J Mater Sci: Mater Med DOI 10.1007/s10856-013-5102-3
In vivo evaluation of Zr-based bulk metallic glass alloy intramedullary nails in rat femora Kazuhiro Imai • Sachiko Hiromoto
Received: 29 August 2013 / Accepted: 18 November 2013 Ó Springer Science+Business Media New York 2013
Abstract Zr-based bulk metallic glasses (BMG) show high corrosion resistance in vitro and higher strength and lower Young’s modulus than crystalline alloys with the similar composition. This study aimed to perform an in vivo evaluation of Zr65Al7.5Ni10Cu17.5 BMG. Osteotomy of the femur was done in rats and stabilized with intramedullary nails made of Zr65Al7.5Ni10Cu17.5 BMG, Ti– 6Al–4V alloy, or 316L stainless steel. Systemic and local effects of each type of nail were evaluated by measuring the levels of Cu and Ni in the blood and the surrounding soft tissue. Changes of the surface of each nail were examined by scanning electron microscopy (SEM). Healing of the osteotomy was evaluated by peripheral quantitative computed tomography and mechanical testing. No increase of Cu and Ni levels was recognized. Surface of the BMG showed no noticeable change, while Ti–6Al–4V alloy showed Ca and P deposition and 316L stainless steel showed surface irregularities and pitting by SEM observation. The stress strain index, maximum torque, torsional stiffness, and energy absorption values were larger for the BMG than those for Ti–6Al–4V alloy, although there was no significant difference. The Zr-based BMG can promote osteotomy healing as fast as Ti–6Al–4V alloy, with the possible advantage of the Zr-based BMG that bone bonding is less likely, allowing easier nail removal compared with
K. Imai (&) Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan e-mail: [email protected] S. Hiromoto Biomaterials Unit, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan
Ti–6Al–4V alloy. The Zr-based BMG is promising for the use in osteosynthetic devices that are eventually removed.
1 Introduction Metallic implants used for fracture management need to be nontoxic, resistant to corrosion, durable, strong, as well as having low Young’s modulus and good biocompatibility. Ti–6Al–4V alloys and 316L stainless steel are commonly used for osteosynthetic devices such as plates, intramedullary nails, and screws. When such materials are implanted into patients to treat fractures, they are subjected to repeated load by daily activities, and are directly exposed to body fluids that are highly corrosive to metallic substances. Under such conditions, these implants are sometimes attacked by corrosion or fractured by corrosion fatigue and fretting corrosion fatigue [1]. To prevent failure, bone plates have been made quite bulky and a large incision is necessary to implant such plates, leading to occasional difficulty in closing the wound. Likewise, intramedullary nails need to have a large diameter to prevent failure. However, such nails interfere with the blood supply by occupying a large area of the medullary canal, which may lead to disturbance of healing. Therefore, one of the disadvantages of the current metallic osteosynthetic devices is their insufficient strength. Another disadvantage is an excessively high Young’s modulus. The Young’s modulus of currently available metallic devices is around 100 GPa and is much higher than that of cortical bone (15–20 GPa). This sometimes leads to stress shielding and absorption of the bone around osteosynthetic devices made of these materials [2–6]. Bulk metallic glass (BMG) alloys, i.e. amorphous alloys, are materials without any long-range atomic order
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J Mater Sci: Mater Med Table 1 Mechanical properties of 316L stainless steel, Ti–6Al–4V alloy, and Zr65Al7.5Ni10Cu17.5 BMG Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation (%)
Young’s modulus (GPa)
316L stainless steel [7]
[175
[480
[40
203
Ti–6Al–4V alloy [8]
853
950
10–15
108–116
Zr65Al7.5Ni10Cu17.5 BMG [9]
–
1,500–1,700
–
70–80
Human bone (femur) [10–13]
80
120
0.7–1.0
15–20
that are prepared by solidification of a liquid melt at a sufficient high speed to suppress the nucleation of crystals. Conventional metallic biomaterials such as Ti–6Al–4V alloy and 316L stainless steel are crystalline alloys. Crystalline alloys suffer from the presence of grain boundaries, dislocations, and segregations. When crystalline alloys are subjected to loading, grain boundaries and dislocations can easily undergo failure. In addition, crystalline alloys have slip planes which are moved by shear stress and allow plastic deformation, and the presences of dislocations enhance the deformation. In contrast, BMG alloys have a random atomic structure and do not contain any segregations and defects. In BMG alloys, slip planes are not generated and elastic deformation continues even under considerable stresses because such deformation is caused by mass movement of the constituent atoms. Accordingly, BMG alloys have a higher strength and a lower Young’s modulus than crystalline alloys. The ultimate tensile strength of Zr-based BMG is 1,500–1,700 MPa, which is approximately twice the ultimate tensile strength of Ti–6Al–4V alloy and 3 times that of 316L stainless steel (Table 1) [7–13]. Also, the Young’s modulus of Zr-based BMG is 70–80 GPa (Table 1), which is closer to that of bone than the values of any conventional materials. The fatigue strength (fatigue limit) of Zr-based BMG was ranged from 560 to 980 MPa while the yield strength was ranged from 1,500 to 1,900 MPa [14]. The fatigue behavior was affected by the fabrication process, surface condition, partial crystallization and Poisson’s ratio [15, 16]. Thus, the Zr-based BMG with high Poisson’s ratio and without inclusion can show comparable or higher fatigue strength than conventional metallic biomaterials such as Ti–6Al–4V alloy (510 MPa) [8]. Zr-based BMG showed high corrosion resistance and good in vitro and in vivo biocompatibility [17, 18]. The Zr65Al7.5Ni10Cu17.5 BMG showed high corrosion resistance in physiological environments [19–24], and Zr52.5 Cu17.9 Ni14.6Al10.0 Ti5.0 BMG showed excellent electrochemical properties in phosphate-buffered saline electrolysis [25]. Zr60.14Cu22.3Fe4.85 Al9.7Ag3 BMG showed very low cytotoxicity to cultured cells as well as Ti–6Al–4V alloy [18]. Powder consolidified Zr76.6Al3.5Ni7.6Cu12.3 BMG showed equivalent fatigue strength to that of pure Ti (Grade 2) [26]. However, the
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behavior of Zr-based BMG has been rarely investigated in vivo. Accordingly, the aim of this study was to evaluate the behavior of Zr65Al7.5Ni10Cu17.5 BMG in vivo and compare it with the results for Ti–6Al–4V alloy and 316L stainless steel. In rats, the femur was subjected to osteotomy and then stabilized using intramedullary nails made of Zr65Al7.5 Ni10Cu17.5 BMG, Ti–6Al–4V alloy, or 316L stainless steel. Systemic and local effects were evaluated by measuring the levels of Cu and Ni in the blood and in the surrounding soft tissue. In addition, the surface changes of the nails were examined by scanning electron microscopy (SEM). Finally, osteotomy healing was evaluated by peripheral quantitative computed tomography (pQCT) and mechanical testing.
