Definitions Biomaterials encompasses all synthetic and natural materials used during orthopaedic procedures Basic definitions load a force that acts on a body stress definition intensity of an internal force calculation force / area units Pascal's (Pa) or N/m2 strain definition relative measure of the deformation of an object calculation change in length / original length units none Mechanical property definitions elastic deformation reversible changes in shape to a material due to a load material returns to original shape when load is removed plastic deformation irreversible changes in shape to a material due to a load material DOES NOT return to original shape when load is removed toughness definition amount of energy per volume a material can absorb before failure (fracture) calculation area under the stress/strain curve units joules per meter cubed, J/m3 creep increased load deformation with time under constant load load relaxation decrease in applied stress under conditions of constant strain hysteresis (energy dissipation) characteristic of viseoelastic materials where the loading curve does not follow the unloading curve the difference between the two curves is the energy that is dissipated finite element analysis breaking up a complex shape into triangular or quadrilateral forms and balancing the forces and moments of each form to match it with its neighbor Material Strength: Stress vs Strain Curve Derived from axially loading an object and plotting the stress verses strain curve Elastic zone the zone where a material will return to its original shape for a given amount of stress "toe region" applies to a ligaments stress/strain curve represents straightening of the crimped ligament fibrils Yield point the transition point between elastic and plastic deformation Yield strength the amount of stress necessary to produce a specific amount of permanent deformation Plastic zone the zone where a material will not return to its orginal shape for a given amount of stress Breaking point the object fails and breaks Ultimate (Tensile) strength defined as the load to failure Hooke's law when a material is loaded in the elastic zone, the stress is proportional to the strain Young's modulus of elasticity measure of the stiffness (ability to resist deformation) of a material in the elastic zone calculated by measuring the slope of the stress/strain curve in the elastic zone a higher modulus of elasticity indicates a stiffer material Young's Modulus of Metals and Biologics Relative values of Young's modulus of elasticity 1. Ceramic (Al2O3) 2. Alloy (Co-Cr-Mo) 3. Stainless steel 4. Titanium 5. Cortical bone 6. Matrix polymers 7. PMMA 8. Polyethylene 9. Cancellous bone 10. Tendon / ligament 11. Cartilage Material Descriptions Brittle material a material that exhibits linear stress stain relationship up until the point of failure undergoes elastic deformation only, and little to no plastic deformation examples PMMA ceramics Ductile Material undergoes large amount of plastic deformation before failure example metal Viscoelastic material a material that exhibits a stress-strain relationship that is dependent on duration of applied load and the rate by which the load is applied (strain rate) a function of the internal friction of a material examples ligaments bone Isotropic materials possess the same mechanical properties in all directions example golf ball Anisotropic materials possess different mechanical properties depending on the direction of the applied load examples ligaments bone Metal Characteristics Fatigue failure failure at a point below the ultimate tensile strength secondary to repetitive loading depends on magnitude of stress and number of cycles Endurance limit defined as the maximal stress under which an object is immune to fatigue failure regardless of the number of cycles Creep phenomenon of progressive deformation of metal in response to a constant force over an extended period of time Corrosion refers to the chemical dissolving of metal. Types include galvanic corrosion dissimilar metals leads to electrochemical destruction mixing metals 316L stainless steel and cobalt chromium (Co-Cr) has highest risk of galvanic corrosion can be reduced by using similar metal crevice corrosion occurs in fatigue cracks due to differences in oxygen tension 316L stainless steel most prone to crevice corrosion titanium least prone to crevice corrosion fretting corrosion description a mode of destruction at the contact site from the relative micromotion of two materials or two components clinical significance common at the head-neck junction in hip arthroplasty most common cause of mid-stem failure in modular revision type stems arthroplasty involving modular implants are at risk for fretting corrosion and failure between the components of the final implant increased risk with the increased number of interfaces between the various components Specific Metals Titanium uses fracture plates screws intramedullary nails some femoral stems advantages very biocompatable forms adherent oxide coating through self passivation corrosion resistant low modulus of elasticity makes it more similar to biologic materials as cortical bone disadvantages poor resistance to wear (notch sensitivity) (do not use as a femoral head prosthesis) generates more metal debris than cobalt chrome Stainless Steel (316L) components primarily iron-carbon alloy with lesser elements of chromium molybdenum manganese nickel advantages very stiff fracture resistant disadvantages susceptible to corrosion stress shielding of bone due to superior stiffness Cobalt alloy components cobalt chromium molybdenum advantages very strong better resistance to corrosion than stainless steel Specific Non-Metals Ultra-high-molecular-weight polyethylene advantages tough ductile resilient resistant to wear disadvantages susceptible to abrasion wear usually caused by third body inclusions thermoplastic (may be altered by extreme temperatures) weaker than bone in tension other gamma irradiation increases polymer chain cross-linking which improves wear characteristics decreases fatigue and fracture resistance Polymethylmethacrylate (PMMA, bone cement) functions used for fixation and load distribution in conjunction with orthopaedic implants functions by interlocking with bone may be used to fill tumor defects and minimize local recurrence properties 2 component material powder polymer benzoyl peroxide (initiator) barium sulfate (radio-opacifier) coloring agent (green chlorophyll or blue cobalt) liquid monomer DMPT (N,N-Dimethyl para-toluidine, accelerator) hydroquinone (stabilizer) advantages reaches ultimate strength at 24 hours strongest in compression Young's modulus between cortical and cancellous bone disadvantages poor tensile and shear strength insertion can lead to dangerous drop in blood pressure failure often caused by microfracture and fragmentation Silicones polymers that are often used for replacement in non-weight bearing joints disadvantages poor strength and wear capability responsible for frequent synovitis Ceramics advantages best wear characteristics with PE high compressive strength disadvantages typically brittle, low fracture toughness high Young's modulus low tensile strength poor crack resistance characteristics Bone Bone composition composed of collagen and hydroxyapatite collagen low Young's modulus good tensile strength poor compressive strength hydroxyapatite stiff and brittle good compressive strength Mechanical properties advantages strongest in compression a dynamic structure remodels geometry to increase inner and outer cortex to alter the moment of inertia and minimize bending stresses disadvantages weakest in shear Failure (fracture) tension usually leads to transverse fracture secondary to muscle pull compression due to axial loading leading to a crush type fracture bone is strongest in resisting compression bending leads to butterfly fragment torsion leads to spiral fracture the longer the bone the greater the stresses on the outer cortex under torsion Ligaments & Tendons Characteristics viscoelastic with nonlinear elasticity displays hysteresis (see definition above) Advantages strong in tension (can withstand 5-10% as opposed to 1-4% in bone) Disadvantages demonstrate creep and stress relaxation