2.79J/3.96J/BE.441/HST522J MATERIALS: BONDING AND PROPERTIES M. Spector, Ph.D. Massachusetts Institute of Technology Harvard Medical School Brigham and Women’s/Massachusetts General Hosp. VA Boston Healthcare System CHEMICAL BONDING Primary ? Metallic 100 kcal/mol ? Covalent 200 ? Ionic 10-20 Secondary ? van der Waals 1-2 ? Hydrogen 3-7 ? Hydrophobic 1-2 Interactions MATERIALS WITH PRIMARY ATOMIC BONDS + + + + + + _ _ _ _ Metallic (electron “glue” or “cloud”) -metals -100 kcal/mol C C C C H H H H H H H H Ionic (attraction of positive and negative ions) -ceramics -calcium phosphates -10-20 kcal/mol Covalent (shared- pair electrons) -polymers -biological macromolec. (e.g., proteins) -200 kcal/mol M M M M M M e - e - e - e - e - e - e - e - e - e - e - e - e - e - COMPOSITION OF METALS (%) Stainless Steel Cobalt Chromium Titanium Fe Co Ti Cr (17-20%) Cr (27-30) Al (5.5-6.5) Ni (10-17) Mo (5-7) V (3.5-4.5) Mo (2-4) Ni (2.5) Fe,C,O (0.5) C (0.03) Fe, C, Mn, Si (<3.1) Mn, P, S, Si (<2.8) METALS M M M M M M e - e - e - e - e - e - e - e - e - e - e - e - e - e - M M e - e - e - Fracture surface METAL SURFACE M M M M M M e - e - e - e - e - e - e - e - e - e - e - e - e - e - Free surface Electropositive FORMATION OF METALLIC OXIDE M M M M M M e - e - e - e - e - e - e - e - e - e - e - e - e - e - Surface O = O = O = THE METALLIC OXIDE (CERAMIC) SURFACE OF METALS Metal Oxide 10nm-10μm ORTHOPAEDIC METALS ADVANTAGES DISADVANTAGES Stainless Strength Potential for corrosion Steel Ease of manuf. High mod. of elasticity Availability Cobalt- Strength High mod. of elasticity Chromium Rel. wear resist. Titanium Strength Poor wear resistance Low modulus Corrosion resist. METALS FOR TJA: PAST, PRESENT, AND FUTURE 1900-1940 1940-1960 1970 1980 1990 2000 2010 ? ? Stainless Steel ? ? Cobalt-Chromium Alloy ? ? Titanium ? ? Oxinium Oxinium? (Smith &Nephew Orthopaedics; oxidized zirconium) is the first new metal alloy in orthopaedic surgery in 30 years. METALS FOR TJA: PAST, PRESENT, AND FUTURE 1900-1940 1940-1960 1970 1980 1990 2000 2010 ? ? Stainless Steel ? ? Cobalt-Chromium Alloy ? ? Titanium ? ? Oxinium Selection Criteria ? ? Inertness/Biocompatibility ? ? Strength ? ? Lower Modulus ? ? Scratch-resist. ? ? Lubricatious ? ? Non-Allergen. ORTHOPAEDIC METALS ADVANTAGES DISADVANTAGES Stainless Strength Potential for corrosion Steel Ease of manuf. High mod. of elasticity Availability Cobalt- Strength High mod. of elasticity Chromium Rel. wear resist. Titanium Strength Poor wear resistance Low modulus Corrosion resist. Oxinium Scratch-resist. ? Low modulus Oxinium ASTM B550 Zr Nb (2.5%) Composition of Orthopaedic Metals Metal Substrate How is the Ceramic Surface Produced on Oxinium?: Oxidation Process ? Wrought zirconium alloy device is heated in air. ? Metal transforms as oxide grows; not a coating. ? Zirconium Oxide (Zirconia ceramic) is ~5 μm thick. Oxygen Enriched Metal Original Surface Air 500 o C Oxygen Diffusion Ceramic Oxide Oxygen Enriched Metal Zirconium metal alloy is heated in air Oxygen diffuses into the metal surface Surface becomes enriched in oxygen Surface transforms to ceramic oxide G. Hunter, S&N Co-Cr ALLOY VERSUS Zr-Nb ALLOY: THICKNESS OF THE OXIDE Co-Cr alloy Chromium oxide 0.01 μm 500 times thicker Zr-Nb alloy 5 μmZirconium oxide Ceramic Metal Oxinium Fatigue Testing of Oxinium Femoral Components ? Fatigue strength the same as for Co-Cr devices. ? Supports 4.4 kN (1000 lbf) in 10 Mcycle fatigue test. ? Tested worst-case: thin condyle, no bone, full