Influence of low dissolved oxygen concentration in body fluid on corrosion fatigue behaviors of implant metals

In their previous study, the authors carried out a fatigue test for AISI 316, 316L stainless steels and COP¹ alloy in a living animal body and observed a remarkable deterioration in the fatigue durability of these metals. In that study, it was concluded that the reason the corrosion resistance of the metals was reduced in the living body was that the low concentration of dissolved oxygen gas in the body fluid (the partial pressure pO₂; 28–78 mmHg) was insufficient to form the chromium oxide passivation film on the metal surface, and the base metal (iron) was released into the environmental fluid in ionic form. In this paper, with the concentration of dissolved oxygen gas in a physiological normal saline solution being set equivalent to that of living body fluid, fatigue tests on AISI 316 were made to simulate the stress corrosion behavior of the metal in the living body. As a result, remarkable deterioration of fatigue strength was observed in the low O₂ concentrated normal saline solution, which was the same as that in the living animal body.     

Electrochemistry of galvanic couples between carbon and common metallic biomaterials in the presence of crevices

In vitro experiments were conducted upon some common metallic biomaterials and carbons, both isolated or forming galvanic couples, in a cell specially designed for crevice corrosion studies. The alloys examined were AISI 316L stainless steel, Ti6AI4V and Co-Cr-Mo. The types of carbon were glassy carbon and carbon fibre-reinforced carbon. The surface modifications were evaluated by SEM, AES and ESCA-XPS analyses. AISI 316L stainless steel suffered localized corrosion in open-circuit experiments whilst the other materials remained unattacked. Galvanic currents between metal-carbon couples were measured by zero resistance ammetry. The carbon-metal area ratio was 1:1. The results showed that 316L stainless steel and the Co-Cr-Mo alloy were prone to accelerated corrosion, whilst the Ti6AI4V alloy remained unattacked. The galvanic corrosion currents were also predicted using mixed potential theory from polarization curves obtained for each material. The experimental and theoretical values showed good agreement for the stainless steel and Co-Cr-Mo alloy. Long-term immersion tests with the same couples showed that the only metal not to suffer degradation was the Ti6AI4V alloy.     

In vivo human electrochemical properties of a NiTi-based alloy (Nitinol) used for minimally invasive implants

The development of a homemade device for in vivo human determination of the open circuit potential (OCP) of Nitinol is described. Pseudo-reference electrodes (316L stainless steel and Pt) were initially tested and validated in vitro using simulated body fluids. As judged from the excellent electrochemical responses in terms of both accuracy and precision, the most ideal system comprised the combination of sterilized Pt (pseudo-reference) and Nitinol (working) needle-shaped electrodes. The average in vivo human OCP determined from independent measurements on six human patients with indication of direct arterial surgery was -0.334 +/- 0.030 V/SCE. This value was in good agreement with data recorded in vitro using simulated body fluids (-0.313 +/- 0.003 V/SCE in AFNOR S90-701 artificial saliva; -0.334 +/- 0.001 V/SCE in artificial urine; -0.239 +/- 0.007 V/SCE in Ringer's solution). The thin surface film protecting the bulk NiTi alloy is therefore not susceptible to active dissolution at rest as long as the break down potentials (>0.0 V/SCE) so far reported are well above OCP measured in this study. These results highlight the importance of evaluating the corrosion resistance of Nitinol under realistic conditions (mechanical loads, wear and fatigue) in order to establish multifaceted mechanisms that might lead to accelerated dissolution and failure of implanted stents.     

Effects of cyclic bending and physiological solution on plasma-sprayed hydroxylapatite coatings of varying crystallinity.

