Patent Description:
The invention further relates to a set comprising at least two dental self-tapping devices.

The term dental self-tapping device is understood to mean a dental implant as well as a dental tapping drill bit for pre-drilling a bore for a dental implant.

When placing dental implants, it is of paramount importance that the implant is properly anchored in the bone. For this purpose, so-called self-tapping implants are used.

Self-tapping implants are designed to be screwed directly into the bone during surgery. First the implantation site is exposed then a hole is drilled into the bone. The implant and a driver mount connected thereto are moved to the implantation site and the apical end of the implant is driven into the bore. As the implant is driven (rotated), it simultaneously taps threads in the bone and is thereby screwed into these threads.

A typical self-tapping implant has an apical end which taps the threads into the bone and a proximal end for connection with an abutment designed to support a dental prosthesis. The apical end is usually provided with one or more flutes provided with a cutting edge.

The implant body generally comprises an internal passage which is on the one hand designed to cooperate with a connection portion of a cover- or healing screw, and on the other hand - after removing the healing screw - with an abutment designed to support a dental prosthesis.

The self-tapping flute is conventionally created by milling parallel to the longitudinal axis of the implant. A flattened surface is formed on each side of the flute where the flute meets the thread formed on the external surface of the implant. As the implant is driven into the bore, the bone chips collect in the flute and the flattened surface compresses the bone chips as the bone chips are pushed forward, whereby the bone chips do not need to be removed, moreover, the compressed bone chips entering the flute increase stability.

However, the inventor of the present invention recognised that, despite the above function, the flattened surface of the conventional straight flute also has a disadvantage, as the flattened surface does not cut the bone while the implant is driven, instead it fractures it, making it more difficult for the living bone tissue to heal and osseointegrate with the implant.

Spiral flutes are also used, but in case of the existing implants the spiral flutes only slightly improve the self-tapping properties of the implant. An example of an implant with a spiral flute is shown in <CIT>. According to this solution the cross-section of the flute perpendicular to the longitudinal axis of the implant has a wider inner portion and a bottle neck shaped outer portion, which the patentee in his commercial activities refers to as the shape of a lion's claw. The inventor of the present invention, however, recognized that the lion claw-like design (bottle neck profile) is rather disadvantageous, since the internally curved edges do not participate in the self-tapping, here the cutting surface that is perpendicular to the tangential direction of travel of the edges during screwing of the implant is reduced, since the edges are facing inwardly (in the direction of the axis of the flute), rather than in the tangential direction of travel during rotation.

Other commercially available implants, such as Alpha Bio-Tec's Spiral™, use a slightly spiral flute, but the spiral is extremely steep and lies at a very small angle with the axis of the implant, whereby there is no cutting edge instead, it serves to guide the implant during screwing.

Traditionally, even with self-tapping implants, the implant site is pre-drilled with successively larger diameter dental drills before implantation, and the implant is screwed into a pre-drilled bore close to or of the same diameter as the implant <NUM> diameter. The gradual expansion of the diameter of the drill bore with several drills is important for several reasons. Firstly, the self-tapping properties of the threads of conventional dental drills are not good enough. Secondly, if a drill with a larger diameter is used at the start, more heat is generated due to the frictional forces, causing the bone tissue to overheat, which will lead to bone necrosis and subsequently to the inability of the dead surrounding bone tissue to osseointegrate with the implant. To avoid this, in harderjawbones, typically <NUM> to <NUM> drills are used to create the desired bore to match the diameter of the implant. This requires both a lot of tools (dental drills) and makes the implantation procedure cumbersome and lengthy, which is uncomfortable for both the dentist and the patient. In addition, the many successive drillings also impose a significant thermal load on the bone, as there is typically no time to wait for the bone to cool down between each drilling, leading to necrosis of the living bone tissue, which also inhibits osseointegration.

<CIT> provides a dental implant, having a body portion, a first thread portion, a head portion, a plurality of second thread portions and a plurality of trenches. The body portion of the dental implant defines a drilling axial direction in a longitudinal direction thereof. The trench of the dental implant is formed at a radial side of the head portion and the body portion, and passes through the cutting faces and the cumulative faces from the head portion to the first thread portion. <CIT> shares with the invention the concern for relieving pain and speed wound healing - paragraph [<NUM>] and to reduce the drilling steps- paragraph [<NUM>].

