Axial Compressive Stiffness Research Articles

Effect of Crosslinks on the Stability of the Spine and the Pedicle Screw Fixation.

To evaluate the effect of crosslinks on the stability of the spine and pedicle screws. Compression fracture of the L1 vertebra was produced in 30 fresh thoracic and lumbar vertebrae samples obtained from adult sheep, which were divided into 3 groups (n=10)with lot-drawing method. Four screws were fixed onto the superior and inferior pedicles of vertebral arch close to the fractured vertebrae, with different number of crosslinks (0 in Group A,1 in Group B, and 2 in Group C) on the rods. After fixation, the samples were subject to 10 000 times of fatigue test with 1.5 Hz load on the HY-3080 computer-control electronic universal test machine and HY-1000NM computer-control torsion test machine. The axial compressive stiffness, maximum pullout strength,and range of motion (ROM) of 6 directions, i.e., flexion, extension, left and right lateral bending, and left and right axial rotation of the 3 groups were measured and compared. There were no statistically significant differences in axial compressive stiffness as well as the ROM of flexion, extension, and left and right lateral bending (all P>0.05). The maximum pullout strength was significantly smaller in Group A and Group B than in Group C [(129.56±29.63)N vs.(294.67±23.25) N,P=0.000;(254.02±36.29)vs.(294.67±23.25)N, P=0.006]. The ROM of left axial rotation was the highest in Group A(13.35°±1.06°), followed by Group B(12.23°±1.06°)and Group C (11.04°±0.74°)(F=13.44, P=0.000; Group B vs. Group A, P=0.000; Group B vs. Group C, P=0.001; Group C vs. Group A,P=0.000). The ROM of right axial rotation was also the highest in Group A(13.56°±1.15°), lower in Group B (12.39°±1.01°) and the lowest in Group C (10.81°±0.51°) (F=21.91, P=0.000; Group B vs. Group A,P=0.002; Group B vs. Group C, P=0.001; Group C vs. Group A, P=0.000). Crosslinks may reinforce the pullout strength of the screws and improve the axial stability of the spine.

Biomechanical comparison of long, short, and extended-short nail construct for femoral intertrochanteric fractures

ObjectivesShort and long cephalomedullary (CM) nails are commonly used construct for fixation of intertrochanteric (IT) fractures. Each of these constructs has its advantages and its shortcomings. The extended-short (ES) CM nail offers a hybrid between long and short nail design that aims to combine their respective benefits. The goals of this study were to (1) biomechanically evaluate and compare construct stiffness for the long, short and ES constructs in the fixation of IT fractures, and to (2) investigate the nature of periprosthetic fractures of constructs implanted with these various designs. MethodsEighteen synthetic femora were used to evaluate three types of fracture fixation constructs. Axial compression, bending, and torsional stiffness were reported for both stable and comminuted IT fracture models. All comminuted fracture constructs were loaded to failure in axial compression to measure failure loads and evaluate periprosthetic fracture patterns. ResultsStiffness were similar among constructs with few exceptions. Axial stiffness was significantly higher for the short nail compared to the long nail for the comminuted model (p=0.020). ES nail constructs exhibited a significantly higher failure load than short nail constructs (p=0.039). Periprosthetic fractures occurred around the distal interlocking screw in all constructs. ConclusionsNail length and position of interlocking screw did not alter the biomechanical properties of the fixation construct in the presented IT fracture model. Periprosthetic fractures generated in this study had similar patterns to those seen clinically. This study also suggests that if a periprosthetic fracture is to occur, there is an increased probability of it happening around the site of the interlocking screw, regardless of nail design.

