The present invention pertains generally to bone saw blades and methods for manufacturing bone saw blades. More particularly, the present invention pertains to bone saw blades having a hard, wear resistant coating on their cutting surface. The present invention is particularly, but not exclusively, useful for a bone saw blade having a ductile cutting section that is coated with a wear resistant coating and a hard, wear-resistant hub for attaching the blade to a power tool.
It is often necessary to surgically resect a portion of a patient""s bone. To perform this procedure, an opening or pathway to the bone is necessarily required to expose the bone. To minimize the size of this pathway, specially designed bone saw blades are generally used in the resection procedure. More specifically, a typical bone saw blade has a thin, flat, elongated shape with a cutting edge at one end. The thin, flat design minimizes the size of the required pathway and allows the blade to be held against a cutting guide during the cut to ensure an accurate, straight cut. The cutting edge is generally oriented along a direction that is orthogonal to the direction of blade elongation and contains a plurality of teeth. Thus, when the blade is inserted into the pathway, the cutting edge can be pressed against the surface of the bone that requires resection.
At the other end of the bone saw blade, the blade contains a hub section for attachment to a hand operated power tool. The power tool imparts a reciprocating motion to the blade causing the teeth of the blade to move back and forth along a cutting line that is co-linear with the cutting edge. During this process, the blade is subjected to several forces. The teeth and portions of the blade near the teeth often experience impact type forces as the oscillating teeth strike the hard bone. If the teeth are too hard and brittle, the impact forces can cause cracks in the teeth (or portions of the blade near the teeth) which will propagate and lead to a brittle fracture of the blade. In a worst case scenario, one or more of the teeth or very small particulates may break away from the blade, remain in the patient, and may result in xe2x80x9cmetalosisxe2x80x9d.
In addition to impact type forces, the surfaces of the teeth are also subjected to wear type forces that can cause material removal and galling of the teeth. These processes tend to cause an unwanted dulling of the teeth and cutting edge. On the other hand, unlike the forces exerted on the teeth at the cutting section, the thin shank of the blade (i.e. the portion of the blade between the cutting section and the hub section) is generally exposed to twisting and bending forces during a cut that tend to distort the shape of the blade. To minimize this distortion, the shank is preferably made of a relatively strong and tough material.
At the hub section of the blade, oscillation forces are transmitted from the power tool to the blade. It is to be appreciated that the surface of the hub section is subjected to wear type forces that can cause material removal and galling. Unfortunately, these processes tend to cause a loose, sloppy fit between the blade and the power tool, causing an inaccurate cut. Additionally, like the shank, the hub section is often exposed to twisting and bending forces during a cut that can distort the shape of the blade. Thus, the hub section of the blade is preferably made of a hard, strong material to prevent surface wear and minimize distortion.
Importantly, the strength, hardness, and ductility of many engineering materials can be selectively altered using heat treating, annealing, and cold working processes. Annealing is a thermal treatment that is often used to increase the ductility and toughness (at the expense of hardness) of steel (including stainless steels). Metallurgically, annealing involves subjecting a material to an elevated temperature to reduce dislocations, vacancies and other metastable conditions in the material. On the other hand, cold working a steel by processes such as drawing or rolling increases the dislocation density in the material, and thus, increases the strength and hardness (at the expense of ductility) of the material. Thus, a wide range of mechanical properties is obtainable for a given material through the selective use of cold working and annealing processes.
Heretofore, a typical procedure for manufacturing a bone saw blade has been to stamp the blade from a cold-rolled sheet of stainless steel having a hardness in excess of 42 on the Rockwell C scale (Rc 42). Next, while the blade is still hard, the teeth are machined. Unfortunately, in this cold rolled condition, the teeth lack ductility and toughness. To prevent brittle fracture in or near the teeth during subsequent use, the entire blade is typically annealed resulting in a blade having a substantially uniform hardness of between, for example, approximately Rc 49 to Rc 51. Although this annealing treatment imparts some ductility to the teeth, the surfaces of the teeth and hub section are also softened leading to excessive wear. Another drawback that occurs when the entire blade is annealed is that the strength of the blade shank is significantly reduced increasing the tendency of the blade to distort during use.
