High-strength and Corrosion Resistant Alloy for Patient-Specific Bioresorbable Bone Fixation Hardware
T2018-394 Novel ternary and quaternary Mg-Zn-Ca-based alloys and a heat treatment process for producing bioresorbable bone fixation implants with improved mechanical and corrosion properties
Mg-Zn-Ca-based alloys are the most promising alloy system for bone implant applications mainly due to their superior biocompatibility. For fixation applications, the ideal is a material strong enough to hold while healing (2-4 months) yet being fully resorbed after the bones have healed (6-24 months). For patient-specific (3D-printed) fixation hardware made of such alloys, various heat treatment processes have been employed to enhance the mechanical or corrosion properties of fixation devices. Most of these efforts have focused on enhancement of a single property such as mechanical strength, biocompatibility or biocorrosion. However, for optimal results, the use of heat treatment for developing improved Mg-Zn-Ca-based bone fixations should address all of the following: (i) proper choice of alloy chemical composition; (ii) proper choice of the heat treatment process and parameters; (iii) asessment of mechanical properties; and (iv) assessment of biocorrosion properties after heat treatment.
This invention is a joint development of the University of Toledo and Ohio State University. It comprises a process in which a novel Mg-Zn-Ca-based alloy is cast and then heat-treated - in particular, solution-treated, quenched and age-hardened. The chemical composition of the cast Mg-Zn-Ca alloy is chosen to obtain the optimum age hardening effect after the heat treatment process. Heat treatment processes and parameters are also chosen carefully to optimize results. For example, the alloy was aged at different age hardening temperatures to determine the temperature that results in the highest mechanical and corrosion resistance. Addition of Mn to the alloy was found to further enhance mechanical and corrosion properties. The ternary or quaternary alloy is coated with a biocompatible ceramic coating using micro arc oxidation (MAO) followed by additional ceramic layering and a sintering process. This coating process determines when resorption begins, allowing for a tailored biocorrosion rate specific to the patient's needs.
- Skeletal fixation and other Mg alloy implantables
- Age hardening duration of 2-5 hours demonstrates optimum mechanical properties, with the tensile and compressive yield strengths of the alloy age-hardened for 3 hours being 1.4 and 1.9 times those of the as-cast alloy, respectively.
- The heat-treated alloy has less negative potential and lower corrosion density than the as-cast alloy. The corrosion rate of the heat-treated alloy aged for 2-5 hours is approximately half that of the as-cast alloy.
- The grain size of the alloy after addition of Mn (67 µm) is much lower than the Mn-free alloy (216 µm), yielding improved mechanical and corrosion properties.
- In vitro electrochemical corrosion tests at pH 7.4 and 37º C demonstrate significantly lower corrosion rates for the heat-treated alloy after the addition of Mn.
- The corrosion rate of the heat-treated alloy at pH 7.4 and 37º C was reduced from 8.7 mm/year to 0.03 mm/year after the MAO coating process.