WO2009054650A1

WO2009054650A1 - Dental fixture

Info

Publication number: WO2009054650A1
Application number: PCT/KR2008/006134
Authority: WO
Inventors: Sang-Oh Park
Original assignee: Osstemimplant Co., Ltd.
Priority date: 2007-10-22
Filing date: 2008-10-17
Publication date: 2009-04-30

Abstract

To ensure more stable implanting, the present invention provides a dental fixture for being inserted into a bone tissue having a cortical bone and a cancellous bone to form an artificial dental root by being rotated based on a central axis thereof, comprising: a second part being implanted to the cancellous bone and having second screw threads formed on the outer circumference thereof; and a first part being implanted to the cortical bone and having continuous screw threads and independent screw threads formed on the outer circumference thereof, wherein the heights between the vertexes and the roots of the continuous screw threads and the independent screw threads are different from each other.

Description

Description DENTAL FIXTURE

Technical Field

[1] The present invention relates to a dental fixture and, more specifically to a dental fixture that prevents the bone resorption and facilitates the osseointegration by removing the pressure given to a cortical bone. Background Art

[2] The screw type fixture used in dentistry is inserted into a bone tissue to form an artificial dental root. When the fixture is inserted into the bone tissue and is integrated with the neighboring bone tissue, a prosthesis that is an artificial tooth is put into an abutment which is coupled thereon. The fixture is made of titanium to improve the integration with the neighboring bone tissue. Screw threads are formed from the upper end to the lower end of the outer circumference of a body portion of the fixture. The fixture is also used as a fixing member for fixing the prosthesis on the bone not only in dentistry but also in orthopedics.

[3] On the other hand, the bone tissue to which the fixture is inserted and implanted is divided into a cortical bone and a cancellous bone. The cortical bone is a relatively thin film that is relatively hard and generally encloses the cancellous bone. The cancellous bone is a relatively soft bone tissue on the inner side of the cortical bone. When the fixture is inserted into and implanted to the bone tissue, the length of the part of the fixture that comes into contact with the cancellous bone is longer than the part of the fixture that comes into contact with the cortical bone. Disclosure of Invention Technical Problem

[4] One of the serious problems that occur when the general screw type fixture is used is the bone resorption. The bone resorption means the phenomenon that the bone tissue around which the fixture is implanted is reduced in amount and is deteriorated. The bone resorption attenuates the fixing force of the fixture so as to deteriorate the stability of the prosthesis and cause the damage of the prosthesis.

[5] Especially, the bone resorption may cause the deposition of tartar that generates an inflammation on the gum tissue that encloses the fixture that is used in dentistry, or make the neighboring gum tissue be curved downwardly along the exposed end of the fixture. Therefore, the bone resorption not only deteriorates the stability of the prosthesis, but also spoils the beauty thereof.

[6] The biological cause of the bone resorption has not been found out yet clearly.

However, it is recognized that the bone resorption is facilitated by both of the excessive stimulus and the low stimulus that are generated at the time of the concentration of stress due to the uneven distribution of the stress that acts on the bone tissue adjacent to the fixture.

[7] Also, it is understood that when a large amount of pressure is given to the cortical bone and the cancellous bone according to the shape of the screw threads formed on the outer surface of the fixture at the time of implanting, the born resorption is generated.

[8] Especially, the screw threads that are installed on the outer circumference of the general fixture are arranged from the upper end to the lower end with the same space between each other, being equal in size. However, such a general fixture causes the problem of serious bone resorption since the characteristics of the hard cortical bone and the soft cancellous bone are not considered properly. Technical Solution

[9] To resolve the foregoing problem, in accordance with a desirable embodiment of the present invention, there is provided a dental fixture for being inserted into a bone tissue consisting of a cortical bone and a cancellous bone to form an artificial dental root by being rotated based on a central axis thereof, comprising: a second part being implanted to the cancellous bone and having second screw threads formed on the outer circumference thereof; and a first part being implanted to the cortical bone and having continuous screw threads and independent screw threads formed on the outer circumference thereof, wherein the heights between the vertexes and the roots of the continuous screw threads and the independent screw threads are different from each other.

[10] Also, in accordance with another embodiment of the present invention, there is provided a dental fixture for being inserted into a bone tissue consisting of a cortical bone and a cancellous bone to form an artificial dental root by being rotated based on a central axis thereof, comprising: a first part being implanted to the cortical bone and having first screw threads formed on the outer circumference thereof; and a second part being implanted to the cancellous bone and having second screw threads formed on the outer circumference thereof, wherein the vertical distance between the vertex of the first screw thread and the central axis is smaller than the vertical distance between the vertex of the second screw thread and the central axis.

