Takamasa Yoshikawa

A biomechanical comparison between cortical bone trajectory fixation and pedicle screw fixation

PurposeThere have been several reports on the pullout strength of cortical bone trajectory (CBT) screws, but only one study has reviewed the stability of functional spine units using the CBT method. The purpose of this study was to compare vertebral stability after CBT fixation with that after pedicle screw (PS) fixation.MethodsIn this study, 20 lumbar spine (L5–6) specimens were assigned to two groups: the CBT model group that underwent CBT screw fixation (n = 10) and the PS model group that underwent pedicle screw fixation (n = 10). Using a six-axis material testing machine, bend and rotation tests were conducted on each model. The angular displacement from the time of no load to the time of maximum torque was defined as range of motion (ROM), and then, the mean ROM in the bend and rotation tests and the mean rate of relative change of ROM in both the bend and rotation tests were compared between the CBT and PS groups.ResultsThere were no significant differences between the CBT and PS groups with regard to the mean ROMs and the mean rate of relative change of ROMs in both the bend and rotation tests.ConclusionIntervertebral stability after CBT fixation was similar to that after PS fixation.

Are Deer and Boar Spines a Valid Biomechanical Model for Human Spines

Objective: To examine the validity of using cadaveric spines of deer or boars for biomechanical experiments as substitutes for the cadaveric spine of humans. Materials and Methods: Five specimens of the L3-4 functional spinal unit of human cadavers, mature deer and mature boars were prepared according to 3 models: 1) normal model, 2) injured model and 3) pedicle screw fixation model and they were evaluated in 8-direction bending and 2-direction rotation tests. The mean ROM in bending and rotation tests of each specimen and the rate of relative change of ROM were calculated. Results: Flexibility of cadaveric spine of deer and boars was slightly higher than that of cadaveric spine of humans in the bending and rotation tests, but the rates of relative change of ROM in the rotational and bending tests were similar across species. Conclusions: It is reasonable to use cadaveric spines of deer and boars as a model of the human cadaveric spine in biomechanical experiments.

Do the Position and Orientation of the Crosslink Influence the Stiffness of Spinal Instrumentation

Study Design: Biomechanical study of double-level pedicle screw constructs with or without crosslinks (CL) in an unstable model. Objectives: The purpose of this study is to investigate the optimal position and orientation of the CL. Summary of Background Data: Several reports have described biomechanical research on such CL, but no definite consensus has been reached regarding the effects. Very few studies have examined the position and orientation of the CL. The question of where and how the CL should be clinically set remains unanswered. Methods: Ten cadaveric lumbar spines (L3–L5) of boars were used and 7 models were prepared by the sequential damage and spinal instrumentation of each specimen. Bending stiffness was measured in flexion, extension, lateral bending, and axial rotation for each model using 6-axis material tester under torque of 0 to ±3 N m. Results for each configuration were compared using analysis of variance and the Turkey-Kramer test. Results: In flexion, extension, and lateral bending, 7 models showed similar stiffness with no significant differences. In axial rotation, stiffness increased significantly (P<0.05) in the cephalic, central, caudal, and oblique CL models in comparison with that of the no CL model, and stiffness of the horizontal 2 CL and oblique 2 CL models was significantly higher than that of cephalic, central, caudal, and oblique CL models (P<0.05). However, no significant differences in stiffness were seen between cephalic, central, and caudal CL models, between the central and oblique CL models, or between the horizontal and oblique 2 CL models. Conclusions: Concomitant use of CLs significantly increased axial rotational stiffness, even though stiffness in flexion, extension, and lateral bending was not increased. In addition, stiffness in axial rotation significantly improved with the use of 2 CLs instead of a single CL, and stiffness was unchanged by position and orientation of CL.

Experimental Study of Critical Stresses of Fe-28Mn-6Si-5Cr SMA Under Various Temperature Conditions

The influence of environmental temperature on the critical stresses of Fe-28Mn-5Cr-6Si SMA under various loading conditions was experimentally investigated. From the experimental results, the stress-induced martensitic transformation of this material occurs before the yielding under uni-axial tensile, compressive, and simple torsional conditions below 135 °C. These critical stresses of this material reverse each other above 135 °C. Therefore the deformation below 135 °C is more suitable for utilizing the shape memory effect or the recovery stress and the loading above this temperature can easily realize the plastic working of this material.

