Biomaterials in Orthopaedic Practice
Derek R. Jenkins, MD
Douglas C. Moore, MS
Neither of the following authors nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter: Dr. Moore and Dr. Jenkins.
This chapter is adapted from Mann KA, Allen MJ: Biomaterials in Orthopaedic Practice, in O’Keefe RJ, Jacobs JJ, Chu CR, Einhorn TA, eds: Orthopaedic Basic Science, ed 4. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2013, pp 69-85.
Keywords: biomaterials; engineering; preclinical
INTRODUCTION
Biomaterials are composed of synthetic and natural substances used in orthopaedic implants and devices applied in biological systems. The field of biomaterials represents a convergence of biology, chemistry, physics, engineering, and medicine and is relatively young, at less than 100 years old, but has come in recent years to significantly impact healthcare and the economy.1,2,3,4 Biomaterials are used in trauma surgery in plates, screws, and intramedullary rods, and as prostheses in joint replacement surgeries, but also as sutures, adhesives, and drug-eluting devices. The main classes of synthetic biomaterials are metals, polymers, ceramics, and composites. As the world population ages and standard of living improves, it can be expected that the field of biomaterials will become more and more prominent in the practice of orthopaedics, and so it is paramount as responsible surgeons that we have a fundamental understanding of basic principles to guide us in our ethical and effective employment of these technological foundations of our practice.
BIOCOMPATIBILITY
Interactions between a biomaterial and the host biological system will lead to success or failure of the material. Failure can be due to the biomaterial’s local or systemic adverse effect on the biological system or due to the biological system’s adverse effect or degradation of the biomaterial. Biocompatibility is a term used to describe the functional integration of the biomaterial into the host biological system.5 Biocompatibility is affected by chemistry of the biomaterial, leachables, shape, mechanics, and design. Some of the early work on the materials used in orthopaedics was done in concert with dentistry, including poly methyl methacrylate, cobalt-chromium-molybdenum and titanium alloys. A biomaterial can be inert, interactive, or viable. Inert biomaterials are bulk materials having little or no adverse biologic response. Interactive biomaterials are passive materials designed to elicit a specific biologic response such as bony ingrowth. Viable biomaterials integrate or attract cells and are subsequently remodeled or resorbed. It is important to note that a biomaterial may be inert in bulk form but can elicit a local adverse effect if corrosion occurs in the host biological environment, or if mechanical wear produces particulates of a certain size and concentration. It is also important to recognize that there is often a change over time in the interaction of host to biomaterial, and biomaterial to host, and this biocompatibility is a dynamic relationship.6,7
At the cellular level, biocompatibility is influenced by the toxic effects of leachates, microbiologic or endotoxin contamination, mechanical interaction at the implant-tissue interface, and direct cell-biomaterial interactions. The vast majority of
orthopaedic implants are fabricated from a short list of metals, polymers, and ceramics (Table 1). All are well characterized and most have decades of successful use.
orthopaedic implants are fabricated from a short list of metals, polymers, and ceramics (Table 1). All are well characterized and most have decades of successful use.
TABLE 1 Most Common Orthopaedic Biomaterials | ||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||
MECHANICAL BEHAVIOR OF ORTHOPAEDIC BIOMATERIALS
Electromagnetic interactions hold atoms together in ascending order of strength of bonding through van der Waals forces, hydrogen bonding, metallic forces, covalent interactions, and ionic forces. Materials are formed when these forces combine atoms or covalently bound atoms (molecules). How the atoms or molecules are organized and bound will determine their mechanical properties. Atoms are about the scale of 0.2 nm in diameter, molecules range from 1 to 100 nm, and molecules are organized into much larger structures. Structural units at the surface of a material are acted upon by different forces than those inside the material bulk and as such have a surface energy which leads to reactivity and unique properties.
