WO1998049948A1

WO1998049948A1 - Medullary-cavity drill head

Info

Publication number: WO1998049948A1
Application number: PCT/EP1998/002518
Authority: WO
Inventors: Thomas Gausepohl; Arthur W. Martin; Dietmar Pennig
Original assignee: Orthofix S.R.L.
Priority date: 1997-05-06
Filing date: 1998-04-29
Publication date: 1998-11-12
Also published as: DE19719051A1; AU7529398A

Abstract

A medullary-cavity drill head, preferably for removable attachment to a flexible drive shaft, has a distal-end cutting portion and a proximal-end shank portion for connection to the drive shaft; the drill head has a central bore for guidance along a guide wire within the medullary canal of a bone, such as a fractured femur or tibia. The cutting portion has plural equally spaced teeth with distally exposed cutting edges which generate a frustoconical surface of distal cutting action. The cutting edges determine a maximum diameter Dmax of cutting action, and all other surfaces of the teeth and remaining parts of the drill head are such as to generate profiles of revolution that are at least relieved from Dmax and therefore at offset within the geometric cylinder of cutting-edge action.

Description

MEDULLARY-CAVITY DRILL HEAD

BACKGROUND OF THE INVENTION

The invention relates to a medullary-cavity drill head having a cutting part and a shank part for connection to a drive shaft, as well as a central bore for accommodating a guide wire. Before an intramedullary nail can be inserted into a long bone, such as a femur or a tibia, it is necessary to gain longitudinal access to, and to enlarge and define, the bore of the medullary cavity, so that a stainless-steel nail can then be inserted into the thus-prepared medullary canal, the insertion being with X-ray monitoring, in the event of a fractured long bone, such as a femur or a tibia. In use, and after the bone has been opened at one end, and a guide wire has been installed via the opened end of the bone, a rotary driving mechanism, such as a cannulated motor driven by compressed-air, is chuck- connected to the shank part of the drill head, so that in the course of a drilling advance in the bone, the drill head rides the guide wire, while the free external end of the guide wire is allowed to pass through and beyond the cannulation of the motor.

Medullary-cavity drill heads of the indicated nature are known in the prior art. They usually comprise an elongate twist-drill head or reamer which has angularly spaced, elongate, side-cutting ribs; in use, these side-cutting ribs generate a geometric cylinder of constant diameter over the entire length of the drill head. Driven rotation of the head, with distally directed advance into a medullary canal, results in bone debris which must be removed without entry between the side-cutting ribs and the bore which is generated by the end-cutting distal end of the drill or reamer. Excessive friction and local bone- tissue necrosis are the consequence of failure to remove the bone dust and/or chips contained in such debris. And, to avoid or reduce the likelihood of a necrosis problem, it has been the practice to employ a succession of drill or reamer heads, of progressively larger bore-generating size, in sequential in- out cycles of ever-increasing diameter. For example, to achieve an ultimate bore diameter of 12.5 mm, a first bore-enlarging drill-head or reamer may be size-selected for 9.5-mm diameter. Then, a 10-mm drill head or reamer replaces the initial drill or reamer, for a second in/out cycle to achieve a 0.5- mm increment of bore-enlarging diameter, and so on, in successive cycles of 0.5-mm incrementally increased bore diameter; thus, as many as six progressively larger drill heads or reamers are used before the 12.5-mm drill head or reamer is used to achieve the final (i.e., the desired) bore size. Apart from the fact that such a recycled procedure consumes a considerable length of time, the repeated changing of the drilling or reaming tool has also been found to be disadvantageous since, of course, it was always necessary to work once again (i.e., to re-bore) with attendant risk of trauma for the patient whose body system has been subjected to such recycling abuse.

BRIEF STATEMENT OF THE INVENTION

It is a primary object of the invention to provide a medullary-cavity drill- head construction which will allow for a desired diameter of medullary bore to be achieved immediately, i.e., on the first in/out cycle of a drilling operation, while at the same time so configuring the new medullary-cavity drill head as to remove drilled-bone debris without presenting the problem of necrosis of remaining bone tissue.

It is also an object to achieve the foregoing object with a medullary cavity drill head which will safely permit a desired medullary-cavity bore enlargement to final-bore size in a single cycle of entry into and removal from the enlarged bore, and at a substantially faster rate of drill-head advance into and withdrawal from the enlarged bore, all without having engendered a problem of bone-chip or bone-dust removal.