Advantageous Effects

[11] The present invention is provided to resolve the aforesaid problems and is able to be applied industrially by providing a dental fixture which is inserted while minimizing the pressure given to the cortical bone to minimize the bone resorption and facilitate the osseointegration. Brief Description of the Drawings

[12] The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

[13] Fig. 1 is a schematic diagram of a fixture in accordance with the first embodiment of the present invention;

[14] Fig. 2 is a schematic diagram of a fixture in accordance with the second embodiment of the present invention;

[15] Fig. 3 is a graph showing the results of the comparison experiment of the fixtures of the first embodiment and the second embodiment of the present invention;

[16] Fig. 4 is a schematic diagram showing a modified embodiment of the third embodiment of the present invention;

[17] Fig. 5 is a schematic diagram showing another modified embodiment of the third embodiment of the present invention;

[18] Fig. 6 is a schematic diagram showing another modified embodiment of the third embodiment of the present invention;

[19] Fig. 7 is a schematic diagram of the fixture in accordance with the third embodiment of the present invention; and

[20] Fig. 8 is a graph showing the results of the comparison experiment of the fixtures of the first embodiment and the third embodiment of the present invention. Best Mode for Carrying Out the Invention

[21] The desirable embodiments of the present invention will now be explained in further detail with reference to accompanying drawings. Mode for the Invention

[22] Fig. 1 is a schematic diagram and an expansion of a fixture in accordance with the first embodiment of the present invention.

[23] As shown in Fig. 1, the fixture in accordance with the present embodiment has screw threads that are divided into a first part 110 and a second part 120 along the direction of a vertical axis to solve the problem of the bone resorption. It is desirable that the first part 110 is disposed on the cortical bone and the second part 120 is disposed on the cancellous bone when the installation of the fixture 100 is completed.

[24] Here, the distance in an axial direction Ia between the vertexes of the screw thread peaks of the first part 110 is smaller than the distance in an axial direction Ib between vertexes of the adjacent peaks of the second part. That is, it is desirable to make the distance in an axial direction between the vertexes of the adjacent peaks of the first part 110 narrow by adding independent screw threads between the screw threads that are formed to be continuous with the second part 120 in the first part 110. [25] Likewise, the first part 120 which has dense screw threads because the independent screw threads are added is inserted into the hard cortical bone, prevents the concentration of the stress and transmits the load evenly to the adjacent bone tissue to prevent the bone resorption. Also, the second part 120 wherein the distance between the screw threads is formed to be relatively broad is inserted into and is stably fixed to the relatively soft cancellous bone (not-illustrated).

[26] To insert the fixture into the bone tissue, an implant hole of a proper size is formed on the bone tissue by using a drill firstly. Then, threads are formed on the inner circumference of the implant hole by rotating and inserting the fixture into the implant hole. It is desirable to make the outer diameter Dd of the implant hole correspond to the middle position between the vertex and the root in consideration of the convenience of the insertion (Please, refer to line Id of Fig. 1). Therefore, the empty space that is formed between the screw thread and the bone tissue after the fixture is inserted is filled up with the bone tissue that grows as time passes, and the fixture and the bone tissue are integrated with each other.

[27] However, when the dense screw threads on the upper part of the fixture are inserted into the bone tissue after the screw threads formed on the lower part of the fixture are inserted into the bone tissue, the rotary torque increases abruptly. Such increase in the rotary torque might not only deteriorate the osseointegration but also facilitate the bone resorption by giving a considerable amount of pressure to the bone tissue.

[28] Also, the outer diameter of the implant hole corresponds to the middle position between the vertex and the root bottom of the screw thread and as illustrated in Fig. 1, because the separated distance e between the root bottom and the outer diameter of the implant hole is large, a large amount of empty space is formed. Therefore, there is a problem that it takes a long period of time for the bone tissue to grow up to the root and thus the osseointegration is delayed.

[29] The second embodiment of the present invention will be explained in further detail.

[30] Fig. 2 is a schematic diagram of the dental fixture in accordance with the second embodiment of the present invention.