Most orthopaedic implants are used in load-bearing applications, to replace damaged or diseased tissue (eg, joint replacement), or to provide structural support during healing (eg, internal fixation). As such, they must have structural properties appropriate for their intended applications. They must be sufficiently strong and have sufficient fatigue resistance to withstand any applied loads, and it is important that their stiffness is tuned to positively influence healing. Compliant implants stimulate bone formation through load sharing and, in the case of fracture fixation, controlled micromotion. Implant structural properties are determined via modeling (eg, finite element analysis), or laboratory testing. There are numerous ASTM standards for implant testing. Testing of unique configurations is routinely reported in the literature.
Implant structural properties are a function of (1) component geometry and (2) the mechanical properties of the material(s) used for fabrication. Component geometry is determined during design, within the anatomic and functional constraints of the application. The mechanical properties of materials are a function of their internal structure—specifically, the atoms and molecules they contain, and how the atoms and molecules are organized at the crystalline and microstructural levels. Material properties are determined empirically; they cannot be predicted analytically.
It is also important to distinguish between the concepts of bulk and surface properties of biomaterials as both are important to the success or failure of the material. Mechanical bulk properties dictate a material’s ability to carry loads over multiple cycles over time without deformation or failure. Surface properties of biomaterials will determine interactions between the material and its host environment.
BULK MATERIAL PROPERTIES
Bulk material properties depend on the types of atoms and molecules it contains and also their arrangement, called microstructure. Microstructure concerns very small scales of the order 10-4 to 10-9 m. Bulk properties that depend on the type or composition of the material and not on microstructure are called intrinsic properties and include density and heat capacity. Bulk properties that depend on the microstructure of the material are called extrinsic properties and include yield strength and stiffness.
LOAD, STRESS, AND STRAIN
When a force or load is applied to an object, it can translate (move), rotate, and deform. To quantify material properties and make comparisons between different materials, it is important to standardize the shape and size of materials to be tested. Specifically, to discuss the concepts of stress and strain, we prepare a sample such that the cross section perpendicular to the applied force is the same, such as a cylinder or rectangular prism, and apply a force perpendicular to a pair of opposite faces of this material.
The most widely used tests to characterize material properties are monotonic tensile tests. Monotonic tests provide information about a material’s ability to withstand a single maximum applied load. In these tests, force is measured as material specimens are elongated until they break. The specimens are carefully machined to precise diameters and lengths, and the data
are reported in terms of applied force per unit cross-sectional area, or stress (σ = F/A0), and deformation per unit length, or strain (ε = (Lf − L0)/L0) (Figure 1). Stress-strain plots are important graphical representations of a material’s inherent mechanical properties.
are reported in terms of applied force per unit cross-sectional area, or stress (σ = F/A0), and deformation per unit length, or strain (ε = (Lf − L0)/L0) (Figure 1). Stress-strain plots are important graphical representations of a material’s inherent mechanical properties.
ELASTIC BEHAVIOR
Typical stress-strain plots include an initial linear region where stress increases proportionally with strain, followed by a nonlinear region that continues to the point of failure (Figure 1). The linear region reflects the fact that at low strains many materials behave elastically, recovering completely when the load is removed. The constant of proportionality, or slope, of the linear elastic region is defined as the modulus of elasticity, E (E = σ/ε). The modulus of elasticity reflects the stiffness of a material. Materials with a relatively high modulus of elasticity deform less under load than those with a low modulus of elasticity.11,12,13,14
Materials used as load-bearing implants should have elastic properties similar to the tissue to which they are adjacent. This would prevent differential motion at the interface between dissimilarly elastic materials, which could lead to loosening and failure. It follows then that titanium alloys are particularly good materials for total hip stems, and that poly methyl methacrylate is a useful intermediary between the more stiff cobalt-chromium-bearing surfaces and the adjacent host bone.
