Medical Device Link.

CUTTING-TOOL DESIGN

PHOTO BY JEFFREY A. DAVIS PHOTOGRAPHY.

The preparation of bone for implants invariably requires cutting tools. It has often been observed that the design of these instruments is not in line with the design of the implants due to the lack of cooperation between different manufacturers. Bringing the cutting-tool design and grinding in-house can provide the orthopedic implant manufacturer with better designs, which could lead to an enhanced market position. Software and automation technologies now make in-house operations easier to set up and run, too. This article will cover what an implant manufacturer needs to know in order to set up an in-house operation: the materials, tool types, and various uses for these tools.

Bone cutting is one of the oldest surgical procedures. These days, one of the most commonly used cutting procedures is drilling holes into bones for inserting orthopedic implants. Other common cutting tools are blades for cutting, severing, and bone reduction. Surgical cutting tools, which are ground on a computer numerical control (CNC) tool grinder, usually have a positive cutting geometry and can be divided into rotary and flat tools. Most rotary tools are powered instruments, while the flat surgical cutters are used manually. A great deal of the research in metal cutting over the last 100 years has been applied to the geometry and design of surgical cutting tools.

Types of Materials

Stainless steel is the most common type of steel used for medical purposes, due to its anticorrosion nature. Common types of stainless steel for surgical cutting tools are found in the series 400 Martensitic alloys, which can be heat treated. This material is also easy to machine and is very strong and resilient. Series 17 represents the heat-treated Martensitic stainless, and is widely used in surgical instruments. The series 300 Austenitic has more corrosion resistance and is used for knives and scalpels. Some of the available arthroscopic burs are actually made in high-speed steel, typically in M2 grade.

Types of Tools

Bone Drill

Drills and Twist Drills. The knowledge gained in metal cutting has helped to shape the different geometries for drill points and flute shapes suitable for drilling into bone. One of the main goals in engineering medical drills is minimizing the thermal injury to the surrounding bone and reducing the cortical penetration. Studies have shown that temperatures of more than 50°C are associated with irreversible changes in the structure and physical properties in the bone. Many orthopedic operations involve drilling and tapping before the insertion of screws into the bone. Another common purpose for drills is to produce holes in preparation for implants and wires.

There are several types of drill points, including conical, split points, three-flute points, and trocar points. Available drill bodies include solid stainless steel, tubular drills for removing drill debris, and cannulated bodies with trocar points. Other drill bits cover fixation stakes and trepanning tools.

One distinguishing feature of a surgical drill point is the point angles, which are lower than industrial drilling tools. Point angles range around 90° and go down as far as 75°.

Special software with three-dimensional simulation is available to design the flute shape, the outside diameter clearance, the point, and the point thinning. Software allowing extensive free programming in order to accommodate the specialties in surgical tools and user-defined features would be invaluable in grinding these types of tools. The simulation software for cutting tools has to be adapted specifically for medical tools. This is because most medical instruments have a custom-designed blank shape that needs to be ground, so it must have the capability to import a CAD file for the blank on which the grinding can subsequently be simulated. Other capabilities must include individual two-dimensional flute generating programs, which are particularly useful for specific flutes, depending on how much bone or tissue is penetrated.

Bone Taps. Bone taps can be produced completely by machining alone, but the more demanding surgeons prefer to use a tap that has precision-ground points for better penetration. These taps can be manufactured on a multiaxis grinder, including the flutes and the thread form. Such taps are extremely high quality and are usually reusable. Taps are used to prethread the bone. The depth of tapping is relative to the density of the bone. In very dense bone, about two-thirds of the screw length should be tapped, and in very loose bone, just a few threads are sufficient. In the design of the tap, the lead and thread form has to conform to the screw thread.

Orthopedic Bur

Burs and Spherical Burs. Materials used for surgical burs are high-grade stainless steel and high-speed steel grade. Typical shapes include oval, acorn, barrel (cylinder-ball), round (spherical), and pear-shaped. The head diameter ranges from 0.5 to 10 mm or larger. The number of teeth that the bur has is relative to the level of recommended revolutions per minute. They range from four to as high as 12, but the most common types have six or eight teeth. Most surgical burs have a cross cut which acts as a chip breaker. Some burs are left-hand spiraled, so that the surgeon can use the other hand and still experience the same cutting behavior.

This surgical bur has six teeth and is sectioned into two groups of three. Surgical burs are generally sectioned in this way so that the front of the bur still has sufficient flute depth for a good cut.

The shank types available are tubular, solid, welded, or a two-piece construction where two tubes are laser-welded together.

A surgical bur is sectioned in the front. It is a ball bur, which is a common bur for joint repairs.

Burs are used in arthroscopic or other orthopedic surgeries, mainly on knees and shoulders. Shaping, polishing, and smoothing are the most common applications for round burs. Surgeons usually prefer round burs, because they offer superior control during shaping. They run in handpieces with high revolutions per minute. One of the goals in designing burs is to reduce tissue adherence and temperature reduction during use.

Cranial procedures. Trepanning is a form of surgery in which a hole is drilled into the skull. Then a router is inserted into the hole and routes a larger plate, which is then temporarily removed, providing surgical access to the brain. The drills are usually called perforators and come in a one-piece version for use with a hand drill, or in a two-piece construction for use with a motorized device. In the case of a powered handpiece, a mechanism will prevent the perforator from touching the brain tissues.