flexion. *Tsai et al., SFB 2001 Image removed due to copyright considerations. ADVANTAGES OF OXINIUM Weds the best of a ceramic with the best of a metal. ? Scratch resistant: less abrasive wear of PE ? More lubricatious: lower friction may result in less adhesive wear of PE; better patella articulation ? Much lower modulus than Co-Cr alloy (similar to Ti): lower stiffness and less stress shielding ? Non-allergenic WEAR PROCESSES Adhesive wear particle adherent to metal Abrasive plowing wear Crack propagated by cyclic loading results in fatigue (delamination) wear PE Component Metal Asperity WEAR PROCESSES Abrasive wear PE Component Metal Asperity Solution is a scratch-resistant metal/ceramic counterface; X-linked PE may not be the solution ? Profound effect of a single scratch; wear due to the ridge of metal bordering an scratch EFFECT OF A SINGLE SCRATCH ON PE WEAR 10-fold increase in PE wear when the ridge bordering the scratch exceeded 2μm in height (This type of scratch is not noticeable by eye.) No PE wear if the metal ridge is removed Dowson, et al., Wear (1987) Image removed due to copyright considerations. 100 μm 50 μm Scratches on Retrieved Co-Cr Femoral Condyles Scanning Electron Microscopy Ant- -post movement Ridge of metal Ridge of metal >2μm SOURCES OF PARTICLES THAT CAUSE SCRATCHES ON CONDYLES ? Bone ? PMMA (bone cement) ? Wear and corrosion products from modular junctions ? Prosthetic coatings (viz., plasma sprayed Ti) Is ceramic-on-PE the answer ? Alumina or zirconia heads Image removed due to copyright considerations. IF CERAMIC IS THE ANSWER How to obtain the benefit of ceramic-on-PE articulation in TKA? Bulk ceramics do not have the necessary mechanical properties for TKA. Answer: A new metal alloy, zirconium niobium (Oxinium), the surface of which can be oxidized to form zirconium oxide (zirconia), a durable scratch- resistant ceramic. Oxinium ? May be a more innovative a development than cross-linked PE. ? One of only 2 materials developed principally for TJA (the other is hydroxyapatite). Image removed due to copyright considerations. CHARACTERISTICS OF OXIDES THAT AFFECT THEIR PERFORMANCE ? Thickness ? Adherence to metal substrate ? Porosity/density/strength of the oxide Oxide (Ceramic) Metal Co-Cr ALLOY VERSUS Zr-Nb ALLOY THICKNESS OF THE OXIDE Co-Cr Chromium oxide 0.01 μm 500 times thicker Zr-Nb 5 μmZirconium oxide Ceramic Metal Chromium Oxide layer COMPARISON OF THE OXIDE THICKNESSES ON Co-Cr AND Zr-Nb 2 μm Co-Cr Alloy Zr-Nb Alloy Zirconium Oxide layer 0.01 μm thick 5 μm thick Typical scratch in the Co-Cr surface Thicker oxide layer (500x thicker) to protect against scratches. Interface between oxide and metal: -no voids -no imperfections Prof. L.W. Hobbs, MIT Zirconium oxide Zirconium metal ADHERENCE OF Zr OXIDE TO THE METAL Cr 2 0 3 Zr0 2 thickness V Benezra, et al. MRS Symp., 1999 Transmission Electron Microscopy Image removed due to copyright considerations. Zirconium oxide Zirconium metal STRENGTH OF Zr OXIDE V Benezra, et al. MRS Symp., 1999 “Brick Wall Tough” Rectangular crystals of Zr0 2 Transmission Electron Microscopy Image removed due to copyright considerations. -10 0 10 20 30 0246 Number of Cycles (millions) M e an Ti bi al W e ar ( m m 3 ) Cast CoCrMo Oxidized Zr- Smith & Nephew Orthopaedics Wear of PE with OxZr versus CoCr Condyles Knee Simulator Study WEAR PROCESSES Materials Issues Adhesive Abrasive PE Metal Fatigue (delamination) wear WEAR PROCESSES Materials Issues Adhesive Abrasive PE Metal Fatigue (delamination) wear