Patent Application
20160296241
This invention relates to material cutting tools and more particularly to surgical saw blades.
Many orthopedic surgical procedures involve the cutting of bones during arthroplasty or while performing osteotomy and bone re-alignment procedures. In the past, these operations were performed with manual and gigli types of saws. The modern surgical procedures have further evolved by replacing these instruments with electrical powered saws, and especially the oscillating or sagittal saws. These tools impart a back-and-forth oscillating movement to the blade by exerting a cutting action in both directions of the movement. Surgical saws reach cutting speeds of up to 20,000 strokes per minute. Saw blades used in surgical procedures generally have the shape of a rectangular thin sheet metal with cutting teeth in one of the narrow edges and the tool attachment in the opposite end.
Bone cutting presents several problems for the surgeon. The chief problem being an increase in temperature caused by friction during the cutting process. Other problems include excessive vibration and noise which present risks for the surgeon rather than the patient. Therefore, an ideal saw-blade would be one that provides accurate bone cuts without generating too much heat, noise and vibration.
Multiple studies have shown that the rise in temperature of bone during cutting can cause thermonecrosis of the bone resulting in the subsequent failure of the installed implant. The threshold temperature is 47° C. and temperatures above this value may cause osteonecrosis. The time at which bone is held at a temperature value above the threshold is another very important parameter that can affect the response of bone to the cutting operation. It widely accepted in the surgical field that the maximum holding time above 47° C. is 1 minute. Cutting process performed above these levels can result in osteonecrosis with irreversible bone damage and detrimental effects on the solidarity of the implant.
Medical experience and research has shown that the temperature of the bone during cutting without other mitigating actions can increase up to a maximum of 450° C. For this reason, very often surgeons will use saline solution to cool down the cutting area resulting in average temperature of 68C and maximum of about 100° C. These temperatures are still above the threshold of 47° C. and inevitable will negatively impact the bone recovery after the surgery.
Most of the research in avoiding high temperatures during orthopedic surgery has been focused on the drilling tools due to the fact that the drilled holes will host a screw which will support the surgical implant. In this case, osteonecrosis will cause bone failure releasing the screw, and, as consequence, the entire implant may move out of place with serious consequences for the patient. On the other hand, little attention has been paid to the improvement of saw blades since these instruments are generally used in cutting bone fragments. Efforts have been made to develop an internally cooled blade where saline solution is fed between two layers of sheet metal but this invention has not been able to find a practical use in surgical rooms. Most of the development has been focused in modifying the teeth design in order to improve the cutting performance and reduce the heat generation.
The design of the saw blade, especially of the cutting edge, as well as the surface condition of the blade can enhance or reduce the negative factors discussed above. The increase in bone temperature is an effect of the heat generated during the cutting process that, in turn, is the result of the friction forces between the blade and the bone.
The present invention provides a saw blade with a tooth configuration that offers improved cutting action and less thermal buildup, during surgical procedures involving the cutting of bone.
The surgical saw blade is generally a planer rectangular body that comprises an improved cutting edge, a midsection and a drive transmission attachment. The drive transmission attachment defines the proximal end of the blade and can be adapted for attachment to a conventional power tool for oscillatory motion. The surface of the drive transmission attachment is not in contact with bone.
The midsection forms the body of the saw blade. The midsection is a generally rectangular planer body defining a pair of longitudinal edges and a distal edge. The midsection will normally not be in contact with bone during the cutting operation and preferably is thinner than the distal edge. The distal edge of the midsection defines the cutting edge of the saw blade.
The cutting edge is configured as a series of triangular blades or teeth having a base disposed along the cutting edge of the blade and adjacent sides culminating at the apex in a tip. The face of each tooth is sharpened at both adjacent sides of the tooth triangle so that the full cutting action of an oscillating or reciprocating blade is achieved. Odd numbered teeth will have sharpened cutting edges on one face of the tooth while even numbered teeth will have cutting edges on the opposite face.
