METAL PARTICLES AND METAL IONS
Metal wear particles are produced by the in vivo removal of surface asperities, a.k.a. surface protrusions, when bearing surfaces move and act against each other. Metal particles are also created by fretting between components and by aseptic loosening [1].
Metal Particles
- Size
- larger than atoms
- smaller than polyethylene wear particles
- Volume produced
- lower than polyethylene wear particles
Corrosion of metal particles creates metal ions through electrochemical processes [2]. All ions carry either a positive or a negative electrical charge. Because of their electrical charge metal ions easily bond with naturally existing proteins creating ion-protein complexes. Worries and concerns about metal ions are actually about ion-protein complexes and the potential to activate the immune system. With current generation metal-on-metal bearing couples, the levels of cobalt and chromium ions are the main concern for immune system activation.
Metal Ions
- Size
- slightly larger than atoms OR
- slightly smaller than atoms
- extremely small
- Produces through
- corrosion of metal in the body
- corrosion rate related to volume & surface area of exposed metal
Proper control of material selection, design and manufacturing processes reduces corrosion, fretting, and wear, therefore reducing metal particle and metal ion production.
BIOLOGIC CONCERNS
Because of the low rate of production, the low volume produced, and the size of metal particles, they are not typically a large concern. Since the critical biologic load is not easily or quickly reached, the human body’s own defenses are usually able to eliminate metal particles and in turn, metal ions. It could be argued that the increased metal ion levels found in the blood and urine of many metal-on-metal bearing recipients is simply a sign of the effective biologic handling of these wear products.
The liver and the spleen are the major waste management systems of the human body. It is reasonable to expect that any toxic effects from metal particles would be seen here first [1], most likely in the form of small localized clumps of particles. An autopsy study of a patient 30 years post-metal-on-metal total hip replacement showed no evidence of metal particle accumulation or organ damage [3]. Repeated searches of the orthopaedic literature have not found any reports of organ damage or toxicity in otherwise healthy patients.
Biologic Issues
- toxicity
- carcinogenicity
- hypersensitivity
Cobalt and chromium ions are of the greatest concern in metal-on-metal THR patients. In animal models, elevated levels of these metal ions are associated with increased carcinoma rates [4,5]. However, these cancer rates have not been seen in humans with metal-on-metal THR nor is it clear how to best translate cancer rates from animal studies to human populations. There are published exposure levels from the EPA for chromium, but these refer to inhaled and ingested levels not implanted sources of metal ions. Only one article has specifically examined cancer risk after metal-on-metal THR [6]. Visuri et al found that the metal-on-metal THR group did not have any significantly increased risk of cancer when compared to the metal-on-polyethylene THR group.
Hypersensitivity to metal ions appears to have the potential to affect a larger portion of the orthopaedic population. Indeed, metal hypersensitivity is higher in the total joint population, regardless of bearing type, than in the general population [4,7,8,9]. Although there are some concernst that hypersensitivity may affect aseptic loosening and therefore failure, it remains unclear if hypersensitivity plays any role at all [1,4,9,10,11,12]
Relationships between Metal Particles, Metal Ions, & Biologic Issues – UNPROVEN
- no reliable reports of organ toxicity due to metal-on-metal THR
- metal ions from THR have not been shown to cause cancer in humans
- hypersensitivity is a minimal risk
- unclear if hypersensitivity contributes to implant failure
DESIGN CONSIDERATIONS
Design Issues
- materials
- surface finish
- clearance
- sphericity
As already discussed, metal ions are primarily produced from metal particles and metal particles are primarily products of wear. Therefore, when wear is reduced, ion production is also reduced. Current metal-on-metal, second-generation, bearing designs are optimized to address wear issues from first-generation designs. Advances in metallurgy and stricter tolerances in surface finish, clearance, and sphericity reduce wear particle production [13].
