SCREW WITH A VARIABLE LOCKING ANGLE AND A CORRESPONDING LOCKING SYSTEM

Abstract

A locking system can comprise a screw head without threads and a thread created only inside a bore of a bone plate. The machining of thread geometry can thus pertain to the screw body, while the screw head can be formed by curved surfaces and contact surfaces by simple machining process. A plurality of screws can be implemented, and each bone plate can have a larger number of bores for accepting the screws than the number of screws. The said locking screw can be used with other bone plates having continuous or partial threads created inside their bores.

Claims

1. A screw with a variable locking angle into a bone plate, the screw comprising: a screw head with carved surfaces and contact surfaces, as well as a centrally placed star-shaped socket to receive a fastening tool, located on an upper side of said screw head; and a screw body ending with a tip portion to enter a bone first, where said screw body is equipped with a self-cutting thread which extends from a joint between the screw head and the screw body to the tip portion of the screw, wherein said screw head is formed from a rotationally symmetric truncated cone of height h, with a base radius Rout on which said star-shaped socket is centrally positioned and where a cone side is inclined at an angle Y relative to an axis of said rotationally symmetric truncated cone which coincides with a screw axis of the screw, wherein n of the carved surfaces, where n≥4, are carved into said rotationally symmetric truncated cone and extend from an upper edge of the screw head to a bottom edge of said screw head, wherein said carved surfaces are interconnected with the contact surfaces of the rotationally symmetric truncated cone, both extending from the upper edge to the bottom edge of the screw head, wherein the upper edge is partly formed from points of the contact surfaces that lie on the base radius Rout, and from upper edges of the carved surfaces in a way that a highest point of each said carved surface lies on a radius Rin in a plane parallel to a plane containing the base radius Rout, provided that Rin<Rout, wherein the bottom edge is formed from ends of contact surfaces and the carved surfaces and leads to the joint between the screw head and the screw body, wherein the screw head, with the carved surfaces and the contact surfaces, is formed without any thread on the carved surfaces and the contact surfaces, and wherein the contact surfaces and the carved surfaces are adapted to be cut into and damaged by a thread of a bore when the screw is inserted into the bore.

2. The screw according to claim 1, wherein the number of carved surfaces n distributed around the perimeter of the screw head is selected to be n=6, 8, 10, or 12, wherein the value Y is selected to be 5°<Y<30°.

3. The screw according to claim 2, wherein the angle Y is 9°.

4. The screw according to claim 1, wherein a maximum applicable locking angle for the screw is 20° measured with respect to the screw axis in all directions.

5. The screw according to claim 1, wherein each of the contact surfaces is flat and linear along an entire length and an entire width of the contact surface.

6. A locking system consisting of: one or more screws whose screw head, which is conically-shaped, has a hardness that is lower than a hardness of a material used to form a bone plate; and the bone plate with a geometry adapted for bone fixation, with at least two bores located within said geometry, where each said bore has a thread of a conical shape with a slope δ in relation to a bore axis of the bore and which is configured to receive the screw head of the screw, wherein each of the one or more screws includes: the screw head, with carved surfaces and contact surfaces, as well as a centrally placed star-shaped socket to receive a fastening tool, located on an upper side of said screw head, and a screw body ending with a tip portion to enter the bone first, where said screw body is equipped with a self-cutting thread which extends from a joint between the screw head and the screw body to the tip portion of the screw, wherein said screw head is formed from a rotationally symmetric truncated cone of height h, with a base radius Rout on which said star-shaped socket is centrally positioned and where a cone side is inclined at an angle Y relative to an axis of said rotationally symmetric truncated cone which coincides with a screw axis of the screw, wherein n of the carved surfaces, where n≥4, are carved into said rotationally symmetric truncated cone and extend from an upper edge of the screw head to a bottom edge of said screw head, wherein said carved surfaces are interconnected with the contact surfaces of the rotationally symmetric truncated cone, both extending from the upper edge to the bottom edge of the screw head, wherein the upper edge is partly formed from points of the contact surfaces that lie on the base radius Rout, and from upper edges of the carved surfaces in a way that a highest point of each said carved surface lies on a radius Rin in a plane parallel to a plane containing the base radius Rout, provided that Rin<Rout, wherein the bottom edge is formed from ends of contact surfaces and the carved surfaces and leads to the joint between the screw head and the screw body, wherein the screw head, with the carved surfaces and the contact surfaces, is formed without any thread on the carved surfaces and the contact surfaces, wherein an inner bore slope δ is greater than or equal to the angle Y of the screw head with the corresponding contact surfaces and carved surfaces, and wherein the system is adapted to implement a locking process whereby insertion of the screw, at a selected angle in a range from −20° to 20° relatively to the bore axis, causes the thread of the bore to be cut into the contact surfaces and carved surfaces of the screw head causing damages on the contact surfaces and the carved surfaces that keep the screw locked into the bone plate.

