Difference In Bending Stiffness Research Articles

Biomechanical comparison of titanium alloy additively manufactured and conventionally manufactured plate-screw constructs

ABSTRACTAimTo biomechanically compare the bending stiffness, strength, and cyclic fatigue of titanium additively manufactured (AM) and conventionally manufactured (CM) limited contact plates (LCP) of equivalent dimensions using plate-screw constructs.MethodsTwenty-four 1.5/2.0-mm plate constructs (CM: n = 12; AM: n = 12) were placed under 4-point bending conditions. Data were collected during quasi-static single cycle to failure and cyclic fatigue testing until implants plastically deformed or failed. Bending stiffness, bending structural stiffness, and bending strength were determined from load–displacement curves. Fatigue life was determined as number of cycles to failure. Median test variables for each method were compared using the Wilcoxon rank sum test within each group. Fatigue data was also analysed by the Kaplan–Meier estimator of survival function.ResultsThere was no evidence for a difference in bending stiffness and bending structural stiffness between AM and CM constructs. However, AM constructs exhibited greater bending strength (median 3.07 (min 3.0, max 3.4) Nm) under quasi-static 4-point bending than the CM constructs (median 2.57 (min 2.5, max 2.6) Nm, p = 0.006). Number of cycles to failure under dynamic 4-point bending was higher for the CM constructs (median 164,272 (min 73,557, max 250,000) cycles) than the AM constructs (median 18,704 (min 14,427, max 33,228) cycles; p = 0.02). Survival analysis showed that 50% of AM plates failed by 18,842 cycles, while 50% CM plates failed by 78,543 cycles.Conclusion and clinical relevanceAdditively manufactured titanium implants, printed to replicate a conventional titanium orthopaedic plate, were more prone to failure in a shorter fatigue period despite being stronger in single cycle to failure. Patient-specific implants made using this process may be brittle and therefore not comparable to CM orthopaedic implants. Careful selection of their use on a case/patient-specific basis is recommended.

Research on the effect of double sealing gaskets arrangement on the mechanical behavior of the segment longitudinal joints with large diameter shield tunnel

The sealing gasket of the traditional double waterproof system of the large diameter shield tunnel consists of two sealing gaskets which are arranged on the outside of the bolt hole(SGOH) or arranged on both sides of the bolt hole(SGBH). The different arrangement of the double sealing gaskets has significant effects on the mechanical behaviors of the longitudinal joints of the segment. In this paper, based on the energy equivalence principle, the actual distribution stress of the compression zone of the segment longitudinal joints was simplified equivalently as rectangular distribution stress. Then, a progressive mechanical model was established for the longitudinal joint, taking into account the effect of double sealing gaskets arrangements. Furthermore, the proposed mechanical model was validated through former experimental results and other analytical models. Finally, the effect of different arrangements of the double sealing gaskets on the mechanical behavior of the longitudinal joints of the segment was compared and analyzed. The results showed that the difference in the arrangement of the double sealing gaskets results in varying effects of the additional bending moment on the joint bending deformation under sagging and hogging moments, leading to the unequal bending stiffness of the joint. The difference in bending stiffness caused by the double sealing gasket arrangements is approximately 100∼150 MN·m/rad. To optimize the mechanical behaviors of the joints, the double sealing gaskets should be arranged on both sides of the bolt hole(SGBH), with the bolt located at the center of both the longitudinal joints and the core compressive zone of the longitudinal joints.

Radiographic, Biomechanical and Histological Characterization of Femoral Fracture Healing in Aged CD-1 Mice.

With a gradually increasing elderly population, the treatment of geriatric patients represents a major challenge for trauma and reconstructive surgery. Although, it is well established that aging affects bone metabolism, it is still controversial if aging impairs bone healing. Accordingly, we investigated fracture healing in young adult (3-4 months) and aged (16-18 months) CD-1 mice using a stable closed femoral fracture model. Bone healing was analyzed by radiographic, biomechanical and histological analysis at 1, 2, 3, 4 and 5 weeks after fracture. Our results demonstrated an increased callus diameter to femoral diameter ratio in aged animals at later time points of fracture healing when compared to young adult mice. Moreover, our biomechanical analysis revealed a significantly decreased bending stiffness at 3 and 4 weeks after fracture in aged animals. In contrast, at 5 weeks after fracture, the analysis showed no significant difference in bending stiffness between the two study groups. Additional histological analysis showed a delayed endochondral ossification in aged animals as well as a higher amounts of fibrous tissue at early healing time points. These findings indicate a delayed process of callus remodeling in aged CD-1 mice, resulting in a delayed fracture healing when compared to young adult animals. However, the overall healing capacity of the fractured femora was not affected by aging.

