BONE ANCHORING ELEMENT

Abstract: A direct connected bone anchoring element for prosthetic structures comprising an implant (10) to be inserted into bone tissue (11) the end surface (12) of the implant, which is the outer end surface of the implant in the implanted position thereof, which is the outer end surface of the implant in the implanted position thereof, has a cover element (22) cupola shaped at the upper surface thereof, detachably connected to said end surface, said cover element being intended to be covered by soft tissue (28) and to join the bone surface radially outside the implant. Under the cupola shaped upper side the cover element surrounds a neck portion (15) of the implant, which forms said end surface.

Description: Bone Anchoring Element The invention relates to a direct connected bone anchoring element for prosthetic structures comprising an implant to be inserted into bone tissue the end surface of the implant which is the outer end surface of the implant in the implanted position thereof, being arranged for attaching the prosthetic structure. It is prior art to anchor permanently e.g. dental prostheses or crown and bridge structures to anchoring elements integrated into the bone, implants, via extension pieces or spacers penetrating through the mucous membrane, said implants most often being cylindrical and screw- shaped. These implants are inserted by a separate surgical operation and are completely covered by the mucous membrane. After a minimum critical ingrowth period a further surgical operation is effected in order to connect the implants to the actual prosthetic structure via said exten- sion pieces of suitable length. This so called two-step method is utilized in order to reduce the failure frequency which is considered too high in a one-step method wherein the implant is allowed to penetrate the mucous membrane covering the bone, already at the installation of the implant. The higher failure frequency in the one-step method is due to the load transferred to the sensitive interface between bone tissue and implant via the portion of the implant which is located in the oral cavity above the mucous membrane, already during the initial period of the ingrowth phase. In addition, there is the risk of an infection in the interface between implant and soft tissue easily propagating along the implant wall and destructing the bone tissue before ingrowth has taken place. However, recently controlled studies have been pre- sented wherein good results with the one-step technique have been demonstrated provided that the portion of the implant penetrating the mucous membrane does not project too much from the surface of the mucous membrane and thus will be exposed to too large loading forces and that this portion of the implant also is kept completely free from micro-organisms continuously so that infection with accompanying destruction of the tissue will not arise. Provided that these conditions are satisfied it is thus very likely that the one-step technique also in a large scale will provide as good results as the two-step technique. This means that both techniques will be applied on their respective indication fields. In the two-step method the implant is located in the bone tissue so that the most superficial portion thereof will be at the same level as or slightly above the bone surface where ingrowth is allowed to take place with the implant covered by soft tissue and unloaded for a period which is sufficiently long in order that the implant shall be intimately anchored, "get firmly rooted", in surrounding bone tissue. A second surgical operation is effected in order to uncover the implant by punching an aperture in the covering mucous membrane or by cutting through said membrane and to connect extension pieces to the implant. The purpose of the present invention is to provide a more rational and cheaper method, an improved ingrowth into the most superficial portion of the bone and the periosteal and connective tissue located on the bone surface. For said purpose the bone anchoring element of the kind referred to above has been given the characterizing features of claim 1. Contrary to prior art types of implant intended for the two-step method the implant described herein can integrate with complete covering by mucous membrane and nevertheless be uncovered without applying extension elements, which provides several advantages : - Reduction of costs and simplification of the manufacture by elimination of the spacer screw (and spacer socket) . - The implant is suitable for one-step technique as well as two-step technique. - The diameter of the implant can be considerably reduced without increased risk of fracture due to the fact that the blind hole for the fastening screw has considerably smaller diameter than the hole for a spacer screw. This increases the indication domain considerably as far as yaws having a small bucko-lingual width are concerned. - Edge cutting with accompanying destructive bone cutting is eliminated. - Simplified secondary operation with two-step tech- nique since the covering element of the implant can easily be palpated and the neck of the implant is easily available for direct connection to the prosthetic structure. X-ray check in order to certify correct connection between implant components is eliminated, and complicating bone over-growth is precluded. - There is no need of turning up a larger flap, which increases the patient comfort, spears the yaw bone and makes the surgery simpler. - The joint between implant and spacer is eliminated. This