BACKGROUND OF THE INVENTION Field of the Invention This invention relates to surgical instruments generally and more specifically to electrically powered, handheld rotary screwdrivers for surgical use.

Description of the Related Art Power assisted tools, including screwdrivers, are commonly employed by orthopedic surgeons in the surgical environment. The design of such tools has commonly been dominated by convention or historical context of power tools, with the result that many of the surgeons tools resemble those of the carpenter or woodworker. In many procedures such designs are perfectly adequate; however, more precise and delicate procedures are facilitated by more specialized tool designs. Electrically powered screwdrivers powered by batteries are available for various purposes, but tend to follow two basic design groups: a first, familiar design is the basic cylindrical powered driver, with the batteries inserted into the cylindrical handle in the manner of a traditional flashlight, axially end-to-end; the second common design is the Pistol grip design, with a main body and a transverse, pistol-type grip {sometimes activated by a trigger switch, completing the "pistol" metaphor) . Designs in the first category are not easy to manipulate with delicacy, being gripped in the clenched fist like a dagger. Those in the second group are also unwieldy and not easy to use j_n restricted spaces. Neither design is optimized for fine surgical applications. Prior screwdriver designs have generally been intended to apply as much torque as possible, without consideration of the torsional ffailure point of the screws upon which the driver will operate. This has not previously been seen as a serious problem in need of remedy. The need persists for an improved powered screwdriver for surgical applications .

SUMMARY OF THE INVENTION In view of the above problems, the present invention in one aspect is a battery powered, electrically driven screwdriver having an elongated body with a narrow forward portion disposed toward ttie working end, and a relatively larger rearward portion disposed distal from the working end. A tapering transition connects the narrow forward portion with the la_rger rearward portion. At least one fingertip-operable swzLtch is mounted on the forward portion. The at least one fingertip switch is disposed at a position consistent with a pad of a human index finger when the screwdriver is Iheld in a "pencil" style of grip. In some embodiments, the screwdriver is substantially sealed against intrusion of fluids or debris (during surgical use) . In some embodiments the screwdriver is adapted to be disposable after surgical use, to discourage infection. In another aspect, the invention is an electrically driven, hand held screwdriver that includes a drive system adapted to apply limited torque to surgical screws, wherein said torque is limited at or below a predetermined, known maximum torsion capability of specific surgical screws.

These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a powered surgical screwdriver in accordance with the invention; FIG. 2 is a side view of the powered surgical screwdriver of FIG. 1; FIG. 3 is a plan view from above (top view) of the screwdriver of FIGs. 1 and 2; FIG. 4 is a frontal end view of the screwdriver of FIGs. 1-3; FIG. 5 is a end view from the rear of the screwdriver of FIGs. 1-4; FIG. 6 is a plan view from below of the screwdriver of FIGs 1-5; and FIG. 7 is a partially exploded view of the screwdriver of FIGs. 1-6, with internal details exposed to view.