[31] As can be seen in Fig. 2, the fixture 310 of the present embodiment consists of a first part 320 and a second part 330. The first part means the part, most of which is implanted to the cortical bone when the fixture 310 is inserted, and constitutes the upper part of the fixture 310. Continuous screw threads 321 and independent screw threads 322 are formed on the outer circumference of the first part 320.

[32] The continuous screw threads 321 have a lead distance of L31, which is the depth of being implanted when the screw threads are rotated once based on the central axis, and are preferably formed to be continuous with the second screw threads 331 formed on the outer circumference of the second part 330. Also, the vertical distance between the vertex and the root of the continuous screw thread 321 is h31.

[33] The outer diameter of the vertex of the continuous screw thread 321 is D31, and when the outer diameter D31 is 4 mm, the height h₃₁ is preferably 0.02 - 0.5 mm, and more preferably 0.03 - 0.3 mm.

[34] The independent screw threads 322 are arranged between the continuous screw threads 321. That is, the continuous screw threads 321 and the independent screw threads 322 are arranged in turn. The lead distance of the independent screw thread 322 is "L₃₂," and is preferably identical to the lead distance of the continuous screw thread (L₃₁ = L₃₂).

[35] The outer diameter D₃₂ of the vertex of the independent screw thread 322 is smaller than the outer diameter D₃₁ of the continuous screw thread 321 (D₃₁ > D₃₂). That is, the height h₃₂ of the vertex of the independent screw thread 322 is smaller than the height h 3i of the continuous screw thread 321.

[36] The difference of the height is preferably 0.02 - 0.4 mm, and more preferably 0.03 -

0.2 mm.

[37] When the difference of the height is less than 0.02 mm, strong insertion torque is caused in the process in which the fixture 310 is inserted into the cortical bone, and thereby excessive pressure is given to the cortical bone.

[38] Also, when the difference of the height is more than 0.4 mm, the height h₃₂of the vertex of the independent screw thread 322 becomes excessively small, and thus the contact area between the implanted fixture 310 and the bone tissue of the cortical bone gets small and the bonding force is deteriorated at the time of osseointegration. Therefore, it is desirable to have the aforesaid difference of height to increase the bonding force by minimizing the pressure to the cortical bone and maximizing the contact area with the cortical bone.

[39] Also, it is desirable that the distance in an axial direction I₃₁ between the vertexes of the adjacent continuous screw thread 321 and independent screw thread 322 is 0.03 ~ 0.6 mm. When the distance in an axial direction is less than 0.03 mm, the insertion torque is increased extremely and excessive pressure is given to the cortical bone, and when it is larger than 0.6 mm, the contact area of the fixture 310 that comes into contact with the cortical bone is small and thus the bonding force is deteriorated.

[40] On the other hand, the second part 330 is implanted to the cancellous bone and has screw threads formed on the outer circumference thereof. Second screw threads 331 whose vertical distance between the vertexes and the roots thereof has a height h₃₃ that is greater than the aforesaid height h₃₂ are installed on the second part 330. It is desirable that the outer diameter D₃₁ of the second screw thread 331 is identical to the outer diameter of the continuous screw thread 321. It is because the torque increases rapidly when the second part 330 is implanted if the outer diameter of the continuous screw thread 321 is greater than that of the second screw thread 331.

[41] Such a dental fixture in accordance with the second embodiment is implanted to the bone tissue of a human body as follows.

[42] Firstly, an implant hole having a drill line of Fig. 2 as an outer diameter Dd₃₁ is formed on the alveolar bone where the dental fixture will be implanted. Then, the dental fixture 310 is inserted into the implant hole of the bone tissue by being rotated based on the central axis thereof with a predetermined tool. Here, the second part 330 having larger screw threads of the dental fixture 310 is inserted into the implant hole firstly to form threads having a predetermined size on the inner circumference of the implant hole, and then the first part 320 is inserted consecutively into the implant hole of the bone tissue, producing new small threads additionally. Then, as time passes, the tissues of the cancellous bone and the cortical bone grow and osseointegrate with the implanted fixture.

[43] Here, in the case of the dental fixture in accordance with the present embodiment, there is no abrupt increase of insertion torque and the pressure given to the cortical bone is small in the process in which the first part 320 having a different screw shape is inserted after the second part 330 is inserted. That is, because the large screw threads and the small screw threads are arranged in turn on the first part 320, the pressure amount given to the bone is minimized and thereby osseointegration can be facilitated.

