Fixture, a thread maker and a fixture set

Abstract

The invention relates to a fixture, such as a dental fixture, for insertion into a bore hole arranged in bone tissue. The fixture has two condensation portions, which may be designed to provide the same or different tensile strain levels to the cortical and cancellous bone tissue, respectively. The invention also relates to a thread maker for making a female thread in bone tissue prior to insertion of a fixture. The invention further relates to a fixture set, having a thread maker and a fixture.

Claims

1. A fixture for insertion into a bore hole arranged in bone tissue, comprising a threaded first portion provided with at least one apical cutting edge for making a female thread in the bone tissue, a threaded non-cutting second portion located coronally of the first portion and being wider than the first portion with respect to major and/or minor fixture diameter, a threaded third portion located coronally of the second portion and provided with at least one coronal cutting edge for processing the female thread already made by the first portion and for making a separate female thread in the bone tissue, the threaded third portion having a cylindrical profile, a threaded non-cutting fourth portion located coronally of the third portion, the threaded non-cutting portion having a cylindrical profile and being wider than the third portion with respect to major and/or minor fixture diameter; a coronal widening transition portion coronally bordering the threaded third portion and apically bordering the threaded non-cutting forth portion; wherein the threaded third portion includes a thread change such that a single thread spiral, which runs from at least the threaded non-cutting second portion into the threaded third portion, changes into at least a double thread spiral about the at least one coronal cutting edge of the threaded third portion.

2. The fixture as in claim 1, wherein the third portion comprises a thread spiral which upon insertion into the bore hole is received by the female thread made by the first portion.

3. The fixture as in claim 1, wherein said first, second, third and fourth portions are each adapted to be anchored in a bone tissue surrounding a blind bore.

4. The fixture as in claim 1, wherein the fixture is threaded at least along 80% of its axial length.

5. The fixture as in claim 1, wherein the difference in major fixture diameter between the second portion and the first portion is greater than the difference in major fixture diameter between the fourth portion and the third portion.

6. The fixture as in claim 1, wherein the difference in minor fixture diameter between the second portion and the first portion is greater than the difference in minor fixture diameter between the fourth portion and the third portion.

7. The fixture as in claim 1, comprising an apical transition portion which tapers in the apical direction and which is arranged between said first portion and said second portion, a coronal transition portion which tapers in the apical direction and which is arranged between said third portion and said fourth portion, or both.

8. The fixture as in claim 7, wherein the threads in the second portion have the same thread profile as the profile of the threads in the apical transition portion, the threads in the fourth portion have the same thread profile as the profile of the threads in the coronal transition portion, or both.

9. The fixture as in claim 1, wherein each one of said first portion and said second portion is provided with at least one thread spiral, and wherein each one of said third portion and said fourth portion is provided with at least one more thread spiral than said first and second portions and having the same lead as said at least one thread spiral in the first and second portions.

10. The fixture as claimed in claim 9, wherein the number of thread spirals in said fourth portion is a multiple integer of the number of thread spirals in said second portion.

11. The fixture as in claim 1, wherein the smallest spacing between adjacent thread tops in the fourth portion is smaller than the smallest spacing between adjacent thread tops in the second portion.

12. The fixture as in claim 1, wherein the threads in the second portion have the same thread profile as the profile of the threads in the first portion, the threads in the fourth portion have the same thread profile as the profile of the threads in the third portion, or both.

13. The fixture as in claim 12, wherein said thread profile is a microthread profile.

14. The fixture as in claim 1, wherein the threads in the first portion and the second portion have the same top radius, the same apical flank angle and the same coronal flank angle, the threads in the third portion and the fourth portion have the same top radius, the same apical flank angle and the same coronal flank angle, or both.

15. The fixture as in claim 1, wherein the axial length of the threading of the second portion is greater than 1 mm, the axial length of the threading of the fourth portion is about 0.5-4 mm, or both.

16. The fixture as in claim 1, wherein the axial length of the threading of the second portion is greater than 3 mm, the axial length of the threading of the fourth portion is about 0.5-4 mm, or both.

