TORDER INSTRUMENT

Description

[0001] The invention relates to a twisting instrument for a screw-in component with a tool recess, which comprises a drive shaft, a neck and a working part.

[0002] In dental implantology, an endosseous implant body is often used to support the prosthesis, for example, in the fabrication of a single-tooth prosthesis. In this case, the implant body, a type of screw anchor, is screwed into a hole artificially created in the patient's jaw. The screwed-in implant body accommodates an implant post in the finished prosthesis. The post is securely fastened to the implant body with a special clamping device, e.g., a screw or a special threaded bolt. A twisting instrument is used to secure or remove the clamping device.

[0003] A superstructure forming the visible tooth crown is attached directly or indirectly to the implant post thus fixed, e.g. by bonding.

[0004] WO 2017 / 070 335 A1 discloses a screwing tool for screwing in screws for attaching abutments to an implant body. The screwing tool has a drive part, a shaft, and a working part. The working part has a plurality of rounded teeth with concave surfaces between them.

[0005] DE 93 03 828 3 U1 discloses a screwdriver whose working head is spherically shaped and has a hexagonal cross-section. The working head tapers into the driver shaft. To minimize stress on this vulnerable cross-section, one design, for example, provides a predetermined breaking point between the driver shaft and the driver handle. An alternative design provides a torsional breaking point at the connection between the driver shaft and the driver handle.

[0006] ES 2 677 998 A1 also discloses a screwdriver with a ball head. The screwdriver has a meltable zone spaced apart from the ball head.

[0007] US 2015 / 0289953 A1 shows a screwdriver with an ellipsoidal working head mounted on the neck. The recesses for the tooth gaps extend deep into the neck. This has two truncated conical sections and one cylindrical section.

[0008] The present invention is based on the problem of improving a twisting instrument for a screw-in component such that the working part of the twisting instrument engages into the tool recess of the screw-in part in a manner that is safe from overload and reliable, wherein the center lines of the twisting instrument and the screw-in component enclose a bending angle of zero or more degrees.

[0009] This problem is solved with the features of patent claim 1. The working part—as a ball head with six teeth—has a spherical envelope surface that is contacted at least by both the free end and by portions of the tooth heads of the working part. There are six tooth gaps between the teeth. The neck or drive shaft has a predetermined breaking zone. The neck consists of a neck transition zone, a neck main cylinder zone, a neck cone zone, and a neck secondary cylinder zone. Furthermore, the ball head has a rear grip at the transition between the ball head and the neck secondary cylinder zone, the radius contour of which transitions tangentially into the neck secondary cylinder zone.

[0010] The invention provides a twisting instrument with which individual parts of a denture can be screwed together during the assembly and maintenance of a denture. The screws, threaded bolts or the like required for the screw connection have a tool recess which forms a mechanical interface with the working part in the form of an angularly movable coupling. The spherical head-shaped, toothed working part, which has, for example, three to eight teeth, is connected in one piece to a torsionally rigid drive shaft via a neck which is at least torsionally elastic. To use the twisting instrument, a generally manual actuating element is attached to the drive shaft. In the mechanical interface located between the drive shaft and the actuating element, in addition to pressure and tension, a torque is also transmitted for both screwing and unscrewing.

[0011] In the primary embodiment, a two-stage torque monitor is integrated directly into the twisting instrument. For the first stage, the neck is designed to be slim and elastic in terms of its geometric shape, combined with an appropriate selection of materials. Thus, in the final phase of the screwing movement, the user perceives a torsion of more than 10 degrees via the actuating element, which twists the neck elastically but does not cause any further screwing in of the screw or bolt. For this function, the neck is designed as a torsion bar. It can be a smooth cylindrical or truncated cone-shaped bar. If necessary, the neck can also be at least partially a body of rotation, the contour of which is also curved, at least in sections. All transitions between individual neck areas are rounded to minimize any notch stresses that may occur.

