The effect of follower load on the intersegmental coupled motion characteristics of the human thoracic spine: An in vitro study using entire rib cage specimens

The mechanical coupling behaviour of the thoracic spine is still not fully understood. For the validation of numerical models of the thoracic spine, however, the coupled motions within the single spinal segments are of importance to achieve high model accuracy. In the present study, eight fresh frozen human thoracic spinal specimens (C7-L1, mean age 54 ± 6 years) including the intact rib cage were loaded with pure bending moments of 5 Nm in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) with and without a follower load of 400 N. During loading, the relative motions of each vertebra were monitored. Follower load decreased the overall ROM (T1-T12) significantly (p < 0.01) in all primary motion directions (extension: −46%, left LB: −72%, right LB: −72%, left AR: −26%, right AR: −26%) except flexion (−36%). Substantial coupled motion was found in lateral bending with ipsilateral axial rotation, which increased after a follower load was applied, leading to a dominant axial rotation during primary lateral bending, while all other coupled motions in the different motion directions were reduced under follower load. On the monosegmental level, the follower load especially reduced the ROM of the upper thoracic spine from T1-T2 to T4-T5 in all motion directions and the ROM of the lower thoracic spine from T9-T10 to T11-T12 in primary lateral bending. The facet joints, intervertebral disc morphologies, and the sagittal curvature presumably affect the thoracic spinal coupled motions depending on axial compressive preloading. Using these results, the validation of numerical models can be performed more accurately.     

The effect of lateral eccentricity on failure loads,kinematics, and canal occlusions of the cervical spine in axial loading

Current neck injury criteria do not include limits for lateral bending combined with axial compression and this has been observed as a clinically relevant mechanism, particularly for rollover motor vehicle crashes. The primary objectives of this study were to evaluate the effects of lateral eccentricity (the perpendicular distance from the axial force to the centre of the spine) on peak loads, kinematics, and spinal canal occlusions of subaxial cervical spine specimens tested in dynamic axial compression (0.5 m/s). Twelve 3-vertebra human cadaver cervical spine specimens were tested in two groups: low and high eccentricity with initial eccentricities of 1 and 150% of the lateral diameter of the vertebral body. Six-axis loads inferior to the specimen, kinematics of the superior-most vertebra, and spinal canal occlusions were measured. High speed video was collected and acoustic emission (AE) sensors were used to define the time of injury. The effects of eccentricity on peak loads, kinematics, and canal occlusions were evaluated using unpaired Student t-tests. The high eccentricity group had lower peak axial forces (1544±629 vs. 4296±1693 N), inferior displacements (0.2±1.0 vs. 6.6±2.0 mm), and canal occlusions (27±5 vs. 53±15%) and higher peak ipsilateral bending moments (53±17 vs. 3±18 Nm), ipsilateral bending rotations (22±3 vs. 1±2°), and ipsilateral displacements (4.5±1.4 vs. −1.0±1.3 mm, p<0.05 for all comparisons). These results provide new insights to develop prevention, recognition, and treatment strategies for compressive cervical spine injuries with lateral eccentricities.     

Evaluation of the intervertebral neck injury criterion using simulated rear impacts

The Intervertebral Neck Injury Criterion (IV-NIC) is based on the hypothesis that intervertebral motion beyond the physiological limit may injure spinal soft tissues during whiplash, while the Neck Injury Criterion (NIC) hypothesizes that sudden changes in spinal fluid pressure may cause neural injury. Goals of the present study, using a biofidelic whole cervical spine model with muscle force replication, were to correlate IV-NIC with soft-tissue injury, determine the IV-NIC injury threshold, and compare IV-NIC and NIC. Using a bench-top apparatus, rear-impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Pre- and post-whiplash flexibility tests measured the soft tissue injury threshold, i.e. significant increases in the intervertebral neutral zone (NZ) or range of motion (ROM) above corresponding baseline values. Extension IV-NIC peaks correlated well with NZ and ROM increases at C0-C1 and at C3-C4 through C7-T1 (r=0.64 and 0.62 respectively, p<0.001). Average IV-NIC injury thresholds (95% confidence limits) varied among the intervertebral levels and ranged between 1.5 (1.1, 1.9) at C5-C6 and 3.4 (2.4, 4.4) at C7-T1. The NIC injury threshold was 8.7 (7.7, 9.7) m2/s2, substantially less than the proposed threshold of 15 m2/s2. Results support the use of IV-NIC for determining the cervical spine injury threshold and injury severity. Advantages of IV-NIC include the ability to predict the intervertebral level, mode, severity, and time of the cervical spine soft-tissue injury.     