Structural analysis and testing of a miniature flexible joint under pressure and vector loading

The structural characteristics of a miniature tactical motor flexible joint subjected to pressure and vector loading were investigated using finite element analysis and bench test. Three-dimensional non-linear finite element analysis was conducted using ANSYS Code Version 11.0. The axial deflection, vectoring torque, and stress distributions in elastomeric and reinforcement rings were presented. The predicted values were consistent with the test data. Results indicate that the axial compressive stiffness increased gradually and nonlinearly with pressure, while the angular stiffness remained nearly constant in the vectoring angle range from 0° to 6°. Under pressure loading, the elastomeric shear stress was negative, high at both sides, and low at the center of the cross-section, while the reinforcement hoop stress was compressive at the inner radius and tensile at the outer radius. The compressive stress was also high. The flexible joint exhibited higher stress level with altered stress distribution when subjected to additional vector loading. Existing empirical formulas for reinforcement hoop compressive stress were determined to be not applicable to the miniature flexible joint, which significantly overestimated the stress caused by pressure and underestimated the stress caused by vectoring.

Biomechanical study of the influence of stability for the pedicle screws fixation by injured vertebral screw when the pedicle cortex perforation

To explore the impact of pedicle cortex perforation on the stability of internal fixation of the vertebral body fracture, and to compare the stability of the vertebrae with pedicle cortex perforation after the injured vertebra transpedicular screw fixation by different ways. A total of 36 fresh thoracic and lumbarvertebrae samples of adult sheep (T₁₃-L₁) were equally divided into four groups (A, B, C and D) by using the random number table method. The vertebral compression fracture was performed in the L1 vertebral body of the four groups using the Chiba's method. Four pedicle screws were fixed on the upper and lower injured vertebrae of four groups. In addition, the group C was fixed into a pedicle screw through the injured vertebrae; D group was set two pedicle screws through the injured vertebrae. Then the samples of group B, C, and D were removed a quarter of either side of lateral T₁₄ thoracic pedicle, which was considered as the pedicle cortex perforation model. Four groups were performed fatigue test of 10 000 times by (300 ± 105)N load. The drawing force of the screw and the stability of injured thoracolumbar vertebrae were measured and the differences in every group were compared. The axial compressive stiffness and maximum drawing force of screws in the other three groups were significantly higher than those in group B (all P=0.000). The maximum range of motion in four directions of group B were significantly larger than those of the other three groups (all P=0.000). The stiffness and the drawing force in groups C and D were significantly larger than those in the group A (all P=0.000), and the maximum range of motion in four directions of the two groups were lower than that of group A (P=0.002, P=0.005). Every testing indicator in group C had no significant difference when compared with group D (P>0.05). The pedicle cortex perforation seriously affects the stability of the fractured vertebral body. The injured vertebra transpedicular screw can increase the stability of internal fixation.

Analytically evaluating the influence of crack on the mesh stiffness of a planetary gear set

Time-varying mesh stiffness, caused by the change of tooth contact number and contact position, is one of the main sources of vibration of a gear transmission system. In order to comprehensively understand the vibration properties of a planetary gear set, it is necessary to evaluate the mesh stiffness effectively. When a crack happens in one gear, the mesh stiffness will decrease and consequently the vibration properties of the gear system will change. This change of vibration can be characterized through dynamic simulation of a gearbox and processed further to detect the crack severity and location. In this paper, the potential energy method is used to analytically evaluate the mesh stiffness of a planetary gear set. A modified cantilever beam model is used to represent the external gear tooth and derive the analytical equations of the bending, shear and axial compressive stiffness. A crack propagation model is developed and the mesh stiffness reduction is quantified when a crack occurs in the sun gear or the planet gear.

A biomechanical comparison of unilateral and bilateral String-of-Pearls™ locking plates in a canine distal humeral metaphyseal gap model.

