Stainless 316L popular for orthopaedic surgical implants

By Srikumar Chakraborty, ex ASP/SAIL, Freelance Consultant

The chemical composition of biomedical types of austenitic stainless steel AISI 316L, ASTM F55, and F138 consist of 17–20% Cr, 12–15% Ni, and 2–3% Mo. This composition allows the product to maintain an austenitic structure from cryogenic temperatures up to the melting point. . The Cr enhances its passivation ability, while the Mo improves the pitting corrosion resistance of the stainless steel. This grade with carbon below 0.03% is the most commonly used for biomedical applications, The lower carbon level in 316L reduces carbide (Cr–C) precipitation at the grain boundaries, thereby minimising corrosion. Because austenitic stainless steel cannot be hardened by heat treatment, its mechanical properties can only be improved using cold deformation methods.

Complex application

For years, orthopaedic surgeons, with the support of scientists, engineers, metallurgists and researchers, have relied extensively on stainless steel 316L for surgical fixation implants to treat patients with acute orthopaedic injuries. However, surgical implementation is merely one step in an intricate and complex process. Prior to the arrival of a stainless steel implant to the operating room, a comprehensive product development process that closely assessed the various physical and chemical properties of the implant has taken place. Beyond ensuring the inherent stability of the implant, developers also establish compatibility. The field of biomaterials is both fascinating and challenging due to the various complexities of its potential applications and the need to improve the quality of life.

Clinical significance

Much has changed in the field of orthopaedic implants. Patient outcomes have been improved by newer designs, improved materials, surgical innovations, and improvements that optimise cost and lifecycle. Despite these advances, areas of concern remain. Peer-reviewed data and non-biased implant research are essential for the deployment of newer devices that are safe and effective.

The importance of understanding how to select the correct implant based on the task at hand cannot be overstated. Implant design continues to evolve and tackle the challenges of cost, reliability, longevity, and infection prevention.

Characteristics & properties of orthopaedic alloys

Quality issues such as thermal conductivity, density, malleability, corrosion, and specific heat are thoroughly considered for alloys applied in orthopaedic applications. Increased density confers increased strength and stiffness. These are two fundamentally important concepts when considering the functionality of an implant. For instance, in the case of a hip replacement femoral stem, the biomaterial’s physical compliance should match the adjacent bone to inhibit stress shielding and/or bone loss resulting from a lack of loading. Material selection is a major criterion in the development of any metal surgical device. The main alloys utilised are described as follows.

Stainless steel 316L

Chemical composition: C 0.03 max, Mn 2.0 max, Si 0.75 max, Cr 16-18, Ni 10-14, Mo 2-3, P 0.045max, S 0.03 max. Density 8.02 gm/Cm2, modulus of elasticity in tension 200GPa, melting point 1390 – 1440° C, rolling/forging temperature 1050- 1260° C.

Parts are made from rolled, forged or cast products, CNC machined, then polished to achieve a fine surface finish. Stainless steel 316L has been employed in the design of countless implants, including plates, rounds, screws, sliding hip screws, flexible nails, early-generation rigid intra-medullar nails, and Cerclage cables and wire used to fix/stabilise fractured bone fragments. This material does not corrode in the oxygen-rich environment of the body

Mechanical properties: The modulus of elasticity describes the ability of a material to deform under stress both linearly and predictably. Stainless steel provides a high modulus of elasticity, giving excellent construct rigidity. For fractures requiring stable fixation, this makes stainless steel appealing as an implant material.

Machining and surface finishing: Originators of open reduction and stable fixation in Europe sourced many of their early implants locally. In many cases, implants were developed in very basic machining environments and built-to-spec on a case-by-case basis.

Early intra-medullar nail rods were made of stainless steel, including tri-flange nails, among others. Many early intra-medullar nails were successful, but some went on to atrophic non-union due to the marked stiffness of these nails. In an attempt to improve these complications, some nails were “slotted” or given a horseshoe-like cross-section to make them less rigid. Fracture care with hardware has evolved. In the 1970s through the 1990s, rigid constructs were preferred. In the past 20 years, there has been a shift away from complete rigidity, with modern fracture fixation theories favouring a less rigid construct to enhance bone healing.

Stainless 316L polished to a high degree of smoothness has been used in many arthroplasty. Plates and round parts are ductile enough to be altered in the operating theatre.

CoCrMo alloy

Chemical composition: Co 58.9–69.5%, 27.0–30% Cr, 5.0–7.0% Mo, and small amounts of other elements (Mn, Si, Ni, Fe and C), complying with the ASTM standards of F-75 or F-1537 for cast alloys and wrought alloys, respectively. Co-Cr-Mo alloys are one of the most useful alloys for biomedical applications in areas like dental and orthopaedic implants because of their excellent mechanical properties and biocompatibility.