What is missing from this picture? Adhesive Abrasive PE Metal Joint Fluid* Fatigue (delamination) wear * What role does the joint fluid play in the tribology of TJA? WEAR PROCESSES Materials Issues WEAR IN TOTAL JOINT ARTHROPLASTY Tribology ? Lubrication – Depends on amount, composition and mechanical properties of joint fluid ? Friction – Better the lubrication lower the friction ? Wear – Lower the friction, less wear Wear testing of a total knee replacement prostheses in a “knee simulator.” Bovine serum; not water How good a lubricant is the patient’s joint fluid? Image removed due to copyright considerations. Image removed due to copyright considerations. COMPOSITION AND MECHANICAL PROPERTIES OF JOINT FLUID IN PRIMARY AND REVISION TKA ? Differences in certain compositional features and certain mechanical properties of joint fluid from revision cases when compared to the properties of fluid from patients before TKA. ? How well can joint fluid lubricate TKA? D. Mazzucco and M. Spector, J. Orthop. Res. 2002;20:1157-1163 D. Mazzucco, et al., Biomat., (In press) z COMPOSITION OF JOINT FLUID Metal Joint Fluid Hyaluronic Acid Protein Phospholipid PE The amount and composition and properties of joint fluid in TKA patients vary widely; this could explain why some pts. have high wear. Solution; Metal with lower friction even in presence of abnormal joint fluid. z COMPOSITION OF JOINT FLUID Metal Joint Fluid Hyaluronic Acid Protein (Lubricin) Phospholipid PE Two types of Lubrication: ?Fluid Film ?Boundary Layer z WEAR PROCESSES Fluid Film Lubrication Metal Joint Fluid Hyaluronic Acid, HA Fluid Film Lubrication ; surfaces separate – no friction and no wear; due to viscosity of fluid (HA conc. and MW), topography of counterfaces, and velocity: TKA? PE z WEAR PROCESSES Fluid Film Lubrication Metal Joint Fluid Hyaluronic Acid, HA Fluid Film Lubrication ; determine the patient’s fluid viscosity (HA conc. and MW); benefit of HA injection? PE z WEAR PROCESSES Boundary Layer Lubrication Metal Joint Fluid Protein Lipid PE Boundary Layer Lubrication; protein and lipid adsorb to the surfaces to decrease friction and reduce adhesive wear; can contribute to reducing abrasive and fatigue wear z WEAR PROCESSES Boundary Layer Lubrication Metal Joint Fluid Protein Lipid PE Boundary Layer Lubrication; Determine the protein and lipid content of the joint fluid; employ a metal counterface that will best adsorb the lipid and protein; Oxinium ADVANTAGES OF OXINIUM Weds the best of a ceramic with the best of a metal. ? Scratch resistant: less abrasive wear of PE ? Better lubricity than Co-Cr alloy: lower friction may result in less adhesive wear of PE; better patella articulation ? Much lower modulus than Co-Cr alloy (similar to Ti): lower stiffness and less stress shielding ? Non-allergenic WEAR IN TOTAL JOINT ARTHROPLASTY Tribology ? Lubrication – Depends on amount, composition and mechanical properties of joint fluid ? Friction – Better the lubrication lower the friction ? Wear – Lower the friction, less wear FRICTION APPARATUS Cantilever Arm Ground Strain Gauge Dead Weight Metal Disk To Computer PE Pin Coef. of friction (μ)=lateral force/normal force 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 C o ef f i ci en t o f F r i c t i o n Co-Cr Alloy Water Serum 0.063±0.004 0.054±0.002 14% dec.* * Wear of PE in serum< 1 / 3 wear in water Friction of Oxinium with PE versus Co-Cr Mazzucco & Spector ice-ice AC-AC 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 C o ef f i ci en t o f F r i c t i o n Co-Cr Alloy Oxinium Water Serum Serum 0.063±0.004 0.054±0.002 0.046±0.001 14% dec.