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Orthopedic implant design success is defined by material choice as much as it is by size and shape. Bioresorbable composites, absorbable metals, superelastic alloys and ultra-high-strength wires are giving engineers new options in thinking about the ways implants are made and function.
“For engineers navigating aggressive size, strength and performance targets, material class selection can be just as critical as geometry,” said Adam Griebel, Senior R&D Engineer at Fort Wayne Metals.
There are many tools available to reach higher strength and stiffness requirements, from processing and alloying to strategic material substitution. At Fort Wayne Metals, that toolbox includes several advanced materials.
Absorbable magnesium alloys. Polymers are common absorbable materials, but magnesium offers an interesting alternative. The material is inherently resorbable but exhibits metal-like mechanical properties that are more familiar to device designers.
“While magnesium cannot match titanium or cobalt-chrome in absolute strength or stiffness, it significantly outperforms many absorbable polymers,” Griebel said.
Compared to materials like polylactic acid, magnesium can offer five to ten times the strength and roughly an order of magnitude higher stiffness, which reduces unwanted device flex. Griebel pointed out that magnesium exhibits substantially higher toughness than polymeric alternatives when it’s properly alloyed and processed.
Pure magnesium alone is generally insufficient for load-bearing orthopedic applications, particularly when transitioning from titanium-based designs. Alloying is therefore essential.
Unlike titanium, however, magnesium does not benefit from a library of standardized, off-the-shelf medical alloys. Fort Wayne Metals has therefore focused on alloying magnesium with elements that are both mechanically beneficial and biologically familiar.
“Calcium, zinc and manganese improve magnesium’s performance while also being essential nutrients that the body needs to survive,” Griebel said.
Fort Wayne Metals’ reps describe the company’s proprietary magnesium alloy, ZXM100 (it contains zinc, calcium and manganese), as a “vitamin metal.”
ZXM100 is available in multiple product forms, including wire, ground bar and sheet, allowing engineers to begin evaluating how absorbable metals might fit into future device concepts.
Superelastic beta titanium. Superelasticity has also transformed medical device design with nitinol serving as the go-to material. Griebel said its ability to reversibly transform between austenite and martensite enables recoverable strains of up to 8%, far exceeding conventional metals.
However, nitinol’s nickel content presents challenges for the estimated 15% of the population that has nickel sensitivity or allergy, a reality that has driven interest in alternative materials that can deliver superelastic behavior without containing nickel.
Fort Wayne Metals addressed this challenge by developing a patent-protected beta titanium alloy containing a titanium-niobium-zirconium-hafnium-tin alloy. Although the alloy doesn’t match nitinol’s superelastic capability, Griebel said it achieves 3% to 4% recoverable strain, representing a substantial improvement over conventional titanium or stainless steel, which typically offer only 0.5% to 1%.
“This nickel-free superelastic alloy is now available in R&D quantities across multiple product forms, giving engineers a new option when biocompatibility concerns limit the use of nitinol,” he said.
Ultra-high-strength refractory metal wires. When engineers reduce the size of a device, particularly its diameter, they sacrifice structural stiffness. At that point, maintaining device performance often requires moving to a material with higher inherent stiffness and strength.
One approach is to tailor the manufacturing process itself. How a material is processed can have a significant impact on its mechanical properties.
“A good example is our 4TITUDE product, for which standard commercially pure Grade 4 titanium is processed in a way that delivers very high strength that often exceeds that of cold-worked Ti-6Al-4V bar in the typical 3mm to 6 mm diameter range,” Griebel said. “Similar processing strategies can be applied across a wide range of materials.”
According to Griebel, one potential step-change in performance comes from advanced processing techniques such as continuous ECAP (equal-channel angular pressing). This severe plastic deformation process introduces large shear strains to create a nanograined crystal structure, resulting in higher strength, improved fatigue life and potentially enhanced bone attachment.
“While still in the early stages of development,” Griebel said, “the approach represents a promising new direction.”
Strength can also be increased through alloying and by adding specific elements to a base material, it’s possible to achieve substantial gains in mechanical performance. For example, Griebel said, if nitinol-like superelastic behavior is needed but higher strength is required, elements such as niobium and yttrium can be introduced.
“We’ve developed an alloy using this approach that more than doubles the superelastic plateau strength,” he added. “In practical terms, that added strength enables devices such as bone staples to be designed with a much lower profile.”
Strengthening Bioresorbable Implants
For decades, orthopedic implant design has operated under a familiar constraint: Metal must be used if mechanical strength is required because there has been no fully resorbable polymer or composite capable of carrying the required loads.
According to Ville Ellä, D.Sc., R&D Director for New Technology at Arctic Biomaterials, that paradigm is changing.
Arctic Biomaterials focuses on the processing of biocomposites and biopolymers for medical applications and resorbable fixation. At the center of this platform is the X3 Fiber Composite, a material system designed to address the strength limitations of bioresorbable implants.
The system’s reinforcement element comprises constituents already present in human bone. As the composite degrades, it provides strength and drives bioactivity, and releases familiar mineral components rather than foreign byproducts.
As the material resorbs, the fiber surface becomes surrounded by a hydroxyapatite/carbonate layer that actively interacts with surrounding bone tissue. This development promotes bone formation during degradation to deliver biological benefits that are similar to ceramic-based composites but without sacrificing mechanical performance.
Preclinical results support this dual function. In animal studies, CT imaging at four weeks shows a closed cortex around the implant site. Importantly, this biological performance is paired with mechanical properties that are six times stronger than existing resorbable polymers and composites.
The elastic modulus of the X3 Fiber exceeds that of native bone to provide structural support while also remaining well below that of metals to reduce the risk of stress shielding. The result is a material that carries load when needed, then gradually transfers the load back to the healing bone.
The composite can be used to produce screws, plates, staples, pins, intramedullary devices, cages, wedges, buttons and mesh-like structures. Fiber placement can be tailored to reinforce high-stress regions, allowing engineers to design implants that are strong where strength is needed most.
From a manufacturing standpoint, Arctic Biomaterials offers multiple material formats to support different production strategies. Injection-moldable pellets with short fibers provide a cost-efficient solution for many implants, with raw material accounting for only a small fraction of the overall implant cost.
For more advanced designs that require higher strength capacities, tape from the company’s X3 Fiber composite can be winded, applied with tape laying methods or compression molded to achieve material utilization rates above 95%.
Arctic Biomaterials offers orthopedic engineers a design space that no longer requires permanent metal.
“Our mission is to deliver a material platform that combines high mechanical strength with complete bioresorbability, enabling stable fixation, natural bone healing, and ultimately, no second surgery,” Ellä said.
