pullout strength to pedicle screws of the C2. Spinous process screws may provide an alternative to pedicle screws fixation, especially with unusual anatomy or stripped screws.

Biomechanics of sports-induced axial-compression injuries of the neck.

Head-first sports-induced impacts cause cervical fractures and dislocations and spinal cord lesions. In previous biomechanical studies, researchers have vertically dropped human cadavers, head-neck specimens, or surrogate models in inverted postures. To develop a cadaveric neck model to simulate horizontally aligned, head-first impacts with a straightened neck and to use the model to investigate biomechanical responses and failure mechanisms. Descriptive laboratory study. Biomechanics research laboratory. Five human cadaveric cervical spine specimens. The model consisted of the neck specimen mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head. Head-first impacts were simulated at 4.1 m/s into a padded, deformable barrier. Time-history responses were determined for head and neck loads, accelerations, and motions. Average occurrence times of the compression force peaks at the impact barrier, occipital condyles, and neck were compared. The first local compression force peaks at the impact barrier (3070.0 ± 168.0 N at 18.8 milliseconds), occipital condyles (2868.1 ± 732.4 N at 19.6 milliseconds), and neck (2884.6 ± 910.7 N at 25.0 milliseconds) occurred earlier than all global compression peaks, which reached 7531.6 N in the neck at 46.6 milliseconds (P < .001). Average peak head motions relative to the torso were 6.0 cm in compression, 2.4 cm in posterior shear, and 6.4° in flexion. Neck compression fractures included occipital condyle, atlas, odontoid, and subaxial comminuted burst and facet fractures. Neck injuries due to excessive axial compression occurred within 20 milliseconds of impact and were caused by abrupt deceleration of the head and continued forward torso momentum before simultaneous rebound of the head and torso. Improved understanding of neck injury mechanisms during sports-induced impacts will increase clinical awareness and immediate care and ultimately lead to improved protective equipment, reducing the frequency and severity of neck injuries and their associated societal costs.

Effect of Multilevel Open-Door Laminoplasty and Laminectomy on Flexibility of the Cervical Spine

A biomechanical comparison of 2 commonly used posterior surgical procedures for spinal cord decompression in the cervical spine: laminoplasty (open door) and laminectomy. To delineate differences in cervical motion after laminoplasty (2-level and multilevel) and laminectomy. Cervical spondylotic myelopathy is a common spinal cord disorder in persons aged 55 years or older. Laminectomy and laminoplasty are the 2 common posterior-based techniques used for decompression of spinal cord. There is lack of adequate literature data on the intersegmental rotations at the operated and adjacent levels. Five human cadaveric specimens were tested sequentially as follows: (1) intact, (2) laminoplasty at C5-C6, (3) laminoplasty at C3-C6, and (4) laminectomy at C3-C6, each subjected to 2 N·m moments in flexion/extension, right/left lateral bending, and right/left axial rotation. For laminoplasty, the laminae of the involved vertebrae were stabilized with standard 10-mm plates and screws. The total and segmental motions of the specimens were measured before and after the surgical procedures. Statistical analysis was performed using repeated measures analysis of variance, with P < 0.05 as the level of significance. Two-level laminoplasty led to minimal decrease (<7% in the 3 loading modes) in C2-T1 motion. Multilevel laminoplasty resulted in a minimal increase during lateral bending (4%) and axial rotation (6%). During flexion/extension, both C4-C5 and C2-C3 showed a decrease of 20% (P > 0.05) and 17% (P > 0.05) after 2-level and multilevel laminoplasty, respectively. Laminectomy resulted in a statistically significant (P < 0.05) increase in the C2-T1 range of motion compared with the intact condition during the 3 loading modes (21% in flexion/extension, 8% in lateral bending, and 15% in axial rotation). Both 2-level and multilevel laminoplasty preserved the C2-T1 range of motion. Laminectomy resulted in a significant increase in C2-T1 motion due to the loss of the posterior structures.