2 Materials and methods 2.1 Implant materials Zr65Al7.5Ni10Cu17.5 (at%) BMG rods were prepared using the copper mold casting method. First, a 10 g buttonshaped ingot of Zr65Al7.5Ni10Cu17.5 was prepared with arcmelting in an argon atmosphere with alloying elements of Zr, Al, Ni, and Cu with 99.9 at% purity. The ingot was crashed into small pieces, packed into a quartz nozzle, and re-melted using a high-frequency induction furnace in a vacuum. The electric power applied to the furnace was controlled to have the constant temperature of the melt. Subsequently, the melt in the nozzle was ejected with argon gas pressure into a copper mold with a diameter of 2.0 mm and a length of 40 mm. The structure of the alloy was analyzed by X-ray diffraction with Cu Ka radiation to confirm that the entire structure of the alloy was really BMG. The BMG nails were cut to a length of 35 mm and were polished with 600 grid SiC paper in distilled water. Then the nails were rinsed ultrasonically in distilled water and acetone, dried for a few days to generate air-formed passive film, and then sterilized in a high-pressure steam sterilizer (BS-305; Tomy, Tokyo, Japan). It was confirmed that none of these procedures had any influence on the BMG structure of the alloy. For the compared materials group, K-wires made of Ti–6Al–4V alloy (Mathys Medical
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Ltd., Bettlach, Switzerland) with a diameter of 2.0 mm, and K-wires made of 316L stainless steel (Mathys Medical Ltd., Bettlach, Switzerland) with a diameter of 2.0 mm were prepared. The K-wires were cut to a length of 35 mm and were sterilized in a high-pressure steam sterilizer.
2.2 Surgical procedure All animal experiments were carried out in compliance with and approved by the Institutional Review Board of The University of Tokyo. All possible steps were taken to avoid animal suffering at each stage of the experiment. Fourteen-week-old male Wistar rats were used in the experiment. Each animal was housed in a separate cage (22 9 33 9 13 cm) and received a standard diet. Thirtytwo rats weighing 380–490 g (417 ± 27: mean ± standard deviation) were randomly assigned to three groups. Zr65Al7.5Ni10Cu17.5 BMG nails were inserted into the medullary canal of the femur in 14 animals (BMG group), while K-wires made of Ti–6Al–4V were implanted as an intramedullary nail in 14 animals (Ti–6Al–4V group). In the other 4 rats, K-wires made of 316L stainless steel were implanted (316L group). Under intraperitoneal anesthesia (75 mg/kg of ketamine hydrochloride and 10 mg/kg of xylazine), the left femur was exposed between the vastus lateralis and the hamstrings. An osteotomy was cut with a fine-toothed circular saw at 13 mm from the top of the greater trochanter. The medullary canal was reamed from the osteotomy site in the proximal direction and distally from the top of the greater trochanter to the level of the distal condyle using 1.5 and 2.0 mm diameter drill bits (Mathys Medical Ltd., Bettlach, Switzerland) inserted into an electric drill (Makita Driver; Makita, Aichi, Japan). The osteotomy was manually reduced and the intramedullary nail was inserted from the trochanter towards the femoral condyle. Next, the wound was irrigated with normal saline and then closed in layers. The contralateral femur was left intact. Postoperatively, each animal was allowed to move free activity in its own cage.
2.3 Radiography Osteotomy healing was examined in the BMG group and in the Ti–6Al–4V group. Radiographs were taken with a CMB-2 (Softex, Kanagawa, Japan) immediately after surgery and every 4 weeks during the follow-up period using a tube voltage of 56 kV and a tube current of 66 mAs at a focus-to-film distance of 65 cm. Fuji RX-U film was employed (Fuji Film, Tokyo, Japan).
2.4 Investigation of systemic and local effects At 12 weeks after implantation, 5 ml of blood was taken from the descending aorta under anesthesia (intraperitoneal injection of 45 mg/kg of ketamine hydrochloride and 6 mg/kg of xylazine), and the blood levels of Cu and Ni were measured. After the animals were killed under anesthesia (intraperitoneal injection of 120 mg/kg of ketamine hydrochloride and 16 mg/kg of xylazine), the periosteum and 3 g of gluteal muscles that had been in contact with the implanted nail were harvested. Then, the content of Cu and Ni in the harvested tissues was measured by graphite furnace atomic absorption spectroscopy (Hitachi Z-8100; Hitachi, Tokyo, Japan). 2.5 Surface analysis of the implants After euthanasia, the bilateral femora were excised and the intramedullary nails were removed. The harvested bones were stored at -70 °C. After the extracted nails were lavaged for 20 s with 30 ml of ultrafiltration detergent in purified water, they were fixed with carbon tape and stored in an airtight container. The surface of the implanted nails was examined by SEM with a Philips XL30FEG (Philips, Eindhoven, The Netherlands) operating at 20 kV. Materials deposited on the surface of the nails were identified and semi-quantified by energy dispersion X-ray spectroscopy (EDS: DX-4; EDAX Japan, Tokyo, Japan). 2.6 Evaluation of osteotomy healing Osteotomy healing was quantitatively evaluated by pQCT and by mechanical testing in the BMG group and the Ti– 6Al–4V group. The femora were scanned using a pQCT system (XCT Research SA?; Stratec Medizintechnik GmbH, Pforzheim, Germany) and a voxel size of 0.08 9 0.08 9 0.5 mm3 was employed. At each osteotomy site, axial images of three consecutive cross-sections with an interslice distance of 0.2 mm were obtained. The results for these three consecutive sections were averaged, and the mean value was used for evaluation. A threshold value of 690 mg/cm3 was used to define cortical bone. The bone mineral density of cortical bone (CoBMD, mg/cm3), the bone mineral content of cortical bone (CoBMC, mg/mm), and the cross-sectional area of cortical bone (CoCSA, mm2) were measured. The stress strain index (SSI) was also calculated as shown below; SSI =
2 i¼l;n R ri
aCD=ND=rmax
where ri is the pixel position from the center, rmax is the maximum distance of a voxel from the center (mm), a is the area of a voxel (mm2), CD is the measured volumetric
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J Mater Sci: Mater Med Table 2 Cu and Ni whole-blood levels in Zr-based BMG group, Ti– 6Al–4V group, and 316L group at 12 weeks after implantation Zr-based BMG group (n = 14)
Ti–6Al–4V group (n = 14)
316L group (n = 4)
Cu (lg/dl)
147.8 ± 43.2
153.1 ± 38.3
145.8 ± 13.6
Ni (lg/dl)
\0.10
0.11 ± 0.03
\0.10
Values are given as mean values ± standard deviations
Table 3 The mean levels of Cu and Ni contained in the collected surrounding tissues in BMG group, Ti–6Al–4V group, and 316L group at 12 weeks after implantation BMG group (n = 14)
Ti–6Al–4V group (n = 14)
316L group (n = 4)
Cu (lg/g)
10.2 ± 12.6
22.3 ± 42.6
10.4 ± 8.0
Ni (lg/g)
2.9 ± 2.4
2.3 ± 1.0
\2.0
Values are given as mean values ± standard deviations 2
cortical density (mg/cm ), and ND is the normal cortical density (1,200 mg/cm2). The femora were thawed at a room temperature for 2 hours just prior to mechanical testing. Both ends of each bone were embedded in Wood’s metal (U-70; Asahi Metal, Osaka, Japan) to ensure consistent alignment of the bone axis with the axis of the torsion testing machine. An axial torsion test was performed at an angular velocity of 15 degrees/minute in external rotation without any axial loading using a torsion testing machine (STTM-A; T. S. Engineering, Kanagawa, Japan). The maximum torque was defined as that at which the ultimate torque was reached. Torsional stiffness was defined as the ratio of the maximum torque to the angle at that time. Energy absorption was calculated from the torque-angular displacement curve and was used as a measure of the energy required to cause failure of the bone. Undecalcified specimens of the femora were prepared and stained with Villanueva bone stain. Animals were given the following bone labels: oxytetracycline (50 mg/ kg, intraperitoneally) on 28 days before death, and alizarin complexone (20 mg/kg, intraperitoneally) on 14 days before death. 2.7 Statistical analysis Data from the BMG and the Ti–6Al–4V group were compared by the unpaired Student’s t test. Differences were considered significant at p values less than 0.05. Results are reported as the mean and standard deviation.
