Muscular contributions to dynamic dorsoventral lumbar spine stiffness

Spinal musculature plays a major role in spine stability, but its importance to spinal stiffness is poorly understood. We studied the effects of graded trunk muscle stimulation on the in vivo dynamic dorsoventral (DV) lumbar spine stiffness of 15 adolescent Merino sheep. Constant voltage supramaximal electrical stimulation was administered to the L3–L4 interspinous space of the multifidus muscles using four stimulation frequencies (2.5, 5, 10, and 20 Hz). Dynamic stiffness was quantified at rest and during muscle stimulation using a computer-controlled testing apparatus that applied variable frequency (0.46–19.7 Hz) oscillatory DV forces (13-N preload to 48-N peak) to the L3 spinous process of the prone-lying sheep. Five mechanical excitation trials were randomly performed, including four muscle stimulation trials and an unstimulated or resting trial. The secant stiffness (k _y = DV force/L3 displacement, kN/m) and loss angle (phase angle, deg) were determined at 44 discrete mechanical excitation frequencies. Results indicated that the dynamic stiffness varied 3.7-fold over the range of mechanical excitation frequencies examined (minimum resting k _y = 3.86 ± 0.38 N/mm at 4.0 Hz; maximum k _y = 14.1 ± 9.95 N/mm at 19.7 Hz). Twenty hertz muscle stimulation resulted in a sustained supramaximal contraction that significantly (P < 0.05) increased k _y up to twofold compared to rest (mechanical excitation at 3.6 Hz). Compared to rest, k _y during the 20 Hz muscle stimulation was significantly increased for 34 of 44 mechanical excitation frequencies (mean increase = 55.1%, P < 0.05), but was most marked between 2.55 and 4.91 Hz (mean increase = 87.5%, P < 0.05). For lower frequency, sub-maximal muscle stimulation, there was a graded change in k _y, which was significantly increased for 32/44 mechanical excitation frequencies (mean increase = 40.4%, 10 Hz stimulus), 23/44 mechanical excitation frequencies (mean increase = 10.5%, 5 Hz stimulus), and 11/44 mechanical excitation frequencies (mean increase = 4.16%, 2.5 Hz stimulus) when compared to rest. These results indicate that the dynamic mechanical behavior of the ovine spine is modulated by muscle stimulation, and suggests that muscle contraction plays an important role in stabilizing the lumbar spine. This study was presented, in part, at the 31st Annual Meeting of the International Society for the Study of the Lumbar Spine, New York, NY, May 11–14, 2005.     

Axial rotation mechanics in a cadaveric lumbar spine model: a biomechanical analysis

Background contextPostoperative patient motions are difficult to directly control. Very slow quasistatic motions are intuitively believed to be safer for patients, compared with fast dynamic motions, because the torque on the spine is reduced. Therefore, the outcomes of varying axial rotation (AR) angular loading rate during in vitro testing could expand the understanding of the dynamic behavior and spine response.PurposeTo observe the effects of the loading rate in AR mechanics of lumbar cadaveric spines via in vitro biomechanical testing.Study designAn in vitro biomechanical study in lumbar cadaveric spines.MethodsFifteen lumbar cadaveric segments (L1–S1) were tested with varying loading frequencies of AR. Five different frequencies were normalized with the base line frequency (0.125 Hz n=15) in this analysis: 0.05 Hz (n=6), 0.166 Hz (n=6), 0.2 Hz (n=10), 0.25 Hz (n=10), and 0.4 Hz (n=8).ResultsThe lowest frequency (0.05 Hz) revealed significant differences (p<.05) for all parameters (torque, passive angular velocity, axial velocity [AV], axial reaction force [RF], and energy loss [EL]) with respect to all other frequencies. Significant differences (p<.05) were observed in the following: torque (0.4 Hz with respect to 0.2 Hz and 0.25 Hz), passive sagittal angular velocity (SAV) (0.4 Hz with respect to all other frequencies; 0.166 Hz with respect to 0.25 Hz), axial linear velocity (0.4 Hz with respect to all other frequencies), and RF (0.4 Hz with respect to 0.2 Hz and 0.25 Hz). Strong correlations (R²>0.75, p<.05) were observed between RF with intradiscal pressure (IDP) and AR angular displacement with IDP. Intradiscal pressure (p<.05) was significantly larger in 0.2 Hz in comparison with 0.125 Hz.ConclusionsEvidences suggest that measurements at very small frequencies (0.05 Hz) of torque, SAV, AV, RF, and EL are significantly reduced when compared with higher frequencies (0.166 Hz, 0.2 Hz, 0.25 Hz, and 0.4 Hz). Higher frequencies increase torque, RF, passive SAV, and AV. Higher frequencies induce a greater IDP in comparison with lower frequencies.     