A further object is to provide an improved medullary-cavity drill-head construction which lends itself to tracking an installed guide wire and to rotary drive from a flexible shaft such that if necessary, the path of drill advance may be curvilinear, without sacrifice of the ability to make a single-pass bore enlargement to intended final diameter and without encountering noticeable necrosis of bone tissue.

The invention achieves these objects in an intramedullary-cavity drill- head construction wherein the only cutting edges are at the distal end of the head, and wherein the geometric cutting circle of the front end is of greater diameter than the diameter of a geometric circle generated upon rotation of any other part of the drill head. Thus, in the proximal direction of drill-head profiling i.e., rearward from the cutting edge or edges, the body of the drill head is convergent to juncture with a proximal-end stem or shank portion that is configured for suitably keyed removable connection to the distal end of a tubular flexible drive shaft for imparting rotation to the drill head.

Stated in other words, a drill head of the invention comprises (i) a relatively short distal-end cutting part having plural equally spaced and distally operative cutting-tooth formations, and (ii) a shank or proximal-end part of greater longitudinal extent than the cutting part. Upon drill-head rotation, the cutting part describes an outer circle of diameter which corresponds to the internal diameter of the desired bore in the bone. Thus, the drill head in its preferred embodiment tapers generally conically, from a maximum or cutting diameter at its front distally-cutting end, with taper convergence toward the more proximal region of its preferably integral connection to the shank part. Illustratively, the shank part may have a diameter of 10 mm, while the cutting part may have an external cutting-circle diameter of 12.5 mm. Furthermore, the distal-end cutting part has at least two and preferably three teeth having cutting edges at equal angular spacing. Each cutting edge is integrally backed by an inclined flank having a land immediately behind the cutting edge, as viewed in the rotary direction of cutting-edge advance, for cutting action. As distinguished from prior drill-head or reamer configurations, there is no further cutting edge beyond that which, for each tooth formation, is sharply defined at intersection of the land with an undercut surface that establishes one side of an inclined surface which is the rotationally forward face of the tooth and which establishes one side of an inclined flute. The flute provides a somewhat helically directional passage for proximally directed withdrawal of debris resulting from the distally advancing zone of canal-drilling (canal-boring) operation within a bone. For each tooth formation, the land provides immediate backing support for the cutting edge. The land has a relatively low angle of clearance with respect to the surface of revolution generated by the cutting edge, and a first heel is defined by adjacent surfaces of the land and the flank, with the flank providing backing support for the cutting edge via the land, and with the flank surface being at a greater angle of rake or clearance with respect to the frusto- conical surface of revolution generated by the cutting edge. Tooth-body support for the flank includes a more steeply inclined surface which defines a second heel adjacent the flank surface and which also establishes another flute-defining surface at the trailing side of the tooth. When viewed in the direction of tool rotation, the integrally supporting body of each tooth formation is seen to be inclined backward, in general conformance with the slope of the indicated undercut slope; moreover, this supporting body also extends radially outward thereby enabling drill dust (debris) to be directionally conveyed, away from the situs of cutting action. The undercut front surface and the back surface which create the flute slope more steeply proximal than the flank surface, resulting in a larger free space, namely, the flute, for debris accommodation, proximally removed from the distally advancing zone of cutting action. The distal end of the central bore of the medullary-cavity drill head is surrounded by a planar annular surface, which serves as a bearing surface for the olive of the guide wire, and thus does not have a cutting action, so that as a result the olive of the guide wire is not damaged.

In an embodiment which has been found to be successful in practice, three cutters have proven advantageous.

Also advantageously, the central bore of the drill head has sufficient running clearance with the guide wire, so that when driven via a flexible shaft, the drill head will readily follow any slight curvature of long tubular bone structure, in that the central bore of the drill head is of longitudinally short design. As a result, a one-pass drilled bore can be achieved in virtually identical conformation to the curvature of the bone.

