Biocompatibility and degradation properties of WE43 Mg alloys with and without heat treatment: In vivo evaluation and comparison in a cranial bone sheep model☆
Introduction
The ideal biomaterial for osteosynthesis of bone fractures in orthopedic and maxillofacial surgery should meet several criteria: mechanical stability, biocompatibility, radiopacity and biodegradability. Currently, titanium implants possess many of these characteristics, with the exception of biodegradability; this lack, although negligible in the majority of cases, can be a tremendous disadvantage when the hardware needs to be removed due to long-term complications such as infection, exposure/fistulization (particularly following radiation therapy), interference with skeletal growth or risk of intracranial/orbital dislocation (pediatric cases), intolerance, thermal sensitivity, or interference with radiological assessment methods (in particular MRI), among other clinical scenarios (Chaya et al., 2015a, Chaya et al., 2015b, Charayeva et al., 2015, Marukawa et al., 2016). Surgical removal of internal fixation devices, in fact, increases the financial burden on hospitals and the health care system, exposes patients to increased stress and risks (Chaya et al., 2015b, Schumann et al., 2013, Shaller et al., 2016) and may be technically challenging, particularly when the hardware is fractured, the anatomy distorted and/or when the surrounding bone overgrows encasing the plate and/or screws.
To overcome the limits of permanent implants, resorbable osteosynthesis devices, mainly composed of poly-α-hydroxy acids, such as poly-l-lactide (PLLA) and poly-glycolic acid (PGA) polymers, have been introduced in clinical settings beginning in the 1990s (Schumann et al., 2013), mainly for treatment of pediatric cases. A primary drawback of those polymers is their poor mechanical strength, which makes them unsuitable for use in load-bearing sites, limiting their applications almost exclusively in reconstruction/recontouring of the cranial vault (i.e. cranioplasty) (Schumann et al., 2013, Shaller et al., 2016). Furthermore, biodegradable PLGA copolymers plates are difficult to be molded, requiring heating (by hot water or dedicated devices) to become ductile and malleable, the screws are brittle and not self-tapping, and their acidic degradation by-products can induce long-term sequelae of inflammation, subcutaneous tissue swelling, and foreign body reaction (Schumann et al., 2013, Shaller et al., 2016, Bergsma et al., 1995).
Magnesium (Mg) and its alloys are a promising alternative to poly(α-hydroxy acid) polymers for the fabrication of biodegradable osteosynthesis devices. Magnesium, a natural element in the human body (half of which is stored in bone), and in its ionic form Mg2+ is an important cofactor for many enzymes involved in DNA repair and for several metabolic pathways, making it an active promoter of healing processes and osteogenesis (Shaller et al., 2016, Bergsma et al., 1995, Weizbauer et al., 2014, Hartwig, 2001). Magnesium and its alloys have unique characteristics that make them ideal candidates to become the first biodegradable metallic osteosynthesis devices to find clinical application in orthopedic and craniomaxillofacial surgery due to the following: 1) mechanical properties comparable with those of cortical bone (Weizbauer et al., 2014, Zhao et al., 2016), providing enough stability for use in load-bearing fracture sites, while reducing the mechanical stress on bone leading to “bone stress shielding” phenomena (Marukawa et al., 2016, Weizbauer et al., 2014); 2) good balance between mechanical stability, ductility, malleability and degradation rate (Chaya et al., 2015a, Chaya et al., 2015b, Chaya et al., 2015b, Charayeva et al., 2015); 3) potential to support bone formation (osteoconduction) and facilitate osteogenesis (Weizbauer et al., 2014, Janning et al., 2010, Witte et al., 2007); and 4) biocompatibility and antibacterial properties (Li et al., 2014).
Lambotte (1932) in 1932 was the first to report the use of a magnesium bone implants. However, the high resorption rate of pure magnesium devices, along with the concerning gas formation associated with their degradation, which was often associated with subcutaneous emphysema and gas gangrene, discouraged the use of Mg and its alloys diverting the attention on more stable and biocompatible metals (steel, vanadium, titanium, etc.). In the last decade, advances in Mg alloying through grain refinement, coating, combination with different elements (calcium, neodymium, zirconium, rare earth elements, etc.) have substantially improved the degradation and biocompatibility issues reported in the past (Charayeva et al., 2015, Shaller et al., 2016, Al-Samman and Li, 2011, Ullmann et al., 2013, Imwinkelried et al., 2013), and Mg alloys have gained renovated interest as materials for biodegradable osteosynthesis devices.