[0012] The second level of torque monitoring is a predetermined breaking zone. This represents the weakest point between the working part and the drive shaft. If the torque specified for the screw connection is exceeded, the neck separates from the drive shaft at the predetermined breaking point. The breaking point is located well outside the already mounted prosthetic restoration, allowing the neck, together with the working part, to be pulled out of the implant post, for example, with the help of tweezers – without exerting any force.

[0013] Further details of the invention emerge from the subclaims and the following description of schematically illustrated embodiments. Figure 1: Perspective view of a twisting instrument; Figure 2: Enlarged view of the working part according to Figure 1; Figure 3: Longitudinal section of the semi-finished working part after turning, enlarged 20 times; Figure 4: Longitudinal section of the working part after the groove has been created to produce the tooth gaps, enlarged 20 times; Figure 5: Cross-section of the working part at the point of largest diameter, enlarged 20 times; Figure 6: Partial view of the instrument neck with partial section, enlarged 30 times; Figure 7: Side view of a forming tool, enlarged 20 times; Figure 8: Engagement of the working part in the screw-in component in section through the external hexalobular teeth, enlarged 20 times; Figure 9: Engagement of the working part in the screw-in component in section through the external hexalobular tooth gaps, enlarged 20 times; Figure 10: as Figure 8 , however, the center line of the working part includes a bending angle of 22 degrees with the center line of the screw-in part; Figure 11: as Figure 9, however, the center line of the working part includes a bending angle of 22 degrees with the center line of the screw-in part; Figure 12: Perspective view of a twisting instrument with a predetermined breaking zone in the form of an annular groove in the drive shaft; Figure 13: Longitudinal section of Figure 12 Figure 14: Perspective view of a twisting instrument with a predetermined breaking zone in the form of a groove; Figure 15: Longitudinal section of Figure 14 ; Figure 16: Perspective view of a twisting instrument with a predetermined breaking zone in the form of a notch; Figure 17: Longitudinal section of Figure 16 ; Figure 18: Perspective view of a twisting instrument with a predetermined breaking zone in the form of a cone; Figure 19: Longitudinal section of Figure 18 ; Figure 20: Perspective view of a twisting instrument with a predetermined breaking zone in the form of a circumferential groove with a circular cross-section; Figure 21: Longitudinal section of Figure 20; Figure 22: Perspective view of a twisting instrument with a predetermined breaking zone in the form of a Mattheck double contour; Figure 23: Longitudinal section of Figure 22 ; Figure 24:perspective view as Figure 22 , but with a protective sleeve surrounding the Mattheck double contour; Figure 25: Longitudinal section to Figure 24 ; Figure 26:perspective view as Figure 20 , but with handle; Figure 27: Longitudinal section of Figure 26 .

[0014] The Figure 1 shows a dental twisting instrument (1) consisting of a drive shaft (10) that is at least partially standardized, a slender instrument neck (20), and an external hexalobular ball head (61) as the working part (60). The twisting instrument (1) can be used to screw a prosthesis to an implant body in the context of dental implantology for the production of a prosthetic tooth replacement.

[0015] The one-piece twisting instrument (1), for example, is made of an austenitic cobalt-based alloy with the material designation COCr20Ni15Mo7. This biocompatible, non-magnetic material has a high yield strength and is resistant to corrosion and hydrogen embrittlement. It is resistant to aging and possesses high fatigue strength.

[0016] The twisting instrument (1) is hardened at 530°C for three hours, for example, in argon or high vacuum, and then polished. Depending on the length of the hardening process, the material's shear modulus is between 75 and 82 GPa.

[0017] An alternative material is high-speed steel HS2-9-1-8. Its main alloying components are, on average, 1% carbon, 2% tungsten, 9% molybdenum, 1% vanadium, and 8% cobalt.

