Sterilizable Non-Lubricated Rotary Instrument for Dental and Medical Use

The present invention relates to dental/medical mechanically power driven instruments, and more particularly, the present invention relates to a non-lubricated dental handpiece having an anti-friction bearing assembly wherein at least the rolling elements are formed of ceramic material.

In recent years, there is an increasing interest in the heat sterilization of powered instruments for use in medical and dental applications to limit the transmission of communicable diseases. Such powered instruments, over the course of their useful lives, may be subjected to many cycles of sterilization. Commercially desirable powered instruments, or handpieces, must be capable of withstanding repeated sterilization cycles without adversely affecting their operability.

Certain powered instruments, such as a dental handpiece, may have a high speed turbine rotatably supported in the proximal end of the handpiece by a pair of anti-friction bearing assemblies to accommodate radial and thrust loads applied in some dentistry procedures. These bearings are quite small, having rolling elements, or balls, about one millimeter in diameter. The raceways and balls are usually fabricated of stainless steel.

In many high speed dental handpieces, air-driven turbines rotate tools, such as drills and burrs, at velocities exceeding 400,000 rpm, and some as high as 500,000 rpm. Even without the application of sideloads, these rotational velocities cause substantial amounts of friction to develop in the bearing assembly. In virtually all commercially available dental handpieces, friction is reduced by lubricating the bearing assembly either continuously, as by flowing a lubricant mist about the assembly, or by periodic lubrication, such as during cleaning and sterilization of the handpiece.

While lubrication of the bearing assemblies ameliorates somewhat the friction problems encountered, the need for lubrication creates other problems. For instance, it is difficult to insure the delivery of the precise amount of lubricant needed. Often, this is a result of different approaches to lubrication applied by users in the field. Lubricants also create problems in sterilization.

When high speed dental handpieces were first introduced in the 1960's, the life of the typical lubricated anti-friction bearing assembly was less than about 10 hours. Over the years, some progress has been made in the manufacture of small anti-friction bearings so that, today, a commercially satisfactory dental handpiece must be capable of surviving 2,000 hours of life cycle testing. A life cycle test involves five seconds of no rotation, acceleration to design speed and running (with a five second sideload applied at nine ounces) for a combined start-run-sideload time of 10 seconds, resulting in a total cycle time of 15 seconds. The same life cycle test procedure also applies to so-called low speed handpieces, i.e. those operating in a range of 5,000-60,000 rpm.

Thorough sterilization of handpieces generally involves the application of heat, such as in an autoclave or a chemiclave. In a typical autoclave, the handpiece, including its bearing assemblies, is subjected to a steam heat environment at a temperature of about 275.degree. F. In a chemiclave, a vapor phase alcohol is used where some water and formaldehyde is also present. Typical sterilization time is about 15 to 20 minutes in both autoclave and chemiclave sterilization procedures. In a so-called dryclave procedure, temperatures may be as high as 375.degree. F.

The high temperatures applied to handpieces during the aforementioned sterilization procedures exacerbate the corrosive effects of the moisture and chemicals on the bearing assemblies, and since many of the bearing assemblies incorporate retainers usually made of phenolic with organic fibers, for spacing the balls apart, the integrity of the retainers is also adversely affected by the heat and chemical reactions.

To place in perspective the life shortening effects of heat sterilization, it is estimated that a typical high speed dental handpiece which is not subjected to sterilization would last a dentist about 20 years with a minimum of repair (10-20% replacement of parts); whereas, the same handpiece, subject to heat sterilization as described heretofore, would last 1-2 years, requiring major repairs (80-90% replacement of parts).

