The Role of Metals in Prosthetic and Orthotic Technology Ben Hertzberg

Introduction Prosthesis: replaces part of the body Orthosis: augments part of the body Prosthesis includes: Artificial Limbs Prosthetic Eyes Replacement Joints Facial Prosthesis Artificial Hearts Neuroprosthetics

Important Materials Characteristics StrengthStiffness Fatigue Resistance Density Corrosion Resistance Biocompatibility (more on that later) Ease of Fabrication Cost

Chart of Materials Properties MetalTensile Strength (MPa) Yield Strength (MPa) StiffnessHardness Corrosion Resistance Density (g/cm 3 ) Titanium ModerateLow-MediumVery High4.5 Ti-6Al-4V ModerateVery HardVery High4.85 Gold Very, very lowMedium Hardness Very, Very High19.32 Stainless Steel HighMedium Hardness High8.03 Tungsten HighVery HardHigh18.82 Magnesium Alloys Very lowMedium Hardness Low-High Co-Cr Alloys Very HighLow-MediumVery High

Special Requirements for Specific Technologies Lower-body prostheses and orthoses will need to support huge amounts of weight Also need good fatigue resistance Also need good fatigue resistance Other characteristics require more precise control Precise control of elasticity Precise control of elasticity Low Density is Important

Requirements for Implant Materials Biocompatibility Tissue-compatible Tissue-compatible Corrosion-resistant Corrosion-resistant Similar elasticity to nearby tissue Similar elasticity to nearby tissue Potential Problems: Allergic reaction Allergic reaction Toxicity Toxicity Other immune system responses Other immune system responses

Facial Implants Miniplates can be used to hold together fractured sections of skull Biocompatibility is very important because of danger of immune response Implant can be cemented, press-fitted, or affixed through impaction Facial Implant and X-Ray Picture of Subject

Replacing Large Sections of Bone Even greater tensile strength is required Bone/metal interface is difficult We must ensure there is effective load transfer between bone and implant

Bone/Metal Interface How do we get bone to grow to and connect with implant metal? Interfacial loosening is leading cause of skeletal implant failure

Problems With Bone/Metal Integration Fibrosis – growth of fibrous connective tissue on surface of implant Fibrosis can interfere with healing damage from implantation surgery Immune system response Abrasion of metal Comparison of Fibrosis in Ti Alloys Extracellular Titanium Debris

Osseointegration Connection between bone and implant Important materials properties: Frictional Coefficient Bio-inert/-active components Progressive growth of bone near Ti implant

Useful Materials: Titanium Ti-6Al-4V is most common alloy used High resistance to corrosion Strong oxide layer that renews itself More easily shaped and more malleable than steel Strong general biocompatibility Stiffness comparable to bone Microstructure of Ti-6Al-4V

Biocompatibility of Titanium Past evidence suggested that titanium might be dangerous Causes fibrosis Metal particles are abraded off Immune system response detected Studies show it is safe, however This opens door for use of titanium in implants Extracellular Titanium Debris Pigmented Cellular Debris

Osseointegration in titanium TiAlV produces very little fibrous tissue – bone can grow normally This can be enhanced by use of thin films Comparison of Fibrosis in Ti Alloys

Metallic Thin Films & Osseointegration Gold-thiol chemistry allows us to attach amino acids to implant surface that encourage bone growth Using amino acid RGD produces significantly thicker bone shell around implant Almost 40% greater interfacial shear strength Self-Assembled Monolayer on Gold Femur Cross-Section After Implant Removal

Metallic Thin Films & Osseointegration – Part 2 Titanium thin films such as Ti-Ca-P-C-O- (N) can also be used to encourage osseointegration No additional immune response Little frictional coefficient Induces growth & serves as anchoring point for osteoblasts and epitheliocytes Dark Spectrum Micrograph of CaP thin film Microstructure of worn CaP thin film

Useful Materials: Magnesium Lightweight metal with mechanical properties similar to natural bone Pure magnesium metal oxidizes quickly This is beneficial because it allows the implant to degrade; however, it happens too rapidly This is beneficial because it allows the implant to degrade; however, it happens too rapidly Mg + rare earths has dramatically reduced corrosion rate - up to 11 months Mg + rare earths has dramatically reduced corrosion rate - up to 11 months Subcutaneous gas pockets can be removed via syringe Subcutaneous gas pockets can be removed via syringe No evidence suggests magnesium is toxic – high biocompatibility

Magnesium & Osseointegration Since Mg implants are designed to degrade, they need to be able to allow bone growth Magnesium can enhance bone growth by itself – evidence is sketchy Porous magnesium microstructures can enhance osseointegration Porous magnesium has materials properties that are even closer to bone Space-holding particles are typically used to produce porous microstructure SEM Micrograph of magnesium with porous microstructure

Futuristic Technologies Trabacular metal 80% porous Uses tantalum, which is highly biocompatible Allows for high bone ingrowth High soft tissue attachment

Futuristic Technologies Scientists have managed to create an prosthetic limb that connects directly to the brain, giving neural control over the limb’s movements & tactile senses

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