7. The locking system according to claim 6, wherein the height h is selected to be from 0.5 to 1.6 of the thickness of the bone plate and the inner bore slope δ is selected to be from 5° to 30°.

8. The locking system according to claim 6, wherein the thread of each said bore is continuous.

9. The locking system according to claim 6, wherein the thread of each said bore is not continuous but has segments with a thread and segments where the thread is absent.

10. The locking system according to claim 6, wherein a minimum thickness of the bone plate is 1 mm or less.

11. The locking system according to claim 6, wherein the number of carved surfaces n distributed around the perimeter of the screw head is selected to be n=6, 8, 10, or 12, wherein the value Y is selected to be 5°<Y<30°.

12. The locking system according to claim 11, wherein the angle Y is 9°.

13. The locking system according to claim 6, wherein a maximum applicable locking angle for the screw is 20° measured with respect to the screw axis in all directions.

Description

A BRIEF DESCRIPTION OF THE FIGURES

[0041] FIGS. 1 and 3 illustrate a bone plate constructed according to a background art solution.

[0042] FIG. 2 depicts a screw that locks into the said plate, while FIG. 2A illustrates the detail S of the screw head shown in FIG. 2, according to the aforementioned background art solution.

[0043] FIGS. 4A and 4B depict damage that can occur in the plate bores, shown in FIGS. 1 and 3, according to the aforementioned background art solution.

[0044] FIG. 5A represents detail P from FIG. 5C and depicts the newly designed screw head that can be locked into the bore made in the bone plate shown in FIG. 5B.

[0045] FIG. 5B is a cross-section of the bone plate detail, marked with Q, in FIG. 5C.

[0046] FIG. 6A illustrates a position of the locked screw in the case when the screw head is fixed at an angle of 0° in relation to the bore axis.

[0047] FIG. 6B shows exemplary damage that may result to the screw head according to one or more embodiments of the present disclosure.

[0048] FIG. 6C illustrates a position of the locked screw in the case when the screw head is fixed at an angle of 10° in relation to the bore axis.

[0049] FIG. 6D shows exemplary damage that may result to the screw head according to one or more embodiments of the present disclosure.

[0050] FIGS. 7A, 7B, 7C, 7D, 7E and 7F depict a screw head design according to one or more embodiments of the present disclosure in the case when n is selected to be equal to 8, 10 and 12 curved surfaces on the screw head, for a screw of the same head radius Rout with variations in the size of the curved surfaces.