[44] Fig. 3 is a graph which compares the insertion torque of the fixture in accordance with the first embodiment and the fixture in accordance with the second embodiment.

[45] As can be seen in Fig. 3, the following embodiment was conducted to confirm that the pressure given to the cortical bone is less in the technology in accordance with the second embodiment as compared with the first embodiment.

[46] <Comparison Experiment of the Rotary Torque at the Time of Implanting>

[47] The target for implant (not-illustrated) that is similar to a human body and a fixture

100, 310 to be implanted to the target for implant are prepared for the experiment.

[48] The target for implant includes an upper part corresponding to the cortical bone

(thickness: 3 mm, bone mineral density: 50 g/cm²) and a lower part disposed on the lower side of the upper part (thickness: 27 mm, bone mineral density: 30 g/cm²), and is constituted to have an environment similar to that of the actual bone tissue of a human body.

[49] Also, the fixture of the first type 310 has a shape identical to that of the second embodiment, and the fixture of the second type 100 has a shape identical to that of the first embodiment.

[50] Firstly, the specification of the fixture 310 of the second embodiment illustrated in

Fig. 2 is as follows.

[51] - Length in an axial direction between the upper end and the lower end Lt₃i: 11.5 mm [52] - Outer diameter D₃₁: 4 mm

[53] - Lead L₃₁ of the first screw thread 321 of the first part 320: 0.8 mm

[54] - First height h₃₁ of the first screw thread 321 : 0.2 mm

[55] - Lead L₃₂ of the second screw thread 322 of the first part 320: 0.8 mm

[56] - Second height h₃₂ of the second screw thread 322 of the first part 320: 0.1 mm

[57]

[58] Also, the specification of the fixture 100 of the first embodiment illustrated in Fig. 1 is as follows.

[59] - Length in an axial direction between the upper end and the lower end Lt: 11.5 mm

[60] - Outer diameter D: 4 mm

[61] - Lead L of the screw thread 111 of the first part 110: 0.8 mm

[62] - Height h of the screw thread 111 of the first part 110: 0.2 mm

[63]

[64] The experiment was performed under such conditions as follows.

[65] Firstly, an implant hole was formed on the target for implant corresponding to the bone tissue to have an outer diameter Dd, Dd₃₁ of 3.6 mm. The fixture 310 in accordance with the second embodiment and the fixture 100 in accordance with the first embodiment were inserted into different implant holes respectively and the rotary torque according to the implanting depth was measured. The experiment results are as illustrated in Fig. 3.

[66] When referring to Fig. 3, in the case of the fixture 100 in accordance with the first embodiment, the rotary torque rapidly increased at the position of 8.5mm where the insertion of the first part 110 began after the second part 120 was inserted. That is, the insertion torque increased from 26 Ncm to 54 Ncm rapidly.

[67] On the contrary, the insertion torque began to increase gradually at the position of 8.5 mm where the implanting of the first part 320 began after the second part 330 was inserted in the fixture 310 in accordance with the second embodiment. However, the rotary torque increased only gradually thereafter. That is, the insertion torque increased slowly from 26 Ncm to 38 Ncm in the second embodiment.

[68] Likewise, in the second embodiment, the insertion torque did not increase abruptly even while the first part was being inserted after the second part was inserted differently from the first embodiment, and the pressure given to the cortical bone was obviously small. Also, it can be inserted into the bone tissue more smoothly to avoid damaging the bone tissue, especially the cortical bone and facilitate the osseoin- tegration.

[69] On the other hand, the second embodiment described above has the following effects other than the aforesaid effects.

[70] Firstly, the outer diameter of the implant hole is formed to correspond to the ap- proximate middle line between the vertex and the root of the screw thread of the first part as mentioned above.

[71] Therefore, in the first embodiment, the outer diameter Dd of the implant hole is formed to correspond to the approximate middle position between the root and the vertex of the screw thread of the first part. That is, the distance between the root and the implanting line Id becomes e. Therefore, when the fixture 100 is inserted into the bone tissue, the bone tissue is not filled up from the root of the second part 120 to the implanting line Id, and a relatively large amount of empty space is formed. If there is a large amount of empty space, there is a problem that it takes a long period of time for the bone tissue to grow.

[72] On the contrary, in the second embodiment, the outer diameter of the first part 320 is smaller than that of the second part 330. Here, the implant hole having an outer diameter Dd₃₁ that corresponds to the middle position between the vertex and the root of the small screw thread of the first part 320 is formed on the bone tissue. Accordingly, the distance f between the root of the second part and the diameter line ld₃i of the implant hole becomes shorter than that of the first embodiment (f < e).