3 Results
Fig. 1 Surface SEM images of the intraosseous part of the BMG nail a Before implantation, b after 12 weeks of implantation
3.1 Blood levels of Cu and Ni When the systemic effects were assessed in the BMG group, the mean whole blood Cu level was 147.8 lg/dl and the mean whole blood Ni level was less than 0.10 lg/dl. In the Ti–6Al–4V group, the mean whole blood Cu level was 153.1 lg/dl and the mean whole blood Ni level was 0.11 lg/dl. In the 316L group, the mean whole blood Cu level was 145.8 lg/dl and the mean whole blood Ni level was less than 0.10 lg/dl. No increase of the blood levels of
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Cu or Ni was recognized in the BMG group compared with the Ti–6Al–4V group or the 316L group (Table 2). 3.2 Levels of Cu and Ni in the soft tissues When the local effects were assessed, the mean levels of Cu and Ni in the surrounding tissues in the BMG group were 10.2 and 2.9 lg/g, respectively. In the Ti–6Al–4V group, the mean levels of Cu and Ni were 22.3 and 2.3 lg/g,
J Mater Sci: Mater Med Fig. 2 EDS spectrum of the intraosseous part of the BMG nail after 12 weeks of implantation. The surface contained small amounts of Ca and Si
respectively. In the 316L group, the mean levels of Cu and Ni were 10.4 lg/g and less than 2.0 lg/g, respectively. No increase of Cu or Ni in the soft tissues was recognized in the BMG group compared with the Ti–6Al–4V group or the 316L group (Table 3). 3.3 Surface analysis of the implants When evaluation of the corrosion resistance and durability of the BMG in vivo was performed, neither breakage nor pitting corrosion of the harvested BMG nails was noted. Observation by SEM revealed nothing but polishing scars that were also observed by SEM before implantation (Fig. 1a, b). Semi-quantitative examination of the surface composition by EDS revealed that small amount of Ca and Si was detected on the intraosseous parts of the BMG nails (Fig. 2). In contrast, the intraosseous parts of the Ti–6Al– 4V alloy nails were coated with laminated deposits on SEM after implantation (Fig. 3a, b), and substantial amounts of Ca and P were noted by EDS (Fig. 4). The intraosseous parts of 316L stainless steel nails showed surface irregularities and micro-pits with thin corrosion products layers on the SEM image after implantation (Fig. 5a, b), and considerable deposition of sulfur was recognized by EDS (Fig. 6). 3.4 Radiographic analysis
Fig. 3 Surface SEM images of the intraosseous part of the Ti–6Al– 4V alloy nail. a Before implantation, b after 12 weeks of implantation. Laminated deposits can be observed
Callus formation became apparent on radiographs at 4 weeks. After 12 weeks, 5 out of 14 osteotomies in the BMG group and 3 out of 14 osteotomies in the Ti–6Al–4V group had healed with bony bridging across the osteotomy
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J Mater Sci: Mater Med Fig. 4 EDS spectrum of the intraosseous part of the Ti–6Al– 4V alloy nail after 12 weeks of implantation. Substantial amounts of Ca and P were detected
ends. In the other animals, partial bridging with callus formation was seen (Fig. 7a, b, c). 3.5 Evaluation of osteotomy healing
Fig. 5 Surface SEM images of the intraosseous part of the 316L stainless steel nail. a Before implantation, b after 12 weeks of implantation. Surface irregularities in the form of pits can be seen
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The CoBMD of the osteotomy site measured by pQCT at 12 weeks was 1036 ± 74 mg/cm3 in the BMG group, while it was 1046 ± 70 mg/cm3 in the Ti–6Al–4V group. The difference between the groups was not significant (P = 0.87). The CoBMC of the BMG group was 14.4 ± 3.7 mg/mm, while that of the Ti–6Al–4V group was 12.0 ± 5.8 mg/mm, and this difference was also not significant (P = 0.37). The CoCSA of the osteotomy site measured by pQCT at 12 weeks was 13.8 ± 3.0 mm2 in the BMG group, while that of the Ti–6Al–4V group was 11.7 ± 5.9 mm2. Once again, the difference was not significant (P = 0.40). The SSI value was higher for the BMG group, with an index of 21.5 ± 5.6 compared to 15.7 ± 9.6 for the Ti–6Al–4V group, but the difference also failed to reach significance (P = 0.19). The maximum torque measured in the axial torsion test was 0.34 ± 0.19 Nm in the BMG group, and a higher strength was attained compared with that for the Ti–6Al–4V group (0.18 ± 0.16 Nm). However, the difference was not significant (P = 0.14). The torsional stiffness was 0.044 ± 0.035 Nm/degree in the BMG group, which was higher compared than that in the Ti–6Al–4V group (0.020 ± 0.021 Nm/degree), but the difference was also not significant (P = 0.19). Furthermore, the energy absorption was 2.2 ± 2.2 Nm degree in the BMG group and was higher than that in the Ti–6Al–4V group (1.0 ± 0.9 Nm degree), but the difference was not significant (P = 0.27) (Fig. 8).
J Mater Sci: Mater Med Fig. 6 EDS spectrum of the intraosseous part of the 316L stainless steel nail after 12 weeks of implantation. The surface contained a considerable amount of S
Fig. 7 Radiographs of femora with the Zr-based BMG (a) and Ti– 6Al–4V alloy (b) (c) nails inserted into the medullary canal, which were obtained at 12 weeks after osteotomy. a, b The osteotomy has
united with bridging by new bone formation. c Non-union with partial bridging by callus
The histology and fluorochrome bone labeling observation of the osteotomy site showed the abundant osteoid surface lined with alizarin complexone labeling. Oxytetracycline labeling was lacking, indicating that it had been absorbed because of high bone turnover and remodeling (Fig. 9a, b).
and inserted nail made of stainless steel, titanium, or polyethylene into the medullary canal to stabilize the osteotomy. Their study concluded that titanium nail with a bending rigidity close to the intact femur had a higher maximum bending load and fracture energy at 12 weeks than both rigidly or softly nailed fracture. They speculated that the presence of a nail more rigid than the bone itself might interfere with effective remodeling at the fracture area. Young’s modulus of titanium is 108–116 GPa and still much higher than that of bone, while the Young’s modulus of Zr-based BMG is 70–80 GPa and closer to that of cortical bone.
