Living hard tissue replacement prepared by superplastic forming of a calcium phosphate base

A living hard tissue replacement is prepared by subjecting a calcium phosphate base ceramic material to superplastic forming. Also provided is a method for preparing a composite body comprising the step of joining a cover of a calcium phosphate base ceramic material to a substrate of a heterogeneous material by superplastic forming. Superplastic forming is often performed by hot isostatic pressing. The living hard tissue replacement and composite body are useful as artificial dental roots, dental crowns, and bones.

This invention relates to living hard tissue replacements obtained by 
shaping a calcium phosphate base ceramic material having biological 
activity by superplastic forming, and a method for preparing the same. It 
also relates to a method for preparing a composite body by joining a 
calcium phosphate base ceramic material to a material of different type 
through superplastic forming. 
BACKGROUND OF THE INVENTION 
A variety of filling and repairing materials have been utilized to restore 
the function and configuration of a deficient part of a living body. 
Typical filling and repairing materials for living bodies include 
artificial bones and analogues such as artificial dental roots and crowns 
as well as artificial joints. They are generally known as living hard 
tissue replacements. 
These living hard tissue replacements are required to be mechanically 
strong, tough, and stable in living bodies and should have high affinity 
to living bodies. Another important factor is ease of shaping because a 
living hard tissue replacement has to be a custom-made part conforming to 
an individual patient's deficient site. 
The biological affinity used in this context means how a living hard tissue 
replacement adapts itself to and merges or assimilates with the 
surrounding living tissue where the replacement is embedded or implanted. 
Thus, a material having high biological affinity is scarcely recognized as 
xenobiotic by the surrounding tissue. Particularly when such material is 
used as an artificial bone, it can promote osteogenesis from the 
surrounding bone to eventually form a firm bond between itself and the 
bone tissue. 
Among the currently available artificial bone materials, those featuring 
high mechanical strength and in vivo stability are metals such as titanium 
and zirconium, alloys containing such metals, and ceramics such as 
alumina, silicon nitride, and zirconia. The materials having high 
mechanical strength and in vivo stability, however, have low biological 
affinity, that is, are unlikely to assimilate with living tissue, 
resulting in an extended cure time and poor adherence to the living 
tissue. In addition, they must be extracted and removed by surgical 
operations after they have performed their duties. 
Typical of known materials having high biological affinity are calcium 
phosphate base ceramic materials including apatite, especially 
hydroxyapatite and tricalcium phosphate. Apatite has the best biological 
affinity as understood from the fact that bone is essentially composed of 
apatite if organic components are excluded. 
As previously described, most living hard tissue replacements have a 
particular shape and a complex profile depending on individual patients. 
In particular, oral surgical implants such as dental roots and crowns 
widely vary in shape depending on individual patients and sites. In the 
prior art, an implant of desired shape is generally prepared from calcium 
phosphate base ceramic material, typically apatite by molding the material 
by an injection molding or casting technique, followed by sintering and 
reshaping. These manufacturing methods, however, have many drawbacks 
including lack of dimensional precision, difficulty to change the shape, 
surface defects induced by reshaping, and losses of strength due to 
stresses. 
Further, it is critical for implants to adhere to living bones. For 
promoted adherence, it is important to control the surface nature of 
implants. However, ordinary processing techniques have limited freedom to 
control the surface nature, for example, to achieve a mirror finish or a 
rough surface. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to eliminate the drawbacks of 
prior art living hard tissue replacements and to provide a novel and 
improved living hard tissue replacement which can be readily formed to the 
desired shape, surface nature, and dimensions in conformity to an 
individual patient. 
Another object of the present invention is to provide a method for 
preparing such a living hard tissue replacement. 
A further object of the present invention is to provide a method for 
preparing a composite body having increased mechanical strength which is 
applicable as a living hard tissue replacement. 
The inventors have found that calcium phosphate base ceramic materials show 
superplastic nature. The present invention is predicated on this finding. 
The superplasticity of ceramics is the nature that ceramics show extremely 
high ductility under low stresses at a temperature substantially lower, 
e.g., by 500.degree. C., than the sintering or forging temperature as 
described in the literature, for example, Journal of the JSTP, 29, 326 
(Mar. 1988); Ceramics, 24, 2 (1989); and Tetsu to Hagane (Iron and Steel), 
75, 3 (1989). Typical prior art ceramic materials known to show 
superplastic nature are Y-TZP (yttria-stabilized tetragonal ZrO.sub.2 
polycrystals) and ZrO.sub.2 --Al.sub.2 O.sub.3 systems. To take advantage 
of plastic deformation, extrusion molding and thin plate molding have been 
attempted on them. Attempts have also been made to diffusion bond two 
pieces of the same material by superplastic forming. 
