Background of Invention Many types of screwdrivers and screwdriver blade constructions exist for driving a screw into a workpiece. Perhaps the most common is a single blade screwdriver. The blade of these screwdrivers has two side walls that both taper slightly. The groove on the screws that receive these blades have straight and parallel walls.

When one rotates the screwdriver blade held within a screw's groove or recess, half of one side of the blade pushes one side of the groove and the other half of the other side of the blade pushes the other side of the groove. The opposing forces exerted by the workpiece are transformed to the screw against the tip of the rotating blade. These forces may cause the blade tip to cam out of the screw groove.

Such cam-out may damage the screwdriver blade and the screw. In some instances, the blade may damage the workpiece if the blade cams out and travels into the workpiece. The damage can be extensive especially if, as is common, the user is forcing the screwdriver into screw. If the workpiece is a piece of wood or metal, the damage often can be repaired. The screw driver of the present invention drives surgical screws so the workpiece is tissue. Damage to surrounding tissue usually is not acceptable. Therefore, it would be highly desirable to have a new and improved screwdriver blade construction that greatly reduces or eliminates cam out.

To alleviate this problem, many screwdrivers and screws do-not use a single blade. A Phillips head is a common replacement for the single slot head. There are also other heads with cruciform grooves. In the surgical setting, the screwdriver blade matches the grooves on the screws. Therefore, the blade fits the screw groove.

Many screwdrivers have blade tips in which both side walls taper such that the blade is narrower at its distal end than at its proximal end. Receiving recesses on screws tend to have parallel, vertical walls.

If the blade tip is sized properly relative to the width of the screw recess, the tip should have an interference fit with the screw recess. However, the tapered or angled blade face meets the vertical screwdriver tip wall at an angle. When one drive the screw, the angled face of the tip meets the vertical face of the recess. As a result, this angled attach can contribute to cam out.

Summary of the Invention The principal object of the present invention is to provide a new and improved screwdriver blade construction that greatly reduces the risk of blade cam out. The improved screwdriver blade construction of the present invention meets this object. Each of the four drive members has a planar front face that is parallel to the vertical face of a screw's recess that is driven when advancing the screw. Each drive member also has an angled rear face that is parallel to the vertical face of a screw's recess that is not driven when advancing the screw. Thus, the drive members taper so that they have interference fits with the screw recesses. However, the tip's driving face is parallel to the driven face of the screw recess. This decreases the potential for cam-out.

This and other objects and features of this invention and its manner of attaining them will become apparent, and the invention itself

will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings.

Brief Description of Drawings FIG. 1 is a perspective view of a motor driven screwdriver having a blade which is constructed in accordance with the present invention.

FIG. 2 is a side plan view of the screwdriver of FIG. 1.

FIG. 3 is an end plan view of the blade of FIG. 1.

FIG. 4 is a perspective view of a prior art recessed screw adapted to receive the blade of FIG. 1.

FIG. 5 is a side plan view of the prior art recessed screw of FIG.

4.

FIG. 6 is a top plan view of the prior art recessed screw of FIG.

1.

FIG. 7 is a bottom end plan view of the prior art screw of FIG. 4.

Detailed Description of the Preferred Embodiments The blade construction of the present invention may be used on a manual or motor-driven screwdriver. FIG. 1 shows the blade end of a motor driven surgical screw driver 10. The screwdriver 10 comprises a motorized driver 14 to which a screwdriver blade 16 attaches. The screwdriver blade 16 includes a shank 18 whose proximal end is received within the driver 14. The shank's distal end includes a blade tip 20 that engages a bone screw 12 (FIGS. 4-7) and rotates it. As FIGS. 1 and 3 show, the tip 20 is cruciform in transverse cross section with four elongate and equally spaced-apart drive members 22-25.

The prior art bone screw 12 (FIGS. 4-7) comprises a head 140 having a centrally disposed recess 142 that engages the tip 20 of the screwdriver 10. The recess 142 has a cruciform contour in transverse cross section. A centrally-disposed, cylindrically shaped bottom slot 144 and four equally spaced-apart engageable grooves 146-149 extending radially outwardly from bottom slot 144 define the contour. The grooves 146-149 are symmetrical about the bottom slot 144 and spaced apart 90° around the longitudinal axis (L) of the screw 12. Each groove 146- 149 is substantially identical in configuration to the other grooves.

Therefore, only groove 146 is described in greater detail.

The groove 146 includes an arcuate base or floor member 160 (FIGS. 4-6) that slopes slightly upwardly from a bottom wall lip 164 of the bottom slot 144. The floor member 160 terminates at an outer periphery lip 162 that is slightly inset from the outer peripheral of the head 140. A pair of upstanding wall members 150 and 152 extend upwardly from the floor member 160. Each wall member terminates at the top surface of the head 140.