DETAILED DESCRIPTION OF THE INVENTION FIGs. 1 through 6 show external views of a typical embodiment of an electrically powered, low torque, surgical screwdriver in accordance with the indention. FIG. 7 shows an exploded view exposing internal details. As seen in FIG. 1, the screwdriver has a cozntoured, elongated body 10 with a forward ("drive") end 1_2 and a rearward end 14 that differ from one ano~ther in thickness and shape. As the end view FIG. 5 shows, the thicker, rearward end 14 has increased bulk and. volume which is sufficient to accommodate batteries and an electric motor; the narrow, forward end 12 has decreased cross section to allow fine manipulation within confined spaces, as well as to facilitate manual grip between the thumb and index finger (or a combination of index and second fingers) in a "pencil" style of grip. Either a fixed drive blade or a collet 16 is provided at the forward end 12 to engage and drive surgical screws. Preferably, a collet 16 is provided that can accept a variety of different, interchangeable drive bl ades of different sizes and configurations. Preferably, the body is not generally cylindrical. As seen in the end view, the body preferably presents not a round but a more oval end view or cross section,, with a longer dimension between top 18 and bottom 20 than from side to side. This shape is preferred to a cylinder, for reasons discussed later. The screwdriver body 10 is prreferably tapered as shown in the figures, from a wider rearward end 14 to the relatively more narrow forward neck: 22, terminating in forward end 12. The transition from wider to more narrow is preferably accomplished by tapering curve 24 disposed primarily along the lower forward portion of the body. By contrast, the top side 18 preferrably follows a more generally linear path. The sweeping curve of taper 24 complements and comfortably engages "the muscle between a human thumb and index finger (adductor pollicis muscle) . A beveled shoulder 26 is preferably disposed along or near the taper 24 to provide a tactile index of position and to enhance positive grip. Forward and reverse polarity control switches (30 and 32, respectively) are disposed on the top of the forward neck 22, specifically positioned to be comfortably accessible to the tip or end pad of a user' s index finger when the screwdriver is held in a natural, pencil style grip. These control switches allow the user to select "forward" or "reverse" turn directions, and are preferably sealed, push-button switches disposed in close proximity to one another. Specifically, the distance between the forward and reverse controls is chosen such that the index finger of an average human hand may move between said controls easily by flexing the index finger. Significantly, the index finger can move between the controls without changing the hand's grip on the screwdriver. In a preferred embodiment, switches 30 and 32 are sealed by a membrane to prevent blood or other contaminants from entering into the internal mechanism of the powered screwdriver. The drawings also show the internal workings of one embodiment of the driver, including battery clips 39, batteries 40 and a D.C. electric motor 42. Batteries 40, preferably transversely mounted as shown, provide drive voltage to the D.C. motor. A polarity control circuit controlled by forward/reverse control switches 30 and 32 allows on/off and polarity reversal controls of the motor. (As discussed above, control switcties are mounted on the forward neck of the driver). D.C. motor 42 preferably drives an offset drive transmission system 46, which transmits torque to a drive shaft 48. (In the particular embodiment shown in FIG. 7 thie transmission and motor are housed together in a unitary mechanism, as shown and as discussed below) . The shaft 48 is preferably coupled to the quick release collet 16 at the front of the driver, to facilitate quick interchange of drive tools. The driver is thus not limited (in general) to a particular screw system such as Phillips, Allen head, star socket or flat blade, but can accept a variety of interchangeable blades. Particular embodiments may be limited to specific blades, for reasons discussed below in connection with limited torque embodiments. The overall structure of the screwdriver body is preferably not symmetrical about the rotstable axis of the actual screw drive shaft 48, but rath.er the axis of drive shaft 48 is offset from the center axis of the wider, rearward portion of the screwdriver body. Internal and external features of the screwdriver are designed to encourage this offset geometry while efficiently utilizing the available volume. Battery clips 39 are preferably included, capabZLe of receiving cylindrical batteries in an orientation transverse to the axis of the driver body. For example, in the embodiment shown the batteries are mounted -with their axes transverse to the screw drive axis; not, as in most powered drivers, generally in line with said axis. This arrangement reduces overall length and uses internal volume efficently, while keeping the balance point close to the support point. The drive motor 42 is preferably coupled to the screw drive shaft 48 via an offset gear train 46 or similar transmission mechanism. Because the rotatable drive shaft 