4 Discussion Utvag et al. [27] reported the effects of nail rigidity on fracture healing. They created osteotomy of the rat femur
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Fig. 8 Mechanical parameters of osteotomy healing in the BMG group (n = 6) and the Ti–6Al–4V group (n = 6). Error bars represent one standard deviation from the mean
Fig. 9 Osteotomy site after 12 weeks of the BMG alloy implantation. a Note the abundant osteoid surface. Villanueva bone stain, b the osteoid surface was lined with alizarin complexone labeling (arrows). Oxytetracycline labeling (asterisks) was lacking, indicating it had been absorbed because of high bone turnover and remodeling
Although our study did not show any significant differences in healing of the osteotomy healing on the basis of pQCT or torsional testing, the average value of each parameter was larger for the BMG group than that for the Ti–6Al–4V group. In addition, the radiographic bone union
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rate of the BMG group was higher than that of the Ti–6Al– 4V group at 12 weeks. From these data, intramedullary nails made of Zr-based BMG promoted osteotomy healing as fast as Ti–6Al–4V alloy for application in osteosynthetic devices. The corrosion resistance of alloys also depends on their atomic structure. In crystalline alloys, grain boundaries and segregations cause irregularities of the surface oxide film, leading to corrosion of the alloy. In BMG, the grain boundary-free structure allows a homogeneous and defectfree surface oxide film to be formed. Therefore, BMG alloys generally show higher corrosion resistance than crystalline alloys with the similar composition [28]. There are several studies which evaluated BMG in vitro. Biocompatibility of Ti-based Ti45Zr38Ni17 BMG was evaluated in vitro. The study indicated that osteoblasts, cultured in presence of Ti-based BMG, differentiated and synthesized bone matrix [29]. The study to investigate mechanical properties and in vitro corrosion behavior of Ti40Zr10Cu38Pd12 BMG reported that BMG showed high hardness, relatively low Young’s modulus, good wear resistance and excellent corrosion behavior [30]. Biodegradability, mechanical behavior, and cytocompatibility of Mg72Zn23Ca5 BMG were evaluated and compared to crystalline Mg70Zn23Ca5Pd2 alloy. Mg72Zn23Ca5 BMG showed higher corrosion resistance and improved mechanical behavior. Neither Mg72Zn23Ca5 BMG nor crystalline Mg70Zn23Ca5Pd2 alloy were cytotoxic [31]. The previous literature contains only one study on the evaluation of BMG alloys in vivo [32], while another report has indicated that metallic intravascular stents with an amorphous oxide coating show excellent corrosion resistance both in vitro and in vivo [33]. Our investigation was the first to assess a BMG intramedullary nail and evaluate its influence on osteotomy healing in vivo. Some osteotomies did not reach solid union, although partial bridging with callus was seen. One of the reasons for delayed union might be rotational instability [34], since stabilizing the osteotomy with a nail combined with
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interlocking pins to eliminate rotational instability is reported to improve bone healing [35]. Stabilizing the osteotomy with a nail combined with interlocking pins needs to be investigated in the future. The main limitation of the present study is that we used the Zr-based BMG containing nickel, which is carcinogenic. No systemic or local leaching was recognized by measuring Ni levels in the blood and the surrounding soft tissue, but Ni-free alloy might be more appropriate for the use in biomaterials. Therefore, future studies should investigate Ni-free BMG. The SEM and EDS data from this study indicated that 316L stainless steel tended to suffer from general and local corrosion probably with the preferential adsorption of proteins containing –SH group. Ti–6Al–4V alloy was liable to the formation of calcium phosphate deposits on its surface, while Zr65Al7.5Ni10Cu17.5 BMG was almost biologically inert. The SEM and EDS investigations of medical devices made of 316L stainless steel have shown that the corrosion forms a layer that consists of sulfur, phosphorus, and calcium [36, 37]. The excellent biocompatibility of titanium and titanium alloys may lead to osseointegration and strong bone-to-metal bonding may cause complications. Detaching tests performed after implantation of Ti–6Al–4V plates into the tibiae of rabbits showed that Ti alloy bonds directly to bone after about 8 weeks [38]. Vresilovic et al. [39] reported a higher rate of complications during removal of titanium pins than stainless steel pins in patients with slipped capital femoral epiphysis and attributed this difference to stronger bonding of titanium pins to the bone. In addition, a retrospective review of 45 patients undergoing removal of femoral intramedullary nails showed that removal of titanium nails required a significantly longer operating time than removal of the stainless steel nails, even though all the nails were removed intact without complications [40]. Therefore, biomaterials with low in vivo reactivity like Zr65Al7.5 Ni10Cu17.5 BMG might be more appropriate for osteosynthetic device that must eventually be removed. In this study, the BMG and titanium alloy nails were compared, but in the next study, 316L stainless steel should be compared because it is usually eventually removed. Several problems need to be overcome before adopting this material for clinical application. One problem is that even larger pieces than we tested would need to be manufactured. Cu36Zr48Ag8Al8 BMG rods with a diameter of 25 mm produced by Cu mold casting [41] and Zr55Cu30 Ni5Al10 BMG rods with a diameter of 30 mm produced by a cap-cast technique [42] are promising alloys. Another problem is the potential limitations of the Zr-based BMG concerning mechanical properties. The fatigue behavior is affected by the fabrication process, surface condition and partial crystallization [15, 16]. Therefore, high quality material should be required. In addition, Zr-based BMG is
less ductile, brittle, and high fatigue crack growth at notches. In this study neither breakage nor crack of the BMG was noted, but further in vivo studies are necessary.
5 Conclusions In this study, intramedullary nails made of Zr65Al7.5 Ni10Cu17.5 BMG were implanted in rat femora after osteotomy for 12 weeks, and systemic and local effects, surface changes of the nails, and osteotomy healing were evaluated. No increase of Cu and Ni levels was recognized in the blood and the surrounding soft tissue, indicating that there were no systemic and local effects. The SEM and EDS data indicated that Zr-based BMG was almost biologically inert. Radiographic analysis and mechanical testing indicated that intramedullary nails made of Zr-based BMG promoted osteotomy healing as fast as Ti–6Al–4V alloy for application in osteosynthetic devices. BMG alloys can be a promising new metallic biomaterial for osteosynthetic implants but further in vivo studies are necessary. Acknowledgments The authors are grateful to Prof. Takao Hanawa and Prof. Isao Ohnishi for valuable discussions. This work was funded by the grant in aid for Scientific Research received from Japan Society for the Promotion of Science and Technology. Conflict of interest
The authors have no competing interests.