Stabilization of the Atlantoaxial Joint With C1-C3 Lateral Mass Screw Constructs: Biomechanical Comparison With Standard Technique

Anatomically and biomechanically, the atlantoaxial joint is unique compared with the remainder of the cervical spine. To assess the in vitro stability provided by 2 C2 screw sparing techniques in a destabilized model of the atlantoaxial joint and compare with the gold standard system. The 3-dimensional intervertebral motion of 7 human cadaveric cervical spine specimens was recorded stereophotogrammetrically while applying nonconstraining, nondestructive pure moments during flexion-extension, left and right axial rotation, and left and right lateral bending. Each specimen was tested in the intact state, followed by destabilization (odontoidectomy) and fixation as follows: (1) C1 and C3 lateral mass screws rods with sublaminar wiring of C2 (LC1-C3 + SW), (2) C1 and C3 lateral mass screws rods with a cross-link in the C1-2 interlaminar space (LC1-C3 + CL), (3) C1 and C3 lateral mass screw rods alone (negative control), and (4) C1 lateral mass and C2 pedicle screws rods augmented with C1-2 interspinous wire and graft (LC1-PC2, control group). Compared with the intact spine, each instrumented state significantly stabilized range of motion and lax zone at C1-2 (P < .001, 1-way repeated-measures analysis of variance). LC1-C3 + SW was equivalent to LC1-PC2 during flexion and lateral bending and superior to LC1-C3 + CL during lateral bending, while LC1-C3 + CL was equivalent to LC1-PC2 only during flexion. In all other comparisons, LC1-PC2 was superior to both techniques. From a biomechanical perspective, both C2 screw sparing techniques provided sufficient stability to be regarded as an alternative for C1-2 fixation. However, because normal motion across C2-3 is sacrificed, these constructs should be used in patients with unfavorable anatomy for standard fixations.

Effect of Two-Level Total Disc Replacement on Cervical Spine Kinematics

Biomechanical study using human cadaver spines. To characterize kinematics of cervical spines implanted with total disc replacement (TDR) at 2-levels referencing the implanted and adjacent levels. Cervical TDR is an appealing alternative to fusion particularly when treating multilevel disease, where the advantages of maintaining motion and reducing adjacent level stresses with TDR are compelling. To our knowledge there are no biomechanical studies evaluating multilevel cervical TDR. Six human cadaveric cervical spine specimens (C3-C7, age: 57 +/- 12 years) were tested (i) intact, (ii) after TDR (Discover, DePuy, Raynham, MA) at C5-C6, and (iii) after additional TDR at C6-C7. Specimens were subjected to flexion/extension, lateral bending and axial rotation (+/-1.5 Nm). Segmental range of motion (ROM) was measured using optoelectronic instrumentation and fluoroscopy. Insertion of TDR at C5-C6 increased flexion/extension ROM of the implanted segment compared with intact (8.6 +/- 1.0 vs. 12.3 +/- 3.3 degrees , P < 0.025). The TDR maintained ROM to intact levels in lateral bending (7.4 +/- 2.6 vs 6.0 +/- 1.6, P > 0.025) and axial rotation (5.5 +/- 1.9 vs. 6.0 +/- 2.9, P > 0.025). The TDR at C5-C6 did not affect ROM at the adjacent levels. Implantation of a second TDR at C6-C7 maintained the ROM at that segment to intact values in flexion/extension (9.6 +/- 4.3 vs. 11.2 +/- 5.5, P > 0.025), lateral bending (6.1 +/- 4.0 vs. 4.1 +/- 2.1, P > 0.025), and axial rotation (6.7 +/- 3.6 vs. 5.5 +/- 3.7, P > 0.025). The second TDR at C6-C7 did not affect the ROM of the prosthesis implanted at C5-C6. Two-level TDR at C5-C6-C7 did not affect the ROM at C4-C5 in flexion/extension or axial rotation, however, in lateral bending a small increase occurred (8.9 +/- 3.6 vs. 10.1 +/- 4.5, P < 0.025). Cervical TDR at 2 levels can provide near-normal mobility at both levels without destabilizing the implanted segments or affecting adjacent segment motions. These observations lend support to the notion that single or multilevel cervical TDR may be advantageous when compared to fusion.

