METALS
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- Metals are materials that contain metallic + non-metallic elements and have freely circulating electrons
- Presence of non-metallic elements creates slight deformation in crystal, increasing yield strength and resistance to plastic deformation
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- Cold working (aka strain hardening). Done below recrystallization temp. Increases yield stress and UTS but sacrifices ductility - "Make metal more deformed so it cannot deform anymore"
- A material that has received prior deformation will be stronger than an undeformed material.
- Annealing = Heat to above recrystallization temperature and plastically deform the metal ➔ increases ductility
- Forging [Combination of cold working and annealing] = Manufacturing process to optimize material properties using pressure. Can be cold or hot working (relative to recrystallization temp.)
- Alloying = Adding materials to interpose within crystals to prevent further dislocations to increase stiffness
- Quenching = Heated metal suddenly immersed in cold water or oil to improve hardness
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- Iron 62%, Chromium 18%, Molybdenum 3%, Nickel 16%, Carbon 0.03% (L = Low Carbon) ["iron chromium MNC"]
- It is a metal alloy - consisting of metals and non-metallic elements in a sea of free ions
- Chemical name is 316L = 3 Molybdenum, 16 nickel, L carbon, the rest are iron and chromium
- Bioinert, stiffer than bone, used in plates and screws
- (+) Stiff Young's modulus of 190, cheap
- (-) Risk of corrosion, risk of nickel allergy
- Some new implants add more nitrogen to make it stronger - Orthinox (Stryker Exeter)
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- Orthinox Stainless Steel (added nitrogen)
- Bana: "In recent years there have been attempts at replacing the nickel content of stainless steel with nitrogen to diminish the possibility of a nickel allergy developing.
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- Cobalt 61%, Chromium 20-30%, Molybdenum 6-10%, Nickel, Carbon, Tungsten MNC ["Cobalt chromium MNC"]
- Bioinert, Biocompatible
- (+) Stiffer than steel, Young's modulus is 210, used for bearing surface
- (-) Better corrosion profile than SS but susceptible to galvanic corrosion compared to titanium. Due to being stiff, risk of stress shielding
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- Titanium 89%, Aluminum 6%, Vanadium 4%, Others 1%
- 6Al-4V = 6% aluminum, 4% vanadium
- Lower Young's modulus 110
- (+) Similar YM (110) to bone, less stress shielding, has self passivation with oxide layer ➔ less prone to corrosion
- (-) Notch sensitivity
- Concerns of Vanadium toxicity, thus newer iterations contain Niobium, zirconium [NZ]
- Vanadium toxicity.pdf
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- Element 73
- 80% porous, consistent pore size with 100% interconnecting pores to facilitate biological bony ingrowth. Used in revision cases
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- Femur = Cobalt Chrome
- Tibia
- Tibia is usually titanium
- If mobile bearing - tibia is cobalt chrome
- THR materials
- Shell - titanium alloy
- Hip ball - cobalt chrome
- Stem - titanium
POLYMERS
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- Acts as a grout, not a glue, with no adhesive properties
- Space-filling, load-transferring material
- No chemical bond between PMMA and bone or implant interface
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- RAPID SAM
- 5 powder = Radioopacifier (barium), Abx (Genta or vanco), Polymer (PMMA), Initiator (benzoyl peroxide), Dye
- 3 liquid = Stabilizer = Hydroquinone, ACCELERATOR = N,N-Dimethyl-P-toluidine, Monomer (methacrylate)
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- Addition polymerization
- CF. PE is condensation polymerization
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- Mixing
- Waiting (aka stringy) phase
- Working (aka doughy) phase - viscosity suitable for implantation
- Setting - hardening
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- Dough time = starts from mixing and ends when cement will not stick to unpowdered glove (i.e., start of working/doughy phase)
- Setting time = beginning of mixing until surface temperature is half maximum
- Working time = difference between dough time and setting time
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- Increased mixing speed, temperature, and humidity all accelerate setting time
- Faster mixing speed ➔ shorter setting time
- Higher temperature ➔ shorter setting time
- Higher humidity (which increases temperature) ➔ shorter setting time
- Incorrect powder-to-fluid ratio
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- Cementing technique
- Air voids create stress risers
- Cement mantle thickness
- Porosity reduces strength — hence mixing in vacuum
- Contents of cement
- Adding antibiotics decreases strength
- Increased radio-opacifier reduces strength
- High mixing speed causes monomer evaporation ➔ fewer monomers ➔ weaker cement
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- Strong in compression
- Weak in tension and shear
- Brittle, therefore notch sensitive
- Viscoelastic
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- Systemic - Fat embolism, cementing syndrome
- Local - Thermal and chemical necrosis of bone, occlusion of nutrient artery ➔ bone necrosis, third-body wear from retained fragments
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- Characterized by hypoxia, hypotension, unexpected loss of consciousness (in spinal patients)
- Can occur during cementation, prosthesis insertion, or joint reduction
- Pathology is controversial - fat and bone marrow particle embolization (more accepted theory) or monomer embolization
- 3 grades
- Grade 1 = arterial saturation < 94% or systolic BP fall > 20%
- Grade 2 = SpO2 < 88% or systolic BP fall > 40%
- Grade 3 = Asystole or major cardiovascular collapse requiring CPR
bone cement and hip fractures.pdf260.9KB
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- Inform anesthesia prior to cementing
- Anesthesia can:
- Pre-oxygenate
- Ensure no hypotension prior to cementing
- Ortho:
- Thorough pulse lavage to remove fat contaminants
- Use cement plug to limit distal spread of cement
- Use suction catheter during cementing to reduce pressure in IM canal
- Avoid rigorous pressurization in high-risk patients