The following patents relate to dental handpieces. U.S. Pat. No. 3,071,861 to Saffir discloses a dental handpiece turbine assembly utilizing jewel bearings. See FIGS. 1 and 3. U.S. Pat. No. 4,568,642 to DeForrest discloses a jewel bearing for accepting thrust loads. See FIG. 4. U.S. Pat. No. 4,153,993 to Kataoka discloses a dental handpiece turbine bearing assembly utilizing air bearings and opposing magnets. U.S. Pat. No. 3,421,224 to Brehm discloses a dental handpiece turbine assembly fabricated of plastic materials. U.S. Pat. No. 3,248,792 to Staunt discloses a dental handpiece having a bearing ball retainer (FIGS. 19 and 20) fabricated of plastic material. U.S. Pat. No. 3,469,318, also issued to Saffir, is of general interest with respect to dental handpiece bearing assemblies, as is U.S. Pat. No. 4,249,896 issued to Kerfoot.

Anti-friction bearing assemblies fabricated of materials other than steel are disclosed in the following patents. U.S. Pat. No. 1,332,176 to Heindlhofer discloses an anti-friction bearing assembly having rolling elements fabricated of materials such as agate, quartz, topaz, garnets, emeralds, sapphires, and diamonds. U.S. Pat. No. 2,158,156 to Schroder discloses a bearing assembly fabricated completely of ceramic materials. U.S. Pat. No. 2,534,929 to Schultz discloses a ball bearing assembly which utilizes beryllium copper races in combination with glass balls and which can operate at low speeds under light load without lubrication. U.S. Pat. No. 3,097,897 to Taylor discloses an anti-friction bearing assembly composed of refractory materials, such as dense alumina, and raceways of dense titanium carbide cermet either with or without a spacer element to keep the balls separated in a raceway. U.S. Pat. No. 4,770,549 to Rokkaku and U.S. Pat. No. 4,792,244 to Yamashita disclose ceramic bearing assemblies of relatively recent vintage.

With the foregoing in mind, a primary object of the present invention is to provide a durable, non-lubricated, sterilizable handpiece.

Another object of the present invention is to provide a dental handpiece capable of withstanding the hostile environments encountered in heat sterilization procedures and of delivering a satisfactory service life with minimal maintenance.

As a further object, the present invention provides an improved dental handpiece which overcomes the limitations of known commercially available handpieces by being capable of operating without lubrication and of being sterilized repeatedly in high temperature hostile environments.

More specifically, the present invention overcomes the problems associated with known handpieces and achieves the foregoing objects by utilizing a bearing construction which does not require lubrication and which can withstand repeated exposure to high temperature sterilization procedures. In brief, this is accomplished by providing, in a powered hand held instrument having a proximal end portion adapted to mount a rotary tool for operating upon a portion of anatomy, a bearing assembly which includes an annular inner raceway, an annular outer raceway, and a plurality of rolling elements contained between the raceways and formed of a hard, dense, wear-resistant, inert material cooperable with the raceways to accommodate without lubrication high rotational speeds of the tool and repeated sterilization procedures without intervening lubrication.

The foregoing and other objects, features and advantages of the present invention should become apparent from the following description, when taken in conjunction with the accompanying drawings which:

FIG. 1 is a fragmentary perspective view of a dental handpiece embodying the present invention;

FIG. 2 is an enlarged cross-sectional view taken on line 2--2 of FIG. 1; and

FIG. 3 is a further enlarged cross-sectional view taken on line 3--3 of FIG. 2.

Referring now to the drawings, FIG. 1 illustrates a dental handpiece 10 which embodies the present invention. Although the present invention has particular utility and will be described hereinafter with respect to a dental handpiece, it should be apparent that the present invention also has applicability to other handpieces, such as bone reamers, and other powered instruments used in a variety of medical applications wherein a rotary tool is used in close proximity with a portion of the anatomy, such as plaster cast rotary saws.