Nevertheless, it has never been reported that calcium phosphate base 
ceramic materials including apatite and tricalcium phosphate show 
superplastic nature. We have first discovered that calcium phosphate base 
ceramic materials show superplastic nature. 
According to a first aspect of the present invention, there is provided a 
living hard tissue replacement obtained by the superplastic forming of a 
calcium phosphate base ceramic material. 
According to a second aspect of the present invention, there is provided a 
method for preparing a living hard tissue replacement comprising the step 
of superplastic forming a calcium phosphate base ceramic material. 
Preferably, the ceramic material has an average grain size of up to 10 
.mu.m, and the superplastic forming is carried out at a temperature of 
500.degree. to 1,600.degree. C. 
According to a third aspect of the present invention, there is provided a 
method for preparing a composite body comprising the step of joining a 
calcium phosphate base ceramic material and a material of different type 
by superplastic forming.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The ceramic materials used in the practice of the present invention include 
a variety of ceramic materials based on calcium phosphate although apatite 
and tricalcium phosphate are often used. Preferred are apatite series 
calcium phosphates having a stoichiometric composition: Ca.sub.10 
(PO.sub.4).sub.6 X.sub.2 wherein X is a hydroxyl group or a halogen atom 
(e.g., fluoro and chloro). The most preferred calcium phosphate materials 
are hydroxyapatite and fluoroapatite, typically having an atomic calcium 
to phosphorus (Ca/P) ratio of from 160/100 to 175/100. Also useful is 
tricalcium phosphate Ca.sub.3 (PO.sub.4).sub.2. 
In the practice of the present invention, these calcium phosphate base 
ceramic materials are used in sintered form. The sintered body may contain 
sintering aids such as Al.sub.2 O.sub.3, SiO.sub.2, MgO and CaO in an 
amount of up to 5% by weight of the body. 
The calcium phosphate base ceramic materials in sintered form preferably 
have an average grain size of up to 10 .mu.m. It is to be noted that the 
average grain size may be measured by means of a scanning electron 
microscope (SEM) by determining an average grain area, and determining the 
average diameter of a phantom circle having the average grain area. The 
average diameter is the average grain size. Less superplasticity is 
exerted with an average grain size in excess of 10 .mu.m. Better results 
are obtained from ceramic materials having an average grain size of up to 
2 .mu.m, more preferably up to 1 .mu.m, most preferably up to 0.7 .mu.m, 
while the minimum grain size is usually of the order of 0.005 .mu.m. 
Since the sintered body approximately maintains its average grain size 
during superplastic forming to be described later, the average grain size 
of the sintered body after superplastic forming is somewhat or little 
changed from that before superplastic forming. It should be understood 
that if the sintered body is subjected to superplastic forming while 
compressing under a unidirectional stress, the grains are generally 
recognized to undergo distortion and orientation. 
The shape and dimensions of the sintered body are not critical since they 
may be determined for a particular purpose. The average grain size of the 
sintered body is generally of the order of 10 to 100% of that of the 
finally formed article. 
The stock materials from which sintered bodies are formed are preferably 
apatite and tricalcium phosphate as previously defined. The stock 
materials may be either biotic apatites collected from bones and teeth of 
various vertebrate animals or synthetic apatites prepared by dry and wet 
processes. Powder stock material of calcium phosphate base ceramic 
material such as apatite and tricalcium phosphate is sintered to form a 
sintered body having superplasticity. The powder stock material preferably 
has a surface area of about 1 to about 100 m.sup.2 /g as measured by the 
BET method. The stock material may be blended with sintering aids as 
previously described. 
The powder stock material is then shaped. For shaping, the material is 
subjected to unidirectional pressing under a pressure of about 1 to about 
3,000 kg/cm.sup.2 and then to cold isostatic pressing (CIP) under a 
pressure of about 1,000 to about 10,000 kg/cm.sup.2. 