[73] That is, because the outer diameter line ld₃i of the implant hole is disposed to be adjacent to the root of the screw thread in the second embodiment, the part that is not filled up with the bone tissue between the screw threads and the bone is minimized in the second part 230, 330. Accordingly, there are advantages that the bone tissue not only osseointegrates with the fixture 210, 310 quickly but also is bonded with the fixture 210, 310 surely. That is, the fixture 210, 310 has a sufficient contact area with the bone tissue and thus is able to improve the bonding force with respect to the bone tissue.

[74] Likewise, it is possible to prevent the abrupt increase of the insertion torque and prevent excessive generation of heat due to friction at the time of implanting by making smaller steps on the outer diameter of the first part 220, 320 as compared with the second part 230, 330 and making small the contact area of the first part as a whole.

[75] Also, it is possible to improve the osseointegration as well as prevent the bone resorption by dispersing the partial concentration of stress in the first part 220, 320 after the fixture 210, 310 is implanted. That is, it is possible to prevent the bone resorption and facilitate osseointegration of the cortical bone by dispersing the concentration of the stress on the cortical bone to the cancellous bone and letting the stress distributed evenly on the entire portion of the fixture 210, 310.

[76] On the other hand, the fixture 310 in accordance with the second embodiment can be modified as follows to achieve the objective of the present invention. As the basic constitution is identical to that of the second embodiment, explanation thereof will be omitted. [77] Firstly, it is described that the outer diameter of the continuous screw thread 321 is identical to that of the second screw thread 331 in the embodiment mentioned above, but it is possible that the outer diameter of the continuous screw 321 is different from that of the second screw thread 331.

[78] On the other hand, as can be seen from Fig. 4, it is possible that the continuous screw thread 321 is a two-thread screw thread. That is, it is also possible to form the continuous screw 321 to be a two-thread screw and reduce the pressure to the cortical bone according to the circumstances in the case of a patient whose cortical bone is hard. Here, the outer diameter D₃₂ of the vertex of the independent screw thread 322 is smaller than the outer diameter D₃₁ of the continuous screw thread 321 (D₃₁' > D₃₂').

[79] Of course, it is possible to form the independent screw thread 322 to be a two-thread screw.

[80] Besides, it is possible to form the continuous screw thread 321 and the independent screw thread 322 to become screw threads of three or more-threads, if it is to achieve the objective of the present invention.

[81] Besides, as can be seen from Figs. 5 and 6, the height of the independent screw can be increased or decreased along the vertical direction.

[82] When referring to Fig. 5, the independent screw threads 422 have heights h2 that are reduced as they go towards the lower side thereof. Therefore, because the heights of the independent screw threads 422 that are adjacent to the second part 430 are relatively low, the excessive increase of the insertion torque can be more prevented at an early stage in which the first part 420 is inserted. That is, because the heights of the independent screw threads 422 increase gradually as they get distant from the second part 430, the increase amount of the insertion torque becomes more gradual. This is suitable for a patient who has a high bone mineral density especially. That is, it is desirable to increase the heights of the independent screw threads 422 slowly as they go toward the upper side thereof for a patient who has a high bone mineral density and thus has a hard cortical bone.

[83] On the contrary, when referring to Fig. 6, it is shown that the heights of the independent screw threads 522 are decreased gradually as they go toward the upper side thereof.

[84] Fig. 7 is a schematic diagram and an expansion of a dental fixture in accordance with the third embodiment of the present invention.

[85] As can be seen from Fig. 7, the dental fixture 210 includes the first part 220 and the second part 230 that are distinguished along the longitudinal direction thereof. When the fixture 210 is implanted, most of the first part 220 formed on the upper part thereof is implanted to the cortical bone, and the first part has plural first screw threads 221 having vertexes 221a and roots 221b formed thereon. Of course, it is possible that a part of the lower end of the first part 220 is implanted to the cancellous bone.

[86] The lead distance which is the implanted depth when the first part is rotated once based on the central axis C is "L₂i". Also, the vertical distance between anyone vertex 221a of the first screw thread 221 and the root 221b adjacent to the same vertex 221a is a first height h₂₁.