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will be described in detail for a preferred embodiment and for a modification thereof, all in conjunction with the accompanying drawings, in which:

Fig. 1 is a side view in elevation of a drill head of the invention, with phantom outline of a guide wire for guidance of the drill head, and drive mechanism including a flexible shaft for rotary drive of the drill head on the guide wire, to enlarge the bore of a medullary canal in which the guide wire has been installed;

Fig. 2 is a front-end view of the drill head of Fig. 1; Fig. 2A is a view similar to Fig. 2, to show a modification; Fig. 3 is a perspective view of the front end of the drill head of Fig. 1, for the case of tilting the front end toward the viewer; Fig. 4 is a view in projection, in the manner of a Mercator projection, to a geometric cylindrical surface of maximum diameter D_max of the cutting circle generated upon driven rotation of the drill head of Figs. 1 to 3, wherein the rotation is clockwise about the central axis of the drill head, as viewed in the distal direction of cutting advance within a medullary canal; and wherein the projection places the geometric circle of maximum diameter of cut at the upper margin, and the reduced diameter of the stem portion of the drill head at the lower margin;

Fig. 5 is a greatly enlarged and simplified top view as in Fig. 2, to enable labelling of identifying features of multiple teeth; and Fig. 6 is a sectional view, taken at 6-6 in Fig. 5.

DETAILED DESCRIPTION:

In the drawings, a medullary cavity drill head 10 is seen to comprise a distal-end cutting portion 11 and a shank portion 12 of greater longitudinal extent than the cutting portion 11. The cutting portion 11 has a maximum cutting-circle diameter D_max which is greater than the circle diameter generated by any other part of the drill head; for example, the cylindrical part of the shank portion 12 may suitably have a diameter in the range of 75 to 85 percent of the cutting-circle diameter D_max, for a drill-head length L which is approximately three times the cutting-circle diameter D_max. The lower or proximal end of the shank portion features a transverse slot or groove formation 13, for selectively keyed connection of the drill-head 10 to a flexible drive shaft 14. For a major fraction (L7L) of its length to its distal end, the drill head features a central bore 15, for directional guidance by a guide wire 16 that will be understood to have been installed in the medullary canal of an elongate bone, such as a fractured femur or tibia that has been diagnosed to require stabilization by an intramedullary nail. As shown, the central bore 15 enlarges at a counterbore 17 which spans the drive-keying groove formation and which is otherwise open at the proximal end of the drill head; flexible shaft 14 is tubular, with a bore to receive guidance from guide wire 16, and drive means 18 is shown to include chuck means 19, providing selective driving engagement to the.fJexible*haft said drive means being cannulated with a central passage for axially guided displacement of the totality of means 18, 19, 14 and the drill head 10, in the course of driven drill-head rotation, whether in the distally directed advancing (bone-cutting) phase or in the non-cutting retraction phase of the single-pass in/out feeding cycle contemplated in use of the invention.

At the distal end of the drill head, the central bore 15 opens within an annular flat 20 (Fig. 2) which assures that drill-head abutment with an "olive" formation 16' at the distal end of the guide wire signals the full extent of drill- head 10 advance. The flat annular surface 20 assures that the olive cannot be "chewed" by distal-end cutting action. In Fig. 2A, a similar flat 20' exists to protect against olive destruction, but the "flat" in Fig. 2A comprises arcuate segments 20' in a single radial plane of harmless limiting abutment with the "olive", the segments 20' being angularly spaced at each cutting-edge distal- end intercept therewith, for the same purpose and result as if the "flat" were a circumferentially continuous annulus 20 (Fig. 2).

In the preferred embodiment shown in the drawings, the distal-end cutting portion of the drill head is defined by three like generally distally and radially directed teeth 21, 22, 23 at equal angular spacing about the central axis 24 of the drill head. Each tooth has a sharp cutting edge 25 which, on rotation, geometrically describes a frusto-conical surface, extending from a minimum radius at the radial plane of flat 20 (or flats 20'), to a maximum radius which, on drill-head rotation, defines the geometric circle of maximum cutting diameter D_max, described above. The angle of geometric frusto-conical divergence from a radial plane, defined upon drill-head rotation, is designated α. in Fig. 1. More particularly, for the embodiment shown, the cutting edge or lip 25 of a tooth, e.g., the tooth 21 which in Fig. 1 is seen in side elevation, is formed by the intersection of an undercut flat surface 26 which appears as a straight line, inclined at an angle α₂ to the longitudinal axis 24. As used herein, the word "undercut" is to be understood, in the context of a clockwise-driven distally advancing working cut, meaning that the cutting edge 25 leads or is ahead of the undercut surface 26, which defines one side of a flute relation (A) between tooth 21 and tooth 22, wherein tooth 21 trails tooth 22 in the course of driven rotation. The other angular side of the flute relation (A) between leading tooth 22 and its next successive tooth 21 is in large part defined by a similarly sloped trailing surface 27 of tooth 22; and a like trailing surface 27 of tooth 21 essentially defines the angular width of tooth 21, as well as the "forward" side of the flute formation (C) between teeth 21 and 23. In similar fashion, the third flute formation (B) between teeth 22 and 23 is defined by the undercut forward surface 26 of tooth 22 and the similarly inclined slope of the back or trailing surface 27 of tooth 23.