Phase Separation and Nematic Order in Lyotropic Solutions: Two Types of Polymers with Different Stiffnesses in a Common Solvent.

The interplay of the isotropic-nematic transition and phase separation in lyotropic solutions of two types of semiflexible macromolecules with pronounced difference in chain stiffness is studied by Density Functional Theory and Molecular Dynamics simulations. While the width of the isotropic-nematic two-phase coexistence region is narrow for solutions with a single type of semiflexible chain, the two-phase coexistence region widens for solutions containing two types of chains with rather disparate stiffness. In the nematic phase, both types of chains contribute to the nematic order, with intermediate values of the order parameter compared to the corresponding single component solutions. As the difference in bending stiffness is increased, the two chain types separate into two coexisting nematic phases. The phase behavior is rationalized by considering the chemical potentials of the two components and the Gibbs excess free energy. The geometric properties of the chain conformations under the various conditions are also discussed.

Curvature-Induced Spatial Ordering of Composition in Lipid Membranes.

Phase segregation of membranal components, such as proteins, lipids, and cholesterols, leads to the formation of aggregates or domains that are rich in specific constituents. This process is important in the interaction of the cell with its surroundings and in determining the cell's behavior and fate. Motivated by published experiments on curvature-modulated phase separation in lipid membranes, we formulate a mathematical model aiming at studying the spatial ordering of composition in a two-component biomembrane that is subjected to a prescribed (imposed) geometry. Based on this model, we identified key nondimensional quantities that govern the biomembrane response and performed numerical simulations to quantitatively explore their influence. We reproduce published experimental observations and extend them to surfaces with geometric features (imposed geometry) and lipid phases beyond those used in the experiments. In addition, we demonstrate the possibility for curvature-modulated phase separation above the critical temperature and propose a systematic procedure to determine which mechanism, the difference in bending stiffness or difference in spontaneous curvatures of the two phases, dominates the coupling between shape and composition.

Strain rate dependency of fractures of immature bone

Radiological features alone do not allow the discrimination between accidental paediatric long bone fractures or those sustained by child abuse. Therefore, there is a clinical need to elucidate the mechanisms behind each fracture to provide a forensic biomechanical tool for the vulnerable child. Four-point bending and torsional loading tests were conducted at more than one strain rate for the first time on immature bone, using a specimen-specific alignment system, to characterise structural behaviour at para-physiological strain rates. The bones behaved linearly to the point of fracture in all cases and transverse, oblique, and spiral fracture patterns were consistently reproduced. The results showed that there was a significant difference in bending stiffness between transverse and oblique fractures in four-point bending. For torsional loading, spiral fractures were produced in all cases with a significant difference in the energy and obliquity to fracture. Multiple or comminuted fractures were seen only in bones that failed at a higher stress or torque for both loading types. This demonstrates the differentiation of fracture patterns at different strain rates for the first time for immature bones, which may be used to match the case history given of a child and the fracture produced.

Comparative failure analysis of allograft constructs stabilized with compression plates utilizing unicortical versus bicortical screws with or without PMMA

Orthopaedic internal fixation is subject to high stress concentration around screw holes leading to screw loosening. We compared the screw pullout strength (n=8), and bending and torsional stiffness (n=6) for cadaveric (young adult donors age 45years or less) diaphyseal bone constructs with a gap and stabilized with Zimmer 4.5 narrow stainless steel dynamic compression plates with 4.5mm cortical stainless steel screws placed in a unicortical or bicortical manner with and without intramedullary PMMA. The pullout strength for unicortical fixation with PMMA (4.3±.6kN) was not significantly different with bicortical fixation with PMMA (5.3±1.5kN), n=8, P>.5; and for the group with bicortical fixation without PMMA (4.3±1.1kN), n=8, P>.5. The difference in bending stiffness for the group with two unicortical screws fixation with PMMA (324.1±59.7kN/m) was also not statistically significant when compared with the groups with four bicortical screws fixation without PMMA (278.7±89.6kN/m) and two bicortical screws fixation with PMMA (347.9±66.7kN/m), n=6, P>.1. The difference in torsional stiffness for the group with two unicortical screws fixation with PMMA (173±44Nm/deg) was not statistically significant when compared with the groups with four bicortical screws fixation without PMMA (190±22Nm/deg), two bicortical screws fixation with PMMA (179±36Nm/deg), and two bicortical screws fixation with PMMA (193±31Nm/deg), n=6, P>.5. The use of PMMA in allograft fixation appears to allow the use of fewer screws, and perhaps unicortical fixation without significant compromise of strength. Clinically, two unicortical screws with PMMA may replace the use of two bicortical screws without PMMA, or two bicortical screws with PMMA may replace four bicortical screws. The clinical relevance would be the ability to use fewer screws and reduce violation of the far cortex of the allograft.