Thoracic spinal kinematics is affected by the grade of intervertebral disc degeneration,but not by the presence of the ribs: An in vitro study

BACKGROUND CONTEXTThoracic spinal three-dimensional kinematics is widely unknown. For the evaluation of surgical treatments and the complete validation of numerical models, however, kinematic data of the thoracic spine are essential.PURPOSETo identify possible effects of rib presence and grade of intervertebral disc degeneration on thoracic spinal kinematics including three-plane helical axes and instantaneous centers of rotation.DESIGN/SETTINGRadiological grading of intervertebral disc degeneration and in vitro tests using n=8 human thoracic functional spinal units of the segmental levels T1–T2, T3–T4, T5–T6, T7–T8, T9–T10, and T11–T12, respectively, were performed with as well as without ribs to analyze the specific kinematic properties.METHODSSpecimens were loaded with pure moments of 5 Nm and constant loading rates of 1°/s in flexion/extension, lateral bending, and axial rotation. Optical motion tracking was performed to visualize helical axes and instantaneous centers of rotation on three-plane X-rays and to evaluate primary ranges of motion (ROMs) and coupled motions.RESULTSMotion segments with no or mild disc degeneration showed reproducible kinematics in all motion planes, whereas medium or severely degenerated specimens offered high variations and shifts of the rotational axes to the distal direction as well as lower ROM. Coupled motions were generally not detected.CONCLUSIONSWith progressing disc degeneration, the rotational axes show higher variation and tend to shift in distal direction, especially in flexion/extension with a shift to the anterior direction, whereas rib resection does not affect thoracic spinal kinematics but its stability. Rib resections as part of spinal deformity treatment destabilize the thoracic spine, but do not alter its kinematics. Young and healthy discs, however, could be affected by surgical treatments of the thoracic spine regarding thoracic spinal kinematics.     

Physiological axial compressive preloads increase motion segment stiffness, linearity and hysteresis in all six degrees of freedom for small displacements about the neutral posture.

The stiffness of motion segments, together with muscle actions, stabilizes the spinal column. The objective of this study was to compare the experimentally measured load-displacement behavior of porcine lumbar motion segments in vitro with physiological axial compressive preloads of 0, 200 and 400 N equilibrated in a physiological fluid environment, for small displacements about the neutral posture. These preloads are hypothesized to increase stiffness, hysteresis and linearity of the load-displacement behavior.At each preload, displacements in each of six degrees of freedom (+/-0.3 mm AP and lateral translations, +/-0.2 mm axial translation, +/-1 degrees lateral bending and +/-0.8 degrees flexion/extension and torsional rotations) were imposed. The resulting forces and moments were recorded. Tests were repeated after removal of posterior elements. Using least squares, the forces at the vertebral body center were related to the displacements by a symmetric 6 x 6 stiffness matrix. Six diagonal and two off-diagonal load-displacement relationships were examined for differences in stiffness, linearity and hysteresis in each testing condition.Mean values of the diagonal terms of the stiffness matrix for intact porcine motion segments increased significantly by an average factor of 2.2 and 2.9 with 200 and 400 N axial compression respectively (p<0.001). Increases for isolated disc specimens averaged 4.6 and 6.9 times with 200 and 400 N preload (p<0.001). Changes in hysteresis correlated with the changes in stiffness. The load-displacement relationships were progressively more linear with increasing preload (R(2)=0.82, 0.97 and 0.98 at 0, 200 and 400 N axial compression respectively). Motion segment and disc load-displacement behaviors were stiffer, more linear and had greater hysteresis with axial compressive preloads.     

