Compressive Preloads Research Articles

Structural behavior of human lumbar spinal motion segments

The objectives of this study were to obtain linearized stiffness matrices, and assess the linearity and hysteresis of the motion segments of the human lumbar spine under physiological conditions of axial preload and fluid environment. Also, the stiffness matrices were expressed in the form of an ‘equivalent’ structure that would give insights into the structural behavior of the spine. Mechanical properties of human cadaveric lumbar L2-3 and L4-5 spinal motion segments were measured in six degrees of freedom by recording forces when each of six principal displacements was applied. Each specimen was tested with axial compressive preloads of 0, 250 and 500 N. The displacements were four slow cycles of ±0.5 mm in anterior–posterior and lateral displacements, ±0.35 mm axial displacement, ±1.5° lateral rotation and ±1° flexion-extension and torsional rotations. There were significant increases with magnitude of preload in the stiffness, hysteresis area (but not loss coefficient) and the linearity of the load-displacement relationship. The mean values of the diagonal and primary off-diagonal stiffness terms for intact motion segments increased significantly relative to values with no preload by an average factor of 1.71 and 2.11 with 250 and 500 N preload, respectively (all eight tests p<0.01). Half of the stiffness terms were greater at L4-5 than L2-3 at higher preloads. The linearized stiffness matrices at each preload magnitude were expressed as an equivalent structure consisting of a truss and a beam with a rigid posterior offset, whose geometrical properties varied with preload. These stiffness properties can be used in structural analyses of the lumbar spine.

Compressive preload improves the stability of anterior lumbar interbody fusion cage constructs.

Insertion of an anterior lumbar interbody fusion cage has been shown to reduce motion in a human spine segment in all loading directions except extension. The "stand-alone" cages depend on compressive preload produced by anular pretensioning and muscle forces for initial stabilization. However, the effect that the in vivo compressive preload generated during activities of daily living has on the construct is not fully understood. This study tested the hypothesis that the ability of the cages to reduce the segmental motions in flexion and extension is significantly affected by the magnitude of the externally applied compressive preload. Fourteen specimens from human lumbar spines were tested intact and after insertion of two threaded cylindrical cages at level L5-Sl. They were subjected to flexion and extension moments under progressively increasing magnitudes of externally applied compressive follower preload from 0 to 1200 N. The range of motion at level L5-S1 after cage insertion was compared with the value achieved in the intact specimens at each compressive preload magnitude. The cages significantly reduced the L5-S1 flexion motion at all preloads (p < 0.05). They decreased flexion motion by 29% to 43% of that of the intact specimens for low preloads (0 to 400 N) and by 69% to 79% of that of the intact specimens under preloads of 800 to 1200 N. In extension, in the absence of an externally applied preload, the cages permitted 24% more motion than the intact segment (p < 0.05). In contrast, they reduced the extension motion at preloads from 200 to 1200 N. Under preloads of 800 to 1200 N, the reduction in extension motion after cage placement was 42% to 48% of that of the intact segment (p < 0.05). The reduction of motion in both flexion and extension after cage placement was significantly greater at preloads of 800 to 1200 N compared with the motion reductions at preloads of < or =400 N (p < 0.05). In contrast to the observed extension instability under anular tension preload only, the two-cage construct exerted a stabilizing effect on the motion segment (a reduction in segmental motion) in flexion as well as extension under externally applied compressive preloads of physiologic magnitudes. The external compressive preload significantly affected the stabilization provided by the cages. The cages provided substantially more stabilization, both in flexion and in extension, at larger preloads than at smaller preloads. The study suggests that the segment treated with an anterior lumbar interbody fusion cage is relatively less stable under conditions of low external compressive preload. The magnitude of preload required to achieve stabilization with stand-alone cages may be only partially achieved by anular pretensioning. Since the magnitude of the preload across the disc space due to muscle activity can vary with activities of daily living, supplemental stabilization of the cage construct may provide a more predictably stable environment for lumbar spine fusion.

Effect of coarse aggregate size and matrix quality on ITZ and failure behavior of concrete under uniaxial compression

Effects of coarse aggregate size and water/cement ( w / c ) ratio of the matrix on the formation of interfacial transition zone (ITZ) and subsequently on the failure process of concrete under uniaxial compression were studied. For this purpose, a series of experiments were designed and carried out on mortars with two different w / c ratios containing single spherical steel aggregates of different sizes. The ITZ properties and the failure process of concrete were investigated through tensile strength tests both before and after compressive preloading, stress–axial strain, stress–volumetric strain and stress–lateral strain measurements. It was observed that ITZ becomes critical for larger aggregates and lower w / c ratio mortar matrices. The negative effect of smooth surface texture of the aggregate and the large difference between aggregate and matrix moduli of elasticity on the properties of ITZ is of paramount importance for low w / c ratio composites. The effect of reduced bond properties of ITZ relative to its matrix was reflected in the lower critical stress levels for the low w / c ratio composites with larger aggregates.

