Biomaterials Used in Orthopedic Implants

Reviewed by: Name: AGNES PurwidyantriStudent ID No: D 0228005Basics of Orthopedic Implants

Bone PropertiesDensity 2.3g/cm3Tensile Strength 3-20MPaCompressive Strength 15,000 psiShear Strength 4,000 psiYoungs Modulus 10-40 MPaOrthopedics TermsOsteoconductive The property of a material that allows for the possible integration of new bone with the host bone. Osteoinductive Characteristic in materials that promote new bone growth. Bioresorbable The ability of a material to be entirely adsorbed by the body.Trochanter The second segment of the leg, after the coxa and before the femur

Screw TypesOBLIQUE SCREWS In subtrochanteric and high femoral fractures oblique screws may be required to be inserted up the femoral neck Screws are 4.5mmX150mm

CANNULATED SCREW Screw Sizes6.5mm X 102mm4.5 X 12.5mm

Screw TypesCANNULATED SCREW A bulbous ended nail with cannulated 12.5 mm screws is shown here successfully stabilizing a subtrochanteric non-union of the femur following a failed Gamma nail

Screw TypesTRANSVERSE SCREWS In most subtrochanteric and upper femoral fractures it is much easier to insert transverse screws in the upper femur, than use oblique screws up the neck of the femur.

Screw TypesTransverse Screws

Screw TypesRemoving the Femoral Head Once the hip joint is entered, the femoral head is actually dislocated from the acetabulum and the femoral head is removed by cutting through the femoral neck with a power saw.

The steps involved in replacing a diseased hip with an uncemented artificial hip begin with making an incision on the side of the thigh to allow access to the hip joint.Reference: Medical Multimedia Group (http://www.sechrest.com/mmg/)Example caseHip & Knee Replacements8Reaming the Acetabulum Attention is then turned towards the socket, where using a power drill and a special reamer, the cartilage is removed from the acetabulum and the bone is formed in a hemisperical shape to exactly fit the metal shell of the acetabular component.

Inserting the Acetabular Component Once the right size and shape is determined for the acetabulum, the acetabular component is inserted into place. In the uncemented variety of artificial hip replacement, the metal shell is simply held in place by the tightness of the fit or by using screws to hold the metal shell in place. In the cemented variety, a special epoxy type cement is used to anchor the acetabular component to the bone.

Preparing the Femoral Canal To begin replacing the femur, special rasps are used to shape the hollow femur to the exact shape of the metal stem of the femoral component.

Inserting the Femoral Stem Once the size and shape are satisfactory, the stem is inserted into the femoral canal. Again, in the uncemented variety of femoral component the stem is held in place by the tightness of the fit into the bone (similar to the friction that holds a nail driven into a hole drilled into wooden board - with a slightly smaller diameter than the nail). In the cemented variety, the femoral canal is rasped to a size slightly larger than the femoral stem, and the epoxy type cement is used to bond the metal stem to the bone.

Attaching the Femoral Head The metal ball that makes up the femoral head is attached.

The Completed Hip Replacement

The steps involved in replacing a diseased knee with an artificial knee begin with making an incision on the front of the knee to allow access to the knee joint.

Shaping the Distal Femoral Bone Once the knee joint is entered, a special cutting jig is placed on the end of the femur. This jig is used to make sure that the bone is cut in the proper alignment to the leg's original angles - even if the arthritis has made you bowlegged or knock-kneed. The jig is used to cut several pieces of bone from the distal femur so that the artificial knee can replace the worn surfaces with a metal surface.