17. The fixture as in claim 1, wherein the axial length of the threading of the second portion is greater than 4 mm, the axial length of the threading of the fourth portion is about 13 mm, or both.

18. The fixture as in claim 1, wherein in said first portion, the largest radial distance from the fixture axis to a thread top of said apical cutting edge is r.sub.t, in said second portion the smallest radial distance from the fixture axis to a thread top is R.sub.t, in said third portion the largest radial distance from the fixture axis to a thread top of said coronal cutting edge is R′.sub.t, in said fourth portion the smallest radial distance from the fixture axis to a thread top is R″.sub.t, and wherein r.sub.t<R.sub.t, r.sub.t<R′.sub.t, and R′.sub.t<R″.sub.t.

19. The fixture as in claim 18, wherein the ratio $\frac{R_t-r_t}{r_t}$ is in the range of 0.01-0.3, wherein the ratio $\frac{R_t^{″}-R_t^{\prime }}{R_t^{\prime }}$ is in the range of 0.01-0.1, or both.

20. The fixture as in claim 18, wherein the ratio $\frac{R_t-r_t}{r_t}$ is in the range of 0.06-0.3, wherein the ratio $\frac{R_t^{″}-R_t^{\prime }}{R_t^{\prime }}$ is in the range of 0.01-0.03, or both.

21. The fixture as in claim 18, wherein the ratio $\frac{R_t-r_t}{r_t}$ is in the range of 0.06-0.1, wherein the ratio $\frac{R_t^{″}-R_t^{\prime }}{R_t^{\prime }}$ is in the range of 0.01-0.02.

22. The fixture as in claim 1, wherein in said first portion the largest radial distance from the fixture axis to a thread bottom of said apical cutting edge is r.sub.b, in said second portion the smallest radial distance from the fixture axis to a thread bottom is R.sub.b, in said third portion the largest radial distance from the fixture axis to a thread bottom of said coronal cutting edge is R′.sub.b, in said fourth portion the smallest radial distance from the fixture axis to a thread bottom is R″.sub.b, and wherein r.sub.b<R.sub.b, r.sub.b<R′.sub.b, and R′.sub.b<R″.sub.b.

23. The fixture as claimed in claim 22, wherein the ratio $\frac{R_b-r_b}{r_b}$ is in the range of 0.01-0.3, wherein the ratio $\frac{R_b^{″}-R_b^{\prime }}{R_b^{\prime }}$ is in the range of 0.01-0.1, or both.

24. The fixture as claimed in claim 22, wherein the ratio $\frac{R_b-r_b}{r_b}$ is in the range of 0.06-0.3, wherein the ratio $\frac{R_b^{″}-R_b^{\prime }}{R_b^{\prime }}$ is in the range of 0.01-0.03, or both.

25. The fixture as claimed in claim 22, wherein the ratio $\frac{R_b-r_b}{r_b}$ is in the range of 0.06-0.1, wherein the ratio $\frac{R_b^{″}-R_b^{\prime }}{R_b^{\prime }}$ is in the range of 0.01-0.02, or both.

26. The fixture as in claim 1, wherein said fixture is a dental fixture for arrangement in a jawbone.

27. The fixture as claimed in claim 26, wherein the fixture is adapted for arrangement in the mandible such that each one of said first, second, third and fourth portions is anchored in the mandible.

28. The fixture as claimed in claim 26, wherein the fixture is adapted for arrangement in the maxilla such that each one of said first, second, third and fourth portions is anchored in the maxilla.

29. The fixture as in claim 26, wherein the length of the fixture is 5-19 mm.

30. The fixture as in claim 1, comprising an apical transition portion which tapers in the apical direction and which is arranged between said first portion and said second portion and a coronal transition portion which tapers in the apical direction and which is arranged between said third portion and said fourth portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating a stress/strain relationship for cortical bone.

(2) FIG. 2 is a graph illustrating a stress/strain relationship for cancellous bone.

(3) FIG. 3 illustrates a fixture according to at least one example embodiment of the invention.

(4) FIG. 4 illustrates a fixture according to at least another example embodiment of the invention.

(5) FIG 5a illustrates in cross-section a detail of a fixture according to at least one example embodiment of the invention.