* 15% dec. * Wear of PE in serum< 1 / 3 wear in water Friction of Oxinium with PE versus Co-Cr Mazzucco & Spector .03 .04 .05 .06 .07 .08 .09 .1 Dy n a m i c Co e f . a- P B S b- Se r u m c- H A - l ow d- H A - h i g h e- P h o s - l ow f-P h o s -h i g h g- P r ot - l ow h- P r o t - h i g h Lubricant Ox-Zr Co-Cr Oxinium versus CoCr Alloy for All Lubricants 2-factor ANOVA; p<0.0001; power=1 Mazzucco & Spector 0.00 0.02 0.04 0.06 0.08 0.10 D y n a m i c C o e ffi c i e n t o f F r i c ti o n TKA OA Results: Joint Fluid Lubrication Within each group, samples are arranged in the order they were obtained Bars represent standard deviation Serum Water 3x difference in wear rate Mazzucco & Spector ADVANTAGES OF OXINIUM Weds the best of a ceramic with the best of a metal. ? Scratch resistant: less abrasive wear of PE ? More lubricatious: lower friction may result in less adhesive wear of PE; better patella articulation ? Much lower modulus than Co-Cr alloy (similar to Ti): lower stiffness and less stress shielding ? Non-allergenic Decrease in the Stress in the Distal Femur after TKA due to the Stiffness of the Co-Cr Femoral Component: Finite Element Analysis M. Angelides, et al., Trans. Orthop. Res. Soc., 13:475 (1988) Image removed due to copyright considerations. J.D. Bobyn, et al., Clin. Orthop., 166:301 (1982) Bone Loss due to Stress Shielding under a Femoral Component: Canine Model Image removed due to copyright considerations. RADIOGRAPHIC BONE LOSS AFTER TKA* ? Retrospective radiographic analysis of 147 TKAs. – 3 designs – Cemented and porous-coated, non-cemented ? Determination of whether bone loss was evident in the post-op radiographs. – 3 examiners * Mintzer CM, Robertson DD, Rackemann S, Ewald FC, Scott RD, Spector M. Bone loss in the distal anterior femur after total knee arthroplasty. Clin Orthop. 260:135 (1990) Bone Loss After TKA: Radiographic Study A-P Radiograph Lateral Radiograph C.M. Mintzer, et al., Clin Orthop. 260:135 (1990) Sites at which changes in bone density was evaluated. Image removed due to copyright considerations. Image removed due to copyright considerations. 1 year post-op Bone Loss Under the Femoral Component of a Total Knee Replacement Prosthesis: Stress Shielding C.M. Mintzer, et al., Clin Orthop. 260:135 (1990) Image removed due to copyright considerations. Image removed due to copyright considerations. BONE LOSS UNDER THE FEMORAL COMPONENT OF TKA ? Bone loss occurred in the majority of cases (68% of patients). ? Bone loss occurred within the first post- operative year and did not appear to progress. ? Bone loss was independent of implant design and mode of fixation (i.e., cemented vs. non-cemented). C.M. Mintzer, et al., Clin Orthop. 260:135 (1990) EFFECT OF BONE LOSS ON BONE STRENGTH How much bone loss needs to occur before it is detectable in a radiograph? ? Radiographic evidence of bone loss in the distal femur = 30% reduction in bone density.* How does bone loss affect bone strength? ? Bone strength is proportional to density 2 . ? Therefore a 30% decrease in bone density means a 50% decrease in bone strength. *D.D. Robertson et al., J. Bone Jt. Surg. 76-A:66 (1994) BONE LOSS UNDER THE FEMORAL COMPONENT OF TKA Conclusion ? Bone loss occurs in the distal anterior femur post-TKA due to stress shielding related to the stiffness of the cobalt- chromium alloy component C.M. Mintzer, et al., Clin Orthop. 