Blade For Bone Reamer

Abstract

A bone-cutting blade with a convex scraper edge, rotatable about an axis which is in the plane of the blade and which intersects the midpoint of the convex scraper edge, the convex scraper edge having spaced notches therein, alternating with arcuate blade edge segments, with segments one side of the midpoint of the arcuate blade corresponding in position along the blade edge with notches on the opposite side of the midpoint.

Description

RELATED APPLICATION

This application discloses an improvement over a bone cutting blade disclosed in my copending application filed Aug. 22, 1969, Ser. No. 852,226.

FIELD OF THE INVENTION

This invention relates generally to blades for rotary bone cutters, particularly of the variety designed for forming a hemispherical socket in a bone, e.g., in hip surgery.

BACKGROUND OF THE INVENTION

In hip surgery, the procedure known as cup arthroplasty includes the reshaping of the acetabulum of the hip to provide a nicely rounded cavity therein. This has been done in various ways heretofore, and my aforesaid patent application discloses a rotary cutter and blade for accomplishing this purpose in an improved manner. The blade is a flat, tool steel plate, with an arcuate shearing edge, and is rotated in the bone cavity to generate hemispherical surface of revolution therein. An electric drill is used to drive the holder for the blade, and it is the common practice to use for this purpose self-contained battery power for this drill, with the drill housing sealed against any possibility of sparking such as might cause ignition of any explosive gases that might be present in the operating room. Such drills are generally of relatively low power and limited in torque; and in some cases the bone in the region of the socket to be constructed has become so dense and hardened that the torque available from a drill of the type mentioned has been somewhat limited for the bone condition encountered. For this reason particularly, but also because a reduced torque requirement is always desirable, the purpose of the present invention is to materially reduce the torque requirement of the blade mentioned above.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention the arcuate cutting of the earlier blade, mentioned hereinabove, is formed on opposite sides of its midpoint with spaced notches, alternating with arcuate blade edge segments, with the notches on one side of the midpoint corresponding generally in position with the arcuate segments on the opposite side of the midpoint. Together, the segments cover the full area of the hemispherical cavity, but each arcuate half of the blade engages and cuts along a series of segments whose summed length is materially less, e.g., half, or somewhat more, of the full length of the arcuate half of the blade. The overall torque requirement to cut through the bone can thereby be reduced as much as half; and with a given torque availability, the cutting force availability can thus be doubled. With this improvement, relatively lightly powered battery operated drills such as are now commercially available, and such as are admirably suited to the work because of compactness and low weight are capable of generating the torque necessary to cut dense and hard bone easily and satisfactorily.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a reamer in accordance with the invention, with parts broken away;

FIG. 2 is a side elevational view of the cutter, in a position at 90.degree. from FIG. 1, parts being broken away;

FIG. 3 is a front elevational view of the cutter as seen from the left toward FIG. 2;

FIG. 4 is a side elevational view, to double scale, of the blade of the cutter of FIGS. 1-3;

FIG. 5 is a view of the blade, as seen looking toward the right in FIG. 4;

FIG. 6 is a fragmentary perspective view of the central portion of the cutter blade; and

FIG. 7 is a largely diagrammatic view showing the cutter blade in a simple basic form.

DESCRIPTION OF A PREFERRED EMBODIMENT

In the drawings, a rotary bone cutter has a head 10 of substantially hemispherical form with a convex front surface of revolution 11, and a flat rearward surface 12. A hub 13 with a reduced coaxial coupling pin 14 projects axially from surface 12. Pin 14 is received in a socket 15 in the end of a somewhat tapered shank 16, and is connected to the latter by a roll pin 17. Shank 16 is to be understood as adapted at its opposite end for coupling to a rotary driver, preferably an electric surgical drill.

The center of curvature C of the convex front face of the head 10 is preferably spaced somewhat rearwardly of the rearward head surface 12, so that while the head can be described as generally or substantially hemispherical, its preferred form is just a trifle under a full 180.degree. hemisphere.