(6) FIG. 5b illustrates in cross-section a detail of a fixture according to at least one example embodiment of the invention.

(7) FIG. 6 illustrates in cross-section a detail of a fixture according to at least another example embodiment of the invention.

(8) FIG. 7 illustrates in cross-section a detail of a fixture according to at least yet another example embodiment of the invention.

(9) FIG. 8 illustrates a fixture set according to at least one example embodiment of the invention, the fixture set comprising a fixture and a thread maker according to at least one example embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) FIG. 1 is a graph illustrating a stress/strain relationship for cortical bone. In an article by McCalden R. W. et al. the relationship between ultimate strain and age is presented (McCalden R. W. et al., Age-related changes in the tensile properties of cortical bone, The Journal of Bone and Joint Surgery, Vol. 75-A. No. 8, August 1993). From the article, one learns that the ultimate strain is substantially linearly dependent on the person's age. For instance, an 80 year old person has in cortical bone an ultimate strain of about 0.015, a 50 year old person has an ultimate strain of about 0.025, while a 20 year old person has an ultimate strain of about 0.035. For cortical bone the yield strain is about half the ultimate strain. For instance, with reference to FIG. 1, in a 20 year old person, for a strain up to about 0.018, the stress/strain relationship could be linear and represents an elastic deformation of the bone. The interval between 0.018 and 0.035 is non-linear and represents a plastic deformation of the cortical bone. Similarly, for an 80 year old person, a strain up to 0.008 would correspond to the linear relationship and the interval between 0.008 and 0.015 would correspond to the non-linear relationship in FIG. 1.

EXAMPLE

(11) Screw shaped fixtures, manufactured from commercially pure titanium, grade 4, were used. In order to reduce a possible grinding effect during insertion the fixtures had a turned surface. The endosseous part of the fixtures comprised three different portions; one leading (cutting) portion, one transition portion with a gradual increase in diameter and one trailing (condensation) portion. The bone bed was drilled to a final burr diameter of 3.3 mm corresponding to the core diameter (2r.sub.b) of the cutting portion of the fixture. When the fixture was inserted the cutting features created a cavity in the bone which was congruent with the fixture shape of the cutting portion. When the transition portion entered the bone it created a gradual increase in the strains in the surrounding bone without cutting. When finally the condensation portion entered the bone the predetermined bone condensation was obtained. The fixtures were installed with a standardized rotation speed of 20 revolutions/minute. Two types of test fixtures were used; one where the increase in diameter was 0.15 mm (referred to as “Group 0.15”) and another with a diameter increase of 0.05 mm (referred to as “Group 0.05”). The control fixtures had no diameter increase.

(12) The fixtures were inserted in tibia of rabbits. Test fixtures were always inserted in the left leg and control fixtures in the right leg. Group 0.15 fixtures were installed proximally in the proximal tibia metaphysis. Group 0.05 fixtures were installed distally in, the proximal tibia metaphysis.

(13) After 3.5 weeks, all fixtures were subjected to removal torque (RTQ) tests. The peak RTQ was investigated with a computerized control RTQ device, in which the values were transmitted at a frequency of 100 per second to the computer via a control box.

(14) The fixture head was connected to the instrument, and an increasing reverse torque was applied to all the fixtures until failure of the bone-fixture interface occurred. The first peak values of resistance to reverse torque rotation were recorded in Ncm.

(15) Prior to the animal experiment a 2D axisymmetric finite element model of the trailing portion of the fixture and the surrounding bone was developed. The fixture and the bone were modelled in a CAD software Pro/Engineer (PTC Corporate Needham, Mass. USA) and then transferred into the finite element software ANSYS 12.01 (ANSYS, Inc. Canonsburg, Pa., USA). The strain in the bone was induced by radial displacement of the fixture surface by 0.025 mm and 0.075 mm simulating a diameter increase of 0.05 mm and 0.15 mm respectively. The simulated maximum principal strain in the surrounding bone for Group 0.15 fixtures was ˜0.045 (0.15 mm divided by 3.3 mm=0.045). For group 0.05 fixtures the maximum principal strain obtained was ˜0.015 (0.05 mm divided by 3.3 mm=0.015).

