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CERAMIC ENDOSSEOUS IMPLANT
CROSSREFERENCE TO RELATED
This application is a continuation-in-part of U.S. patent application No. 787,623, filed Apr. 4, 1977, now abandoned.
BACKGROUND OF THE INVENTION io
1. Field of the Invention
This invention relates to improvements in an endosseous implant used in dental and orthopedic treatment.
2. Prior Art
Surgical techniques involving the use of an implant 15 (screw implant, blade implant, pin implant, etc.) into bone tissue are extensively utilized in dental and orthopedic surgery as a result of the progress made in somatological engineering.
In prior art endosseous implant, the emphasis was on 20 increasing the strength of the implant in the structure. For instance, the present inventors relied on structural considerations for the implant of the previous U.S. Patent Application Ser. No. 550,186 wherein a nut was fitted over the head portion of a screw type implant 25 (and, if necessary, the bottom portion of the implant passed through bone tissue). The device was screwed into the bone tissue and the screw-tightening force of the nut pressed the implant into contact with the bone tissue so as to cause the implant to resist the repeated 30 external force to which the implant was then subjected.
It is an object of the present invention to obtain stabilized post-implantation strength from a binding force between the tissue of the living being and the implant. This binding force is a result of new bone and connec- 35 tive tissue of the living infiltrating deep into the implant surface. In this manner, proliferation and ossification of the bone and connective tissues in a net-like working arrangement is achieved. Another object of the invention, as will become apparent from the description that 40 follows, is to regulate the amount of the incoming new bone and connective tissues by the selection of opening diameters of the micro-apertures located in the ceramic material and in a biodegradable material, and making either of the tissues larger in quantity than the other or 45 making both of them uniform in quantity. In this manner the rigid bonding force inherent in the bone tissue and the elastic bonding force inherent in the connective tissue are brought into conformity with the state of the affected region in which the implant is to be set. 50
SUMMARY OF THE INVENTION
In order to achieve these and other objects, the invention includes a combined structure of two members in the form of a ceramic outer member and a ceramic inner 55 core member. Structural considerations allow for penetration of the outer member by new bone and connective tissue. The composite structure is used to compensate for a possible reduction in the mechanical strength of the outer member due to its inherent structure. 60 Added strength is achieved with the aid of the core material which is either rigid and close ceramics, or single crystal ceramics. Two species are briefly mentioned as a means provided by the other member for permitting the penetration of such new bone tissue and 65 new connective tissue thereinto. In one species the surface of the outer member in contact with a bone tissue has microapertures distributed in a net-like working
arrangement over the area extending from the surface to the inside of the member, the micro apertures suitable in diameter to allow penetration of new bone tissue and connective tissue into it. In the other species the outer member contains a biodegradable material able to be broken down by the bone tissue into a linking form from the surface to the inside' of the member.
The invention will now be described with reference to preferred embodiments thereof, shown by way of example only, reference being made to the accompanying drawings.
BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1 is an exploded perspective view showing a dental screw type implant in one embodiment of the invention;
FIG. 2 is a microscopically enlarged diagrammatic view showing the surface of the outer member;
FIG. 3 is a microscopically enlarged sectional diagrammatic view showing the inside of the outer member;
FIG. 4 is a sectional view showing a prosthetic structure of an artificial tooth using the implant of the invention;
FIG. 5 is an exploded perspective view showing a dental screw type implant in another embodiment of the invention;
FIG. 6 is a microscopically enlarged diagrammatic sectional view showing the inside of the wall thickness of the outer member;
FIG. 7 is a longitudinal sectional view showing the prosthetic structure of an artificial tooth using the implant of the second embodiment of the invention;
FIG. 8 is a microscopically enlarged diagrammatic sectional view of the inside of the wall thickness of the outer member, the view showing the state in which new bone tissue and connective tissue penetrate into the outer member;
FIG. 9 is a microscopically enlarged diagrammatic view showing the pre-implantation state of the inside of the outer member of another embodiment of the invention;
FIG. 10 is a similar view to FIG. 9, and shows the state of the inside of the outer member after the member is implanted;
FIG. 11 is a sectional view showing the prosthetic structure of an artificial tooth using the dental pin type implant in still another embodiment of the invention; and
FIG. 12 is a sectional view showing another embodiment of the invention as applied to an artificial hip joint.