Several in vitro and in vivo studies have been conducted testing different Mg alloys for biocompatibility, gas formation, degradation rate, and mechanical stability. However, comparison of results yielded by different studies is challenging because of the substantial differences in Mg alloys composition and manufacturing methods, as well as variety of protocols used (in vitro studies, in vivo studies, different solutions for corrosion test, different animal models). These studies do show that degradation rate is higher in vitro than in vivo (Shaller et al., 2016, Imwinkelried et al., 2013, Martinez Sanchez et al., 2015), probably because of the lack of physiologic buffering systems counteracting the degradation processes encountered in in vitro experiments; corrosion and strength loss are influenced by local environment in vivo, being accelerated in tissues highly perfused and with higher water content (muscle, periosteum) and reduced when the implant is encased in bone (Chaya et al., 2015a, Witte et al., 2006, Willbod et al., 2011); gas formation as result of Mg degradation remains a biocompatibility concern when it exceeds the physiologic clearance mechanism (especially during the first weeks after implant positioning) producing gas pockets (Chaya et al., 2015a, Chaya et al., 2015b, Charayeva et al., 2015, Marukawa et al., 2016, Shaller et al., 2016). Such pockets may result in subcutaneous emphysema (Zhao et al., 2016, McCord et al., 1942) and/or impair the bone healing processes (Marukawa et al., 2016).
In this metallurgical characterization and in vivo study, a highly translational large animal model (sheep) was utilized to test the biocompatibility of WE43 Magnesium alloy in two different conditions, as-cast (Control) and heat treated: hot rolled followed by T5 temper, referred to as T5 alloy in this study (Experimental). Both alloys present the same composition comprising the addition of alloying elements (yttrium [Y], heavy rare earth elements [RE], and zirconium [Z]) to Mg in order to obtain products with more favorable mechanical properties and corrosion behavior; the experimental Mg alloy went through further processing by heat treatment to obtain an artificially aged product (WE43-T5) with different degradation properties characteristics. Both the as-cast and T5 alloys, in the form of cylindrical rods measuring 10 mm diameter and 5 mm thick, were positioned subperiosteally on the nasal-frontal region of each animal in contact with the bone without violating the cortical layer. Endpoints of the study were: 1) physicochemical alloy characterization; 2) assessment of local biocompatibility of the two alloys by nondecalcified histological analysis including bone, implant and surrounding soft tissue (periosteum and deep subcutaneous tissue); and 3) evaluation of regional effects of the implants were tested by inductively coupled plasma mass spectrometry (ICP-MS) of regional lymph-nodes for detection of metal deposition.
Section snippets
Samples
Two different compositions of WE43 magnesium alloy, indicated as as-cast and T5 temper, were used in the study. Both the magnesium alloys tested have the same nominal composition accounting: yttrium (Y) 4 wt.%, RE 3.3 wt.%, and zirconium (Zr) 0.5 wt.%, where RE represents rare earth elements. The alloy was manufactured at the facility of Magnesium Elektron North America (Madison, IL, USA). WE43-T5 was artificially aged by heat treatment at 210 °C for 48 h, to obtain the desired peak aged (T5
Physicochemical characterization
The microstructure of as-cast WE43 alloy consists of bulk α-Mg phase with eutectic mixture oriented along grain boundaries, as observed in Fig. 1a. The eutectic phase is dissolved in α-Mg in WE43-T5 as observed in Fig. 1b. The grain size of the as-cast and T5 specimens was measured from the optical micrographs to be 54.1 ± 18.6 and 24.8 ± 12.5 μm, respectively. Close observation of Fig. 1c shows a lamellar structure of the eutectic mixture in as-cast WE43. EDS analysis in Fig. 1d indicates that
Discussion
Mg and its alloys represent a promising biomaterial to create resorbable osteosynthesis devices for orthopedic and maxillofacial purposes; with a tensile and compressive strengths of ∼300 MPa and specific density of 1.7–2 g/cm3, Mg alloys are suitable to be used in load-bearing sites, and mimic the mechanical properties of the human cortical bone (Marukawa et al., 2016, Zhao et al., 2016). The major advantages of Mg alloys, compared to the titanium-based rigid fixation osteosynthesis currently
Conclusion
Biocompatibility of as-cast and T-5 WE43 Mg alloys has been tested in vivo in a sheep model. The main findings are summarized as follows: Both tested alloys showed complete biocompatibility clinically and histologically. Surprisingly, no hydrogen gas pocket formation was observed, a data which is in neat contrast with reports in the current literature. This observation is attributed to the small size implant in the large animal model and requires further investigations to be confirmed.
The T-5
Acknowledgments
The authors acknowledge U.S. Army Research Laboratory Cooperative Agreement W911NF-11-2-0096 for partially supporting this research by providing materials. The views presented in this article do not necessarily represent the views of the funding agencies or the government of the United States.
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The study was not supported by any grant, the authors acknowledge U.S. Army Research Laboratory Cooperative Agreement W911NF-11-2-0096 for partially supporting this research by providing materials.