DETAILED DESCRIPTION

[0051] Embodiments of the disclosed subject matter can relate to a screw (10) with a variable angle of locking into a bone plate (30), where the head (20) of the said screw is locked into a bore (31) of the plate (30). Embodiments of the disclosed subject matter can also involve a corresponding locking system of screw (10)-plate (30) as illustrated in FIG. 5C. The plate (30) may be formed of an arbitrary geometry, but for simplicity reasons the plate (30) is illustrated in FIG. 5C with a series of evenly spaced bores (31). The spacing of each bore (31) should be such to allow optimum plate (30) fixation to the desired bone. According to embodiments of the present disclosure, each of the bores (31) is made with a continuously machined thread (32), with a slope, for instance, as shown in FIG. 5B. FIG. 5B shows one continuous thread (32), carved conically at an angle δ relative to the bore (31) axis where the side of the cone accompanying the thread (32) is marked with straight line Δ. According to one or more embodiments, it may be desirable that the angle δ is greater than or equal to angle Y, which can define the cone of the screw head (20) which is suitably locked by the corresponding thread (32) when the value is about 2 Y. In practice, the value δ can be chosen in the range from 5° to 30° as shown in FIGS. 5A and 5B.

[0052] Embodiments of the present disclosure, however, are not limited to tapered threads (32) illustrated in FIG. 5B, but can also include cylindrical threads of a constant cross-section (δ=0). However, a due care may need to be undertaken when choosing dimensions of a corresponding screw (10) whose head (20) is locked by said thread (32). In addition to the already mentioned continuous conical and cylindrical geometry, the thread (32) can be machined in a way which is not continuous but has segments which are threaded and segments where the thread is absent, i.e., with the thread made only on segments of the bore (31). If there is a need for such a technical solution, it is also possible to imagine a plate (30) which combines bores (31) with several different types of previously mentioned threads (32). For machining simplicity and for obtaining optimum locking performances, for instance, the processing of all bores (31) of the bone plate (30) can be performed in such a way that all threads (32) are selected to be of conical type where δ≈2 Y, which can significantly reduce the production costs of such plates (30). According to one or more embodiments of the present disclosure, the plate (30) can be made of a material exhibiting hardness higher than the hardness of the screw head (20), preferably of titanium used for medicinal purposes, e.g., grade 5, and for the corresponding screw (10) or screw head (20) of titanium grade 2. FIG. 5C illustrates such a screw (10) before insertion into the plate (30).

[0053] In one or more embodiments of the present disclosure, the screw (10) can consist of a screw head (20) with machined curved surfaces (24) and contact surfaces (25), as well as a centrally placed socket shaped like a star (26) for receiving the fastening tool into, located on the upper side of said head, as shown in FIG. 7A. The star-shaped socket (26) can be of an arbitrary geometry suitable for receiving a fastening tool tip and made in a way which prevents the tools from popping out of this socket, e.g., a Phillips or an Allen type tool which enables accurate handling by the operator. The screw body (13) ends with a tip (12) of the screw (10) that enters the bone first. The screw body (13) is equipped with a self-cutting thread (14) which extends from the joint (16) between the screw head (20) and the screw body (13) to the said screw tip (12).

[0054] However, the head (20) of the screw (10), illustrated in FIG. 5A as a detail in FIG. 5C, can represents another patentable distinction of one or more embodiments of the disclosed subject matter. As mentioned before, fewer screws (10) are often used in surgical interventions than there are bores (31) with threads (32) made on bone plates (30). Bearing this in mind, a locking system that has more bores (31) created in a simpler and more economical way on plates (30), while the more demanding machining may be required for the screws (10) only—has obvious advantages in the art, rendering the locking system cheaper.

[0055] The screw head (20), according to one or more embodiments of the disclosed subject matter, can be formed from a rotationally symmetric truncated cone of height h, see FIGS. 5C and 5A as well as FIGS. 7A-7F. This truncated cone is directed with its missing tip towards the tip (12) of the screw (10). The base of this rotating cone is designed with radius Rout, see FIGS. 7A-7F, and is projecting slightly outwards in a way that the star-shaped socket (26) is situated within the said cone. The side of the cone is inclined at the angle Y in relation to the longitudinal axis of the said cone, as indicated in FIGS. 7C and 5A. The straight line πc is a line that lies on the side of the cone in at least two different points and cuts the cone rotational axis in just one point. Consequently, each such straight line πc is inclined at Y angle in relation to the longitudinal axis of the said cone, see, for example, FIG. 5A.