eliminates annoying odour, if any, and infection involving risk of bone destruction in case said joint is located adjacent the bone edge. - Optimal aesthetics by elimination of the spacer and by the fact that the end surface of the implant in particu- larly delicate cases can be bevelled (towards the buccal side) . - The thickness of the soft tissue can be controlled during ingrowth by the shape of the covering element being controlled. - Considerably smaller critical height of the prosthetic structure by connection thereof directly to the implant, which facilitates the prosthetics and increases the indication domain. - Minimal risk of buccal screw apertures through angled passages. Preferably, the implant is made of a material which has a sufficient degree of biocompatibility and strength in order to be permanently implanted as anchoring element for prostheses, crowns, or bridges. It can consist of ceramics, metal, or tissue tolerant plastics or combinations thereof. An example of a suitable metal is pure titanium which is prepared so that the surface thereof is completely free from organic material and which has been given a suitable surface roughness by turning, blasting or in another way. It can also be coated on the surface thereof with a material which optimizes the ingrowth thereof to the most intimate connection with surrounding bone, e.g. titanium, which is applied by so called plasma spraying, or hydroxyl apatite. Preferably, the implant is screw-shaped. The invention will be described in more detail with reference to the accompanying drawings in which FIG. 1 is a side view, partly a vertical cross-sectional view, of a prior art screw implant in the embodiment which at present is most commonly appearing on the market, FIG. 2 is a side view, partly a vertical cross-sectional view, of an implant according to the invention, FIG. 3 is a side view, partly a vertical cross-sectional view, of the implant according to the invention pro- vided with a cover element, FIG. 4 is a side view, partly a vertical cross-sectional view, of the implant in FIG. 3 after an aperture having been punched in covering soft tissue and the cover element having been removed, FIG. 5 as a side view, partly vertical cross-sectional view, of the implant in FIG. 3 after the soft tissue having been laid down around the neck of the implant, FIG. 6 is a side view, partly a vertical cross-sec- tional view, of the implant according to the invention provided with a cover element of another embodiment, FIG. 7 is a fragmentary perspective view of an embodiment of the end surface of the implant according to the invention, and FIG. 8 to 11 are views illustrating different embodiments of the connection surface of the implant according to the invention. The prior art implant disclosed in FIG. 1 comprises a screw which is made of or coated with titanium or another biocompatible material. It has a screw threaded portion 10 which is screwed into a hole which has been drilled in the bone 11, so that the end surface 12 thereof will be placed substantially at the same level as the surface of the bone. The implant has a hexagonal portion on the end surface 12 in order that a tool can be engaged therewith when the implant is to be screwed into the bone the implant also having a threaded blind hole 14 for the attachment of an extension piece or spacer when the two-step method described above is applied. In FIG. 1 the implant is pro- vided with a cover screw 13 which is screwed into the threaded blind hole 14. The implant according to the invention which is disclosed in FIG. 2 has a screw threaded portion 10 and a cylindrical neck portion 15 which has been made smooth by turning and may be polished. The neck portion can have a length of 1 to 10 mm but preferably of 1 to 3 mm. The curved surface of the neck portion at the transition from the threaded portion of the implant is flush with the crests of the threaded portion but preferably diverges slightly towards the end surface 12 of the implant . By this arrangement there is provided a tight connection to the adjacent bone edges when the implant is being screwed into the bone without detrimental overpressure being created in the bone tissue. At the same time a sufficient screwing resistance is obtained in order to impart stability to the implant . The diameter of the implant can vary from some tenths mm and upwards but in oral prosthetical function preferably ranges from 2,0 to 8,0 mm. Preferably the implant is made self-tapping. In the centre of the upper end surface of the implant there is provided a threaded axial blind hole 19 the diameter of which can vary in dependence on the dimension of the fastening screw which shall retain the prosthetic struc- ture, and the diameter of the implant. At the upper end of the blind hole 19 there is provided an internal hexagonal portion 19A for engagement with a tool by means of which the implant shall be screwed into the bone. The end surface 12 also can be provided with supplementary means for lock- ing the prosthetic structure attached to the end surface against rotation. Such means can comprise one or more grooves 20 or blind holes 21 as shown in FIG. 7, corresponding ridges or cylindrical or conical projections on the prosthetic structure to be attached to the end surface of the implant engaging therewith. The rotational locking can also be provided by means of a raised or countersunk portion around the fastening screw with four or more walls wherein a corresponding patrix or matrix attached to the prosthetic structure is exactly fitted. Due to a well