The inventor of the present invention recognized that the self-tapping property of the implant could be enhanced by creating a cutting edge with a large surface area instead of a flattened surface at the location where the flute meets the thread. The surface area of the cutting edge depends on the angle of inclination of the flute, i.e. the angle between the flute and the implant axis, and the depth of the flute. The inventor of the present invention recognized that in order to obtain a suitable cutting edge, a minimum inclination angle of <NUM> degrees is required (wherein the angle is measured from the direction of the neck portion of the implant).

The inventor of the present invention also recognized that as the depth of the flute is increased, the diameter of the flute must be increased as well so that the flute widens continuously outwardly (i.e. away from the central axis of the implant) and the flute's cross-section should not have a bottle neck or lion's claw-shaped outer portion, as this reduces the cutting edge and worsens the self-tapping property rather than improving it. In addition, it is difficult to create a bottle neck profile from a manufacturing point of view.

<CIT> has a first disadvantage that it does not provide enhanced cutting edge with the large surface instead of a flattened surface, because the third objective of <CIT> see paragraph [<NUM>] departs from the findings of the invention, because <CIT> does not aim to enhance the cutting edge of the self-tapping device, as the invention does, but rather to better guide bone scraps to approach the defeat portion which lacks of bone, so as to reduce the requirement for filing the bone graft materials.

The inventor of the present invention also recognized that it is sufficient to pre-cut a thread corresponding to the diameter of the implant in the harder cortical bone tissue of the jaw bone, and to create a smaller diameter pre-drilled bore in the underlying softer spongiosa bone tissue in case an implant with an enhanced self-cutting flute according to the invention is to be screwed therein. By making a narrower bore in the spongiosa bone tissue, the thermal stress on the bone tissue and thus the risk of necrosis of the bone can be reduced. The inventor has recognized that it is preferred to form a thread and self-tapping flute in only a proximal portion of a dental drill bit corresponding to a thread and self-tapping flute in the proximal threaded part of the dental implant according to the invention, since this is sufficient to allow a thread to be cut for the implant in the harder cortical bone tissue of the bone prior to implantation of the implant according to the invention.

<CIT> has a second disadvantage that it does not provide for optimization of the self-tapping within the meaning explained in the precedent paragraph, said optimization referring to how the self-tapping device must be adapted for both the harder cortical bone tissue and the underlying softer spongiosa bone tissue.

Based on the above findings, the objective of the invention is to provide a dental self-tapping device that is free from the disadvantages of the prior art solutions. The objective of the invention is in particular a dental self-tapping device that is optimized for self-tapping.

According to the invention, the above objectives were achieved by a dental self-tapping device comprising a body having a longitudinal axis t, the body having a proximal neck portion N, a proximal-central straight portion S of substantially constant diameter and a distal conical portion C of decreasing diameter, the body comprising a core and a thread provided thereon, a flute is formed in the thread, which flute runs spirally around the core and an axis of the flute is at an angle with the longitudinal axis. The angle measured from a proximal end of the body is <NUM> to <NUM> degrees, preferably <NUM> to <NUM> degrees, and the diameter of the flute in at least a portion of the body is <NUM> to <NUM>%, preferably <NUM> to <NUM>%, of the diameter of the body.

Since the flute spirals around the core, the axis of the flute also runs in a spiral shape and varies from point to point. The angle between the tangent of the axis and the longitudinal axis of the body is measured by shifting the tangent or the longitudinal axis of the body parallel to itself so that the tangent and the longitudinal axis intersect.

The shape of the cross-section of the flute perpendicular to the axis of the flute is preferably a circular arc having a centre angle of maximum <NUM> degrees. This means that the width of the flute increases monotonically from the direction of the core outwards and does not have a glass neck profile, thus maximising the efficiency of the cutting edge during the driving of the dental self-tapping device.

The flute is preferably formed with a spherical rotary burr.

The dental self-tapping device preferably comprises an even number of flutes - preferably two or four flutes - spaced evenly around the outer surface of the body.

The angle of the axis of the flute with the longitudinal axis is constant or varying along the longitudinal axis.

The dental self-tapping device can be a dental implant or a dental bone tapping drill bit.

In case of a dental implant, the outer diameter of the threads on at least the proximal half, preferably on at least the proximal two-thirds of the threaded part of the body is constant, and at least the distal third of the threaded part of the body tapers conically towards the distal end of the dental implant <NUM>.

