Differences In Bending Stiffness Research Articles

Biomechanical validation of novel polyurethane-resin synthetic osteoporotic femoral bones in axial compression, four-point bending and torsion

In addition to human donor bones, bone models made of synthetic materials are the gold standard substitutes for biomechanical testing of osteosyntheses. However, commercially available artificial bone models are not able to adequately reproduce the mechanical properties of human bone, especially not human osteoporotic bone.To overcome this issue, new types of polyurethane-based synthetic osteoporotic bone models have been developed. Its base materials for the cancellous bone portion and for the cortical portion have already been morphologically and mechanically validated against human bone. Thus, the aim of this study was to combine the two validated base materials for the two bone components to produce femur models with real human geometry, one with a hollow intramedullary canal and one with an intramedullary canal filled with synthetic cancellous bone, and mechanically validate them in comparison to fresh frozen human bone.These custom-made synthetic bone models were fabricated from a computer-tomography data set in a 2-step casting process to achieve not only the real geometry but also realistic cortical thicknesses of the femur. The synthetic bones were tested for axial compression, four-point bending in two planes, and torsion and validated against human osteoporotic bone.The results showed that the mechanical properties of the polyurethane-based synthetic bone models with hollow intramedullary canals are in the range of those of the human osteoporotic femur. Both, the femur models with the hollow and spongy-bone-filled intramedullary canal, showed no substantial differences in bending stiffness and axial compression stiffness compared to human osteoporotic bone. Torsional stiffnesses were slightly higher but within the range of human osteoporotic femurs.Concluding, this study shows that the innovative polyurethane-based femur models are comparable to human bones in terms of bending, axial compression, and torsional stiffness.

Tailorable elasticity of cantilever using spatio-angular functionally graded biomimetic scales

Cantilevered beams are of immense importance as structural and sensorial members for a number of applications. Endowing tailorable elasticity can have wide ranging engineering ramification. Such tailorability could be possible using some type of spatial gradation in the beam's material or cross section. However, these often require extensive additive and subtractive material processing or specialized casts. In this letter, we demonstrate an alternative bio inspired mechanical pathway, which is based on exploiting the nonlinearity that would arise from a functionally graded distribution of biomimetic scales on the surface using an analytical approach. This functional gradation is geometrically sourced and could arise from either spatial or angular gradation of scales. We analyze such a functionally graded cantilever beam under uniform loading. In comparison with uniformly distributed scales, we find significant differences in bending stiffness for both spatial and angular gradations. Spatial and angular functional gradation share some universality but also sharp contrasts in their effect on the underlying beam. A combination of both types of gradation in the structure can be used to alternatively increase or decrease stiffness and therefore a pathway to tailor the elasticity of a cantilever beam relatively easily. These results give rise to an architected framework for designing and optimizing the topography of leveraged solids.

The influence of mini-fragment plates on the mechanical properties of long-bone plate fixation.

Objective:Mini-fragment plates (MFPs) are increasingly used in fracture surgery to provide provisional fixation. After definitive fixation, the surgeon decides whether to remove the plates or leave them in place as additional fixation, based on the perceived biomechanical influence of the MFP. However, there are no current biomechanical studies to guide this decision. Therefore, the purpose of this study was to evaluate the influence of MFPs on the four-point bending and torsional stiffness of long bone transverse and simple wedge fracture fixation constructs.Methods:Fourth-generation composite bone cylinders were cut to produce transverse (AO-OTA classification 12-A3) and simple wedge (AO-OTA classification 12-B2) fracture models. The specimens were fixed using a low-contact dynamic compression plate (LC-DCP) and MFPs. Specimens were tested in four-point bending and torsion utilizing 3 different MFP orientations.Results:No statistically significant differences in bending stiffness were found between control and MFP groups for transverse fracture constructs. MFPs significantly increased the bending stiffness for wedge fracture constructs under certain loading conditions. This increase was observed when MFPs were positioned both orthogonal (85.1% increase, P = .034) and opposite (848.2% increase, P < .001) to the LC-DCP. MFPs significantly increased the torsional stiffness for both transverse and wedge fracture constructs when MFPs were positioned both orthogonal (transverse: 27.7% increase, wedge: 16.7% increase) and opposite (transverse: 28.4%, wedge: 24.2% increase) to the LC-DCP.Conclusions:Our results indicate that including MFPs in definitive fixation can increase the bending and torsional stiffness of a long-bone fracture fixation construct. This suggests that the biomechanical influence of MFPs should be considered. However, clinical studies will be required to test the applicability of these findings to the clinical setting.

Stiffness gradients facilitate ovipositor bending and spatial probing control in a parasitic wasp.