Ability of chicken spermatozoa to undergo acrosome reaction after liquid storage or cryopreservation

The effects of in vitro storage on the sperm's ability to undergo the acrosome reaction (AR) have never been studied in avian species despite its major importance for reproduction management.The ability of chicken sperm to undergo the AR was measured after liquid storage at 4 °C and after cryopreservation, and its relationship with other semen quality parameters, including viability, mass motility and objective motility parameters measured by computer semen analyser (CASA) was analysed in two different flocks. The percentage of intact acrosome-reacting spermatozoa (IAR) was dramatically decreased by 48 h liquid storage (loss of 2/3 among the spermatozoa initially able to undergo the AR) whereas motility, viability and morphological integrity were reduced by 10-15%. By contrast, cryopreservation did not affect the induction of AR in flock 1 (29% IAR) whereas it was strongly affected in flock 2 (7% IAR). Motility parameters, viability and morphology were considerably altered by freezing in every case (more that 50% loss). Positive correlations were found between the percentage of intact acrosome-reacting spermatozoa and viability, mass motility and many objective motility parameters.Our results showed that the sperm's ability to undergo the AR was much more affected than other sperm functions after storage at 4 °C, while cryopreservation only had an effect in semen with the lowest initial quality. These results raise questions regarding the specific features of chicken sperm biology that must be taken into account in the treatment of semen.     

In-vivo 6 degrees-of-freedom kinematics of metal-on-polyethylene total hip arthroplasty during gait

Knowledge of accurate in-vivo 6 degree-of-freedom (6-DOF) kinematics of total hip arthroplasty (THA) during daily activities is critical for improvement of longevity of the components. Previous studies assessed in-vivo THA kinematics using skin marker-based motion analysis. However, skin markers are prone to move with respect to the underlying bones. A non-invasive dual fluoroscopic imaging system (DFIS) based tracking technique has been used to avoid skin artifacts and provide accurate 6-DOF kinematic measurement. This study aimed to quantify in-vivo 6-DOF THA kinematics during gait using DFIS. Twenty eight well-functioning THAs were evaluated during treadmill gait under DFIS surveillance. The maximum translations of the femoral head were 0.46±0.10 mm and 0.45±0.10 mm during the stance and swing phases (p=0.57), respectively. The range of hip flexion was from 8.7° to 47.6°, adduction from 3.0° to 12.5° and external rotation from 19.2° to 29.7°. The THA was flexed, externally rotated and adducted throughout the gait. The magnitudes of the femoral head translations were found to be within the manufacture tolerance of the components, suggesting that in-vivo hip “pistoning” during gait cycle may be minimal in well-functioning THAs. The 6-DOF kinematics could be used as the baseline knowledge for further improvement of wear-testing of hip implant, implants manufacturing and implant positioning during surgery.     

