Each step in the creation of a metal medical prosthesis presents particular challenges. Choosing the right abrasive at each stage—from raw castings all the way to final prostheses—is essential to achieving the required dimensions and surface finishes.

Although femoral and tibial knee implants, hip implants, and bone and tube plates are substantially different, the stages in their production processes are similar enough that one can meaningfully generalize about the best abrasive products to employ at each step. To start, liquid metal is poured into the central hole of a mold, which branches off into channels for each part. After cooling, the mold is sepa-rated. This article discusses the properties and benefits of certain abrasives for cropping castings from these molds, called investment trees; removing the gates, or leftover metal, from when the liquid metal was poured into the mold; grinding contours, profiles, and interior surfaces; and polishing the implant. It also provides some basic operational tips and guidance in choosing an abrasives supplier.

Cropping Castings

When cropping investment castings from an investment tree, it is important to cut as close to the casting as possible without damaging it. Alloys used for orthopedic implants are typically quite expensive, and any gate material left on the part must be ground away and ultimately becomes waste. By contrast, gate material left on the investment tree can usually be recycled. Fines (also called grinding swarf) are seldom recycled, so the kerf (cut) should be minimized.

Other requirements include straight cuts and no burning. These requirements can be achieved with free-cutting, reinforced-bond cutoff wheels with either high-performance zirconia alumina or seeded-gel (SG) ceramic abrasive blends. The development of ceramic SG grain technology enhances the productivity and life of abrasive products made with previously available abrasive grain technology.

The proper zirconia alumina or SG wheel for cropping castings has rough sides and is specifically engineered for foundry applications. Cutoff wheels made to these specifications can last three to five times longer than conventional aluminum oxide cutoff wheels and cut up to 40% faster, especially on the difficult-to-grind materials used in orthopedic implants.

In an effort to minimize kerf losses, manufacturers sometimes select a cutoff wheel for cropping that is too thin to remain stable in the cut. Once a wheel becomes unstable, it begins to cut off-track and continues to veer even further in the wrong direction. The result could be worse than kerf loss. A crooked cut produces excessive gate material if the cut is too far from the part or produces a ruined part if the cut is inadvertently made into the part. A gate that is positioned so that the cut is made along its narrowest dimension enhances cut quality and reduces the heat generated by the cutting process.

Gate Removal

Once parts are cut from the investment tree, the remaining gate material must be removed from the part. This process commonly employs a coarse-grit abrasive belt. The actual grinding can be robotic, manual with some sort of pressure assistance, or manual offhand. The manual offhand technique is a method of grinding in which the part is held in the operator's hands and placed against a moving abrasive tool—in this case, a moving belt.

During gate removal, prosthesis manufacturers typically look for rapid, burn-free stock removal from durable abrasive belts. To best meet these requirements, they can use either a cool-cutting, advanced SG ceramic abrasive belt or a zirconia alumina abrasive belt. Depending on the operation, these belts should either have a heavy Y-weight or an extraheavy H-weight polyester backing. The extreme pressures generated by pressure-assisted grinding generally mandate the use of H-weight backing and strong belt joints. Y-weight–backed belts are best for offhand operations.

The latest advanced SG belts feature grains that are less blocklike than earlier SG grains and present more uncompromising cutting edges to parts, enabling more aggressive grinding. A functional grinding aid (lubricant) known as supersize can further enhance the productivity and cool cutting of an advanced SG belt and is often used for grinding alloys used in medical castings.

In tests, the latest generation of SG belts have delivered from 50 to 250% improvement over other SG and ceramic-coated belts. For example, these belts lasted 2.5 times longer than other ceramic belts when removing gates from cobalt-chrome knee implants.

Zirconia alumina grains also resist dulling and provide cool cuts on stainless steel and on titanium alloys. Belts with this grain are cost-effective rivals of many available ceramic belts.

A 36-grit belt is common for gate removal. However, if a part's material is very hard, such as tool steel, using a finer grit size can sometimes produce higher metal-removal rates than belts with standard grit sizes. A 40-grit belt is generally the finest grit size used in gate removal, unless there is little material to remove.

Contour Grinding

Once the gate has been removed from a casting, the initial shape of the implant must be refined on the raw casting's surface. Although some manufacturers use machining to contour parts, grinding is the quickest method to achieve dimensional accuracy as well as the required finish. Although belt grinding using robotic or offhand methods is common, it can also be accomplished using conventional abrasive or superabrasive grinding wheels.

The requirements for contour grinding using abrasive belts are similar to those for gate removal: rapid, burn-free stock removal from long-lasting abrasive belts. The grinding operation must also afford some degree of flexibility.

Because the requirements are similar to those for gate removal, the recommended belts are the same: a cool-cutting, advanced SG ceramic abrasive belt or a zirconia alumina abrasive belt. Advanced SG belts with the supersize option and a Y-weight polyester backing are usually the most productive and cost-effective choice. If zirconia alumina belts are selected, they too should have a heavy Y-weight backing. Refinement of the removed gate area and initial contouring usually requires belts in the 60–120-grit range. It is sometimes possible to complete contour grinding with an 80-grit product, because a 100-mm engineered-abrasive polishing belt will remove 80-grit scratches on some metals and alloys. Many operations have been able to remove this step and move directly to profile grinding.