[0056] In order to obtain a screw head shape according to one or more embodiments of the present disclosure, the manufacturing process may be considered. Screws (10) are most often produced using CNC (Computer Numerical Control) technology and it is therefore may be useful to use tool trajectories that are compatible with the production technology. The screw head (20) according one or more embodiments of the present disclosure, when viewed from the upper perspective, see FIGS. 7A-7F, is a part with side grooves, i.e., curved surfaces which have been machined out. According to one or more embodiments of the present disclosure, the head (20) is formed by milling curved surfaces (24) of the truncated cone as shown in FIG. 5A. Each of the curved surfaces (24) is formed as a portion of the cylindrically shaped surface, where the cylinder itself has a radius Rc and where the axis of that cylinder lies on straight line H. The straight line II is inclined at an angle Y relative to the longitudinal axis of the cone into which curved surfaces (24) are carved as shown in FIG. 5A. According to one or more embodiments of the present disclosure, n equally curved surfaces (24) are carved into said cone, as illustrated in FIGS. 7A-7F. The bundle of straight lines π which are inclined at an angle Y forms vertices of a regular polygon with n sides, when these lines intersect the plane perpendicular to the longitudinal axis of the cone. According to one or more embodiments, it may be desirable that n is equal or greater than 4, for instance, when n=8, 10 or 12. When curved surfaces (24) are carved, then the truncated cone surfaces that remain between them are portions of the side of the truncated cone that connect them and represent the contact surfaces (25). In this way, alternately shaped curved surfaces (24) and the contact surfaces (25) are extended from the upper edge (22) to the bottom edge (23) of the head (20), where said head (20) passes into the joint (16) between the head (20) and the screw body (13).

[0057] FIGS. 7A-7F show projections of the characteristic radii Rout, Rin, and Rc′ into the plane perpendicular to the longitudinal axis of the cone and the corresponding design of the screw head (20) with such a geometry disclosed in Table 1. The size of Rout represents the radius of the cone base, Rin is the smallest distance from the upper edge (22), i.e., the highest point of the curved surface (24) on the screw head (20) to the screw (10) axis. Value Rc′ represents a projection cylindrical curvature Rc of the surfaces (24) into the plane perpendicular to the cone axis. L represents the largest distance between two contact surfaces (25) located closer to the bottom edge (23) where the cylinder with radius Rc plunges deeper into the cone, and l represents the smallest distance between the contact surfaces (25) located closer to the upper edge (22) of the head (20). D represents the maximum distance between two adjacent curved surfaces (24), and d is the smallest distance, close to the bottom edge (23). Value A represents the area of contact surface (25) of the screw designed in this way. A side-by-side comparison of the screw head (20) performances was made for the constant values of Rout and Rin; only parameter Rc was varied by approx. 15% and the number of curved surfaces (24) is selected to be 8, 10 and 12. The parameters of such variations for 2.4 mm screw are shown in Table 1 below:

TABLE-US-00001 TABLE 1 n Rout Rin Rc l L D d A (FIG.) [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm.sup.2]  8 (7A) 1.524 1.285 0.750 0.952 0.970 0.229 0.068 0.184  8 (7B) 1.524 1.285 0.875 1.010 1.028 0.167 0.006 0.107 10 (7C) 1.524 1.285 0.425 0.724 0.740 0.226 0.098 0.202 10 (7D) 1.524 1.285 0.525 0.811 0.829 0.137 0.006 0.089 12 (7E) 1.524 1.285 0.325 0.612 0.622 0.182 0.079 0.162 12 (7F) 1.524 1.285 0.375 0.672 0.686 0.120 0.013 0.083

[0058] When designing the screw, according to embodiments of the disclosed subject matter particular care of the surface A layout may be taken, specifically of the contact surface (25) area once the machining of the cone head (20) is finished, where said contact surfaces (25) have a key function in the locking process. The locking process is shown in a series of FIGS. 6A-6D. FIG. 6A is a side view of the plate (30) into which the screw (10) is locked at an angle of 0°. FIG. 6B shows damage (29) that occurs on the contact surfaces (25) and the curved surfaces (24) of said screw head (20). The line of damage (28) ascent coincides with the thread (32) created in the bore (31). In the same way FIG. 6C represents a side view of the plate (30) into which the screw (10) is locked at an angle of 10°. FIG. 6D illustrates damage (29) that occurs on the contact surfaces (25) and the curved surfaces (24) of said screw head (20) during locking at an angle of 10°. The damage occurs on the screw head (20) made of a material that is softer than the material of the plate (30).

