Variations of stiffness and strength along the human cervical spine

The load-displacement response and strength of the mid (C2-C5) and lower (C5-T1) cervical regions were determined for combinations of sagittal loads, in vitro. In unpaired t-test comparisons, the mid cervical region was significantly stiffer in compression and extension than the lower region. In tests to failure, failure in six out of seven mid cervical specimens resulted from flexion alone, while combined compression-flexion was required to fail five of the eight lower cervical specimens. Post-test dissections revealed no regional differences in the pattern of failure. In addition to sagittal tests, the load-displacement responses of three-vertebrae cervical specimens were measured with the upper body axially rotated with respect to the lower body. The effect of this pre-torsion was to diminish the zone of low slope near zero load for axial, shear, and flexion motion. Three of the four axially rotated specimens failed in flexion without added compression. These controlled load-displacement measurements of cervical spine specimens describe for the first time the continuous flexion-compression response up to failure, and suggest that consideration of the biomechanics of three apparently distinct mobile regions of the cervical spine (C1-C2, C2-C5, C5-T1) may facilitate the interpretation of hazardous conditions and the diagnosis of injury. These data also provide basic information for the in vitro investigation of passive cervical spine protection such as helmets and head-rests, suggesting that the head should be kept in a non-rotated position to reduce risk of injury.     

Ultimate strength of the lumbar spine in flexion--an in vitro study

The ultimate strength in flexion of 16 lumbar functional spinal units (FSU) was determined. The specimens were exposed to a combined static load of bending and shearing in the sagittal plane until overt rupture occurred (simulated flexion-distraction injuries). The biomechanical response of the FSU was measured with a force and moment platform. Mechanical displacement gauges were used to measure vertical displacements (flexion angulation) of the specimens. Photographs were taken after each loading step for determination of horizontal displacements and the centre of rotation. The lumbar FSU could resist a combination of bending moment and shear force of 156 Nm and 620 N respectively, before complete disruption occurred. The tension force acting on the posterior structures was 2.8 kN. The flexion angulation just before failure was 20 degrees and the anterior horizontal displacement between the upper and lower vertebrae was 9 mm. The centre of rotation was located in the posterior part of the lower vertebral body. The bone mineral content in the vertebrae appeared to be a good predictor of ultimate strength of the lumbar FSU. Knowledge of the biomechanical response of the lumbar spine under different static traumatic loads is a first step to better understand the injury mechanisms of the spine in traffic accidents.     

Analysis of large compression loads on lumbar spine in flexion and in torsion using a novel wrapping element

Axial compression on the spine could reach large values especially in lifting tasks which also involve large rotations. Experimental and numerical investigations on the spinal multi motion segments in presence of physiological compression loads cannot adequately be carried out due to the structural instability and artefact loads. To circumvent these problems, a novel wrapping cable element is used in a nonlinear finite element model of the lumbosacral spine (L1-S1) to investigate the role of moderate to large compression loads on the lumbar stiffness in flexion and axial moments/rotations. The compression loads up to 2,700 N was applied with no instability or artefact loads. The lumbar stiffness substantially increased under compression force, flexion moment, and axial torque when applied alone. The presence of compression preloads significantly stiffened the load-displacement response under flexion and axial moments/rotations. This stiffening effect was much more pronounced under larger preloads and smaller moments/rotations. Compression preloads also increased intradiscal pressure, facet contact forces, and maximum disc fibre strain at different levels. Forces in posterior ligaments were, however, diminished with compression preload. The significant increase in spinal stiffness, hence, should be considered in biomechanical studies for accurate investigation of the load partitioning, system stability, and fixation systems/disc prostheses.     