This study investigated the effects of cyclic bending stress levels and testing in simulated physiological solutions or air on the integrity of plasma-sprayed hydroxylapatite (HA) coatings of two different crystallinities. Hydroxylapatite-coated commercially pure (CP) Ti rods were evaluated by immersion testing in Hank's Balanced Salt Solution (HBSS) and by rotating bending in air and HBSS. Static immersion testing of nonstressed specimens resulted in significant microcracking of coating surfaces after 42 days. Specimens cyclically tested at bending stresses above the yield strength of Ti experienced low cycle fatigue failure of the Ti substrates prior to spallation of the HA coatings. Coatings tested at 1 x 10(6) cycles with interface bending stresses of 180 MPa displayed increased surface microcracking, but no bulk coating spallation. Coatings cycled in HBSS displayed greater amounts of microcracking and surface alteration than samples cycled in air. There was no apparent relation between HA crystallinity and mechanical integrity under cyclic bending stresses.     

基于ABAQUS及疲劳试验的烧结股骨柄设计研究

目的确保精锻Ti-6Al-4V钛合金烧结股骨柄柄体能达到植入后支撑人体体重及运动循环载荷的要求。方法通过对Worst Case下的烧结股骨柄进行数值模拟,设计3种烧结涂层区域不同(不同的应力规避区)的股骨柄,并对其分别进行疲劳试验。结果通过数值模拟,Worst Case下的烧结股骨柄的最大主应力为432.8 MPa;方案一中股骨柄外侧烧结涂层至股骨柄肩部的距离为47 mm(规避处的应力值为309.493 MPa),疲劳试验结果为6组全部断裂,循环载荷次数均为2.3×10~6次上下;方案二中股骨柄外侧烧结涂层至股骨柄肩部的距离为44 mm(规避处的应力值为277.673 MPa),疲劳试验结果为2组断裂,循环载荷次数均为4.1×10~6次上下,其余4组通过5×10~6次循环试验;方案三中股骨柄外侧烧结涂层至股骨柄肩部的距离为41 mm(规避处的应力值为240.755MPa),6组均通过5×10~6次循环试验,股骨球头垂直偏移量最大的一组的偏移值为1.55 mm。结论方案三的精锻Ti-6Al-4V钛合金烧结股骨柄在植入后能达到支撑人体体重和运动时循环载荷的要求。     

医用钛合金模拟体液预浸后的疲劳性能及其数值模拟

目的 研究新型低弹模Ti-3Zr-2Sn-3Mo-25Nb（TLM）钛合金在模拟体液（simulated body fluid,SBF）干预下的腐蚀疲劳性能。方法 以Ti-6Al-4V（TC4）钛合金为对照组,测定两种钛合金在SBF中的电化学腐蚀极化曲线,并对预腐蚀后的TC4钛合金和TLM钛合金试样进行旋转弯曲疲劳试验,利用实验数据建立加载应力幅与疲劳断裂循环次数之间的关系,绘制应力-寿命曲线,通过分析试样的腐蚀疲劳微观断口形貌分析其断裂机制,并结合有限元软件对钛合金试件进行疲劳分析。结果 应力退火状态下TC4钛合金自腐蚀电位低于热处理后的TLM钛合金,而TLM钛合金对循环应力的变化更为敏感。对比仿真结果与试验结果显示,应力退火状态下的TC4钛合金与热处理后的TLM钛合金相比疲劳强度更高,抵抗裂纹扩展的能力更强,而耐腐蚀性能反之。相对于未预腐蚀处理试件,SBF预浸泡后的TLM钛合金脆性增加,疲劳性能有所降低。结论 通过对比分析,说明试验结果可靠性高,COMSOL有限元软件能够很好预测钛合金材料的疲劳寿命。     

The electrochemical evaluation of a Zr-based bulk metallic glass in a phosphate-buffered saline electrolyte

Bulk metallic glasses (BMGs) represent an emerging class of materials with an amorphous structure and a unique combination of properties. The objectives of this investigation were to define the electrochemical behavior of a specific Zr-based BMG alloy in a physiologically relevant environment and to compare these properties to standard, crystalline biomaterials as well as other Zr-based BMG compositions. Cyclic-anodic-polarization studies were conducted with a Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 (at %) BMG in a phosphate-buffered saline electrolyte with a physiologically relevant oxygen content at 37 degrees C. The results were compared to three common, crystalline biomaterials: CoCrMo, 316L stainless steel, and Ti-6Al-4V. The BMG alloy was found to have a lower corrosion penetration rate (CPR), as compared to the 316L stainless steel, and an equivalent CPR, as compared to the CoCrMo and Ti-6Al-4V alloys. Furthermore, the BMG alloy demonstrated better localized corrosion resistance than the 316L stainless steel. However, the localized corrosion resistance of the BMG alloy was not as high as those of the CoCrMo and Ti-6Al-4V alloys in the tested environment. The excellent electrochemical properties demonstrated by the BMG alloy are combined with a low modulus and unparalleled strength. This unique combination of properties dramatically demonstrates the potential for amorphous alloys as a new generation of biomaterials.     

