Updated by Dennis S. Walsh, Application Engineer, Saint-Gobain Abrasives
For an orthopedic knee implant to be comfortable and effective, it must feel as much like a natural knee as possible; this means that the surface of a knee implant must be smooth like bone, and that is where contour grinding comes in. When you take a step, squat down, or jog a short distance, the first thing that you think about probably isn’t contour grinding, but a person with a knee replacement reaps the benefits of correct contour grinding every time they bend their knee.
The effectiveness of knee implants depends on the shape, accuracy, design and finish of the model from the factory. A skilled surgeon can place these implants in a patient’s leg for optimal mobility, but these devices only work as well as the finished product performs. That’s why contour grinding is so important.
Whether you use automated machinery or do the detail work by hand, there are many considerations to make when contour grinding. Here are a few:
Refine the Initial Cast
Although a machine may investment cast the titanium or zirconium implant to a precise shape, it takes the detailed work of a more finely tuned process to achieve the perfect fit. Every knee implant patient is unique, so each knee implants come in different shapes and sizes for a comfortable fit. Some companies even custom make implants for you that match the exact shape of your existing bone.
Many manufacturers use machining to refine and finish knee implants, but contour grinding is the fastest way to get the most accurate fit and the smoothest finish. Manufacturers commonly use automated machines and handheld tools with belts to create high-quality workpieces.
Find the Right Grit
If you are using a belt, grit matters. Normally, you might use abrasive belts with a grit range of 60-120 grit for removal or refinement of a gate area or initial contouring. An 80-grit belt can complete the roughing/intermediate process so long as you polish out any scratches with a X100 engineered-abrasive (NORaX) or finer polishing belt.
Making sure your belts have advanced abrasives, grinding aids and the right backing which allows you to have precise control over your grinding and more predictable costs and timelines. Consider using seeded-gel ceramic (Blaze) abrasives, a super-sized grinding aid and Y-weight polyester backing for the most cost-effective initial grinding and a engineered abrasive (NORaX) for finishing the orthopedic knee implant.
Use a Dressing Stick
A dressing stick is a vital tool for grinding knee implants. Slightly dress down the edges of your belt with a boron nitride or a coarse-grit silicon carbide dressing stick. Dressing down the belt prevents the edges from gouging the implant, which could cause costly mistakes.
Control Your Belt Speeds
Paramount belt speed improves conformability and prevents flat spots or edges. This is critical for knee implants, as natural human knees are curved, precise joints. Selecting the optimum belt speed can also prevent heat damage to the implant, increase the life of the belt and affect your company's productivity.
It's best to aim for belt speeds between 4,500 and 6,000 surface feet per minute for cobalt chrome implants. Keep your belt speed between 2,000 and 3,500 SFPM for titanium or zirconium pieces.
Consider reducing your belt speed by 500 SFPM but not lower than the minimum SFPM recommended to increase the belt’s ability to conform to the shapes within the implant. Lower belt speeds can/will reduce the creation of facets and flat spots when you move too quickly. While it may be tempting to save time by increasing your belt speed, this will result in costly mistakes. Remember, slow and steady wins the race.
Using Contact Wheels
Contact wheels for contour grinding offer an alternative to belt grinders. Acquire a contact wheel comprised of serrated rubber with ashore “A” and a durometer hardness of 65 to 90 for roughing and intermediate grinding. Look for Contact wheels with a cross-serrated wheel-face pattern with a shore “A” durometer of 50 – 65 ; this pattern provides the proper conformability for contours/radii plus will produce the precise finish.
These tips and tricks can help you contour grind an implant right the first time, saving time and money, but also increasing a person's quality of life. Make these considerations and know that you are not only upholding your company's reputation, but also helping someone move more comfortably.