The shaped material is usually sintered at a temperature of 500.degree. to 
1,600.degree. C., especially 700.degree. to 1,200.degree. C. for about 3 
minutes to about 30 hours. During sintering, concurrent hot pressing or 
hot isostatic pressing (HIP) is preferably carried out for densification 
purposes. These pressing techniques favor a pressure of 50 to 5,000 
atmospheres. The atmosphere may be an inert gas, air, hydrogen or vacuum. 
The material may be calcined at about 700.degree. to about 1,350.degree. C. 
for about 3 minutes to about 30 hours before sintering. 
In this way, there is obtained a sintered body with the above-described 
average grain size, preferably having a relative density of at least 70%, 
more preferably at least 90%, and most preferably at least 99.5%. 
The next step is to process the sintered material by superplastic forming. 
The forming temperature ranges from 500.degree. C. to a temperature lower 
than the sintering temperature, preferably lower by 50.degree. C. More 
illustratively, the temperature ranges from 600.degree. to 1,300.degree. 
C., especially from 800.degree. to 1,200.degree. C. The superplastic 
forming in the first form of the invention uses a mold with a cavity 
having the shape and dimensions corresponding to the intended implant and 
is performed by subjecting the sintered calcium phosphate base Q ceramic 
material to extrusion shaping or embossing. During the process, both the 
sintered material and the mold should be heated to a temperature at which 
the sintered material exhibits superplasticity. The sintered calcium 
phosphate base ceramic material subject to superplastic forming is 
preferably in the form of thin plates or sheets, granules, and powder 
particles. Superplastic forming causes thin sheets or granules to fuse 
together to form a continuous crystalline structure at their interface, 
resulting in a very firm junction. 
The compression rate, compression force, and deformation during 
superplastic forming vary with a particular technique employed. Preferred 
forming parameters include a compression rate of 0.01 to 50 mm/min., a 
compression force of 1 to 2,000 MPa, more preferably 1 to 500 MPa, 
especially 1 to 100 MPa, and a deformation of about 0.1 to about 1.5 in 
true strain. 
In the case of embossing, the sintered calcium phosphate base ceramic 
material which has been shaped to approximately the desired shape, for 
example, the dental crown shape may be used. In this case, the deformation 
to be achieved by superplastic forming or embossing is minimized, with the 
benefit of ease of forming. 
The superplastic forming may also be carried out by a hot pressing 
technique using a mold or a hot isostatic pressing (HIP) technique. 
Further, the superplastic forming may be repeated any times if desired. 
In general, the thus formed calcium phosphate base ceramic material has 
experienced a change in grain size as low as 100% or less. It is sometimes 
observed that the grains have undergone slippage along the grain boundary, 
deformation, and orientation. 
In the second form of the present invention, a sintered calcium phosphate 
base ceramic material as previously described and a material of different 
type, that is, other than the calcium phosphate base ceramic material are 
placed one on another to form a stack and joined together by superplastic 
forming. More particularly, the two materials are placed in pressure 
contact at a predetermined temperature to provide a diffusion or solid 
phase bond. 
The temperature, compression rate, compression force, deformation, and 
other parameters for joint formation are substantially the same as 
previously described for the superplastic forming. 
Pressure contact may be achieved between a calcium phosphate base ceramic 
material and a substrate of a different material by pressing the stacked 
materials in close contact using a press or punch or by extruding or 
reverse extruding the calcium phosphate base ceramic material through a 
die using a punch formed of the substrate material. Pressure contact may 
also be accomplished by rolling and drawing combined with extrusion. 
The superplastic forming may be repeated any time if desired. 
The superplastic forming joins the sintered calcium phosphate base ceramic 
material and the heterogeneous material together into a composite body 
having a bond strength as high as 30 to 1,000 MPa, especially 100 to 1,000 
MPa although the bond strength depends on the type of the heterogeneous 
material. 
The calcium phosphate base ceramic material in the composite body sometimes 
experiences a change in grain size as previously described. 
The material of different type to be joined is not particularly limited as 
long as it is different from the calcium phosphate base ceramic material. 
It may be selected from various ceramic materials, various metals, various 
glasses, composites thereof, and other suitable materials. The material to 
be joined may or may not exhibit plastic deformation during pressure 
joining. Also, it may or may not exhibit superplastic deformation. If the 
material to be joined has superplastic nature, then it is possible to join 
more than two materials together at the same time. Most often, the 
material to be joined has high mechanical strength and functions as a 
substrate or support. 