短节段DRFS脊柱内固定术后即刻刚度的变化及其意义

目的测试短节段猪脊柱标本在完整、失稳后及内固定后的刚度变化。方法选择12节段猪脊柱标本12具,测定其前屈、后伸、侧屈及旋转时的刚度;切除椎间盘、小关节及前后纵韧带松解脊柱,测定其各项运动刚度;用DRFS脊柱内固定系统进行短节段内固定后重复测试各项运动刚度。结果与完整脊柱的刚度（前屈、后伸、侧屈及旋转的刚度分别为0.389±0.305,1.090±0.355,1.012±0.301,1.232±0.441,1.103±0.414,1.013±0.402）相比,失稳后脊柱各项运动的刚度（前屈、后伸、侧屈及旋转的刚度分别为0.216±0.218,0.278±0.204,0.255±0.124,0.409±0.169,0.633±0.218,0.626±0.216）均明显减少（P〈0.05）,而内固定后脊柱各项运动的刚度（前屈、后伸、侧屈及旋转的刚度分别为（0.568±0.351,0.679±0.151,0.759±0.314,0.729±0.311,1.006±0.304,0.975±0.218）均明显高于失稳后脊柱,与完整脊柱前屈、侧屈及旋转运动无明显差异,后伸运动的刚度明显高于完整脊柱（P〈0.01）。结论短节段脊柱内固定后各项运动的刚度明显高于脊柱失稳后,与完整标本相比除后伸运动的刚度明显增高外,其余各方向的运动刚度相近。     

Occlusion of the lumbar spine canal during high-rate axial compression

BACKGROUND CONTEXTWhile burst fracture is a well-known cause of spinal canal occlusion with dynamic, axial spinal compression, it is unclear how such loading mechanisms might cause occlusion without fracture.PURPOSETo determine how spinal canal occlusion during dynamic compression of the lumbar spine is differentially caused by fracture or mechanisms without fracture and to examine the influence of spinal level on occlusion.STUDY DESIGNA cadaveric biomechanical study.METHODSTwenty sets of three-vertebrae specimens from all spinal levels between T12 and S1 were subjected to dynamic compression using a hydraulic loading apparatus up to a peak velocity between 0.1 and 0.9 m/s. The presence of canal occlusion was measured optically with a high-speed camera. This was repeated with incremental increases of 4% compressive strain until a vertebral fracture was detected using acoustic emission measurements and computed tomographic imaging.RESULTSFor axial compression without fracture, the peak occlusion (O_max) was 29.9±10.0%, which was deduced to be the result of posterior bulging of the intervertebral disc into the spinal canal. O_max correlated significantly with lumbar spinal level (p<.001), the compressive displacement (p<.001) and the cross-sectional area of the vertebra (p=.031).CONCLUSIONSSpinal canal occlusion observed without vertebral fracture involves intervertebral disc bulging. The lower lumbar spine tended to be more severely occluded than more proximal levels.CLINICAL SIGNIFICANCEClinically, intermittent canal occlusion from disc bulging during dynamic compression may not show any radiographic features. The lower lumbar spine should be a focus of injury prevention intervention in cases of high-rate axial compression.     

Comparative biomechanical investigation of a modular dynamic lumbar stabilization system and the Dynesys system

