FIXTURE FOR ANCHORING IN BONE TISSUE

Abstract: A fixture intended for implantation, for anchoring in bone tissue such as the bone tissue of the jaw, in order to carry a prosthesis such as a tooth crown or the like. The fixture (1) is provided with a surface coating of tissue-friendly material, at least in the area intended to come into contact with the bone tissue, and has a cylindrical shaft provided with external threading (8, 9) and a cylindrical head (5). The fixture (1) is arranged to be screwed into a previously prepared hole in the bone tissue, the diameter of said hole being substantially in agreement with the core diameter of the shaft section and the threaded shaft section (8, 9) of the fixture (1) has at least one thread-free zone (10) arranged to be brought into contact with nerves present in the bone tissue.

Description: FIXTURE FOR ANCHORING IN BONE TISSUE The invention relates to a fixture intended for implanta¬ tion, for anchoring in bone tissue such as the bone tissue of the jaw, in order to carry a prosthesis such as a tooth crown or the like, which fixture is provided with a surface coating of tissue-friendly material, at least in the area intended to come into contact with the bone tissue, and has a cylindrical shaft provided with external threading and a cylindrical head, said fixture being arranged to be screwed into a previously prepared hole in the bone tissue, the diameter of said hole being substantially in agreement with the core diameter of the shaft section. A number of factors must be taken into consideration when implanting fixtures in the form of screws in bone tissue such as in the jaw-bone, to provide attachment for a super¬ structure such as a dental prosthesis or the like. A prima¬ ry factor is of course that integration of the fixture shall occur as favourably as possible with minimum risk of infection and mechanical damage in the implantation region. Furthermore, it should be possible for the fixture to be fitted relatively quickly and simply, its installation should permit rapid integration and the fixture and bandage should naturally be such as to minimize the risk of dis¬ turbance during the integration period. One known technique, for instance, is to use a fixture comprising a cylindrically threaded shaft with a cylindri¬ cal head, the height of which is a fraction of the shaft diameter, and that diameter of which is slightly greater than the shaft diameter, a short and gently curved tran¬ sition being arranged between shaft and head. The head may be provided with a central axial hole and may be provided at its free end with formations facilitating assembly of a superstructrue, for instance in the form of a crown or a base for a crown or the like. At installation a hole is prepared in the jaw-bone, the bore diameter corresponding substantially to the core diameter of the shaft. The bore may be formed on or of the fixture itself, for instance at the lower end of the shaft. The threaded shaft of the fixture may suitably be arranged to cut the thread in the jaw-bone for the shaft threading. The technique desribed above has been used with success for many years. However, in recent years symptoms have been observed in certain cases, particularly when such fixtures are applied in the lower jaw, entailing motoric and/or sensory loss of feeling, often in the area of the lower lip. After thorough studies of these cases it has been established that the loss of feeling is caused by a nerve - nervus alveolaris interior - located in the mandible coming into contact with the external threading of the implanted fixture. This probably causes irritation of the nerve, thereby disturbing the distal area supplied by the nerve. It has now proved possible to eliminate these disturbances by means of the fixture proposed according to the inven¬ tion, the primary characteristic of which is that the threaded shaft section of the fixture has at least one zone without threading. According to the invention this thread-free zone is arrang¬ ed in the mid-section of the external thread so that it will be located within the marrow cavity of the jaw-bone after installation. When the fixture according to the invention is installed by being screwed into the jaw-bone, the nerve is lifted out of the way and is subsequently replaced, i.e. when the fixture has been finally installed, against the thread-free zone of the fixture. According to a suitable embodiment of the invention the thread-free zone has a smooth outer surface, is suitably arranged in the mid-section of the external thread and covers at least two thread pitches in the external threa¬ ding. According to a suitable embodiment the diameter of the thread-free zone substantially coincides with the core diameter of the shaft and is connected to respective threa¬ ded sections by gently curved portions. The invention will be described more fully in the following with reference to an embodiment shown in the accompanying drawings in which Figure 1 shows a view in perspective of a fixture according to the invention. Figure 2 shows a diagrammatic sketch of a mandible, Figure 3 shows a section through the mandible shown in Figure 2 - Figure 4 and 5 show the saiaa section as Figure 3 with the fixture inplace in the mandible. The fixture shown in Figure 1 is generally shaped as a screw. The fixture 1 comprises an insertion end 2, suitably provided with slits 3 running axially upwards and having cutting edges to enable the screw itself to cut a thread in the bone tissue. The fixture comprises a neck portion 4 and a head 5. The head 5 has a central hole 6 with internal threading 7 for attachment of a superstructure such as a crown or the like, which is then fitted in said bore 6 when the fixture 1 has become integrated into the bone tissue. The fixture 1 is also provided with a threaded shaft sec- tion comprising an upper, external thread 8 adjoining the head 5, and a lower external thread 9. Between the upper and lower external threads 8 and 9 is a thread-free zone 10, the diameter of which is substantially in agreement with the core diameter of the shaft. The thread-free zone 10 is joined to the upper thread 8 and the lower thread 9 by gently curved portions 10a and 10b, respectively. Figure 2 shows a diagrammatic sketch of a mandible 11 including nerves 12. From the section through the mandible shown in Figure 3 it can be seen that the nerve 12, in this particular case nervus alveolaris interior, is normally located in the marrow cavity 13 of the mandible 11. When the fixture according to the invention is installed by being screwed into the jaw-bone, the nerve 12 is lifted out of the way - see Figure 4 - and is subsequently replaced - as shown in Figure 5 - against the thread-free zone 10 of the fixture which has a substantially smooth and uniform outer surface. The previously observed irritation of the nerve is thus avoided since the nerve is no longer in contact with the threaded part of the fixture. Irritation of the nerve and the resultant drawbacks are thus avoided. To install the fixture 1 a hole is suitably prepared in the bone tissue with a diameter substantially corresponding to the core diameter of the shaft. The fixture 1 is then screwed into this hole, cutting a groove for its external thread, preferably to a depth illustrated in Figures 4 and 5. During installation of the fixture the nerve is thus lifted out of the way and is then replaced against the thread-free, smooth zone 10 of the shaft after completed installation. The fixture then becomes integrated into the bone tissue over a suitable period of time and once osseo- integration has been achieved the proposed superstructure can be fitted on the fixture in known manner. The invention has been described with reference to a fix¬ ture for carrying a dental prosthesis. However, it can of course also be used for anchoring other types of prostheses in bone tissue where it is feared that nerve irritation may occur against the external thread. The thread-free zone 10 shown in the drawings may of course be different from that shown, i.e. it may have less ex- tension in the longitudinal direction of the screw. Howe¬ ver, it is important that there are a number of thread pitches on each side of the thread-free zone, and the thread-free zone should suitably be located approximately mid-way along the fixture in order to ensure that the fixture is located in the marrow cavity after installation. Several thread-free zones may of course also be arranged along the threaded portion of the shaft. The fixture may suitably be manufactured of titanium and the shaft thread is suitably M 3.75.