Biomechanical evaluation of surgical constructs for stabilization of cervical teardrop fractures

Background context Cervical flexion teardrop fractures (CFTF) are highly unstable injuries, and the optimal internal fixation construct is not always clearly indicated. Purpose The purpose of the current study was to determine whether the type of fixation construct (anterior, posterior, or combined) or number of joint levels involved in fixation (one or two) affected the relative stability of a CFTF injury at C5–C6. Study design/setting Human cadaveric cervical spine specimens were mechanically tested under displacement control in the intact state and after creation of CFTF at C5–C6 with stabilization using five different instrumentation constructs. Joint stiffness and intervertebral translation of the constructs were compared with the intact state and normalized (instrumented/intact) to assess relative differences across the five constructs. Methods Spine specimens were mechanically tested in the intact state during flexion, extension, lateral bending, and axial rotation. CFTF was created at C5–C6 by creating an osteotomy at C5 and transecting the posterior ligaments and intervertebral disc. Specimens were tested with anterior, posterior, and combined single-level constructs (C5–C6). Then, a corpectomy was performed at C5, and specimens were retested with the two-level constructs (C4–C6; anterior and anterior-posterior). Joint stiffness and intervertebral translations were computed. Results All five fixation constructs resulted in joint stability that was as good as or better than that of the intact specimens. Relative stiffness of the constructs differed depending upon the motion type considered, though the two-level anterior-posterior construct typically provided the greatest stability. Intervertebral translation along the major axis was reduced the most for both of the combined instrumentation systems, although there were few changes in total intervertebral translation across the five constructs. Conclusions All five constructs restored stability comparable to that of the intact specimens. The significance of the relative differences in constructs for the in vivo spine is unclear and warrants further clinical investigation.

Value of intraoperative true lateral radiograph of C2 pedicle for C1-2 transarticular screw insertion

Background context Transarticular C1-2 screws are widely used in posterior cervical spine instrumentation. Injury to the vertebral artery during insertion of transarticular Cl-2 screw remains a serious complication. Use of a computer-assisted surgery system decreases this complication considerably. However, this system encounters problems in ensuring complete accuracy because of positional variations during preoperative and intraoperative imaging generation. Therefore, intraoperative fluoroscopy still is one of the commonly used methods to guide insertion of transarticular Cl-2 screw. Evaluation of a true lateral radiographic view of the C2 pedicle for screw trajectory during C1-2 transarticular screw insertion may help to minimize this potential complication. Purpose To evaluate the value of intraoperative true lateral radiograph of the C2 pedicle for screw trajectory during C1-2 transarticular screw insertion. Study design To compare the height of the C2 pedicle area allowing instrumentation on true lateral view radiograph of the C2 pedicle and computed tomographic (CT) scan with multiplanar reconstruction. Methods Twenty embalmed human cadaveric cervical spine specimens were used to insert a total of 40 C1-2 transarticular screws using Magerl and Seemann technique. One side of the C2 transverse foramen was filled with radiopaque material (lead oxide) to simulate the artery and to demarcate the danger zone for better visualization on radiography. Measurements and calculation of the mean and standard deviation of the height of the area allowing instrumentation of the C2 pedicle were done on true lateral view radiograph of the C2 pedicle, the sagittal and 30° sagittal views relative to the frontal plane passing exactly through the center of the C2 pedicle of CT scans. Student t test was applied to calculate the statistical significance of measured values. Statistical significance was defined as p≤.001. Results On true lateral radiographic views of the C2 pedicle, the height of the area allowing instrumentation of the pedicle was 7.75±0.92 mm (right) and 7.64±0.63 (left), p≥.36. Using sagittal CT scan views, the height of pedicles was 7.71±0.7 mm (right) and 7.58±1.01 mm (left), p≥.23. On 30° sagittal CT scan views, the height of pedicles was 7.84±1.00 mm (right) and 7.76±1.02 mm (left), p≥.27. The p value was ≥.78, ≥.56, and ≥.49 for true lateral radiographic view and sagittal CT scan view, true lateral radiographic view and 30° sagittal CT scan view, and sagittal CT scan view and 30° sagittal CT scan views, respectively. On lateral view of cervical spine, the decline angle of the transarticular screw was 51.3±0.50° (right) and 50.68±0.41° (left), p≥.17. Mean decline angle was 51±0.43°. On the anteroposterior (AP) view, radiograph median angle was 6.87±0.53° (right) and 6.0±0.59° (left), p≥.25. Mean median angle was 6.44±0.62°. Conclusions True lateral radiographic views of the pedicles provide useful information for defining screw trajectory intraoperatively. Using this view along with AP and lateral view of cervical spine and preoperative three-dimensional CT scan may narrow the margin of error in this delicate area.