Though the recessed screw 12 is prior art, the following discussion of the screw in greater detail may help those of ordinary skill appreciate the present invention. The screw 12 (FIGS. 4-7) includes a body 130 having a set of spaced apart (120°) biting or cutting surfaces 172,182, and 192 (FIG. 7). The cutting surfaces allow the screw 12 to cut into a hard surface, such as a cranium bone. The body 130 also includes a set of spaced apart threads 174 and 176 (FIGS. 4 and 5) that direct the screw 12 along its longitudinal axis as it spirals downwardly into the receiving surface.

The screw 12 may be manufactured in different lengths that range between about. 157 mm to about. 709 mm. The minimum slot depth of the screw 12 is about. 028 mm. The maximum drill point depth

of the screw 12 is about. 040 mm. The head 140 of the screw has a slight rounded shape and an overall diameter of about 0.1086 mm. The threads 174 and 176 are spaced apart by about 0.0334 mm and each thread is about 0.0025 mm wide at its edge. The screw's"B"dimension varies with its overall length between about 0.75 mm for a screw having a 0.709 millimeter length to about 0.60 mm for a screw having a 0.157 millimeter length. The distal end 170 of the screw 12 is smoothly rounded having a cylindrical shape tapered outwardly into the biting surface 172,182, and 192.

The overall individual width of each of the grooves 146-149 is about 0.224 mm. The diameter of the bottom slot 144 is about 0.0340 mm.

When the user inserts the tip 20 in the recess 142 of the screw 12, the drive members 22-25 engage the grooves 146-149 of the recess 142. As one applies a fastening or clockwise torque to the blade 16, each driver member 22-25 pushes into its associated wall. For example, drive member 22 pushes associated walls 150. The drive member stays in contact with wall 152. As torque acts on the blade 16, the screw 12 is driven spirally downwardly along its longitudinal axis into the receiving body.

In the exemplary embodiment, the shank 18 and blade tip 20 (FIGS. 1-3) are one piece. An intermediate taper section 19 tapers inwardly from the shank 18 to the tip 20. Thus, as FIGS. 1 and 2 show, the tip section has a smaller diameter than the shank in the exemplary embodiment. The distal end 30 of the blade has a slightly rounded profile 31 that conforms to the screw's arcuate floor member 160 (FIGS.

4-6).

As best seen in FIG. 2, the screwdriver blade tip 20 includes a cylindrical body member 38 having four distal-end cut-out areas for

defining the drive members 22-25. The drive members 22-25 are arranged symmetrically about the distal end 30 of the tip 20, spaced apart 90° around the longitudinal axis (A) of the screwdriver blade 16.

Each drive member 22-25 is substantially similar in configuration. In the exemplary embodiment, each has a front side planar wall, such as front side walls 42-45 and a back side tapered or curved wall, such as back side walls 32-35. FIG. 2 shows drive member 22 in detail along with parts of drive members 23 and 25. Front wall 22, for example is vertical. By vertical, applicant means that it is parallel to the screwdrivers longitudinal axis A. Both the side walls 150 and 152 of the screw's groove 142 are also vertical, i. e., parallel to the screw's longitudinal axis L. Therefore, tapered or curved wall 32 has a very gentle slope. At section 36, wall 32 is planar but slightly angled to the plane of front wall 42. The tapered wall of the exemplary embodiment has a curve at 37. Section 36 is flat or planar. Wall 32 can begin having a curvature above the maximum distance that the blade penatrates into the screw recess. Further, though the wall 32 has a curved section, one is not necessary. The wall can have an abrupt end.

Ideally, the driving members 22-25 engage the grooves 146- 149 in an interference fit. As the user inserts the screwdriver tip into the grooves, the front and rear walls 42 and 32 contact walls 150 and 152 of the grooves. Because of the angle between walls 42 and 32, pushing the tip further into the grooves creates an interference fit. This interference fit secures the drive member in the slot.

Once the interference fit exists, the user can move the screw with the screwdriver. Gravity should not cause the screw to fall off.

The interference fit also prevents the drive member from camming out of the groove. The vertical front face 42 is parallel to the vertical face 150 of the screw's groove 146. Therefore, the entire surface

of whatever length of face 42 that extends into the groove is in contact with the wall 150 of screw groove 142. Further, because walls 42 and 150 are both parallel and vertical, transmitted torque from the driving member on the screw create no vector in the vertical (parallel to the screw and blade's axes L and A), no force tends to urge the blade out of the screw groove.

When one applies counterclockwise torque on the screw 12 to remove it, the tapered wall could have a slight tendency to cam out. That should not be a problem, however. First, less force is usually required to remove a screw. Therefore, whatever vector that exists in the vertical direction has a small magnatude. This minimizes the cam-out tendency.

Second, when removing a screw, the user does not apply as much transverse force on the screw. Therefore, even if the tip cams out, it is less likely that the user will jam the tip into the patient.