The hemispherical head 10, and the hub 13, for approximately three-fourths of the depth of the latter, are split longitudinally on a diametrical plane, forming a diametrical slot 20, adapted to receive the flat, tool steel blade 22. The blade 22 is shaped in general resemblance to the head and hub, with alternating notches and segments, its cutting edge being convex in form, as described later, and with a shank 24 extending from its base edge 25, so as to seat into the bottom of the slot 20. The hub is drilled and tapped on the axis of the center C to receive a shoulder bolt 27, and the blade 22 is drilled as at 28 to receive and be positioned accurately by this bolt 27. Tightening of the bolt 27 clamps the split head and knife in solid assembly. The blade may be quickly removed and replaced by taking out and replacing the bolt 27.

The radius r of the convex blade edge 30, drawn from center C, is slightly greater than that of the convex head surface 11, so that the blade edge projects a slight marginal distance beyond the latter for proper cutting, the cutting depth being controlled by the proximity of the hemispherical surface 11. The blade edge is oppositely beveled on opposite sides of its center or midpoint 31 to give a suitable clearance angle for each half of the blade, as designated at 32 and 33, and so as to form two shear or scraper edges 32' and 33', respectively. Rotation of the head spins the two convex scraper edges against the cartilage and/or bone to take a fine hemispherical shear type cut therein, which may be progressively deepened as desired. The blade produces fine cuttings or scrapings, which are disposed of as presently described. It will be particularly noted that the curved blade edges 32' and 33' move normally to the bone, and depth of cut is controlled to be uniform along the entire blade edges by the uniform projection distance thereof beyond the hemispherical guide surface 11. Maximized cutting rate can be achieved with projection distance small enough to avoid gouging or chatter.

Preferably, the blade slot 20 is bisected by a diametrical plane P of the head, so that the two blade edges 32' and 33' are equidistantly positioned, by distances equal to the half-thickness of the blade, ahead of this diametrical plane. Maximum uniformity of cutting by the two oppositely beveled half-lengths of the blade is thereby achieved, and a highly uniform hemispherical socket obtained.

Alongside each knife edge 32' and 33', the head 10 is formed with a trough, passage, or groove 40. As will be seen, each of these grooves extends from a point just beyond the midpoint of the blade, back past said midpoint and angularly down or back alongside the opposite half of the blade. The groove opens to the blade on one side, and intersects the convex surface 11 on the other, in a curved line 42, opening through a discharge notch 44 in the back surface 12 of the head, adjacent its periphery.

It will be seen especially from FIG. 3, that the groove 40, positioned immediately in advance of the scraping edge of the blade, receives and gathers scrapings and cuttings from the blade, and that these will pack into, flow along, and be continuously discharged from the groove 40 and its exit notch 44 in the back of the head 10 during the operation of the drill.

Irrigation is not required, and since the cuttings and scrapings pass continuously out of the cutter and the bone cavity being formed, there is no need for the surgeon to stop his work to clean out the cutter.

Irrigation is not required, and since the cuttings and scrapings pass continuously out of the cutter and the bone cavity being formed, there is no need for the surgeon to stop his work to clean out the cutter.