To compare the in vitro biomechanical performance of two String-of-Pearls (SOP) plate constructs in a canine distal humeral metaphyseal gap model. Seven pairs of canine cadaveric humeri, including the elbow joints, were prepared. One group consisted of a unilateral medially placed SOP plate with bicortical screws (UNI). The second group consisted of bilateral caudo-medial and caudo-lateral SOP plates applied with monocortical screws (BI). A 2 cm ostectomy was performed immediately proximal to the supratrochlear foramen. Constructs were tested in torsion and axial compression. The UNI constructs had significantly lower stiffness in torsion and axial compression than the BI group. However, UNI constructs had a significantly higher ultimate strength than BI constructs. All UNI constructs failed by bending of the transcondylar screw and SOP plate. All BI constructs failed by axial pullout of the distal most screws. In stabilizing canine supracondylar humeral fractures as modeled here, both the UNI model and the BI model demonstrated biomechanical advantages. The incorporation of a transcondylar screw through the medial plate appears to be beneficial to construct strength.

Effect of Axial Compression Ratio and Concrete Strength on Seismic Performance of Concrete Filled Steel Tube Beam-Column Joints

Based on finite element analysis o f concrete filled steel tube beam-column joints under the single axial compression ratio and concrete strength, further research was done to analyze the seismic performance of concrete filled steel tube beam-column joints under different axial compression ratio and concrete strength. Beam-column joint which is connected by bolts with welding extended steel sheets at the beam root was analyzed. The results show that with the increase of axial compression ratio, strength and stiffness degradation of the joint accelerated gradually. Axial compression ratio at 0.3, 0.4 are appropriate values for joints specimen, load-displacement hysteresis curve of joint specimens is relatively plump and shows good seismic performance. Chance of concrete strength also had effect on seismic performance of joint specimen, but in contrast it is not so obviously.

Opening Wedge Distal Femur Osteotomy: Biomechanical Study of a Biplane Fixation Using Conventional Implants

Opening Wedge Distal Femur Osteotomy: Biomechanical Study of a Biplane Fixation Using Conventional Implants

Research on the Dynamics of the Double Helical Gear Transmission

Against the transmission characteristics of the double helical gear pair, considering the time-varying mesh stiffness, the bearing radial and axial stiffness, the tensile and compressive stiffness of the transmission shaft, the bending stiffness of that, error excitations, and corresponding dampings, a double helical gear pair bending-torsion-axis coupling dynamic model was established by using the lumped parameter method based on the gear meshing theory and Lagrange equations. Based on the model, the dynamic response of double helical gear pair was solved, and taking the right side of helical gear as an example, the frequency spectrum characteristics of dynamic meshing force of the right side of helical gear were mainly analyzed. This research establishes the foundation for dynamic performance optimizations and reliability designs of the double helical gear pair transmission system in the future.

Mechanical behavior of fixation components for periprosthetic fracture surgery

BackgroundReliable periprosthetic fracture treatment needs detailed knowledge on the mechanical behavior of the fixation components used. The holding capacity of three conventional fixation components for periprosthetic fracture treatment was systematically investigated under different loading directions. MethodsLocking compression plates were fixed to a 7cm long part of diaphyseal fresh frozen human femur with either a single 1.7mm cerclage cable, a 5.0mm monocortical or a bicortical locking screw (n=5 per group). Constructs were loaded in lateral, torsional and axial direction with respect to the bone axis in a load-to-failure test. Corresponding stress distribution around the screw holes was analyzed by finite element modeling. FindingsBoth screw fixations revealed significantly higher stiffness and ultimate strength in axial compression and torsion compared to the cerclage (all P<0.01). Ultimate strength in lateral loading and torsion was significantly higher for bicortical screws (mean 3968N SD 657; mean 28.8Nm SD 5.9) compared to monocortical screws (mean 2748N SD 585; mean 14.4Nm SD 5.7Nm) and cerclages (mean 3001N SD 252; mean 3.2Nm SD 2.0) (P≤0.04). Stress distribution around the screw hole varied according to the screw type and load direction. InterpretationFixation components may be combined according to their individual advantages to achieve an optimal periprosthetic fracture fixation.