[0018] The drive shaft (10) of the twisting instrument (1) is a tool section designed for a rotating or oscillating movement. It has a straight, cylindrical shaft section (11) with a flattened portion (41) and a groove (17) at its free end. The diameter of the shaft section measures 2.35 mm. In the exemplary embodiment, it has a length of 12.5 mm. A handpiece (80) in the form of an interchangeable handle is attached to the shaft section (11) to use the twisting instrument (1), see. Figures 26 and 27 The flattened portion (41) serves to transfer the torque manually applied to the handpiece (80) to the twisting instrument (1). A spring-loaded pin in the handpiece (80) engages the groove (17) of the shaft section (11), which serves as a rear grip, and securely holds the twisting instrument (1) in the handpiece (80). This shaft section (11) is described as Type 1 in DIN EN ISO 1779.

[0019] Of course, a ratchet mechanism can also be integrated into the handpiece (80). This mechanism allows, for example, in a confined working space, to initiate a rotational movement in a specific direction into the screw-in component (90) through repeated, sector-by-sector pivoting movements of the handpiece (80). Furthermore, the ratchet mechanism can be combined with a torque limiter.

[0020] The shaft section (11) is followed by an instrument neck, e.g. 16.4 mm long, which is referred to as the neck (20) below. The neck (20) consists of a neck transition zone (21), a main neck cylinder zone (31), a neck cone zone (35) and a secondary neck cylinder zone (36). In the neck transition zone (21), the shaft section (11) tapers towards the main neck cylinder zone (31). A diameter reduction takes place from e.g. 2.35 mm to e.g. 1.3 mm. In order to keep the notch stress caused by the diameter jump as small as possible, the neck transition zone (21) is designed using Mattheck's tensile triangle method, cf. Figure 6 .

[0021] The Figure 6 shows the neck transition zone (21) located between the shaft section (11) and the predetermined breaking zone (40). In the upper, uncut half of the Figure 6it can be seen that the neck transition zone (21) consists of a first (23), second (24) and third transition zone region (25). The first transition zone region (23), which adjoins the shaft section (11), has a tapered straight cone with a cone angle of 90 degrees. The second transition zone region (24), which adjoins the first transition zone region (23), has a further tapered straight cone with a cone angle of 45 degrees. The third transition zone region (25), which adjoins the second transition zone region (24), also has a tapered straight cone with a cone angle of 22.5 degrees. A transition radius of e.g. 0.5 mm is provided between each individual cone.

[0022] The length of the individual transition zone areas (23-25) results from the construction by means of the - in the section area according to Figure 6Three differently hatched tension triangles (26-28) shown in dashed lines. All three tension triangles each form an isosceles triangle. The first tension triangle (26), which is right-angled, has a hypotenuse that extends like the contour of a 45° chamfer from the edge of the shaft section (11) to the level of the outer diameter of the neck main cylinder zone (31). The acute-angled second tension triangle (27) is connected to the half of the hypotenuse of the first tension triangle (26) closest to the centerline, with its leg closest to the shaft section (11). The angle enclosed between the hypotenuse and one leg measures 22.5 degrees. The third tension triangle (28) nestles against the half of the hypotenuse of the second tension triangle (27) closest to the centerline, with its leg closest to the shaft section (11). Its other cathetus is at the level of the outer diameter of the neck main cylinder zone (31).The two half hypotenuses of the tension triangles (26) and (27), together with the hypotenuse of the third tension triangle (28), form the low-stress Mattheck contour (29). Of course, the Mattheck contour (29) can be approximated by a continuous curve.

[0023] The cylindrical neck main cylinder zone (31) has a length of e.g. 10.15 mm.

[0024] In the rear area of ​​the neck main cylinder zone (31) there is a predetermined breaking zone (40). The center of the predetermined breaking zone (40) is located, for example, 2.25 mm in front of the shaft section (11). The predetermined breaking zone (40) has, for example, three to six flattened portions (41) distributed equidistantly around the circumference of the neck main cylinder zone (31). Figures 1 and 6In the exemplary embodiment, four flattened portions are selected. Each flattened portion is flat and has the shape of a rectangle. The rectangular edge length aligned parallel to the center line (3) has a length of, for example, 0.5 mm. The rectangular edge length perpendicular to the center line (3) measures, for example, 0.75 mm. The flattened portions (41) of the predetermined breaking zone (40) lying adjacent to one another along the circumference do not contact one another, although their centroids lie on a plane that is intersected perpendicularly by the center line (3). The flattened portions thus form a square profile with rounded corner areas.