Trajectory of instantaneous axis of rotation in fixed lumbar spine with instrumentation

BackgroundSeveral studies showed instantaneous axis of rotation (IAR) in the intact spine. However, there has been no report on the trajectory of the IAR of a damaged spine or that of a fixed spine with instrumentation. It is the aim of this study to investigate the trajectory of the IAR of the lumbar spine using the vertebra of deer.MethodsFunctional spinal units (L5–6) from five deer were evaluated with six-axis material testing machine. As specimen models, we prepared a normal model, a damaged model, and a pedicle screw (PS) model. We measured the IAR during bending in the coronal and sagittal planes and axial rotation. In the bending test, four directions were measured: anterior, posterior, right, and left. In the rotation test, two directions were measured: right and left.ResultsThe IAR of the normal model during bending moved in the bending direction. The IAR of the damaged model during bending moved in the bending direction, but the magnitude of displacement was bigger compared to that of the normal model. In the PS model, the IAR during bending test hardly moved. During rotation test, the IAR of the normal model and PS model located in the spinal canal, but the IAR of the damaged model located in the posterior part of the vertebral body.ConclusionsIn this study, the IAR of damaged model was scattering and that of PS model was concentrating. This suggests that higher mechanical load applied to the dura tube and nerve roots in the damaged model and less mechanical load applied to that in the PS model.

124 Influence of Temperature for Yield Surface of AZ31 Magnesium Alloy with Anisotropy

1.緒言 省エネルギー対策のために産業機械や輸送機器といった 機械の軽量化が求められる現代において,軽金属材料が注目 されている.軽金属材料の中でも,実用金属中最も比重が小 さく,高い比強度を有しているマグネシウム合金の機械構造 用材料への応用が期待されている. 最密六方格子の結晶構造をもつマグネシウム合金には底 面すべり,非底面すべりおよび変形双晶といった複数の変形 機構が存在し,それぞれ臨界分解せん断応力(以下 CRSS) の大きさが異なる.また常温において塑性ひずみが小さいマ グネシウム合金は,押出しや圧延といった素材成形時に発生 する著しい集合組織によって降伏曲面上に異方性があらわ れる.したがって,マグネシウム合金は塑性加工の制御が困 難である.すなわち,異方性を有するマグネシウム合金の塑 性加工を制御する上で,集合組織による降伏曲面の異方性を 明らかにすることは重要である. マグネシウム合金にみとめられる複数の変形機構は,それ ぞれ CRSS の大きさや温度依存性が異なる.そのため,環境 温度と負荷条件によって降伏応力と変形機構が異なる.すな わち,集合組織の影響下にあるマグネシウム合金押出し材の 降伏曲面が,環境温度によって変化するものと予測される. そこで本研究では,顕著な集合組織をもつ AZ31 マグネシウ ム合金押出し材の,降伏曲面に対する環境温度の影響を明ら かにすることを目的とした.

EXPERIMENTAL STUDY ON DEFORMATION MECHANISM OF AZ31 MAGNESIUM ALLOY UNDER VARIOUS TEMPERATURE CONDITIONS

The objective of this study was to experimentally determine the properties of magnesium alloys during plastic deformation under various temperature regimes. Proportional loading tests combining both axial and torsional loads were performed on cylindrical AZ31 Mg alloys in a range from room temperature to 673 K, where superplasticity has a high probability of occurrence, and the crystalline structures of the samples were observed after these tests. The above observations indicate that the deformation mechanisms in AZ31 Mg alloy are dominated by twinning deformation at 300 K and 423 K, and by grain boundary sliding promoted by dynamic recrystallization at 523 K and higher temperatures.

Mechanical Properties of Bulk Glassy Metal after Deformation under High Temperature Conditions

Bulk glassy metal is an alloy with the vitreous amorphous structure. Because of various excellent properties, this material is expected to use as an alternative structural material for several engineering applications very well. Although bulk glassy metal is very little deformed plastically in the room temperature, it shows the huge super-plastic behavior over the high temperature. However, there is not many reports mentioned about the mechanical properties of bulk glassy metal after plastic deformation under high temperature condition. From the above point of view, in this study, we have investigated the lower bound of temperature at which Zr55Cu30Al10Ni5 bulk glassy metal can be plastically deformed in uniaxial tensile load. Furthermore, it is focused on the strength property of bulk glassy metal in the room temperature after deformed under various high-temperature conditions. In the experimental result, when this material was heated at temperature of 685[K] or higher, this material crystallized and the mechanical strength in room temperature drastically decreased to 200[MPa], although this material as cast had the strength over 1500[MPa]. However, this material showed sufficiently the plastic deformation at temperatures of 643[K] and the strength in room temperature after cooling was equal to as cast. It is supposed that the strength depend on its atomic structure, i.e., amorphous or crystalline, and the change of its structure is affected strongly by heating process.