The goal of non-fusion stabilization is to reduce the mobility of the spine segment to less than that of the intact spine specimen, while retaining some residual motion. Several in vitro studies have been conducted on a dynamic system currently available for clinical use (Dynesys^®). Under pure moment loading, a dependency of the biomechanical performance on spacer length has been demonstrated; this variability in implant properties is removed with a modular concept incorporating a discrete flexible element. An in vitro study was performed to compare the kinematic and stabilizing properties of a modular dynamic lumbar stabilization system with those of Dynesys, under the influence of an axial preload. Six human cadaver spine specimens (L1–S1) were tested in a spine loading apparatus. Flexibility measurements were performed by applying pure bending moments of 8 Nm, about each of the three principal anatomical axes, with a simultaneously applied axial preload of 400 N. Specimens were tested intact, and following creation of a defect at L3–L4, with the Dynesys implant, with the modular implant and, after removal of the hardware, the injury state. Segmental range of motion (ROM) was reduced for flexion–extension and lateral bending with both implants. Motion in flexion was reduced to less than 20% of the intact level, in extension to approximately 40% and in lateral bending a motion reduction to less than 40% was measured. In torsion, the total ROM was not significantly different from that of the intact level. The expectations for a flexible posterior stabilizing implant are not fulfilled. The assumption that a device which is particularly compliant in bending allows substantial intersegmental motion cannot be fully supported when one considers that such devices are placed at a location far removed from the natural rotation center of the intervertebral joint.     

Dynamic cervical plates: biomechanical evaluation of load sharing and stiffness.

STUDY DESIGN: An in vitro biomechanical study using a simulated cervical corpectomy model to compare the load-sharing properties and stiffnesses of two static and two dynamic cervical plates. OBJECTIVES: To evaluate the load-sharing properties of the instrumentation with a full-length graft and with 10% graft subsidence and to measure the stiffness of the instrumentation systems about the axes of flexion-extension, lateral bending, and axial torsion under these same conditions. SUMMARY OF BACKGROUND DATA: No published reports comparing conventional and dynamic cervical plates exist. METHODS: Six specimens of each of the four plate types were mounted on ultra-high molecular weight polyethylene-simulated vertebral bodies. A custom four-axis spine simulator applied pure flexion-extension, lateral bending, and axial torsion moments under a constant 50 N axial compressive load. Load sharing was calculated through a range of applied axial loads up to 120 N. The stiffness of each construct was calculated in response to +/-2.5 Nm moments about each axis of rotation with a full-length graft, a 10% shortened graft, and no graft. ANOVA and Fisher's post hoc test were used to determine statistical significance (alpha < or = 0.05). RESULTS: The two locked cervical plates (CSLP and Orion) and the ABC dynamic plate were similar in flexion-extension, lateral bending, and torsional stiffness. The DOC dynamic plate was consistently less stiff. The Orion plate load shared significantly less than the other three plates with a full graft. Both the ABC and the DOC plates were able to load share with a shortened graft, whereas the conventional plates were not. CONCLUSIONS: All plates tested effectively load share with a full-length graft, whereas the two dynamic cervical plates tested load share more effectively than the locked plates with simulated graft subsidence. The effect of dynamization on stiffness is dependent on plate design.     

In vitro evaluation of stiffness and load sharing in a two-level corpectomy: comparison of static and dynamic cervical plates

Background contextAnterior cervical plating has been accepted in corpectomy and fusion of the cervical spine. Constrained plates were criticized for stress shielding that may lead to subsidence and pseudarthrosis. A dynamic plate allows load sharing as the graft subsides. Ideally, the dynamic plate design should maintain adequate stiffness of the construct while providing a reasonable load sharing with the strut graft.PurposeThe purpose of the study was to compare dynamic and static plate kinematics with graft subsidence.Study design/settingThe study designed was an in vitro biomechanical study in a porcine cervical spine model.MethodsTwelve spines were initially tested in intact condition with 20-N axial load in 15 degrees of flexion and extension range of motion (ROM). Then, a two-level corpectomy was created in all specimens with spines randomized to receive either a static or dynamic plate. The spines were retested under identical conditions with optimal length and undersized graft. Range of motion and graft loading were analyzed with a one-way analysis of variance (p<.05).ResultsBoth plates significantly limited ROM compared with the intact spine in both graft length conditions. In extension graft, load was significantly higher (p=.001) in the static plate with optimal length, and in flexion, there was a significant loss of graft load (p=.0004). In flexion, the dynamic plate with undersized graft demonstrated significantly more load sustained (p=.0004).ConclusionsBoth plates reasonably limited the ROM of the corpectomy. The static plate had significantly higher graft loads in extension and significant loss of graft load in flexion, whereas the dynamic plate maintained a reasonable graft load in ROM even when graft contact was imperfect.     