Metals For ImplantsMust be corrosion resistantMechanical properties must be appropriate for the desired applicationAreas subjected to cyclic loading must have good fatigue properties -- implant materials cannot heal themselvesORTHOPEDICS MATERIALS1. MetalOrthopedic Devices with MetalPlates and screws, Pins and Wires, rods (temporary)Total joints (permanent)Clips and staplesMetals Used in ImplantsThree main categories of metals for orthopedic implantsstainless steelscobalt-chromium alloystitanium alloysMaterial looked at in this project:Magnesium FoamGenerally about 12% chromium (316L, Fe-Cr-Ni-Mo)High elastic modulus, rigidLow resistance to stress corrosion cracking, pitting and crevice corrosion, better for temporary useCorrosion accelerates fatigue crack growth rate in saline (and in vivo)Intergranular corrosion at chromium poor grain boundaries -- leads to cracking and failureWear fragments - found in adjacent giant cells

Stainless SteelCobalt Based AlloysCo-Cr-MoUsed for many years in dental implants; more recently used in artificial jointsgood corrosion resistanceCo-Cr-Ni-MoTypically used for stems of highly loaded implants, such as hip and knee arthroplastyVery high fatigue strengths, high elastic modulusHigh degree of corrosion resistance in salt water when under stressPoor frictional properties with itself or any other materialMolybdenum is added to produce finer grains

Titanium and Titanium AlloysHigh strength to weight ratioDensity of 4.5 g/cm3 compared to 7.9 g/cm3 for 316 SSModulus of elasticity for alloys is about 110 GPaNot as strong as stainless steel or cobalt based alloys, but has a higher specific strength or strength per densityLow modulus of elasticity - does not match bone causing stress shieldingTitanium AlloysCo-Ni-Cr-Mo-Ti, Ti6A4V Poor shear strength which makes it undesirable for bone screws or platesTends to seize when in sliding contact with itself or other metals Poor surface wear properties - may be improved with surface treatments such as nitriding and oxidizing

Best PerformanceTitanium has the best biocompatibility of the three.Metal of choice where tissue or direct bone contact required (endosseous dental implants or porous un-cemented orthopedic implants)Corrosion resistance due to formation of a solid oxide layer on surface (TiO2) -- leads to passivation of the materialMetallic FoamTypes of metallic foamsSolid metal foam is a generalized term for a material starting from a liquid-metal foam that was restricted morphology with closed, round cells.Cellular metals:A metallic body in which a gaseous void is introduced.Porous metal: Special type of cellular metal with certain types of voids, usually round in shape and isolated from each other.Metal Sponges: A morphology of cellular metals with interconnected voids.

Why Foam? (Mg Foam)Open cellular structure permits ingrowths of new-bone tissue and transport of the body fluidsStrength & Modulus can be adjusted through porosity to match natural bone propertiesRequirements for Porous ImplantPore Morphology (Spherical)Pore Size (200m - 500m)PorosityHigh Purity (99.9%)BiocompatibilityBioresorbableBiocompatibleOsteoconductiveOsteoinductiveProperties of bone can be easily attained using varying processing techniquesWhy Magnesium?Processing the Mg by Powder Metallurgy TechniquesPowder Mg powder99.9% purity particle size 180mBinder: Ammonium BicarbonateSpherical Shape99.0% puritySize between 200m 500m

Processing StepsBlend powders until a homogenous mixture is attained.Uniaxially press at 100MPa into green compactsHeat treat at 200C for 5hrs, for binder burnoutSinter at 500C for 2hrsResults From Processing

Optical Micrograph of Porous Mg:Small isolated micropores distributed in the wall of the interpenetrated macropores.The micropores are on the order of microns, while the macropores are in the range of 200m 500mResults of ProcessingSEM Micrograph of Mg:Micropores result from the volume shrinkage during sintering and are to small for bone growthMacropores are made on the appropriate size level to promote the ingrowths of new-bone tissues and transport of body fluid

Adsorption and Toxicity Adsorption Rates for MgThe bone will adsorb around 40% of the Mg in the screw per year.From this the lifetime of the screw would be between 5 7 years before no traces are left.ToxicityRecommended dosage of Mg per day is 350mg60% of Mg in the body is found in bonesIn recent studies, a diet rich in Mg resulted in increases in bone density in postmenopausal womenRelatively low toxicity issues, but in vivo testing would clarify.ComparisonsMaterialDensityYoungs ModulusTensile StrengthEstimated Cost RankingBone2.310 403 20NaStainless Steel7.91962901Co Alloys8.92113454Ti Alloys4.51052003Mg Foam2.3310.4762.8432Thank You!!