(16) In all sites the removal torque of the test fixtures was higher than that of the corresponding control fixtures. See Table 1.

(17) TABLE-US-00001 TABLE 1 Comparison between removal torque for test fixtures and control fixtures. Average torque Test Average torque Control Removal Ncm (Std) Ncm (Std) Tibia proximal 26.0 (6.89) 16.8 (7.83) (Group 0.15) Tibia distal 23.0 (5.31) 17.2 (5.29) (Group 0.05)

(18) Strain in cortical bone from rabbits has been measured by Shunmugasamy V. C. et al. and presented in an article (Shunmugasamy V. C. et al., High strain rate response of rabbit femur bones. Journal of Biomechanics, 2010; 43: 3044-3050). The ultimate strain of rabbit cortical bone was measured to be about 0.02.

(19) In the present study the fixtures were just supported by cortical bone. It should be noted that the Group 0.15 fixtures gave rise to strains (0.045) which exceeded the ultimate strain (˜0.02) of cortical rabbit bone. In spite of this there was no evidence of reduced removal torque. On the contrary the removal torque of the experimental fixtures was higher than that of the control fixtures which were designed not to produce static strains in the bone. It is striking that the very highest removal torque was obtained for Group 0.15 fixtures for which the strains induced by far exceeded the ultimate strains. From the values in Table 1, one can simply calculate that for Group 0.15 fixtures the removal torque was increased by 55%, and for Group 0.05 fixtures the removal torque was increased by 34%. Obviously, the stresses in the bone, which were induced during fixture insertion, are maintained for a considerable time.

(20) This study indicates that an increased strain provides better initial fixture stability, it is also noticeable that increased strain provides a better stability after 3.5 weeks.

(21) In the above-mentioned article by McCalden R. W one learns that the ultimate strain is substantially linearly dependent on the person's age. The above discussed ultimate strain (˜0.02 of rabbits) can be seen for a 70 year old person. While the rabbit experiments in the above discussed example showed a successful result for a strain of 0.045, which by far exceeds the ultimate strain of cortical rabbit bone (2¼ times the ultimate strain of cortical rabbit bone), and also exceeds the ultimate strain of cortical bone of a 70 year old human, it is anticipated that an even higher strain would be successful in a younger person's cortical bone. For a 20 year old person, it would correspond to applying a strain of about 0.08 (2¼ times the ultimate strain 0.035 of a 20 year old person). For a child or adolescent the ultimate strain is even higher, for instance 0.04, which means that a strain of 0.09 could be applied. The rabbit study in the above example did not measure the upper limit for suitable static radial strain, but since the Group 0.15 fixtures surprisingly provided an even better result than the 0.05 fixture, it is reasonable to assume that even higher strains relative to the ultimate strain may be suitable for cortical bone.

(22) While the above study analyzed the strain in cortical bone, an analogy may be made to strains in cancellous bone. Thus, similarly to the previous explanations with regard to providing a tensile strain in cortical bone above the yield strain, a beneficial biological response may also be triggered by providing a tensile strain in cancellous bone above the yield strain of the cancellous bone.

(23) FIG. 2 is a graph illustrating a stress/strain relationship for cancellous bone. The behavior of the graph up to the yield point is similar to that of FIG. 1, i.e. a linear relationship is presented. However, the curved part above the yield point is different and more stretched. According to Gibson, the yield strain is about 0.06 for cancellous bone (Gibson, J. Biomechanics, Vol. 18, No. 5, pp 317-328, 1985). Drawing conclusions from an article by Kold S. et al. (Kold S. et al., Compacted cancellous bone has a spring-back effect. Acta Orthopaedica Scandinavica, 2003; 74(5): 591-595) the yield strain for cancellous bone may be even higher. According to Kold S. et al. a bore hole of 5.0 mm in diameter was made in cancellous bone. The bone was then compacted by expanding the bore to 5.6 mm, after which the bone sprung back. During the compaction, the tensile strain ΔD/D on the cancellous bone was therefore 0.6/5=0.12. Thus, the yield strain in cancellous bone is multiple that of the yield strain in cortical bone. In addition, the plastic deformation of cancellous bone is much more stretched than for cortical bone. Thus, since a strain level of 0.1 is considered by the inventors to be suitable for cortical bone tissue, at least for some age groups, a strain level of 0.3 should be suitable for cancellous bone tissue.