Claims: C X a X m s 1. A fixture (1) intended for implantation, for an¬ choring in bone tissue such as the bone tissue of the jaw, in order to carry a prosthesis such as a tooth crown or the like, which fixture (1) is provided with a surface coating of tissue-friendly material, at least in the area intended to come into contact with the bone tissue, and has a cylin¬ drical shaft provided with external threading (8, 9) and a cylindrical head (5), said fixture (1) being arranged to be screwed into a previously prepared hole in the bone tissue, the diameter of said hole being substantially in agreement with the core diameter of the shaft section, w h e r e i n the threaded shaft section of the fixture (1) has at least one thread-free zone (10). 2. A fixture as claimed in claim 1, w h e r e i n the thread-free zone (10) has a smooth outer surface. 3. A fixture as claimed in claims 1-2, w h e r e i n the thread-free zone (10) is arranged in the mid-section of the external thread - seen in the longitudinal direction of the shaft - and covers at least two thread pitches in the external threading. 4. A fixture as claimed in claims 1-3, w h e r e i n the diameter of the thread-free zone (10) substantially coincides with the core diameter of the shaft. 5. A fixture as claimed in claims 1-4, h e r e i n the thread-free zone (10) is connected to the threaded sections (8) and (9) respectively by way of gently curved portions (10a, 10b). 6. A fixture as claimed in claims 1-5, c o m p r i s¬ i n g an open end provided with cutting edges (3) in the form of longitudinal slits.

Inventors: BRAANEMARK PER-INGVAR, BRAANEMARK, PER-INGVAR
Applicants: MEDEVELOP AB
Application year: 1991
Main classification: A61C8
IPC codes: A61C8