Casting better bones: almost routine today, surgically-implanted castings are regularly replacing knee, hip and shoulder joints with amazing results.

Lee Majors' "$6 Million Man" and Lindsey Wagner's "Bionic Woman" wowed TV viewers in the 1970s with the portrayal of the future of medical science. The ability to fix humans with replacement parts, and turn them into superior beings was thought by most (outside of the medical community) to be science-fiction-fantasy at that time.

Twenty five years later, science-fiction-fantasy is reality every day as artificial hearts and lungs are joining surgically implanted hip, knee and shoulder prostheses to augment nature's work and fix our broken bodies. In fact, knee, hip and shoulder replacements in particular are increasing and are expected to zoom as Baby Boomers try to feel "young" longer and opt for surgically implanted parts to replace their joints worn down from years of hard use.

According to the National Center for Medical Statistics, 152,000 Americans (83,000 women and 69,000 men) had hip replacements in 2000. Of these, 9% were under the age of 45. By 2030, an estimated 272,000 Americans will have the surgery annually.

This article takes a look at the surgically implanted knee, hip and shoulder prostheses, and how cast components are assisting surgeons in rebuilding these parts of the human body.

Cast Metal Replacing Bone

Implants have been a part of our society for more than 3000 years, when the first dentists replaced teeth with ivory and wood. Orthopedic implants first became a reality at the turn of the 20th century when surgeons began using pins in joints to assist movement. In 1938, British surgeons performed the first hip replacement. In 1951, the first knee replacement was performed.

While these first orthopedic surgeons paved the road for implants, medical science has come a long way in the last 50 years as today's implants are revolutionary in both design and materials.

Today's implants for knees, hips and shoulders are constructed of a combination of castings, forgings and/or plastics. The choice of manufacturing process and material for each component in an implant prostheses is dependent on several variables, including design, component size and required strength of the system in application. Guidelines detail what product fits best with each manufacturing method.

"Casting is the cost-effective method to produce implants with special geometries, angles, radiuses or configurations," said David Schroeder, project manager in the Biomaterials Div. of Biomet, Inc., a manufacturer of surgical implants headquartered in Warsaw, Indiana. According to Schroeder, castings are the product of choice for implants with complex geometries in today's modern implant era.

Castings used in implant applications are investment cast. After the castings are cleaned and finished, they are heat-treated and sometimes hot isostatic pressed (a process in which the castings are placed under intense pressure to eliminate all measurable porosity) to increase mechanical properties. After nondestructive testing ensures properties and chemical specifications, the final components are sterilized and shipped to surgeons.

Investment casting allows designers to integrate many features into components (such as screw holes) that otherwise would have to be machined, This then follows that implant designers have freedom with investment casting to produce a variety of shapes and sizes to fit unique human bone structures.

The unique designs result in near-net-shape components that require little to no post-casting processes before being shipped to the ultimate customer. As a result, Schroeder said, many of the decisions between cast and forged implant components are based on the design intricacies of the component.

For example, Fig. 1. shows implants used to replace a hip. The sphere-shaped components are a cast hip socket ball and cast hip socket shells. Investment cast, these implants require no machining on the outside and minimal machining and/or grinding on the inside to provide a tight fit with the stem and ball. The top socket, which is meant for a second hip replacement, has cast-in holes for the screws that help hold the socket in place.

Also factoring into the design geometry for implants is weight. Typical weight for an implant system for a knee, hip or shoulder is between 15-20 oz. The goal for surgeons and manufacturers is to replicate the weight of the bone system it is replacing so as to not disturb the body and its balance. Investment casting allows designers to put the weight of a component in the areas that must withstand stresses, while eliminating weight (and reducing wall thickness) in the areas that don't need it.

Implant Material

Beyond the dimensional requirements of the implants are the mechanical property requirements and material choice. Each of the materials used for implants must meet the industry standard American Society of Testing and Measurement (ASTM) F75 specification for mechanical and physical properties. One of the keys to this standard is strength. According to the American Academy of Orthopedic Surgeons (AAOS), a 150-lb person places 800 lb of pressure on his/her knee every step they take.

The other critical property is wear resistance. According to AAOS, hip replacements are expected to last at least 10 years and knee replacements 15-20 years.

Material choice also is critical from a biological point of view because it must be compatible with the human body. The implants must function in the body without creating a local or systemic rejection response. In addition, today's implants are designed to enable the body and, specifically, human bone to grow around them.

As seen in Fig. 1, the rough surface of the cast hip socket shells is designed into the component to promote bone growth. According to the AAOS, today's cast implants can be textured on the surface "to be conducive to attracting new bone growth...The bone actually grows into the surface of the implant."

The two cast materials most often used to produce the implants are:

Cobalt-Based Alloys--Offering excellent corrosion resistance, these alloys contain 50% or more cobalt, chromium (up to 29%), molybdenum (up to 6%), and a high level (compared to other casting alloys) of carbon (up to 1.2%) in wear-resistant applications. The material's ultimate tensile strength can achieve 110 ksi and it maintains excellent corrosion resistance to all chemical corrosives even at temperatures up to 2100F. Other applications for these alloys include valves, turbine components and wear plates.