Biomechanical contributions of upper cervical ligamentous structures in Type II odontoid fractures

Fractures of the odontoid present frequently in spinal trauma, and Type II odontoid fractures, occurring at the junction of the odontoid process and C2 vertebrae, represent the bulk of all traumatic odontoid fractures. It is currently unclear what soft-tissue stabilizers contribute to upper cervical motion in the setting of a Type II odontoid fracture, and evaluation of how concomitant injury contributes to cervical stability may inform surgical decision-making as well as allow for the creation of future, accurate, biomechanical models of the upper cervical spine. The objective of the current study was to determine the contribution of soft-tissue stabilizers in the upper cervical spine following a Type II odontoid fracture. Eight cadaveric C0-C2 specimens were evaluated using a robotic testing system with motion tracking. The unilateral facet capsule (UFC) and anterior longitudinal ligament (ALL) were serially resected to determine their biomechanical role following odontoid fracture. Range of motion (ROM) and moment at the end of intact specimen replay were the primary outcomes. We determined that fracture of the odontoid significantly increases motion and decreases resistance to intact motion for flexion–extension (FE), axial rotation (AR), and lateral bending (LB). Injury to the UFC increased AR by 3.2° and FE by 3.2°. ALL resection did not significantly increase ROM or decrease end-point moment. The UFC was determined to contribute to 19% of intact flexion resistance and 24% of intact AR resistance. Overall, we determined that Type II fracture of the odontoid is a significant biomechanical destabilizer and that concurrent injury to the UFC further increases upper cervical ROM and decreases resistance to motion in a cadaveric model of traumatic Type II odontoid fractures.     

Instantaneous centers of rotation for lumbar segmental extension in vivo

The study aimed to map instantaneous centers of rotation (ICRs) of lumbar motion segments during a functional lifting task and examine differences across segments and variations caused by magnitude of weight lifted. Eleven healthy participants lifted loads of three different magnitudes (4.5, 9, and 13.5 kg) from a trunk-flexed (~75°) to an upright position, while being imaged by a dynamic stereo X-ray (DSX) system. Tracked lumbar vertebral (L2-S1) motion data were processed into highly accurate 6DOF intervertebral (L2L3, L3L4, L4L5, L5S1) kinematics. ICRs were computed using the finite helical axis method. Effects of segment level and load magnitude on the anterior-posterior (AP) and superior-inferior (SI) ICR migration ranges were assessed with a mixed-effects model. Further, ICRs were averaged to a single center of rotation (COR) to assess segment-specific differences in COR AP- and SI-coordinates. The AP range was found to be significantly larger for L2L3 compared to L3L4 (p=0.02), L4L5 and L5S1 (p<0.001). Average ICR SI location was relatively higher – near the superior endplate of the inferior vertebra – for L4L5 and L5SI compared to L2L3 and L3L4 (p≤0.001) – located between the mid-transverse plane and superior endplate of the inferior vertebra – but differences were not significant amongst themselves (p>0.9). Load magnitude had a significant effect only on the SI component of ICR migration range (13.5 kg>9 kg and 4.5 kg; p=0.049 and 0.017 respectively). The reported segment-specific ICR data exemplify improved input parameters for lumbar spine biomechanical models and design of disc replacements, and base-line references for potential diagnostic applications.     

Investigation of impact loading rate effects on the ligamentous cervical spinal load-partitioning using finite element model of functional spinal unit C2–C3

The cervical spine functions as a complex mechanism that responds to sudden loading in a unique manner, due to intricate structural features and kinematics. The spinal load-sharing under pure compression and sagittal flexion/extension at two different impact rates were compared using a bio-fidelic finite element (FE) model of the ligamentous cervical functional spinal unit (FSU) C2–C3. This model was developed using a comprehensive and realistic geometry of spinal components and material laws that include strain rate dependency, bone fracture, and ligament failure. The range of motion, contact pressure in facet joints, failure forces in ligaments were compared to experimental findings. The model demonstrated that resistance of spinal components to impact load is dependent on loading rate and direction. For the loads applied, stress increased with loading rate in all spinal components, and was concentrated in the outer intervertebral disc (IVD), regions of ligaments to bone attachment, and in the cancellous bone of the facet joints. The highest stress in ligaments was found in capsular ligament (CL) in all cases. Intradiscal pressure (IDP) in the nucleus was affected by loading rate change. It increased under compression/flexion but decreased under extension. Contact pressure in the facet joints showed less variation under compression, but increased significantly under flexion/extension particularly under extension. Cancellous bone of the facet joints region was the only component fractured and fracture occurred under extension at both rates. The cervical ligaments were the primary load-bearing component followed by the IVD, endplates and cancellous bone; however, the latter was the most vulnerable to extension as it fractured at low energy impact.     