Analysis of Silicone Rubber Elastomer Elastic Failure in Small-Size Flexible Joint at Low Temperature

In order to study the performance on silicone rubber elastomer in small-size flexible joint at different temperatures, a series tests for two miniature flexible joints (joint-SR1 and joint-SR2) with the same structure and different formula of silicon rubber elastomer (phenyl silicon rubber and dimethyl silicon rubber) were conducted. Axial compressive stiffness and bending stiffness at various temperatures (-60 °C~20 °C) and vessel pressures (1MPa~8MPa) were studied. Results show that the axial compressive stiffness and bending stiffness of joint-SR1 remain stable throughout the temperature range of-60 °C~20 °C, while those remain stable throughout the temperature range of-40 °C~20 °C and become to be infinite at-50 °C due to the elastic failure of elastomer material for the joint-SR2. Analysis indicates that the reason that elastomer of joint-SR2 lost elasticity at-50 °C is not glass-transition but crystallization. The phenomenon of elastic failure did not appear throughout the temperature range-60 °C~20 °C for the joint-SR1 elastomer is due to the existence of phenyl side-group substitute. The modified chain content of silicone rubber not only affects the process of crystallization inhibition but also the crystallization rate. The rubber with phenyl or other bulky side-group could destroy the crystal of silicone rubber at low temperatures, which is beneficial to be applied in small-size flexible joint.

A Biomechanical Evaluation of Three Drop Wire Configurations

Evaluate effect of adding drop wires to single-ring constructs. Biomechanical study Single ring circular external skeletal fixator constructs stabilizing a Delrin segment bone model. Eight replicates of 5 constructs made with 66 mm complete rings, 1.6 mm olive wires, and a 15.9-mm diameter Delrin rod were loaded in axial compression, craniocaudal and mediolateral bending, and torsion. Constructs tested were: (1) base single-ring construct; (2) single-ring construct with a drop wire mounted on fixation bolts; (3) single-ring construct with a drop wire mounted on 1-hole posts; (4) single-ring construct with a drop wire mounted on 2-hole posts; and (5) 2-ring construct. Construct stiffness for each mode of loading was compared using repeated measures ANOVAs (P ≤ .05). Axial compression and torsion: the 2-ring construct was stiffer than all others. Drop wire constructs were stiffer than the single-ring construct, but not significantly different from each other. Craniocaudal bending: the 2-ring construct was stiffest with the 2-hole post construct being stiffer than all except the 2-ring construct. Mediolateral bending: the 2-hole post construct was stiffer than the 2-ring construct, which was stiffer than the 1-hole post construct, which was stiffer than the fixation-bolt construct, which was stiffer than the single-ring construct. Drop wires improved stiffness of single-ring constructs in all loading modalities. Positioning the drop wire farther from the ring surface significantly improved craniocaudal and mediolateral bending stiffness, but did not affect axial compression and torsional stiffness.

Strength and stiffness of post-heated columns repaired with ferrocement and fibre reinforced polymer jackets

This paper presents the results of an experimental study carried out to compare the effectiveness of ferrocement and fibre reinforced polymers (FRPs) jackets for the repair of post-heated square and circular reinforced concrete columns. The suite of test specimens comprised (a) non-heated and non-repaired; (b) post-heated and non-repaired and (c) post-heated and repaired, columns. Glass fibre reinforced polymer (GFRP), carbon fibre reinforced polymer (CFRP) and ferrocement jackets were used to repair the heated columns. All the columns were tested under axial compression. The test results covering the axial compressive strength, stiffness (secant stiffness), ductility, deformation and energy dissipation for the non-heated and non-repaired, and post-heated and non-repaired columns, were explored and compared with those of the post-heated columns repaired with FRP and ferrocement jackets. The test results showed that the FRP jackets increased the compressive strength, ductility, deformation ability and energy dissipation capacity of post-heated columns but did not increase the stiffness. However, the ferrocement jackets enhanced both the strength and stiffness of the post-heated columns. It is concluded that a possible combination of ferrocement and FRP jackets is the optimum solution to restore the required strength, stiffness and ductility following structural damage from a fire.