[87] The distance between the central axis C and the vertex 221a of the first screw thread is R₂i, and the outer diameter of the first screw thread 221 is D₂i. It is desirable that the vertexes 221a of the first screw thread 221 are disposed in the same distance from the central axis C, and the straight line connecting each vertex 221a is parallel to the central axis C.

[88] Also, the distance in an axial direction I₂₁ between the vertexes 221a of the adjacent first screw threads 221 of the first part 220 is preferably 0.03 - 0.6 mm, and more preferably 0.3 mm.

[89] When the distance in an axial direction 1_2! is less than 0.03 mm, the first screw threads 221 become excessively dense, and the insertion torque gets increased extremely. Accordingly, it is not desirable because excessive pressure is given to the cortical bone.

[90] Also, when the distance in an axial direction 1_2! is more than 0.6 mm, the distance between the screw threads of the first screw threads 221 becomes broader and thus there is a risk that the vertex thereof 221a might give too much stress to the cortical bone. That is, the density of the first screw threads 221 should be maintained to be proper to prevent excessive stress from being focused on anyone part of the cortical bone and make the stress be distributed on the entire portion of the cortical bone properly.

[91] On the other hand, the second part 230 is formed on the lower part of the fixture 210 to be implanted to the cancellous bone, and have the second screw threads 231 formed on the outer circumference thereof. Of course, it is possible that anyone part of the second part is implanted to the cancellous bone.

[92] The implanted depth of the second part 230 when the fixture 210 is rotated once based on the central axis C can be considered to be a lead distance, "L₂₂ " Also, the vertical distance between anyone vertex 231a of the second screw thread 231 and the root 231b adjacent to the same vertex is a second height h₂₂.

[93] Also, the radius from the central axis C to the vertex 231a of the second screw thread

231 is R₂₂, and the outer diameter of the second screw thread 231 is D₂₂. It is desirable that such vertexes 231a of the second screw threads 231 are separated from the central axis C in the same distance and thus the straight line connecting each vertex 231a becomes parallel to the central axis C.

[94] On the other hand, it is desirable that the radius R_2! between the vertex of the first screw thread 221 of the first part and the central axis C is smaller than the radius R₂₂ between the vertex of the second screw thread 231 of the second part 230 and the central axis C (R₂₁<R₂₂).

[95] If the vertical distances of the second part and the first part are identical, excessive stress shall be focused on the cortical bone due to the abrupt increase of the insertion torque. Such points are as evidenced in the experiments to be described hereinafter.

[96] Likewise, making the radius R₂₁ of the first part 220 be smaller than that of the second part 230 is to prevent the abrupt increase of the insertion torque due to the increase of the number of screw threads when the first part 220 is inserted after the second part 230 is inserted.

[97] On the other hand, it is desirable that the radius R₂₁ of the first part 220 is smaller than the radius R₂₂ of the second part 30 by 0.02 0.4 mm, and more preferably by 0.03 - 0.2 mm. If the difference of the two radiuses is less than 0.02 mm, there is little difference between the first part 220 and the second part 230, and thus there is a risk that the reduction of the insertion torque that is sought to be achieved will be hardly obtained.

[98] Also, if the difference of the two radiuses is greater than 0.4 mm, the distance between the vertex and the root of the first screw thread 221 protruding from the outer circumference of the first part 220 becomes small and thus the function as a screw is lost greatly, there is a risk that the supporting force for supporting the bone tissue will be weakened later, and it takes a long period of time to be recovered after the surgical operation because the empty space between the surface of the first part 220 and the bone tissue becomes large after implanting and thus the amount of the bone tissue that should grow up to the surface of the first part 230 becomes larger.

[99] Also, it is desirable that the first height h₂₁ which is the vertical distance between the vertex 221a of the first screw thread 221 of the first part 220 and the root 221b adjacent to the same vertex 221a is smaller than the second height h₂₂that is the vertical distance between the vertex 231a of the second screw thread 231 of the second part 230 and the root 231b adjacent to the same vertex 231a. In the second part 230 that is firstly inserted, a large amount of bone tissue is filled up between the second screw threads 231 to make the initial supporting force firm, and in the first part 220 that is inserted later, a smaller amount of bone tissue is resorbed as compared with the second part 230, but the stress is transmitted to the cortical bone evenly and the concentration of the stress is prevented instead.

