Medical fastener

Abstract

A dual-start fastener is disclosed having two threads helically wound about its shaft. The thread tooth profile (in section) constitutes substantially parallel upper and lower facets on each thread flank. A first thread has a crest diameter greater than a second thread. A distal end of the fastener includes self-tapping cutters formed on the threads which force shards of bone to advance into shallow troughs adjacent the cutters for subsequent resorption by adjacent osseous tissue.

Claims

1. A medical fastener, comprising: an elongate shaft having a proximal end and a distal end, a first thread helically wound around said shaft, said first thread having facets defining surfaces as upper and lower flanks, said upper and lower flanks oriented in substantially parallel relationship, a second thread helically wound around said shaft, said second thread having facets oriented substantially diametrically opposite from said first thread facets, a self-tapping cutter having a talon shaped cutting element with a sharp leading edge that extends from the shaft, a blunted trailing edge and a trough operatively communicating therewith to receive bone shards therein, wherein said second thread facets are substantially parallel to each other, and said first thread has a crest diameter greater than said second thread crest diameter, and wherein the thread flanks of the first thread and the thread flanks of the second thread are oriented at a right angle relative to said elongate shaft, whereby the thread flanks of the first thread and the thread flanks of the second thread are configured to form a series of square-wave teeth profile within a bone.

2. The fastener of claim 1 further comprising a self-tapping first cutter on said distal end to remove bone shards during fastener installation.

3. The fastener of claim 2 wherein said first self-tapping cutter supports a removal cutter (56) to excise bone during fastener removal.

4. The fastener of claim 1 wherein said facets include means to resist shear forces imposed on cancellous bone adjacent thereto.

5. The fastener of claim 4 wherein said means to resist shear forces is embodied as serrations disposed on at least one said facet.

6. The fastener of claim 1 including a thread angle of substantially zero.

7. The fastener of claim 1 wherein a second self-tapping cutter is disposed on a distal end of said second thread.

8. The fastener of claim 7 including a third self-tapping cutter located on a distal end of one of said threads.

9. The fastener of claim 1 including a pilot on said distal end.

10. A medical fastener, comprising: an elongate shaft having a proximal end and a distal end, a first thread helically wound around said shaft, said first thread having facets defining surfaces as upper and lower flanks, said upper and lower flanks oriented in substantially parallel relationship defining a zero thread angle, a second thread helically wound around said shaft, said second thread having facets oriented substantially diametrically opposite from said first thread facets, a self-tapping cutter having a talon shaped cutting element with a sharp leading edge that extends from the shaft, a blunted trailing edge and a trough operatively communicating therewith to receive bone shards therein, wherein said second thread facets are substantially parallel to each other, and said first thread has a crest diameter greater than said second thread crest diameter, and wherein the thread flanks of the first thread and the thread flanks of the second thread are capable of forming a series of square-wave teeth profile within a bone.

11. A medical fastener comprising: a dual start thread, a plurality of cutters symmetrically disposed about a fastener at an insertion end, at least two cutters forming a smaller thread crest diameter and a remaining cutter finishing a larger thread crest diameter, said cutters each including a leading edge and a trailing edge, said leading edge and trailing edge combine to form a talon-like, pointed contour at an extremity of said cutter remote from said shaft, said cutters collectively forming at least two thread patterns where all facets of said threads are parallel to each other and all thread angles defined by the conjunction of adjacent upper and lower facets of each said thread is zero.

12. A self tapping threaded fastener comprising, in combination: an elongate shaft having a proximal and distal end, and threads circumscribing said shaft, said distal end having threads interrupted by a cutter, said cutter comprising a sharp leading edge which extends from the shaft, said cutter further including a trailing edge blunted when compared to said leading edge, said leading edge and trailing edge combine to form a talon-like, pointed contour at an extremity of said cutter remote from said shaft, whereby shards of material cut by said cutter curl from said edges.

13. The fastener of claim 12 wherein a trough is located in the shaft adjacent a base of said cutter, said trough contoured as an elongate oval having curved end walls and parallel linear side walls, between which said trough resides, a leading edge of said trough being sharp to effect a smooth surface while tapping.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a fastener along a side thereof according to the present invention.

(2) FIG. 2 is another perspective view, detailing the top, proximal end.

(3) FIG. 3 is another perspective view, detailing the bottom, distal end.

(4) FIG. 4 is another perspective view of a distal end of the fastener, detailing the starting threads, centering pilot and self-tapping cutter structure.

(5) FIG. 5 is a differing perspective of FIGS. 3 and 4.

(6) FIG. 6 is a perspective view of serration-like “V” shaped grooves on the facets to resist shear.

(7) FIG. 7 is a sectional view of the faster, taken along its longitudinal axis.

DESCRIPTION OF PREFERRED EMBODIMENTS

(8) Referring to the drawings, where like numerals reflect like parts, reference numeral 10 is directed to the fastener (screw) of the present invention. The screw 10 has an elongate shaft 13, substantially cylindrical along its length with a linear long axis 100 at its center. The screw may have a head 6 provided with a drive face 5 on its top surface. The face 5 can be contoured to accommodate a drive socket, screw driver tip, “torx” fitting, alien wrench, etc. to advance the screw.

