HK1259987B - Medical fastener - Google Patents

Description

Medical fasteners

This application is a divisional application of application number 201380051546.3, filed on September 30, 2013, and entitled “Medical Fastener”.

Technical Field

The present invention generally relates to fasteners. More specifically, the present invention relates to threaded fasteners. Most specifically, these threaded fasteners (or screws) are particularly useful for embedding unique and typically inhomogeneous materials under dynamic loads. A common example would be bone found in orthopedic, veterinary, and dental environments.

Background Art

In the past, homogeneous materials without dynamic loading could be reliably fastened. However, when the homogeneity changes and when dynamic loading varies, puzzling fastener problems arise. The skeleton is an example of a non-homogeneous structure subjected to dynamic loading.

Skeleton includes bone tissue.Compact bone is a form of bone tissue, and the feature of this bone tissue is greater than another bone tissue (also known as cancellous bone, trabecular bone or spongy bone) density.This cancellous bone has the surface area larger than compact bone, but it is more flexible, weaker and not too hard.When a screw is embedded in the bone with these two characteristics, the holding force (for example, pull-out strength) of screw is subject to the ability limitation that screw is retained in this cancellous bone, because cancellous bone is the most fragile component.However, this fastener still must be designed to penetrate this compact bone.Prior art sawtooth thread (industry standard) cannot solve the heterogeneity problem of bone under dynamic load.

One result is that the prior art fails to provide a reliable threaded portion that engages the patient's bone in a manner that hinders loosening of the threaded portion. Bone is a remarkable structure whose hardness and elasticity vary with both age and location. The load on the fastener must accommodate not only these constraints, but also the dynamics of the forces generated by the patient during daily activities. Prior art buttress threads are unable to cope with these demands, resulting in the fastener loosening or even pulling out.

Therefore, industry practice is to provide long cancellous screws with a greater pitch density to increase holding force. These screws typically have a shaft that is fully or partially threaded along its length and threads with a constant crest diameter along the majority of its length (excluding the initial distal end), with the shaft diameter being equal to the crest diameter. Therefore, the pull-out resistance is limited by the volume of cancellous bone between the screw teeth and their junction with the adjacent cancellous bone tissue.

Buttress threads are currently the industry standard. Acme threads are sometimes used, but Acme threads are simply buttress threads in which the exposed sharp angles of the threads are truncated at the top. This does not remedy the fragility of buttress threads. Unfortunately, buttress threads are only suitable when the load forces acting on the fastener are in one direction. (Bhandari, Design of Machine Elements (2007), p. 204). In cases where these load forces are multi-directional, or non-unidirectional and axial, failures may occur. One manifestation of buttress thread failure is "toggling" where the fastener acts on bone and enlarges the hole in which the fastener resides, leading to failure.

Screw insertion time is a function of screw length and the resistance of the medium to the advancement of the screw. In the operating room, shorter surgeries are believed to yield better results in terms of sterility and shorter patient exposure to surgery. To this end, several patents have proposed double-ended, double-threaded fasteners having two different crest diameters ("high-low"), one crest diameter for each thread. Because the industry has still adopted buttress thread geometries, they have not solved the major problem of embedding into heterogeneous bone tissue under multi-directional dynamic loads. See exemplary U.S. Patents 6,743,233, 5,743,914, and 5,720,766. Surprisingly, these patented technologies have not been adopted by the industry.

Summary of the Invention

The medical fastener of the present invention provides both easier insertion resulting in low friction and a greater volume of cancellous bone adjacent to the fastener. This increases pull-out strength and reduces insertion time. An important consequence is that the cancellous bone adjacent to the fastener is not subjected to the elevated temperatures caused by high insertion torque due to undesirable friction. Osteonecrosis is known to occur when temperatures exceed temperatures as low as 116 degrees Fahrenheit, compromising pull-out strength and leading to complete failure of the fastener.

The reduced effort during installation provides better feedback to the surgeon, allowing for an improved feel and signal that the fastener is properly installed. Currently, surgeons are known to sense high insertion torque as a signal that the fastener has been securely purchased and therefore the desired quality of fixation has been achieved.