The enlarged and simplified top view of Fig. 5 and the fragmentary section of Fig. 6 enable a more detailed accounting for important details of the preferred embodiment of the invention. For each of the teeth 21, 22, 23, the cutting edge 25 is defined by intersection of a land 29 with the undercut surface 26. The land has a clearance angle α₃ with the geometric radial plane 30 of maximum-diameter cut that is described, upon drill-head rotation in the clockwise direction, suggested by arrow 31. Suitably, the land 29 may be generated by one of three equally angularly indexed generally radial surface- grinding operations, each along a path which truncates the distally convergent distal body-surface portion or flank 32 of each tooth at the cutting end of the drill head; such grinding of land 29 therefore establishes a first heel 33 between body-surface portion of flank 32 and land 29, and the angle α₄ illustratively establishes a rake angle or clearance and backing support for land 29 and its cutting edge 25. The body-surface portion 32 derives further tooth-body support and backing from the maximum tooth width between the leading and trailing surfaces 26, 27 of the tooth, as clearly shown in Fig. 6 wherein a second heel 34 is defined at intersection of surfaces 27, 32; and an angle α₅ is shown to mark the heel-defining relation between surfaces 27 and 32.

Various of the surfaces which have been identified in Fig. 5 will be recognized in the Mercator-like projection of Fig. 4, which displays the cutting edges 25 at intercept with the geometric cylinder to which the projection is made, these intercepts being in spaced relation along the top margin of the projection, and which displays cutting-head features that are proximally offset to the lower margin of the projection, i.e., to the location of reduced stem diameter, D_stem. The cutting edges 25 terminate on the rotationally generated circle of maximum diameter (D_max) of the drill head 10, and in Fig. 4 these edges show schematically as dots 25, on the geometric cylinder to which the projection is made. Thus defined, all other features of the projection will be understood to be "behind" or "within" the geometric Dmax cylinder of the projection, and surfaces already described in connection with Figs. 5 and 6 have been identified by reference numbers, where applicable. In the described embodiment, the cutting part is formed by three cutters or teeth 21, 22, 23. Each cutter or tooth, e.g., tooth 21 , has a cutting edge or lip 25, which is formed with a sharp edge, in the direction of rotation of the tool. The cutting edge 25 is adjoined by a supporting surface or land 29, which is delimited on its rear side by a heel 33, the heel 33 at the same time forming the border between the supporting surface (land 29) and a flank 32. The supporting surface (land 29) for each cutting edge or lip 25 is slightly inclined to define a clearance or relief angle α₃ from the geometric frustum of a cone that is generated by the cutting edge 25 in the course tool rotation. The flank 32 is adjoined by a more steeply sloped surface 27, which together with the next-adjacent cutting tooth, for example, the cutting tooth 23, forms a flute (C) which creates a relatively large free space through which the drill debris can flow out toward the rear, i.e., proximally away from the distally advancing zone of cutting action. The transition from the flank 32, to the surface 27 is defined by a second heel 34, so that there are different slopes here between flank 32 and surface 27.

The medullary cavity drill head 10 itself is made from a metal of suitable strength, such as titanium or stainless steel, although it is also possible to make the actual cutting edge 25 more efficient by using a sintered carbide inlay, to provide definition of land-29 and undercut 26 at their intersection to define the cutting edge 25.

For a better understanding of the present drawings, the maximum diameter D_max of the cutting portion 11 of the medullary cavity drill head 10 may, illustratively, be 12.5 mm, while the external diameter of the shank portion 12 is 10 mm. The flexible drive shaft which is to be connected to the lower end of the shank portion 12 may have a diameter of 8 mm, so that when viewed in overall terms, there is sufficient free space to remove the drill debris, without danger of fouling between a cut surface and any part of the drilling tool. For maximum usefulness to the orthopedic surgeon or to his hospital, the described medullary-cavity drill head may be supplied as a kit containing a variety of sizes of D_max, coordinated with commercially available intramedullary- nail sizes, for example, the sizes of nail diameters 10 mm, 11.5 mm, 12.5 mm, 13.5 mm and 14.5 mm. Further illustratively, in the case of all sizes, including particularly the drill-head size having a D_max of 12.5 mm and a shaft diameter of 10 mm, and an overall length of 35 mm, satisfactory angle relations may be: For αι-in the range 20° to 30°, and preferably 25°.