Profile Grinding

After contour grinding, the next step is to give the device its final shape (profile). Profiling is typically done using grinding wheels on a computer numerical control (CNC) machine. The requirements in this grinding step are rapid and accurate stock removal using a repeatable process, as well as durable grinding wheels that produce an improved intermediate finish.

Three types of grinding wheels are suitable for profiling medical implants:

Grinding Interior Surfaces

When prostheses have interior surfaces—holes, recesses, etc.—they are typically ground using an offhand technique. Such surfaces require rapid stock removal, leading to a fine finish. These requirements can be met using SG ceramic grain mounted points.

Mounted points come in various shapes (pointed, spherical, cylindrical, etc.) and have shafts attached to help them fit into the chucks of grinding tools. Because the mounted points are composed of the same materials as SG ceramic grain grinding wheels, they provide the same benefits: twice the removal rate and an operational life three to five times longer than conventional aluminum oxide mounted points.

CNC grinding machines may be capable of grinding interior surfaces of prostheses. CBN electroplated superabrasive tools can also be used for these surfaces. It is important to consider that spindle rotations per minute (rpm) must be very high in order to maintain a productive grinding speed when using these small-diameter tools. Half-inch tools should operate at a minimum of 25,000 rpm to reach approximately 3300 surface ft/min.

Choices for Polishing

Polishing is usually done by robots or by offhand techniques. The goal is to remove grind lines left over from the previous processes and to prepare the implant surface for buffing to a mirror finish. A variety of tools can achieve the right polish, depending on an implant's material and shape and the prosthesis manufacturer's polishing techniques. The process should employ durable abrasives that enable flexibility and consistency throughout while minimizing heat generation and reducing the need for buffing. Several abrasive products are ideal for polishing implants.

Abrasive Polishing Belts. Highly engineered, three-dimensional, coated abrasive polishing belts last longer than conventional abrasive belts in fine grit sizes. They offer significantly higher performance in cut, life, and finish consistency compared with other products. Engineered-abrasive belts also offer significantly cooler operating temperatures and the ability to reduce polishing steps. Virtually all robotic applications use these types of polishing belts, and most offhand applications have adopted this technology as well.

Engineered-abrasive belts with aluminum oxide grains work well for polishing cobalt chrome. Engineered-abrasive belts with silicon carbide grains are ideal for polishing titanium. Grit sizes from 100 to 5 µm (or even smaller) are available for polishing implants. Grit sizes of 100, 80, and possibly 65 µm are capable of removing scratches and smoothing the surface produced by previous contouring steps.

A flexible X-weight cotton-backed belt is often used for the initial sequence. An extraflexible J-weight cotton backing works well for the last one or two belt stages. However, robotic polishing may allow for the use of X-weight cotton-backed belts for the entire sequence.

Portable-File Polishing Belts. For reaching the interior surfaces of implants, portable-file polishing belts (with the same specifications as the polishing belts just described) are available.

Nonwoven Belts. A good alternative to engineered abrasive belts is an ultrafine, surface-blending, nonwoven belt. This type of belt consists of a nonwoven web of nylon fibers impregnated throughout with abrasive grains and bonded with synthetic resins. The design produces a cushioned, three-dimensional material that is approximately 1¼8 in. thick and can last up to seven times longer than fine grit–engineered abrasive belts. The uniform dispersion of abrasive throughout the web provides a continuous supply of new grains as the old grains and fibers wear away. These belts have been shown to produce a finish comparable to 6-µm engineered-abrasive belts but with up to 7-to-1 belt-life advantage.

Nonwoven Wheels. Nonwoven wheels with the same basic construction as nonwoven belts can reach into recesses and can be formed into specific shapes.

Foam Buffing Wheels. Recently introduced to the market is a foam buffing wheel with self-contained buffing compounds, eliminating the need to apply liquid or bar-type buffing compounds. This wheel is capable of producing very fine finishes approaching zero microinches. The process is completely dry and greatly reduces the amount of cleaning needed.

Cartridge Rolls. Engineered-abrasive straight and tapered cartridge rolls are made of the same highly engineered coated abrasive as engineered polishing belts. These small, rotary tools fit onto a steel mandrel that in turn fits into the chuck of a rotary power tool. The rolls can easily reach and polish interior surfaces such as the inside box section of a posterior stabilized knee implant.

Mounted Points. Cotton-fiber mounted points are other small rotary tools capable of producing highly polished finishes on difficult-to-reach interior surfaces without changing their geometries. However, these points generally do not have the grit-size range of engineered cartridge rolls.

Diamond Hand Pads. Diamond hand pads may be used for touch-up, but they can also be used for dressing and renewing engineered polishing belts that have accidentally become contaminated with metal or other materials.

Conclusion

In manufacturing orthopedic implants, it is critical to select the proper abrasive at each step. Although each type of implant will have its own specifications, care should be taken to select the abrasive that will help achieve the required dimensions and surface finish. Manufacturers should familiarize themselves with the benefits and drawbacks of different abrasive techniques as well as the materials that are best suited to their specific requirements.

Ed Reitz is senior corporate application engineer for Saint-Gobain Abrasives (Worcester, MA). He can be reached at [email protected].

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