Metallurgy
The cobalt-chrome alloys currently in use may be wrought or cast alloys, and they may be high or low carbon content; both of which affect wear characteristics. The magnitude and distribution of surface carbides, a factor in wear resistance, is markedly affected by carbon content [14,15,16,17]. Various studies have shown differing results, clearly systematic evaluation of alloys and carbon content are needed [13]. Results range from higher carbon alloys are associated with less wear [15,18], to carbon content does not affect wear [19], and from cast alloys are more resistant than high carbon wrought alloys [18] to wrought alloys are superior to cast [20].
Design Variables
Smooth, consistent surface finish on both bearing surfaces reduces the production of metal particles and metal ions through decreased wear. Smoother surface finishes, less rough surface finishes, have fewer surface asperities and therefore have reduced wear rates; there is simply less to wear off. Sphericity and clearance are closely related since both are dependent on femoral head geometry. Sphericity is controlled by the manufacturing process and refers directly to the smooth roundness of the femoral head. Poor sphericity causes poor clearance and increased wear. Clearance is the difference between the three-dimensional radius of the femoral head and the acetabular cup. Proper clearance results in increased polar contact at the apex of the femoral head, but not in the equatorial region. Low clearance results in decreased wear, but if clearance is too low, synovial fluid cannot enter the interface to act as lubricant. Additionally, wear particles will not be ejected from between the bearing surfaces, causing more wear to be produced.
SUMMARY
Unfortunately, there are no hard and fast conclusions. It seems clear that excessive metal particle production, and in turn metal ion generation, is not a good thing. But what is “excessive”?
If “excessive” metal particle and metal ion levels can be defined as the point where adverse effects occur for the general metal-on-metal THR population, then it is also clear that current metal-on-metal bearing surfaces do not generate “excessive” levels. It may be that in the very long-term, some patients are not as well served by metal-on-metal THR as might be hoped. But given the drive for further knowledge and development in the orthopaedic industry, science, and practice, innovative solutions will be discovered.
References
- Urban et al. J Arthrop 2004; 19(8) Suppl 3: 94-101
- Adami et al. Annali di chimica(Italy), 93(1-2):1-10, 2003
- Sieber et al. JBJS 1999; 81B(1): 46-50
- Lewis & Sunderman. CORR 1996; 329S: S264-S268
- Memoli et al. J Orthop Res 1986; 4: 346-355
- Visuri & Koskenvuo. Orthopedics 1991; 14: 137-142
- Huo & Cook. JBJS 2001; 83A(10): 1598-1610
- Long et al. J Arthroplasty 2004; 19(8)Suppl 3: 29-34
- Willert et al. Hypersensitivity around metal/metal hips. Trans Second International Conference on Metal-Metal Hip Prostheses: Past Performance and Future Directions; Montreal, June 2003.
- Heisel et al. JBJS 2003; 85A(7): 1366-1379
- Jazrawi et al. Am J Orthop 1998; 27(4): 283-292
- Willert et al. Particle disease due to wear of metal alloys. Findings from retrieval studies. New York: Raven Press; 1993. p.129-46.
- Campbell et al. CORR 2004; 418: 98-111
- Chan et al. CORR 1999; 333: 96-107
- Firkins et al. Trans Orthop Res Soc 1998; 23: 370
- Medley et al. CORR 1996, 329 Suppl: S148-S159
- Schmidt et al. CORR 1996; 329S: S35-S47
- St John et al. “Cobalt-Base Alloys for Biomedical Applications” ASTM 1999; 1365: 145-155
- Chan et al. CORR 1999; 369: 10-24
- Streicher et al. Proc Inst Mech Eng H 1996; 210: 223-232
Copyright (c) 2005 Susan G Capps, PhD
What you need to know…
There is a lot of discussion about “minimally invasive total hip arthroplasty” (MISTHA) – in the media, on the internet, at orthopaedic industry meetings, and at orthopaedic surgeon meetings, but what is MISTHA? What are the possible benefits; is it better than conventional total hip arthroplasty? What are the possible problems? What does recent research tell us about MISTHA? This tutorial will answer these questions without presenting an advocacy or skeptical viewpoint.
What is MISTHA?
