Joints are Big Business
Spare parts for baby boomers
By Robert B. Aronson
For the over-50 segment of the population, "I just got a joint replacement" has become a common statement. There are two major reasons for joint replacement. About 60% to 70% of the operations are the result of age deterioration arthritis. Bones and cartilage wear or become brittle and no longer function as originally designed. The other major reasons for joint replacement are birth defects and accidents.
Manufacturing is the hero. The success of this medical technology is due chiefly to improvements in manufacturing, including more precise machining, material advances, and better control of manufacturing processes.
There are two categories of orthopedic joints: large and small. The small include finger, wrist, elbow, and toes. These joints experience lighter loads and require a more limited range of motions. The big joints, the hip, knee, shoulder, and ankle are more complex, both in design and function. They take greater loads, have to accommodate a wider range of motions, and require greater accuracy both in manufacture and installation.
Because of the consequences of failure, reliability is a key issue in joint design and manufacture. According to Robert Churinetz, vp of global operations, Wright Medical Technology (Arlington, TN), "Obvious errors are generally found during initial lab testing. The problem comes with long-term use. An accurate check of a joint design may require following about 500 patients for 10 years. It takes that long before we know how a design change has influenced performance."
There is no way to do accelerated testing. The variables in a human that influence how the body will react to the implant include: age, physical condition, life-style, and even geographic location. These factors all determine how the joint will wear and how the body will react to the joint material.
Making these joints begins with components made from forgings or lost-wax castings. Then they go through traditional roughing and finishing machining operations. There is also considerable manual polishing and grinding, although much of this work is being taken over by robots as their reliability increases. This is followed by passivating and sterilization. The joints are made to very tight tolerances, often to an accuracy of 0.001" (0.3 mm).
Materials are a major manufacturing decision. They have to be long lasting, very machinable, and totally corrosion-free. There are not many materials that meet those criteria, so there is not a lot of latitude in the decision process. The metals used include precipitation-hardened materials and 400 and 600 series stainless along with alloys of cobalt, chromium, and molybdenum. Joint manufacturers also use titanium, both commercially pure and various alloys.
The nonmetallics are chiefly silicone elastomers and ultra-high molecular weight polyethylene. These materials are used chiefly for joint-bearing applications such as in the hip joint to receive the metallic ball portion of the joint. Fingers are "hinges" or spacers and are made from silicone elastomers.
One of the newer material developments is the use of ceramics for hip bearings. Designers have long wanted to take advantage of ceramics' hardness, but it is also quite brittle, which means the potential for catastrophic failure. "The newer ceramics are tougher, which minimizes the failure potential," says Richard Tarr, vice president, research and emerging technologies, DePuy Orthopedics (Warsaw, IN).
Finish on ceramics is better. With metals, there are microscopic peaks and valleys. With ceramics there are only valleys, which means a less abrasive surface of 5 µin. or less.
"Another new material is a glass-type alloy," explains Tarr. "Its benefit is that it can be made into joint elements by casting or molding at lower temperature. Manufacturing cost is, therefore, lower. It also can be formed with greater accuracy so the part is near net shape, minimizing machining operations."
Biologic reactions have to be carefully considered. Some of the problems with joints are caused by a break-down of the implant materials. When even small elements of the insert material break free of the joint, the long-term result may be joint failure. The free material may cause reactions between the implant and the natural bone and tissue that could cause the joint to loosen and eventually fail. For this reason, much research has gone into maintaining the stability of the insert material and minimizing friction within the joint that could cause breakdown of the material.
Attachment methods for hip and knee prostheses are keyed to the patient's physical condition. In one case, the ends of the joint that are inserted into the bone have a smooth finish. They are placed into the bone and secured with a cement, poly-methylmethacrylate.
In the other technique, the implant surfaces are porous or roughened. Instead of cement, the bond relies on the bone to grow and adhere to the rough surface. The rough surface can be created in a number of ways, including plasma spray, sintering beads to create porosity, or rough grinding. More recently a substance called hydroxyapatite is sprayed on the insert to encourage bone growth. The bone-growth technique is favored. However, with some older patients or those with bone disease, where healthy bone growth is limited, the cement technique is used.
There are also variations in the design of the bearing surfaces for knee and hip joints. Originally the prevailing design was a metal ball riding against a polyethylene lined cup for hips. The knee joint bearing surface is metal on the femur riding against a polyethylene lined tray on the tibia or shin bone. However, developments in machining techniques and materials have caused a move to metal-on-metal or ceramic joints.
New superfinishing processes with tolerances in the millionths of an inch, and highly accurate honing, provide greater sphericity have minimized friction. At the same time machine tool programming has led to better overall control of the machining processes.
The surgical procedure and joint positioning requires as much, if not more, equipment than the joint itself. Unlike machining an engine block, it is not possible to get the patient into a nice snug fixture. The surgeon has to accurately measure the exposed bone with the patient attached.
There are a number of variables including matching patient anatomy and load distribution. The design of the joint and the actions of the surgeon have to take into account how the body load is transmitted to the implant and how load is transmitted from the implant to the rest of the body. The process is called "load sharing." It's not just a matter of making the strongest possible implant, but how the implant distributes the load. If the implant takes all the load, the supporting bone becomes "lazy" and will not sustain its mass. This could lead to joint failure.
Basically the surgeon relies on X-rays of the joint area taken before the operation to judge the size and shape of the implant used. Each type of joint is supplied with a set of instruments that the surgeon uses to make the cuts and position the joint. Then a set of trial implants is used to establish alignment before the actual joint is secured.
More advanced systems use tomographic 3-D X-rays of the joint before surgery. During surgery, a display system overlays the pre-surgery image over projected images of the exposed joint area. The surgeon then uses these imposed images to make the cuts and position the implant properly. This process called Navigational Orthopedic Surgery, is being developed by DePuy.
Robots, once touted as the surgeon's assistant, have not performed as predicted in aiding surgical procedures. Many in the medical profession believed that the use of robots would greatly simplify these operations and improve accuracy. However, experience did not bear that out. "Robots could not give the fine positioning and feedback necessary to match the human hand," says Tarr. "Plus, the robots required a larger field to work in. As a result, the chance of infection is greater and healing time longer. Now, research is going in the other direction," he continues. "Instead of exposing large areas of the joint, surgeons are working though smaller incisions, utilizing electronic signals to guide them. The result is reduced chance for infection, faster healing, and less discomfort for the patient."
The future of joint replacement surgery will see an ongoing series of evolutionary changes. "We are constantly looking for ways to improve reliability," says Churinetz. "But patients need to understand that we are dealing with a mechanical device that wears out, no matter how well made it is."
Although research continues on refining the mechanical joints, industry experts predict the next big breakthrough will be "orthobiologics," or encouraging the body to repair itself. Many companies are working with new bone grafting materials, and materials that will encourage cartilage growth.
This article was first published in the May 2003 edition of Manufacturing Engineering magazine.