DESIGN AND MANUFACTURING
PROCESS FOR ARTIFICIAL JOINTS
The goal of total joint design is to produce a long-lasting implant that restores normal anatomy and function to the greatest possible extent in a prosthetic situation. To this end, all of the anatomic and physiologic characteristics of the joint are considered. The appropriate shape of the implant is then formulated. The infinite number of human "sizes" must also be considered and proportionally-sized prostheses developed.
In the process of designing the artificial joint, not only the shape of the articulation must be considered but also the means by which the prosthesis will be fixed to the bone. Currently fixation is accomplished by either bone growing into a prosthesis or with bone cement. Different shapes of implants are better adapted to bone ingrowth and other shapes to cement. Bone ingrowth prostheses cannot always be optionally fixed with cement. Frequently a joint will be designed with two versions available - one which can be fixed with bone ingrowth and one which can be cemented.
After the design of the articulation and method of fixation are decided upon, complex and detailed blue prints are drawn. From these prints, manufacturers are able to produce the artificial joints. Many other decisions must be considered along the way. These include:
1. What type of metal should be used?
· Which is best accepted by the body and has the least chance for future untoward biologic consequences?
· Which will wear the least with time in a human environment?
· Which is the strongest and will not fatigue and break?
· Which is the most like bone and will not detract from bone's desirable biologic properties?
2. What plastic should be used?
· Which one wears best?
· Which one deforms least with time?
· Which one is best accepted by the body?
3. Are any other materials better than metal and plastic?
· Do ceramic, fiber composite, etc. have a role?
The success of joint replacement has been directly related to the accuracy with which the surgical procedure was performed. Therefore, it is important that the surgeon be able to technically accomplish the surgery with reproducible accuracy. To assist in this, instruments must be provided with the artificial joint to assist with its implantation. These instruments must be designed so they are easily and efficiently used in the operating room and compliment the prosthetic design.
After the design is complete, the manufacturing process begins. Manufacturing has been helped immeasurably in the last few years with CAD-CAM systems. These Computer Assisted Design - Computer Assisted Manufacturing systems assist skilled engineers in the orthopaedic factory. Implants are made usually in one of two methods: 1. Investment casting, or 2. Machining.
Investment casting is the process where a specific shape is made (i.e. part of the total joint). A ceramic mold is then made using this shape. After the mold is complete, the shape is removed. A prosthesis is formed by pouring molten metal into the mold. After the metal is hard, the ceramic mold is broken to release the prosthesis. Thus, an investment (the mold) is made to yield the prosthesis. This gives the prosthesis its rough shape. It is then polished, and a porous coating is added if desired. The porous ingrowth surface is attached by setting the beads or fiber (wire) mesh on the prosthesis. The prosthesis is then heated to just below its melting point with pressure being applied to the porous surface. This process, called sintering, creates small points of melted metal where the wire mesh or beads touch the underlying large prosthesis. This welds them securely into place. After the prosthesis is cool, final polishing of any articulating surface occurs, and the new joint is ready.
Some metals and shapes (particularly titanium) are machined into final shape much faster and more accurately than can be done with casting. With this machining technique, a design is fed into a computer-driven milling machine or lathe. A solid piece of the appropriate metal is then manipulated through the cutting instruments to turn out the final shape. Once the shape is final, a porous surface can be sintered on if desired. Final polishing completes the process, and a finished prosthesis is ready for implantation.
The polyethylene bearing surface for total joint replacement is the same material which is found in plastic bags or tubing, but its chemical structure is slightly altered. It is ultra high in molecular weight, giving it some unique properties. It is slippery which is important for joint motion. It is very strong and wear resistant to withstand the stress of walking. It is also very well accepted by the body. It is shaped to the desired form by either molding or machining. Machining is similar to the processes described above for the metallic portion of the prosthesis.
The metals used in artificial joints today are alloys made of different elements to achieve a strong biologically compatible implant. The two most commonly used are vitalium and titanium. Vitalium is an alloy of chromium, cobalt, iron, and other trace elements. Titanium is usually an alloy of titanium, vanadium and aluminum. It can also be used as a pure element. Both of these metals have many advantages for orthopaedic application. Stainless steel is used for some orthopaedic needs but is too weak to be used to make artificial joints.
Obviously, many parts can be simultaneously manufactured with either the casting or machining technique. After the prothesis is made, it must be packaged in a sterile fashion. This is usually accomplished by wrapping the artificial joint in many layers of protective plastic, paper, or boxes. The boxed product is then sterilized by special radiation techniques. This does not change the shape, composition, or usability of the prosthesis.
The prosthesis can now be delivered to the surgeon for implantation.
Very high standards of manufacturing are kept to insure the best product possible. Many quality control steps exist along the production path. Sizes, shapes, finish, and sterility are all checked and rechecked. This is good for the consumer but also escalates the cost of the prosthesis.