PLASTIC BEHAVIOR AND FAILURE
The elastic limit, or yield point, marks the transition from elastic to plastic behavior. At the yield point the internal structure of the material begins to rearrange irreversibly, resulting in a permanent residual strain that is not lost on unloading. Closely related to the yield point is the yield strength, which is defined as the stress that results in plastic deformation of the material. Beyond the yield point molecular rearrangement results in a generalized decrease in the cross-sectional area along the length of the specimen. Ultimately, the plastic deformation localizes to a small region of the specimen that constricts (“necks”) preferentially on the way to failure, and stress decreases with increasing strain. Tensile strength reflects the maximum stress a material can withstand before failing, and breaking strength is the stress when the material finally breaks catastrophically.
 FIGURE 1 Schematic of tensile test. Samples machined to precise diameters and lengths (L0) are loaded to failure in tension and the data are reported in terms of applied force per unit cross-sectional area, or stress (σ = F/A0), and deformation per unit length, or strain (ε = (Lf − L0)/L0). Typical stress-strain plots include an initial linear region where stress increases proportionally with strain. The slope of the linear elastic region is defined as the modulus of elasticity, E (E = σ/ε). The yield strength (YS) reflects the stress that results in plastic deformation of the material. The tensile strength (TS) is the maximum stress the material can withstand before failing, and the breaking strength (BS) is the stress when the material finally fails catastrophically. |
Other important mechanical properties that are reflected in stress-strain curves are ductility and toughness. Ductility is a material’s ability to withstand plastic strain before fracture. Ductility may be reported as elongation at failure, (Lf − L0)/L0, or reduction in cross-sectional area (A0 − Af)/A0 of the necked region. In general, metals and polymers are ductile because they exhibit significant plastic deformation before failure. Ceramics, on the other hand, are brittle because they fracture with little, if any, plastic deformation. Toughness is a measure of the amount of energy required to break a material. Toughness is determined by integrating the area under the load-displacement curve. Resilience is the elastic energy stored in a unit volume of stressed material. It represents the area under the stress-strain curve extending from zero strain to the strain at which the yield point is reached. Hardness is determined by deformation on a material surface caused by a small indenter tip under a constant load over a certain period of time. Hardness is measured on a relative scale and increases as the relative deformation decreases.
FATIGUE
The stresses a material can withstand under cyclic loading are much less than the yield or ultimate strengths determined via monotonic tensile testing. This is important because implants
routinely experience cyclic loading, especially weight-bearing implants in the lower extremity and spine. Fracture toughness measures the ability of a material to resist crack propagation. Fatigue failure occurs by crack propagation, driven by locally high stresses at the crack tip with each loading cycle. The cracks usually start at surface irregularities where bending or torsion leads to high tensile stresses, and catastrophic failure occurs when the cross-sectional area is reduced below the ability to carry the applied load. In general, the number of cycles to failure is inversely proportional to the applied stress.
routinely experience cyclic loading, especially weight-bearing implants in the lower extremity and spine. Fracture toughness measures the ability of a material to resist crack propagation. Fatigue failure occurs by crack propagation, driven by locally high stresses at the crack tip with each loading cycle. The cracks usually start at surface irregularities where bending or torsion leads to high tensile stresses, and catastrophic failure occurs when the cross-sectional area is reduced below the ability to carry the applied load. In general, the number of cycles to failure is inversely proportional to the applied stress.
The ability of a material to withstand fatigue is determined by cyclically loading specimens to different levels of maximum stress (S) and recording the number of cycles to failure (N). Plotted logarithmically, S-N has an inverse sigmoid shape (Figure 2). Specimens fail quickly when the cyclic stress is between the tensile strength and yield strength (low cycle fatigue), and after increasingly more cycles as the stress is reduced below the yield strength (high cycle fatigue). In many materials there is a lower limit of cyclic stress that will cause fatigue failure. The stress corresponding to this lower limit is called the endurance limit, or fatigue limit.
The rate of the applied strain also affects mechanical properties. Over a long period of time, a constantly applied stress below yield strength can cause deformation called creep.