Corrosion fatigue behaviors of two biomedical Mg alloys – AZ91D and WE43 – In simulated body fluid

Magnesium alloys have been recently developed as biodegradable implant materials, yet there has been no study concerning their corrosion fatigue properties under cyclic loading. In this study the die-cast AZ91D (A for aluminum 9%, Z for zinc 1% and D for a fourth phase) and extruded WE43 (W for yttrium 4%, E for rare earth mischmetal 3%) alloys were chosen to evaluate their fatigue and corrosion fatigue behaviors in simulated body fluid (SBF). The die-cast AZ91D alloy indicated a fatigue limit of 50 MPa at 10⁷ cycles in air compared to 20 MPa at 10⁶ cycles tested in SBF at 37 °C. A fatigue limit of 110 MPa at 10⁷ cycles in air was observed for extruded WE43 alloy compared to 40 MPa at 10⁷ cycles tested in SBF at 37 °C. The fatigue cracks initiated from the micropores when tested in air and from corrosion pits when tested in SBF, respectively. The overload zone of the extruded WE43 alloy exhibited a ductile fracture mode with deep dimples, in comparison to a brittle fracture mode for the die-cast AZ91D. The corrosion rate of the two experimental alloys increased under cyclic loading compared to that in the static immersion test.     

Deep rolling of titanium rods for application in modular total hip arthroplasty

Compressive residual stresses are commonly introduced into the near-surface regions of morse taper junctions of modular hip endoprostheses to prolong fatigue life. An increasing number of publications report that contamination of shot-peened surfaces can lead to enhanced corrosion and third body wear. This study evaluates deep rolling of titanium alloy rods as a possible alternative to shot peening. Ten rods of Ti6Al7Nb alloy with a diameter of 15 mm were deep rolled with various rolling parameters. The resulting surface topography and residual contamination was analyzed using a scanning electron microscope (SEM). The near-surface residual stress states after deep rolling were characterized by means of X-ray diffraction. The roughness of the surfaces before deep rolling was about R(z) = 14 microm, and after deep rolling surface roughness values of R(z) 0.4-7.5 microm were achieved. The results of the SEM and EDAX analyses of the sample surface showed no evidence of surface contamination by particles or abrasion products caused by any process. At a pressure of 300 bar, compressive stress reached the maximum of -1150 MPa at a depth of 0.1 mm. Deep rolling thus allows a smooth and particle-free surface to be obtained and therefore shows promise as a surface treatment for mating surfaces of morse tapers in modular hip endoprostheses.     

The influences of electrical potential and surface finish on the fatigue life of surgical implant materials.

The influence of both imposed anodic potential in Ringer's solution and surface finish on the fatigue lives of annealed 316 type stainless steel and annealed pure titanium were measured and statistically compared to fatigue data run in air. The applied potentials in simulated extracellular fluid approximated conditions existing within the body while also producing the types of surface defects actually found on removed long time implants within the time interval of the accelerated R. R. Moor fatigue tests. Differentiating tests were run at single levels of applied cyclic stress well above the endurance limits. In Ringer's solution, the fatigue life of the 316 stainless steel decreased with increasing applied potential, and at +500 mV was significantly shorter than when run in air. At each condition, the 316 stainless steel was independent of initial surface finish. In contrast, the fatigue life of titanium improved rapidly with increasingly fine surface finishes. Furthermore, compared to air, the application of +500 mV in Ringer's solution improved the life of the rough surface finished material and markedly increased to number of cycles to failure for the electopolished specimens.     