The illustrated handpiece 10 includes a handle portion 11 and a head 12 which rotatably mounts a tool, such as the burr 13 illustrated in FIG. 1. As best seen in FIG. 2, the head 12 is hollow and contains a turbine T having a plurality of peripheral buckets 14 mounted on a shaft 15. The burr 13 is releasably captured in a collet 16 connected to the shaft 15. The turbine T is rotated by air supplied by a conduit 17 extending lengthwise in the handle 11 and connected to a source of air under pressure, such as at a pressure of 30-35 psi.

The shaft 15 is rotatably mounted in the head 12 by means of upper and lower anti-friction bearing assemblies 18 and 19, respectively. The upper and lower anti-friction bearing assemblies 18 and 19 are of identical construction. Accordingly, reference hereinafter will be made to the lower anti-friction bearing assembly 19, it being understood that the same description applies to the upper bearing assembly 18.

As described thus far, the dental handpiece 10 is of conventional design and construction. A more detailed description of its structure, assembly and operation may be found in U.S. Pat. No. 4,249,896 issued to Kerfoot, the disclosure of pertinent portions of which is incorporated by reference herein.

As disclosed in the subject Kerfoot patent, the upper and lower anti-friction bearing assemblies 18 and 19 may be fabricated of stainless steel, such as the high-carbon chromium steel designated as SAE52100 stainless steel. Each bearing assembly, such as the lower bearing assembly 19, has an annular inner raceway 19a mounted to the shaft 15, an annular outer raceway 19b surrounding the inner raceway 19a, and a plurality of rolling elements 19c engaged between the inner and outer raceways 19a and 19b, respectively. The rolling elements 19c mount the inner raceway, and hence the shaft 15, collet 16, and burr 13 for rotation relative to the outer raceway 19b and its mounting head 12.

Preferably, as described in the Kerfoot patent, the rolling elements 19c are preloaded a predetermined amount in accordance with good design practice. In the embodiment of FIG. 2, the inner raceway of the upper anti-friction bearing assembly 18 is preloaded in the upward direction, and the inner raceway 19a of the lower bearing assembly 19 is preloaded in the axially downward direction. This preloading also preloads the rolling elements 19c in the lateral direction, i.e. transverse to the axis of rotation of the burr 13.

In the embodiment illustrated in FIG. 2, the rolling elements 19c are balls of a spherical shape. It should be apparent, however, that the present invention contemplates the use of rolling elements of other shapes and configurations, such as cylindrical, frusto-conical, and the like. As will be discussed, however, spherical elements formed of certain ceramic materials have been tested and found to provide a desirable service life.

In the aforedescribed Kerfoot patent, the rolling elements 19c are maintained in spaced relation between the inner and outer raceways by a retainer or spacer element, not shown. One of the purposes of the retainer is to prevent the rolling elements from contacting one another and, in so doing, generating friction and heat, thereby adversely affecting the performance and life of the bearing assembly. While the present invention eliminates the need for retainers, if used, the retainer or spacer may be fabricated of either a metallic or a non-metallic material, such as a phenolic resin, or of any of the modern ultra-high molecular weight engineering grade plastic materials commonly in use such as polyamides, polyimides and polycarbonates sold under the registered trademark TORLON.

As discussed heretofore, in the conventional dental handpiece, the bearing assemblies are lubricated either by a continuous supply of a lubricant which is contained within the air supplied to the turbine and circulated about the bearing assemblies, or by lubricating the bearing assemblies periodically, such as when the handpiece is taken out of service for general maintenance or for sterilization. Often, as recommended by some manufacturers, lubrication occurs after sterilization, but this is undesirable because the lubricant may contain contaminants that nullify the sterility of sterilization procedure. Furthermore, as discussed heretofore, repeated sterilization of the handpiece reduces the useful life of the bearing assemblies by accelerating their degradation.

The present invention overcomes the aforementioned limitations of the conventional handpiece, and provides a handpiece which is durable, sterilizable and operates without lubrication. These advantages and others, are realized by fabricating at least the rolling elements of a non-metallic material, such as a ceramic material. With rolling elements of such material, the retainer may even be eliminated entirely.

