Tissue Microstructure
Tissue Microstructure and Mechanical Properties
Overview of the Lecture
Lecturer: John L. Ricci, PhD, FBSE
Affiliation: Division of Biomaterials and Regenerative Biology, Department of Molecular Pathobiology, New York University College of Dentistry
Why Study Mechanical Properties of Tissues?
The understanding of mechanical properties is crucial for interfacing biomaterials with tissues.
Knowing the comparative properties allows for better insights into their interactions.
Finite Element Analysis (FEA) Modeling of 3 or 4 Unit Fixed Prosthesis
Assign Physical Properties to Each Material:
Elastic Modulus
Poisson’s Ratio
Assign Force, Magnitude, Direction to Elements
Failure Points:
The four unit restoration is likely to fail at the highest stress points.
Location of failure: unsupported unit interfaces with adjacent units.
Characteristic: Smallest cross-sectional area corresponds to the highest stress (illustrated by blue arrows).
Mechanical Properties of Biomaterials
All Tissues are Viscoelastic
Viscoelasticity: Mechanical properties depend on the rate of loading; tissues become stiffer at higher strain rates.
Key Concepts
Creep: Materials change shape under a constant load.
Stress Relaxation: The load decreases at constant strain.
Anisotropy: Tissues exhibit different mechanical properties in various directions.
Tissue Microstructure
Understanding tissue microstructure is essential to grasp the mechanical properties.
All tissues are composed of cells and extracellular matrix (ECM).
The ECM is a composite consisting of several components and it determines mechanical properties.
Bone Structure and Organization
General Composition:
Bone Structure is a 3-D organization of:
Cells
Extracellular Matrix
Inorganic Components:
Hydroxyapatite mineral represented as
Organic Components:
Type I collagen mixed with various other proteins.
Stability:
Bone mineral maintains stability in body fluid at physiological pH but dissolves in low pH environments.
Can be stabilized by certain ions (e.g., fluoride) and can turn into ceramic at high temperatures.
Denaturation of Bone Collagen:
Heat: Above 40-45°C
Enzymatic Action: Protease enzymes (collagenases, gelatinases, metalloproteinases)
Microscopic Forms of Bone:
Woven Bone:
Immature, temporary, highly cellular structure.
Lamellar Bone:
Layered, remodeled, mature structure with fewer cells.
Types of Bone Structure
Haversian Bone
Trabecular Bone
Cortical vs. Trabecular Arrangement
Cell Types:
Osteoblasts: Mesenchymal-origin cells that synthesize and mineralize bone.
Osteocytes: Mature bone cells derived from osteoblasts.
Osteoclasts: Multinucleated cells formed from fused monocyte precursors that resorb bone through protease degradation and acid dissolution.
Mechanical Properties of Bone
Cortical and cancellous bone's mechanical properties vary based on structure and organization.
Bone Repair: Focus on Primary Bone Formation in a dog premolar tooth socket observed at 3 weeks.
Enamel and Dentin versus Bone
E (Elastic Modulus):
Cortical Bone = 17 GPa
Cancellous Bone = 1.5 - 5 GPa
Titanium Alloy = 110 GPa
Anisotropic Nature: Bone has different mechanical properties based on orientation.
Failure Modes: Bone is weaker under tension compared to compression.
Clinical Observations: Human Mandible
Dentate versus Edentulous structures.
Implant Stability in Cancellous Bone:
5 x 13mm Implant placed in density 12 sawbones (slight over-drilling) performing Finite Element Analysis.
Stress Results: Under Axial Loading:
Stress Concentration resulting in:
Maximum Stress = 3.8 MPa
Maximum Stress = 4.6 MPa
For side load silhouette vs. V thread:
Max Stress = 91.9 MPa
Max Stress = 140.2 MPa.
Soft Tissue Mechanical Properties
Ligaments and Tendons
These structures are highly organized with a linear orientation and function primarily in tension.
Composed of organized Type I collagen (with some Type III) and minimal amounts of proteoglycans.
Functional Roles:
Tendons: Connect muscle to bone.
Ligaments: Connect bone to bone.
General Mechanical Comparison of Bone and Soft Tissue
Soft Tissues in Compression:
Cartilage serves as a composite of 3-D organized collagen reinforcement (Type II) paired with large quantities of proteoglycans (hydrogel).
These function primarily to absorb shocks and facilitate movement of bone on bone.
Properties of Cartilage:
Exhibits a mechanical response in compression; contains a large toe region related to collagen organization, characterized by low stiffness and notable creep and stress relaxation behavior.
Specific Soft Tissue Properties
Periodontal Ligament (PDL) Properties:
Multicomponent structure operating under both compression and tension.
High cellularity, extensive vasculature, and nerve supply.
Mechanical Properties of Mineralized Tissues
Comparison of Tissues
Mineralized Tissues (e.g., enamel, dentin, bone) function optimally in compression but poorly in tension.
Their properties are determined by several factors including form, orientation, and microstructure.
Comparative Mechanical Properties:
Cortical bone's properties are akin to those of dentin, whereas enamel is stiffer and more brittle.
Cancellous bone has a variable range of properties depending on its density, demonstrating lower stiffness while being able to endure large strains.
Hard and Soft Tissue Summary
Hard Tissues: Tend to function entirely in compression with limited capacity for tensile resilience.
Soft Tissues: Tendon and ligament are primarily tensile and exhibit differing mechanical behaviors due to microstructural organization.
Soft Tissue Mechanics
Tissues like skin and gingiva exhibit very low stiffness, able to deform (strain) up to 40-50%.
Keratinized epithelium exhibits greater stiffness compared to non-keratinized.
Skin Specific Stress/Strain Characteristics
The stress/strain curve for skin reveals a consistent toe region attributed to the random orientation of fibers.
Gingiva Mechanical Properties
Gingiva: Notably compliant, which is important for obtaining accurate impressions in dental procedures. Proper denture retention relies on accurate impressions that maintain a thin, uniform film of saliva.
Conclusion
Understanding tissue mechanical properties is essential for developing restorative materials and implants.
It is imperative to compare properties of biomaterials with those of bodily tissues.
Contact Information:
John.Ricci@NYU.edu