If the dental self-tapping device is a dental bone tapping drill bit, it comprises a wider diameter proximal portion <NUM> having a first diameter and a narrower diameter distal portion <NUM> having a second diameter smaller than the first diameter, wherein the thread and spirally running flute are formed on the wider diameter proximal portion <NUM>.

The wider diameter proximal portion <NUM> of the dental bone tapping drill bit preferably has a length of <NUM>-<NUM>, more preferably <NUM>-<NUM>, and a diameter of <NUM>-<NUM>.

The narrower diameter distal portion <NUM> preferably has a length of <NUM> to <NUM>, a diameter of less than <NUM>, preferably less than <NUM>, more preferably not more than <NUM>.

The invention also relates to a set of dental self-tapping devices, according to Claim <NUM>.

Further details of the invention are described by way of exemplary embodiments with reference to the drawings, wherein:.

<FIG> show different views of an embodiment of a dental self-tapping device according to the invention, which is a dental implant <NUM> in this example. The dental implant <NUM> has a body <NUM> with a longitudinal axis t. The body <NUM> comprises a core <NUM> and thread <NUM> formed thereon, as well as a proximal neck portion <NUM>, N without a bone-cutting thread. In the context of the present invention thread <NUM> means one or more threads, for example, single thread <NUM> or double threads <NUM> or triple threads <NUM> can run around the core <NUM> on the body <NUM>. The design of the threads <NUM> can be any design, for example so-called buttress thread design.

At least one flute <NUM> forming an angle α with the longitudinal axis t and running spirally around the body <NUM> is formed in the thread <NUM>, preferably with a spherical rotary burr <NUM>. The implant <NUM> preferably contains an even number of flutes <NUM> - preferably two or four flutes <NUM> - which are evenly spaced from each other around the outer surface of the body <NUM>. This eliminates the tendency for the implant <NUM> to tilt during screwing, which would result in the implant being placed at an angle to the desired position.

Preferably, the implant <NUM> is formed from a template body illustrated with a dashed line, the template body having a proximal neck portion <NUM>, N, a proximal-central straight portion S of substantially constant diameter and a distal conical portion C of decreasing diameter. The thread <NUM> is cut into this template body <NUM>', a flute groove 16a of which delimits the core <NUM>. An embodiment is also conceivable in which the entire part apart from the neck portion <NUM>, N of the template body <NUM>' is conical, but the advantage of the proximal-central straight portion S is that after the implantation of the implant <NUM>, the straight portion S does not exert a tensioning, wedging effect on the surrounding bone tissue, while the distal conical portion C facilitates the driving of the implant into the bone tissue. In the case of the finished implant <NUM>, it is particularly advantageous if the outer diameter of the threads <NUM> is constant on at least the proximal half of the threaded part of the body <NUM> (this is the largest diameter d<NUM>) and at least the distal third of the body <NUM> tapers conically towards a distal end <NUM> of the implant.

The flute <NUM> can be formed on the threaded body or on the template body <NUM>' before cutting the threads <NUM>, as illustrated in <FIG>. In <FIG> an internal threaded channel 13a can be observed in the cross-section of the template body <NUM>' which serves for connecting a healing screw and later an abutment.

The diameter d<NUM> of the neck portion <NUM>, N is chosen to accommodate a standard size internal threaded channel <NUM>. For example, the diameter of the neck portion <NUM>, N is approximately <NUM>. Consequently, the diameter d<NUM> of the neck portion <NUM>, N of the finished implant <NUM> is about <NUM>, which is also the maximum diameter of the core <NUM>.

The maximum d<NUM> diameter of the portion of the template body <NUM>' apart from the neck portion <NUM>, N (in this case the diameter of the straight portion S) is the maximum diameter d<NUM> of the body <NUM> of the implant <NUM>, which corresponds to the maximum diameter of an enveloping surface of the thread <NUM>. This is chosen according to the implantation site. Implants are usually manufactured with a range of diameters d<NUM>. The inventor of the present invention has found that the various needs can be well covered by selecting the diameter d<NUM> of each of the at least one dental implant <NUM> is selected from a respective list of diameters, each list comprising at least three diameters d<NUM> for each of the at least one dental implants <NUM> that forms an increasing series with a step interval of at least <NUM>, preferably with a step interval of <NUM> to <NUM>, more preferably with a step interval of <NUM>, and wherein the cores <NUM> of the dental implants are substantially identical for all the selected diameters d2 from the list. For example, at least three dental implants <NUM> with the maximum external diameters d<NUM> of the bodies <NUM> form an increasing series with a step interval of at least <NUM>, preferably with a step interval of <NUM> to <NUM>, more preferably with a step interval of <NUM>, and all the afore-mentioned dental implants <NUM> having substantially the same core <NUM>. For example, the embodiment in <FIG> where d<NUM> = <NUM>, the embodiment in <FIG> where d<NUM> = <NUM> and the embodiment in <FIG> where d<NUM> = <NUM> form such a series, where the diameter d<NUM> of the neck portion18, N in all three embodiments is <NUM>.