The dynamic flexion/extension properties of the lumbar spine in vitro using a novel pendulum system

The biomechanical properties of the ligamentous cadaver spine have been previously examined using a variety of experimental testing protocols. Ongoing technical challenges in the biomechanical testing of the spine include the application of physiologic compressive loads and the application of dynamic bending moments while allowing unconstrained three-dimensional motion. The purpose of this study was to report the development of a novel pendulum apparatus that addressed these challenges and to determine the effects of various axial compressive loads on the dynamic biomechanical properties of the lumbar functional spinal unit (FSU). Lumbar FSUs were tested in flexion and extension under five axial compressive loads chosen to represent physiologic loading conditions. After an initial rotation, the FSUs behaved as a dynamic, underdamped vibrating elastic system. Bending stiffness and coefficient of damping increased significantly as the compressive pendulum load increased. The apparatus described herein is a relatively simple approach to determining the dynamic bending properties of the FSU, and potentially disc arthroplasty devices. It is capable of applying physiologic compressive loads at dynamic rates without constraining the kinematics of the joints, crucial requirements for testing FSUs in vitro.     

Spinal stability and role of passive stiffness in dynamic squat and stoop lifts

The spinal stability and passive-active load partitioning under dynamic squat and stoop lifts were investigated as the ligamentous stiffness in flexion was altered. Measured in vivo kinematics of subjects lifting 180 N at either squat or stoop technique was prescribed in a nonlinear transient finite element model of the spine. The Kinematics-driven approach was utilized for temporal estimation of muscle forces, internal spinal loads and system stability. The finite element model accounted for nonlinear properties of the ligamentous spine, wrapping of thoracic extensor muscles and trunk dynamic characteristics while subject to measured kinematics and gravity/external loads. Alterations in passive properties of spine substantially influenced muscle forces, spinal loads and system stability in both lifting techniques, though more so in stoop than in squat. The squat technique is advocated for resulting in smaller spinal loads. Stability of spine in the sagittal plane substantially improved with greater passive properties, trunk flexion and load. Simulation of global extensor muscles with curved rather than straight courses considerably diminished loads on spine and increased stability throughout the task.     

Nonlinear gross response analysis of a lumbar motion segment in combined sagittal loadings

A 3-D nonlinear mathematical model is used to analyze the mechanical response of a lumbar L2-3 motion segment including the posterior elements when subjected to combined sagittal plane loads. The loadings consist of axial compression force, anterior and posterior shear forces, and flexion and extension moments. The facet articulation is modelled as a general moving contact problem and the ligaments as a collection of uniaxial elements. The disk nucleus is considered as an inviscid fluid and the annulus as a composite of collagenous fibers embedded in a matrix of ground substance. The presence of axial compression force reduces the segmental stiffness in flexion whereas a reverse trend is predicted in extension. In the presence of axial compression with and without sagittal shear force, flexion considerably increases the intradiscal pressure while extension reduces it. In other words, under an identical compression force, disk pressure is predicted to be noticeably larger in flexion than in extension. The segmental mechanical response in extension loadings is markedly influenced by the changes in the relative geometry of the articular surfaces at the lower regions. Finally, the deformation of the bony structures plays a significant role in the segmental mechanics under relatively large loads.     

Calibration of hyperelastic material properties of the human lumbar intervertebral disc under fast dynamic compressive loads

Under fast dynamic loading conditions (e.g. high-energy impact), the load rate dependency of the intervertebral disc (IVD) material properties may play a crucial role in the biomechanics of spinal trauma. However, most finite element models (FEM) of dynamic spinal trauma uses material properties derived from quasi-static experiments, thus neglecting this load rate dependency. The aim of this study was to identify hyperelastic material properties that ensure a more biofidelic simulation of the IVD under a fast dynamic compressive load. A hyperelastic material law based on a first-order Mooney-Rivlin formulation was implemented in a detailed FEM of a L2-L3 functional spinal unit (FSU) to represent the mechanical behavior of the IVD. Bony structures were modeled using an elasto-plastic Johnson-Cook material law that simulates bone fracture while ligaments were governed by a viscoelastic material law. To mimic experimental studies performed in fast dynamic compression, a compressive loading velocity of 1 m/s was applied to the superior half of L2, while the inferior half of L3 was fixed. An exploratory technique was used to simulate dynamic compression of the FSU using 34 sets of hyperelastic material constants randomly selected using an optimal Latin hypercube algorithm and a set of material constants derived from quasi-static experiments. Selection or rejection of the sets of material constants was based on compressive stiffness and failure parameters criteria measured experimentally. The two simulations performed with calibrated hyperelastic constants resulted in nonlinear load-displacement curves with compressive stiffness (7335 and 7079 N/mm), load (12,488 and 12,473 N), displacement (1.95 and 2.09 mm) and energy at failure (13.5 and 14.7 J) in agreement with experimental results (6551 ± 2017 N/mm, 12,411 ± 829 N, 2.1 ± 0.2 mm and 13.0 ± 1.5 J respectively). The fracture pattern and location also agreed with experimental results. The simulation performed with constants derived from quasi-static experiments showed a failure energy (13.2 J) and a fracture pattern and location in agreement with experimental results, but a compressive stiffness (1580 N/mm), a failure load (5976 N) and a displacement to failure (4.8 mm) outside the experimental corridors. The proposed method offers an innovative way to calibrate the hyperelastic material properties of the IVD and to offer a more realistic simulation of the FSU in fast dynamic compression.     