Effect of compressive follower preload on the flexion–extension response of the human lumbar spine

Traditional experimental methods are unable to study the kinematics of whole lumbar spine specimens under physiologic compressive preloads because the spine without active musculature buckles under just 120 N of vertical load. However, the lumbar spine can support a compressive load of physiologic magnitude (up to 1200 N) without collapsing if the load is applied along a follower load path. This study tested the hypothesis that the load–displacement response of the lumbar spine in flexion–extension is affected by the magnitude of the follower preload and the follower preload path. Twenty-one fresh human cadaveric lumbar spines were tested in flexion–extension under increasing compressive follower preload applied along two distinctly different optimized preload paths. The first (neutral) preload path was considered optimum if the specimen underwent the least angular change in its lordosis when the full range of preload (0–1200 N) was applied in its neutral posture. The second (flexed) preload path was optimized for an intermediate specimen posture between neutral and full flexion. A twofold increase in flexion stiffness occurred around the neutral posture as the preload was increased from 0 to 1200 N. The preload magnitude (400 N and larger) significantly affected the range of motion (ROM), with a 25% decrease at 1200 N preload applied along the neutral path. When the preload was applied along a path optimized for an intermediate forward-flexed posture, only a 15% decrease in ROM occurred at 1200 N. The results demonstrate that whole lumbar spine specimens can be subjected to compressive follower preloads of in vivo magnitudes while allowing physiologic mobility under flexion–extension moments. The optimized follower preload provides a method to simulate the resultant vector of the muscles that allow the spine to support physiologic compressive loads induced during flexion–extension activities.

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° lateral bending and ±0.8° 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 6×6 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.

Effect of compressive preload on the anterior lumbar interbody fusion cage construct stability

Avinash G. Patwardhan, PhD, Gerard Carandang, MS, Hines, IL, USA; Alexander J. Ghanayem, MD, Maywood, IL, USA; Benjamin Cunningham, MD, Troy, MI, USA; Frank M. Phillips, MD, Chicago, IL, USA; Leonard I. Voronov, MD, Maywood, IL, USA; Robert Havey, BS, James Simonds, BS, Hines, IL, USA; Kevin Meade, PhD, Tae-Hong Lim, PhD, Chicago, IL, USA; Thomas M. Gavin, CO, Burr Ridge, IL, USA; Michael R. Zindrick, MD, Hinsdale, IL, USAIntroduction: Anterior lumbar interbody fusion (ALIF) using threaded cylindrical interbody fusion cages is used to treat segmental instability in symptomatic lumbar disc degeneration. The construct should minimize the unstable segment motion during activities of daily living to promote a solid fusion in proper alignment. The compressive load on the lumbar spine ranges from 200 N to 1,200 N during activities of daily living. Although previous cadaveric studies have applied physiologic levels of bending moments, the effect of in vivo compressive loads on fusion constructs has not been investigated. Without a realistic compressive preload, the loads on the implant in the ex vivo experiment differ significantly (both in magnitude and direction) for those in vivo [1]. The purpose of this study was to quantify the effect of compressive preload on the response of the ALIF cage construct and adjacent segments. A unique experimental model was used to subject human cadaveric specimens to loads of in vivo magnitudes [2].Materials and methods: Thirteen specimens (L1–sacrum; age, 52.15 years) were tested in flexion, extension and lateral bending with and without an L5–S1 ALIF cage construct. To simulate in vivo conditions, a follower preload was applied using cables [2]. The follower load cables maintained a compressive preload throughout the range of motion without compromising the mobility of the spine. The specimens were tested under an increasing compressive preload (0 to 1,200 N) with bending moments applied to L1 (0 to 8 Nm flexion, 0 to 6 Nm extension, 0 to 3 Nm lateral bending). Three-dimensional motions were measured using an optoelectronic motion monitoring system for at least two complete cycles of moment application. After intact specimen testing, two threaded, cylindrical interbody cages were inserted anteriorly at L5–S1 and the specimen was retested. Radiographic templating was used to select the cage size. Fusion cages of 13, 15 and 17 mm diameter were used in 2, 10 and 1 specimens, respectively. The load-displacement curves gave the range of motion (ROM) and stiffness at the instrumented and adjacent segments in flexion, extension and lateral bending. The ROM and stiffness after cage insertions were compared with the intact values using repeated-measures analysis of variance.Results: The ALIF cages decreased the L5–S1 ROM in the sagittal and frontal planes (p<.05). Their effectiveness improved with increasing compressive preload in flexion and extension. The cages decreased L5–S1 flexion ROM by 27% to 41% of intact for low preloads (0 to 400 N), and by 68% to 78% of intact under preloads of 800 to 1,200 N (p<.05). In extension, the ALIF cages permitted more than the intact segment with no compressive preload (p<.05), whereas the motion decreased by 48% at preloads of 800 to 1,200 N (p<.05). In lateral bending, the cages decreased the motion by 40% relative to intact at 400 N preload as compared with a 15% decease with no preload. At L4–L5 and L3–L4, there were few significant changes in ROM after insertion of the cages at L5–S1.Discussion: This study tested interbody cages under loads closely mimicking in vivo conditions. The response of the ALIF cage construct in flexion-extension under low compressive preloads is consistent with previous studies that tested the construct under zero or low compressive preloads [3,4]. The lack of segmental stabilization of the ALIF cage construct in extension with no preload has been observed in previous studies [3,4] and may be the result of a separation at the cage–end plate interface in extension. In contrast, the segmental stability of the ALIF cage construct in flexion and extension was significantly greater under high compressive preloads that correspond to standing and walking. This emphasizes the importance of preload for cage stability that may be only partially achieved by annular pretensioning. Relaxation of the annulus may be an important issue, because collagenous soft tissues relax to approximately 20% to 30% of the maximum load over time. Therefore, supplemental stabilization or postoperative immobilization may play an important role in protecting the ALIF cage construct during activities of daily living that induce minimal or low levels of compressive preloads on the lumbar spine.