The length L of the implant also depends on the implantation site, the figures show medium length implant <NUM> shown having a length L of <NUM>, but the implant <NUM> according to the invention can also be produced with a different length L if required.

Due to the spiral shape of the flute <NUM>, a cutting edge 22a and 22b is created on each side of the flute <NUM> on the thread <NUM>. Thus, when the implant <NUM> is rotated clockwise and counter clockwise, one of the cutting edges 22a, 22b is cutting. The cutting edge 22a is performing cutting in case of clockwise rotation, which is therefore called the forward cutting wing, while counter clockwise rotation causes the cutting edge 22b to cut, which is therefore called the reverse cutting wing.

The size and thus the efficiency of the cutting edges 22a, 22b depends on the angle α and the diameter of the flute <NUM>.

The angle α formed by the flute <NUM> and the longitudinal axis t of the implant <NUM>, measured from the proximal end of the body <NUM> (i.e. from the end at the neck portion <NUM>, N), is, between <NUM> to <NUM> degrees, for example <NUM> to <NUM> degrees, or <NUM> to <NUM> degrees. The angle α along the longitudinal axis t may be constant or varying. The angle α may be measured at any distance from the distal end <NUM> of the implant <NUM>. The direction of the axis of the flute <NUM> varies from point to point as a function of the distance, since the flute <NUM> spirals up the body <NUM> of the implant <NUM>. Consequently, the angle α shown in <FIG> is illustrative, on the one hand, the longitudinal axis t and the axis of the flute <NUM> define skew lines which can be made to intersect by parallel translation, and on the other hand, the axis of the flute <NUM> is at a different angle to the plane of the sheet of <FIG> from point to point along the spiral line of the flute <NUM> around the core <NUM>. In <FIG>, one leg of the angle α (shown vertically) is the longitudinal axis t of the body <NUM>, and the other leg is the tangent line of the axis of the flute <NUM> in the projection corresponding to the side view of <FIG>, since the tangent line also forms an angle with the plane of <FIG>.

The diameter of the flute <NUM> in at least a portion of the body <NUM>, preferably in the apical (distal) two-thirds portion, is <NUM>-<NUM>%, preferably <NUM>-<NUM>%, of the diameter d<NUM> of the body <NUM>. In the proximal portion close to the neck portion <NUM>, N the thread grove 16a of the thread <NUM> is less deep in order to allow sufficient wall thickness in the remaining core <NUM> despite the proximal threaded channel <NUM> provided for the attachment of the abutment. For this reason, the flute <NUM> in this portion is not a continuous trough-shaped groove, but is formed by flute portions 20a at the outer periphery of the thread <NUM>, referred to as micro flutes. In the central portion following the proximal portion there is no such obstacle, therefore here the diameter of the flute <NUM> is preferably <NUM>-<NUM>% of the diameter d<NUM> of the body <NUM>, preferably <NUM>-<NUM>%.

In order to provide the maximum cutting edge 22a, 22b area, the width of the flute <NUM> (i.e. the dimension of the flute <NUM> perpendicular to its axis) increases monotonically outwardly from the direction of the core <NUM>, in other words, the cross-section of the flute <NUM> perpendicular to the axis of the flute <NUM> has a circular arc shape (see in particular <FIG> and <FIG>), the central angle of the circular arc being at most <NUM> degrees.

This means that in the case of the flute <NUM> of the invention, there is no bottle neck in the flute <NUM> profile.