[0025] The rectangular edges lying transversely to the center line (3) are adjoined - in the longitudinal direction of the twisting instrument (1) - by flattening runs (42), the concave curved surfaces of which are here partial surfaces of a cylinder, the individual diameter of which measures, for example, 1 mm.

[0026] The cross-section of the predetermined breaking zone (40) is at least 18% smaller than the regular cross-section of the neck main cylinder zone (31).

[0027] The neck cone zone (35), which adjoins the main neck cylinder zone (31) at the front, reduces its diameter to, for example, 1.1 mm towards the working part (60). Starting from the main neck cylinder zone (31), the neck cone zone (35) encloses a cone angle of 2.86 degrees. The neck cone zone (35) has a length of, for example, 4 mm. Adjoining it is a secondary neck cylinder zone (36), which is, for example, 1 mm long and extends to the working part (60).

[0028] The Figures 12-27 show twisting instruments that are used in the area of ​​the Figures 26 and 27in the handpiece (80) have different predetermined breaking zones (40). In front of each predetermined breaking zone (40) there is a second, 0.9 mm wide drive flattened portion (45), the distance of which from the center line (3) corresponds to the center line distance of the first drive flattened portion (15). The second drive flattened portion (45) has the task of serving as a drive surface for introducing a torque after the predetermined breaking zone (40) bursts. The second drive flattened portion (45) ends at the front - like the first drive flattened portion (15) - in a flat front stop surface (46), for example. Both front stop surfaces (16, 46) are spaced from each other by 4.8 mm, for example.

[0029] In the Figures 12 and 13 Immediately behind the second drive flat (45) there is an annular groove (51) that serves as a predetermined breaking zone, e.g., 0.725 mm deep and e.g., 0.9 mm wide. The annular groove (51) has a rectangular cross-section.

[0030] The Figures 14and 15 Instead of the annular groove (51), they feature a groove (52) of equal width, the base contour of which is a semicircle. The groove (52) is also 0.725 mm deep.

[0031] The predetermined breaking zone of the Figures 16 and 17 consists of a short cylindrical section (54) and a notch (53), the notch flanks of which form an angle of 45 degrees with the center line (3). The cylindrical section (54) located in front of the sharp notch (53) has a diameter of 1.35 mm and a length of 0.53 mm. The minimum diameter of the predetermined breaking zone in the area of ​​the 90° notch is 0.9 mm. This minimum diameter applies to all variants of the Figures 12-27 .

[0032] The Figures 18 and 19Between the second (45) and the first drive flat (15), there is a notched cone (55) of approximately 3.21 mm in length, whose cone angle is approximately 24.4 degrees. The narrowest point of the notched cone (55) ends in a flat, partial annular surface immediately in front of the second drive flat (45).

[0033] In the Figures 20 and 21 The predetermined breaking zone forms a circumferential groove (56) with a circular cross-section. The radius of the groove contour measures, for example, 1.25 mm. There is a transition radius of, for example, 0.25 mm toward the drive shaft (10).

[0034] Each predetermined breaking zone of the Figures 22-25 is a circumferential groove with a Mattheck double contour (57). The simple Mattheck contour (29) is in connection with the neck transition zone (21) of the Figure 6 described. In the case of Figures 22-25two Mattheck contours (29) are arranged next to each other or mirrored in such a way that the narrowest tensile triangles of both contours contact each other at their acute-angled corners.

[0035] The predetermined breaking zone of the Figures 24-25 is surrounded by a protective sleeve (59) that fills the circumferential groove (57). The cylindrical outer wall of the protective sleeve (59) has a diameter that is, for example, 0.15 mm smaller than the diameter of the drive shaft (10). In order to securely hold the protective sleeve (59) on the front part of the drive shaft (10), there is a small retaining recess (58) in the second drive flat (45), e.g. in the form of a short bore, into which the material of the protective sleeve (59) engages in a form-fitting manner.

