Effect of sterilization on 3-point dynamic response to in vitro bending of an Mg implant

Abstract

Background: The aim of the study is to characterize a biomedical magnesium alloy and highlighting the loss of mechanical integrity due to the sterilization method. Ideally, when using these alloys is to delay the onset of degradation so that the implant can support body loads and avoid toxicological effects due to the release of metal ions into the body.

Methods: Standardized procedures according to ASTM F-1264 and ISO-10993-5 were used, respecting detailed methodological controls to ensure accuracy and reproducibility of the results, this testing methodology is carried out in accordance with the monographs of the Pharmacopoeia for the approval of medical devices and obtaining a health registration. An intramedullary implant (IIM) manufactured in magnesium (Mg) WE43 can support loads of the body in the initial period of bone consolidation without compromising the integrity of the fractured area. A system with these characteristics would improve morbidity and health costs by avoiding secondary surgical interventions.

Results: As a property, the fatigue resistance of Mg in aggressive environments such as the body environment undergoes progressive degradation, however, the autoclave sterilization method drastically affects fatigue resistance, as demonstrated in tests carried out under in vitro conditions. Coupled with this phenomenon, the relatively poor biocompatibility of Mg WE43 alloys has limited applications where they can be used due to low acceptance rates from agencies such as the FDA. However, Mg alloy with elements such as yttrium and rare earth elements (REEs) have been shown to delay biodegradation depending on the method of sterilization and the physiological solution used. With different sterilization techniques, it may be possible to keep toxicological effects to a minimum while still ensuring a balance between the integrity of fractured bone and implant degradation time. Therefore, the evaluation of fatigue resistance of WE43 specimens sterilized and tested in immersion conditions (enriched Hank's solution) and according to ASTM F-1264, along with the morphological, crystallinity, and biocompatibility characterization of the WE43 alloy allows for a comprehensive evaluation of the mechanical and biological properties of WE43.

Conclusions: These results will support decision-making to generate a change in the current perspective of biomaterials utilized in medical devices (MDs), to be considered by manufacturers and health regulatory agencies. An implant manufactured in WE43 alloy can be used as an intramedullary implant, considering keeping elements such as yttrium-REEs below as specified in its designation and with the help of a coating that allows increasing the life of the implant in vivo.

Keywords: Biological evaluation; Fatigue resistance; Intramedullary implant; Morphological characterization; Toxicity.

Fig. 1

General dimensions of tension specimen.(mm)

Fig. 2

Test specimens (ASTM F1264–03) 3 pointing bending test scheme (A1.4, A4.2)

Fig. 3

a System used in simple bending tests for the WE43 9.54ø × 114 mm specimen. b Load applicator rollers

Fig. 4

a Test specimen for fatigue flexion tests at 3 points. b System used to perform a fatigue flexion test. c Instron 1011 machine configuration with polypropylene chamber assembly for specimen immersion

Fig. 5

Load ratio from 0 to − 1 vs cycles in time(s)

Fig. 6

Average stress-strain curve obtained by simple tension. (Offset method)

Fig. 7

Stress-strain curve of the 3 specimens tested at simple tension

Fig. 8

Average values of force vs displacement, simple flexion

Fig. 9

Values of force vs displacement, 3 tests with simple flexion

Fig. 10

shows the detrimental effect of autoclave sterilization when comparing test 1 (Green) that receives sterilization and acclimatization, against the result of test 2 (Blue) without sterilization, in both tests the protocol of aseptic control and acclimatization.

Fig. 11

a Element mapping by SEM-EDS (O, Mg, Y, La, Nd, Gd, Fe, Ni and Cu) distribution analysis corresponding to those second phase particles. b Diffraction pattern image; under examination by TEM (Bright field, dark field). c Second phases identified in WE43, α-Mg, Gd9.08 Mg45.9, and Gd0.75 Mg0.25, HRTEM images were taken from specific sites marked with A & B. Lower left image, a cluster of nanoparticles of uniform size and quasi-spherical geometry approximately 5 nm. d XRD patterns of the WE43

Fig. 12

The effect of WE43 extracts in the viability of L-929 murine fibroblasts. The graph represents results obtained after 48 h incubation with non-serial dilutions of WE43 extracts as assayed by the neutral red uptake assay (n = 4). Data are expressed as media ± standard deviation in percentage relative to the control cells (treated with cell culture medium). *P ≤ 0.05

Fig. 13

a Microstructure. b Approach to grain size for an approach to grain boundaries. (Detail a) c the mesh represents a real microstructure obtained from the examination through SEM-EDS

Fig. 14

a Directional deformation. b Displacement of elements (grains)

Fig. 15

a Deformation concentration towards grain boundaries. b Equivalent elastic deformation stimulating the propagation of the cracks