Characterization and prediction of rate-dependent flexibility in lumbar spine biomechanics at room and body temperature

Background contextThe soft tissues of the spine exhibit sensitivity to strain-rate and temperature, yet current knowledge of spine biomechanics is derived from cadaveric testing conducted at room temperature at very slow, quasi-static rates.PurposeThe primary objective of this study was to characterize the change in segmental flexibility of cadaveric lumbar spine segments with respect to multiple loading rates within the range of physiologic motion by using specimens at body or room temperature. The secondary objective was to develop a predictive model of spine flexibility across the voluntary range of loading rates.Study designThis in vitro study examines rate- and temperature-dependent viscoelasticity of the human lumbar cadaveric spine.MethodsRepeated flexibility tests were performed on 21 lumbar function spinal units (FSUs) in flexion-extension with the use of 11 distinct voluntary loading rates at body or room temperature. Furthermore, six lumbar FSUs were loaded in axial rotation, flexion-extension, and lateral bending at both body and room temperature via a stepwise, quasi-static loading protocol. All FSUs were also loaded using a control loading test with a continuous-speed loading-rate of 1-deg/sec. The viscoelastic torque-rotation response for each spinal segment was recorded. A predictive model was developed to accurately estimate spine segment flexibility at any voluntary loading rate based on measured flexibility at a single loading rate.ResultsStepwise loading exhibited the greatest segmental range of motion (ROM) in all loading directions. As loading rate increased, segmental ROM decreased, whereas segmental stiffness and hysteresis both increased; however, the neutral zone remained constant. Continuous-speed tests showed that segmental stiffness and hysteresis are dependent variables to ROM at voluntary loading rates in flexion-extension. To predict the torque-rotation response at different loading rates, the model requires knowledge of the segmental flexibility at a single rate and specified temperature, and a scaling parameter. A Bland-Altman analysis showed high coefficients of determination for the predictive model.ConclusionsThe present work demonstrates significant changes in spine segment flexibility as a result of loading rate and testing temperature. Loading rate effects can be accounted for using the predictive model, which accurately estimated ROM, neutral zone, stiffness, and hysteresis within the range of voluntary motion.     

Strain of the facet joint capsule during rotation and translation range-of-motion tests: an in vitro porcine model as a human surrogate

BACKGROUND CONTEXTPrior data about the modulating effects of lumbar spine posture on facet capsule strains are limited to small joint deviations. Knowledge of facet capsule strain during rotational and translational intervertebral joint motion (ie, large joint deviations) under physiological loading could be useful as it may help explain why visually normal lumbar spinal joints become painful.PURPOSEThis study quantified the strain tensor of the facet capsule during rotation and translation range-of-motion tests.STUDY DESIGN/SETTINGStrain was calculated in isolated porcine functional spinal units. Following a preload, each specimen underwent a flexion/extension rotation (F/E) followed by an anterior/posterior translation (A/P) range-of-motion test while under a 300 N compression load.METHODSTwenty porcine spinal units (10 C3–C4, 10 C5–C6) were tested. Joint flexion/extension was imposed by applying a ±8 Nm moment at a rate of 0.5°/s, and translation was facilitated by loading the caudal vertebra with a ±400 N shear force at a rate of 0.2 mm/s. Points were drawn on the exposed capsule surface and their coordinates were optically tracked throughout each test. Strain was calculated as the displacement of the point configuration with respect to the configuration in a neutral joint position.RESULTSCompared to a neutral posture, superior-inferior strain increased and decreased systematically during flexion and extension, respectively. Posterior displacement of the caudal vertebra by more than 1.3 mm was associated with negative strains, which was significantly lower than the +4.6% strain observed during anterior displacement (p≥.199). The shear strain associated with anterior translation was, on average, −1.1% compared to a neutral joint posture.CONCLUSIONSThese results demonstrate that there is a combination of strain types within the facet capsule when spinal units are rotated and translated. The strains documented in this study did not reach the thresholds associated with nociception.CLINICAL RELEVANCEThe magnitude of flexion-extension rotation and anterior-translation may glean insight into the facet capsule deformation response under low compression (300 N) loading scenarios. Further, intervertebral joint motion alone, even under low compression loading, does not appear to initiate a clinically relevant pain response in the lumbar facet capsule of a nondegenerated spinal joint.     