For α₂~in the range 20° to 30°, and preferably 25°. For α₃~in the range 4° to 6°, and preferably 5° to a cutting edge tangent to the cutting cone.

For α₄-in the range 25° to 35°, and preferably 30°. For αjHn the range 15° to 25°, and preferably 20°.

Also preferably, the sum of angles α₂, α₄, and as is advantageously in the range 80° to 90° and preferably about 85°. The surface width of land surface 29 is suitably in the range 0.75 mm to 2.0 mm, and preferably about 1.5 mm. The central bore 15 of the drill head is recommended for a diameter of 3.7 mm, for running-clearance guidance along a guide wire of 3.4-mm diameter.

For a surgical operation on a femur, wherein the operation is to use a drill head of the invention, to prepare for ultimate insertion of a cannulated nail of 10-mm diameter, the procedure commences with the patient in a supine position, and with the proximal end of the bone exposed, a longitudinal incision of approximately 10-cm length is made above the tip of the greater trochanter, using a pointed instrument to gain opening access to the medullary canal, via cancellous bone structure at the proximal end of the bone. After thus opening the canal, a 3.4-mm guide wire with an olive tip is inserted, it being understood that the olive is an integrally deformed distal end of the guide wire. Once the guide wire becomes seated and centered on the supracondylar roof at the distal end of the femur, drilling commences by selecting a 12.5 size drill head of the invention and a flexible drive. Most cannulated drive motors are unidirectional, and therefore the drilling operation is performed in a single in/out cycle of driven rotation, which proceeds at a speed between 75 and 150 rpm, usually about 100 to 125 rpm. Very little opposition is sensed in the course of feeding the tool down the medullary cavity, all the way to contact with the olive; in that the one-step tool is almost self-feeding, so that a full drilling stroke accomplishes ultimate bore-diameter (12.5 mm) in 15 to 20 seconds. The one-step tool and guide wire are readily removed by withdrawal from the now-widened femoral canal, in a two-step procedure (a) in which the motor, the flexible drive shaft and the one-step tool of the invention are (as an interconnected unit) removed from the wire, and (b) in which the cannulated 10-mm diameter nail is assembled to the wire with passage to the olive, prior to removal of the wire, in olive contact with the distal end of the nail, thereby assuring nail protection of the newly prepared bore, e.g., against otherwise potentially harmful contact of the olive with the newly prepared bore, thus enabling smooth reinsertion of the nail alone, without the wire and its olive. It is believed that the drilling efficiency of the drill head of the invention is attributable to the fact that the cutting edges of the teeth are the only parts of the drill head having direct contact with the medullary canal. The outer ends of these cutting edges determine D_max of the bored canal, and all other features of the drill head have at least some clearance with the bored diameter D_max. For example, the approximately 5° clearance α₃ of land 29 begins at the cutting edge, making a reduction from the conical surface reserved exclusively for the cutting edge. Also, as will be apparent from the end view of Figs. 2, 2A, and 5, an immediate relief behind the outer diameter of cutting-edge action is readily provided when the peripheral profile of the end view is generated as an eccentrically developed circular arc 40, over the angular spread β and about an eccentric center 41 , i.e., between a first angular limit 41 of D_max generation at angular intercept with cutting edge 25, and a second angular limit 42 of reduced-diameter generation behind the cutting-edge intercept.

Claims

MEDULLARY CAVITY DRILL HEADWHAT IS CLAIMED IS:

1. A medullary cavity drill head, preferably for attachment to a flexible drive shaft, having a cutting part and a shank part for connection to the drive shaft, as well as a central bore for accommodating a guide wire, wherein the cutting part has a greater external diameter than the shank part and is arranged at the front end of the shank part, the cutting part having at least two cutters, which taper conically towards the shank part from their greatest, upper external diameter.

2. The medullary cavity drill head as claimed in claim 1 , wherein each cutter has a cutting edge which is directed transversely to the longitudinal axis of the medullary cavity drill head and is adjoined by a flank which is inclined toward the rear, as seen in the direction of rotation.