(24) FIG. 3 illustrates a fixture 2 according to at least one example embodiment of the invention. The fixture 2 comprises a coronal end 4 and an apical end 6. Extending coronally from the apical end 6 is a threaded first portion 10 provided with at least one apical cutting edge 11 for making a female thread in the bone. For instance there may be two, three, four or more cutting edges, suitably evenly distributed around the central axis of the fixture 2. The entire first portion 10 may be threaded or, as illustrated in FIG. 3, an apical section of the first portion 10 may be non threaded. The first portion 10 may be substantially cylindrical or, as illustrated in FIG. 3, tapering towards the apical end 6. Another alternative is to have a substantially cylindrical coronal section of the first portion 10 and a tapering apical section. For the purpose of providing the desired strain to the bone tissue, it does not matter which of the shapes the first portion 10 has. Instead, with respect to dimensioning the strain, what matters is the width of the first portion 10 at the coronal end 11a of the apical cutting edge 11. This width is what will determine the depth of the female thread created in the bone. A threaded non-cutting second portion 14 of the fixture 2 which is overdimensioned in relation to the depth of the female thread will create the strain in the bone.

(25) An apical transition portion 12 is located between the first portion 10 and the second portion 14. Thus, the apical transition portion 12 borders apically to the first threaded portion 10 and coronally to the second threaded portion 14. The apical transition portion 12 lacks cutting edges and widens the fixture 2 in the coronal direction. The second portion 14 is substantially cylindrical and will therefore, when arranged in the female bone thread which has been cut by the apical cutting edge 11, provide a substantially static tensile strain to the surrounding bone tissue. The second portion could be made slightly widening in the coronal direction in order to compensate for any grinding effect.

(26) In the illustrated embodiment, both the minor fixture diameter and major fixture diameter have increased from the first threaded portion 10 to the second non-cutting threaded portion 14. In other words,

(27) $\frac{R_t-r_t}{r_t}>0,\frac{R_b-r_b}{r_b}>0,$
wherein,

(28) r.sub.t is the largest radial distance from the central fixture axis C to a thread top 22 of said apical cutting edge 11 in said first portion 10,

(29) R.sub.t is the radial distance from the central fixture axis C to a thread top 22 in said second portion 14,

(30) r.sub.b is the largest radial distance from the central fixture axis C to a thread bottom 24 of said apical cutting edge 11 in said first portion 10,

(31) R.sub.b is the radial distance from the central fixture axis C to a thread bottom 24 in said second portion 14.

(32) Suitably, the above ratios may be in the range of 0.01-0.3, such as in the range of 0.06-0.3, suitably as in the range of 0.06-0.1.

(33) Thus, the above ratios provide a measure of the tensile strain which may be provided to the bone by the apical strain-creating zone 26 (which comprises the first portion 10 and the second portion 14 and the intermediate apical transition portion 12). Within the apical strain-creating zone 26, the first portion 10 may be regarded as a leading portion, while the second portion 14 may be regarded as a trailing or condensation portion. The apical strain-creating zone 26 is suitably configured and dimensioned to affect cancellous bone tissue.

(34) The fixture 2 is also provided with a coronal strain-creating zone 28, which comprises a threaded third portion 16, a coronal non-cutting transition portion 18 and a threaded non-cutting fourth portion 20. Within the coronal strain-creating zone 28, the third portion 16 may be regarded as a leading portion and the fourth portion 20 may be regarded as a trailing or condensation portion. The coronal strain-creating zone 28 is suitably configured and dimensioned to affect cortical bone tissue.

(35) More specifically, in the illustration of FIG. 3, the third portion 16 having at least one coronal cutting edge 17 borders to the second portion 14. Thus, the third portion 16 can be regarded as starting with the apical end 17b of said coronal cutting edge 17 and terminating with the coronal end 17a of said coronal cutting edge 17.