[0036] In this way, if the predetermined breaking zone breaks, the protective sleeve (59) remains attached to the front part of the twisting instrument (1), so that there is no risk of injury to the patient if the handpiece (80) slips out.

[0037] The Figures 26 and 27 show a handpiece (80) into which the Figure 20 The twisting instrument shown is inserted securely and cannot be twisted. In the sectional view of the Figure 27 It is clearly visible that the predetermined breaking zone is located in the central area of ​​the receiving bore (81) of the handpiece (80). This ensures that if the predetermined breaking zone breaks, there is no risk of injury to the user of the twisting instrument (1).

[0038] The working part (60) is a hexalobular ball head (61). The latter is suitable for torque-transmitting engagement in a tool recess (92) of a screw-in component (90) with a hexalobular socket according to DIN EN ISO 10664. The hexalobular ball head (61) has six teeth (63) on its circumference, between which there are six tooth gaps (66), see also Figure 2 The tooth tips (64) of the teeth (63) have tip roundings that directly transition into the tooth root roundings in the area of ​​the tooth gaps (66). The working part (60) transmits the torque to the hexagon socket (92) of the screw-in component (90) via the flanks (65) of its teeth (63).

[0039] As a semi-finished working part, the working part (60) consists of a ball formed on the front end of the neck (20) or the neck secondary cylinder zone (36) - located on the center line (3), cf. Figure 3 The surface of this sphere represents the surface of the finished product Figures 3-5dashed envelope surface (62). It has a diameter of e.g. 1.38 mm

[0040] In a first step, according to Figure 3 , at the transition to the neck secondary cylinder zone (36), a rear grip (67) is created, which enables the center line (3) of the twisting instrument (1) to be tilted by a bending angle (7) of up to 22 degrees relative to the center line (99) of the screw-in component (90). The rear grip (67) has a radius contour, the radius of which measures, for example, 0.55 mm. The radius contour merges tangentially into the neck secondary cylinder zone (36). The center of the radius contour is located, for example, 0.4 mm behind the center point of the envelope surface (62) lying on the center line (3).

[0041] In a further operation, the ball of the semi-finished working part is toothed using the forming tool (100), see. Figure 4. Depending on the tool type, the toothing is machined by grinding or milling or formed in a rolling process using the forming tool (100).

[0042] For this purpose, the forming tool (100) has a profile disc (102) formed on a tool shank (101), cf. Figure 7. The profile disc (102), which has a diameter of 4 mm, for example, has a central tooth gap profile (105) and two edge-side tooth tip profiles (107). The tooth gap profile (105) is part of a torus surface, the diameter of the gap circle (106) of which is 0.74 mm, for example. The edge-side tooth tip profiles (107) are each also part of a torus surface, the diameter of the tooth tip circle (108) of which is 0.22 mm, for example. The centers of the tooth tip circles (108) are each spaced from the center of the gap circle (106) by 0.29 mm, for example, parallel to the center line (3). They are also offset radially outwards from the center of the gap circle (106) by 0.384 mm. The tooth tip circles (108) each contact the gap circle (106) at a point of contact. At the two points of contact, both circles (106, 108) have identical tangents.

[0043] To form the six teeth (63) and the corresponding tooth gaps (66), the forming tool (100) is moved along a Figure 4 The tooth gap base contour (71) shown in the figure is guided. The latter consists of a front, straight gap base area (72), a middle, curved gap base area (73), and a rear, also curved gap base area (74). All three gap base areas (72-74) merge tangentially into one another.

[0044] The middle gap base area (73) is a circular arc that covers an angle of 40 degrees and 26 minutes.

[0045] The circular arc has a radius of, for example, 0.515 mm. The center of the central gap base area (73) corresponds to the center of the spherical envelope (62). The foremost point of the central gap base area (73) lies on a fictitious radius ray (75) that forms an angle of 50 degrees with the center line (3).

[0046] The straight front gap base area (72) adjoins the frontmost point of the central gap base area (73). The latter ends at the free end (76) of the working part (60). The straight front gap base area (72) forms an angle of 40 degrees with the center line (3).