In the preferred embodiment of the second form wherein the calcium 
phosphate base ceramic material covers a substrate of different material, 
two materials are held in close contact at a predetermined temperature and 
pressure by hot isostatic pressing (HIP), thereby achieving a diffusion or 
solid phase bond therebetween. The superplastic ceramic material such as 
calcium phosphate material as the cover layer undergoes superplastic 
deformation during the HIP process. 
For the HIP process, the temperature generally ranges from 500.degree. C. 
to lower than the sintering temperature, usually from 500.degree. C. to 
1,600.degree. C. The compression force generally ranges from 1 to 2,000 
MPa, usually from 1 to 500 MPa. The heating/pressing time may vary over a 
wide range of from about 6 seconds to 1,500 minutes. The deformation 
ranges from about 0.1 to about 1.5 in true strain. 
Referring to FIGS. 1 to 3, an assembly of a substrate and a cover is shown 
in perspective views to illustrate the method for preparing a composite 
body using the HIP process. The elements involved in this method are a 
substrate 11, a cover 12 formed of superplastic ceramic material, a 
surrounding/pressurizing medium 23 in the form of ceramic powder, and an 
enclosure 24 in the form of a glass envelope for enclosing the foregoing 
three elements in a stationary manner. 
The substrate 11 is preferably formed of a material which can withstand 
repetitive stresses and has a flexural strength of 200 MPa or higher, 
sometimes as high as 1500 MPa and a high melting point of 700.degree. C. 
or higher. Also preferably, the substrate material undergoes little 
deformation, say, a deformation of less than 10%, during superplastic HIP 
joining. Preferred examples of the substrate material include metals and 
ceramics, which should be non-harmful to living bodies in the case of 
living hard tissue replacements. The metals include elemental metals such 
as Ti, W, Mo, Au, and Pt and alloys such as Ni-Cr, Ni-Ti, Fe-Cr-Al, Ti-Zr, 
Fe-Ni-Cr, Ti-Al-V, Co-Cr, Co-Cr-Mo, Ti-Mo, and stainless steel. The useful 
ceramics include high strength ceramics such as zirconia, SiC, SiN, BN, 
and Al.sub.2 O.sub.3. 
The cover 12 is of a superplastic ceramic material which is selected from 
biologically active calcium phosphate base ceramic materials such as 
apatite and tricalcium phosphate as previously described. More 
particularly, the cover 12 is formed from a sintered ceramic material 
having a predetermined grain size. The configuration and dimensions of the 
cover 12 may be determined in accordance with the configuration and 
dimensions of the substrate 11. 
It is desired that the difference in thermal expansion coefficient between 
the substrate 11 and the cover 12 be within 50%, preferably within 40%, 
more preferably within 10%. 
The ceramic powder used as the surrounding/pressurizing medium 23 includes 
zirconia, alumina, BN, SiC, SiN, WC, and partially stabilized zirconia 
although high temperature ceramics which do not melt at the superplastic 
forming temperature involved in the HIP process, especially at 700.degree. 
C. or higher temperatures are preferred. A proper choice should be made of 
the particle size of the ceramic powder 23 since the cover 12 joined to 
the substrate by superplastic forming can have a surface roughness 
corresponding to the selected particle size of the surrounding powder. 
Therefore, the ceramic powder may have a particle size of about 10 to 
about 500 .mu.m. The ceramic powder layer 23 is generally about 0.5 mm to 
about 50 mm thick. 
The glass envelope 24 may be formed of silica glass, boron oxide glass, 
silicate glass, borosilicate glass, germanate glass, phosphate glass or 
the like. It is generally about 100 .mu.m to about 5,000 .mu.m thick. 
These elements are assembled as follows. First, a suitable support material 
is shaped and finished to form a substrate or support 11 of predetermined 
configuration and dimensions depending on the intended use. Also a calcium 
phosphate base ceramic material is sintered by the CIP or HIP process to 
form a cylindrical cover or sleeve 12. The cover or sleeve 12 generally 
has a wall thickness of up to 10 mm, preferably 10 .mu.m to 3 mm. Then the 
substrate 11 is fitted and received in the cover 12 as shown in FIG. 1. 
Preferably the substrate 11 is snugly fitted in the cover 12 with the gap 
therebetween being as small as 0.3 mm or less. 