This medical fastener benefits from a double thread structure in which two helical threads are helically intertwined along a shaft. Each thread has its own self-tapping starter tip, which feeds bone shards into at least one shallow groove integrally formed with the fastener's shaft. Preferably, multiple grooves are provided, each having a minimum depth for receiving the guided shards in a thin layer. This thin deposit of bone shards can thus still be absorbed by adjacent bone, minimizing the risk of necrotizing tissue and otherwise ulcerating the shards.

As described above, the two thread paths can be unified to have the same overall pitch density of a conventional fastener, if desired. However, the teeth on the two thread paths (when viewed in cross-section) are a radical departure from the prior art in many important respects. For example, the upper and lower facets on each tooth are substantially parallel. In addition, the thickness of each tooth is minimal, approximately blade-like ("thick gauge") thickness. This construction benefits the fastener by having greater strength than the surrounding material into which it is embedded, and significantly reduces the torque required for installation compared to the prior art.

In addition, because the two thread paths have different crest diameters, the kinematics of the fastener deviate significantly from conventional buttress threads. In the presence of two different crest diameters, the bone embedded in the fastener is not held by a core of cancellous bone having a constant "plug" diameter (i.e., a crest diameter equal to the buttress thread and shaft diameter), but rather by residual bone surrounding both the smaller and larger crest diameters, thereby creating a "snake-like" or "sawtooth-like" continuum of cancellous bone that interweaves between the two crest diameters.

The result is a bone "tooth" geometry that mimics a square wave; in other words, a blunt, serrated profile or battlement with alternating merlons and crenels. This results in a significant increase in the volume of cancellous bone retained around the fastener. This benefit is not present in the double-ended patents listed above. Furthermore, the bone teeth formed by the fastener of the present invention abut the cylindrical shaft portion between adjacent fastener teeth. This avoids the prior art pressure points where the bone tapers between adjacent fastener teeth.

The pitch (pitch) of a double-start, double-threaded structure provides important benefits, wherein the thread flanks have parallel upper and lower facets or flanks. The pitch of a helical thread is preferably at least twice the pitch of a conventional cancellous bone serration fastener. Because the two helical threads are staggered and because of the different crest diameters of the two threads, more volume of cancellous bone is available for retention, resulting in greater pull-out strength.

Equally important is the symmetry of two helical threads that are 180 degrees out of phase. This means that, in cross-section, a smaller crest diameter is essentially directly opposite a larger crest diameter anywhere along the length. Therefore, loads applied to the fastener react simultaneously to both a larger crest and a smaller crest anywhere along the thread pattern, with loads parallel to the long axis (especially when offset) reacting through the blade-like threads that oppose and deflect the force. Loads not parallel to the long axis of the fastener are more effectively distributed by the blade structure along with the smooth shaft because pressure points are minimized.

That is, unlike a sawtooth thread, the space between each thread does not form a "V" shaped focal point (or fulcrum), which previously defined both the stress point along the bone tissue and the location of a failure point on the fastener. Instead, the cancellous bone tissue encounters a smooth, cylindrical wide area of the fastener shaft between adjacent threads. The adjacent abutment of the cancellous bone tissue with these smooth areas improves the load distribution along more of the bone surface without the "pressure" points previously defined by the sharp angles of the prior art sawtooth threads. Therefore, forces transverse to the long axis are similarly relative and deflected. Although so taught, it should now be clear that tilting forces are also similarly directed.

Each thread has a flank with an upper facet and a lower facet. Unlike a buttress thread, which has triangular (sometimes truncated) flanks in cross-section, the upper and lower facets of each of the flanks of the present fastener are very closely spaced and preferably substantially parallel in cross-section, resulting in a very thin, blade-like thread. A thread angle is defined as the angle between adjacent upper and lower flanks.

The blade-like threads (with substantially parallel upper and lower facets) result in a thread angle close to 0 degrees. (Serration fastener thread angles are typically 60 degrees for both single-start and double-start threads.) Consequently, the thin threads also result in minimal bone resection during installation, leading to more bone retention and easier installation. When combined with a double thread/head configuration, significant benefits are achieved.