岩石力学 大谷石を用いたピーク強度以降のカイザー効果に及ぼす拘束圧の影響

In order to clarify the influence of confining pressure on the Kaiser effect in post-failure region, acoustic emission (AE) experiments were performed under triaxial loading conditions using Oya-tuff. Triaxial compressive pre-loading was performed until post-failure region, and then re-loading test was performed under different confining pressure. The rapidly increasing point of cumulative AE event count was determined as the peak point of AE event count rate increment. The estimated stress was compared with pre-stress. The main results are summarized as follows. (1) In the case of the re-loading test performed under the same confining condition, the remarkable AE increase appeared at the differential stress level at the previous residual strength. (2) In the case of the re-loading test under the lower re-confining pressure than the pre-loading test, the estimated stress is smaller than differential stress at previous residual strength. (3) In the case of the re-loading test under higher confining pressure, the estimated stress was equivalent to the differential stress at the previous stress peak.

A modified ultrasonic linear motor

A prototype for modified multi-mode ultrasonic linear motor has been developed. In contrast to the other multi-mode ultrasonic linear motors which use single layer piezoelectric ceramics as actuators bonded on the surface of elastic body, it uses actuators made of multi-layer piezoelectric actuators clamped in elastic body with compressive pre-load. With this design, not only peel-off problem that might occur on other multi-mode motors is prevented, but also speed and load are higher.

Analytical and Experimental Investigations of Prestressed Laminated Composite Beams with Constrained Viscoelastic Damping Layer

The damping characteristics of prestressed composite beams with con strained viscoelastic damping layers on one surface of the beam are investigated both ana lytically and experimentally. The analytical method involves derivation of finite element equations for the analysis of beam and plate structural elements with continuous or discon tinuous constrained viscoelastic damping layer with prestress. The experimental method is an impulse-frequency response technique which is used on preloaded laminated beams. Results for loss factors and natural frequencies of [90/90/0/90/90] and [903/03/90 3]s T300/934 graphite-epoxy laminated beams with a 3M SJ-2052X constrained viscoelastic tape attached on one surface are presented. Numerical results based on the finite element prediction agree reasonably well with the experimental results under different tensile and compressive preloads.

FATIGUE CRACK GROWTH IN DUCTILE MATERIALS UNDER CYCLIC COMPRESSIVE LOADING

Abstract— Mode I fatigue crack growth has been studied in notched specimens of 7017‐T651 aluminium alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field and on subsequent compressive cyclic loading in laboratory air, a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented.