The cutter as heretofore described, excepting for brief reference to the blade notches is disclosed in my aforementioned application. For a basic understanding of the present invention, please refer to the diagrammatic FIG. 7 of this application. The arcuate, convex edge 30 of the blade 22 is divided into two halves by a midpoint 31 coinciding with the axis of rotation A-A'. The convex edge 30 is shown to subtend somewhat less than 180.degree. of angle about the center C of the edge 30, and, for simple example, each half of the edge 30, on opposite sides of midpoint 31, has two arcuate segments 50, alternating with two notches 51. The segments will be understood to be oppositely beveled on opposite side of the midpoint 31, as explained in connection with FIGS. 1- 6. The segments in this simple example are of the same lengths as the notches 51, though in the preferred example, FIGS. 1- 6, the segments 50 exceed the notches in length by substantially 2to 1. As seen in simple basic example of FIG. 7, the first segment 50 of the lower half of the blade edge 30 has one end at the midpoint 31. It is followed by a notch 51, then a second segment 50, and a second notch 51. (More segments and notches are desirable in practice.) The upper half of the blade edge 30 beings at midpoint 31, with a notch 51, followed by a segment 50, a second notch 51, and a second segment 50. Thus, a segment on one-half of the convex blade edge corresponds in position along the arc with a notch in the other half. It will be clear that if the blade is rotated on axis A-A', the cutting segments on one half will cut spaced bands in the hemispherical bone cavity, and the cutting segments on the other half will cut the remaining or intervening "bands," so that a complete substantially hemispherical cut will be made. It is convenient to adopt as a definition for these "bands," that used in works on Solid Geometry, i.e., "the portion of the surface of a sphere included between two parallel planes is called a "zone." The two ends of each segment and of each notch of each blade half thus define a "zone." And for the simple example of FIG. 7, these zones Z meet edge to edge along parallel planes such as p in FIG. 7. The zone defined and occupied by each segment on each blade half thus meets and fits precisely between adjacent zones defined notches on the other blade half. If, however, the segments are of greater lengths than the notches, as in FIG. 1-6, the zones for the segments slightly overlap the zones defined by the notches. There is thus a factor of safety assuring that cutting will occur continuously about the complete hemisphere.

Turning now to the preferred embodiment of FIGS. 1-6, it will be immediately observed that a larger number of segments 50 and notches 51 have been used, and also, as suggested hereinabove, that the segments 50 have been made substantially double the length of the notches 51. Also, as will be clear from the drawings, the zones defined by the segments at least slightly overlap the zones defined by the notches in the opposite half of the convex blade. Clean cutting and stable nonchattering cutting, with assurance of cutting continuously, without leaving a narrow strip of bone between successive segments on the two halves of the blade, are thereby achieved.

A further improvement in the region of the midpoint 31 of the convex blade is also employed. Note that the segment 50 (FIG. 5) at the midpoint of the blade is above the midpoint for most of its length, but extends for a short portion 50a of its length below the midpoint 31, and that this last mentioned portion of the segment has an opposite bevel from the remainder of the segment. The segment, thus divided into two oppositely beveled portions, assures clean cutting at the center.

It will be clear that various changes in design in the number, length, location, and arrangement of the segments or teeth 50 and the notches or gaps 51 may be made within the broad scope of the invention. The present blade, which performs successfully, has a center-to-center spacing angle, from notch to notch, of 18.degree. 15', and the notches are substantially 0.10 inch in length along the arc. The blade is double scale in the patent drawings in FIGS. 5 and 6 (prior to reduction in printing).

Claims

I claim:

1. A rotary shearing blade for a rotary bone cutter adapted to rotate it on an axis substantially in the plane thereof, comprising a flat plate having a convex arcuate edge struck from a center point on said axis of rotation, said arcuate edge being oppositely beveled on opposite sides of its midpoint, the two halves of said beveled arcuate edge, on opposite sides of said midpoint, having spaced notches therein, alternating with beveled arcuate edge segments, with segments on one of the halves corresponding substantially in position along the arcuate edge with notches in the other half, such that the segments on the two halves of the arcuate edge coact to cut a continuous substantially hemispherical surface of revolution when rotated on said axis, with each half cutting a plurality of spaced zones thereof which intervene between zones cut by the other half.

2. The subject matter of claim 1, wherein the beveled edge segment on one arcuate edge half of the blade extends to the midpoint of the arcuate blade edge, and is immediately adjoined, at said midpoint, by an oppositely beveled edge segment on the other arcuate edge half of the blade.

3. The subject matter of claim 1, wherein blade segments on each side of the midpoint of the blade define and occupy zones about the axis of rotation which at least meet adjacent zones defined and occupied by notches on the opposite side of the midpoint of the blade.

4. The subject matter of claim 3, with blade segments longer than blade notches, in such arrangement that zones defined and occupied by blade segments on each side of the midpoint of the blade at least meeting zones defined and occupied by the blade notches on opposite sides of the midpoint of the blade.

5. The subject matter of claim 4, wherein the zones occupied on each side of the midpoint of the blade at least slightly overlap zones defined and occupied by blade notches on opposite sides of the midpoint of the blade.