Influence of the crash pulse shape on the peak loading and the injury tolerance levels of the neck in in vitro low-speed side-collisions

The aim of the present in vitro study was to investigate the effect of the crash pulse shape on the peak loading and the injury tolerance levels of the human neck. In a custom-made acceleration apparatus 12 human cadaveric cervical spine specimens, equipped with a dummy head, were subjected to a series of incremental side accelerations. While the duration of the acceleration pulse of the sled was kept constant at 120 ms, its shape was varied: Six specimens were loaded with a slowly increasing pulse, i.e. a low loading rate, the other six specimens with a fast increasing pulse, i.e. a high loading rate. The loading of the neck was quantified in terms of the peak linear and angular acceleration of the head, the peak shear force and bending moment of the lower neck and the peak translation between head and sled. The shape of the acceleration curve of the sled only seemed to influence the peak translation between head and sled but none of the other four parameters. The neck injury tolerance level for the angular acceleration of the head and for the bending moment of the lower neck was almost identical for both, the high and the low loading rate. In contrast, the injury tolerance level for the linear acceleration of the head and for the shear force of the lower neck was slightly higher for the low loading rate as compared to the high loading rate. For the translation between head and sled this difference was even statistically significant. Thus, if the shape of the crash pulse is not known, solely the peak bending moment of the lower neck and the peak angular acceleration of the head seem to be suitable predictors for the neck injury risk but not the peak shear force of the lower neck, the peak linear acceleration of the head and the translation between head and thorax.

Correlation Between Neck Injury Risk and Impact Severity Parameters in Low-Speed Side Collisions

In vitro acceleration study on human cadaveric cervical spine specimens. To investigate the correlation between the risk to sustain a structural cervical spine injury and vehicle-related impact severity parameters. Impact severity parameters, such as the peak acceleration of the vehicle, its mean acceleration, and its velocity change, are often used to predict the whiplash injury risk or to objectify the patient's symptoms even though their correlation to injury is still not well understood. In a series of three in vitro experiments, a total of 18 human cadaveric cervical spine specimens were subjected to incremental side accelerations until structural injury occurred. While the duration of the acceleration pulse was kept constant throughout all three experiments, its shape was varied: In Experiment I, the acceleration pulse had a fast increase up to the maximum value and a fast decrease down to zero (fast-fast). Experiment II was characterized by a slow increase and fast decrease (slow-fast), and Experiment III was characterized by a fast increase and a slow decrease (fast-slow). The specimens of Experiment II (slow-fast) sustained structural injury at a significantly higher peak acceleration of the sled (4.6 g on average) than those of Experiments I (fast-fast) (2.6 g) and III (fast-slow) (3.1 g). In contrast, mean acceleration and velocity change of the injuring impacts were almost the same in all three experiments. The injury risk to the cervical spine was predictable by the mean acceleration of the sled and since the duration of the crash pulses was constant also by its velocity change but not by its peak acceleration.