adjusted neck length the implant can be implanted with covering mucous membrane and can integrate unloaded and without infectious contamination via the oral cavity while the implant by selection of correct neck length in relation to the thickness of the mucous membrane can be easily palpated and uncovered by punching or in another way, but in order to avoid the existing relatively great risk of perforation of the mucous membrane there is mounted to the implant according to the invention a cover element 22, FIG 3, which is attached by means of a screw 23 in the threaded hole 19 which is intended for the fastening screw of the prosthetic structure. This cover element has a gently rounded cupola shaped upper surface 24 and comprises a body of a tissue tolerant at least partly plastic material which is attached to the head of the screw 23. The screw and the head thereof can consist of titanium. The head engages the end surface 12 of the implant a washer 25 being located therebetween. The tissue tolerant plastics can comprise a non-resorbable or alternatively a partly or completely resorbable polymer. The cover element can be perforated in the portion thereof which is located outside the end surface of the implant. These perforations partly shown at 27, should have a size which allows the formation of new tissue, which contributes both to the nutrition of the mucous membrane and the ingrowth process in the bone tissue adjacent the neck portion of the implant. The plastics (polymer) should have a consistence which is close to the consistence of the connective tissue in order that the risk of perforation of the mucous membrane within 3 to 6 months after implantation shall be prevented. It should also be so plastic at least at the periphery thereof that it can be deformed without elastic return and without cracking or fracturing. It should also be possible to reshape - reduce - the plastics by means of a pair of scissors or a scalpel. FIG. 3 discloses how the implant and the cover element thereof is covered by connective tissue 28. When the implant has been anchored in the bone by ingrowth 3 to 6 months after the implantation the mucous membrane shall be perforated by a cut or a circular aperture in order to uncover the neck portion 15 of the implant. In order to facilitate the identification of the implant at punching the upper end surface of the cover element can form an aperture or a depression which can be palpated by means of a sond or the like. Uncovering of the cover element by punching causes the smallest possible damage of the tissue and thus the best possible healing at the shortest possible time in the important "sealing region" around the portion of the implant which penetrates the soft tissue. When the mucous membrane has been perforated and the cover element has been removed, see FIG. 4, the soft tissue is laid down and takes the place of that part of the cover element which surrounded the neck portion 15 of the implant as disclosed in FIG. 5 where a cover screw 29 of metal or plastics has been attached by screw to the implant. The described embodiment of the implant means that the spacer screw which is a necessary element in the two- step systems available on the market will be superfluous. As a consequence thereof the implant system is substantially rationalized and cheapened. Moreover, the manufac- ture of the implant will be simplified due to the fact that no threaded central hole for a spacer screw in the threaded portion of the implant will be necessary. This means also that the strength of the implant will be increased and that the implant can be given a smaller diameter, which in turn means that the implant can be inserted by surgery in regions having a smaller bone width than that required by implants available so far. Another decisive advantage is that the connective joint provided between implant and spacer at the level of the bone surface of two-step implants available on the market now will be eliminated. This provides obvious advantages for optimal ingrowth in this sensitive border zone region. The only joint which is provided is the connection joint between the implant and the prosthetic structure which exists also in other systems. 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Above and outside the cap the plastic body of the cover element 22 is located said body consisting of plastics. Said latter material can be applied separately in relation to the central cap. In the embodiment of FIG. 6 the cap 34 can form an annular bead or projections on the inside surface thereof to engage an annular groove or depressions on the outside surface of the neck portion 15 for attachment of the cover element to the implant. Thus, the central fastening screw 23 can be dispensed with. The neck portion 15 can be so conical (diverging towards the end surface) that the end surface of the implant has a diameter which is substantially larger than the diameter of the rest of the implant in order that the neck portion shall harmonize with the overlaying prosthetic structure and in order that a sufficiently dimensioned fastening screw can be used for attaching said structure due to the fact that the threaded hole 19 can be made larger. In the embodiment of the implant according to the invention which is disclosed in FIG. 8, the end surface 12 is bevelled at an angle ranging from 5° to 45°. The threaded central hole 19 in the end surface is perpendicular to the end surface. The intention is that the implant having an angled end surface can be installed in a direction where actual bone mass is