Establishing validity of the fundamentals of spinal surgery (FOSS) simulator as a teaching tool for orthopedic and neurosurgical trainees

BACKGROUND CONTEXTPedicle screw placement is a demanding surgical skill as a spine surgeon can face challenges including variations in pedicle morphology and spinal deformities. Available CT simulators for spine pedicle placement can be very costly and hands-on cadaver courses are limited by specimen availability and are not readily accessible.PURPOSETo conduct validation of a simulated training device for essential spine surgery skills.DESIGNCross-sectional, empirical study of physician performance on a surgical simulator model.SAMPLESpine attending physicians and residents from four different academic institutions across the United States.OUTCOME MEASURESPerformance metrics on two surgical simulator tasks.METHODSAfter IRB approval, an inexpensive ($30) simulator was developed to test two main psychomotor tasks (1) creation of the pedicle screw path with a standard gearshift probe without cortical breaks and (2) the ability to palpate for the presence or absence of cortical breaches as well as determine the location of wall defects. Orthopedic and neurosurgery residents (N=72) as well as spine attending surgeons (N=26) participated from four different institutions. To test construct validity, performance metrics were compared between participants of different training status through one-way analysis of variance and linear regression analysis, with significance set at p<.05.RESULTSSpine attending surgeons consistently scored higher than the residents, in the screw trajectory task with triangular base (p=.0027) and defect probing task (p=.0035). In defect probing, performance improved with linear trend by number of residency training years with approaching significance (p=.0721). In that task, independent of institutional affiliation, PGY-2 residents correctly identified an average of 1.25±0.43 fewer locations compared with attending physicians (p=.0049). More than 80% of the spine attendings reported they would use the simulator for training purposes.CONCLUSIONSThis low-cost fundamentals of spine surgery simulator detected differences in performances between spine attending surgeons and surgical residents. Programs should consider implementing a simulator such as fundamentals of spine surgery to assess and develop pedicle screw placement ability outside of the operating room.     

Combined flexion and compression negatively impact the mechanical integrity of the annulus fibrosus

Purpose

To observe the effect of static flexion, in combination with compression, on the intralamellar and interlamellar matrix properties of the annulus fibrosus.

Methods

C3/C4 cervical functional spinal units of porcine specimens were selected. Following preloading, all specimens were loaded under 1200 N axial compression in either a neutral or static end range flexion posture (15º) for 2 h. Following loading, six annulus samples were dissected from each disc: four single-layer and two multi-layer samples. The multi-layer samples underwent peel tests to quantify the mechanical properties of the interlamellar matrix while the single-layer samples underwent tensile tests to quantify the mechanical properties of the intralamellar matrix. Statistical comparisons between properties were performed to determine differences between postural condition, extraction location, and extraction depth.

Results

Flexion elicited a decrease in lamellar adhesive strength (p = 0.045) and in single-layer failure strain (p = 0.03) when compared to a neutral posture. Flexion also had extraction depth-specific effects namely increased intralamellar matrix stiffness in the inner annulus when compared to neutral (p = 0019). Flexion also resulted in a significant decrease in toe region strain for the inner region of the annulus (p = 0.035). The inner region of the annulus was shown to have a significant increase in stress at 30% strain when compared to the outer region after flexion (p = 0.041).

Conclusion

The current findings suggest that the mechanical properties of the interlamellar and intralamellar matrices are sensitive to flexion, creating an environment that promotes an increased potential for damage to occur.