As mentioned above, double threads/starts reduce the pitch of each thread pattern by at least half to maintain the same overall pitch density. However, for a double-start/thread structure, the angle of attack (lead angle) of the thread pattern also changes by a factor of 2. This also changes the "ramp angle" (i.e., the spiral rate of the helical thread) by a factor of 2. A helix angle can be defined as the angle between a line approximating the slope of an upper facet and an intersection line transverse to the long axis of the fastener. For example, for a single-thread fastener, the helix angle is typically 11 degrees in the prior art. For a double-start, double-thread design, the helix angle is alternatively approximately 21 degrees.

Therefore, the applied force vector is not only resisted by the larger volume of cancellous bone retained, but also redirected and deflected by the flanks of the thread at different, small angles of inclination. Importantly, in the present invention, these forces are redirected into the adjacent and larger amount of cancellous bone. In summary, a thread angle of 0 degrees maintains more bone, and a higher helix angle directs the force into the larger amount of bone at an optimal angle.

Perhaps paradoxically, this structure, combined with a self-tapping cutter head, reduces the effort and time required for installation while increasing the surgeon's tactile feedback and simultaneously increasing holding force. As is well known, a double-threaded structure doubles the shaft's axial advancement with each rotation of the shaft. The same applies here, but with the cutter head. The surgeon again gains a feel for the fastener's screw advancement thanks to a confluence of features including an improved lead angle, varying crest diameters, and closely spaced, substantially parallel, thin facets on opposing sides of the threads. In cross-section, these facets are substantially perpendicular to the long axis of the fastener.

Double-start threads intuitively should have at least one cutter head for initiating and forming each thread pattern. For symmetrical progression, the cutter heads should be deployed in pairs, diametrically spaced relative to each other. The cutter heads should also be in pairs. In contrast, in the present invention, three cutter heads are preferably provided for the two thread paths, but these three cutter heads are symmetrically arranged 120 degrees around the fastener, each with its own chip groove. The first and second cutter heads are arranged on each of the two threads, 120 degrees apart, and the two cooperate to form the smaller crest diameter, while the third cutter head completes the cutting of the larger crest diameter thread upstream of the first two.

The thread geometry of the present invention minimizes insertion force by reducing heat-generated friction. This allows the surgeon tactile feedback and reduces the effort required to deploy the fastener. This, combined with a shallow tool-feed debris storage groove, centering guides, and improved thread cutting features, keeps friction low and maintains the fastener aligned and oriented to prevent wandering from the preferred path.

The distal end of many fasteners includes a buffer zone (long groove) designed to help penetrate the entire bone, thereby defining a "self-tapping" fastener. A buttress thread fastener with a self-tapping feature typically includes a long groove that is straight or at least nearly colinear with the axis of the screw. Therefore, as the fastener advances, the cutting edge transports bone debris toward the head of the fastener, which has entered the path of the helical thread. This bone debris accumulates along the thread teeth and increases insertion torque and friction, which therefore generates additional heat. The debris also makes the fastener more difficult to insert and provides a weak joint between the bone and the fastener.

In the present invention, the cut debris curls away from the cutting edge and is fed into a preformed groove integrally formed with the fastener and parallel to its long axis on an annular outer periphery. That is, as the fastener advances, the long groove pushes the debris downward into the adjacent groove. This results in a precise gap between the fastener and the portion of the bone that forms the "bone tooth." (The bone block that engages the threaded fastener should be referred to as the bone tooth). Therefore, the joint between the bone and the fastener is essentially free of cuttings and provides healthier bone tissue adjacent to the fastener to prevent additional bone trauma. The debris in the groove adjacent to the uncut bone tissue is available for absorption and feeding, thereby creating a healthier, stronger interconnection.

Another problem associated with buttress threads is that the area between the threads of the fastener is the only area anchored in the bone, and due to the design constraints associated with it, this area is difficult to optimize. In other words, once the pitch and the crest and root diameters are chosen, the thread pattern is fixed. Because the metal of the fastener is orders of magnitude stronger than the bone it holds, bone trauma is always involved in the event of a failure. This is partly due to too much fastener material being embedded and too little bone being retained.