Importance of nonlinear and multivariable flexibility coefficients in the prediction of human cervical spine motion

The flexibility matrix currently forms the basis for multibody dynamics models of cervical spine motion. While studies have aimed to determine cervical motion segment behavior, their accuracy and utility have been limited by both experimental and analytical assumptions. Flexibility terms have been primarily represented as constants despite the spine's nonlinear stiffening response. Also, nondiagonal terms, describing coupled motions, of the matrices are often omitted. Currently, no study validates the flexibility approach for predicting vertebral motions; nor have the effects of matrix approximations and simplifications been quantified. Therefore, the purpose of this study is to quantify flexibility relationships for cervical motion segments, examine the importance of nonlinear components of the flexibility matrix, and determine the extent to which multivariable relationships may alter motion prediction. To that end, using unembalmed human cervical spine motion segments, a full battery of flexibility tests were performed for a neutral orientation and also following an axial pretorque. Primary and coupled matrix components were described using linear and piecewise nonlinear incremental constants. A third matrix approach utilized multivariable incremental relationships. Measured motions were predicted using structural flexibility methods and evaluated using RMS error between predicted and measured responses. A full set of flexibility relationships describe primary and coupled motions for C3-C4 and C5-C6. A flexibility matrix using piecewise incremental responses offers improved predictions over one using linear methods (p<0.01). However, no significant improvement is obtained using nonlinear terms represented by a multivariable functional approach (p<0.2). Based on these findings, it is suggested that a multivariable approach for flexibility is more demanding experimentally and analytically while not offering improved motion prediction.     

Dynamic in vivo 3D atlantoaxial spine kinematics during upright rotation

Diagnosing dysfunctional atlantoaxial motion is challenging given limitations of current diagnostic imaging techniques. Three-dimensional imaging during upright functional motion may be useful in identifying dynamic instability not apparent on static imaging. Abnormal atlantoaxial motion has been linked to numerous pathologies including whiplash, cervicogenic headaches, C2 fractures, and rheumatoid arthritis. However, normal C1/C2 rotational kinematics under dynamic physiologic loading have not been previously reported owing to imaging difficulties. The objective of this study was to determine dynamic three-dimensional in vivo C1/C2 kinematics during upright axial rotation. Twenty young healthy adults performed full head rotation while seated within a biplane X-ray system while radiographs were collected at 30 images per second. Six degree-of-freedom kinematics were determined for C1 and C2 via a validated volumetric model-based tracking process. The maximum global head rotation (to one side) was 73.6 ± 8.3°, whereas maximum C1 rotation relative to C2 was 36.8 ± 6.7°. The relationship between C1/C2 rotation and head rotation was linear through midrange motion (±20° head rotation from neutral) in a nearly 1:1 ratio. Coupled rotation between C1 and C2 included 4.5 ± 3.1° of flexion and 6.4 ± 8.2° of extension, and 9.8 ± 3.8° of contralateral bending. Translational motion of C1 relative to C2 was 7.8 ± 1.5 mm ipsilaterally, 2.2 ± 1.2 mm inferiorly, and 3.3 ± 1.0 mm posteriorly. We believe this is the first study describing 3D dynamic atlantoaxial kinematics under true physiologic conditions in healthy subjects. C1/C2 rotation accounts for approximately half of total head axial rotation. Additionally, C1 undergoes coupled flexion/extension and contralateral bending, in addition to inferior, lateral and posterior translation.     