260:135 (1990) BONE LOSS DUE TO STRESS SHIELDING Potential Problems ? Complicates revision arthroplasty due to the loss of bone stock. ? May place the prosthesis at risk for loosening. ? May place the distal femur at risk of fracture. Solution ? Oxinium TKA. – Oxinium has approximately ? the stiffness of Co-Cr alloy, therefore there should be less stress shielding and less bone loss. ADVANTAGES OF OXINIUM Weds the best of a ceramic with the best of a metal. ? Scratch resistant: less abrasive wear of PE ? More lubricatious: lower friction may result in less adhesive wear of PE; better patella articulation ? Much lower modulus than Co-Cr alloy (similar to Ti): lower stiffness and less stress shielding ? Non-allergenic METAL SENSITIVITY IN PATIENTS ? 10-15% of population have dermal sensitivity to metal (14% to Ni) ? Metals known as sensitizers: – Ni > Co and Cr >>> Ti and V ? 60% of pts. with failed TJRs were metal sensitive vs. 25% with well-functioning implants – Did metal sensitivity cause failure or did the failed implant cause metal sensitivity? Hallab, Merritt, Jacobs, JBJS 83-A:428 (2001) WHAT ARE CERAMICS? ? Compounds of metallic and nonmetallic (e.g., oxygen) elements. ? Ceramic materials: Alumina (aluminum oxide) Zirconia (zirconium oxide) ? Metal oxides on metallic materials: Chromium oxide (on Stainless Steel and Co-Cr alloys) Titanium oxide (on Titanium and Titanium alloy) Zirconium oxide (on Zr-Nb alloy) ADVANTAGES OF CERAMICS ? Dense/hard (scratch resistant) Related to the character of the ionic bonding ? Ability to be polished to an ultra smooth finish CHARACTERISTICS OF OXIDES THAT AFFECT THEIR PERFORMANCE ? Adherence to metal substrate Related to the mismatch in bonding (oxides comprise ionic and covalent bonds in contrast to metallic bonds) ? Porosity/density ? Thickness POLYMERS Polyethylene C C Polymethylmethacrylate C C H H H H H CH 3 H COOCH 3 ORTHOPEDIC POLYMERS ADVANTAGES DISADVANTAGES UHMWPE Relatively high Subject to oxidation wear resistance PMMA Polymerization Low fatigue strength in vivo (for load-bearing applications) Micrometer Level Fusion defects due to incomplete consolidation are cracks that can be propagated by fatigue (delamination) wear. MOLECULAR STRUCTURE OF POLYETHYLENE ULTRAHIGH MOLECULAR WEIGHT POLYETHYLENE 10-30nm C C H H H H Crystallites Amorphous Region “Tie” Molecules MOLECULAR STRUCTURE OF POLYETHYLENE Nanometer Level ? “Tie” molecules bind PE crystallites ? Mechanical properties are related to the number of tie molecules (fracture occurs through the amorphous region comprised of tie molecules) ? Mechanical bonding between PE particles is due to entanglement of molecular chains ? Reinforcing elements (e.g., fibers) added to PE are only effective if PE bonds to them Intrinsic Factors ? Molecular weight distribution ? Cross-linking ? Crystallite size, shape, and orientation ? Degree of crystallinity ? Number of tie molecules POLYETHYLENE WEAR AND STRENGTH “Tie” Molecules Crystallites 10-30nm EFFECT OF GAMMA RADIATION ON PE: OXIDATION C C H H H Gamma Radiation O 2 O 2 O 2 O 2 O 2 C C H H H H O 2 O GAMMA-RADIATION INDUCED MODIFICATION OF POLYETHYLENE C C C C C C Cross-linking * Small peak in IR **Large peak in IR C C C O C C O Oxidation Aldehyde* from C-C cleavage Ketone** from C-H cleavage from C-H cleavage From Sutula, Sperling, Collier, Saum, Williams. “Delamination and White Band: Impact of Gamma Sterilization in Air and Material Consolidation” AAOS 1995 Orlando Image removed due to copyright considerations