In contrast, the present invention maximizes the bone engaged while minimizing the threads of the fastener, which cannot be achieved with buttress threads and other common threads using common production processes. The result is less bone trauma and less bone removed, resulting in increased bone strength to better retain the fastener.

The present invention abandons conventional thinking and production techniques in pursuit of new and desired functions that can be achieved from thread profiles.

Therefore, manufacturers have adopted a simple and very fast production process to produce screws that do not function as well as common wooden screws.

The present invention (especially with improved cutting edge, debris removal, reduced tooth width, negligible thread angle, improved force distribution along the thread facets and less bone removal) creates new bone joining characteristics while also providing the surgeon with the best possible feel and torque sequence during initial threading.

The facets on the sides of the fastener may also include a mechanism to resist swelling of bone tissue adjacent to the fastener parallel to the facets in response to loads. With a thread angle of 0 degrees, the primary redirecting force vector is caused by the reaction to the larger helix angle. Therefore, the facets' resistance to loads parallel to the facets is shear, and this resistance can be enhanced by a series of serrations applied to the facets.

Preferably, the serrations are configured as a series of tooth-like protrusions and recesses extending helically or concentrically on the face of the facet. Cancellous bone is elastic, and this elasticity varies with position and patient age. During installation, the cancellous bone is slightly compressed and, when decompressed after installation, pushes into the serrations, becoming embedded within them, thereby resisting the shear forces of the cancellous bone as it bulges under load. The component of force applied parallel to the facet surface encounters resistance caused by the serrations.

The cutting head of the present invention is also equipped with a second, active cutting site, distinct from the previously mentioned cutting site. These screws often require removal, and in some cases, extraction is difficult due to subsequent bone growth. The second cutting site is active only during the removal process and removes bone that has formed around the screw.

Purpose of the Invention

An object of the present invention is to provide an improved cancellous bone screw fastener.

Another object is to provide a better feel for the surgeon by lowering the insertion torque and reducing the resistance to installation, thereby providing lower friction and faster insertion.

Another object is to provide a fastener which is suitable for mass production technology.

Another object is to provide a fastener that exhibits enhanced holding power in cancellous bone while minimizing bone trauma.

Another object is to increase the volume of bone that receives the fastener.

From a first perspective, an object is to provide a fastener having threads with upper and lower sides that are substantially parallel to one another, in order in particular to minimize bone displacement of the anchoring.

From another advantageous perspective, an object of the present invention is to provide a cancellous bone fastener having double-start threads.

Viewed from another advantage, an object is to provide a fastener having at least two helical thread flights, each helical thread having a different crest diameter.

Another object of the present invention is to form a profile of a bone whose cross section looks like a square wave sawtooth.

Viewed from another advantage, it is an object of the present invention to provide a fastener having an improved lead angle and helical thread which, in combination with the previously enumerated objects, results in a beneficial force distribution under load.

Viewed from another advantage, it is an object of the present invention to provide a fastener having self-tapping tips that advance excised bone fragments into an integrally formed groove and away from the threads of the fastener.

Viewed from another advantage, it is an object of the present invention to provide a fastener having a substantially zero thread angle.

Viewed from another advantage, it is an object of the present invention to provide a threaded fastener having side facets that are substantially parallel to one another.

Viewed from another advantage, it is an object of the present invention to provide a fastener in which the root diameter of a threaded portion is substantially equal to a diameter of an unthreaded shaft portion.

Viewed from another advantage, it is an object of the present invention to provide a helix angle greater than that of a single-start threaded fastener.

Viewed from another advantage, it is an object of the present invention to provide a tool head that facilitates fastener removal and installation.

From another advantage, an object of the present invention is to provide resistance to cancellous bone bulging under load by increasing the surface area between the bone and the thread facets. One way to increase the surface area and increase the resistance to bulging and the resulting shear is to modify the surface of these facets.

BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

FIG. 5 is a different perspective view of FIG. 3 and FIG. 4 .

FIG. 6 is a perspective view of a sawtooth "V" shaped groove on a facet for resisting shear.

FIG. 7 is a cross-sectional view of the fastener taken along its longitudinal axis.