(36) Unless the bore in the bone has been prepared so as to have a greater diameter at the coronal cortical bone compared to the apical cancellous bone, then the second portion 14 will provide the above discussed strain also to the cortical bone during insertion of the fixture 2. However, as the coronal cutting edges 17 of the third portion 16 follows the second portion 14 during insertion of the fixture 2, the strain in the bone will at least temporarily be relieved since the female bone threads will be cut deeper by the third portion 16 rather than being condensed by the second portion 14.

(37) Similarly to the apical strain-creating zone 26, the thread profile of the coronal strain-creating zone 28 remains unchanged and both the minor and major fixture diameters are increased from the third portion 16, via the coronal non-cutting transition portion 18, to the fourth non-cutting portion 20. In other words,

(38) $\frac{R_t^{″}-R_t^{\prime }}{R_t^{\prime }}>0,\frac{R_b^{″}-R_b^{\prime }}{R_b^{\prime }}>0,$
wherein,

(39) R′.sub.t is the largest radial distance from the central fixture axis C to a thread top 22 of said coronal cutting edge 17 in said third portion 16,

(40) R″.sub.t is the radial distance from the central fixture axis C to a thread top 22 in said fourth portion 20,

(41) R′.sub.b is the largest radial distance from the central fixture axis C to a thread bottom 24 of said coronal cutting edge 17 in said third portion 16,

(42) R″.sub.b is the radial distance from the central fixture axis C to a thread bottom 24 in said fourth portion 20,

(43) Suitably, the above ratios may be in the range of 0.01-0.1, such as in the range of 0.01-0.03, suitably in the range of 0.01-0.02.

(44) FIG. 4 illustrates a fixture 50 according to at least another example embodiment of the invention. The apical strain-creating zone 52 of the fixture 50 in FIG. 4 corresponds to the apical strain-creating zone 26 of the fixture 2 in FIG. 3. However, the coronal strain-creating zone 54 of the fixture 50 in FIG. 4 is different from the coronal strain-creating zone 28 of the fixture 2 in FIG. 3.

(45) The coronal strain-creating zone 54 of the fixture 50 shown in FIG. 4 has a third threaded portion 56 provided with at least one cutting edge 57, which is coronally followed by a fixture-widening coronal transition portion 58, which in turn is coronally followed by a threaded non-cutting fourth portion 60. About halfway along the axial extension of the third portion 56, the threading is changed. The single thread spiral 62 which runs from the first portion 64, via the apical transition portion 66 and the second portion 68, to the third portion 56 changes into a double thread spiral 63 having the same lead as the single thread spiral 62, but half the pitch of the single thread spiral 62. The smaller axial top-to-top distance of the double thread spiral 63 enables the stiffness of the fixture 50 to be increased, thereby improving the ability of the fixture 50 to transmit loads more evenly to the cortical bone tissue, which reduces the risk of marginal bone resorption.

(46) Furthermore, the thread depth of the double spiral 63 is smaller than the thread depth of the single spiral 62. For instance, the large single thread spiral 62 may be a macrothread, while the smaller double thread spiral 63 may comprise microthreads. Nevertheless, the radius of curvature of the thread top and the angle of the thread flanks may be substantially the same for both types of threading. An example of profiles will be discussed later in connection with FIGS. 5a and 5b.

(47) Continuing with FIG. 4, the coronal cutting edge 57 will thus cut two female thread spirals in the bone, one of which is new and one of which is a processing/deepening of the female bone thread already created by the apical cutting edge 65.

(48) The following widening caused by the coronal transition portion 58 and the static strain provided by the fourth non-cutting portion 60 follows the previously discussed principles. Thus, the width of the fourth portion 60 compared to the width of the third portion 56 at the coronal cutting edge 57 will provide a measure of the tensile strain transmittable to bone tissue. It should be understood that since the third portion 56 and fourth portion 60 has a double thread spiral, which for explanatory purposes will now be referred to as spirals A and B, different strain-effects may be created. The strain caused by spiral A depends on the difference in width of spiral A in the third and fourth portions 56, 60. The strain caused by spiral B depends on the difference in width of spiral B in the third and fourth portions 56, 60. Thus, if as illustrated in FIG. 4, said differences between the third and fourth portions 56, 60 are the same for spiral A and spiral B, there will be no differentiation in strain between the two spirals. However, if, when comparing the fourth portion 60 with the third portion 56, spiral A has a larger increase in width than spiral B, then spiral A will provide a larger strain to the bone.