The substrate 11 has substantially the same configuration as the final 
product. More particularly, the substrate 11 may be provided with grooves, 
recesses, ridges, or rims on its surface if desired. In the illustrated 
example, the substrate 11 is a cylinder formed with a circumferential 
groove at an axial intermediate. The configured substrate 11 may be 
prepared by sintering, casting, and/or machining. 
Also, the cover 12 has substantially the same outer configuration as the 
final product. However, the cover 12 may take a simple configuration 
having a smooth outer surface while necessary surface irregularities can 
be omitted at this point. This leads to efficiency of manufacture because 
only the substrate 11 need precise shaping and the cover 12 can have a 
simple configuration. The simple configuration allows the cover to be 
formed to a uniform wall thickness, resulting in the final coating with a 
uniform thickness. 
Next, the assembly of substrate 11 fitted in cover 12 is covered with 
ceramic powder 23 and enclosed in the glass envelope 24 as shown in FIG. 
2. 
The envelope is then loaded in a conventional HIP apparatus which applies 
heat and pressure to the envelope, creating a strong junction between 
substrate 11 and cover 12. There results an integral or composite body 1 
in which a coat 15 is closely bonded to the substrate 11 as shown in FIG. 
3. At the end of HIP forming, the glass envelope 24 is broken and the 
composite body 1 is taken out. 
During the HIP process, the substrate 11 is little deformed and the coat 15 
of uniform thickness closely conforms to the outer contour of substrate 
11. The isostatic pressing minimizes the chance of poor bond. 
The coat 15 on the composite body 1 generally has a thickness of from about 
1 .mu.m to about 10 mm, preferably from 1 .mu.m to 5 mm, more preferably 
from 10 .mu.m to 3 mm. The coat 15 does not peel away even when it is 
relatively thick as several millimeters. 
The coat 15 can be finished to a mirror or rough surface through a proper 
choice of the particle size of the surrounding ceramic powder 23. For 
increased surface area and biological activity, the coat should have a 
rough surface having a surface roughness Rmax of about 10 to about 500 
.mu.m. 
The superplastic junction using an HIP process has been described. In the 
practice of the invention, a hot pressing (HP) process may also be used to 
establish a junction insofar as the substrate 11 having substantially the 
same configuration as the final composite body 1 can be combined with a 
cover 12 of a simple configuration capable of fitting over the substrate. 
The HP process involves the steps of engaging the substrate 11 with the 
cover 12 in place, embedding the assembly in the ceramic powder 23 in a 
press mold, and forcing a punch thereto. The press mold is preferably 
formed of the same or similar material as the ceramic powder. Then a bond 
is established to a degree substantially equal to the HIP process. 
The composite body thus obtained may be utilized as parts at least 
partially embedded in a living body, for example, artificial dental parts 
such as dental roots and crowns, artificial bones (including general 
bones, skull, ossiculum auditus, jaw bone, cartilagines ansi, etc.), bone 
replacements, artificial joints, fracture fixtures, artificial valves, and 
artificial blood vessels as well as medical equipment, for example, 
hypodermic implanting equipment such as dialysis shunts, living body 
embedding equipment such as pacemakers, and other living body indwelling 
equipment. The composite body is best suited as living hard tissue 
replacements. 
Now, the application of the composite body of the invention to an 
artificial dental root and crown as a typical living hard tissue 
replacement is described. 
The artificial dental root and crown are preferably of the construction in 
which a sintered calcium phosphate base ceramic material is joined to the 
surface of a substrate by superplastic forming. 
FIG. 4 shows an artificial dental root and crown according to one preferred 
embodiment of the invention. An artificial dental root 3 to which a crown 
4 is secured through a damping member 5 by cement layers 6 and 7 is 
implanted in an alveolar bone 71. 
The dental root 3 includes a root substrate 31 and a coat layer 35 bonded 
to the substrate surface. The root 3 on the outer surface is formed with a 
plurality of tabs or threads 33. The tabs 33 have an anchoring function 
and a function of defining gaps between the root 3 and the alveolar bone 
71. The root 3 does not directly integrate with the surrounding alveolar 
bone 71, but a neoblastic bone which will form around the root. The 
provision of tabs 33 promotes the growth of a new bone, ensuring a firm 
bond between the root 3 and the alveolar bone 71. The shape of tabs 33 is 
not critical, and they may be circumferential rings, screw threads or 
discrete protuberances on the outer surface of the root 3. The tabs 33 are 
generally 100 .mu.m to 3 mm in radial height. 