The effect of axial compression and distraction on cervical facet mechanics during anterior shear,flexion, axial rotation,and lateral bending motions

The subaxial cervical facets are important load-bearing structures, yet little is known about their mechanical response during physiological or traumatic intervertebral motion. Facet loading likely increases when intervertebral motions are superimposed with axial compression forces, increasing the risk of facet fracture. The aim of this study was to measure the mechanical response of the facets when intervertebral axial compression or distraction is superimposed on constrained, non-destructive shear, bending and rotation motions. Twelve C6/C7 motion segments (70 ± 13 yr, nine male) were subjected to constrained quasi-static anterior shear (1 mm), axial rotation (4°), flexion (10°), and lateral bending (5°) motions. Each motion was superimposed with three axial conditions: (1) 50 N compression; (2) 300 N compression (simulating neck muscle contraction); and, (3) 2.5 mm distraction. Angular deflections, and principal and shear surface strains, of the bilateral C6 inferior facets were calculated from motion-capture data and rosette strain gauges, respectively. Linear mixed-effects models (α = 0.05) assessed the effect of axial condition. Minimum principal and maximum shear strains were largest in the compressed condition for all motions except for maximum principal strains during axial rotation. For right axial rotation, maximum principal strains were larger for the contralateral facets, and minimum principal strains were larger for the left facets, regardless of axial condition. Sagittal deflections were largest in the compressed conditions during anterior shear and lateral bending motions, when adjusted for facet side.     

Quantitative evaluation of facet deflection,stiffness, strain and failure load during simulated cervical spine trauma

Traumatic cervical facet dislocation (CFD) is often associated with devastating spinal cord injury. Facet fractures commonly occur during CFD, yet quantitative measures of facet deflection, strain, stiffness and failure load have not been reported. The aim of this study was to determine the mechanical response of the subaxial cervical facets when loaded in directions thought to be associated with traumatic bilateral CFD – anterior shear and flexion. Thirty-one functional spinal units (6 × C2/3, C3/4, C4/5, and C6/7, 7 × C5/6) were dissected from fourteen human cadaver cervical spines (mean donor age 69 years, range 48–92; eight male). Loading was applied to the inferior facets of the inferior vertebra to simulate the in vivo inter-facet loading experienced during supraphysiologic anterior shear and flexion motion. Specimens were subjected to three cycles of sub-failure loading (10–100 N, 1 mm/s) in each direction, before being failed in a randomly assigned direction (10 mm/s). Facet deflection, surface strains, stiffness, and failure load were measured. Linear mixed-effects models (α = 0.05; random effect of cadaver) accounted for variations in specimen geometry and bone density. Specimen-specific parameters were significantly associated with most outcome measures. Facet stiffness and failure load were significantly greater in the simulated flexion loading direction, and deflection and surface strains were higher in anterior shear at the non-destructive analysis point (47 N applied load). The sub-failure strains and stiffness responses differed between the upper and lower subaxial cervical regions. Failure occurred through the facet tip during anterior shear loading, while failure through the pedicles was most common in flexion.     

Biomechanical analysis of the anterior cervical fusion

This paper presents a biomechanical analysis of the cervical C5–C6 functional spine unit before and after the anterior cervical discectomy and fusion. The aim of this work is to study the influence of the medical procedure and its instrumentation on range of motion and stress distribution. First, a three-dimensional finite element model of the lower cervical spine is obtained from computed tomography images using a pipeline of image processing, geometric modelling and mesh generation software. Then, a finite element study of parameters' influence on motion and a stress analysis at physiological and different post-operative scenarios were made for the basic movements of the cervical spine. It was confirmed that the results were very sensitive to intervertebral disc properties. The insertion of an anterior cervical plate influenced the stress distribution at the vertebral level as well as in the bone graft. Additionally, stress values in the graft decreased when it is used together with a cage.     