Many parasitic wasps use slender and steerable ovipositors to lay eggs in hosts hidden in substrates, but it is currently unknown how steering is achieved. The ovipositors generally consist of three longitudinally connected elements, one dorsal and two ventral valves that can slide along each other. For the parasitic wasp Diachasmimorpha longicaudata, it has been shown that protraction of the ventral valves causes incurving of the ventral valves towards the dorsal one, which results in a change in probing direction. We hypothesize that this shape change is due to differences in bending stiffness along the ovipositor. Alignment of the stiff tip of the dorsal valve with a more flexible ventral S-shaped region situated just behind the tip straightens this S-bend and results in upwards rotation of the ventral tip. We show that the S-shaped region of the ventral valves has a low bending stiffness because it contains soft materials such as resilin. In contrast, the large cross-sectional area of the dorsal valve tip area probably results in a high bending stiffness. Elsewhere, the dorsal valve is less stiff than the ventral valves. Our results support the hypothesis that the interaction between the stiff dorsal valve portion and the more flexible S-shaped region co-determines the configurational tip changes required for steering the ovipositor in any desired direction along curved paths in the substrate. This provides novel insights in the understanding of steering mechanisms of the hymenopteran ovipositor, and for application in man-made probes.

A study on the mechanical characteristics of the EBM-printed Ti-6Al-4V LCP plates in vitro.

PurposeThe electron beam melting (EBM) Ti-6Al-4V material technology has been developed over a short time period. It was introduced through a research to develop Ti-6Al-4V implants for patients, but EBM printed locking compression plates have not been used for clinical implants. The main purpose of this study is to find whether the EBM Ti-6Al-4V plate suit for clinical implants.MethodsFirst, we scanned an AO-locking compression plate (LCP) and printed LCP samples using EBM. Next, we evaluated the EBM plate surface roughness through optical microscopy as well as the LCP and EBM plates’ mechanical characteristics using the ASTM standard, which is commonly used to test the mechanical properties of bone plates subject to bending. Each sample was examined using a single-cycle four-point bending test and hardness testing to acquire data on bending stiffness, bending strength, bending structural stiffness, and hardness.ResultsThe results show significant differences in bending stiffness, bending strength, bending structural stiffness, and hardness between the samples using EBM and the original LCP plates. The EBM-printed samples’ surface roughness was 0.49 ± 0.02 μm. The mean hardness of the LCP sample was 266.67 HV10 ± 5.8, and the EBM-printed sample mean hardness was 341.1 HV10 ± 1.93. The EBM samples’ bending stiffness was 87.67%, which is greater than using the LCP plates’; and the bending strength was 190.7% greater, the bending structural stiffness was 73.2% greater, and the hardness was 27.9% greater.ConclusionsThe results show that the EBM plates’ general mechanical strength was significantly greater than the LCP plates. An EBM plate is advantageous for clinical implants because it can be customized with great potential for improvement.

Compliant flexural behaviour in laser sintered nylon structures: Experimental test and Finite Element Analysis – correlation

The aim of this paper is to present a simple and easily tuneable method of creating compact structures with gradient flexural properties using Selective Laser Sintering (SLS). The method makes use of a sine-wave ridged pattern commonly found in nature which produces a compliant bending pattern along various directions defined by the reference axes. These patterns have been adopted due to the ease with which it is possible to parametrically control the height, width, pitch and thickness of the designed structure. A series of 3-point bending tests and finite element analyses of flat and ridged specimens have been performed in order to obtain more appropriate material definition for the inclusion of such patterns into more complex structures.Results show that a parametrically defined ridged pattern has the ability to induce passive bending behaviour on structures. Differences in bending stiffness between 20% and 50% were observed for the directions parallel and perpendicular to the ridged pattern lines and a capacity to redistribute stresses across the bending specimen was highly dependent on the ridged pattern direction.

Bidirectional Stiffness Material: A Potential Solution for Passive Shape-Morphing Wings

An ornithopter is a flying device created by mimicking the motion of birds or insects. Its wings play significant roles in determining flight performance and stability. For example, butterfly wings are demonstrated to possess significant differences in bending stiffness when considering the bending direction (up or down). Novel concepts are needed to build a passive shape-morphing wing structure that could change shape smoothly during flapping, and thus enable efficient flight. Traditionally, ornithopter wing structures have employed hinges between rigid wing sections or hinges along the leading edge to introduce the desired wing flexibility. Motivated by the scale structure found in butterfly wings, it was hypothesized that a biomimetic scale-type material system would possess the desired bidirectional stiffness response. A material system was manufactured by 3-D printing of polyhedral-shaped particles, assembling such particles in a dense planar array, and joining them with carbon fibers. In particular, the polyhedral shapes considered were tetrahedra, truncated tetrahedra, and Janus-type tetrahedra made of half soft and hard solid. These systems were tested in single-point load bending. For regular tetrahedra, the assemblies possess the same stiffness in both deflection directions ( !! !!#$ = 1 , due to symmetry), while the assemblies of the truncated tetrahedra ( !! !!#$ = 25.2 ) and of the Janus-type tetrahedra ( !! !!#$ = 4.7 ) exhibited significant bending stiffness asymmetries. These results confirm the initial hypothesis that scale-type structures are responsible for the bending stiffness asymmetry in biological wing structures. The results show that the proposed material system can imitate the function of a wing, and that the passive shapemorphing wing constructed using a bidirectional stiffness material presents a potential solution for ornithopters.