     

DRFS系统的体外生物力学测试

目的 测试DRFS系统固定后脊柱标本的刚度及其椎弓根钉的拔出力,评估DRFS系统临床应用的可靠性.方法 选择猪脊柱标本12具,测试其前屈、后伸、侧屈及旋转时的刚度;制成失稳标本后,重复各项运动刚度测试;用DRFS固定失稳标本后,再重复各项运动刚度测试.选择双节段腰椎标本12对,分别拧入DRFS系统椎弓根钉(固定钉、提拉钉)后,进行拔出测试.结果 在猪脊柱标本前屈、后伸、侧屈及旋转活动的刚度测试中,与完整脊柱的刚度相比较,失稳脊柱各项运动的刚度均明显减小(P<0.05);DRFS系统内固定后,脊柱各项运动的刚度均明显高于失稳脊柱(P<0.05),固定钉的拔出力明显大于提拉钉(P<0.01).结论 DRFS系统内固定术后脊柱标本的刚度明显高于失稳标本.其椎弓根钉可提供足够的提拉力.     

A new dynamic six degrees of freedom disc-loading simulator allows to provoke disc damage and herniation

Purpose

The cause of disc herniation is not well understood yet. It is assumed that heavy lifting and extreme postures can cause small injuries starting either in the inner anulus or from the outside close to the endplate. Such injuries are accumulated over years until its structure is weakened and finally a single loading event leads to a sudden failure of the last few intact lamellae. This paper describes a novel, custom-developed dynamic 6-DOF disc-loading simulator that allows complex loading to provoke such disc damage and herniations.

Methods

The machine’s axes are driven by six independent servomotors providing high loads (10 kN axial compression, 2 kN shear, 100 Nm torque) up to 5 Hz. A positional accuracy test was conducted to validate the machine. Subsequently, initial experiments with lumbar ovine motion segments under complex loading were performed. After testing, the discs were examined in an ultra-high field MRI (11.7 T). A three-dimensional reconstruction was performed to visualise the internal disc lesions.

Results

Validation tests demonstrated positioning with an accuracy of ≤0.08°/≤0.026 mm at 0.5 Hz and ≤0.27°/≤0.048 mm at 3.0 Hz with amplitudes of ±17°/±2 mm. Typical failure patterns and herniations could be provoked with complex asymmetrical loading protocols. Loading with axial compression, flexion, lateral bending and torsion lead in 8 specimens to 4 herniated discs, two protrusions and two delaminations. All disc failures occurred in the posterior region of the disc.

Conclusion

This new dynamic disc-loading simulator has proven to be able to apply complex motion combinations and allows to create artificial lesions in the disc with complex loading protocols. The aim of further tests is to better understand the mechanisms by which disc failure occurs at the microstructural level under different loading conditions. Visualisation with ultra-high field MRI at different time points is a promising method to investigate the gradual development of such lesions, which may finally lead to disc failure. These kinds of experiments will help to better investigate the mechanical failure of discs to provide new insights into the initiation of intervertebral disc herniation. This device will also serve for many other applications in spine biomechanics research.

     

A method to calculate relative spinal motion without digitization

Background contextEuler and projection methods have been used to describe relative spinal motion. In the Eulerian formulation, the exiting method used vector form of Euler angles and only provides an approximation. In the projection method, local coordinate systems constructed with digitization can affect the accuracy of kinematical results. A more consistent data reduction method is desired to calculate relative spinal motion (range of motion) from raw marker data.PurposeTo develop a new data reduction method to calculate relative spinal motion based on arbitrarily oriented local coordinate systems of individual vertebrae, and to simplify experimental procedures in multidirectional testing of spines.Study design/settingThe relative spinal motion was determined from raw marker data using transformation matrices.MethodsIn the Eulerian formulation, the relative motion of a vertebra to its subjacent level was determined using transformation matrices rather than vector operation on Euler angles. In the projection method, the projection axes were determined by tranforming local coordinate systems. Both approaches can be used to analyze raw marker data.ResultsThe new data reduction method was successfully implemented to analyze the raw data acquired on an intact L1–L2 motion segment. There was little difference between the Euler method and projection methods.ConclusionsIn conclusion, an alternative data reduction method in both Euler and projection angles to calculate range of motion for in vitro spine biomechanical studies was presented. The method was validated on a human cadaveric lumbar motion segment under axial torsion, lateral bending, and flexion extension. Because the relative spinal motion does not depend on how local coordinate systems are oriented, the digitization process can be eliminated in most multidirectional flexibility tests. Compared with previous methods, this new method provides more consistent kinematical results and significantly simplifies experimental procedures.     