Quantification of C2 cervical spine rotatory fixation by X-ray, MRI and CT.

Atlanto-axial rotatory displacement is known to be a cause of childhood torticollis and may as well be responsible for chronic neck pain after rear-end automobile collisions. The objective was to determine whether quantification of C2 malrotation is possible by plain radiographs in comparison to CT as the golden standard. MR imaging was evaluated as to whether it was of equal value in the detection of bony landmarks. C2 vertebra of five human cadaveric cervical spine specimens, ligamentously intact, were rotated using a Steinmann pin in steps of 5 degrees up to 15 degrees right and 15 degrees left. Plain radiographs, CT and MRI images were taken in each rotational step. Data were analyzed for quantification of C2 rotation by three independent examiners. A rotation of 5 degrees led to a spinous process deviation (SPD) from the midline of 3 mm as measured on an a.p. plain radiograph. A coefficient of rotation was calculated (1.62 degrees mm(-1)). Data analyzed by three examiners revealed a small coefficient of variation (0.03). MRI and CT measurements showed comparable results for the quantification of rotation; however, in both techniques the 15 degrees rotation was underestimated. Quantification of upper cervical spine malrotation was possible on plain radiographs using the SPD and a rotation coefficient. MRI and CT were equally successful in the assessment of C2 malrotation.

A new acceleration apparatus for the study of whiplash with human cadaveric cervical spine specimens

The biomechanics of whiplash is often studied using cadaveric cervical spine specimens. One of the most important points in this kind of study is to create realistic loading conditions. The aim of the present project therefore was to develop an acceleration apparatus, which allows the study of whiplash with human cadaveric cervical spine specimens under as realistic loading conditions as possible. The new acceleration apparatus mainly consisted of a sled, a pneumatic acceleration unit and a railtrack and offered several unique features to create more realistic loading conditions. Among these features, the possibility to simulate the passive movements of the trunk is of capital importance. In this new apparatus, first, the general feasibility of whiplash experiments was studied, second, the reproducibility of the impacts was quantified and third, the effect of simulated movements of the trunk on accelerations and loads was examined. In the new acceleration apparatus various types of collisions could reproducibly be simulated. Simulated passive movements of the trunk strongly influenced the loading pattern of the neck. Without pivoting a steep increase of all loading parameters could be observed. This increase was less pronounced if pivoting was allowed. In conclusion, biomechanical aspects of whiplash could reproducibly be examined in the new acceleration apparatus. Due to its significant effects on the loading of the neck, pivoting of the trunk should always be taken into account in future experiments on the biomechanics of whiplash in which isolated cervical spine specimens are used.

Anatomic considerations for standard and modified techniques of cervical lateral mass screw placement