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In vivo evaluation of Zr-based bulk metallic glass alloy intramedullary nails in rat femora Kazuhiro Imai • Sachiko Hiromoto
Received: 29 August 2013 / Accepted: 18 November 2013 Ó Springer Science+Business Media New York 2013
Abstract Zr-based bulk metallic glasses (BMG) show high corrosion resistance in vitro and higher strength and lower Young’s modulus than crystalline alloys with the similar composition. This study aimed to perform an in vivo evaluation of Zr65Al7.5Ni10Cu17.5 BMG. Osteotomy of the femur was done in rats and stabilized with intramedullary nails made of Zr65Al7.5Ni10Cu17.5 BMG, Ti– 6Al–4V alloy, or 316L stainless steel. Systemic and local effects of each type of nail were evaluated by measuring the levels of Cu and Ni in the blood and the surrounding soft tissue. Changes of the surface of each nail were examined by scanning electron microscopy (SEM). Healing of the osteotomy was evaluated by peripheral quantitative computed tomography and mechanical testing. No increase of Cu and Ni levels was recognized. Surface of the BMG showed no noticeable change, while Ti–6Al–4V alloy showed Ca and P deposition and 316L stainless steel showed surface irregularities and pitting by SEM observation. The stress strain index, maximum torque, torsional stiffness, and energy absorption values were larger for the BMG than those for Ti–6Al–4V alloy, although there was no significant difference. The Zr-based BMG can promote osteotomy healing as fast as Ti–6Al–4V alloy, with the possible advantage of the Zr-based BMG that bone bonding is less likely, allowing easier nail removal compared with
K. Imai (&) Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan e-mail: [email protected] S. Hiromoto Biomaterials Unit, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan
Ti–6Al–4V alloy. The Zr-based BMG is promising for the use in osteosynthetic devices that are eventually removed.
1 Introduction Metallic implants used for fracture management need to be nontoxic, resistant to corrosion, durable, strong, as well as having low Young’s modulus and good biocompatibility. Ti–6Al–4V alloys and 316L stainless steel are commonly used for osteosynthetic devices such as plates, intramedullary nails, and screws. When such materials are implanted into patients to treat fractures, they are subjected to repeated load by daily activities, and are directly exposed to body fluids that are highly corrosive to metallic substances. Under such conditions, these implants are sometimes attacked by corrosion or fractured by corrosion fatigue and fretting corrosion fatigue [1]. To prevent failure, bone plates have been made quite bulky and a large incision is necessary to implant such plates, leading to occasional difficulty in closing the wound. Likewise, intramedullary nails need to have a large diameter to prevent failure. However, such nails interfere with the blood supply by occupying a large area of the medullary canal, which may lead to disturbance of healing. Therefore, one of the disadvantages of the current metallic osteosynthetic devices is their insufficient strength. Another disadvantage is an excessively high Young’s modulus. The Young’s modulus of currently available metallic devices is around 100 GPa and is much higher than that of cortical bone (15–20 GPa). This sometimes leads to stress shielding and absorption of the bone around osteosynthetic devices made of these materials [2–6]. Bulk metallic glass (BMG) alloys, i.e. amorphous alloys, are materials without any long-range atomic order
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J Mater Sci: Mater Med Table 1 Mechanical properties of 316L stainless steel, Ti–6Al–4V alloy, and Zr65Al7.5Ni10Cu17.5 BMG Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation (%)
Young’s modulus (GPa)
316L stainless steel [7]
[175
[480
[40
203
Ti–6Al–4V alloy [8]
853
950
10–15
108–116
Zr65Al7.5Ni10Cu17.5 BMG [9]
–
1,500–1,700
–
70–80
Human bone (femur) [10–13]
80
120
0.7–1.0
15–20
that are prepared by solidification of a liquid melt at a sufficient high speed to suppress the nucleation of crystals. Conventional metallic biomaterials such as Ti–6Al–4V alloy and 316L stainless steel are crystalline alloys. Crystalline alloys suffer from the presence of grain boundaries, dislocations, and segregations. When crystalline alloys are subjected to loading, grain boundaries and dislocations can easily undergo failure. In addition, crystalline alloys have slip planes which are moved by shear stress and allow plastic deformation, and the presences of dislocations enhance the deformation. In contrast, BMG alloys have a random atomic structure and do not contain any segregations and defects. In BMG alloys, slip planes are not generated and elastic deformation continues even under considerable stresses because such deformation is caused by mass movement of the constituent atoms. Accordingly, BMG alloys have a higher strength and a lower Young’s modulus than crystalline alloys. The ultimate tensile strength of Zr-based BMG is 1,500–1,700 MPa, which is approximately twice the ultimate tensile strength of Ti–6Al–4V alloy and 3 times that of 316L stainless steel (Table 1) [7–13]. Also, the Young’s modulus of Zr-based BMG is 70–80 GPa (Table 1), which is closer to that of bone than the values of any conventional materials. The fatigue strength (fatigue limit) of Zr-based BMG was ranged from 560 to 980 MPa while the yield strength was ranged from 1,500 to 1,900 MPa [14]. The fatigue behavior was affected by the fabrication process, surface condition, partial crystallization and Poisson’s ratio [15, 16]. Thus, the Zr-based BMG with high Poisson’s ratio and without inclusion can show comparable or higher fatigue strength than conventional metallic biomaterials such as Ti–6Al–4V alloy (510 MPa) [8]. Zr-based BMG showed high corrosion resistance and good in vitro and in vivo biocompatibility [17, 18]. The Zr65Al7.5Ni10Cu17.5 BMG showed high corrosion resistance in physiological environments [19–24], and Zr52.5 Cu17.9 Ni14.6Al10.0 Ti5.0 BMG showed excellent electrochemical properties in phosphate-buffered saline electrolysis [25]. Zr60.14Cu22.3Fe4.85 Al9.7Ag3 BMG showed very low cytotoxicity to cultured cells as well as Ti–6Al–4V alloy [18]. Powder consolidified Zr76.6Al3.5Ni7.6Cu12.3 BMG showed equivalent fatigue strength to that of pure Ti (Grade 2) [26]. However, the
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behavior of Zr-based BMG has been rarely investigated in vivo. Accordingly, the aim of this study was to evaluate the behavior of Zr65Al7.5Ni10Cu17.5 BMG in vivo and compare it with the results for Ti–6Al–4V alloy and 316L stainless steel. In rats, the femur was subjected to osteotomy and then stabilized using intramedullary nails made of Zr65Al7.5 Ni10Cu17.5 BMG, Ti–6Al–4V alloy, or 316L stainless steel. Systemic and local effects were evaluated by measuring the levels of Cu and Ni in the blood and in the surrounding soft tissue. In addition, the surface changes of the nails were examined by scanning electron microscopy (SEM). Finally, osteotomy healing was evaluated by peripheral quantitative computed tomography (pQCT) and mechanical testing.