Design and Compression Deformation Analysis of an Innovational Structure with Auxetic Effect

An innovational structure with auxetic effect was designed from inspiration of a re-entrant honeycomb structure firstly proposed by Gibson et.al. In the newly designed structure, yarns were used to replace the straight and tilted ribs of re-entrant honeycomb structure in order to achieve negative Poissons ratio. The compression testing was carried out in the vertical direction of the structure using an INSTRON 5566 tester to understand its deformation behavior. The results show that the difference in bending stiffness between the warp and weft yarns is the main reason for getting negative Poissons ratio of the structure.

Analysis and Comparison of the Biomechanical Properties of Univalved and Bivalved Cast Models

Fiberglass casts are frequently valved to relieve the pressure associated with upper extremity swelling after a surgical procedure or when applied after reduction of a displaced fracture in a child. Although different opinions exist regarding the valving of casts, no research to date has explored the biomechanical effects of this commonly used technique. As cast integrity is essential for the maintenance of fracture reduction, it is important to understand whether casts are structurally compromised after valving. Understanding the effects of valving on cast integrity may help guide clinicians in the technique of valving while minimizing the potential for a loss of fracture reduction. Thirty standardized cylindrical fiberglass cast models were created. Ten models were left intact, 10 were univalved, and 10 were bivalved. All the models were mechanically tested by a 3-point bending apparatus secured to a biaxial materials testing system. Load to failure and bending stiffness were recorded for each sample. Differences in load of failure and bending stiffness were compared among the groups. Unvalved cast models had the highest failure load and bending stiffness, whereas bivalved casts showed the lowest value for both failure load and bending stiffness. Univalved casts had a failure load measured to be between those of unvalved and bivalved cast models. Analysis of variance showed significance when failure load and bending stiffness data among all the groups were compared. A post hoc Bonferroni statistical analysis showed significance in bending stiffness between intact and bivalved models (P < 0.01), intact and univalved models (P < 0.01), but no significant difference in bending stiffness between univalved and bivalved models (P > 0.01). Differences in measured failure load values were found to be statistically significant among all cast models (P < 0.01). Valving significantly decreases the bending stiffness and load to failure of fiberglass casts. Univalved casts have a higher load to failure than bivalved casts. Valving adversely alters the structural integrity of fiberglass casts. This may impair a cast's ability to effectively immobilize an extremity or maintain a fracture reduction.

External Ring Fixation versus Screw Fixation for Ankle Arthrodesis: A Biomechanical Comparison

The use of external ring fixation for ankle arthrodesis offers an alternative to internal screw fixation when bone quality is suboptimal, when complex ankle pathology is present, and as a salvage procedure for complicated cases. Additionally, it permits earlier weightbearing and postoperative adjustment of bone alignment and joint compression. This study was designed to determine whether there are differences in initial ankle joint fixation integrity between these two techniques. Ten fresh-frozen cadaveric lower extremities underwent ankle arthrodesis with a hybrid ring fixator, and the bending and torsional stiffness of each arthrodesed joint was quantified. Stiffness was calculated from angulation measured at the tibiotalar joint using electronic clinometers attached to those bones. The external fixators were then removed and ankle arthrodesis performed using 6.5-mm cannulated screws: three across the ankle joint, one fixing the fibula to the tibia, and one fixing the fibula to the talus. Stiffness testing was then repeated. No statistically significant difference in bending stiffness, torsional stiffness, or joint rotation under torsional load was demonstrable between the two techniques. The similarity in ankle fusion site stability achieved with external ring fixation and an optimized screw fixation technique under the described testing conditions indicates that, in cases in which suboptimal bone quality or complex ankle pathology preclude the use of internal fixation, an external fixator is a viable option. Establishing that the primary stability of external ring fixation is comparable to that of screw fixation provides a basis for its use in cases in which screw fixation may be problematic.