48 is offset with respect to the center of the body, the hand- is able to exert more torque (or resist more torque) by exerting a given force. The torque exerted is increased by increasing the distance from the shaft to the point of application of force. The offset drive tends to increase such distance, the force being exerted on the handle body on the edge further removed from the shaft. The screwdriver of the invention is adapted to be gripped in the manner most commonly employed for gripping pencils or other writing implements ("pencil grip") . As such, the dimensions, proportion and. design of the screwdriver are all relevant in the! r relation to the usual dimensions, proportion and design of the human hand. Specifically, it is well adapted to be held in a position substantially in the traditional "pencil grip". With the index finger positioned at the switches 30 and 32, the balance point of the screwdriver is preferably disposed substantially over the fleshy,- webbed structure between the thumb and the first (index ) finger, resting substantially on the adductor pollici.s muscle, with rotating drive point forward in the position corresponding to that of the pencil or- pen point. The forward, narrow portion of the screwdri_ver rests against and is controlled by the thumb, index; and middle fingers; the index finger can reach the forward control and reverse control button which are foarwardly positioned on top of the narrow, forward neck of the screwdriver, while the middle finger supports the neck from the side. The forward and reverse control switches 30 and 32 are disposed within the reach of a comfortably flexed, average index finger. Furthermore, th& distance between the forward and reverse controls is chosen such that the index finger may move between said controls easily, primarily by flexing the index finger. In addition to the positions of trie controls, other preferred specific features facilitate the use of the pencil grip. The surfaces of the screwdriver are generally rounded and curved, which has been found to enhance comfortable grip. Optionally, a gnurled or textured surface can be employed over s ome or all of the surface to further facilitate grip. As shown in end view (FIG. 5) , the cross section of at least the rearward body portion of the screwdriver is generally oval (but not necessarily perfectly elliptical or symmetrical) , having a wider dimension in one direction than the other. For example, it can suitably be ellipticai, having unequal major and minor axes . It is not required to be perfectly symmetrical. The generally oval form is advantageous in that it allows the hand -fco apply or resist torque more effectively, utilizing the longer dimension for more torque, without necessarily adding excess volume (as would a cylindrical object of the same larger diameter) . The irregularity of the oval form is also more easily gripped than a cylindrical., or regular form. The Center of Gravity of the device, including batteries, is preferably just over or slightly rearward of support point. The support point is defined by the typical position of the adductor pollicis muscle, and given that the index finger pad is positioned to touch both of the forward and reverse controls (3O and 32) on the forward neck portion of the driver. In one embodiment, the distance to the support point is found to be in the range of 4 to 8 centimeters rearward from the rearmost control switch (30 or 32) . The overall shape of the screwdriver of the invention is preferably not encumbered by the addition of any prominent transverse projection such as a "pistol" grip. Pistol grips are known for promoting hiigher torque powered tools, but the prominent projection of such a grip would be more likely to cause interference in a delicate surgical position. Furthermore, the pistol grip does not promote delicate or fine control of the driver. Some variation from proportions described is permissible subject to the constraints set forth herein regarding the pencil grip, control of the forrward/reverse switches, balance point, and dimensions of a human index finger.

Other features of the invention make it well adapted for surgical use. Due to the non-cylindrical, oval shape of the body, the screwdriver of the invention is not likely to roll freely when placed on an inclined plane. This reduces the chances of dropping or losing the instrument during a surgical procedure. Surgical tools are required to be sterile dur±ng each use; this implies that they must either be a) capable of enduring autoclave temperatures, or b) disposable, to prevent re¬ use. The screwdriver of the invention is intended to be disposable (except for recycling of some components) . Accordingly, the exterior casing of the body is assembled by snapping And preferably without use of any screws. One practical method of construction is shown in the partially exploded FIG. 7, which also reveals one internal arrangement of components. Removable screws could encourage a user to replace batteries and thereafter re-use the driver in a manner which is inconsistent with proper surgical sterility procedures . The casing is preferably sealed after assembly, for example by an adhesive, to prevent entry of blood, fluids, or other adulterants into the i_nterior mechanism.