Titanium-Based Alloys--This material is known for its exceedingly high strength-to-weight ratio, excellent mechanical properties and corrosion resistance. The most commonly cast titanium alloy, Ti-6A1-4V, has typical room temperature properties of 135 ksi ultimate tensile strength, 124 ksi yield strength and 12% elongation. Typically, titanium castings are used in pump and valve, aircraft and aerospace applications.

Low nickel and stainless steel alloys also have been used in producing implants, however, stainless steels are not as bio-compatible as the others.

Examining a Knee

According to the AAOS, the knee is considered a hinge joint because of its ability to bend and straighten like a hinged door. The first implant designs used the hinge concept and literally included a connecting hinge between the components. As medical science has found out, however, the knee is more complex than a door hinge.

Recognizing the complexity of the knee joint, newer implant designs attempt to replicate the joint's complicated motions by taking advantage of the two main ligaments running through it--the posterior cruciate and medial collateral--for support. Manufacturers have developed more than 150 different designs of knee replacements, for a variety of sizes and left and right models. But the key to the components that make up these replacements is their ability to withstand at least 800 lb of pressure or more. This combination of required complexity and strength is where cast components become instrumental.

In a total knee replacement, up to three bone surfaces may be replaced--the lower ends of the thigh bone; the top surface of the shin bone; and/or the back surface of the knee cap (Fig. 3). These components are designed so that the metal always articulates against plastic, which ensures smooth movement and minimal wear.

Following is a look at the three components:

Femoral Component--The metal femoral component, which can be cast, curves around the end of the thighbone and has an interior groove so the kneecap can move up and down smoothly against the bone as the knee bends and straightens.

Tibial Component--The tibial component, which can be cast, is a flat metal platform with a polyethylene cushion.

Patellar Component--The patellar component is a dome-shaped piece of polyethylene (plastic) that duplicates the shape of the kneecap anchored to a flat metal plate.

For installation during surgery, the surgeon smooths, cuts and reconfigures the existing bones to fit the shape required for the replacement knee. The implant then is either cemented or pressed into place. New medical technologies using bio-compatible materials for the implant instead of cement promote bone growth around the implant. This makes the joint stronger and closer to a natural movement when fully healed.

With current technology, about 95% of all knee replacements are expected to see a useful life of 15-20 years.

Bionic Man?

Although Bionic fantasies of the 1970s still are a ways from reality, medical industry forecasts show that many of our futures will be supported by cast implants.

According to Biomet, the current orthopedic reconstructive device market is estimated at $2.53 billion. As the Baby Boomer generation ages, medical science too is maturing and keeping pace to meet their needs. As a result, implants are experiencing an 18-20% annual growth rate (AGR), led by knees (12-14% AGR), hips (10-12% AGR) and shoulders (8% AGR).

Based on these statistics, many Americans are taking their first steps toward making the Bionic Man and Woman a reality. When it is your turn, remember you may owe your Bionic" status to metal castings.

For More Information

American Academy of Orthopedic Surgeons website at www.aaos.org.

Handbook on the Investment Casting Process, American Foundry Society, Des Plaines, Illinois (1993).

RELATED ARTICLE: Cast Innovation: Metal-to-Metal Hips

Castings have made the latest innovation in hip replacements possible. The implants, normally consisting of a metal surface (the ball and stem that are anchored in the femur) that interacts with a plastic surface (liner of the implanted cast socket shell), have been redesigned into metal-to-metal hips in two different styles.

In one style, the plastic liner was redesigned as a cast metal Piece. In the second style for larger systems, the shell and the liner are constructed as a single casting. The design lasts longer and provides more wear resistance than plastic.

Casting allows the metal socket to be created as a monoblock component with a large diameter. Its shape includes complicated geometry that would be impossible to machine because of fins designed into the part. Investment casting's near-net-shape results make production possible.

A metal-to-metal hip prostheses reduces wear by 20-100 times as compared to metal-to-plastic.

--Information courtesy of Biomet, Inc. Warsaw, Indiana, and Orthopedics Today

Rapid Prototyping for Custom Cast Implants

Based in Warsaw, Indiana, Biomet, Inc., designs, manufactures and markets orthopedic surgery products, and uses up to 1000 castings/month. Although a majority of these are traditional investment castings, up to 60/month are implantable castings produced via rapid prototyping in less than 2 weeks.

In the rapid prototyping process, an .STL file of the casting design (with gating system) is entered into the prototyping system. The system then builds this design into a wax pattern. The pattern's surface finish then is smoothed before being invested in a ceramic slurry to build a shell mold. Once the mold is built, the pattern is melted out and a traditional investment casting process follows.

The rapid prototyping system eliminates the need to build hard tooling to produce the wax pattern. Thus, it is used to lest components for form, fit and function before a large tooling investment is made. In addition, prototyping is used to produce custom implants. For example, while knees and hips come in variety of sizes that fit 90% of the population, special designs are required. Rapid prototyping allows orthopedic surgeons to specify an exact fit for their patients that can be cast to order.

The Ever-Expanding Medical Casting Market

While this article focused on implant technology and how it is strengthening our bones and joints, castings also are used throughout the medical industry in other forms of prosthetics and equipment, Examples include:

* wrist components;

* toe implants;

* shoulder stems;

* ligament staples;

* surgical instruments;'

* femoral caps for hips;

* mitral heart valves;

* X-ray and microscope equipment housings.