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about the biomaterials used in orthopaedics and trauma and their biomechanics

Transcript of Biomaterials in orthopaedics & trauma

• Biomaterials in Orthopaedics & Trauma Zahid Askar FCPS(Ortho), FRCS (Ortho) Prof of Orthopaedics & Trauma Khyber Medical College, Peshawer
• Study of Biomaterials The physical and biological study of materials and their interactions with the biological environment. Increase use of biomaterials -Their interactions -Increasing Duration and stresses
• Biomechanics The science of movement of a living body, including how muscles, bones, tendons and ligaments work together to produce movement.
• Force applied will lead to deformation and if continued beyond a certain point will lead to ultimate failure The force ----- STRESS and Deformation is known as STRAIN
• Stress & Strain Stress:- Force per unit area Units NM/Sq M or Pascal Strain:- Change in length per unit original length
• Stress Strain Elastic Plastic Yield Stress Ultimate Stress Breaking Stress
• The path to failure
• TENSILE STRENGTH/ ULTIMATE TENSILE STRENGTH - The maximum stress on the curve before breakage (N/M2) YIELD STRESS- Point at which elastic behaviour changes to plastic behaviour. BREAKING STRESS Point at which the substance fails/brakes
• Youngs modulus E Stress /Strain For elastic part of curve or the slope of the elastic part of the curve SI unit = pascal (Pa or N/m2 or m1kgs2). megapascals (MPa or N/mm2) or gigapascals (GPa or kN/mm2)
• DUCTILITY/ BRITTLENESS- The amount by which a material deforms (i.e. the strain that occurs) before it breaks. Represented by %age elongation or reduction in cross section. HARDNESS- The ability of the surface of a material to withstand forces.
• The Yield Point = marks the onset of plastic deformation Plastic Region = Beyond the yield point, irreversible (plastic) deformation takes place
• Elastic Modulus of Common Materials in Orthopaedics Stainless Steel 200 Titanium 100 Cortical Bone 7-21 Bone Cement 2.5-3.5 Cancellous Bone 0.7-4.9 UHMWPE 1.4-4.2
• Relative values of Young's modulus of elasticity (numbers correspond to numbers on illustration to right) 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
• Bone Mechanics Bone Density Subtle density changes greatly changes strength and elastic modulus Density changes Normal aging,Disease,Use,Disuse Figure from: Browner et al: Skeletal Trauma 2nd Ed. Saunders, 1998.
• Bone Biomechanics Bone is anisotropic - its modulus is dependent upon the direction of loading. Bone Type Load Type Elastic Modulus (10 E9 N/m2) Ultimate Stress ( 10 E6 N/m2) Cortical Tension 11.4-19.1 107-146 Compression 15.1-19.7 156-212 Shear 73-82 Cancellous Tension ~0.2-5.0 ~3-20 Compression 0.1-3 1.5-50 Shear 6.6 +/- 1.7
• Material Ultimate Strength Tensile (MPa) Ultimate Strength Compressive (MPa) Yield Strength 0.2% Offset (MPa) Elastic Modulus (MPa) Cortical bone 100 175 80 15,000 Cancellous bone 2 3 1000 Polyethylene 40 20 20 1000 PTFE Teflon 25 500 Acrylic bone cement 40 80 2000 Stainless steel (316 L) (annealed) >500 >200 200,000 Titanium (Al-4V) (alloy F 136) 900 800 100,000 Cobalt chrome (wrought, cold work) 1500 1000 230,000 Super alloys (CoNiMo) 1800 1600 230,000
• ORTHOPAEDIC BIOMATERIALS
• BIOMATERIAL - A non-viable material used in a medical device, intended to interact with biological systems. State of Mutual Coexistance between a Biomaterial and the Physiological Environment Such as Neither has an Undesirable Effect on the Other. . BIOCOMPATIBILITY No host response to the materialBIOINERT
• Ideal Biomaterial Suitable mechanical properties to fulfil its intended function Must not corrode in biologic environment Must not release potentially harmful degradation by-products locally and systemically. To permit fabrication in the optimum design configuration,