Are conventional reconstruction plates equivalent to precontoured locking plates for distal humerus fracture fixation? A biomechanics cadaver study

BackgroundThe optimal plate type and configuration for distal humerus fracture fixation has yet to be defined. Available biomechanical studies show conflicting results. No existing studies compare conventional reconstruction plates to newer precontoured distal humerus locking plates in both parallel and perpendicular configurations. MethodsThree groups of humerus specimens were compared via biomechanical testing in a cadaver model simulating metaphyseal comminution. Group 1 consisted of conventional reconstruction plates in a perpendicular configuration. Group 2 used precontoured locking plates in a perpendicular configuration. Group 3 used precontoured locking plates in a parallel configuration. Each group was tested for stiffness in anterior bending, posterior bending, axial compression, and torsion. The specimens then underwent cyclic loading followed by single load to failure in posterior bending. FindingsThere was no significant difference between the three groups for anterior bending, posterior bending, axial compression, or torsional stiffness. There was no significant difference in load to failure for any of the three groups. Screw loosening was significantly higher in Group 1 when compared to Groups 2 and 3 after cyclic loading. InterpretationIn the early postoperative period, less expensive perpendicular conventional reconstruction plate constructs provide similar stiffness and load to failure properties to newer precontoured locking plate systems regardless of plate configuration.

Rationale of a Test Setup with a Defined COR for Extra-Discal Motion-Preserving Implants with a Low Implant Stiffness

Abstract In the current version of ASTM F2624, the center of rotation (COR) is not specified. Potentially, each device can be tested using a different COR, which subsequently makes a direct design comparison of results difficult. Four posterior dynamic stabilization (PDS) devices (Dynesys, DYN, Zimmer; DSS, Paradigm Spine; and two Aesculap implant concepts) were tested in comparison to a rigid-fixation device and to the native situation of the lumbar spine on fresh-frozen human lumbar spines (L3–L5). The instrumented level was L4–L5. The PDS systems have axial compressive stiffness values ranging from 10 N/mm to 230 N/mm and were all made compatible to connect with the pedicle-screw system. The specimens were loaded in a spinal simulator, applying pure moments for flexion/extension, lateral bending and axial rotation (+/−7.5 Nm) with a defined velocity. The COR was analyzed based on the data measured with a 3-dimensional (3D) motion-analysis system. The effect of the PDS on the location of the COR is most pronounced in the sagittal plane. In general, the higher the implant stiffness, the more the COR shifted in a posterior direction. The DYN had a similar COR to the rigid fixator. However, the PDS systems with low axial compressive stiffness values (range: 10–70 N/mm) showed very similar results on CORs, which are located in the region of the posterior border of the intervertebral disc. In the frontal and transversal plane, the COR was found to be close to the native situation for each system. Therefore, for PDS devices with low implant stiffness, the location of the COR varies only marginally and can be specified for a test setup. An initial proposal that will allow side-by-side comparison for these kinds of PDS systems is given and the feasibility of the new test setup could be proven for all three loading conditions.

Buckling Analysis of the Tube Compression-Bending Member in Elastic-Plastic State with ANSYS

Stability is an important issue in steel structure design.When the steel member is subjected to elastic-plastic instability, the axial compression stiffness reduces with the increasing axial pressure and bending moment at end. Therefore, the authors analyze the process of steel member that is instable in elastic-plastic state in this paper by studying the degradation laws of axial compression stiffness. The eigenvalue buckling and nonlinear buckling analyses of axially compressed member and compression-bending member are carried out by using commercial package ANSYS in this study. The relationship curve between the axial force at end and the axial compression stiffness and the relationship surface among the axial pressure at end, bending moment at end and the axial compression stiffness are determined respectively. The made observations indicate that it is feasible to analyze the process of the steel member that is instable in elastic-plastic state by investigating the degradation properties of axial compression stiffness, which becomes lower with axial pressure at end and bending moment at end.

