Artificial body parts used to be the stuff of the fiction. Nowadays artificial joints at least are routine. An artificial joint is a prosthesis that surgeons insert to replace diseased or worn out joint or cartilage. In nearly all cases an artificial joint relives pain and improves the range of motion. From your knees and hips, to your wrists, elbows and shoulders, if your joints go, you can replace them. An artificial joint has both metal and plastic components. The metal acts as bone and the plastic as cartilage. The metal part can be made in three ways. One can either casts them, forge them or machine them.
Statement of Works
The first method of manufacturing is casting. Casting is the most complex method. The first step is to pressure inject molten wax into an aluminum mould. This makes the pattern, a wax replica of the metal part. Now they build the mould for casting the part in metal. Workers fuse a group of pattern with hot wax to a holder also made of wax. The patterns will shape the cavities in the moulds. They submerge the pattern in thick liquid ceramic which soon hardens into a fragile shell around the wax. Then they sprinkle the shell with the mixture of two minerals, silica sand and zircon, to build up a strengthening crust. The shells now go into a furnace for about 2 hours. The intense heat hardens them and incinerates the wax inside, leaving a cavity in the pattern shape. The shells are now moulds, strong enough to withstand molten metal. That metal is cobalt chrome. In this crucible, the chrome liquefies in about 10 minutes. They remove the moulds from the furnace, and lock them at the top of another one, above the crucible. Then they flip it over. The molten metal now flows from the crucible into the moulds. It solidifies within two minutes. Then they transfer the hot moulds into a cooling bin. Cold air flows around them for about two hours until they are finally cool enough for the workers to handle. Then the workers mount each tree on an automated machine that works like a jack hammer. Each mould disintegrates, freeing the cast metal inside. They use a saw to separate the parts from the tree, then a grinding belt to remove any unnecessary metal that hardened in the channels. Next they laser engrave a unique serial number on each part for tracking purposes. Every part undergoes a thorough inspection. They dust the parts with fluorescent powder, and then rinse it off. Powder residue settles in any flaw and glows in ultraviolet light. After inspection they buff every part until the metal is so shiny, you can see your reflection.
Another method for making metal part is forging. They use a titanium alloy which is more flexible than cobalt chrome, making it better suited for hip joints in particular. They clean the metal and heat it in a 1200 degree furnace. A 600 ton press forms one piece at a time into a preliminary shape. Then they move it to the next dye where they stamp twice to refine and then finalize the shape. The last stamping on the third dye trims off excess metal and adds detail. The forged part now goes into the sand blasting chamber which cleans the metal and gives the surface a matt finish. Now a computer numerical controlled (CNC) milling machine smoothes away the rough edge that trimming the excess metal left behind. This removes all but the last bit. So they get rid of that with a sanding belt. But sanding leaves metal residue behind. So the parts now go for a bath in a water based citric solution. The metal surface has to be perfectly clean in order to stamp on the serial number in ink. Each and every metal part, forged or casted, goes into the Quality Control Department. There using digital vernier calipers, an inspector checks all the dimensions and examines the part under a magnifying glass.
So far we have seen two methods that have been used to manufacture metal parts used in making the artificial joints, casting and forging. Now we will focus on the plastic. The plastic mimics the cartilage. But this is no ordinary plastic; it is medical grade polyethylene, meaning it is formulated to be lightweight but also extremely durable so that it withstands years of wear and tear within the body. The type of metal they use to make the bone portion of the artificial joint depends on how flexible the joint has to be. For example, a knee joint requires less elasticity, so they mould it out of cobalt chrome. A hip joint, on the other hand, has to be more bendable. So for that they use titanium. They either forge the shape in the press or mount the titanium block on a computer guided mill. Under a steady shower of lubricant, the mill's 20 odd machining tools carve the block from all angles. After a primary rough machining, the mill's precision tools finalize the shape. From start to finish, machining just one part takes about half an hour. Now, a robotic arm takes over. It runs the part against the series of sanding belts, their grids progressing from rough to fine. This removes marks the machining tools left on the metal and polishes the surface to a mirror finish. A worker sands the edges manually and verifies the dimensions. Then the parts go for a thorough cleaning. Random samples are selected for wear testing in machines that simulate joint movement complete with fake joint fluid. The factory sprays some parts with titanium powder to help bone tissue cling to them. Then an optical comparator projects the image of the part's coded outline against the technically perfect template. The technician makes sure the part and the template match. To become an artificial joint the metal bone now needs its plastic cartilage and that requires polyethylene powder. A technician weighs out a specific amount of 34.80g, puts it in a mould which is mounted on a press that applies both pressure and heat (190 deg. C) The powder becomes a solid plastic. CNC drilling machine then carves out grooves where the metal part has to be attached. These details are far too intricate for the mould to shape. Then final measurements to ensure the finished part adheres to the engineering specifications are completed. Random samples undergo fatigue testing equivalent to 10 years of wear. Some plastic parts are too complex to be molded out right. Instead, factory machines a block of plastic. CNC Lathe and drilling machines are used. After specific number of movements over specific time period, parts are removed and weighed. By comparing to the initial weight, we can find out how much plastic has worn off. The products are then packaged in a clean room. Care should be taken in handling as a slightest scratch may cause premature ware. They are then packed in foam and transported to a sterilization plant where gamma rays are passed through killing all the microorganisms lurking inside.