(49) For the fixture 50 shown in FIG. 4, the strain levels provided to the bone by the apical strain-creating zone 52 and the coronal strain-creating zone 54 may suitably be in line with the levels discussed in connection with the fixture 2 of FIG. 3. However, other levels may also be achieved depending on how the different zones are configured.

(50) FIGS. 5a-5b illustrate in cross-section a detail of a fixture according to at least one example embodiment of the invention. It may, for instance, be a detail of a coronal strain-creating zone, similar to the one shown in FIG. 4. Alternatively, it could be a detail of an apical strain-creating zone.

(51) The fixture has a leading portion 232 (e.g. a third threaded portion as previously discussed), a coronally widening transition portion 234 and a substantially straight trailing portion 236 (e.g. a fourth threaded portion as previously discussed). The leading portion 232 is provided with macrothreads 238 having thread tops 240 with a certain radius of curvature a. The thread tops 240 are flanked by apical and coronal flank portions 242a, 242b at a certain acute angle γ relative to a plane perpendicular to the central fixture axis. The angle γ lies in the plane containing the fixture axis. In this case the apical and coronal flanks 242a, 242b are illustrated as having the same angle γ. However, in alternative embodiments the coronal and apical flank angles may differ from each other.

(52) Coronally of the macrothreads 238, the leading portion 232 is also provided with double-spiraled microthreads 246 which continue into the transition portion 234 and the trailing portion 236. The microthreads 246 have the same lead as the macrothread 238, the pitch being half the pitch of the macrothread 238. A cutting feature 248 (e.g. the coronal cutting edge as previously discussed) is present at the microthreads in the leading portion 232 to make corresponding female microthreads in the bone tissue. In the illustrated embodiment, throughout the leading portion 232, transition portion 234 and trailing portion 236, the tops 250 of the microthreads 246 have the same radius of curvature as the radius of curvature a of the macrothreads 238. Also, the flank angles of the microthreads 246 correspond to those of the macrothreads 238. The effect of this conformation to the macrothreads 238 will now be explained.

(53) The microthreads 246 are provided as two thread spirals, herein referred to as a first thread spiral 246a and a second thread spiral 246b. The first thread spiral 246a will follow the path of the macrothreads 238. The second thread spiral 246b will make its own path. Thus, when the first thread spiral 246a of the microthreads 246 enters the female bone thread it can theoretically be in full contact with the bone, since the thread tops have the same radius of curvature a and the flanks have the same angles γ as the female bone thread. This means that the initial stability of the fixture can be higher than if the first thread spiral of the microthreads would not fill out the space of the female bone thread. It should be noted that while the cutting features 248 at the microthreads 246 will make a new path for the second thread spiral 246b, it will just adapt the inner areas of the already made female bone thread to conform with the inner areas of the first thread spiral 246a.

(54) FIG. 6 illustrates in cross-section a detail of a fixture 70 according to at least one example embodiment of the invention. In this example, going from a leading portion 72 (i.e. first or third threaded portion), via a transition portion 74, to a trailing portion 76 (i.e. second or fourth non-cutting threaded portion), the radial distance from the fixture axis to the thread tops 78 is constant. Thus, the major fixture diameter remains unchanged. However, the outer surface formed by the thread bottoms 80 (i.e. minor fixture diameter) is changing throughout the different portions. Thus, the outer surface of the transition portion 74 formed by the thread bottoms 80 is conically widened away from that of the leading portion 72. In terms of the previously discussed radial distances (here taking the second and first portions as examples of trailing and leading portions, respectively), R.sub.t=r.sub.t, while R.sub.b>r.sub.b wherein

(55) $\frac{R_b-r_b}{r_b}$
is in the range of 0.01-0.3. Thus, only the thread bottoms 80 provide said radial pressure to cause the desired static strain on the bone tissue.