Tensile Mechanical Properties of Swine Cortical Mandibular Bone

Temporary orthodontic mini implants serve as anchorage devices in orthodontic treatments. Often, they are inserted in the jaw bones, between the roots of the teeth. The stability of the mini implants within the bone is one of the major factors affecting their success and, consequently, that of the orthodontic treatment. Bone mechanical properties are important for implant stability. The aim of this study was to determine the tensile properties of the alveolar and basal mandible bones in a swine model. The diametral compression test was employed to study the properties in two orthogonal directions: mesio-distal and occluso-gingival. Small cylindrical cortical bone specimens (2.6 mm diameter, 1.5 mm thickness) were obtained from 7 mandibles using a trephine drill. The sites included different locations (anterior and posterior) and aspects (buccal and lingual) for a total of 16 specimens from each mandible. The load-displacement curves were continuously monitored while loading half of the specimens in the oclluso-gingival direction and half in the mesio-distal direction. The stiffness was calculated from the linear portion of the curve. The mesio-distal direction was 31% stiffer than the occluso-gingival direction. The basal bone was 40% stiffer than the alveolar bone. The posterior zone was 46% stiffer than the anterior zone. The lingual aspect was stiffer than the buccal aspect. Although bone specimens do not behave as brittle materials, the diametral compression test can be adequately used for determining tensile behavior when only small bone specimens can be obtained. In conclusion, to obtain maximal orthodontic mini implant stability, the force components on the implants should be oriented mostly in the mesio-distal direction.     

Model and in vivo studies on human trunk load partitioning and stability in isometric forward flexions

To resolve the trunk redundancy to determine muscle forces, spinal loads, and stability margin in isometric forward flexion tasks, combined in vivo-numerical model studies was undertaken. It was hypothesized that the passive resistance of both the ligamentous spine and the trunk musculature plays a crucial role in equilibrium and stability of the system. Fifteen healthy males performed free isometric trunk flexions of approximately 40 degrees and approximately 65 degrees +/- loads in hands while kinematics by skin markers and EMG activity of trunk muscles by surface electrodes were measured. A novel kinematics-based approach along with a nonlinear finite element model were iteratively used to calculate muscle forces and internal loads under prescribed measured postures and loads considered in vivo. Stability margin was investigated using nonlinear, linear buckling, and perturbation analyses under various postures, loads and alterations in ligamentous stiffness. Flexion postures significantly increased activity in extensor muscles when compared with standing postures while no significant change was detected in between flexed postures. Compression at the L5-S1 substantially increased from 570 and 771 N in upright posture, respectively, for +/-180 N, to 1912 and 3308 N at approximately 40 degrees flexion, and furthermore to 2332 and 3850 N at approximately 65 degrees flexion. Passive ligamentous/muscle components resisted up to 77% of the net moment. In flexion postures, the spinal stability substantially improved due both to greater passive stiffness and extensor muscle activities so that, under 180 N, no muscle stiffness was required to maintain stability. The co-activity of abdominal muscles and the muscle stiffness were of lesser concern to maintain stability in forward flexion tasks as compared with upright tasks. An injury to the passive system, on one hand, required a substantial compensatory increase in active muscle forces which further increased passive loads and, hence, the risk of injury and fatigue. On the other hand, it deteriorated the system stability which in turn could require greater additional muscle activation. This chain of events would place the entire trunk active-passive system at higher risks of injury, fatigue and instability.     