As of yet, there is no single, standard definition for MISTHA. At least one author has suggested that “minimally invasive” is a misnomer and should be discarded. After all, regardless of the incision size, the acetabulum must be reamed, the femoral head removed, and the femoral canal reamed to prepare for implantation. These actions can hardly be classified as minimally invasive. Other, more descriptive, nomenclature has been suggested: minimal-incision, limited-incision, and less-invasive total hip arthroplasty (THA). In essence, there are a variety of THA procedures with varying incision lengths, incision placements, surgical approaches, and underlying tissue dissection; for simplicity, all of these are referred to as MISTHA in this tutorial.
The primary goal of any surgical innovation is to improve upon the standard surgical procedure, or at the very least to produce results “as good as” what is currently being offered. MISTHA seeks to do this primarily by using smaller incisions and by causing less soft tissue damage under the skin.
Basically, MISTHA procedures can be classified by the number of incisions: one or two. Single incision techniques are usually either posterior-approach or anterior-approach, and incision lengths vary from 2.5 to 5.5 inches (6 to 14 cm). The two-incision technique uses two smaller incisions, 1 to 2 inches (2.5 to 5 cm) in length, one for placement of the acetabular component, the other for insertion of the femoral component. For comparison, a standard THA incision length varies from 5 to 14 inches (13 to 36 cm), with contemporary incisions on the lower end of the range. There is also the difference between cosmetic MISTHA (small incision with standard dissection) and functional MISTHA (small incision with minimal soft tissue disruption).
Incision Lengths
- Single-incision MISTHA @ 2.5 to 5.5″ (6 to 14 cm)
- Two-incision MISTHA @ 1 to 2″ (2.5 to 5 cm)
- Standard THA @ 5 to 14″ (13 to 36 cm)
Continue reading …
Structures and Materials
Let’s begin with the difference between “structures” and “materials”. An easy way to remember the distinction is to visualize a building in its early stages of construction. It looks like a huge, complex, erector-set of connected steel beams. The skeletal building is a structure, its properties and behaviors are functions of its geometry, connections, and material. The properties of the material, steel, are dependent on the alloy content and the processing that created it.
Another illustration: the spine. The spine is a structure made of vertebral bodies and intervertebral discs. The vertebrae are also structures in their own right; primarily made of a cortical bone “shell” and a cancellous bone “filling”. To the naked-eye, cortical bone is a material and cancellous bone is a structure. After all, the beam-and-truss “construction” of cancellous bone is easy to see. The small, individual “beams” and “trusses” are the true bone in cancellous bone. On the microscopic level, neither cortical nor cancellous bone are materials, they’re both structures and the individual components, such as calcium phosphate crystals and collagen, are the materials.
Just as the structural properties of the building under-construction are dependent on the alloy and its processing, the structural properties of the spine are dependent on the bone and its health. Osteoporosis affects the strength of bone through the removal of material and the creation of voids. Age affects the strength of intervertebral discs through changes in collagen.
The Basics
The properties of biological tissues are determined by the analysis of structural testing output, often in the form of load versus displacement diagrams [Figure 1]. Each load versus displacement diagram for biological tissues is unique. Not only do the diagrams differ from type of tissue, such as bone and cartilage, they also differ by location, for example the distal radius and the proximal femur. Load versus displacement diagrams also differ with age and health status, the peak load for osteoporotic bone will be less than for healthy, non-osteoporotic bone. This peak load, known as the ultimate load is the point where force and displacement create irretrievable damage to the biological tissue; it is the load where failure occurs (a.k.a. “failure load”).
“Load”, also known as “force”, is the external pull or push applied to an object; the units of force are Newtons (N) or pounds (lbs) [Figure 2]. “Displacement” is the difference in the position of a particular point from the initial, unloaded condition to the final, loaded condition. “Deformation” is the relative displacement of any two points in a body. Displacement and deformation have units of length; meters (m), feet (ft), or inches (in) [Figure 3].
Stiffness is represented by the slope of the straight line on force-displacement diagrams and has units of force per deformation, Newtons per meter (N/m) or pounds per inch (lb/in). Stiffer materials, like bone, are more resistant to deformation at a given load than less stiff materials like cartilage [Figure 4].