The configuration and dimensions of substrate 31 are not critical. The 
substrate may be configured to the shape and dimensions corresponding to 
the intended artificial dental root as previously described. It is 
preferably formed with tabs corresponding to the tabs 33. It is preferably 
formed from a safe material having high mechanical strength and toughness 
as previously described, inter alia, titanium, titanium alloy, zirconia, 
and monocrystalline alumina (sapphire). 
The coat layer 35 on the root substrate 31 is formed from a calcium 
phosphate base ceramic material and joined to the substrate 31 by 
superplastic forming. The coat layer 35 is initially a hollow cylinder 
having one closed end and one open end formed from a sintered calcium 
phosphate base ceramic material and adapted to fit over the substrate 31. 
The cover cylinder is preferably about 10 .mu.m to about 10 mm thick. The 
superplastic forming causes the cover cylinder of calcium phosphate base 
ceramic material to deform in conformity with the outer contour of 
substrate 11 and to bond to the substrate 11 without leaving a gap. The 
coat layer 35 of uniform thickness is joined to the substrate 11 in this 
way. The coat layer 35 preferably has a thickness of 1 .mu.m to 5 mm, more 
preferably from 10 .mu.m to 2 mm. 
The coat layer 35 should be present at least on those surface areas of the 
artificial dental root 3 which come in contact with a living body, for 
example, the alveolar bone 71, gingival epithelium 72, and subepithelial 
connective tissue 73 in FIG. 4. 
The artificial dental root 3 of the above-mentioned construction is not 
particularly limited in shape. A choice may be made of circular, oval, and 
rectangular column and blade shapes. It is preferred to provide tabs 33 as 
previously described. In general, the artificial dental root 3 has a 
maximum diameter of 2 to 20 mm and a height of 3 to 50 mm. It may be 
configured to any desired dimensions Q in accordance with various 
standards or to special dimensions if necessary. 
Although the artificial dental root 3 shown in FIG. 4 is a one-piece 
member, the invention is also applicable to two-piece dental roots having 
a post core and multiple-piece dental roots having three or more 
components. These roots should have a coat layer of calcium phosphate base 
ceramic material on at least the surface to be in contact with a living 
body. 
The dental crown 4 includes a coronal substrate 41 and a coat layer 45 
bonded to the substrate surface. The coat layer 45 is formed of sintered 
calcium phosphate base ceramic material and joined to the coronal 
substrate 41 by superplastic forming. An initial cover which is fitted 
over the substrate and eventually converted into the coat layer 45 may 
have any appropriate thickness depending on the desired configuration and 
thickness of the coat layer. 
The configuration and dimensions of coronal substrate 41 are not critical. 
The coronal substrate 41 may be configured to a columnar or pyramid shape 
having a recess at its bottom for receiving the root or a combination of 
such shapes. The top side of the coronal substrate 41 may be flat, but 
preferably configured to a complex shape conforming to the intended crown. 
More particularly, the crown configuration largely differs between 
incisive and molar teeth. Thus, the use of a coronal substrate 41 
configured to a shape conforming to a particular incisive or molar tooth 
leads to ease of forming because the sintered calcium phosphate base 
ceramic material requires minimal deformation. The material of which the 
coronal substrate 41 is formed may be selected from the same materials as 
previously mentioned for the root substrate. 
In order that the crown 4 have an aesthetic appearance similar to a natural 
tooth, biological affinity, and no detrimental influence to the gingival 
epithelium 72, the coat layer 45 should preferably surround the entire 
outer surface of the coronal substrate 41. The coat layer 45 generally has 
a thickness of about 1 .mu.m to about 5 mm. 
The crown 4 illustrated in FIG. 4 is a single component crown although the 
present structure is applicable to the outer crown component of a 
two-component crown. 
The artificial dental crown 4 is secured to the artificial dental root 3 
through the damping members 5 by cement layers 6 and 7 as shown in FIG. 4. 
The damping members 5 are effective for damping shocks applied to the root 
during mastication and gnash. Cementing of the crown to the root through 
the damping members allows the crown to fluctuate like a natural tooth. 
The damping members 5 are often formed of synthetic rubber to a thickness 
of about 0.01 to about 4 mm. The cement layers 6 and 7 may be of a 
conventional dental cement. 