Tibiotalocalcaneal Arthrodesis: A Biomechanical Comparision of Multiplanar External Fixation with Intramedullary Fixation

Methods of achieving tibiotalocalcaneal arthrodesis include intramedullary nailing, crossed lag screws, blade plates, and external fixation. While reports in the orthopaedic literature have compared the biomechanical properties of some of these fixation techniques, to our knowledge none has evaluated multiplanar external fixation. The purpose of this study was to compare the biomechanical properties of intramedullary nail fixation and external ring fixation for tibiotalocalcaneal arthrodesis. Tibiotalocalcaneal arthrodesis was performed on ten matched pairs of fresh-frozen human cadaveric legs. A ring fixator stabilized the arthrodesis in one leg from each pair and a 10 mm x 150 mm nail inserted retrograde across the subtalar and ankle joint stabilized the arthrodesis in the contralateral leg. The bending stiffness of the resulting constructs was quantified in plantarflexion, dorsiflexion, inversion, and eversion, and torsional stiffness was measured in internal and external rotation. No difference in bending stiffness between the two constructs was identifiable in any of the four bending directions (p > 0.05). Torsional stiffness was approximately two-fold greater in both internal and external rotation in specimens with the ring fixator arthrodesis than in those with the intramedullary nail (p = 0.002). The ring fixator provides a stiffer construct than a 10 mm x 150 mm intramedullary nail in torsion, but no difference in bending stiffness was demonstrable. Both techniques can provide satisfactory fixation; however, the ring fixator may better minimize rotational joint motion. This study provides a basis for selecting an arthrodesis method that offers optimized fixation.

Bending Stiffness and Pull-Out Strength of Tapered, Variable Pitch Screws, and 6.5-mm Cancellous Screws in Acute Jones Fractures

Stabilization of fifth metatarsal Jones fractures with intramedullary screws is popular, particularly in athletes, because nonoperative treatment involves prolonged casting and a distinct risk of nonunion or delayed union. Conventional lag screws of various diameters are routinely used for Jones fracture fixation. More recently, tapered, headless, variable pitch screws have become available as an option. These screws have the advantage of not having a protruding screw head, but information regarding their performance in Jones fracture fixation is limited. To determine whether differences exist in the mechanical integrity of fifth metatarsals fixed with each type of screw, this study was designed to compare Jones fracture fixation with 6.5-mm partially-threaded lag screws and headless, tapered, variable pitch compression screws with a 4-mm leading-thread diameter and 5-mm trailing-thread diameter. Simulated Jones fractures were created in 20 matched pairs of fresh-frozen fifth metatarsals. One bone from each pair was stabilized with a tapered, variable pitch screw, and the contralateral with a 6.5-mm partially-threaded cancellous lag screw. The stiffness in lateral-to-medial bending of the resulting constructs and the resistance of the screws to pulling out of the distal fragment were quantified. There was no demonstrable difference in bending stiffness between metatarsals fixed with the two types of screws (p = 0.688). The 6.5-mm screw provided significantly higher resistance to pull-out (p = 0.001). Headless, tapered, variable pitch compression screws of the size tested are not entirely comparable to 6.5-mm lag screws in this application. They are effective in resisting bending but do not offer equivalent resistance to thread pull-out.

A Biomechanical Comparison of Composite Fixation and Veterinary Acetabular Plates for Stabilization of Acetabular Osteotomies in Dogs Loaded in Three-Point-Bending Fashion

SummaryAn in vitro study comparing the biomechanical performance and accuracy of reduction for two methods of acetabular osteotomy repair in the dog is presented. Pelves were harvested from five adult mixed breed dogs weighing 25-30 kg. On e hemipelvis from each dog was repaired with the composite fixation and the contralateral hemipelvis was repaired with a 2.0 mm six hole veterinary acetabular plate. Eac h hemipelvis was loaded in a threepoint- bending fashion at a rate of 5 mm/min. An extensometer was placed over the dorso-medial surface of the acetabular osteotomy during loading. Reduction was significantly better for hemipelves stabilized with the composite fixation when compared to hemipelves stabilized with the 2.0 mm six hole veterinary acetabular plate. There was not a significant difference in bending stiffness, distractional stiffness, yield point and maximum load between repair groups.An in vitro comparison was made between a composite fixation method and acetabular plates for the stabilisation of acetabular osteotomies.