[0059] In practice, two extremes can be avoided, i.e., a too large contact surface (25) which can prevent easy occurrence of damage (29), but locks the head (20) into the plate perfectly, and a very small contact surface (25) that locks the head (20) into the plate (30) less than perfectly, but handling such a screw is relatively simple.

[0060] In modelling the screw (10) layouts and cutting-in forces that cause damages (29) the following values were found as optimal for the given screw head design according to one or more embodiments of the disclosed subject matter.

[0061] Table 2 shows the values for screws of 2.4 mm, 3.5 mm and 5.0 mm that are used in practice, with 8, 10 or 12 curved surfaces (24) and acceptable parameters D and d, defining the largest and the smallest spacing between the curved surfaces (24) and thus the amount of the contact surface (25) that participates in the locking process. It should be emphasized that the sizes D and d are directly related to the chosen range of the radius of curvature Rc and the values Rin and Rout.

TABLE-US-00002 TABLE 2 Screw Rout Rin Rc D d [mm] n [mm] [mm] [mm] [mm] [mm] 2.4 8 1.524 1.285 0.750-0.875 0.229-0.167 0.068-0.006 2.4 10 1.524 1.285 0.425-0.525 0.226-0.137 0.098-0.006 2.4 12 1.524 1.285 0.325-0.375 0.182-0.120 0.079-0.013 3.5 8 2.477 2.100 1.400-1.550 0.304-0.241 0.074-0.010 3.5 10 2.477 2.100 0.800-0.910 0.281-0.196 0.095-0.007 3.5 12 2.477 2.100 0.550-0.650 0.276-0.162 0.127-0.007 5.0 8 3.200 2.800 2.125-2.300 0.463-0.408 0.062-0.007 5.0 10 3.200 2.800 1.150-1.275 0.409-0.332 0.081-0.001 5.0 12 3.200 2.800 0.800-0.850 0.347-0.302 0.077-0.028

[0062] A comparison between embodiments of the present disclosure and the locking system disclosed by the document WO2015/020789A1 is given below. The analysis was carried out by performing the cyclic testing of the locking features by bending the screw of 2.4 mm in diameter locked into a 1 mm thick plate with 4 bores by parallelly examining the solution presented in WO2015/020789A1 and embodiments of the present disclosure in a fatigue test (LFV 50-HH, Walter+Bai AG, Switzerland, production year 2006). The selected angles of locking the screws into the plates were 0°, 10°, 15° and 20° in relation to the bore (31) or bore (91) axis, as shown in Table 3, which contains the test parameters:

TABLE-US-00003 TABLE 3 Screw angle Fsr Fa f N Fd F [°] [N] [N] [Hz] [—] [kN] [N] 0 38 32 1, 4 5000 6, 3 6-70 10 33 27 1, 4 5000 6, 3 6-60 15 27 23 1, 4 5000 6, 3 4-50 20 22 18 1, 4 5000 6, 3 4-40

[0063] where the following abbreviations are used:

[0064] Fsr—mean force,

[0065] Fa—force amplitude

[0066] F—force range Fsr+/−Fa,

[0067] f—frequency of cyclic loading,

[0068] N—number of load cycles, and

[0069] Fd—nominal force of the force transducer.

[0070] The results of the bending test, using 3 different samples in each experiment, with the screw locked into the plate under cyclic load are shown in the tables below, where the following notation is used:

[0071] Smax—maximum screw displacement,

[0072] Smin—minimal screw displacement,

[0073] Δs=Smax—Smin,

[0074] Fmax—maximum force at Smax,

[0075] Stiffness—Fmax/Smax ratio.