Regional Atherosclerotic Plaque Properties in ApoE–/– Mice Quantified by Atomic Force, Immunofluorescence, and Light Microscopy

Elucidating regional material properties of arterial tissue is fundamental to predicting transmural stresses and understanding how tissue stiffness influences cellular responses and vice versa. Atomic force microscopy (AFM) was used to measure point-wise the axial compressive stiffness of healthy aortas and atherosclerotic plaques at micron level separation distances. Cross sections of plaques were obtained from a widely used animal model of atherosclerosis (ApoE–/– mice). Median point-wise values of material stiffness were 18.7 and 1.5 kPa for the unloaded healthy wall (n = 25 specimens) and plaque (n = 18), respectively. When the healthy wall was distended uniformly during AFM testing, two mechanically distinct populations emerged from comparisons of normal cumulative distributions, with median values of 9.8 and 76.7 kPa (n = 16). The higher values of stiffness may have been due to extended elastin, which was not present in the plaques. Rather, most plaques were identified via standard and immunofluorescent histology to be largely lipid laden, and they exhibited a nearly homogeneous linear elastic behavior over the small AFM indentations. Understanding the mechanics and mechanobiological factors involved in lesion development and remodeling could lead to better treatments for those lesions that are vulnerable to rupture.

Finite Element Analysis and Experimental Investigation of Carbon Fiber Composite Circular Tube

In this paper, A 3-D numerical model of a circular tube is established, which is made of carbon fiber composite materials whose inner diameter is 43mm and outer diameter is 45mm. The behaviour of axial compression and bending load cases are analyzed under the theory of mechanics of materials and theory of classical laminate plate. Results of axial compressive stiffness and bending stiffness agree well with the experimental data,showing that the FEM simulation used in this paper is an efficient and reliable tool to calculate and predict the behaviour of carbon fiber tube.

A Biomechanical Evaluation of Press-Fit Stem Constructs for Tumor Endoprosthetic Reconstruction of the Distal Femur

This study was designed to assess the biomechanical parameters of the older Kotz Modular Femur Tibia Reconstruction (Stryker Inc, Mahwah, NJ) stem and the newer Restoration and the unfluted Global Modular Replacement System (Stryker Inc, Mahwah, NJ) uncemented stems for use with tumor endoprostheses as well as to assess the optimal reaming technique for insertion of these stems. Fresh-frozen adult femora or composite distal femora were implanted with the uncemented stems. Separate experiments were performed to compare reaming technique and bone resection level. All constructs were mechanically tested for axial compression, lateral bending, and torsional stiffness and torque to failure. Results showed that the biomechanical performance of all the stems were similar with respect to each parameter. Cylindrical reaming was associated with a significantly higher torque to failure than flexible reaming in the diaphysis ( P = .006). Newer uncemented stems provide adequate initial biomechanical stability for implantation in the distal femur.

The effects of local buckling and material yielding on the axial stiffness and failure of uniformly compressed I-section and box-section struts

The behaviour of thin-walled compression members is known to be significantly influenced by the effects of local buckling. Thin-walled sections lose axial compressive stiffness and the compressive carrying capability of the members can be considerably reduced as a result of the effects of local buckling. Finite element simulation is employed in this paper to examine the post-buckled response of thin-walled I-section and box-section struts, giving due consideration to the influence of geometric imperfections and to elasto-plastic material behaviour. A detailed account of the growth and redistribution of stresses after local buckling as well as the initiation of yielding and yield propagation throughout loading is given in the paper. The influence of different in-plane displacement boundary conditions imposed in the simulations at the section plate junctions is also detailed in the paper as well as the effect of yielding on the post-buckled stiffness and failure of the sections. It is shown that the effects of geometrical imperfections are most prominent for strut designs with near simultaneous buckling and yielding and that the ultimate failure and unloading of strut designs, in general, is synonymous with the development of yielding at the section junctions along the full length of the struts and through to the middle surface of the section walls.