DETAILED DESCRIPTION OF THE
In the drawings, FIGS. 1 through 4 show various embodiments of the invention included under the first species. FIGS. 5 through 10 show embodiments of the invention included under the second species. FIGS. 1 through 11 show implants for use in dental endosseous implantation, and FIG. 12 shows an implant for use in artificial joint utilized in orthopedic surgery.
Referring now in detail to the invention in conjunction with the drawings, the implant of the invention comprises an outer member 1 and an inner core member 2. Both members 1 and 2 are tightly bonded together into a combined assembly, i.e., a composite structure. In the embodiment illustrated in FIGS. 1 through 4, both
the outer member 1 and the inner core member 2 are made of polycrystalline sintered ceramics, both members being sintered or cemented into one body. The outer member 1 is a slender cylindrical body and may be formed on the outer periphery with threads 11. The 5 inner core member 2 is formed at the top and thereunder respectively with an artificial tooth receiving portion 21, a flange 22, which is at least not smaller in outer diameter than the outer member 1, and a post portion 23 adapted to be closely fitted into a cavity 12 in the outer 10 member 1, Means are provided in the outer member 1 for permitting the penetration of a new bone tissue and a new connective tissue thereinto. This is shown in FIG. 3 as micro-apertures 13. The apertures 13 are located in the outer member 1 in net-like working arrangement (in 15 a cubic net-like state) from the surface portion toward the inside of the outer member 1, and most of the openings 130 of the micro-apertures 13 in the surface portion are arranged to be on the order of 100 fxm (0.1 mm) in diameter, preferably in the range of 100-500 fim in 20 diameter so as to permit the passage of the new bone tissue thereunder. Alternatively, they may be in the range of 20-100 |un in diameter so as to permit the passage of the new connective tissue therethrough.
Although the ceramics used in the prior art implant of 25 ceramics were higher in affinity to the new bone and new connective tissues than metal and plastics, the new tissues could not penetrate deep into the implant and fix themselves thereto. The reason is that, since ceramics forming the implant are close in texture and lacking in 30 permeability, the prior art ceramics afford no room for the new bone and connective tissues to penetrate therethrough. On the other hand, when unglazed (porous) ceramic is used, such ceramics permit the existence of very small apertures inside thereof. However, it is also 35 equally true that such apertures are very small, i.e., less than 10 /xm. Therefore, it is virtually impossible for the new bone and connective tissue to penetrate therethrough. As will later be described, according to the findings of the present inventors, it is necessary that the 40 apertures be larger than 200 fim (0.2 mm) in effective opening diameter to permit new bone tissue to penetrate through the apertures. Furthermore, it is necessary that in the case of the dental implant that the apertures be in the range of in the range of 200-700 urn, preferably 45 300-500 fim in effective diameter of opening, and that in the case of the orthopedic implant, the apertures be in the range range of 200 ftm-5 mm, preferably 300-500 jam in effective opening diameter. In order for the new connective tissue to penetrate through the micro-aper- 50 tures, their openings must be far smaller in diameter and must be in the range of 20-100 ju.m. In the present invention, it is proposed on the basis of the above findings, that the micro-apertures 13 be larger than 20 fun in the diameter of opening. In the case of the dental implant 55 they should be in the range of 20-700 jam in order to permit both new bone and connective tissues to penetrate in larger percentage. The penetration of the new bone tissue through the implant has the characteristic of imparting rigid bonding to the implant. The penetration 60 of connective tissue through the implant has the characteristic of imparting elastic bonding to the implant because such tissue is fibrous and high in elasticity. Accordingly, when the implant is subjected to overstress, it is necessary that the new connective tissue penetrate 65 in larger percentage through the implant, and conversely when the implant is subjected to less stress and rigid bonding is required, all that is necessary is to make
it possible for the new bone tissue to penetrate in larger percentage through the implant. Alternatively, when it is desired for both the new bone tissue and connective tissue to penetrate in equal percentage through the implant, it is necessary to afford an equal opportunity for both tissues to penetrate the implant. Regulation of the penetration opportunity of both tissues, needless to say, depends upon the distribution percentage chosen for the two opening diameters (20-100 /xm and 200-700 jxm).