ISOTROPY AND ANISOTROPY
Materials are classified as being isotropic or anisotropic depending on whether their mechanical properties are directionally dependent. Isotropic materials have the same material properties in all directions, whereas in anisotropic materials, material properties vary by the direction of loading. In their raw forms, the metals, plastics, and ceramics used in orthopaedic implants are isotropic. However, in practice many are anisotropic. Anisotropy can be introduced into implant materials during manufacturing (eg, forging of metals, or extrusion of UHMWE). Allografts are anisotropic, a reflection of their complex hierarchically organized structure.
 FIGURE 2 Schematic of fatigue test. Carefully machined specimens are cyclically loaded to different levels of maximum stress (σ = F/A0) and the number of cycles to failure (N) is recorded. Specimens fail quickly when the cyclic stress is high, and after more cycles when the cyclic stress is low. In many materials there is a lower limit of cyclic stress that will cause fatigue failure. This lower limit is called the endurance limit (EL), or fatigue limit. TS = tensile strength (TS), YS = yield strength (YS). |
FAILURE CRITERIA
Most orthopaedic implants are designed using computers and analyzed numerically (ie, finite element analysis, FEA) before being fabricated, tested, and placed into service. Material properties obtained from laboratory tests are used as model inputs, and anticipated loads are used as boundary conditions. FEA models return predicted implant stresses during use. Because implants need to maintain their shape during use and withstand repeated loading, design stresses are held below the yield stress and endurance limit, depending on the application.
Biomaterials and implant standards are set by ASTM Committee F04 on Medical and Surgical Materials and Devices. Material standards describe the chemical, mechanical, and metallurgical properties of the raw materials used in fabrication, while device standards define requirements for implant performance in specified laboratory testing conditions.
SURFACE PROPERTIES
Atoms and molecules at the surface of a biomaterial have important particular properties to consider as they will be the interface between material and host. An important question in biocompatibility is how the material influences the response of the host cells and extracellular matrix. Material properties, surface geometry, and local chemistry will all affect host response and incorporation or alternatively failure of the material. Prevention of contamination of a surface is also a concern when we are aiming for a specific host response, whether that be from a biological (bacterial for instance) or chemical (oxidation) process. Contamination or alteration of a material can occur because of contact with packaging, exposure to sterilization methods, from the environment, and from the host.
Properties of surfaces of importance to biomaterials include roughness, morphology, wettability, surface mobility, chemical composition, electrical charge, crystallinity, and modulus.
Tests to evaluate material surfaces include scanning electron microscopy to test morphology and electron spectroscopy to determine chemical analysis.
Tests to evaluate material surfaces include scanning electron microscopy to test morphology and electron spectroscopy to determine chemical analysis.
WEAR AND CORROSION
Wear is defined as the removal of surface material by mechanical motion. Wear is caused by friction, motion, and loading of an interface between materials. Lubrication mechanisms act to reduce friction and wear. There are three main types of wear: abrasive, adhesive, and third-body wear. Abrasive wear occurs when a rougher, harder surface moves against a softer surface, generating particulate debris from the softer surface. Adhesive wear results between two moving objects in contact whereby a thin layer of material is transferred from one surface to another. Wear occurs in the material which donates the film. Third-body wear results when an additional particle is present between the two main bearing surfaces, leading to friction and loss of material at the contact surface.
Corrosion occurs between metal surfaces as a combination of mechanical and chemical reactions.15 Galvanic corrosion occurs at the interface of two dissimilar metals creating an electric potential. Fretting occurs because of micromotion at the interface of two metals, disrupting the passivation layer. Crevice corrosion involves the development of microscopic cracks, which creates a microenvironment that disrupts the passivation layer. Pitting corrosion occurs because of small surface defects in the passivation layer. Intergranular corrosion occurs at the interface of grain boundaries where carbides may be present.16,17
METALS
GENERAL STRUCTURE OF ORTHOPAEDIC METALS AND ALLOYS
Metal alloys are the most frequently used biomaterials in orthopaedic surgery, because of their high strength and load-bearing capacity. Alloys are metals in which additional elements are admixed with a base metal. The quantities of the alloying elements are tightly controlled to optimize material properties and biocompatibility. Mixing is performed by melting the combined elements together under controlled conditions.