７０７５アルミニウム合金切欠き材の疲労き裂発生および伝ぱ挙動に及ぼす予荷重の影響

Fatigue tests were carried out on the preloaded notch-specimens of 7075-T6 aluminum alloy under stress ratio of 0.1 and frequency of 1Hz, and the fatigue crack initiation and propagation behaviors were investigated. The tensile preloading, which corresponds to 80% of fracture load, increases the crack initiation life, while the compressive preloading decreases the life. The effect of preloading is larger when the overload ratio, or the ratio of preload to maximum repeating load, is larger. Through a microarea X-ray analysis near notch root, the compressive residual stress is discovered to be generated by the tensile preloading and the maximum repeating stress at notch root is found to be decreased, which will be the main reason for the increase in fatigue crack initiation life. The crack propagation velocity after crack initiation from notch root is decreased or increased within the preload-affected zone by the tensile or compressive preloading, respectively.

Dynamics of human lumbar intervertebral joints. Experimental and finite-element investigations.

To improve our understanding of the dynamic characteristics of the human lumbar spine, both experimental and finite-element methods are required. The experimental methods included measurement of the axial steady state response, resonant frequencies, and damping of seven lumbar motion segments under an upper-body mass of 40 kg. The influence of the presence of posterior elements and different magnitudes of compression preload on the response was also studied. To supplement the measurements, linear and nonlinear, axisymmetric, and three-dimensional finite-element models of a L2-L3 disc-vertebra unit were developed to predict the free and forced-vibration responses. The step and harmonic loadings in the axial direction were considered for the forced-vibration analysis. The effect of the presence of the body mass and compression preloads were also examined. The results of experimental and finite-element studies were in good agreement with each other. They indicated that the system resonant frequencies are reduced considerably with the addition of a body mass of 40 kg and increase significantly (P less than .005) as the compression preload increases. The compliance at both low and resonant frequencies decreases with increasing compression preload. Under preloads of not more than 680 N, removal of the facet joints tends to decrease slightly the segmental resonant frequencies irrespective of the magnitude of compression preload (P less than .1). The finite-element model studies show quasi-static response under harmonic loads with periods much larger than the fundamental period of the segment and under step loads with slow rising times. Under a step load without the body mass, the nucleus pressure varies with both location and time and reaches a maximum of about 2.5 times that under equivalent static load. The addition of a 40-kg mass, in this case, renders a single degree-of-freedom response, with the pressure remaining nearly constant with location inside the nucleus. The stresses and strains throughout the segment in this case increase approximately twofold in comparison with equivalent static values. Partial or complete removal of the disc nucleus considerably decreases the resonant frequency and increases the corresponding segmental response amplitude (ie, compliance). The results indicate that the most vulnerable element under axial vibration loads is the cancellous bone adjacent to the nucleus space. Fatigue fracture of bone as a cumulative trauma and the subsequent loss of nucleus content likely initiates or accelerate the segmental degenerative processes. The annulus fibers do not appear to be vulnerable to rupture when the segment is subjected to pure axial vibration.

Large compressive preloads decrease lumbar motion segment flexibility

The bending, shear, and torsion flexibilities of 13 intact adult lumbar motion segments (from 11 men, two women, 48-83 years of age) were compared under three different compressive preloads, 0, 2,200, and 4,400 N. Test forces and moments up to 160 N and 16 Nm were applied at the center of the upper end plate of the intact disc. A compressive preload of 2,200 N resulted in a significant decrease in motion segment flexibilities in all seven test directions (p less than 0.06) when compared with results obtained with no preload; the preload decreased flexibility 2.6, 4.5, and 6.1 times in bending, axial torsion, and shear, respectively. These results suggest that studies of internal trunk load-sharing between active and passive tissues during strenuous tasks, which engender large spine compressive loads, should take these changes in spine passive resistance into consideration.

Large compressive preloads decrease lumbar motion segment flexibility

The bending, shear, and torsion flexibilities of 13 intact adult lumbar motion segments (from 11 men, two women, 48-83 years of age) were compared under three different compressive preloads, 0, 2,200, and 4,400 N. Test forces and moments up to 160 N and 16 Nm were applied at the center of the upper end plate of the intact disc. A compressive preload of 2,200 N resulted in a significant decrease in motion segment flexibilities in all seven test directions (p less than 0.06) when compared with results obtained with no preload; the preload decreased flexibility 2.6, 4.5, and 6.1 times in bending, axial torsion, and shear, respectively. These results suggest that studies of internal trunk load-sharing between active and passive tissues during strenuous tasks, which engender large spine compressive loads, should take these changes in spine passive resistance into consideration.