DETAILED DESCRIPTION

Referring to the drawings, wherein like numbers refer to like parts, reference numeral 10 designates a fastener (screw) of the present invention. Screw 10 has an elongated shaft 13 that is substantially cylindrical along its length and has a linear longitudinal axis 100 at its center. The screw may have a head 6 having a drive face 5 formed on its top surface. Face 5 may be contoured to accommodate a drive socket, screwdriver tip, "torx" fitting, allen wrench, etc., to advance the screw.

Multiple helical threads are contemplated, and the accompanying drawings illustrate a two-thread embodiment. A first helical thread having a first thread 12 exhibits a larger crest diameter than a second helical thread having a second thread 24. Preferably, the two threads are spaced 180 degrees apart and share the same pitch. Thus, FIG. 7 shows that, in cross-section, a first, larger crest thread 12 is diametrically opposed to a second, smaller crest thread 24 throughout the entire thread path. Bone "teeth" (cancellous bone) are retained between the threads.

Unlike bone retention by a buttress thread (which simply consists of a "V" shape with a constant dimension from root diameter to crest diameter along the entire fastener), the bone teeth 25 retained in the present invention create a sawtooth or blunt-serrated appearance. Importantly, the high and low areas of the bone teeth are also 180 degrees opposite each other along the entire thread pattern, so that an area of minimum bone volume is diametrically reinforced by its corresponding area of maximum bone volume at 180 degrees.

When faced with loads acting on the fastener, the opposing bone teeth and the larger/smaller crest threads work together to oppose these loads and distribute the load into harmless (manageable) vectors throughout the cancellous bone. In fact, the applied load helps circulate blood throughout the cancellous bone.

Due to the double-start thread structure, the helix angle 20 ( FIG. 7 ) of a double-start thread like this is significantly greater than that of a single-start thread by a certain amount (typically, just under a factor of 2). This helix angle dictates the slope (or gradient) of the threads 12 , 24 as they spiral about the axis 13 . For example, a single-start thread may have an 11-degree helix angle, while the double-start helix angle 20 of the present invention would be 21 degrees (measured from the “horizontal plane,” i.e., transverse to the major axis 100 ). Similarly, the loads faced by the fastener also encounter this larger helix angle and are dispersed into harmless vectors into a larger volume of cancellous bone than would occur in the prior art.

Each thread is formed with a side having an upper facet (closer to the proximal end adjacent to the head 6) and an opposite lower facet. Thread 12 has an upper facet 21 and a lower facet 22; thread 24 has an upper facet 28 and a lower facet 27. Figure 7 reveals facet pairs (21, 22) and (27, 28) that enjoy a substantially parallel relationship. That is, facets 21, 22, 27, 28 are all parallel to each other and substantially perpendicular to the long axis 100. This structure is particularly good at distributing loads parallel to the long axis 100, and the crest diameters of these threads are well suited for loads parallel to but axially offset from the centerline long axis 100, primarily due to the greater volume of cancellous bone retained and its interaction with the different crest diameters. As shown, thread 12 enjoys a larger crest diameter than thread 24.

The torsional load and other loads that are not parallel to the long axis are resisted by the following items: the serpentine line of the cancellous bone that increases in volume around this fastener, the larger helix angle of these thread patterns and the space between adjacent sides.It should be noted that shaft 13 is inserted between the sides.This cancellous bone does not encounter a " V " (as in the sawtooth thread) at the intersection of the facets, but encounters a smooth wide area (expanse) of cylindrical shaft 13.Therefore, there is no " V-shaped " pressure point at this position, and instead there is a smooth tangential registration area between bone and screw.

Since the side facets are parallel to each other, the thread angle (the angle formed by extending adjacent upper and lower facets) is zero. Therefore, there is no pressure point (additional vector) on the cancellous bone caused by this angle.

With a zero thread angle and parallel facets, the load transferred between the cancellous bone and the facets of the lateral surface is shear; that is, parallel to their junction. Resistance to shear loads can be enhanced by texturing the lateral surface, an example of which is shown in FIG6 . Serrations 40, here embodied as a series of "V"-shaped grooves 42, are embedded in one or more facets 21, 22, 27, and 28. These grooves receive cancellous bone 25 therein.

