Metals and Ceramics

Metals and Ceramics

Key Take-aways

• State several examples (general compositions) of metals and ceramics in medical use.
• Know advantages and disadvantages for each type of material.
• Use material properties to explain these (dis)advantages and state the biological outcomes.
• Explain the benefits and drawbacks of corrosion.
• Explain the mechanism of bioactive processes.

Biomaterial Beginnings

• History of biomaterials:
  • Initially, metals were used as implanted materials.
  • Commonly, materials manufactured for non-medical applications (e.g., airplanes, automobiles, clocks, radios) were repurposed for medical use.

Advantages of Metals

• Mechanical Properties:
  • High tensile strength, high yield strength, and high fatigue strength.
  • Controllable shape and properties through processing.
  • Example: Stainless steel can be molded or extruded.
• Biocompatibility:
  • Classified as bioinert, having low reactivity within the body, thus minimizing adverse reactions.
• Imaging:
  • Radio-opaque, allowing for implant tracking through imaging techniques.
• Drawbacks:
  • Risk of potential metal allergy.

Basic Structure of Metals

• Metals have a crystalline structure, which includes the following components:
  • Multicrystalline material: Composed of multiple crystals or grains.
  • Grain size: Refers to the structural units in the crystalline structure.
  • Grain boundaries: Interfaces where two crystal grains meet.
• The size of the grains can be manipulated to improve the mechanical properties of the metal.

Strength Improvement through Grain Structure

• Increasing the number of grain boundaries improves metal strength.
  • Dislocations:
  • Dislocations within grains move easily, causing permanent distortion;
  • Grain boundaries hinder this movement, thus enhancing strength.
  • Smaller grain sizes yield higher strength due to more boundaries.

Common Metals Used in Medical Applications

Applications and Examples of Metals

• Utilization of metals in various medical devices such as:
  • Vascular stents
  • Metal fillings
  • Bone plates
  • Spinal fusion screws

Case Studies

Mitral Valve Replacement

• Approximately 40,000 mitral valves are replaced annually.
• Average cardiac cycle involves 70-75 beats per minute.

Bjork-Shiley Heart Valve

• This particular valve experienced significant failures in the 1980s due to the following reasons:
  • Constructed with a Cr-Co alloy disk attached to a metal flange using two welds.
  • Issues of high failure rates in younger patients must be considered.

ASR XL Hip Replacement

• Over 450,000 hip replacements occur annually in the U.S.
• The hip joint is a major weight-bearing joint, necessitating unimpeded rotation within the acetabulum (socket).
  • A conventional hip replacement features two components: the acetabular cup and a plastic liner (+ stem).
  • The ASR design excluded the plastic liner, allowing for a larger, more stable femoral head.
• Potential challenges and testing methods must be analyzed along with diagnostics.

Metal-on-Metal Wear

• This phenomenon arises from two metal parts rubbing together, leading to:
  • Constant wear releases metal debris into the body, identified through measuring ion concentrations in blood.

Effects of Metal Debris Shedding

• Phagocytosis of debris by cells leads to:
  • Accumulation of metal, termed metallosis, resulting in increased inflammation and aseptic loosening of implants.

Issues with Stiffness in Materials

• The increased stiffness of implant stems results in reduced force transfer to bone, leading to the phenomenon known as stress shielding.
• Lack of adequate loading on normal bones results in tissue degeneration and aseptic loosening, as documented by D.R. Sumner in 2015.

Corrosion in Metals

• Metals' interaction with water can lead to rusting and subsequent degradation via electrochemical reactions.
• Inflammation in the surrounding tissues can exacerbate corrosion effects.

Positive Aspects of Corrosion

• The alloy composition creates stainless steel by forming a protective chrome oxide layer when reacting with oxygen.
• This protective process, termed passivation, develops a thin oxide layer and may be impaired within the body.

Introduction to Ceramics

• Ceramics are defined as inorganic, non-metallic solids composed of various metal or non-metal compounds such as oxides, nitrides, carbides, and zirconia.
• Most applications are in the fields of dentistry and orthopedics.
• Natural ceramics such as bone and teeth consist of hydroxyapatite, primarily made of calcium and phosphate ions.

Ceramics: Advantages and Disadvantages

• Advantages:
  • High compressive toughness with relatively low modulus.
  • Excellent wear resistance and can be polished to an ultra-smooth finish.
  • Classified as bioactive, with some degree of biodegradability.
• Disadvantages:
  • Brittleness and limited toughness in tension; low resorption rates.

Processing of Ceramics

• Ceramics can be produced in various forms: liquid, powder, particles, or scaffolds, depending on the intended application.
• Physical properties are influenced by microstructure.

Microstructure of Ceramics

• Like metals, ceramics feature small crystals, termed grains, where a smaller grain size correlates with increased strength.
• Ceramics show more diversity in microstructures than metals; for instance, glassy ceramics are completely non-crystalline.
• Ceramics typically exhibit high ionic bond strength which contributes to their hardness but also to brittleness, as noted by Ahmadipour et al. in 2016.

Applications of Ceramics

• Common types of ceramics include alumina, zirconia, and calcium phosphates, employed in:
  • Dental screws,
  • Veneers,
  • Bone fillers,
  • Bone scaffolds.

Bioactivity Concepts

Topography Introduction for Bioactivity

• Topography refers to the physical surface features that enhance cell-material interactions.
  • Methods such as grinding, brushing, and blasting can increase surface area on metals to facilitate these interactions.

Osseointegration Process

• Macrostructures are used to stabilize implants prior to osseointegration, which is a gradual process that does not create a fully solid bond.
• Adequate attachment durability is critical, as the removal of implants can lead to bone fracture.

Applications of Osseointegration

• In hip implants, parts requiring movement can be distinguished from static sections.
  • Coating the acetabular surface with metal fibers aids in increasing roughness and integration potential.
  • This technique similarly enables the creation of pores for cellular growth.

Bioglass Ceramics

• Bioglass is composed of SiO2, Na2O, CaO, and P2O5, demonstrating elastic properties comparable to bone (22-40 GPa).
• The specific bioglass known as Bioglass 45S5 generates a bioactive layer of hydroxyapatite within the body.

Bioactive Integration with Bone

• Biointegration: Refers to the chemical bonding that occurs between bone and a ceramic implant, which can lead to a faster and more complete union compared to osseointegration.

Other Interactions with Bone

• Osteoconductive: Describes the ability of a scaffold to support new bone formation.
• Osteoinductive: Indicates the capability to induce the differentiation of stem cells into osteogenic (bone-forming) cells.
• Osteogenic: Involves directing stem cells to deposit new bone material.