In one embodiment, the torque capabilities of the screwdriver's D.C. motor are preferably intentionally limited to a pre-determined maximum torque capability so that the screwdriver will, with adeqiaate battery power, provide up to a predetermined toarque limit before stalling, but will halt if a greater torque is applied. This characteristic is desirable because i~t makes it less likely that a surgical screw can be snapped. For typical applications, for example, surgical screws are commonly used having a shaft diameter of 1.7 millimeter, 4 to 5 mm length, made from titanium alloy (suitably Ti δAluminum /4Vanadium) and having a self tapping thiread for bone. These and other surgical screws are available from Kinamed, Inc. in Camarillo, California. The inventors have discovered that such screws can fa_il by fracture (snap) if excess torque is applied. Accoxdingly, in one embodiment the motor is chosen or modified so that it is incapable of exerting torque in excess of a threshold torque. For example, in a specific embodLment the driver stalls in the neighborhood of 78 inch- ounces. More generally, it is possible to specifically design the driver for a desired threshold torque, for example by using current or voltage limiting D.C. dri-ve circuits. A variety of current or voltage limiting cirrcuits are known in the art of power supply or motor contirols which could be employed for this purpose. Accordingly, in one embodiment the screwdriver of the invention is specifically limited in that it is incapa.ble of exerting torque in excess of a given threshold torque, the threshold torque being a design paramete-e: of the driver and predetermined to be less than the toxque which will likely snap a specific surgical screw. Zt is noteworthy in this respect that the driver is designed for unusual, surgical screws having unusual strength characteristics. Furthermore, it is very important in surgical applications that the screws not be damaged during placement. Thus, this torque ^Limitation is more than usually important for the surgical power driver, as compared to other drivers. In the controlled torque embodiment of the invention, the torque characteristics of the motor is a design parameter dictated by intended application. It should be understood that th_e "threshold torque" as used herein is determined by the specific intended application, and does not merely refer to an arbitrary, inherent motor characteristic, except to the extent that a particular motor characteristic: is intentionally chosen in accordance with the desired application. Accordingly, one embodiment of the invention employs a Model HG650B-052 DC gearmotor from Hennkwell Ind. Co., Ltd. Electrical, physical and performance characteristics are shown in the attached drawings. The characteristics of this motor have been found particularly suitable for the surgical screw driver of the invention. The motors are drriven by battery power in the invention (thus the torque curves labeled "battery" apply) . The invention also includes methods as performed by the apparatus described, including methods for limiting the torque applied to surgical screws during insertion, and other such procedures facilitated or performed with the powered screwdriver described herein. In another aspect, the invention includes a matched system of surgical screws and disposable surgical screwdriver, wherein the disposable screwdriver is designed to have a predetermined threshold of t;°rque before stalling, and the surgical screws matched to the disposable driver are selected to be capable of withstanding at least the predetermined threshold of torque without failure. Both screws and screwdriver of the system are preferably pre-packaged, ready for use, sterilized in a sealed package to be opened only in the sterile surgical environment. Preferably the screws and driver should be packaged in a combination packaging system, but alternatively could be clearly identified by a code or marking system. Details of the forward and reverse polarity switches, D.C. motor, Battery and drive circuits are not shown, as these systems can easily be designed for a particular application by one with skill in the electrical arts, given the important limitations described above. One with skill in the electrical arts will easily understand how forward and reverse directions of a D.C. motor can be controlled by reversing polarity of the current flow, and how to accommodate on/off control of such a simple motor cirrcuit. Batteries having known and stable characteristics should be included in a pre-sealed embodiment of the screwdriver, so that the battery characteristics can be included in the design process as a known and predictable parameter. A sealed plastic housing without removable screws will discourage removal or replacement of the batteries by the user. A further noteworthy aspect of the screwdriver of the invention is that its body is free from any substantial transverse projections or handle, so that said body cannot be gripped in a pistol style of grip. This encourages the intended grip, which in turn facilitates better fine control over the tip during surgery. While several illustrative embodiments of the invention , have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variati_ons and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims. As used herein and in the claims the words "comprising," "having," and "including" should be understood as open ended and not liiruLting, to convey the meaning: "including but not limited to".