In light of the above, it is an object of the present invention to provide a bone saw blade having a strong hub section together with a cutting section that is coated with a hard wear resistant material. It is another object of the present invention to provide methods for manufacturing a stainless steel bone saw. blade having a cutting section with a hardness of between approximately Rc 42 and Rc 58 and a shank and hub section having a hardness between approximately Rc 49 and Rc 63. It is yet another object of the present invention to provide a method for manufacturing a stainless steel bone saw blade having a fracture-resistant and deformation-resistant cutting section together with a strong, wear resistant shank and hub section. Yet another object of the present invention is to provide a stainless steel bone saw blade which is safe to use, does not dull easily, and is comparatively cost effective.
The present invention is directed to a bone saw blade and a method for manufacturing a bone saw blade. In overview, the bone saw blade includes a blade body that is partially coated with a hard wear-resistant coating. In terms of shape, the blade body is formed with a first substantially flat surface and an opposed second substantially flat surface. Between the flat surfaces, the blade body can be characterized as being relatively thin in section. Accordingly, a blade thickness, t, can be defined as the thickness between the flat surfaces. In addition, the thin blade body is elongated defining a longitudinal axis in the direction of elongation. For the present invention, the blade body can be further characterized as having three distinct sections. Specifically, the blade body includes a cutting section at one end of the blade body, a hub section at the opposite end of the blade body and a shank located between the cutting section and the hub section.
Within the hub section, the blade body is preferably formed with one or more recesses, holes or slots for engagement with a hand operated power tool. At the other end of the blade body, the cutting section includes a plurality of teeth that define a cutting edge. Preferably, the cutting edge extends in a direction that is orthogonal to the longitudinal axis and lies within the plane of the thin bone saw blade. The cutting section further includes approximately 3-7 mm of blade that is positioned between the teeth and the blade body. As described further below, a hard, wear resistant coating is applied to the surface of the cutting section.
For the present invention, the blade body is preferably fabricated from a stainless steel material, but can be manufactured using titanium or zirconium alloys. Importantly, the manufacturing method used to prepare the blade is controlled to produce specific mechanical properties within the different blade body sections. In greater detail, the blade is manufactured having a cutting section that is relatively ductile with a Rockwell hardness between approximately Rc 42 and Rc 58. This ductility allows the cutting section including the teeth to accommodate impact type forces without fracture. On the other hand, the shank and hub section are manufactured to be relatively strong and hard having a Rockwell hardness between approximately Rc 49 and Rc 63. The strong shank prevents unwanted distortion of the blade during a cut and the hard hub section inhibits wear and prevents the attachment between the blade and the power tool from becoming loose and sloppy.
In accordance with the methods of the present invention, the blade body is first formed having a substantially uniform Rockwell hardness between approximately Rc 42 and Rc 63 throughout. Next, the surface of the cutting section is coated with a hard, wear resistant coating. Preferably, the coating is a metal nitride coating that is deposited on the cutting section using a cathodic arc process. During the coating process, ion impingement on the surface of the cutting section creates heat that anneals the cutting section. As envisioned for the present invention, this annealing reduces the hardness of the cutting section from a hardness in a range between Rc 49 and Rc 63 to a hardness in a range between approximately Rc 42 and Rc 58, dependent upon the materials being used.
Importantly, in accordance with the methods of the present invention, significant annealing of the shank and hub section is prevented during the coating process. In particular, during the coating process, a plurality of blade bodies are stacked on a fixture. Blade blanks are positioned between adjacent blade bodies within the stack. Each blank has substantially the same shape as the blade bodies with each blank being slightly larger that the blade bodies. The blanks also differ from the blades in that the blanks do not contain a cutting section. Thus, the blanks are somewhat shorter than the blades. With this cooperation of structure, the shank and hub section of each blade are sandwiched between a pair of blanks in the stack. On the other hand, the cutting section of each blade body is left exposed and a gap (having a thickness equal to the thickness, d, of each blank) is established between adjacent cutting sections.
During coating of the cutting sections, the blade bodies and blanks combine together to present a large mass that will absorb the heat that is generated due to ion impingement of the cutting sections. By design, the mass (blades and blanks) is large enough to prevent the heat from raising the mass above the temperature that is required to anneal the material of the blade body. The exposed cutting section, however, is annealed by the heat to a ductile condition. The as-formed strength of the shank and hub sections, however, is maintained through the coating process.
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