A net-like working structure provided by the microapertures 13 of the type described must be formed at least on the outer surface of the member 1 in contact with a bone tissue. As illustrated in FIGS. 1 and 3, the structure has been formed with micro-apertures 13 over the entire thickness of the member 1. Furthermore, the micro-aperture openings 130 on the member 1 outer surface must have diameters in the ranges described above, but the pore diameters inside the member 1 need not necessarily be limited to the ranges described. The reason for this is that even if there may be some differences in aperture diameter, the new bone and connective tissues which penetrate through the openings in the surface of the member 1 can find their way into the inner part. It should be understood that the contact surface of the outer member 1 in which the apertures 13 are provided need not necessarily range over that entire area over which the member 1 comes into contact with the bone tissue, but may cover most of the area. Likewise, the limited range in the opening diameters need not necessarily cover all the diameters of the openings located in the contact surface of the member 1 but may cover most part of the limited range. The porosity of outer member 1 suitable for the apertured structure is properly in the range of 20-50% , and penetration of the new bone and connective tissues is reduced in amount when the porosity is below this range. When the porosity is above this range, machining of the member 1 such as thread cutting becomes difficult because the member increases in brittleness. The method of obtaining a porous structure member 1 having such a porosity is preferably instituted by sintering a mixed material, produced by mixing a vanishing material with a ceramic material. The vanishing material is then burnt out or vaporized from the ceramics, and vanishes and leaves no ashes at all or leaves such ashes, if any, which are quite harmless to the living being. In this manner the pores are formed. Such vanishing materials include polyethylene spherical bodies, acrylate resin fiber chop, and the like.
The inner core member 2 is intended to increase the mechanical strength of the outer member 1 of a porous structure, and accordingly, the material of the member 2 must be solid and strong. The inner core member 2 in this embodiment may be made of a polycrystalline sintered body of ceramics and is shown wherein two members 1 and 2 are sintered into one body with a post portion 23 left inserted into a cavity 12 of the member 1. On the second species shown in FIG. 5, it is possible also to bring the inner core member 2 into mechanical engagement by threads with the outer member 1. In the case of the first embodiment wherein both members are sintered into one body, the two members are, in principle, placed under conjugate sintering conditions. In the second embodiment, the inner core member 2 may be a single crystal body of alumina, and has no such conjugate conditions. Also, in the second embodiment both members 1 and 2 may be bonded to each other by use of a suitable binder, instead of the threaded engagement.
According to an initial object of the inner core member 2, the member should desirably be a single crystalline body which is far superior in strength to polycrystalline ceramics.
A solid core of polycrystalline ceramics, even if it is 5 polished, is on the order of 2500-3500 kg/cm2 in bending strength. In this respect, the solid core of polycrystalline ceramics is far inferior to that of metal. Even though metal is superior in mechanical strength, metal has a great disadvantage in point of practical application 10 in that it has a harmful effect on the human body. Single crystalline alumina ceramics, when used as a solid core, may be as much as several times higher in mechanical strength than polycrystalline ceramics and equal to or higher than metal. Moreover, because single crystal is 15 superior in flexibility to polycrystalline ceramics, the single crystal is highly reliable also in bending strength, which prevents the breaking of an implant due to flexion of the natural bone around the implant.
The member 2 also includes a flange portion 22 pref- 20 erably larger in outer diameter than the outer member 1. This is apparent from the prosthetic structure in FIG. 4, and is based on the idea of preventing bacilli from invading a hole h of a bone tissue b after an operation by covering the opening of the top portion of the hole h 25 with the flange 22.
A description of dental prosthetic procedures according to the embodiment described above will now be given with reference to FIG. 4. A gingival flap f is cut open, a tap hole h is provided in a bone tissue b and an 30 implant i of the invention is screwed into the hole h. The bone tissue b includes hard tissues bi and b3 and a soft tissue b2 therebetween. In the embodiment shown, the implant i does not reach the lower hard tissue b3, but the bottom end of the implant i stops in the soft tissue bz- 35 If it is necessary to fix the implant i to the inside of the bone tissue b, a nut (not shown), which enables the implant i to be fixed to the upper hard tissue bi, is threadedly mated with the implant i and an artificial tooth t is fitted over the top of the implant i through 40 cement c.