(56) FIG. 7 illustrates in cross-section a detail of a fixture 90 according to at least one other example embodiment of the invention. In this example, going from the leading portion 92, via the transition portion 94, to the trailing portion 96, the radial distance from the fixture axis to the thread bottoms 100 is constant. However, the outer surface formed by the thread tops 98 is changing throughout the different portions. Thus, the outer surface of the transition portion 94 formed by the thread tops 98 is conically widened away from that of the leading portion 92. In terms of the previously discussed radial distances (again taking the second and first portions as examples of trailing and leading protions, respectively), R.sub.b=r.sub.b, while R.sub.t>r.sub.t, wherein

(57) $\frac{R_t-r_t}{r_t}$
is in the range of 0.01-0.3. Thus, only the thread tops 98 provide said radial pressure to cause the desired static strain on the bone tissue.

(58) Thus, from FIGS. 6 and 7 it should be understood that it is within the scope of the inventive idea to provide a fixture having on the one hand an apical strain-creating zone in which the major and/or minor fixture diameter is widened coronally and on the other hand a coronal strain-creating zone in which the major and/or minor fixture diameter is widened coronally. Thus, it should be understood that the zones do not have to be widened in the same way. For instance, the apical strain-creating zone may have a widening major fixture diameter and a minor fixture diameter which is constant coronally of the apical cutting edge, while the coronal strain-creating zone may have a widening minor fixture diameter and a major fixture diameter which is constant coronally of the coronal cutting edge.

(59) FIG. 8 illustrates a fixture set 120 according to at least one example embodiment of the invention, the fixture set 120 comprising a fixture 140 and a thread maker 130 according to at least one example embodiment of the invention.

(60) The tread maker 130 or tapper is adapted to be rotated into a bore hole arranged in bone tissue for making a female thread in the bone tissue prior to insertion of the fixture. The thread maker comprises an apical portion 132 and a coronal portion 134. The apical portion is provided with at least one apical cutting edge 133 for making a female thread in the bone having a major diameter d1. The coronal portion 134 being provided with a multiple thread spiral 136 is also provided with at least one coronal cutting edge 135 for making female threads having a major diameter d2. The multiple thread spiral 136 at the coronal portion 134 is in FIG. 8 exemplified as comprising microthreads, however, a single macrothread would be a conceivable alternative.

(61) Thus, in the apical part of the bore hole, where cancellous bone is normally present, the female bone thread will have a smaller major diameter d1 compared to the diameter d2 in the coronal part of the bore hole, where cortical bone is normally present.

(62) The fixture 140 to be inserted into the pre-threaded bore hole, comprises an apical leading portion 142, the major diameter of which is d1, i.e. corresponding to the major diameter of female bone thread in the apical part of the bore hole (the minor diameters also correspond to each other). Thus, the apical leading portion 142 will not exert a radial pressure onto the bone. Bordering coronally to the apical leading portion 142 is an apical transition portion 144 which widens the fixture 140 and will thus apply a pressure to the bone which is gradually increasing until it is leveled out by an apical trailing or condensation portion 146. The apical condensation portion 146 may suitably have said major diameter d2 and will thus provide a static tensile strain to the bone which at the apical parts of the bore hole has only been provided with a female thread of diameter d1.

(63) Because the coronal portion 134 of the thread maker 130 will in the bore hole provide a female thread of diameter d2, i.e. the same as the diameter of the apical condensation portion 146 of the fixture 140, the apical condensation portion 146 will not exert any pressure to the cortical part of the bone when the fixture 140 is inserted, but only to the cancellous part.

(64) Bordering coronally to the apical condensation portion 146 is a coronal transition portion 148 which further widens the fixture to a coronal trailing or condensation portion 150 having a major fixture diameter d3. As the coronal condensation portion 150 enters the coronal part of the bore hole having the female thread with diameter d2, it will because of the larger diameter d3, provide a tensile strain to the cortical bone tissue.

(65) Thus, by appropriately choosing diameters d1, d2 and d3, desired strain levels for the cancellous and cortical bone, respectively, may be accomplished.