Estimation of ACL forces by reproducing knee kinematics between sets of knees: A novel non-invasive methodology

In situ force in the anterior cruciate ligament (ACL) has been quantified both in vitro in response to relatively simple loads by means of robotic technology, as well as in vivo in response to more complex loads by means of force transducers and computational models. However, a methodology has been suggested to indirectly estimate the in situ forces in the ACL in a non-invasive, non-contact manner by reproducing six-degree of freedom (six-DOF) in vivo kinematics on cadaveric knees using a robotic/UFS testing system. Therefore, the objective of this study was to determine the feasibility of this approach. Kinematics from eight porcine knees (source knees) were collected at 30 degrees , 60 degrees , and 90 degrees of flexion in response to: (1) an anterior load of 100 N and (2) a valgus load of 5 N m. The average of each kinematic data set was reproduced on a separate set of eight knees (target knees). The in situ forces in the ACL were determined for both sets of knees and compared. Significant differences (rho<0.05) were found between the source knees and the target knees for all flexion angles in response to an anterior load. However, in response to valgus loads, there was no significant difference between the source knees and the target knees at 30 degrees and 90 degrees of flexion. It was noted that there was a correlation between anterior knee laxity (the distance along the displacement axis from the origin to the beginning of the linear region of the load-displacement curve) and internal-external rotation. These data suggest that in order to obtain reproducible results one needs to first match knees to knees with comparable anterior knee laxity. Thus, an estimate of the in situ forces in the ACL during in vivo activities might be obtainable using this novel methodology.     

Anomalous rate dependence of the preconditioned response of soft tissue during load controlled deformation

It has been observed in load controlled laboratory tests of myocardium and skin that the tissues can exhibit a decrease in nonlinear stiffness with an increase in loading rate: the faster a test is performed, the more compliant is the preconditioned material behavior. This response seems to conflict with what is generally expected of soft tissues based on stretch or strain controlled tests, in which an increased rate of deformation results in a stiffer material response. It is hypothesized that this anomalous behavior has not been observed previously due to the small number of cyclic load controlled mechanical characterization tests that are geared specifically towards viscoelastic tissue response. The goal of this paper is to examine the preconditioned response of soft tissue to load controlled deformation using nonlinear viscoelastic material models including quasi-linear viscoelasticity, and to determine under what conditions this anomalous behavior becomes apparent. Results from this study suggest that this behavior is a true phenomenon unique to load controlled deformations that results from the interplay of nonlinear effects and creep behavior. These results call for increased attention to experimental parameters when testing and modeling nonlinear viscoelastic material behavior.     

Mechanical validation of whole bone composite tibia models

Composite synthetic models of the human tibia have recently become commercially available as substitutes for cadaveric specimens. Their use is justified by the advantages they offer as a substitute for real tibias. The present investigation concentrated on an extensive experimental validation of the mechanical behaviour of the whole bone composite model, compared to human specimens for different loading conditions. The stiffness of the tibias was measured with a torsional load applied along the long axis, and with a bending load applied both in the latero-medial and in the antero-posterior direction. The bending stiffness of the composite tibias matched well with that of the cadaveric specimens. This was not true for the torsional stiffness. In fact, the composite tibias were much stiffer than the cadaveric specimens, possibly due to the structure of the reinforcement material. The inter-specimen variability for the composite tibias was much lower than that for the cadaveric specimens. Thus, it seems that the composite tibias are suitable to replace cadaveric specimens for certain types of test, whereas they might be unsuitable for others, depending on the loading regimen.     

