1. The properties of surgical implant materials are given below. The materials a
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Question
1. The properties of surgical implant materials are given below. The materials are selected based on following limitvalues of criteria listed below. a) Can you identify alternative materials (different from what is specified in the table given under part c of this problem) for hip implants using criteria given below, especially mechanical properties? l. Tissue tolerance, lower limit, Y Tensile strength, lower limit, Yi 95 MPa 4. Fatigue strength, lower limit, Yi 33.25 MPa. Toughness, lower limit, Yi 6. Wear resistance, lower mit. Y-7. 7. Elastic modulus, target value, YLE 14 GPa 8. Specific gravity, target value, 2.1 9. Re ve total cost, upper limit, Y 10 b) Using the table of properties given below,decide the relative importance of each criteria utilizing AHP Property Tissue tolerance (lower-limit property) Corrosion resistance (lower-limit property) Tensile strength (lower-limit property) Fatigue strength (lower hit property) Toughness mit property) Wear resistance (lower mit property) Elastic modulus (target-value property) Specific gravity (target-value property) Cost (upper mit property c) Using the table of material properties, AHP and weight factor of criteria from AHP process of part b, identify the best materials that can be used for hip implants. Corrosion Resistance: This is because body fluids are aqueous salt solutions with concentrations roughly comparable to seawater and with a pH value of 7.4. This environment is hostile and could cause corrosion of many metallic materials. Corrosion resistance is rated according to a scale of 10-1. Tissue Tolerance: In general, biological requirements represent the major constraints in selecting implant materials. The action ofthe implant material on the body tissues can range from toxicity, in which case the implant material is totally rejected, to inertness. This requirement is usually called tissuetoleranceand is difficultto quantify. Forthe sake of comparison, materials are given a rating of 10 for the best material and1for the worst Only materials with tissue tolerance of 7 or more will be considered. Instructor: Dr. Ragu KannanExplanation / Answer
Metal on Metal (MOM) – These are what they sound like. Both the socket and the ball are made of stainless steel, titanium, chromium, cobalt or some combination of these. One sub-type of a MOM hip is a minimally invasive model which usually is smaller in size, so it can be installed with a smaller incision.
Polyethylene and Metal on Polyethylene (MOP) – Polyethylene is basically plastic, so these hips usually have metal structural pieces and a plastic liner where the ball and socket meet. They can also have a metal ball meeting a plastic socket liner. A sub-type of a polyethylene hip is made with a newer plastic called cross linked polyethylene, which is more durable.
Ceramic on Metal (COM), Ceramic on Ceramic (COC), Ceramic on Polyethylene (COP) – Ceramic hips are made of specialized and more durable versions of the same type of material that plates and bowls are made from. There are ceramic on metal, ceramic on ceramic, and ceramic on polyethylene versions. While these are durable, they can be vulnerable to fracture and breaking under big stresses.
Correct Sizing is Key!
It’s very clear from reviewing the medical research on this topic that a poorly sized hip device is a huge problem for many reasons. First, it will reduce the longevity of the device. Perhaps more importantly, it virtually guarantees more harmful wear particles. And since we’re talking about replacing your hip, a good fit is common sense.
Given the modular nature of these hip devices and the wide array of options, a poor fit should never happen. However, in my experience, hardware fitting issues usually happen when the operating room and/or hospital don’t have the correct size in stock on the day of the surgery. So agree before hand on the size of the components and make sure the staff has double-checked to make sure the hospital has that size in stock.
The upshot? All hip replacement devices produce wear particles. But which is best? It’s clear that when metal on metal implants go bad, things can go very bad with a local tissue reaction that can cause pseudotumors and high metal ions levels in the blood. If I had to get my hip replaced, I would cross this type of implant off my list. Ceramics still produce wear particles and have the added problem of fracturing in an active individual. Polyethylene wear particles in a MOP hip can be just as bad as those from a MOM hip. Given that highly cross linked polythylene has the least wear particles, this is likely the winner. However, realize that not as much is known about tissue reaction to cross linked polyethylene, so that recommendation may change with time. In addition, there are newer types of ceramics that once mated with HCLP could produce less debris.
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