Strength, Elasticity, and Viscoelasticity
Although load and strength or stiffness and elasticity are often used interchangeably, they are not the same. Load and stiffness are properties described by force-displacement diagrams while strength and elasticity are properties described by stress versus strain diagrams [Figure 5]. These diagrams are very similar in appearance to load-displacement diagrams, but they represent strength not failure load and elasticity not stiffness.
“Stress” refers to the forces within an object. The intensity of the internal force over the cut section is stress and is defined as the force per unit area over which the force is acting [σ = F/A]. Figure 6 illustrates “normal stress”, meaning stress that is perpendicular to the plane of the cut; stress is measured as force per unit area and has units of MPa (N/m2) or psi (lb/in2). “Strength” is analogous to “failure load”, it is a measure of the compressive force at the exact instant of failure.
“Strain” is unit deformation, a measure of the degree or intensity of deformation. Strain is the ratio of change in length (increase or decrease) to original length [є = ∆l/l], and therefore has no units of measure [Figure 7]. When a body is in tension, its length increases and strain is positive. When a body is in compression, its length decreases and strain is negative.
“Elasticity” is defined by Hooke’s Law, E = σ/ є, and is represented by the slope of the straight line on stress-strain diagrams. Elasticity has units of MPa or psi. Within the elastic range, the straight line portion of the stress-strain diagram, deformation is reversible by simply removing the load; for example, a rubber band after being stretched will return to it’s original shape. In the non-linear portion of the stress-strain curve, deformation is plastic and non-reversible; potters clay is a good example of plastic behavior.
But, few biological tissues obey Hooke’s Law, they actually behave as viscoelastic (a.k.a. viscoplastic) solids. There are two basic properties that describe their behavior: strength and stiffness. Cancellous bone is a viscoplastic biological structure. When cancellous bone is loaded quickly, it behaves as a fairly stiff material, it is resistant to deformation AND it tends to support higher loads. When cancellous bone is loaded slowly, it behaves as a less stiff material. “Visco” because cancellous bone deforms as a function of time. “Plastic” because once it deforms, cancellous bone doesn’t recover much, if any, of its original shape.
Final Thoughts
This is a very brief “taste” of biomechanical engineering as it relates to the properties and behavior of orthopaedic tissues. A more detailed discussion would include advanced testing techniques such as impact and fatigue testing, properties like ductility and toughness, and influences from age, gender, and disease.
Please feel free to contact me with any thoughts or inquiries.
Biomechanical engineering describes the properties and the behavior of biological tissues using mechanical engineering principles and terms. Words such as “load”, “force”, “stress”, “strength” and “stiffness” populate marketing materials, white papers, journal articles, and popular media. Bone, cartilage, ligaments, and implants are described using this vocabulary. Unfortunately, these terms are frequently misused and/or misunderstood by both authors and readers. This mini-tutorial on biomechanical engineering will clarify some of the more common language and its engineering definitions.
Structures and Materials
Let’s begin with the difference between “structures” and “materials”. An easy way to remember the distinction is to visualize a building in its early stages of construction. It looks like a huge, complex, erector-set of connected steel beams. The skeletal building is a structure, its properties and behaviors are functions of its geometry, connections, and material. The properties of the material, steel, are dependent on the alloy content and the processing that created it.
Another illustration: the spine. The spine is a structure made of vertebral bodies and intervertebral discs. The vertebrae are also structures in their own right; primarily made of a cortical bone “shell” and a cancellous bone “filling”. To the naked-eye, cortical bone is a material and cancellous bone is a structure. After all, the beam-and-truss “construction” of cancellous bone is easy to see. The small, individual “beams” and “trusses” are the true bone in cancellous bone. On the microscopic level, neither cortical nor cancellous bone are materials, they’re both structures and the individual components, such as calcium phosphate crystals and collagen, are the materials.
Just as the structural properties of the building under-construction are dependent on the alloy and its processing, the structural properties of the spine are dependent on the bone and its health. Osteoporosis affects the strength of bone through the removal of material and the creation of voids. Age affects the strength of intervertebral discs through changes in collagen. Continue reading …
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