[0047] The rear gap base area (74) begins at the rear end of the middle gap base area (73). It has a radius of, for example, 2.175 mm. The rear gap base area (74) ends in the rear grip (67) of the working part (60).

[0048] After each traverse of the tooth gap base contour (71), the semi-finished work part is pivoted by 60 degrees for the next forming process. This creates a toothing as shown in Figure 5 as a cross section through the center of the enveloping surface (62) and in Figure 2 shown in a perspective view. After Figure 2The tip (76) of the working part (60) is a star-shaped head (77), the smallest diameter of which measures, for example, 0.18 mm. The curvature of the star-shaped head (77) corresponds to the curvature of the enveloping surface (62).

[0049] All dimensions refer to a hexagon socket for screws according to DIN EN ISO 10664, type no. 5.

[0050] Depending on the abutment type, the screw-in component (90) is a screw, a threaded bolt, or the like. The head of the screw or at least one end of the threaded bolt has a tool recess (92) that represents a hexalobular socket according to DIN EN ISO 10664, cf. Figures 8-11 According to these figures, the tool recess (92) has a 30° chamfer (94) in the area of ​​its opening (93), the width of which is less than half the depth (98) of the usable area of ​​the tool recess (92). The width of the 30° chamfer (94) is Figures 9 and 11 e.g. 0.36 mm.

[0051] The tool recess (92) has a base (95) in the shape of a right cone, with a cone angle of, for example, 118 degrees, and a tip (76) located farther from the opening (93) of the tool recess (92) than the cone base line. The twisting instrument (1) rests on this base (95) during each screwing or unscrewing of the screw-in component (90), ensuring secure engagement of the working part (60) in the tool recess (92).

[0052] Alternatively, a base (96) can be provided in the tool recess (92), the base surface of which is spherically curved, see the dashed arc in the Figures 9 and 10The curvature of the base (96) has a radius of, for example, 0.7 mm. The center of the curvature is located, for example, 0.265 mm below the flat end face of the screw-in component (90) surrounding the tool recess (92). If necessary, the base can also be a differently curved surface of revolution or even a lowered flat surface.

[0053] To screw the screw-in component (90) into the implant body (not shown in the figures), the twisting instrument (1) is inserted with its working part (60) into the tool recess (92) of the screw-in component (90). On the one hand, the teeth (63) of the working part (60) engage in the tooth gaps of the tool recess (92) with minimal play. On the other hand, the tip (76) or the head star (77) of the working part (60) rests against the base (96) of the screw-in component (90), see. Figures 8-11 .

[0054] In the Figure 8 and 10the working part (60) of the twisting instrument (1) is cut in the area of ​​the tooth heads (64) of the external hexalobular teeth (63), while in the Figures 9 and 11 The screw-in component (90) is shown pivoted, ideally, by 30 degrees around its center line (99). Accordingly, the twisting instrument (1) has also been pivoted, ideally, by 30 degrees around its center line (3).

[0055] The cardan error, which increases with increasing bending angle (7), causes, at least theoretically, an offset and a change in the angle of the center line (99) of the twisting instrument (1) between the Figures 8 and 9 as well as 10 and 11. This is neglected here.

[0056] Towards the end of the screwing-in process, the tightening torque increases abruptly. In order to make reaching the maximum tightening torque on the handpiece (80) of the twisting instrument (1) tactilely perceptible for the user, the neck (20) of the twisting instrument (1) twists Figure 1 by 10-12 angular degrees, without any measurable plastic deformation occurring in the predetermined breaking zone (40). List of reference symbols:

[0057] 1Torsion instrument, screwdriving instrument 3Midline 7Flexion angle, angle between (3) and (99) 10Drive shaft 11Shaft section, cylindrical 15Drive flat, first 16End stop surface 17Groove 20Neck, instrument neck 21Neck transition zone 23First transition zone area 24Second transition zone area 25Third transition zone area 26First pull triangle, right-angled 27Second pull triangle 28Third pull triangle 29Mattheck contour 31Neck cylinder zone, neck main cylinder zone 35Neck cone zone, neck section 36Neck cylinder zone, neck secondary cylinder zone, neck section 40Predetermined breaking zone 41Flats, four, flat 42Flat runouts 45Drive flat, second 46End stop surface 51Annular groove 52Groove 53Notch, 90° notch 54Cylinder section 55Notched cone, cone 56Circular groove 57Mattheck double contour; circumferential groove 58Retaining recess 59Protective sleeve 60Working part 61Spherical head, external hexalobular head 62Enveloping surface, spherical, ball 63Teeth, external hexalobular teeth 64Tooth tips 65Flanks, tooth flanks 66Tooth gaps 67Undercut 71Tooth gap base contour 72Anterior gap base area, straight 73Middle gap base area, curved 74Posterior gap base area, curved 75Radius ray of (60), fictitious 76Free end, tip 77Head star 80Handpiece, handle, actuating element 81Hole 90Screw-in component, screw 91Head, hexalobular socket 92Tool recess, hexalobular socket, Torx ®< 93Opening 94Chamfer, 30° chamfer 95Bottom of (92), conical 96Bottom, spherically curved 98Depth, penetration depth of (92) 99Center line of (90) 100Form tool 101Tool shank 102Profile disc 105Tooth gap profile, center 106Gap circle, circle 107Tooth tip profile, edge side 108Tooth tip circles, circles

Claims

1. Twisting instrument for a screw-in component (90) with a tool recess (92), which comprises a drive shaft (10), a neck (20) and a working part (60), - wherein the working part (60) - as a spherical head (61) having six teeth (63) - has a spherical enveloping surface (62) which is contacted at least both by the free end (76) and by partial regions of the tooth-own tooth heads (64) of the working part (60), - wherein there are six tooth gaps (66) between the teeth (63), - wherein the neck (20) or the drive shaft (10) has a predetermined breaking zone (40), - wherein the neck (20) consists of a neck transition zone (21), a neck main cylinder zone (31), a neck cone zone (35) and a neck secondary cylinder zone (36), and - wherein the spherical head (61) has a rear grip (67) at the transition of the spherical head (61) to the neck subcylinder zone (36), the radius contour of which merges tangentially into the neck subcylinder zone (36).

2. Twisting instrument according to claim 1, characterised in that the drive shaft (10) is of type 1 according to DIN EN ISO 1797, which is provided at its free end with a drive flattening (15) and a groove (17).

3. Twisting instrument according to claim 1, characterised in that the neck transition zone (21) comprises a first (23), second (24) and third transition zone region (25), wherein the first transition zone region (23) adjoining the drive shaft (10) is a tapering straight cone with a cone angle of 90 degrees, wherein the second transition zone region (24) adjoining the first transition zone region (23) is a tapering straight cone with a cone angle of 45 degrees of angle, and wherein the third transition zone region (25) adjoining the second transition zone region (24) is a tapering straight cone with a cone angle of 22.5 degrees of angle.

4. Twisting instrument according to claim 1, characterised in that the neck cylinder zone (31) adjoining the drive shaft (10) has a length which corresponds to at least 6.6 times the diameter of the neck cylinder zone (31) and in that the neck section located between the neck cylinder zone (31) and the working part (60) has a length which is at least 5.5 times the smallest diameter of this neck section.

5. Twisting instrument according to claim 1, characterised in that the predetermined breaking zone (40) is arranged in the rear region of the neck cylinder zone (31) adjoining the neck transition zone (21).

6. Twisting instrument according to claim 1, characterised in that the predetermined breaking zone (40) has four flattenings (41) arranged equidistantly on the circumference of the neck cylinder zone (31) adjoining the neck transition zone (21).

7. Twisting instrument according to claim 6, characterised in that the adjacent flattenings (41) of the predetermined breaking zone (40) do not contact each other.

8. Twisting instrument according to claim 1, characterised in that the spherical head (61) is an external hexagonal spherical head.