聚-L-乳酸/β-磷酸三钙复合物生物可吸收骨折内固定棒的体内降解和组织相容性

将β-磷酸三钙(β-TCP)与聚-L-乳酸(PLLA)复合得到PLLA/β-TCP复合材料,用注射成型方法制备出可吸收骨折内固定棒,然后通过分子量、质量变化、扫描电镜观察、弯曲强度变化和组织学方法等研究可吸收骨折内固定棒的体内降解过程。结果表明,降解初期聚乳酸的分子量有大幅度下降,重量损失滞后。随着降解的进行,可吸收棒表面逐渐粗糙,内部逐渐出现微孔和小“沟壑”,弯曲强度从初始的151MPa下降至12周的106MPa。组织学分析显示,PLLA/β-TCP复合材料具有良好的组织相容性。     

Research on an Mg–Zn alloy as a degradable biomaterial

In this study a binary Mg–Zn magnesium alloy was researched as a degradable biomedical material. An Mg–Zn alloy fabricated with high-purity raw materials and using a clean melting process had very low levels of impurities. After solid solution treatment and hot working the grain size of the Mg–Zn alloy was finer and a uniform single phase was gained. The mechanical properties of this Mg–Zn alloy were suitable for implant applications, i.e. the tensile strength and elongation achieved were ～279.5 MPa and 18.8%, respectively.The results of in vitro degradation experiments including electrochemical measurements and immersion tests revealed that the zinc could elevate the corrosion potential of Mg in simulated body fluid (SBF) and reduce the degradation rate. The corrosion products on the surface of Mg–Zn were hydroxyapatite (HA) and other Mg/Ca phosphates in SBF. In addition, the influence caused by in vitro degradation on mechanical properties was studied, and the results showed that the bending strength of Mg–Zn alloy dropped sharply in the earlier stage of degradation, while smoothly during the later period.The in vitro cytotoxicity of Mg–Zn was examined. The result 0–1 grade revealed that the Mg–Zn alloy was harmless to L-929 cells. For in vivo experiments, Mg–Zn rods were implanted into the femoral shaft of rabbits. The radiographs illustrated that the magnesium alloy could be gradually absorbed in vivo at about 2.32 mm/yr degradation rate obtained by weight loss method. Hematoxylin and eosin (HE) stained section around Mg–Zn rods suggested that there were newly formed bone surrounding the implant.HE stained tissue (containing heart, liver, kidney and spleen tissues) and the biochemical measurements, including serum magnesium, serum creatinine (CREA), blood urea nitrogen (BUN), glutamic-pyruvic transaminase (GPT) and creatine kinase (CK) proved that the in vivo degradation of Mg–Zn did not harm the important organs. Moreover, no adverse effects of hydrogen generated by degradation had been observed and also no negative effects caused by the release of zinc were detected. These results suggested that the novel Mg–Zn binary alloy had good biocompatibility in vivo.     

新型镁合金材料的特性及临床应用

镁是人体必需的常量元素。镁合金具有与人体正常骨组织的密度和弹性模量相接近、可降解性、良好的生物相容性及一定的机械强度等特点,近年来在医学领域作为新型可降解金属生物材料引起了广泛的关注,目前临床上主要探索了用作骨科处置所用的内置物材料及心血管支架材料等。新型镁合金由于其降解速度较慢,可在较长时间保持其完整性,并且表现出较好的机械强度及促进骨新生的作用,具有良好的应用前景。     

In vitro and in vivo studies on bioabsorbable ultra-high-strength poly(L-lactide) rods.