• Ideal Biomaterial Be like the natural and mimic its biomechanical properties Not elicit a response- Bioinert Elicit a favourable response- Biocompatible Economical and Reproducible
• Implants- Uses Help Substitution
• Mechanical Properties of Bone Youngs Modulus(E) 17.0 GN/m2 Ultimate Tensile Strength (UTS) 0.132GN/m2 Compressive Strength(c) 0.192 GN/m2 Shear Modulus(K) 2.01 GN/m2 Poissons Ratio() 0.3
• Materials used in Orthopaedics Metals -Stainless Steel 316L. -Co-Cr-Mo. -Ti-6AL-4V. Ceramics- Alumina/Zirconium Polymers- UHWPE, PMMA, Silicones.PEEK
• Stainless steel-(316L) Iron- 60%, Chromium- 20% Nickel- 14% Molybdenum- 3% Carbon- 0.03% Manganese, Silicon,P,S,- 3%
• Functions Iron Chromium/Nickel/ Molybdenum- Carbon- Manganese, Silicon - Strength Corrosion Strength Manufacturing Problems The chromium forms an oxide layer when dipped in nitric acid to reduce corrosion and the molybdenum increases this protection when compared to other steels.
• Stainless Steel Strong Cheap Relatively ductile Relatively biocompatible
• High Youngs modulus 200 GPascals (10 that of bone) Leads to stress shielding of surrounding bone which can cause bone resorption. susceptible to corrosion
• Titanium and its alloys Ti 6AL-4V ELI (Grade 23) Ti 6Al-4V (Grade 5) Excellent resistance to corrosion Youngs modulus Stronger than stainless steels MRI complaint
• Disadvantages opoor resistance to wear o Can be brittle i.e. less ductile generates more metal debris than cobalt chrome
• Cobalt-Chromium-Molybdenum alloy (Co-Cr-Mo) COBALT-BASED ALLOYS Two main types of cobalt-based alloys A cast alloy A wrought alloy, Also known as Vitallium (or in Britain, "Stellite") is often applied to both alloys.
• Advantages strength and corrosion resistance high abrasion resistance Superior to stainless steel Disadvantages More expensive to manufacture cannot be contoured at the time of surgery.
• USES Usually for bearing surfaces THR Metal-on-metal devices.
• Material Elastic Yield U.Tensile Modulus Strength Strength (GN/m2) (MN/m2) (MN/m2) 316L 200 795 965 Co-Cr-Mo 210 950 1450 Ti-6Al-4V 105 895 1173 Mechanical Properties of Orthopaedic Alloys
• POLYMETHYLMETHACRYLATE (PMMA) Prepolymerized methylmethacrylate( powder) Liquid monomer Exothermic Reaction 10 min at 23 0 C . 60 0 C in the center of the material and 40 0 C at the surface. A grouting agent Good in compression Hard but brittle
• 2 component material Powder polymer benzoyl peroxide (initiator) barium sulfate (radio-opacifier) Liquid monomer DMPT (accelerator) N,N- Dimethyl-p-toluidine hydroquinone (stabilizer)
• The curing process is divided into 4 stages: a) mixing, The mixing can be done by hand or with the aid of centrifugation or vacuum technologies. b) sticky/waiting, c) working, and d) hardening. It is recommended that the unopened cement components are stored at 73 F (23 C) for a minimum of 24 h before use.
• First generation cementing technique 1)- Hand mixing 2)-Minimal preparation of the femoral canal 2)-Digital application of cement. Second generation cementing techniques 1)-Preparation ,packing and drying of the femoral canal 2)-Distal cement restrictor 3)-Pulsatile irrigation, 4)-Retrograde insertion of cement with a cement gun. Third generation cementing techniques 1)-Cement is prepared using a vacuum-centrifugation( reduces porosity). 2)-The femoral canal is irrigated with pulsatile lavage and then packed with adrenaline soaked swabs. 3)-Insertion and pressurisation of the cement in a retrograde fashion Fourth generation cementing techniques Insertion using distal and proximal centralizers to ens