Reamers. In typical reamer applications, the reamer hole is predrilled with a bone drill. A 4.5 mm diameter is typical. A guidewire is then inserted into the hole. A cannulated hip reamer is used to overream the guidewire up to the depth needed. There are also stepped reamers to prepare the bone to accept specially shaped implants. A spherical cutter, also known as a reamer, is used to shave the inside cup of a hip socket. This spherical reamer has the shape of half a sphere and has multiple teeth, similar to a bur.

Shaver Blades. Rotary shaver blades are used to remove tissue and debris, and for bone resection. They always run on powered handpieces. They consist of an outer stationary tube with a small window towards the front of the tool, and an inner rotating tube with a smaller window in the identical position. Both windows have the shape of cutting teeth, and when they are moved against each other, they perform the desired “grabbing” action. The debris is then moved up the tube by suction assistance. Another type of inner rotating part is often a drill-type shaft with spiral flutes, which act as an updraft for the debris.

Surgical Rasp

Rasps. Rasps are used for bone surgery, bone correction, hump removal, and other orthopedic procedures. The term “diamond chip cut” in rasps comes from grinding a crosscut across the already angled main cut, creating a diamond pattern. Those cuts usually run perpendicular to each other. Other patterns only have a small chip breaker in the main cut. Some types of rasps with narrow tooth profiles are disposable and cannot be used repeatedly, since they may harbor harmful debris that clinical cleaning does not fully eradicate.

Another category of rasps are hip broaches. These are a set of shaped rasps that progressively increase in size and are used to internally shape the femoral canal during hip replacements.

Saw Blades. Bone saws, bone saw blades, and reciprocating blades are used in many medical devices for orthopedic procedures, amputations, and removing casts. These blades oscillate back and forth only a few degrees at a very rapid rate, causing the cutting action. For removing casts, this type of saw will not cut flesh—it just jiggles the skin. Sometimes, these blades are referred to as sagittal blades. Procedures requiring these blades include hip and knee replacements, spinal surgery, and hand, wrist, elbow, and shoulder surgeries.

Software and Lean Manufacturing

Surgical rasps are used for orthopedic procedures such as amputations and bone reductions, as well as facial cosmetic procedures.

Lean manufacturing in medical tool grinding basically refers to smaller lot sizes and quicker setup. This can only be achieved with a suitable software package. A good grinding software suite includes flute, 3-D tool, and machine simulation. The tools should be able to be designed off-line on a Windows-based PC using individual standard and special grinding operations. A vast array of modules offering special functions specifically designed for surgical tools may be needed. Additionally, the operator or tool designer should be able to generate these specialty grinding modules on his or her own. All modules should have pictorial association, and the user should be able to launch a PDF file for more detailed grinding instructions.

A 3-D simulator should have the ability to export a STEP file (.stp format) in two- or three-dimensional format, which in turn can be imported in any commercially available CAD software. Within CAD, the tool designer can create a customer or shop print with dimensional details, including a 3-D model. A 3DViewer package may also be a benefit. It is one tool that can help zoom, rotate, section, pan, measure distances and angles, and perform other functions as needed.

Efficient networking, standardized file management, and file sharing are other characteristics of a good software suite for surgical tool grinding.

Here a half-round steady rest gives full support during fluting and outer diameter relieving of a bone drill to increase cycle time. Bone drills are used in procedures such as fracture repair, inserting implants, and joint replacements. These drills are made of stainless steel.

Short batches in CNC tool grinding have traditionally been a problem, due to insufficient software and general machine drift. New systems are now available to make short runs more attractive, so that some of the philosophy of lean manufacturing can be transferred to CNC tool grinding. Workflow and workplace organization, standardization, and waste reduction are now properly addressed in surgical tool manufacturing. This means making surgical tools is no longer looked at as an art, but rather as a standardized process.

Lean manufacturing encourages short batches, which enable rapid setup and quick arrival at the first-good tool. It is imperative here that the software provided with the tool grinding machine includes machine animation that can be run on a desktop so that dry-running of the machine can be avoided. This saves substantial time for each part produced in short lots.

Automation

A three-dimensional machine simulation of grinding a surgical bur using a steady rest system. Surgical burs are mainly used on joint repairs or replacements such as knees, hips, shoulders, and elbows.

Some manufacturers are moving toward unattended production to save costs. With the right equipment, the production of rotary surgical cutting tools, blades, and rasps can be unattended. A pick-and-place loader, which is part of the basic machine design, can hold the tools vertically in a cassette, or rack. This vertical storage usually works well with tool lengths up to 12 in. Hydraulic grippers transfer the blank from the loader compartment to the grinding area. A pick-and-place loader has a cassette table which lifts hydraulically. Then the loader arm comes into position and grips the blank, the table goes back down, and then the loader arm swings 90° into a horizontal position and feeds the blank into the grinding area. It is imperative that the automatic loading system is positively reliable, especially in the case of long parts, which may not always be perfectly straight.

If the blanks are longer than 12 in., a horizontal magazine loader is recommended. It has a larger capacity than the pick-and-place system and is suitable for long-shank bone drills.

Automatic fixtures for nonround cutting instruments such as saw blades and rasps are also available and have proven to be very successful in unattended manufacturing.

Conclusion

With the onset and spread of applicable CNC grinders, the manufacturing of surgical cutting tools has become competitive. Expanding your product line to include these types of tools will be beneficial not only to your bottom line, but for your knowledge of the medical tooling industry as a whole.

Eric Schwarzenbach is the president of Rollomatic Inc. (Mundelein, IL).