Ultra-high-strength poly(L-lactide) (PLLA) rods were fabricated using a drawing technique. Rods with a diameter of 3.2 mm and a draw ratio of 2.5:1 showed initial bending strength and modulus values of 240 MPa and 13 GPa, respectively. The purpose of this study was to investigate the in vitro and in vivo degradation of PLLA rods with a draw ratio of 2.5:1. The greater the rod diameter, the longer the bending strength was maintained in phosphate buffered saline at 37 degrees C. The bending strength retention of rods (diam. 3.2 mm) implanted in the subcutis of rabbits was almost equal to that of rods in the in vitro study, while those rods implanted in the medullary cavity of rabbit femora showed a slightly lower bending strength retention. Molecular weight was reduced to the greatest extent in the medullary cavity, followed by in the subcutis and in vitro. The weight of PLLA rods in the medullary cavity was reduced by 22% at 52 weeks and by 70% at 78 weeks after implantation. Histologically, no inflammatory or foreign body reaction was observed in the medullary cavity for 52 weeks. The drawn PLLA rods maintained a bending strength exceeding that of human cortical bone in the medullary canal for a period of 8 weeks, suggesting that the drawn PLLA rods may be useful in the repair of fractured human bones.     

Strength and strength retention vitro,of absorbable,self-reinforced polyglycolide (PGA) rods for fracture fixation

The initial shear strength and changes in flexural strength of self-reinforced, absorbable polyglycolide (PGA) composite rods, submerged in distilled water (at 37°C) for a period of 6 wk, were investigated. The recently developed self-reinforced absorbable material consists of an absorbable polymeric matrix reinforced with fibres of the same polymer.The initial shear strength of self-reinforced cylindrical PGA rods with a diameter of 3.2 mm was 250 MPa and the initial flexural strength of the rods was 370 MPa. During the first week of immersion the level of flexural strength decreased very little i.e. to 320 MPa. The loss of flexural strength increased after 1 wk immersion. However, after 3 wk it was 90 MPa. After 5 wk the flexural strength decreased to the level of strength of cancellous bone i.e. 10–20 MPa.The γ-irradiation of the PGA rods (total dosage 2.5 Mrad) decreased the initial bending strength to 300 MPa but the hydrolytic behaviour of the rods was not changed.The in vitro strength and the strength retention of self-reinforced PGA rods are clearly better than the corresponding values for self-reinforced glycolide/lactide copolymer rods which we developed recently. Self-reinforced PGA rods are now used routinely in Helsinki University Central Hospital in the treatment of certain types of cancellous bone fracture.     

Tensile characteristics of ten commercial acrylic bone cements

The mechanical properties of acrylic bone cement, used in orthopedic surgery, are very influential in determining successful long-term stability of a prosthesis. A large number of commercial formulations are available, differing in chemical composition and physical properties of both powder and monomer constituents. In this study, the static and dynamic tensile characteristics of a number of the most commonly used bone cements (Palacos R, Simplex P, CMW 1 & 3, Sulfix-60, Zimmer Dough), along with some newer formulations (Endurance, Duracem 3, Osteobondtrade mark and Boneloc), have been investigated under the same testing regimes. Testing was performed in air at room temperature. Significant differences in both static and fatigue properties were found between the various bone cements. Tensile tests revealed that Palacos R, Sulfix-60, and Simplex P had the highest values of ultimate tensile strength, closely followed by CMW 3, while Zimmer Dough cement had the lowest strength. Fatigue testing was performed under stress control, using sinusoidal loading in tension-tension, with an upper stress level of 22MPa. The two outstanding cements when tested in these cyclic conditions were Simplex P and Palacos R, with the highest values of Weibull median cycles to failure. Boneloc bone cement demonstrated the lowest cycles to failure. While the testing regimes were not designed to replicate exact conditions experienced by the bone cement mantle in vivo, there was a correlation between these results and clinical outcome.     