As described above, cancellous bone can both compress and expand. During installation, the bone is slightly compressed, after which it expands and fills the grooves 42. Shear loads (parallel to the facet surfaces) are countered by the increased friction caused by this structure. The grooves 42 can form a spiral, can be concentric, or alternatively can be viewed as simply a textured, knurled, or matte surface. Thus, the protrusion of the cancellous bone is countered by this increased friction.

4 and 5 show the guide 59, the cutting head 200, and the concave bone fragment retaining groove 55, which is substantially parallel to the long axis 100 of the screw 10 on an annular outer surface of the cylindrical shaft 13. The guide 59 features a tapered tip that transitions to the cylindrical shaft 13 at an angle, creating a radial guide portion 60. The primary bone cutting head 200 originates on the "left-hand" side of the groove 55. This means that as the fastener is advanced into the bone (conventionally via "clockwise" or "right-hand" rotation), the active surface of the cutting head 200 removes the cutting head. Each cutting head 200 has an active helical surface that extends only a short distance (one or more threads) toward the proximal end of the screw.

The actual cutting of the bone fragments by the cutting head 200 is caused by a sharp cutting leading edge 53 formed at the bottom of the cutting head 200, and has a slightly blunt cutting trailing edge 51. Therefore, a concave groove 55 placed near the cutting head 200 receives the fragments cut off by the blunt trailing edge 51 from the sharp cutting leading edge 53. The grooves 55 are each strategically placed at the bottom of the cutting head. The grooves 55 look like very shallow elongated ovals, with curved end walls 49 on which the grooves 55 taper upward to the shaft 13, and parallel linear side walls 48 between which the grooves 55 have their maximum depth.

The groove is sized to receive only a thin layer of bone fragments. This allows the uncut adjacent cancellous bone an opportunity to absorb these fragments and minimizes the likelihood of thicker fragments being deposited, which could otherwise cause necrosis due to lack of blood circulation. The sharp edge 53 and the blunt edge 51 combine to form a roughly claw-like profile. The fragments curl from the cutting edge and transition along path 57 to the groove 55.

4 includes (on one opposite end of the cutting head 200) at least one sharp cutting surface 56 that is effective only when the fastener is rotated to remove it (in this example, counterclockwise.) Historically, fasteners have sometimes been difficult to remove where a site has been subject to active restorative bone growth, and this cutting head 56 facilitates easier removal of the fastener.

Preferably, three cutting tips 200 are used to form the bone teeth 25 by removing bone fragments and placing them in the grooves 55. The cutting tip closest to the distal end is the first contributor to forming all of the threads 12, while the next other cutting tips closer to the proximal end cooperate to remove additional fragments, thereby providing clearance for the smaller diameter threads 24 and ultimately the larger diameter threads 12.

These features combine to form a clean-cutting thread and move the debris along a debris path 57, pushing the debris into an adjacent groove 55 and out of the path of the advancement screw as it spirals into the bone. The leading edge 61 of the groove 55 at the shaft (root) 13 is sharp and forms a precise fit within the inner diameter of the pre-drilled pilot hole by providing an active scraping action as the screw 10 is rotated into the pre-drilled pilot hole. This scraping action pushes the debris downward into the groove 55. The benefit here is that the cutting edges 51, 53 cause the debris to curl forward and follow the contour of the groove 55 toward and along the path 57 away from the advancement threads.

In other words, as the fastener is inserted clockwise (CW), the cutting edge 53 gradually scrapes away thin strips of bone, which are then severed by the trailing edge 51, causing the fragments to advance forward and be pushed by the cutter head 200 in direction 57 over the scraping edge 61 of the groove 55. This creates a true self-tapping screw and also prevents debris from being drawn into the advancing screw threads and bone. This results in much lower cutting pressures, cleaner threads, and less damage to the bone. (If the debris cannot escape, it will be drawn into the path of the threads and pressed into the surrounding bone. If this occurs, the trapped fragments may trigger an inflammatory process, which causes the immune system to attack the debris as foreign matter during final absorption, resulting in gaps near the threads and ultimately loosening of the screw threads 12 and 24.)

