about the biomaterials used in orthopaedics and trauma and their biomechanics
Text 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.
Response to Load
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
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
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
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.
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.
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
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