The flute <NUM> of the implant <NUM> is preferably formed with a spherical rotary burr, which is illustrated by the ball marked with the reference number <NUM> in <FIG>, <FIG> and <FIG>. It can be seen that in the implant <NUM> having larger diameter d<NUM> and comprising the thread <NUM> of greater depth, the flute <NUM> is fraised with a spherical rotary burr <NUM> having a larger diameter d<NUM>, since the diameter d<NUM> of the spherical rotary burr <NUM> gives the diameter (maximum width) of the flute <NUM>. Preferably, the diameter (maximum width) of the flute <NUM> is the same as the diameter d<NUM> of the <NUM> spherical rotary burr <NUM> with which the flute <NUM> is formed. For this, when milling the flute <NUM>, the spherical rotary burr <NUM> is pressed into the thread <NUM> at a depth equal to its radius (that is, half of the diameter d<NUM>). This can also be determined afterwards, from the examination of the flute <NUM>. The radius of the spherical rotary burr <NUM> (which is half the diameter d<NUM>) is ideally chosen so that it is not greater than the thread depth of the thread <NUM>, and when milling the flute <NUM>, the spherical rotary burr <NUM> is preferably pressed into the thread <NUM> at a depth equal to its radius at most, so that the flute <NUM> is made in the thread <NUM> and not in the core <NUM>. The inner part of the flute <NUM> is not expanded by the spherical rotary burr <NUM>, i.e. the cross-section of the flute <NUM> perpendicular to the axis of the flute <NUM> is in the shape of a circular arc corresponding to the circumference of the spherical rotary burr <NUM>, the centre angle of which circle is at most <NUM> degrees due to the fact that the spherical rotary burr <NUM> is pressed in at a depth corresponding to its radius at most into the thread <NUM>, thus no glass neck is formed at the outer edge of the thread <NUM>, instead the width of the flute <NUM> increases monotonically outwardly in a direction substantially perpendicular to the axis of the flute <NUM>, whereby a maximum cutting edge 22a, 22b area can be provided.

<FIG> illustrates another embodiment of the dental self-tapping device according to the invention, which is a dental bone tap drill <NUM>. The dental bone tap drill <NUM> has a body <NUM> with a wider diameter proximal portion <NUM> having a core <NUM> and a thread <NUM> formed thereon, and a narrower diameter distal portion <NUM> connected to the wider diameter proximal portion <NUM>. The thread <NUM> of the wider diameter proximal portion <NUM> is at an angle β with the longitudinal axis T of the body <NUM>, and includes at least one, preferably two or four flutes <NUM> spiralling around the body <NUM>, such as described in relation to the preceding embodiments.

The length of the wider diameter proximal portion <NUM> of the dental bone tap drill <NUM> is preferably <NUM>-<NUM>, more preferably <NUM>-<NUM>, and its diameter is preferably <NUM>-<NUM>. Preferably, the external design of the wider diameter proximal portion <NUM> of the tap drill <NUM> is substantially identical to the external design of the wider diameter proximal portion <NUM>, preferably the proximal third, of an implant <NUM> according to the invention, so that the tap drill <NUM> can be used to tap a thread into the jaw bone prior to implantation for the wider diameter proximal portion <NUM> of the corresponding implant <NUM>.

The narrower diameter distal portion <NUM> is preferably a guide shaft, the length of the guide shaft is selected such that the combined length of the wider diameter proximal portion <NUM> and the narrower diameter distal portion <NUM> is substantially equal to the length L of the implant <NUM> for which the tap drill <NUM> is used to cut the threads prior to implantation. Accordingly, the length of the narrower diameter distal portion <NUM> is typically between <NUM> and <NUM>, depending on the length L of the associated implant <NUM>. Preferably the diameter of the narrower diameter distal portion <NUM> is less than <NUM>, more preferably less than <NUM>, more preferably up to <NUM>, for example <NUM>, so that it can be easily inserted into a bore predrilled by a conventional pilot drill.

The process for implanting the implant <NUM> according to the invention is illustrated in <FIG> shows a cross section of the patient's lower or upper jawbone <NUM>. The outer thicker cortical tissue <NUM> is typically <NUM> - <NUM> thick, in extreme cases up to <NUM>. It is denser and harder in texture compared to the underlying softer spongiosa tissue <NUM>.

For the implant <NUM> of the invention, a conventional pilot drill with a diameter of about <NUM> can first be used to create a primary bore <NUM> having a depth equal to the length L of the implant <NUM> (<FIG>). In the next step, a wider secondary bore <NUM> (<FIG>) is formed, preferably with a three-stage step drill bit, at the location of the primary bore <NUM>. The three-stage step drill bit has a distal part <NUM> having the smallest diameter, preferably corresponding to the diameter of the pilot drill, which is typically <NUM> - <NUM> (e.g. about <NUM> in diameter), its middle part <NUM> has a diameter of a few tenths of a mm larger (preferably <NUM> - <NUM> larger in diameter, e.g. about <NUM> in diameter), and its proximal part <NUM> has a further few tenths of a mm larger diameter (being preferably <NUM> to <NUM> larger in diameter, for example having a diameter of about <NUM>).