The generation of heat during the cutting process is largely due to friction between bone being cut and the saw blade. In accordance with the invention the teeth are slightly bent inwardly or outwardly. The teeth are bent sufficiently to provide a kerf that is greater than the thickness of the blade body. In this manner there is minimum contact between the bone, the cutting edges of the teeth and the surfaces of the midsection resulting in less heat generated due to friction ring the cutting procedure. In addition debris from the cutting procedure can accumulate between the midsection surface of the blade and the wall of the bone creating more friction and heat buildup. The kerf created by the bent teeth is larger than the thickness of the midsection so that there is little or no contact with the bone and the surface of the midsection. In a preferred embodiment the midsection is thinner than the distal edge of the blade further insuring minimum contact between the bone and the surface of the midsection.
Other advantages and features of the invention will become apparent from following description taken in conjunction with the following drawings.
Certain terminology will be used in the following description for convenience in reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement. The word “distally” shall mean directed towards the patient, and the word “proximally” shall mean directed away from the patient. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.
It has been found that heat generated by the configuration of the teeth of the saw blade of the present invention is unexpectedly less and therefore, there is a lower danger of bone necrosis. In accordance with the invention the improved tooth configuration is achieved by bending the teeth 20 inwardly or outwardly with respect to the base 22 of the tooth.
Referring to
In an alternate embodiment of the tooth configuration of the invention the teeth are bent outwardly. This configuration is illustrated in
The midsection 16 defines the body of the blade and is not necessarily involved in the actual cutting of the bone. It and serves merely as a vehicle for the teeth 20. Surfaces 17 may be contiguous with the wall of the bone during the cutting procedure. In the prior art the cutting edge 14 and the midsection 16 are of equal thickness and are substantially equal to the kerf of the surgical saw blade 10. During the cutting procedure the surfaces 17 are substantially contiguous with the wall of the bone and generate heat during the oscillation of the surgical saw blade 10. Moreover additional friction between the surfaces 17 and cut away particles of bone that become lodged between the surfaces and the bone will generate even more heat.
Most conventional surgical saw blades have a midsection thickness equal to the cutting edge. As illustrated in
Several factors have to be considered when selecting the material for the surgical blade in view of the medical applications for which the saw blade will be used. Martensitic stainless steel (410 or 420) is preferred for the surgical saw blade. Martensitic stainless steel is preferred because it is resistant to corrosion, can be easily machine in annealed condition, and possesses a high ultimate tensile strength and very good edge-keeping ability in the hardened condition. Surface condition of the surgical blade is a consideration in reducing friction and heat buildup in bone.
Tests have shown that a lower roughness of the metal surface results in lower friction coefficient and, as consequence, lower temperature increase. The best surface condition can be achieved by a polishing process but processing times and associated costs are unjustifiably high. Some products are electro polished but this process, while assuring a low surface roughness, rounds off the cutting edges thus reducing the cutting ability.
Coating the saw blade with a material having a low coefficient of friction, similar to the process commonly used with conventional drill bits, will be beneficial to the goal of maintaining low cutting temperatures. Among coating materials commercially used with drill bits are hard ceramic materials such as titanium nitrate that is applied as a thin layer on the order of 5 micrometers. Titanium nitrate provides a hard low friction surface that reduces heat buildup due to friction. A preferred coating material is an a-C:H material distributed under the trade name Balinit™ Triton. This material provides a thinner, on the order of 0.2 micrometers, hard diamond like, surface having a lower coefficient of friction than titanium nitrate 0.1-0.2 on steel versus 0.4 for TiN. Low friction coatings as applied to the surgical saw blade of the invention further reduces heat buildup due to friction.
From the foregoing it will be seen that the danger of bone necrosis due to frictional heat buildup during a cutting procedure will be reduced by the surgical saw blade of the present invention. The contact between bone and surfaces of the cutting teeth is reduced by bending the cutting teeth to form a configuration in which a large portion the surfaces of the teeth and blade body do not contact the bone during the cutting procedure and thus there is less friction caused by movement of the blade.