At the atomic level, solid metals form densely packed crystalline lattices in which individual atoms are bonded together by the shared cloud, or sea, of delocalized free valence electrons. The most common lattice packing arrangements in metals are body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) (Figure 3).
At the microscopic level, metals are composed of small crystals, called grains. As molten metal is cooled, individual, randomly oriented grains nucleate and grow until they encounter neighboring grains. Depending on the solubility of the alloying elements, neighboring grains may have different elemental compositions and different crystal structures, or phases. The interfaces between adjacent grains are called grain boundaries. The material properties of metals are determined by their atomic and microscopic structures. Metallic bonding confers strength and hardness, as well as malleability and ductility. The microstructure (crystalline structure and grain size) influences most mechanical properties. In general, stiffness, yield strength, tensile strength, and fatigue strength all increase with decreasing grain size.
Implant alloys are initially manufactured as raw castings (ingots). Most implants are processed to near-final implant form by casting or forging, and then finished by machining (Figure 4). Casting involves remelting the alloy, pouring it into a mold, and cooling it under controlled conditions. During forging, a solid blank (cast ingot, bar, or rod) is heated until it is malleable and then it is shaped to final form by compression in a progressive series of dies. Hot forging results in metal that is stronger than cast or machined metal parts. Other, less common fabrication methods include powdered metallurgy, where powdered alloy is fused by sintering in heated dies, computer-controlled machining, or mechanical forming by rolling, drawing, or extrusion. The choice of fabrication method depends on the shape and complexity of the implant, characteristics of the alloy being used, intended use of the implant, and cost.
Manufacturing processes influence implant structural properties by altering the microstructure of the alloy. During casting, the rate of cooling can be used to control grain size, to a degree, because grain growth is a function of time and temperature. More rapid cooling yields a finer grain size, whereas slower cooling yields larger grains. During forging, the individual grains are distorted in the direction of the metal flow or plastic deformation, conforming to the shape of the implant as it is progressively shaped. The microstructural changes associated with forging increase tensile strength, yield strength, fatigue strength, and ductility. Final profile shaping is done via numerically controlled machining and/or abrasive grinding.
Once an implant is fabricated, heat treatment may be used to alter the mechanical properties of the alloy. The forming of implants can introduce residual stresses and work hardening. These can be relieved by annealing, which involves heating the part above the recrystallization temperature for a set period of time, followed by cooling at a controlled rate. Annealing increases ductility and reduces hardness. Raw castings typically contain internal voids and microporosity, which reduce fatigue resistance. Casting quality can be improved by a process called hot isostatic pressing, which involves simultaneous heating and pressurization with an inert gas in a high-pressure containment vessel. Hot isostatic pressing consolidates the casting by solid state diffusion.18
After bulk processing, several techniques are used for surface finishing. Nitriding and ion implantation are used to harden the load-bearing surface of stainless steel and titanium implants to increase wear resistance. Nitriding involves the exposure to a nitrogen-rich gas (eg, ammonia) at elevated temperatures.19,20
The dissociated nitrogen diffuses into the surface of the part and bonds with the base metals to yield thin layers or iron or titanium nitride. Grind lines, scratches, and pits are removed by polishing with engineered abrasives. Articulating surfaces are buffed to a final mirror-like finish by a rotating cloth wheel that is impregnated with fine abrasive compounds.
The dissociated nitrogen diffuses into the surface of the part and bonds with the base metals to yield thin layers or iron or titanium nitride. Grind lines, scratches, and pits are removed by polishing with engineered abrasives. Articulating surfaces are buffed to a final mirror-like finish by a rotating cloth wheel that is impregnated with fine abrasive compounds.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