If the outer thicker cortical tissue <NUM> of the patient's jawbone <NUM> is relatively thin (e.g., between <NUM> and <NUM>), the implant <NUM> according to the invention comprising the thread <NUM> with the flute <NUM> can be screwed into the secondary bore <NUM> without the use of an additional pre-drill due to its increased self-tapping ability.

In case of harder bone, i.e. outer thicker cortical tissue <NUM>, it is preferable to use the tap drill <NUM> of the invention to drill a tertiary bore <NUM> (<FIG>). The narrower diameter distal portion <NUM> of the tap drill <NUM> is a guide shaft, the diameter of which preferably does not exceed the diameter of the distal part <NUM> of the secondary bore <NUM> formed by the step drill bit, so that when the drill bit is inserted, the distal end <NUM> does not cut the bone, but only serves to position the wider diameter proximal portion <NUM>. In <FIG>, for illustrative purposes, the wider diameter proximal portion <NUM> of the tap drill <NUM> is shown as having a smaller diameter than the proximal portion of the tertiary bore <NUM> in order to allow the wider diameter proximal portion <NUM> of the tap drill <NUM> to be visible within the tertiary bore <NUM>, but in reality, the proximal portion of the tertiary bore <NUM> is formed by the wider diameter proximal portion <NUM> of the tap drill <NUM>. The tap drill <NUM> cuts a thread (and creates a diameter) in the outer thicker cortical tissue <NUM> corresponding to the wider diameter proximal portion <NUM> of the implant <NUM> to be inserted, so that when the implant <NUM> is inserted, the wider diameter proximal portion <NUM> of the implant <NUM> can be screwed into the tertiary bore <NUM> without substantial cutting of the outer thicker cortical tissue <NUM>. The implant <NUM> only taps significantly into the softer spongiosa tissue <NUM> while creating a fourth bore <NUM> corresponding to the external size of the implant <NUM>. The inventor of the present invention has recognized that it does not cause any problem to significantly widen the diameter of the fourth bore <NUM> in the softer spongiosa tissue <NUM>, as the softer spongiosa tissue <NUM> is easier to cut, and the resulting bone chips can be partially removed from the inside of the wider diameter fourth bore <NUM> by compressing the surrounding softer spongiosa tissue <NUM>, and can be partially compressed in the flutes <NUM> of the implant <NUM>. Thus, even with the outer thicker cortical tissue <NUM> (up to <NUM>-<NUM> thick), the conventional pilot drill, the three-stage step drill bit <NUM> described above and the tap drill <NUM> of the invention are sufficient for inserting the implant <NUM> when using the dental implant <NUM> and the dental tap drill <NUM> according to the invention, as opposed to the state-of-the-art implantation protocol, where more pre-drills of up to <NUM>-<NUM> different diameters are used after the pilot drill to create increasingly larger diameter bores. Multiple drilling puts a heavy thermal load on the bone tissue, which can lead to bone necrosis and consequently reduced osseointegration.

Claim 1:
A dental self-tapping device (<NUM>,<NUM>) comprising a body (<NUM>) having a longitudinal axis (t,T), the body (<NUM>, <NUM>) having a proximal neck portion N, a proximal-central straight portion S of substantially constant diameter and a distal conical portion C of decreasing diameter, the body (<NUM>, <NUM>) comprising a core (<NUM>, <NUM>) and a thread (<NUM>) provided thereon, a flute (<NUM>, <NUM>) having a circular arc shape and the central angle of the circular arc being at most <NUM> degrees, the flute (<NUM>,<NUM>) formed in the thread (<NUM>), which flute (<NUM>, <NUM>) runs spirally around the core (<NUM>, <NUM>) and an axis of the flute (<NUM>, <NUM>) is at an angle (α, β) with the longitudinal axis (t,T), characterized in that the angle (α, β) measured from a proximal end of the body (<NUM>, <NUM>) is <NUM> to <NUM> degrees, preferably <NUM> to <NUM> degrees, and the diameter of the flute (<NUM>, <NUM>) in at least a portion of the body (<NUM>, <NUM>) is <NUM> to <NUM>%, preferably <NUM> to <NUM>%, of the diameter (d<NUM>) of the body (<NUM>, <NUM>).