Kinematics of the upper cervical spine during high velocity-low amplitude manipulation. Analysis of intra- and inter-operator reliability for pre-manipulation positioning and impulse displacements

To date, kinematics data analyzing continuous 3D motion of upper cervical spine (UCS) manipulation is lacking. This in vitro study aims at investigating inter- and intra-operator reliability of kinematics during high velocity low amplitude manipulation of the UCS.Three fresh specimens were used. Restricted dissection was realized to attach technical clusters to each bone (skull to C₂). Motion data was obtained using an optoelectronic system during manipulation. Kinematics data were integrated into specific-subject 3D models to provide anatomical motion representation during thrust manipulation. The reliability of manipulation kinematics was assessed for three practitioners performing two sessions of three repetitions on two separate days.For pre-manipulation positioning, average UCS ROM (SD) were 10° (5°), 22° (5°) and 14° (4°) for lateral bending, axial rotation and flexion–extension, respectively. For the impulse phase, average axial rotation magnitude ranged from 7° to 12°. Reliability analysis showed average RMS up to 8° for pre-manipulation positioning and up to 5° for the impulse phase.As compared to physiological ROM, this study supports the limited angular displacement during manipulation for UCS motion components, especially for axial rotation. Kinematics reliability confirms intra- and inter-operator consistency although pre-manipulation positioning reliability is slightly lower between practitioners and sessions.     

Examining motion in the cervical spine II: characterization of coupled joint motion using an opto-electronic device to track skin markers

Analysis of coupled motion in the cervical spine may be useful in helping to identify injuries. In order to investigate this possibility, the nature of coupled motion in the spine and previous investigations on this subject are reviewed here. An enhanced set of displays are developed for an existing opto-electronic device employed for the non-invasive measurement of movement in the upper spine. This instrument consists of a high resolution motion analysis system which tracks small infrared emitting diodes (IREDs). Kinematic data for the motion of the markers is processed and absolute coordinates for the location of each IRED at any time are tabulated; coupled motion with respect to a fixed calibration frame, as well as for vertebrae relative to each other, is deduced from these. Overall analysis provided by the original device includes assessment of cervical lordosis, thoracic kyphosis, and inter-segmental mobility. Characterization of coupled motion, in particular, involves a series of plots showing principal versus secondary motion. Principal movements include flexion-extension, lateral bending, and axial rotation, corresponding to motion in the sagittal, transverse, and horizontal planes, respectively. Mobility is represented in terms of the direction angles made by virtual vectors orthogonal to the planes made by markers on the head, neck, and shoulders. Development of the enhanced displays and the required refinements are described. Precision of the deduced angles is found to be approximately 1°. This representation of coupled motion is expected to be valuable in improving the accuracy of attempts to identify normal versus pathological motion in the cervical spine.     

Static and dynamic bending responses of the human cervical spine.

The quasi-static and dynamic bending responses of the human mid-lower cervical spine were determined using cadaver intervertebral joints fixed at the base to a six-axis load cell. Flexion bending moment was applied to the superior end of the specimen using an electrohydraulic piston. Each specimen was tested under three cycles of quasi-static load-unload and one high-speed dynamic load. A total of five specimens were included in this study. The maximum intervertebral rotation ranged from 11.0 to 15.4 deg for quasi-static tests and from 22.9 to 34.4 deg for dynamic tests. The resulting peak moments at the center of the intervertebral joint ranged from 3.8 to 6.9 Nm for quasi-static tests and from 14.0 to 31.8 Nm for dynamic tests. The quasi-static stiffness ranged from 0.80 to 1.35 Nm/deg with a mean of 1.03 Nm/deg (+/- 0.11 Nm/deg). The dynamic stiffness ranged from 1.08 to 2.00 Nm/deg with a mean of 1.50 Nm/deg (+/- 0.17 Nm/deg). The differences between the two stiffnesses were statistically significant (p < 0.01). Exponential functions were derived to describe the quasi-static and dynamic moment-rotation responses. These results provide input data for lumped-parameter models and validation data for finite element models to better investigate the biomechanics of the human cervical spine.     

Morphological aspects and biomechanical properties of the vertebroaxial joint (C2-C3).