2 Materials and methods 2.1 Implant materials Zr65Al7.5Ni10Cu17.5 (at%) BMG rods were prepared using the copper mold casting method. First, a 10 g buttonshaped ingot of Zr65Al7.5Ni10Cu17.5 was prepared with arcmelting in an argon atmosphere with alloying elements of Zr, Al, Ni, and Cu with 99.9 at% purity. The ingot was crashed into small pieces, packed into a quartz nozzle, and re-melted using a high-frequency induction furnace in a vacuum. The electric power applied to the furnace was controlled to have the constant temperature of the melt. Subsequently, the melt in the nozzle was ejected with argon gas pressure into a copper mold with a diameter of 2.0 mm and a length of 40 mm. The structure of the alloy was analyzed by X-ray diffraction with Cu Ka radiation to confirm that the entire structure of the alloy was really BMG. The BMG nails were cut to a length of 35 mm and were polished with 600 grid SiC paper in distilled water. Then the nails were rinsed ultrasonically in distilled water and acetone, dried for a few days to generate air-formed passive film, and then sterilized in a high-pressure steam sterilizer (BS-305; Tomy, Tokyo, Japan). It was confirmed that none of these procedures had any influence on the BMG structure of the alloy. For the compared materials group, K-wires made of Ti–6Al–4V alloy (Mathys Medical
J Mater Sci: Mater Med
Ltd., Bettlach, Switzerland) with a diameter of 2.0 mm, and K-wires made of 316L stainless steel (Mathys Medical Ltd., Bettlach, Switzerland) with a diameter of 2.0 mm were prepared. The K-wires were cut to a length of 35 mm and were sterilized in a high-pressure steam sterilizer.
2.2 Surgical procedure All animal experiments were carried out in compliance with and approved by the Institutional Review Board of The University of Tokyo. All possible steps were taken to avoid animal suffering at each stage of the experiment. Fourteen-week-old male Wistar rats were used in the experiment. Each animal was housed in a separate cage (22 9 33 9 13 cm) and received a standard diet. Thirtytwo rats weighing 380–490 g (417 ± 27: mean ± standard deviation) were randomly assigned to three groups. Zr65Al7.5Ni10Cu17.5 BMG nails were inserted into the medullary canal of the femur in 14 animals (BMG group), while K-wires made of Ti–6Al–4V were implanted as an intramedullary nail in 14 animals (Ti–6Al–4V group). In the other 4 rats, K-wires made of 316L stainless steel were implanted (316L group). Under intraperitoneal anesthesia (75 mg/kg of ketamine hydrochloride and 10 mg/kg of xylazine), the left femur was exposed between the vastus lateralis and the hamstrings. An osteotomy was cut with a fine-toothed circular saw at 13 mm from the top of the greater trochanter. The medullary canal was reamed from the osteotomy site in the proximal direction and distally from the top of the greater trochanter to the level of the distal condyle using 1.5 and 2.0 mm diameter drill bits (Mathys Medical Ltd., Bettlach, Switzerland) inserted into an electric drill (Makita Driver; Makita, Aichi, Japan). The osteotomy was manually reduced and the intramedullary nail was inserted from the trochanter towards the femoral condyle. Next, the wound was irrigated with normal saline and then closed in layers. The contralateral femur was left intact. Postoperatively, each animal was allowed to move free activity in its own cage.
2.3 Radiography Osteotomy healing was examined in the BMG group and in the Ti–6Al–4V group. Radiographs were taken with a CMB-2 (Softex, Kanagawa, Japan) immediately after surgery and every 4 weeks during the follow-up period using a tube voltage of 56 kV and a tube current of 66 mAs at a focus-to-film distance of 65 cm. Fuji RX-U film was employed (Fuji Film, Tokyo, Japan).
2.4 Investigation of systemic and local effects At 12 weeks after implantation, 5 ml of blood was taken from the descending aorta under anesthesia (intraperitoneal injection of 45 mg/kg of ketamine hydrochloride and 6 mg/kg of xylazine), and the blood levels of Cu and Ni were measured. After the animals were killed under anesthesia (intraperitoneal injection of 120 mg/kg of ketamine hydrochloride and 16 mg/kg of xylazine), the periosteum and 3 g of gluteal muscles that had been in contact with the implanted nail were harvested. Then, the content of Cu and Ni in the harvested tissues was measured by graphite furnace atomic absorption spectroscopy (Hitachi Z-8100; Hitachi, Tokyo, Japan). 2.5 Surface analysis of the implants After euthanasia, the bilateral femora were excised and the intramedullary nails were removed. The harvested bones were stored at -70 °C. After the extracted nails were lavaged for 20 s with 30 ml of ultrafiltration detergent in purified water, they were fixed with carbon tape and stored in an airtight container. The surface of the implanted nails was examined by SEM with a Philips XL30FEG (Philips, Eindhoven, The Netherlands) operating at 20 kV. Materials deposited on the surface of the nails were identified and semi-quantified by energy dispersion X-ray spectroscopy (EDS: DX-4; EDAX Japan, Tokyo, Japan). 2.6 Evaluation of osteotomy healing Osteotomy healing was quantitatively evaluated by pQCT and by mechanical testing in the BMG group and the Ti– 6Al–4V group. The femora were scanned using a pQCT system (XCT Research SA?; Stratec Medizintechnik GmbH, Pforzheim, Germany) and a voxel size of 0.08 9 0.08 9 0.5 mm3 was employed. At each osteotomy site, axial images of three consecutive cross-sections with an interslice distance of 0.2 mm were obtained. The results for these three consecutive sections were averaged, and the mean value was used for evaluation. A threshold value of 690 mg/cm3 was used to define cortical bone. The bone mineral density of cortical bone (CoBMD, mg/cm3), the bone mineral content of cortical bone (CoBMC, mg/mm), and the cross-sectional area of cortical bone (CoCSA, mm2) were measured. The stress strain index (SSI) was also calculated as shown below; SSI =
2 i¼l;n R ri
aCD=ND=rmax
where ri is the pixel position from the center, rmax is the maximum distance of a voxel from the center (mm), a is the area of a voxel (mm2), CD is the measured volumetric
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J Mater Sci: Mater Med Table 2 Cu and Ni whole-blood levels in Zr-based BMG group, Ti– 6Al–4V group, and 316L group at 12 weeks after implantation Zr-based BMG group (n = 14)
Ti–6Al–4V group (n = 14)
316L group (n = 4)
Cu (lg/dl)
147.8 ± 43.2
153.1 ± 38.3
145.8 ± 13.6
Ni (lg/dl)
\0.10
0.11 ± 0.03
\0.10
Values are given as mean values ± standard deviations
Table 3 The mean levels of Cu and Ni contained in the collected surrounding tissues in BMG group, Ti–6Al–4V group, and 316L group at 12 weeks after implantation BMG group (n = 14)
Ti–6Al–4V group (n = 14)
316L group (n = 4)
Cu (lg/g)
10.2 ± 12.6
22.3 ± 42.6
10.4 ± 8.0
Ni (lg/g)
2.9 ± 2.4
2.3 ± 1.0
\2.0
Values are given as mean values ± standard deviations 2
cortical density (mg/cm ), and ND is the normal cortical density (1,200 mg/cm2). The femora were thawed at a room temperature for 2 hours just prior to mechanical testing. Both ends of each bone were embedded in Wood’s metal (U-70; Asahi Metal, Osaka, Japan) to ensure consistent alignment of the bone axis with the axis of the torsion testing machine. An axial torsion test was performed at an angular velocity of 15 degrees/minute in external rotation without any axial loading using a torsion testing machine (STTM-A; T. S. Engineering, Kanagawa, Japan). The maximum torque was defined as that at which the ultimate torque was reached. Torsional stiffness was defined as the ratio of the maximum torque to the angle at that time. Energy absorption was calculated from the torque-angular displacement curve and was used as a measure of the energy required to cause failure of the bone. Undecalcified specimens of the femora were prepared and stained with Villanueva bone stain. Animals were given the following bone labels: oxytetracycline (50 mg/ kg, intraperitoneally) on 28 days before death, and alizarin complexone (20 mg/kg, intraperitoneally) on 14 days before death. 2.7 Statistical analysis Data from the BMG and the Ti–6Al–4V group were compared by the unpaired Student’s t test. Differences were considered significant at p values less than 0.05. Results are reported as the mean and standard deviation.