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- Increase O2 to 100%
- Aggressive IV fluid resuscitation
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- Reduce porosity and corresponding stress risers
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- Monomers - avoid excessive exposure to vapors as they are highly volatile and flammable
- Must not come into contact with skin or rubber gloves
- Conventional gloves protect for only 1 minute. Additional polyethylene gloves provide 5 more minutes of protection
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- "hand, finger", "prep, plug, gun", "vacuum, pressure, centralizer"
- 1st gen = hand mix, finger pack
- 2nd gen (+3 PGP) = + pulse lavage canal prep, cement gun allowing retrograde injection and cement restrictor plug
- 3rd gen (+3 CVP) = + vacuum mix, pressurization, distal centralizer
- 4th gen + proximal centralizer
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- Prevents formation of long cement plugs, which reduces risk of cementing syndrome and makes future revision easier
- Enhances cement pressurization
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- A = Complete whiteout of cement-bone interface
- B = Slight radiolucency < 50%
- C = Radiolucency 50-99%
- D = Gross radiolucency (e.g., no cement distal to tip, major defects)
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- PMMA, not plastic
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- The ideal cement mantle thickness remains debated, with two main philosophies:
- One advocates for a complete cement mantle of at least 2mm is ideal
- The French paradox involves implanting a canal-filling femoral component in a line-to-line manner with a thin cement mantle. Good results may be attributed to cortical bone supporting the canal-filling stem.
- Both techniques have demonstrated excellent clinical results
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- It was designed to improve cement properties by reducing exothermic reaction during polymerization, lowering residual monomer solubility, and reducing airborne monomer and aromatic amines
- Found to have an unacceptably high failure rate and is not recommended for use
CERAMICS
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- Compounds made of metallic and non-metallic elements. Unlike metals where electrons freely circulate, ceramics have electrons locked in ionic or covalent bonds
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- 4 steps
- Sintering - mix powder with water, then heat and bake to achieve high density
- Transformational toughening by adding tetragonal material (e.g., zirconia)
- Hot isostatic pressing to further increase density [3rd gen]
- Addition of trigonal particles (chromium and strontium) to limit crack propagation
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- 1st gen - Alumina oxide - underwent transformation in vivo, leading to catastrophic failures
- 2nd gen - Added tetragonal zirconia particles [transformational toughening]
- Prevents crack initiation by acting like "airbags" to absorb impact forces
- 3rd gen [Forte] - Added hot isostatic pressing (3rd stage)
- 4th gen [Delta] - Includes chromium and strontium (trigonal particles) to limit crack propagation
- These form platelet-like crystals that dissipate energy by deflecting and neutralizing cracks
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- Bioinert
- (+) High wettability for lubrication
- (-) Brittle (no elastic behavior), risk of catastrophic failure
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- Alumina
- Zirconia
- Biolox
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- Ceramic is a brittle material, and fails by Crack initiation and propagation
- Microscopic cracks can occur from stress
- Cracks may become critical size and it
- Propagates through the structure in a fast manner, failing suddenly
- It fails suddenly, soon after the yield point.
- This is opposed to a ductile material where the cracks keeps getting bigger until the remaining area cannot support the load.
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- "edge loading", "stripe wear", "patch wear", "squeaking"
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- During processing of the liner, an inevitable sharp edge forms at the rim. During hip movement, the femoral head loads against this sharp edge, resulting in edge loading
- Posterior edge loading occurs more frequently than anterior edge loading and may be associated with micro-separation of the femoral head during the swing phase
- Associated with steep cup inclination, young patients, and revision surgery
- Edge loading is a normal mechanism in CoC articulations and is unavoidable. However, it is considered clinically insignificant as the wear volumes produced are too low to generate osteolysis
- Stripe wear progresses to patch wear
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- Has multiple causes and may result from edge loading
- Implant factors - occurs in CoC, not in hard-on-soft bearings
- Patient factors - related to demands placed on hip; more common in young, taller, and heavier patients
- Implant positioning - studies show increased squeaking in implants placed outside the safe zone
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- It is oxidized zirconium, a Smith & Nephew product. It combines a metal alloy core with a ceramic surface (Black)
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- Hydroxyapatite Ca10(PO4)6(OH)2
- A ceramic used as an adjuvant coating on prosthetic cups or stems
- It functions as an osteoconductive agent, readily accepting osteoblasts and reducing gaps
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- HA coating uses plasma spray technique, where HA particles are projected onto metallic material at high temperatures (15,000°) under vacuum, at high speeds (300m/s)
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- Acts as an osteoconductive agent, enabling rapid gap closure between implant and bone
- Provides an optimal surface for osteoblast attachment, facilitating bidirectional gap closure
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- Higher manufacturing costs and risk of coating delamination
- Delaminated HA particles can trigger osteolysis
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Ponder the path of your feet; then all your ways will be sure. Proverbs 4:26