The C2-C3 intervertebral joint must be regarded as a transitional area situated between the upper cervical spine where most rotation of the neck and little flexion and extension occur and the lower cervical spaces where chiefly motion in the sagittal plane and also somewhat rotation take place. Under normal circumstances the range of flexion-extension reaches 11 degrees, slighter than below (19.5 degrees at C5-C6); on the opposite, the range of rotation attains 7 degrees; less than above but much more than below (0 degrees at C5-C6). The motion in the sagittal and coronal planes is relatively poor because of the location of Penning's motor-axis of C2 which runs far from the vertebral body and the lowness of the intervertebral disc. However, the rotation of C2 with respect to C3 is fair by the peculiar inclination of the articular facets which slope sagittally but also coronally and trace a sphere whereupon C2 may move around its motor-centre in any plane. When C2-C3 is surgically fixed by bone graft, the lack of motion is completed by a "compensatory movement" in the upper cervical spaces and especially at the atlantooccipital joint for flexion-extension. In the same way, C2-C3 may improve its mobility especially in the sagittal plane when the inferior partner is blocked by surgical or arthritic fusion.     

Artificial Cervical Vertebra and Intervertebral Complex Replacement through the Anterior Approach in Animal Model: A Biomechanical and In Vivo Evaluation of a Successful Goat Model

This was an in vitro and in vivo study to develop a novel artificial cervical vertebra and intervertebral complex (ACVC) joint in a goat model to provide a new method for treating degenerative disc disease in the cervical spine. The objectives of this study were to test the safety, validity, and effectiveness of ACVC by goat model and to provide preclinical data for a clinical trial in humans in future. We designed the ACVC based on the radiological and anatomical data on goat and human cervical spines, established an animal model by implanting the ACVC into goat cervical spines in vitro prior to in vivo implantation through the anterior approach, and evaluated clinical, radiological, biomechanical parameters after implantation. The X-ray radiological data revealed similarities between goat and human intervertebral angles at the levels of C2-3, C3-4, and C4-5, and between goat and human lordosis angles at the levels of C3-4 and C4-5. In the in vivo implantation, the goats successfully endured the entire experimental procedure and recovered well after the surgery. The radiological results showed that there was no dislocation of the ACVC and that the ACVC successfully restored the intervertebral disc height after the surgery. The biomechanical data showed that there was no significant difference in range of motion (ROM) or neural zone (NZ) between the control group and the ACVC group in flexion-extension and lateral bending before or after the fatigue test. The ROM and NZ of the ACVC group were greater than those of the control group for rotation. In conclusion, the goat provides an excellent animal model for the biomechanical study of the cervical spine. The ACVC is able to provide instant stability after surgery and to preserve normal motion in the cervical spine.     

Biomechanical analysis of the anterior cervical fusion

This paper presents a biomechanical analysis of the cervical C5-C6 functional spine unit before and after the anterior cervical discectomy and fusion. The aim of this work is to study the influence of the medical procedure and its instrumentation on range of motion and stress distribution. First, a three-dimensional finite element model of the lower cervical spine is obtained from computed tomography images using a pipeline of image processing, geometric modelling and mesh generation software. Then, a finite element study of parameters' influence on motion and a stress analysis at physiological and different post-operative scenarios were made for the basic movements of the cervical spine. It was confirmed that the results were very sensitive to intervertebral disc properties. The insertion of an anterior cervical plate influenced the stress distribution at the vertebral level as well as in the bone graft. Additionally, stress values in the graft decreased when it is used together with a cage.     

Gender dependent cervical spine segmental kinematics during whiplash

Clinical and epidemiological studies have frequently reported that female occupants sustain whiplash injuries more often than males. The current study was based on the hypothesis that segmental level-by-level cervical intervertebral motions in females are greater than in males during rear impact. The hypothesis was tested by subjecting 10 intact human cadaver head-neck complexes (five males, five females) to rear impact loading. Intervertebral kinematics were analyzed as a function of spinal level at the time of maximum cervical S-curve, which occurred during the loading phase. Segmental angles were significantly greater (p<0.05) in female specimens at C2-C3, C4-C5, C5-C6, and C6-C7 levels. Because greater angulations are associated with stretch in the innervated components of the cervical spinal column, these findings may offer a biomechanical explanation for the higher incidence of whiplash-related complaints in female patients secondary to rear impact acceleration.