[0076] The comparison of the technical solution presented in FIGS. 1-4B and the new locking system according to one or more embodiments of the disclosed subject matter is elaborated side-by-side in the tables below:

TABLE-US-00004 TABLE 4A background art Screw angle Smax Smin ΔS Fmax Stiffness 0° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.249 0.067 0.183 69.30 277.87 Sample 2 0.364 0.155 0.209 70.80 194.43 Sample 3 0.279 0.106 0.173 68.70 246.41 Mean value 0.297 0.109 0.188 69.60 239.57 Std. dev. 0.060 0.044 0.018 1.082 42.140

TABLE-US-00005 TABLE 4B Present embodiment(s) Screw angle Smax Smin ΔS Fmax Stiffness 0° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.267 0.072 0.195 67.80 253.89 Sample 2 0.262 0.071 0.191 71.30 271.83 Sample 3 0.343 0.137 0.206 71.30 207.96 Mean value 0.291 0.093 0.198 70.13 244.56 Std. dev. 0.045 0.038 0.008 2.021 32.938

[0077] At the very least, for the locking angle of 0° the system according to one or more embodiments of the present disclosure can be significantly simpler to produce.

TABLE-US-00006 TABLE 5A background art Screw angle Smax Smin ΔS Fmax Stiffness 10° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.213 0.064 0.149 60.70 285.04 Sample 2 0.292 0.115 0.177 60.80 208.47 Sample 3 0.419 0.217 0.202 59.70 142.38 Mean value 0.308 0.132 0.176 60.40 211.96 Std. dev. 0.104 0.078 0.027 0.608 71.396

TABLE-US-00007 TABLE 5B Present embodiment(s) Screw angle Smax Smin ΔS Fmax Stiffness 10° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.270 0.092 0.178 58.80 217.82 Sample 2 0.302 0.101 0.200 60.90 201.92 Sample 3 0.254 0.078 0.175 59.00 232.47 Mean value 0.275 0.090 0.185 59.57 217.40 Std. dev. 0.024 0.011 0.014 1.159 15.276

[0078] The results show that the mean value of the maximum screw displacement can be lower in embodiments of the present disclosure, and that stiffness, when the maximum force is taken into consideration for both technical solutions, may exhibit similar values for a locking angle of 10°.

TABLE-US-00008 TABLE 6A background art Screw angle Smax Smin ΔS Fmax Stiffness 15° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.257 0.114 0.143 45.70 177.51 Sample 2 0.209 0.046 0.163 49.60 236.98 Sample 3 0.250 0.081 0.168 49.10 196.60 Mean value 0.239 0.081 0.158 48.13 203.696 Std. dev. 0.026 0.034 0.013 2.122 30.364

TABLE-US-00009 TABLE 6B Present embodiment(s) Screw angle Smax Smin ΔS Fmax Stiffness 15° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.220 0.047 0.172 49.60 225.81 Sample 2 0.258 0.080 0.178 51.40 199.03 Sample 3 0.238 0.068 0.170 49.80 209.60 Mean value 0.239 0.065 0.173 50.27 211.48 Std. dev. 0.019 0.017 0.004 0.99 13.49

[0079] The results show once again that the mean value of the maximum screw displacement can be the same, and that stiffness may be similar in both technical solutions, taking the maximum force of a similar amount into account for a locking angle of 15°.