(9) A plurality of thread flights is contemplated, and the drawings illustrate a two flight embodiment. A first flight having a first thread 12 displays a greater crest diameter than a second flight having a second thread 24. Preferably these two flights are spaced from each other by 180 degrees and enjoy the same pitch. Accordingly, FIG. 7 shows that in section, a first major crest thread 12 always is diametrically opposed by a second minor crest thread 24 all along the thread paths. Bone “teeth” (cancellous bone) has been preserved between threads.

(10) Unlike retained bone from a buttress thread (which merely comprises a “V” shape of constant dimension all along the fastener from the root diameter to the crest), the bone teeth 25 retained in the present invention evokes a saw tooth or crenelated appearance. Importantly the high and low areas of the bone teeth are also 180 degrees opposed all along the thread pattern, so that an area of minimal bone volume on one aspect is diametrically fortified by its corresponding maximal bone volume 180 degrees opposite.

(11) When confronted with loads on the fastener, the opposing bone teeth and major/minor crest threads act in concert to oppose the loads and dissipate the loads into harmless (manageable) vectors through the cancellous bone. In fact, the loads imposed assist in circulating blood through the cancellous bone.

(12) Because of the dual start thread structure, the helix angle 20 (FIG. 7) for a double start thread such as this is greater than a single start thread by an appreciable amount (typically just less than a factor of 2). This helix angle ordains the slope (or ramp) that the threads 12, 24 spiral about shaft 13. For example, a single start thread may have a helix angle of 11 degrees while the present invention's dual start helix angle 20 would be 21 degrees (from “horizontal” i.e. transverse to the long axis 100). Similarly, loads confronted by the fastener also see this greater helix angle and are dissipated into harmless vectors into the greater volume of cancellous bone (than would be the case in the prior art).

(13) Each thread is formed as flanks having upper facets (nearer the proximal end adjacent head 6) and opposing lower facets. Thread 12 has upper facet 21 and lower facet 22; thread 24 has upper facet 28 and lower facet 27. FIG. 7 reveals that the facet pairs (21, 22) and (27, 28) enjoy substantial parallelism. That is facets 21,22,27,28 are all parallel to each other and substantially perpendicular to the long axis 100. This structure is especially adept at diffusing loads parallel to the long axis 100 and the threads' crest diameters are well suited to loads parallel but axially offset from the centerline long axis 100, primarily due to the greater volume of retained cancellous bone and its interplay with the varying crest diameters. As shown, thread 12 enjoys a greater crest diameter than thread 24.

(14) Torsional loads and other loads not parallel to the long axis are countered by: the increased volume of cancellous bone's serpentine meandering about the fastener, the larger helix angle of the thread patterns, and the space between adjacent flanks. Notice shaft 13 is interposed between flanks. The cancellous bone does not see a “V” at the juncture of facets (as in the buttress thread) but rather a smooth expanse of cylindrical shaft 13. Thus, there are no “V-shape” pressure points at that site but instead a smooth area of tangential registry between bone and screw.

(15) Because the flank facets are parallel to each other, the thread angle (the angle formed by the extension of adjacent upper and lower facets) is zero. Thus there are also no pressure points (additive vectors) on the cancellous bone caused by that angle.

(16) With a zero thread angle and parallel facets, loads transferred between cancellous bone and the flanks' facets are in shear; that is parallel to their interface. Resistance to shear loading can be enhanced by texturing the flanks, one example of which is shown in FIG. 6. Serrations 40, embodied here as a series of “V” shaped grooves 42, are embedded into one or more facets 21, 22, 27 and 28. The grooves receive the cancellous bone 25 there within.

(17) As mentioned above, cancellous bone is amenable to both compression and expansion. During installation, the bone is slightly compressed, after which it expands and fills the grooves 42. Shear loads (parallel to the facet surface) are opposed by the increased friction caused by this structure. The grooves 42 may form a spiral, may be concentric or may instead appear only as a textured, knurled or matted surface. Thus, heaving of the cancellous bone is opposed by this increased friction.

(18) FIGS. 4 and 5 show pilot 59, cutters 200 and the concave bone shard retention trough 55 that substantially parallels the long axis 100 of screw 10 on an outer, annular surface of the cylindrical shaft 13. Pilot 59 features a conical tip which angularly transitions to the cylindrical shaft 13, resulting in a radial lead 60. Primary bone cutters 200 start on the “left-hand” side of troughs 55. That means that when the fastener advances into the bone (by tradition via “clockwise” or “right-handed” rotation), the active surface of the cutters 200 remove bone. Each cutter 200 has an active helical surface only a short distance (1 or more threads) towards the proximal end of the screw.