The materials of which the rolling elements are fabricated should be hard, dense, wear-resistant, and inert, in addition to being non-metallic. A preferred material has a density of less than about 5.0 g/cm.sup.3, a hardness of at least about 75 Rockwell C. at 20.degree. C. and a coefficient of dry sliding friction of less than about 0.30 with respect to stainless steel of the type described above. The ceramic material is preferably sintered under heat and pressure into spheres of relatively small diameter, such as about 0.5 to 5.0 millimeters. When assembled into a bearing assembly, such as illustrated, no lubricants are supplied, the bearing assembly being essentially free of lubricants.

Other physical characteristics of a desirable rolling element material include the following, which are typical of silicon nitride used in the tested prototypes to be described:

There are essentially two types of ceramic materials that find utility in the present invention. These materials may be grouped into non-oxide ceramics and oxide ceramics. Of the non-oxide ceramics, preferred materials include nitrides, carbides and borides. The most preferable include: silicon nitride, and silicon carbide. Oxide type ceramics include the following: aluminum oxide, zirconium oxide, zirconia (with or without surface treatment), and zirconia-toughened alumina. More specifically, of these ceramics, preferred materials include the following: boron nitride, Sintered Si.sub.3 N.sub.4 (silicon nitride), Sintered AlSiC, boron carbide, silicon oxynitride, boron oxynitride, titanium boride, boron nitride-titanium boride.

In the illustrated embodiment, only the rolling elements are fabricated of ceramic material. The inner and outer raceways are both fabricated of a bearing grade steel, such as: 440C stainless steel, M-50 bearing steel or SAE52100 steel. While these bearing steels are preferable as metals, other corrosion-resistant metals may be used for the raceways. If desired, however, one or both of the raceways may be formed of a ceramic material, like the ceramic material of the rolling elements, or they may be formed of a ceramic material which is compatible with the ceramic material of the rolling element. Economic considerations and use of conventional equipment augur in favor of the use, at present, of stainless steel for the inner and outer raceways.

In the absence of retainers, the rolling elements may group themselves in a manner illustrated in FIG. 3 as they rotate between the raceways. Thus, they may, and do, contact one another. However, the relatively low coefficient of friction between the rolling elements, (on the order of 0.17) renders unnecessary a retainer to maintain them in spaced apart relation as with conventional bearing assemblies. If desired, however, a retainer of either metallic or non-metallic plastic material such as discussed above may be used in those applications where retainers are indicated.

Silicon nitride has been formed into spheres one millimeter in diameter and assembled into bearing assemblies, without retainers, such as illustrated in FIG. 3. The bearing assemblies were installed in dental handpieces, such as illustrated in FIG. 1. The handpieces were subject to the operating cyclic life test procedure described heretofore. No lubrication was supplied to the bearing assemblies at any time.

The tests revealed that handpieces fabricated according to the present invention could operate satisfactorily over 2000 hours without failure and without lubrication. Satisfactory operation is defined as existing when the rotational speed of the tool remains within 10% of its design speed under no load conditions. Companion tests on handpieces of the same materials, but with heat sterilization occurring at 8 cycles/day intervals, established satisfactory performance for over 1000 hours, with tests continuing satisfactorily as of the date of execution of this application.

In view of the foregoing, it should be apparent that the present invention now provides an improved handpiece particularly suited for use in dental applications but also suitable for use in a variety of medical applications requiring sterilizable rotary tools. The handpiece of the present invention is resistant to the deleterious effects of corrosive heat sterilizing procedures. The handpiece eliminates entirely the need for either continuous or periodic lubrication since it operates free of lubricants. Furthermore, despite repeated sterilization and the absence of lubricants, the handpiece meets the requirements of commercial dependability and durability.

While a preferred embodiment of the present invention has been described in detail, various modifications, alterations and changes may be made without departing from the spirit and scope of the present invention and as defined in the appended claims.