The cast alloy of this grade shows low ductility compared to the forged or rolled products owing to the formation of shrinkage porosity, inter-dendritic segregation, and inclusions at the grain boundaries. Casting technology has improved the strength and ductility of these alloys. Moreover, maintaining the crystal structure of the Co-Cr-Mo alloys to be the phase (FCC-Face Center Cubic) is considerably effective in keeping the Co-Cr-Mo alloys ductile.

Mechanical properties: With a very high modulus of modulus of elasticity, this alloy is not ideal when elastic or plastic material deformation properties are desired. Its surface can be highly polished to obtain an incredibly high surface smoothness, which allows for minimal wear in metal and the ability to withstand extreme force before fracturing.

This alloy can provide machining challenges due to its high modulus of elasticity and low ductility. The most desirable property of cobalt chrome is its highly polishable surface, which decreases abrasive wear on polyethylene bearing surfaces. CoCrMo alloy has the lowest yield strength of 450 MPa due to inter-dendritic inclusions of σ phase at the grain boundaries. The crystal structure of the Co-Cr-Mo alloys will transform from the phase that is stable at high temperatures to the phase that is stable at room temperature. Since Ni is the most common metal sensitizer in the human body, the use of Ni should be avoided or restricted.

Titanium alloy

Chemical composition: While Ti is a metallic element, the majority of orthopaedic Ti implants are proprietary alloy blends, e.g. Ti-6Al-4V. Increasing demand has driven the search for materials that combine strength, biocompatibility, and a long lifetime. Compared to stainless steel and CoCrMo alloys, Ti and its alloys are favoured for their high strength, corrosion resistance and biocompatibility.

Strengthening properties, wear, fatigue, and corrosion resistance are crucial factors influencing the performance and reliability of Ti implants. There have been improvements to Ti-
based biomaterial durability through surface modifications to enhance their biofunction, wear resistance, corrosion resistance, and antibacterial properties. It is critically important for surgical device manufacturers to select a metal alloy that fares well in its destined anatomic location. Orthopaedic hip stems are often made of Ti because of their excellent fatigue strength, lightweight, and ability to reduce stress shielding, which can lead to localised bone loss surrounding the implant. Independent of adjacent metals, Ti forms an outer oxide layer that protects it from corrosion. However, in the presence of mechanical contact with other metals, this Ti oxide layer breaks down and friction results in third-body particulate and wear of the underlying Ti. Therefore Ti alloys do not fare well under sliding contact applications that require load bearing, such as articulating joints. Other disadvantages of pure titanium include its low elastic modules when compared to stainless steel or Co-Cr and low shear strength.

Tantalum

Tantalum implants are less widely used in orthopaedic surgery today but may see expanding use due to material cost and increasing revision rates in arthroplasty

Manufacturing 316L orthopaedic implants

Austenitic stainless 316L is melted in an EAF and finished as cold-worked to increase its mechanical strength. Through severe plastic deformation, ultra-fine-grained 316L alloy exhibits a higher specific strength useful for many applications, including implants. Average grain size is reduced from 30 μm to 0.86 μm after several strain steps. Strengthening and strain hardening increase the hardness. Improved sliding wear resistance is attributed to a transition from micro-cutting to a wedge-forming mode of abrasive wear. Load-bearing orthopaedic implants often fail from pitting-initiated corrosion fatigue. Various test results have revealed enhanced pitting resistance of forged steel, which is confirmed by analysis. This is ascribed to an increase in the grain boundary volume and homogenisation of pit-inducing impurities and non-metallic phases due to severe deformation, which influenced the passive film properties. These model studies on 316L steel demonstrate that severely deformed ultra-fine-grained metals have the potential to deliver improved implant performance.

This low-temperature cold-worked material is then used as a starting material for the manufacture of surgical implants. Additional strength improvement has been reported for 316L by subjecting the cold-worked steel to a low-temperature stress relief process in Improved Properties of 316L by Low-Temperature Stress Relief’ by Hochman, et al, Journal of Materials at page 425-442 (1966). The Hochman, et al article reports improvements in hardness, tensile strength, and yield strength by stress relieving cold-worked specimens of type 316L stainless steel at temperatures of about 750° F (399° C) for approximately two hours. Although some improvement in the mechanical strength of the cold-worked starting material has been achieved by this stress-relief technique, as reported by Hochman, the corrosion fatigue resistance of the stress-relieved starting material is not affected by such stress relieving.

Conclusion

The structural characteristics of 316L stainless steel make it suitable for many load-bearing orthopaedic applications. Most metallic materials are intrinsically susceptible to chemical attack or corrosion from reacting with aqueous physiologic environments surrounding an injury. Implant degradation not only concerns the structural integrity of a device, but may also result in a systemic response. As a result, 316L materials chosen for implantation are both resistant to oxidative or chemical stress and eligible for protective mechanisms such as passivation. For development engineers and manufacturers the material selection process is a multifaceted responsibility factoring in geometry, structural and physiologic stresses and biological environment.

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Appearing in the March 2025 issue of Stainless Steel World Magazine, this Featured Story is just one of many insightful articles we publish. Subscribe today to receive 10 issues a year, available monthly in print and digital formats. – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.

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