utilized in the best manner for a longest possible implant the angled end surface at the same time optimizing the possibilities of attaching the prosthetic structure fragmentarily shown at 35, and that the direction of the fastening screw 36 can be adjusted optimally to the bite conditions. In case the implant is of screw type, i.e. provided with threads particularly the self-tapping implants allow a tolerance which means that the implant can be tightened to a level which is in agreement with the desired direction of the bevelled end surface. This is particularly true if the implant has a small thread pitch. The bevelled oval end surface can be shorted in regions where the radius thereof is larger in order to harmonize with a circular end surface in the prosthetic structure. In a further embodiment disclosed in FIG. 9 the bevelled end surface 12 faces buccally for aesthetic optimization by the joint between the prosthetic structure 35 and the implant being located at a sufficiently low level in relation to the buccal soft tissue surface said joint for the rest not being located too low. A shape exactly fitting the bevelled end surface is imparted to the prosthetic structure. In FIG. 10 there is disclosed an embodiment wherein the bevelled surface comprises only a minor part 12a of the end surface 12 in the basic embodiment described and thus does not include that part of this surface which is occupied by the fastening screw 36. The prosthetic structure 35 also in this case is given a shape fitting exactly the end surface. In a further embodiment, FIG. 11, the end surface as in the basic embodiment is perpendicular to the longitudinal axis of the implant but the central hole is angled between 5° and 45°. At the same time the surface of the structure 35 to be attached against the end surface 12 of the implant is angled at an angle corresponding to that of the central hole 19. By this arrangement the same effect is achieved as in the embodiment presented above wherein it was the intention to utilize the bone mass for the best possible retention by providing an angle between the implant and the prosthetic structure, said structure at the same time being optimally adjusted to the bite conditions. The invention has been described with regard to bone anchoring elements intended to be located substantially in yaw bones for anchoring a prosthesis or bridge structure but the invention is not limited to this particular field; it can be applied also in other connections where a bone anchoring is required.

Claims: CLAIMS 1. Direct connected bone anchoring element for prosthetic structures comprising an implant (10) to be inserted into bone tissue (11) the end surface (12) of the implant, which is the outer end surface of the implant in the implanted position thereof, being arranged for attaching the prosthetic structure, characterized in that said end surface is provided on a neck integral with the rest of the implant said neck being adjusted to have in the implanted position of the implant, the end surface (12) above the surface of the bone tissue at the same level as or below the soft tissue in order to allow ingrowth with complete covering of mucous membrane and to be uncovered without applying extension pieces, and that a threaded blind hole is provided in the neck, opening in the end surface, for attaching by screw the prosthetic structure directly to the neck engaging the end surface . 2. Bone anchoring element characterized in that said end surface (12) is completely or partly angled in relation to the axis of the implant (10) . 3. Bone anchoring element according to claim 2, characterized in that said end surface is partly bevelled or recessed. 4. Bone anchoring element according to any of claims 1-3 characterized in that the blind hole is angled preferably 10 to 30° . 5. Bone anchoring element according to claim 1-4 characterized by a cover element (34) fitting the end sur- face congruently and having high peripheral connective precision. 6. Bone anchoring element according to claim 5, characterized in that the cover element (34) comprises a cap surrounding the neck portion. 7. Bone anchoring element according to claim 5 or 6, characterized in that the cap (34) is detachably connected to the implant (10) by means of interengaging projection and recess on the inside surface of the cap and the outside surface of the neck portion, respectively. 8. Bone anchoring element according to claim 1, characterized in that a cover element (22) which is cupola shaped at the upper side thereof is detachably connected with said end surface (12) , said cover element being intended to be covered by soft tissue (28) and to join the surface of the bone tissue radially outside the implant and to surround below the cupola shaped upper side the neck portion (15) provided on the implant, said neck portion forming said end surface. 9. Bone anchoring element according to claim 8, characterized in that the cover element (22) comprises a body of non-resorbable plastics, for example silicone, or metal, for example titanium. 10. Bone anchoring element according to claim 9 char- acterized in that the body is unperforated. 11. Bone anchoring element according to claim 9, characterized in that the body is perforated. 12. Bone anchoring element according to any of claims 2-11, characterized in that the body is at least partly elastically or plastically deformable.

Inventors: LUNDGREN DAN, LUNDGREN, DAN
Applicants: LUNDGREN DAN, NOBEL BIOCARE AB, LUNDGREN, DAN
Application year: 1997
Main classification: A61C8
IPC codes: A61C8, A61F2