3 Results
Fig. 1 Surface SEM images of the intraosseous part of the BMG nail a Before implantation, b after 12 weeks of implantation
3.1 Blood levels of Cu and Ni When the systemic effects were assessed in the BMG group, the mean whole blood Cu level was 147.8 lg/dl and the mean whole blood Ni level was less than 0.10 lg/dl. In the Ti–6Al–4V group, the mean whole blood Cu level was 153.1 lg/dl and the mean whole blood Ni level was 0.11 lg/dl. In the 316L group, the mean whole blood Cu level was 145.8 lg/dl and the mean whole blood Ni level was less than 0.10 lg/dl. No increase of the blood levels of
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Cu or Ni was recognized in the BMG group compared with the Ti–6Al–4V group or the 316L group (Table 2). 3.2 Levels of Cu and Ni in the soft tissues When the local effects were assessed, the mean levels of Cu and Ni in the surrounding tissues in the BMG group were 10.2 and 2.9 lg/g, respectively. In the Ti–6Al–4V group, the mean levels of Cu and Ni were 22.3 and 2.3 lg/g,
J Mater Sci: Mater Med Fig. 2 EDS spectrum of the intraosseous part of the BMG nail after 12 weeks of implantation. The surface contained small amounts of Ca and Si
respectively. In the 316L group, the mean levels of Cu and Ni were 10.4 lg/g and less than 2.0 lg/g, respectively. No increase of Cu or Ni in the soft tissues was recognized in the BMG group compared with the Ti–6Al–4V group or the 316L group (Table 3). 3.3 Surface analysis of the implants When evaluation of the corrosion resistance and durability of the BMG in vivo was performed, neither breakage nor pitting corrosion of the harvested BMG nails was noted. Observation by SEM revealed nothing but polishing scars that were also observed by SEM before implantation (Fig. 1a, b). Semi-quantitative examination of the surface composition by EDS revealed that small amount of Ca and Si was detected on the intraosseous parts of the BMG nails (Fig. 2). In contrast, the intraosseous parts of the Ti–6Al– 4V alloy nails were coated with laminated deposits on SEM after implantation (Fig. 3a, b), and substantial amounts of Ca and P were noted by EDS (Fig. 4). The intraosseous parts of 316L stainless steel nails showed surface irregularities and micro-pits with thin corrosion products layers on the SEM image after implantation (Fig. 5a, b), and considerable deposition of sulfur was recognized by EDS (Fig. 6). 3.4 Radiographic analysis
Fig. 3 Surface SEM images of the intraosseous part of the Ti–6Al– 4V alloy nail. a Before implantation, b after 12 weeks of implantation. Laminated deposits can be observed
Callus formation became apparent on radiographs at 4 weeks. After 12 weeks, 5 out of 14 osteotomies in the BMG group and 3 out of 14 osteotomies in the Ti–6Al–4V group had healed with bony bridging across the osteotomy
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J Mater Sci: Mater Med Fig. 4 EDS spectrum of the intraosseous part of the Ti–6Al– 4V alloy nail after 12 weeks of implantation. Substantial amounts of Ca and P were detected
ends. In the other animals, partial bridging with callus formation was seen (Fig. 7a, b, c). 3.5 Evaluation of osteotomy healing
Fig. 5 Surface SEM images of the intraosseous part of the 316L stainless steel nail. a Before implantation, b after 12 weeks of implantation. Surface irregularities in the form of pits can be seen
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The CoBMD of the osteotomy site measured by pQCT at 12 weeks was 1036 ± 74 mg/cm3 in the BMG group, while it was 1046 ± 70 mg/cm3 in the Ti–6Al–4V group. The difference between the groups was not significant (P = 0.87). The CoBMC of the BMG group was 14.4 ± 3.7 mg/mm, while that of the Ti–6Al–4V group was 12.0 ± 5.8 mg/mm, and this difference was also not significant (P = 0.37). The CoCSA of the osteotomy site measured by pQCT at 12 weeks was 13.8 ± 3.0 mm2 in the BMG group, while that of the Ti–6Al–4V group was 11.7 ± 5.9 mm2. Once again, the difference was not significant (P = 0.40). The SSI value was higher for the BMG group, with an index of 21.5 ± 5.6 compared to 15.7 ± 9.6 for the Ti–6Al–4V group, but the difference also failed to reach significance (P = 0.19). The maximum torque measured in the axial torsion test was 0.34 ± 0.19 Nm in the BMG group, and a higher strength was attained compared with that for the Ti–6Al–4V group (0.18 ± 0.16 Nm). However, the difference was not significant (P = 0.14). The torsional stiffness was 0.044 ± 0.035 Nm/degree in the BMG group, which was higher compared than that in the Ti–6Al–4V group (0.020 ± 0.021 Nm/degree), but the difference was also not significant (P = 0.19). Furthermore, the energy absorption was 2.2 ± 2.2 Nm degree in the BMG group and was higher than that in the Ti–6Al–4V group (1.0 ± 0.9 Nm degree), but the difference was not significant (P = 0.27) (Fig. 8).
J Mater Sci: Mater Med Fig. 6 EDS spectrum of the intraosseous part of the 316L stainless steel nail after 12 weeks of implantation. The surface contained a considerable amount of S
Fig. 7 Radiographs of femora with the Zr-based BMG (a) and Ti– 6Al–4V alloy (b) (c) nails inserted into the medullary canal, which were obtained at 12 weeks after osteotomy. a, b The osteotomy has
united with bridging by new bone formation. c Non-union with partial bridging by callus
The histology and fluorochrome bone labeling observation of the osteotomy site showed the abundant osteoid surface lined with alizarin complexone labeling. Oxytetracycline labeling was lacking, indicating that it had been absorbed because of high bone turnover and remodeling (Fig. 9a, b).
and inserted nail made of stainless steel, titanium, or polyethylene into the medullary canal to stabilize the osteotomy. Their study concluded that titanium nail with a bending rigidity close to the intact femur had a higher maximum bending load and fracture energy at 12 weeks than both rigidly or softly nailed fracture. They speculated that the presence of a nail more rigid than the bone itself might interfere with effective remodeling at the fracture area. Young’s modulus of titanium is 108–116 GPa and still much higher than that of bone, while the Young’s modulus of Zr-based BMG is 70–80 GPa and closer to that of cortical bone.