[100] It is desirable to make the lead distance L₂₁ of the first part 220 be identical to the lead distance L₂₂ of the second part 230 and make the first and second parts be inserted into the bone tissue in the same distance for the same number of rotations even if the first part 220 is inserted after the second part 230 is inserted (L₂₁=L₂₂). That is, it can be seen that the second screw threads 231 of the second part 230 extend toward the first part 220, while maintaining the same lead, and the independent screw threads are additionally formed between the first screw threads 221 extending toward the first part 220.

[101] Such a dental fixture 210 in accordance with the present embodiment is implanted to the bone tissue of the human body as follows.

[102] Firstly, the implant hole having the drill line of Fig. 7 as an outer diameter Dd_2I is formed on the alveolar bone where the fixture will be implanted. Then, the dental fixture 210 is rotated based on the central axis C and is inserted into the implant hole of the bone tissue with a predetermined tool.

[103] Then, the second part 230 having larger screw threads is inserted into the implant hole and forms a predetermined size of threads on the inner circumference of the implant hole. After the second part 230 is inserted, the first part 220 is inserted consecutively and is inserted into the implant hole while producing new small threads additionally on the threads that are formed by the second screw threads 231.

[104] After the insertion is completed, a proper empty space is produced between the surface of the first part 220 or the second part 230 and the inner circumference of the implant hole, and such an empty space is filled up while the tissues of the cancellous bone and the cortical bone grow as time passes, and the implanted fixture 10 and the bone tissue osseointegrate with each other.

[105] Here, in the dental fixture 10 in accordance with the present embodiment, there is no abrupt increase of the insertion torque in the process in which the first part 20 having a different screw shape is inserted after the second part 30 is inserted and thus the pressure that is given to the cortical bone is small. That is, the outer diameter of the first screw thread 21 of the first part 20 is smaller than the outer diameter of the second screw thread 31 of the second part 30, and thus the pressure given to the bone gets minimized and the effects of facilitating the osseointegration are obtained.

[106] On the other hand, Fig. 8 is a graph that compares the insertion torque of the fixtures in accordance with the first embodiment and the second embodiment of the present invention.

[107] As can be seen in Fig. 8, the comparison experiment of the rotary torque at the time of implanting the fixtures in accordance with the first embodiment and the second embodiment was conducted in an environment that is similar to the bone tissue of a human body. The changing behavior of the entire insertion torque can be recognized through this experiment.

[108] For the aforesaid experiment, a target for implant to which a fixture will be implanted (not-illustrated) and a fixture 100, 210 which will be implanted to the target for implant were prepared. The first type is a fixture 210 that has the same shape as the fixture of the third embodiment illustrated in Fig. 6, and the second type is a fixture 100 that has the same shape as the fixture illustrated in Fig. 1.

[109] The target for implant includes an upper part corresponding to the cortical bone (thickness: 3 mm, bone mineral density: 50 g/cm²) and a lower part disposed on the lower side of the upper part (thickness: 27 mm, bone mineral density: 30 g/cm²), and is constituted to have an environment similar to that of the actual bone tissue of a human body.

[110] The specification of the fixture 210 in accordance with the third embodiment illustrated in Fig. 7 is as follows.

[I l l] - Length in an axial direction Lt_2I between the upper end and the lower end: 11.5 mm

[112] - Outer diameter D2j of the first part 220: 3.8 mm

[113] - Lead L₂i of the first screw thread 221 of the first part 220: 0.8 mm

[114] - First height h₂₁ of the first screw thread 221 : 0.2 mm

[115] - Outer diameter D₂₂ of the second part 230: 4.0 mm

[116] - Lead L₂₂of the second screw thread 231 of the second part 230: 0.8mm

[117] - Second height h₂₂ of the second screw thread 231 : 0.35

[118] Also, the specification of the fixture 100 in accordance with the first embodiment illustrated in Fig. 1 is as follows.

[119] - Length in an axial direction Lt between the upper end and the lower end: 11.5 mm

[ 120] - Outer diameter D : 4 mm

[121] - Lead L of the screw thread 111 of the first part 110: 0.8 mm

[122] - Height h of the screw thread 111 of the first part 110: 0.2 mm

[123] The experiment was performed under such conditions as follows.

[124] Firstly, an implant hole was formed on the target for implant corresponding to the bone tissue to have an outer diameter Dd, Dd₂₁ of 3.6 mm. The fixture 210 in accordance with the third embodiment and the fixture 100 in accordance with the first embodiment were inserted into different implant holes respectively and the rotary torque according to the implanting depth was measured. The experiment results are as illustrated in Fig. 8.