3. The medullary cavity drill head as claimed in claim 1 or 2, wherein a supporting surface, which together with the flank forms a heel, is provided between the flank and the cutting edge.

4. The medullary cavity drill head as claimed in one of the preceding claims, wherein the supporting surface is undercut, as seen in the direction of rotation, toward the flank.

5. The medullary cavity drill head as claimed in one of the preceding claims, wherein the supporting surface is inclined outward.

6. The medullary cavity drill head as claimed in one of the preceding claims, wherein the flank is adjoined by a surface which creates a flute.

7. The medullary cavity drill head as claimed in one of the preceding claims, wherein the transition from the flank to the surface is formed by a heel and the surface is inclined more steeply downward then the flank.

8. The medullary cavity drill head as claimed in one of the preceding claims, wherein the central bore is surrounded by a planar annular surface, which serves as a bearing surface for the olive of the guide wire.

9. The medullary cavity drill head as claimed in claim 1 , which has three cutters.

10. The medullary cavity drill head as claimed in one of claims 1 to 9, in which the material of the drill head is titanium.

11. The medullary cavity drill head as claimed in one of claims 1 to 9, in which the material of the drill head is an alloy in which titanium is the preponderant element.

12. A medullary cavity drill head, comprising an elongate body having a central axis and an open-ended bore on said axis for accommodating a guide wire, said body having (i) a front end with distally operative generally radial cutting formations and (ii) a shank having a rear end formed for connection to a rotary drive shaft, said cutting formations being configured to generate a generally circular cutting of greatest diameter at said distal end upon driven rotation of said body, said shank having a diametral extent less than said greatest diameter, and said body being so configured proximally of said greatest diameter as, in the course of driven rotation, to generate a reducing profile of revolution about the axis, said reducing profile extending from the distal-end location of greatest diameter and in the proximal direction to the point of juncture with said shank.

13. The medullary cavity drill head of claim 12, wherein the number of said generally radial cutting formations is three, in equal-angle spaced relation about the axis.

14. The medullary cavity drill head of claim 13, wherein each generally radial cutting formation comprises a cutting lip contained in a geometric frustoconical surface having a slope angle in the range of 15 to 25 degrees with respect to a radial plane about said axis.

15. The medullary cavity drill head of claim 13, wherein each generally radial cutting formation comprises a cutting lip contained in a geometric frustoconical surface having a slope angle of substantially 20 degrees with respect to a radial plane about said axis.

16. The medullary cavity drill head of claim 14, wherein, in angularly successive adjacency, each cutting lip is succeeded by a ground generally radial land at a clearance angle and by a flank having a primary rake angle (╬▒4) of relief from a plane that is tangent to said frustoconical surface at said cutting lip.

17. The medullary cavity drill head of claim 16, wherein in further angularly successive adjacency, each primary rake angle (╬▒4) is succeeded by a secondary rake angle (╬▒5) of further relief from said tangent plane.

18. The medullary cavity drill head of claim 16, wherein said primary rake angle (╬▒4) is in the range of 25 to 35 degrees.

19. The medullary cavity drill head of claim 15, wherein the material of the drill head is titanium.

20. The medullary cavity drill head of claim 15, wherein the material of the drill head is an alloy in which titanium is the preponderant element.

21. The medullary cavity drill head of claim 16 wherein said clearance angle is in the range 4┬░ to 6┬░, and preferably substantially 5┬░.

22. The medullary cavity drill head of claim 17, in which said primary rake angle (╬▒4) is in the range 25┬░ to 35┬░.

23. The medullary cavity drill head of claim 17, in which said primary rake angle (╬▒4) is substantially 30┬░.

24. The medullary cavity drill head of claim 16, in which each cutting lip is defined by said land at intercept with a flat undercut front surface which makes an angle in the range 20┬░ to 30┬░ in an undercut angel (╬▒2) a geometric plane which includes said central axis and which is perpendicular to the geometric plane of said undercut front surface.

25. The medullary cavity drill head of claim 16, in which said angle (╬▒2) is substantially 25┬░.

26. The medullary cavity drill head of claims 17 and 24, in which the sum of said primary and secondary rake angles plus said undercut angle (╬▒2) is in the range 80┬░ to 90┬░.

27. The medullary cavity drill head of claim 26, in which said sum is substantially 85┬░.