The guide 59 has a substantially rounded arcuate distal end 4 as shown in the drawings. The tapered transition of the guide 59 includes a guide portion 60 leading to the shaft 13. A guide 59 provides a transition between the arcuate distal end 4 and the cylindrical shaft 13 via the cylindrical guide section 60. The arcuate distal end 4, the angled transition 59, and the guide portion 60 force the fastener 10 to remain in the pre-drilled pilot hole and help the fastener find the pilot hole when passing through an opposing sidewall or bone portion, thereby ensuring registration of the fastener 10 through all subsequently drilled pilot holes in the bone, thereby ensuring maximum purchase strength and pullout strength.

In use, a pilot hole is preferably pre-drilled, and the fastener 10 is oriented over the pilot hole. Angle transition 59 (the curved end of guide 59 and cylindrical guide portion 60) nests within the pre-drilled pilot hole. Advancing the fastener in a clockwise (CW) rotation causes the (left-hand) leading cutting edge 53 to incrementally scrape bone fragments away from the fastener. After being shredded by trailing cutting edge 53 and assisted by edge 61 on the periphery of groove 55, trailing edge 51 pushes these fragments downward along path 57 into the recessed portion of groove 55. Thus, the bone is threaded according to the fastener's tooth profile, creating bone teeth 25. As the fastener 10 is advanced into the bone, bone teeth 25 provide positive engagement with the fastener 10 without causing the surgeon, who may use very little force to advance the fastener (unlike the prior art), to experience noticeable friction (harmful heat buildup) or unwanted radial forces.

This provides the surgeon with accurate information about the progress of the operation.The guide portion 60 on the guide accurately tracks the pre-drilled hole without harmful migration and trauma to adjacent bones.

Unlike the prior art, this contact causes a change in force that is perceptible to the surgeon. Thus, the surgeon has a better "feel" to sense and adjust the compression/torque that is most beneficial for the procedure. Incidentally, the same improved tactile feedback exists when the fastener does not have a head but is countersunk instead.

While one illustrative form of the invention has been described and taught thus, it should be understood that modifications are contemplated as a part of the invention as defined by the appended claims.

Claims (15)

1. A medical fastener, comprising: A slender shaft having a proximal end and a distal end; A first thread is helically wound around the shaft, the first thread having small planes defining an upper side surface and a lower side surface, the upper side surface and the lower side surface being oriented in a substantially parallel relationship, and A self-tapping tip has a claw-shaped cutting element with a sharp leading edge and a blunt trailing edge, the sharp leading edge and the blunt trailing edge being combined to form the claw-shaped cutting element, the self-tapping tip having a groove operatively communicating with the claw-shaped cutting element to receive bone fragments in the groove. 2. The fastener of claim 1, further comprising a second thread helically wound around the shaft. 3. The fastener of claim 1, wherein a first self-tapping tip is formed on the distal end for removing bone fragments during fastener installation. 4. The fastener of claim 3, wherein the groove is adjacent to the blade and is used to receive the bone fragments in the groove. 5. The fastener of claim 3, wherein the first self-tapping tip supports a removal tip to cut bone during fastener removal. 6. The fastener of claim 1, wherein the facets include means for resisting shear forces applied to the cancellous bone adjacent to the facets. 7. The fastener of claim 1, comprising a thread angle that is substantially zero. 8. The fastener of claim 2, wherein a second self-tapping tip is disposed on a distal end of the second thread. 9. The fastener of claim 8, comprising a third self-tapping tip located at the distal end of one of the threads. 10. The fastener of claim 6, wherein the means for resisting shear force is embodied as serrations disposed on at least one of the small planes. 11. The fastener of claim 1, comprising a guide located at the distal end. 12. The fastener of claim 2, wherein the second thread is oriented such that its facet is substantially opposite to the facet of the first thread in diameter. 13. The fastener of claim 12, wherein the second thread facets are substantially parallel to each other. 14. The fastener of claim 13, wherein the first thread has a crest diameter larger than that of the second thread. 15. A medical fastener comprising: a self-tapping head having a claw-shaped cutting element having a sharp leading edge and a blunt trailing edge, the sharp leading edge and the blunt trailing edge being combined to form the claw-shaped cutting element, the self-tapping head having a groove operatively communicating with the self-tapping head to receive bone fragments in the groove.