(19) The actual cutting of bone shards by cutter 200 is caused by sharp leading cutting edges 53 formed at the base of the cutter 200 and has slightly blunted trailing cutting edges 51. A concave trough 55 placed near cutter 200 thus receives shards from the sharp leading cutting edge 53 which are severed by the blunted trailing edges 51. The troughs 55 are each strategically placed at the base of the cutters. The troughs 55 appear as very shallow, elongate ovals, having curved end walls 49 where the trough 55 tapers up to the shaft 13 and parallel, linear side walls 48 between which the trough 55 has its greatest depth.

(20) The dimensions of the trough are designed to receive only a thin layer of bone shards there within. This allows uncut adjacent cancellous bone an opportunity to absorb the shards and minimizes the likelihood that a thick deposition of shards are present which, otherwise, may support necrosis by being deprived of blood circulation. Sharp edges 53 and blunted edges 51 combine to approximate a talon-like contour. Shards curl from the cutting edges and transition into the trough 55 along path 57.

(21) FIG. 4 includes at least one sharp cutting surface 56 (on an opposite end of cutter 200) which is effective only when fastener rotation is for removing the fastener, in the present example counterclockwise. Historically, fasteners are occasionally troublesome to remove where the site has enjoyed active, restorative bone growth, and this cutter 56 facilitates easier fastener removal.

(22) It is preferred that three cutters 200 be employed in forming the bone teeth 25 by removing bone shards and placing them into troughs 55. The cutter nearest the distal end is the first contributor in forming all threads 12, while the other, following cutters closer to the proximal end cooperate to remove additional shards providing clearance for minor diameter threads 24 and finally major diameter threads 12.

(23) These features combine to cut clean threads and move the chips along chip path 57 pushing them into an adjacent trough 55 and out of the path of the advancing screw as it is helically turned into the bone. The leading edges 61 of the trough 55 at shaft (root) 13 are sharp and produce a precision fit within an inside diameter of a predrilled pilot hole by providing a positive scraping action as screw 10 turns inside the predrilled pilot hole. This scraping action forces the shards down into the troughs 55. The benefit here is that the cutting edges 51, 53 cause the shards to curl forward and follow the contour of trough 55 forward, and along path 57 away from the path of the advancing threads.

(24) Stated alternatively, as the fastener is inserted clockwise CW, the leading cutting edges 53 progressively shave thin ribbons of bone which are then severed by trailing edge 51, advancing the shards forward and pushed by the cutter 200 over the scraping leading edge 61 of trough 55 in the direction 57. This creates a true self-tapping screw and also prevents the chips from being drawn into the advancing screw threads and the bone. This results in much lower cutting pressure, cleaner threads and less damage to the bone. (If the chips can't get out of the way they get drawn into the path of the threads and get crushed into the surrounding bone. If this happens, the trapped shards can initiate an inflammation process resulting in the immune system attacking these shard chips as foreign bodies with eventual absorption, causing voids next to the threads eventually resulting in loosening of the screw threads 12 and 24.)

(25) The pilot 59 has a substantially rounded, radiused distal end 4 shown in the drawings. The conical transition of pilot 59 includes a lead 60 to shaft 13. A pilot 59 provides a transition between the radiused distal end 4 and the cylindrical shaft 13 via cylindrical lead section 60. This radiused end 4, angular transition 59 and lead 60 force the fastener 10 to remain in the predrilled pilot hole and helps find the pilot hole as it passes through an opposite side wall or portion of bone, thereby assuring registry of the fastener 10 through all subsequent pilot drilled portions of bone assuring maximum purchase and pull out strength.

(26) In use, preferably a pilot hole is predrilled and the fastener 10 is oriented there over. The angular transition 59, radiused end and cylindrical lead 60 of the pilot 59 nest within the predrilled pilot hole. Advancement of the fastener by clockwise rotation (CW) causes the leading (left handed) cutting edges 53 to incrementally shave bone shards away from the fastener, with trailing edge 51 pushing the shards downward along path 57 into the trough 55 concavity after having been chipped off by the trailing cutting edges 51 and assisted by edges 61 on the periphery of the trough 55. Bone is thus threaded in conformance with the fastener's tooth profile resulting in bone teeth 25. As the fastener 10 advances into the bone, the bone teeth 25 provide positive engagement with the fastener 10 without perceptible friction (deleterious heat buildup) or unwanted radial forces experienced by the surgeon who-unlike the prior art-can advance the fastener with very little effort.

(27) This gives the surgeon precise information on the progress of the procedure. The lead 60 on the pilot tracks the predrilled hole precisely without deleterious wandering and trauma to the adjacent bone.

(28) This contact induces a change in force which is perceptible to the surgeon unlike the prior art. The surgeon thus has better “feel” to sense and adjust the compression/torque most beneficial to the procedure. Incidentally, the same improved tactile feedback exists where the fastener does not have a head, but instead is to be countersunk.

(29) Having described an illustrative form of the invention and having been thus informed, it should be apparent that modifications are contemplated as being part of the invention as proscribed by the appended claims.