A finite element study of a lumbar motion segment subjected to pure sagittal plane moments

A nonlinear finite element program has been developed and applied to the analysis of a three-dimensional model of the lumbar L2-3 motion segment subjected to sagittal plane moments. The analysis accounts for both material and geometric nonlinearities and is based on the Updated Lagrangian approach. The disc nucleus has been considered as an incompressible inviscid fluid and the annulus as a composite of collagenous fibres embedded in a matrix of ground substance. Articulation at the facet joints has been treated as a general moving contact problem and the spinal ligaments have been modelled as a collection of nonlinear axial elements. Effects of the loss of intradiscal pressure in flexion and of facetectomy in extension have been analyzed. Comparison of the predicted gross response characteristics with available measurements indicates satisfactory agreement. In flexion relatively large intradiscal pressures are generated, while in extension negative pressures (i.e. suction) of low magnitude are predicted. The stress distribution results indicate that the load transfer path through the posterior elements of the joint in flexion is different from that in extension. In flexion the ligaments are the means of load transfer, while in extension the load is transmitted through the pedicles, laminae and articular processes. In flexion, the inner annulus fibres at the posterolateral location are subject to maximum tensile strain. It is suggested that large flexion moment in combination with other loads is a likely cause of disc prolapse commonly found at this location of the annulus.     

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.     

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.     

Effects of external trunk loads on lumbar spine stability

Stability of the lumbar spine is an important factor in determining spinal response to sudden loading. Using two different methods, this study evaluated how various trunk load magnitudes and directions affect lumbar spine stability. The first method was a quick release procedure in which effective trunk stiffness and stability were calculated from trunk kinematic response to a resisted-force release. The second method combined trunk muscle EMG data with a biomechanical model to calculate lumbar spine stability. Twelve subjects were tested in trunk flexion, extension, and lateral bending under nine permutations of vertical and horizontal trunk loading. The vertical load values were set at 0, 20, and 40% of the subject's body weight (BW). The horizontal loads were 0, 10, and 20% of BW. Effective spine stability as obtained from quick release experimentation increased significantly (p<0.01) with increased vertical and horizontal loading. It ranged from 785 (S.D.=580) Nm/rad under no-load conditions to 2200 (S.D.=1015) Nm/rad when the maximum horizontal and vertical loads were applied to the trunk simultaneously. Stability of the lumbar spine achieved prior to force release and estimated from the biomechanical model explained approximately 50% of variance in the effective spine stability obtained from quick release trials in extension and lateral bending (0.53相似文献   

Contact forces in the tibiofemoral joint from soft tissue tensions: Implications to soft tissue balancing in total knee arthroplasty

Proper tension of the knee’s soft tissue envelope is important during total knee arthroplasty; incorrect tensioning potentially leads to joint stiffness or instability. The latter remains an important trigger for revision surgery. The use of sensors quantifying the intra-articular loads, allows surgeons to assess the ligament tension at the time of surgery. However, realistic target values are missing. In the framework of this paper, eight non-arthritic cadaveric specimens were tested and the intra-articular loads transferred by the medial and lateral compartment were measured using custom sensor modules. These modules were inserted below the articulating surfaces of the proximal tibia, with the specimens mounted on a test setup that mimics surgical conditions. For both compartments, the highest loads are observed in full extension. While creating knee flexion by lifting the femur and flexing the hip, mean values (standard deviation) of 114 N (71 N) and 63 N (28 N) are observed at 0° flexion for the medial and lateral compartment respectively. Upon flexion, both medial and lateral loads decrease with mean values at 90° flexion of 30 N (22 N) and 6 N (5 N) respectively. The majority of the load is transmitted through the medial compartment. These observations are linked to the deformation of the medial and lateral collaterals, in addition to the anatomy of the passive soft tissues surrounding the knee. In conclusion, these findings provide tangible clinical guidance in assessing the soft tissue loads when dealing with anatomically designed total knee implants.     