Long‐term effects of saw osteotomy versus random fracturing on bone healing and remodeling in a sheep tibia model

This article is about the evaluation of possible differences in biomechanical or histomorphological properties of bone healing between saw osteotomy and random fracturing after 6 months. A standardized, 30 degrees oblique monocortical saw osteotomy of sheep tibia was carried out, followed by manual fracture completion of the opposed cortical bone. Fixation was performed by bridge plating (4.5 mm, LCDCP, broad). X-rays were taken immediately after surgery and at the end of the study. Polychrome fluorescent staining was performed according to a standardized protocol in the 2nd, 4th 6th, 10th, 14th, 18th, 22th and 26th week. Ten sheep were comprehensively evaluated. Data for stiffness and histomorphology are reported. The average bending stiffness of the operated bone was higher (1.7 (SD 0.3) with plate (MP) vs. 1.5 without plate) than for the intact bone (1.4 (SD 0.2), though no significant differences in bending stiffness were observed (P>0.05). Fluorescence staining revealed small numbers of blood vessels and less fragment resorption and remodeling in the osteotomy gap. Bone healing after saw osteotomy shows a very close resemblance to 'normal' fracture healing. However, vascular density, fragment resorption, fragment remodeling, and callus remodeling are reduced at the osteotomy.

Influences of dome height and stored elastic energy on the actuating performance of a plate-type piezoelectric composite actuator

The objective of this study is to examine the influences of dome height and stored elastic energy induced by thermal deformation after a curing process on the actuating performance of plate-type piezoelectric composite actuators (PCAs). The PCA is made of woven fabric fiber skin layers and a PZT ceramic core layer. The actuating performance has been evaluated in terms of flexural displacement with and without a load of mass at simply supported and fixed-free boundary conditions. Additionally, the stored elastic energy of the PCA is obtained via a flexural mechanical bending test. The results indicate that in a static state with increasing electric field, PCA-1 with a top layer of three carbon-epoxy plies exhibits better performance than PCA-2 with a top layer of two carbon-epoxy plies and one glass-epoxy ply and PCA-3 with a top layer of one carbon-epoxy ply and two glass-epoxy plies in terms of free displacement as well as load capability due to its increased dome height and stored elastic energy. The behavior of the flexural free displacement of the PCA in a quasi-static state at a drive frequency of 1 Hz shows non-linearity at the simply supported boundary condition, whereas the flexural displacement increases in a nearly linear manner in a fixed-free condition. In a dynamic state above 1 Hz, the magnitude of free displacement for each PCA reaches a peak value at the specific frequency, i.e. the resonance frequency. The corresponding resonance frequency varies among the PCAs due to differences in bending stiffness. Furthermore, as the applied electric field is increased, each PCA displays a slight decrease in resonance frequency because of a PZT ceramic softening effect. On the basis of experimental results, it has been concluded that the flexural displacement of the asymmetrically laminated PCA presented here can be controlled such that applications for pumping devices will be possible subsequent to investigation of the drive frequency along with the applied electric field.

Biomechanical analysis of supracondylar femoral fractures fixed with modern retrograde intramedullary nails.

Several new retrograde supracondylar intramedullary nails have been developed to specifically address fractures of the distal femur. The nails appear clinically effective, but there are few biomechanical data documenting the stability of the fixation or the mechanical stiffness of the different designs. The goal of this study was to assess the torsional and bending stiffness of four designs of intramedullary nails developed for this application. Four nail designs were tested in torsion and bending to determine system stiffness: Ace supracondylar, Richards "five hole" and "multi-hole" supracondylar, and Biomet retrograde. The nails were inserted into cadaveric femurs in which a one-centimeter distraction osteotomy had been created seven centimeters proximal to the condyles. The constructs were then tested on an Instron biaxial testing system. There were no statistically significant differences in bending stiffness among the groups of nails (range 0.79 to 1.18 newtons/meter; p > 0.1). However, the Ace nails (1.10 newtonmeters/degree) did exhibit a statistically lower torsional stiffness compared with the other nails (2.20 to 2.21 newton-meters/ degree; p < 0. 1). No differences were noted as a function of the number of locking holes. The bending stiffness of four currently available designs of retrograde intramedullary nails does not appear to be dependent on design variations. The torsional stiffness did vary among the four designs, but this was not determined by the number of fixation holes provided. It appears that a well-placed retrograde supracondylar nail of modern design should have sufficient stiffness to support the femur and provide stability during fracture healing.

A comparison of cortical bone atrophy secondary to fixation with plates with large differences in bending stiffness

Two internal-fixation plates with large differences in bending stiffness were applied to pairs of intact femora of six adult mongrel dogs to study the osteoporosis induced by rigid fixation. After nine and twelve months of plate application a significant, increased amount of bone atrophy was seen on the rigidly plated side. Biomechanical measurements of specimens from various segments of both femoral diaphyses showed the bones to have similar mechanical properties (as tissue), but different structural properties (as organ). These findings suggest that the osteoporosis due to rigid plate fixation occurs by thinning of the cortex rather than by reduction of the mechanical properties of the osseous tissue.