Three-dimensional biomechanical properties of the human cervical spine in vitro

Summary Our aim was to determine the biomechanical properties of the normal human cervical spine under physiological static loads. The three-dimensional displacements under three pure moments: flexion-extension, left-right lateral bending and left-right axial torsion — were measured in 56 intact functional spinal units (FSUs) taken from between C2 and C7 in 29 human cadavers. For each mode of loading, load-displacement curves were plotted. Then we calculated each neutral zone, range of motion, neutral zone ratio, ratio of coupled motion, limit moment and secant stiffness. The effects of intervertebral disc degeneration and the disc level were also taken into account by the analysis of variance. Our results adequately demonstrated both the non-linearity of load-displacement curves and the neutral zone of the cervical spine in three-dimensional space. At the same time, we found statistically that the stiffness in the three planes are significantly different, as are the stiffnesses in lateral bending of successive different FSUs. However, significant differences of stiffness in different states of disc degeneration were only found in right lateral bending. There were significant differences between levels in ratio of coupled motion under both lateral bending and axial torsion. The loading cycle conditions and the biomechanical responses of principal motion of C1-2 are also reported.     

Biomechanical comparison of calf and human spines

Given the limited availability of human cadaveric specimens and their potential risk of infection, calf spines have been used as substitutes for human spines in the evaluation of spinal implants. Few bio-mechanical data comparing calf and human spines are available, however. The purpose of this study was to determine the biomechanical properties of the calf spine and to compare them with properties previously reported for the human spine. We determined the range of motion, neutral zone, and stiffness of thoracic and lumbar calf spines (T6 to L6) under pure moment loading in flexion-extension, axial rotation, and lateral bending. These properties were shown to be similar to those of the human spine. The results suggest that the calf spine can be used as a substitute for the human spine in some in vitro tests.     

Height and torsional stiffness are most sensitive to annular injury in large animal intervertebral discs

Background contextAcute annulus fibrosus injury has been identified as a contributing factor to intervertebral disc (IVD) degeneration. Injuries as small as those resulting from needle injection result in localized mechanical disruption via fiber breakage, but it is unknown whether these injuries initiate degeneration locally or through changes in the mechanical behavior of the entire disc. However, in vitro biomechanical studies of injury are limited to a single type of injury or measurements in only one or two degrees of freedom.PurposeThe aim of this study is to provide a comprehensive assessment of the joint level mechanical response to IVD injuries of various sizes in a large animal model. We hypothesize that annular injuries will affect disc mechanics differently depending on size, location, and mode of loading. We further hypothesize that a large injury to one side of the disc will induce a bending moment reaction under axial compression, which may decrease spinal column stability.Study designA comprehensive biomechanical study investigating effects of small and large injuries on IVD pressurization and six-degree-of-freedom stiffness behaviors using bovine motion segments.MethodsBovine caudal motion segments were subjected to a series of annular injuries ranging from 21-gauge needle puncture to 10-mm scalpel incisions and evaluated before and after injury with both mechanical testing under multiple degrees of freedom (axial compression, flexion-extension, lateral bending, and torsion) and nucleus pulposus (NP) fluid pressurization tests.ResultsMechanical tests showed that axial torsional stiffness and disc height under resting compressive load were the parameters most sensitive to large annular injury. Bending and compressive stiffnesses, as well as bending moments induced by axial compression, were not significantly changed by scalpel incisions. Additionally, large injuries resulted in altered relaxation behavior after NP pressurization indicative of increases in both radial bulge compliance and fluid flow rates.ConclusionsThese findings suggest that loss of disc height, torsional stiffness, and NP fluid pressurization are the immediate results of acute annular injury and are therefore those properties that IVD repair strategies must strive to restore or maintain. The lack of change in bending stiffness and moment under compression suggests that acute annular tears alone are not sufficient to induce off-axis motion and instability.