Background context: Posterior screw placement techniques have been previously described. Each technique differs with respect to starting point, lateral angulation and sagittal orientation. Purpose: To examine the potential for injury to critical anatomic structures, such as nerve roots and vertebral arteries, during posterior cervical screw placement and to determine safe screw placement. Study design/setting: An anatomic study was conducted to determine the optimal screw angulation for posterior cervical lateral mass screws. S pecimen sample: Ten fresh-frozen human cadaveric cervical spine specimens were used, consisting of four females and six males, ranging in age from 32 to 68 years. Outcome measures: Angular measurements and distance from nerve root and vertebral arteries were measured with a single caliper and recorded. One millimeter of proximity to a vital structure was considered a violation of that structure. Methods: Ten fresh-frozen human cadaveric cervical spine specimens were instrumented from C2 to C7 by a single surgeon. Kirschner wires (2.0 mm) were used to reproduce the Roy-Camille, Anderson and Magerl screw trajectories. The wire was drilled through each lateral mass, simulating overdrill error. Each technique was instrumented according to the original description and with additional modifications. The modification consisted of varying the angle of screw placement in the axial plane of the original description from 0 to 30 degrees. Distances to the closest neurovascular structures were averaged for all assays. Results: The Magerl technique is safe at the standard position and modified positions of 20 degrees and 30 degrees from C3–C6. The Roy-Camille technique frequently violates neurovascular structures below C3, especially the nerve root with more lateral screw angulation. The standard technique is noted to have good bone purchase only at C2 and C3. The Anderson technique is safe at 20 degrees and 30 degrees modified positions from C3–C7. Posterior screw placement at the C7 vertebral level was safe only with a modified Anderson technique of 20 degrees and 30 degrees of lateral screw angulation. Conclusions: The present study indicates that there are significant differences of potential neurovascular injury, which is dependent on the technique used for screw entry, the level instrumented and the angle of screw trajectory in the parasagittal plane.

Bone graft force in anterior cervical fusions without anterior plating

Bone graft force in anterior cervical fusions without anterior plating

Stabilizing potential of anterior cervical plates in multilevel corpectomies.

An in vitro investigation of three-dimensional kinematics of cervical spine models of one- and three-level corpectomy with anterior plate fixation. To evaluate the capability of an anterior plate to stabilize the reconstructed cervical spine under simulated physiologic motions, and to study the effects of fatigue loading. Clinical studies have found high failure rates of multilevel anterior cervical plate fusions, indicating suboptimal stabilization. However, no biomechanical studies have been done to investigate the stabilizing capabilities of long-plate instrumentations in corpectomy models. Seven fresh human cadaveric cervical spine specimens (C2-T1) were used. Flexibility tests consisted of flexion, extension, and bilateral torsion, and lateral bending, each with a pure moment of 0.25, 0.5, 0.75, and 1.0 Nm. Stabilizing potential indices [(MotionIntact-MotionInstrumented)/MotionIntact] for ranges of motion and neutral zones obtained from the flexibility tests, were measured when the specimen was intact and after one-level (C5) and three-level (C4, C5, and C6) corpectomies and anterior plate stabilizations). The stabilizing potential indices were re-measured after a 1000-cycle fatigue loading (1 Nm flexion and extension moments at C5 vertebra at 0.14 Hz). The differences in stabilizing potential indices of range of motion and neutral zone between one-level and three-level plates were not significant before fatigue. However, after fatigue, the stabilizing potential indices significantly decreased (P < 0.05) for the three-level model, but not for the one-level plate model. The capability of an anterior cervical plate to stabilize the spine after three-level corpectomy was significantly reduced with fatigue loading.

Whiplash injuries and the potential for mechanical instability.

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.

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.

Cartilage and synovium of the human atlanto-odontoid joint. An anatomic and histological study.

The surface area, thickness and composition of articular cartilage of the atlanto-odontoid joints were investigated in twenty human cadaveric cervical spine specimens. The specimens were also examined grossly and by light microscopy to determine the location of the synovium. The anterior arch of the atlas and ventral and dorsal articular surfaces of the dens were covered with hyaline cartilage. The mean values of the articular surface areas on the ventral surface of the dens and anterior arch of the atlas were 55.10 and 58.24 mm2, respectively. The mean thickness of the articular cartilage of the anterior arch of the atlas, ventral and dorsal surfaces of the dens was 0.80, 0.81 and 0.82 mm, respectively. Synovial membranes were associated with the joint capsules and surrounding tissues of both anterior and posterior atlanto-odontoid joint spaces, where the synovial membranes were attached to the margins of the articular surfaces of the dens and anterior arch of the atlas anteriorly and the region of the cruciate ligament immediately peripheral to the cartilage region apposed to the dens and dens cartilage itself, posteriorly.