TABLE-US-00010 TABLE 7A background art Screw angle Smax Smin ΔS Fmax Stiffness 20° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.259 0.135 0.124 39.30 151.77 Sample 2 0.188 0.057 0.131 39.00 207.50 Sample 3 0.168 0.042 0.125 39.60 236.07 Mean value 0.205 0.078 0.127 39.30 198.445 Std. dev. 0.048 0.050 0.004 0.300 42.873

TABLE-US-00011 TABLE 7B Present embodiment(s) Screw angle Smax Smin ΔS Fmax Stiffness 20° [mm] [mm] [mm] [N] [N/mm] Sample 1 0.164 0.024 0.141 39.40 239.59 Sample 2 0.156 0.026 0.130 39.00 249.52 Sample 3 0.171 0.018 0.153 39.40 230.34 Mean value 0.164 0.023 0.141 39.27 239.82 Std. dev. 0.007 0.004 0.011 0.23 9.59

[0080] The obtained results show a comparison among background art and one or more embodiments of the present disclosure. The stiffness according to one or more embodiments of the present disclosure can be higher for a locking angle of 20° than in the case of background art.

[0081] From the results shown in Tables 4, 5, 6 and 7 it is possible to conclude that, within the experimental error, the locking system of one or more embodiments of the present disclosure can be, for instance, even better (stiffer) as the inclination of the screw head (20) increases in relation to the bore (31) axis, which is certainly an unexpected result for an average person skilled in the art. This unexpected result, therefore, indicates an inventive step in the presented new technical solution. It is necessary to emphasize once again that the proposed new locking system according to embodiments of the disclosed subject matter can transfer all the machining complexity or processing from the plate (30) to the screw (10), more specifically, to the screw head (20).

REFERENCES

[0082] References of the New Technical Solution (Representative of Examples Only) [0083] 10 screw for the plate 30 [0084] 12 tip of the screw 10 [0085] 13 screw body with a self-cutting thread 14 [0086] 14 self-cutting thread [0087] 16 joint between screw head 20 and screw body 13 [0088] 20 screw head [0089] 22 upper edge of the screw head 20 [0090] 23 bottom edge of the screw head 20 [0091] 24 curved surface formed in the screw head 20 [0092] 25 contact surface between the curved surfaces 24 [0093] 26 star-shaped socket for receiving a fastening tool [0094] 28 line of ascent of damage 29 [0095] 29 damage caused by carving thread 32 into the contact surfaces 25 and curved surfaces 24 [0096] 30 plate [0097] 31 bore for locking the screw head 20 [0098] 32 thread in the bore 31 [0099] Rin the smallest radius of the upper edge 22 of the screw head 20 [0100] Rout the largest radius of the upper edge 22 of the screw head 20 [0101] Rc curvature radius of the machined surface 24 measured perpendicular to the direction of the tool movement along the straight line π [0102] Rc′ projection of Rc on the plane perpendicular to the screw 10 axis [0103] L distance between two adjacent contact surfaces 25 along the edge 23 [0104] l distance between two adjacent contact surfaces 25 along the edge 22 [0105] D contact surface 25 width along the edge 22 [0106] d contact surface 25 width along the edge 23 [0107] h screw head height [0108] π straight line parallel to the cone sides of the screw head 20 [0109] πc straight line situated on the contact surface 25 [0110] Y inclination angle of the straight line II in relation to the screw 10 axis [0111] Δ the slope line of the thread 32 in the bore 31 [0112] δ angle of the slope line A in relation to the bore 31 axis [0113] n number of curved surfaces 24 on the head 20 of the screw 10 [0114] P, Q details

[0115] References of the Background Art [0116] 90 plate [0117] 91 bore for receiving the screw head 101 [0118] 92 upper edge of the bore 91 [0119] 93 bottom edge of the bore 91 [0120] 94 curved section of the bore 91 [0121] 95 line of intersection of sections 94 [0122] 96 contact area at the intersection of sections 94 [0123] 98 line of ascent of damage 99 [0124] 99 damage caused by screw head threading 105 [0125] 100 screw for the plate 90 [0126] 101 screw head with the thread 105 [0127] 102 tip of the screw 100 [0128] 103 body of the screw 100 with the self-cutting thread 104 [0129] 104 self-cutting thread [0130] 105 thread of the screw head 101 [0131] 106 joint between the screw head 101 and the screw body 103 [0132] δ′ inclination angle of curved sections 94 which is cone-shaped, in relation to the bore 91 axis [0133] S detail