颈椎前路微型记忆加压合金板的有限元分析

目的 评估微型记忆加压合金板在前路颈椎椎间盘切除融合术中置入后的生物力学性能,为该内植物的临床应用、术后护理及后续实验研究与改进提供依据。 方法 随机选择一名健康成年女性志愿者,建立C5、C6颈椎-微型记忆加压合金板三维有限元模型,对各工况下内植物上应力集中区域的应力分布情况进行比较分析。 结果 中立位时,该内植物上分布的应力相对较为均匀且应力值较小;前屈状态时,各个区域的平均应力相对其余工况均为最大,最大平均应力位于区域6即下钩部分,为（45.89±5.32）MPa;后伸状态时,区域1、2、3、4即加压部件的应力分布较为均匀（P>0.05）;侧屈工况下,该内植物整体应力分布不均,屈侧平均应力远大于伸侧（P<0.05）;旋转工况下,应力在旋转对侧上部与同侧下部较为集中,左旋转时分别为（23.66±6.24）MPa和（23.62±7.07）MPa,右旋转时分别为（24.16±5.42）MPa和（24.58±5.30）MPa。除中立位外,在其余六种工况下,置入椎体内部的上下钩固定部分受到的应力均较大。 结论 颈前植入该微型记忆加压合金板后,应减少做前屈动作,避免过于频繁或过于激烈的侧屈、旋转动作,保持中立位最为稳妥。在后续研究及改良过程中,可考虑加强内植物上下钩的固定强度。     

Austenitic and duplex stainless steels in simulated physiological solution characterized by electrochemical and X-ray photoelectron spectroscopy studies

A study of oxide layers grown on 2205 duplex stainless steel (DSS) and AISI 316L austenitic stainless steel in simulated physiological solution is presented here in order to establish the possibility of replacement of AISI 316 L with 2205 DSS in biomedical applications. The results of the potentiodynamic measurements show that the extent of the passive range significantly increased for DSS 2205 compared to AISI 316L stainless steel. Cyclic voltammetry was used to investigate electrochemical processes taking place on the steel surfaces. Oxide layers formed by electrochemical oxidation at different oxidation potentials were studied by X-ray photoelectron spectroscopy, and their compositions were analyzed as a function of depth. The main constituents on both the investigated materials were Cr- and Fe-oxides. Atomic force microscopy topography studies revealed the higher corrosion resistance of the DSS 2205 compared to the AISI 316L under the chosen experimental conditions.     

The biocompatibility of materials for internal fixation of fractures

Surgically produced fractures of rabbit tibiae were internally stabilized with intramedullary rods of stainless steel (316LVM), titanium (6A1,4V), polyacetal (Delrin), and polyamide (Nylon 101). Periodic radiographs were taken until sacrifice at 16 weeks after fracture. Structural properties of the tibiae were determined in torsion with the rods in situ, and then the tissue was prepared for histology or microradiography. The results demonstrated that fracture remodeling was adversely affected by the metal rods. New bone was seen to have formed over the ends of the metal rods, and cortical bone resorption was observed in the fracture region, suggestive of transfer of mechanical stress to the rods, resulting in stress shielding of the diaphysis. Negligible osseous response to the polymeric rods was observed; fracture callus remodeling was extensive. The torsional test results demonstrated that fractures with polymeric rods were significantly stronger and tougher than those with metallic rods. With the exception of titanium, the strength of healed fractures was inversely related to the elastic moduli of the implant materials.     

Fracture toughness, strength and slow crack growth in a ceria stabilized zirconia-alumina nanocomposite for medical applications

Mechanical properties and slow crack growth (SCG) behavior of a 10Ce-TZP/Al2O3 nanocomposite currently developed as a biomaterial are considered. Fracture toughness is determined for sharp, long (double torsion) and short (indentation) cracks and a good agreement is found between the two types of cracks. The main toughening mechanism in the nanocomposite is the tetragonal to monoclinic phase transformation of the ceria-stabilized zirconia (Ce-TZP) phase. Transformation at the surface of ground specimens leads to surface compressive induced stresses and an increase in strength. Crack velocity curves (V-K(I) curves) are obtained under static and cyclic fatigue using the double torsion method. The static V-K(I) curve in air reveals the three stages characteristic of stress corrosion with a threshold K(I0) approximately 4.5 MPa m(1/2) and a fracture toughness of 8.8 MPa m(1/2) significantly higher than those of currently used inert bioceramics (i.e., alumina and Y-TZP). A crack growth accelerating effect is shown under cyclic loading, correlated with a decrease in the threshold. However, the cyclic fatigue threshold (4 MPa m(1/2)) still stands above that of current biomedical grade alumina and zirconia.