4 Discussion Utvag et al. [27] reported the effects of nail rigidity on fracture healing. They created osteotomy of the rat femur
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Fig. 8 Mechanical parameters of osteotomy healing in the BMG group (n = 6) and the Ti–6Al–4V group (n = 6). Error bars represent one standard deviation from the mean
Fig. 9 Osteotomy site after 12 weeks of the BMG alloy implantation. a Note the abundant osteoid surface. Villanueva bone stain, b the osteoid surface was lined with alizarin complexone labeling (arrows). Oxytetracycline labeling (asterisks) was lacking, indicating it had been absorbed because of high bone turnover and remodeling
Although our study did not show any significant differences in healing of the osteotomy healing on the basis of pQCT or torsional testing, the average value of each parameter was larger for the BMG group than that for the Ti–6Al–4V group. In addition, the radiographic bone union
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rate of the BMG group was higher than that of the Ti–6Al– 4V group at 12 weeks. From these data, intramedullary nails made of Zr-based BMG promoted osteotomy healing as fast as Ti–6Al–4V alloy for application in osteosynthetic devices. The corrosion resistance of alloys also depends on their atomic structure. In crystalline alloys, grain boundaries and segregations cause irregularities of the surface oxide film, leading to corrosion of the alloy. In BMG, the grain boundary-free structure allows a homogeneous and defectfree surface oxide film to be formed. Therefore, BMG alloys generally show higher corrosion resistance than crystalline alloys with the similar composition [28]. There are several studies which evaluated BMG in vitro. Biocompatibility of Ti-based Ti45Zr38Ni17 BMG was evaluated in vitro. The study indicated that osteoblasts, cultured in presence of Ti-based BMG, differentiated and synthesized bone matrix [29]. The study to investigate mechanical properties and in vitro corrosion behavior of Ti40Zr10Cu38Pd12 BMG reported that BMG showed high hardness, relatively low Young’s modulus, good wear resistance and excellent corrosion behavior [30]. Biodegradability, mechanical behavior, and cytocompatibility of Mg72Zn23Ca5 BMG were evaluated and compared to crystalline Mg70Zn23Ca5Pd2 alloy. Mg72Zn23Ca5 BMG showed higher corrosion resistance and improved mechanical behavior. Neither Mg72Zn23Ca5 BMG nor crystalline Mg70Zn23Ca5Pd2 alloy were cytotoxic [31]. The previous literature contains only one study on the evaluation of BMG alloys in vivo [32], while another report has indicated that metallic intravascular stents with an amorphous oxide coating show excellent corrosion resistance both in vitro and in vivo [33]. Our investigation was the first to assess a BMG intramedullary nail and evaluate its influence on osteotomy healing in vivo. Some osteotomies did not reach solid union, although partial bridging with callus was seen. One of the reasons for delayed union might be rotational instability [34], since stabilizing the osteotomy with a nail combined with
J Mater Sci: Mater Med
interlocking pins to eliminate rotational instability is reported to improve bone healing [35]. Stabilizing the osteotomy with a nail combined with interlocking pins needs to be investigated in the future. The main limitation of the present study is that we used the Zr-based BMG containing nickel, which is carcinogenic. No systemic or local leaching was recognized by measuring Ni levels in the blood and the surrounding soft tissue, but Ni-free alloy might be more appropriate for the use in biomaterials. Therefore, future studies should investigate Ni-free BMG. The SEM and EDS data from this study indicated that 316L stainless steel tended to suffer from general and local corrosion probably with the preferential adsorption of proteins containing –SH group. Ti–6Al–4V alloy was liable to the formation of calcium phosphate deposits on its surface, while Zr65Al7.5Ni10Cu17.5 BMG was almost biologically inert. The SEM and EDS investigations of medical devices made of 316L stainless steel have shown that the corrosion forms a layer that consists of sulfur, phosphorus, and calcium [36, 37]. The excellent biocompatibility of titanium and titanium alloys may lead to osseointegration and strong bone-to-metal bonding may cause complications. Detaching tests performed after implantation of Ti–6Al–4V plates into the tibiae of rabbits showed that Ti alloy bonds directly to bone after about 8 weeks [38]. Vresilovic et al. [39] reported a higher rate of complications during removal of titanium pins than stainless steel pins in patients with slipped capital femoral epiphysis and attributed this difference to stronger bonding of titanium pins to the bone. In addition, a retrospective review of 45 patients undergoing removal of femoral intramedullary nails showed that removal of titanium nails required a significantly longer operating time than removal of the stainless steel nails, even though all the nails were removed intact without complications [40]. Therefore, biomaterials with low in vivo reactivity like Zr65Al7.5 Ni10Cu17.5 BMG might be more appropriate for osteosynthetic device that must eventually be removed. In this study, the BMG and titanium alloy nails were compared, but in the next study, 316L stainless steel should be compared because it is usually eventually removed. Several problems need to be overcome before adopting this material for clinical application. One problem is that even larger pieces than we tested would need to be manufactured. Cu36Zr48Ag8Al8 BMG rods with a diameter of 25 mm produced by Cu mold casting [41] and Zr55Cu30 Ni5Al10 BMG rods with a diameter of 30 mm produced by a cap-cast technique [42] are promising alloys. Another problem is the potential limitations of the Zr-based BMG concerning mechanical properties. The fatigue behavior is affected by the fabrication process, surface condition and partial crystallization [15, 16]. Therefore, high quality material should be required. In addition, Zr-based BMG is
less ductile, brittle, and high fatigue crack growth at notches. In this study neither breakage nor crack of the BMG was noted, but further in vivo studies are necessary.
5 Conclusions In this study, intramedullary nails made of Zr65Al7.5 Ni10Cu17.5 BMG were implanted in rat femora after osteotomy for 12 weeks, and systemic and local effects, surface changes of the nails, and osteotomy healing were evaluated. No increase of Cu and Ni levels was recognized in the blood and the surrounding soft tissue, indicating that there were no systemic and local effects. The SEM and EDS data indicated that Zr-based BMG was almost biologically inert. Radiographic analysis and mechanical testing indicated that intramedullary nails made of Zr-based BMG promoted osteotomy healing as fast as Ti–6Al–4V alloy for application in osteosynthetic devices. BMG alloys can be a promising new metallic biomaterial for osteosynthetic implants but further in vivo studies are necessary. Acknowledgments The authors are grateful to Prof. Takao Hanawa and Prof. Isao Ohnishi for valuable discussions. This work was funded by the grant in aid for Scientific Research received from Japan Society for the Promotion of Science and Technology. Conflict of interest
The authors have no competing interests.
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