[125] When referring to Fig. 8, in the case of the fixture 100 in accordance with the first embodiment, the rotary torque rapidly increased at the position of 8.5mm where the insertion of the first part 110 began after the second part 120 was inserted. That is, the insertion torque increased from 26 Ncm to 40 Ncm rapidly.

[126] On the contrary, the insertion torque increased gradually at the position of 8.5 mm where the insertion of the first part 20 began after the second apart 30 were inserted in the fixture 10 in accordance with the third embodiment, and the rotary torque increased slowly thereafter without any abrupt increase. That is, the insertion torque increased slowly from 32.5 Ncm to 35 Ncm in the third embodiment.

[127] Likewise, there was no abrupt increase of the insertion torque in the transition portion from the second part 30 to the first part 20 in the third embodiment differently from the first embodiment and the rotary torque increased slowly thereafter.

[128] Therefore, there is no abrupt increase of torque in the cortical bone as a whole and the pressure given to the cortical bone is obviously small in the third embodiment. These bring about the advantages that the fixture can be inserted into the bone tissue without any difficulty and without damaging the cortical bone, and facilitate the os- seointegration.

[129] While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims. Industrial Applicability

[130] The present invention is able to be applied industrially by providing a dental fixture which is inserted while minimizing the pressure given to the cortical bone to minimize the bone resorption and facilitate the osseointegration.

Claims

[I] A dental fixture for being inserted into a bone tissue having a cortical bone and a cancellous bone to form an artificial dental root by being rotated based on a central axis thereof, comprising: a second part being implanted to the cancellous bone and having second screw threads formed on the outer circumference thereof; and a first part being implanted to the cortical bone and having continuous screw threads and independent screw threads formed on the outer circumference thereof, wherein the heights between the vertexes and the roots of the continuous screw threads and the independent screw threads are different from each other.

[2] The dental fixture of claim 1, wherein the height of the independent screw thread is smaller than that of the continuous screw thread.

[3] The dental fixture of claim 1, wherein the height of the second screw thread is higher than that of the continuous screw thread.

[4] The dental fixture of claim 1, wherein the continuous screw threads and the independent screw threads are arranged in turn along the direction of the central axis. [5] The dental fixture of claim 1, wherein the outer diameter of the continuous screw thread is substantially identical to the outer diameter of the second screw thread. [6] The dental fixture of claim 1, wherein the heights of the independent screw threads increase as they go toward the upper side thereof along the direction of the central axis. [7] The dental fixture of claim 1, wherein the heights of the independent screw threads decrease as they go toward the upper side thereof along the direction of the central axis. [8] The dental fixture of claim 1, wherein the heights of the independent screw threads are uniform. [9] The dental fixture of claim 1, wherein the height of the continuous screw thread is larger than that of the independent screw thread by 0.02 0.4 mm. [10] The dental fixture of claim 1, wherein the distance in an axial direction between vertexes of the adjacent screw threads of the first part is 0.03 0.6 mm.

[I I] A dental fixture for being inserted into a bone tissue having a cortical bone and a cancellous bone to form an artificial dental root by being rotated based on a central axis thereof, comprising: a first part being implanted to the cortical bone and having first screw threads formed on the outer circumference thereof; and a second part being implanted to the cancellous bone and having second screw threads formed on the outer circumference thereof, wherein the vertical distance between the vertex of the first screw thread and the central axis is smaller than the vertical distance between the vertex of the second screw thread and the central axis. [12] The dental fixture of claim 11, wherein each vertex of the first screw threads is separated from the central axis in the same distance. [13] The dental fixture of claim 11, wherein each vertex of the second screw threads is separated from the central axis in the same distance. [14] The dental fixture of claim 11, wherein the vertical distance between the vertex of the first screw thread and the root adjacent thereto is smaller than the vertical distance between the vertex of the second screw thread and the root adjacent thereto. [15] The dental fixture of claim 1, wherein the vertical distance between the vertex of the first screw thread and the central axis is smaller than the vertical distance between the vertex of the second screw thread and the central axis by 0.02 - 0.4 mm. [16] The dental fixture of claim 11, wherein the distance in an axial direction between the vertexes of the first screw threads is 0.03 - 0.6 mm. [17] The dental fixture of claim 11, wherein the first screw threads comprise continuous screw threads that are formed to be continuous with the second screw threads; and independent screw threads being arranged between the continuous screw threads.