The effect of the point of application of anterior tibial loads on human knee kinematics

Coupled axial tibial rotation in response to an anterior tibial load has been used as a common diagnostic measurement and as a means to load the ligamentous structures during laboratory tests. However, the exact location of the point of application of these loads as well as the corresponding sensitivity of the coupled tibial rotation to this point can have an effect on the function of the soft tissues at the joint. Therefore, the purpose of this study was to determine the effects of four different points of application of the anterior tibial load on the anterior tibial translation and coupled axial tibial rotation. The four points include: (1) geometric point - midway between the collateral ligament insertion sites on the tibia, (2) clinical point - a position that attempts to simulate clinical diagnostic tests, (3) medial point - a position medial to the geometric point and (4) lateral point - a position lateral to the clinical point. A robotic/universal force-moment sensor testing system was used to apply the anterior tibial load at the four points of application and to record the resulting joint motion. Anterior tibial translation in response to an anterior tibial load of 100N was found not to vary between the four points of application of the anterior tibial load at all flexion angles examined. However, internal tibial rotation was found for the lateral point (13+/-10 degrees at 30 degrees of knee flexion) in all specimens and clinical point (8+/-10 degrees at 30 degrees of knee flexion) while external rotation resulted when the load was applied at the medial point (-8+/-7 degrees at 30 degrees of knee flexion). Both internal and external tibial rotations occurred throughout the range of flexion when the tibial load was applied at the geometric point. The results suggest that the clinical point should be used as the point of application of the anterior tibial load whenever clinical examinations are simulated and multi-degree-of-freedom joint and soft tissue function are examined.     

Effects of elastic loads on the contractions of cat muscles

The nerves to plantaris and soleus muscles in the cat were stimulated with maximal single shocks and with random stimulus trains which produced partially fused contractions. In order to obtain information on the mechanism of muscular contraction, the effects of allowing the muscles to shorten against various elastic loads were studied in the time domain and in the frequency domain. When springs of increasing stiffness were placed in series with the muscle, the twitch tension increased greatly. The gain of the frequency response curve was also much greater with stiffer springs. The shape of the frequency response curve for plantaris muscle could usually be described by that expected for a second-order system with two real time constants or rate constants. The rate constants changed in qualitatively similar ways in response to increased stiffness of an elastic load, increased muscle length and increased mean rate of nerve stimulation. These results are in agreement with the hypothesis that the linear responses of muscles working against elastic loads are determined by the values of two rate constants. Thus, of the many processes associated with contraction, only two are rate-limiting: one associated with the viscoelastic properties of muscle and the second associated with the reuptake of Ca into the sarcoplasmic reticulum. Non-linear aspects of muscular contraction are also discussed. These are more prominent in soleus muscle than in plantaris muscle.Graduate student of the Medical Research Council of Canada.Formerly a Post-doctoral Fellow of the Muscular Dystrophy Association of Canada.     

Flexion and extension structural properties and strengths for male cervical spine segments

New vehicle safety standards are designed to limit the amount of neck tension and extension seen by out-of-position motor vehicle occupants during airbag deployments. The criteria used to assess airbag injury risk are currently based on volunteer data and animal studies due to a lack of bending tolerance data for the adult cervical spine. This study provides quantitative data on the flexion-extension bending properties and strength on the male cervical spine, and tests the hypothesis that the male is stronger than the female in pure bending. An additional objective is to determine if there are significant differences in stiffness and strength between the male upper and lower cervical spine. Pure-moment flexibility and failure testing was conducted on 41 male spinal segments (O-C2, C4-C5, C6-C7) in a pure-moment test frame and the results were compared with a previous study of females. Failures were conducted at approximately 90 N-m/s. In extension, the male upper cervical spine (O-C2) fails at a moment of 49.5 (s.d. 17.6)N-m and at an angle of 42.4 degrees (s.d. 8.0 degrees). In flexion, the mean moment at failure is 39.0 (s.d. 6.3 degrees) N-m and an angle of 58.7 degrees (s.d. 5.1 degrees). The difference in strength between flexion and extension is not statistically significant. The difference in the angles is statistically significant. The upper cervical spine was significantly stronger than the lower cervical spine in both flexion and extension. The male upper cervical spine was significantly stiffer than the female and significantly stronger than the female in flexion. Odontoid fractures were the most common injury produced in extension, suggesting a tensile mechanism due to tensile loads in the odontoid ligamentous complex.