The embodiment described above provides an inventive implant i, imbedded in the bone tissue b, in which the surface of the outer member 1 in contact with the bone tissue has numerous openings 130 for micro-aper- 45 tures, most of whose diameters are greater than 20 jim. Because of the apertures 13, it is possible for the outer member 1 to have gas permeability at least on its surface side to permit the penetration of the bone tissue b, particularly the new bone tissue and new connective tissue 50 of the upper hard bone tissue bi, through the Openings 130 into the inner part of the outer member 1. In response thereto, the tissues thus penetrated dispersedly drives the gas existing inside the micro-apertures 13 (CO2 and NH3 are especially abundant in a living being) 55 through openings 130 into the bone tissue b. Penetration of the new bone and connective tissues into the member 1 in such a responsive relation is thus enhanced. This causes the tissues to form a net-like root structure in every part of the member 1 over a long period of time. 60 Thus, as in the case of the prosthetic structure shown in FIG. 4, the member 1 and the bone tissue b, can develop into very firmly bonded relation because of the net-like structure of the roots of the new bone and connective tissues which have extended throughout the member 1. 65 Accordingly, if the outer member 1 should be cracked after the operation, the new bone and connective tissues which have penetrated into the member 1 serve to sup
port the cracked member 1. In this manner the cracked member is prevented from being further cracked. In addition, the new bone and connective tissue serves to strengthen the support characteristics of member 1. In the embodiment in FIG. 4, the direction in which the new tissues penetrate through the member 1 is generally at right angles to the axial direction of the inner core member 2, and accordingly, marked improvement is made in the resistance of the implant i to the accluding impact centered on the implant i. Additionally, since the implant i of the invention is made of ceramics, it is free from toxic properties and does not produce adverse effects on the human body even if implanted in the bone tissue b over a longer period of time. Since the implant i is excellent in durability, it does not cause any chemical deterioration due to secretions. Finally, since the outer member 1 of the invention is reinforced internally with the inner core member 2 of compact polycrystalline body of ceramics or single crystalline ceramics, the implant i provides no possibility of being broken or deformed by the repeated accluding impacts given thereto. Thus it can stand long time use in its implanted state.
A description will now be given of the second species of the invention with reference to FIGS. 5 through 10, wherein like parts are represented by the same reference characters and space. Means for permitting the penetration of the new bone tissue and new connective tissue in this embodiment is shown as a material 14 which is degraded by the new bone and connective tissues that find their way into the member 1. The material 14 takes the place of the micro-apertures 13 in the first embodiment. More specifically, the outer member 1 in FIGS. 5 and 7 includes an apatite sintered body in the form of new biodegradable material 14 in the ceramics of the member 1. More particularly, the outer member 1 in this embodiment is a sintered complex (the term "complex" means that the body produced is made of a plurality of materials, and is different from the term "composite" in the composite structure hereinbefore used) consisting mainly of ceramics 15 and an apatite sintered body 14. The apatite sintered body 14 in FIG. 6 is an aggregate which retains the form of a sphere, polyhedron, fibrous body, etc. and whose grains are linked to one another in a net-like working arrangement. The ceramics 15 in FIG. 6 are an aperture-filling body continuously linked throughout the aggregate for filling the apertures between the grains of the aggregate so as to form a net-like working structure.
Apatite is a general term for a substance represented by chemical formula ... calcium phosphate based hydroxy-apatite represented by ... .(OH)2 is the one which has been found to be most closely related to a living being. This hydroxy-apatite is the chief constituent of minerals of bones and teeth of a vertebrate animal. The hydroxy-apatite can be synthesized and also can be obtained by removing organic matter (protein, polysaccharide, etc.) from the bones and teeth of a living being through combustion, through melting, or through dissolving. The former is termed synthetic apatite and the latter is termed bioapatite.
The apatite in this embodiment may contain either of the above two kinds of apatite. This powder is sintered under pressure into an apatite sintered body 15. It is desirable to have the main constituent of this apatite sintered body to be calcium triphosphate ... which has the property of being physiologically dissolved and absorbed (the term "physiologically dissolve