The artificial dental root according to the invention is not only useful as 
an implant embedded in an alveolar bone when combined with the artificial 
crown as shown in FIG. 4, but also as an intradental implant by inserting 
the artificial root into a natural tooth. 
The artificial dental root combined with the artificial dental crown 
according to the invention is also useful in full and partial denture 
sets, and free-standing tooth replacements. 
The artificial dental root and crown according to the invention may be 
independently used in practice. That is, the artificial dental crown and 
root to be combined with the artificial dental root and crown according to 
the invention, respectively, need not have a coat layer of calcium 
phosphate base ceramic material. The artificial dental root and crown 
according to the invention are effective even when they are combined with 
conventional artificial dental crown and root. 
EXAMPLE 
Examples of the present invention are given below by way of illustration 
and not by way of limitation. 
EXAMPLE 1 
Dental Root 
Hydroxyapatite prepared by a wet process having a Ca/P atom ratio of 1.67 
and a specific surface area of 80 m.sup.2 /g in BET was subjected to 
unidirectional pressing under a pressure of 50 kg/cm.sup.2, and then to 
cold isostatic pressing (CIP) under a pressure of 2,900 kg/cm.sup.2. It 
was then calcined for two hours at 1,000.degree. C. in air and then HIP 
sintered for two hours at 1,000.degree. C. and 2,000 atm. in argon gas, 
obtaining a cylindrical sintered body. This cylindrical sintered body had 
a diameter of 4 mm, a height of 20 mm, a relative density of 99.9%, and an 
average grain size of 0.64 .mu.m. 
The sintered body was subjected to forming in an inert gas atmosphere using 
an artificial dental root-shaped mold of molybdenum base alloy (TZM). The 
forming conditions included a holding temperature of 1,050.degree. C., a 
compression rate of 1.0 mm/min., a compression force of 60 MPa, and a 
deformation of 0.5 in true strain. The forming resulted in an artificial 
dental root of hydroxyapatite. It was observed at the end of forming that 
the average grain size was 1.0 .mu.m and the grains had been distorted and 
oriented. 
EXAMPLE 2 
Dental Crown 
A sintered body of hydroxyapatite was obtained by the same procedure as in 
Example 1 except that the body was dimensioned 7 mm by 7 mm by 7 mm. The 
sintered body was formed by extruding into an artificial dental 
crown-shaped mold of CaO-TiO.sub.2 -ZrO.sub.2 -MgCl.sub.2 ceramic 
material. The forming conditions were the same as in Example 1. It was 
observed at the end of forming that the average grain size was 1.0 .mu.m 
and the grains had been distorted and oriented. 
EXAMPLE 3 
Dental Root 
Hydroxyapatite prepared by a wet process having a Ca/P atom ratio of 1.67 
and a specific surface area of 80 m.sup.2 /g in BET was subjected to 
unidirectional pressing under a pressure of 50 kg/cm.sup.2, and then to 
cold isostatic pressing (CIP) under a pressure of 2,900 kg/cm.sup.2. It 
was then calcined for two hours at 1,000.degree. C. in air and then HIP 
sintered for two hours at 1,000.degree. C. and 2,000 atm. in argon gas, 
obtaining a sintered plate. This sintered plate had dimensions of 25 mm by 
20 mm by 4 mm, a relative density of 99.9%, and an average grain size of 
0.64 .mu.m. 
The sintered plate was subjected to forming by extruding into a mold using 
a dental root substrate as the extruder head. The root substrate was a 
columnar piece of titanium having a diameter of 2 mm and a height of 18 
mm. The forming conditions included a holding temperature of 1,000.degree. 
C., a compression rate of 1.0 mm/min., a compression force of 60 MPa, and 
a deformation of 0.5 in true strain. The forming resulted in an artificial 
dental root having a hydroxyapatite coat layer of 2 mm thick joined to the 
root substrate surface. The bond strength between the substrate and the 
coat layer was 350 MPa. It was observed that the coat layer had an average 
grain size of 1.0 .mu.m and the grains had been distorted and oriented. 
EXAMPLE 4 
Dental Crown 
A sintered plate of hydroxyapatite was obtained by the same procedure as in 
Example 3 except that the plate was dimensioned 25 mm by 20 mm by 4 mm. 
The sintered plate was subjected to forming by extruding into a mold of 
CaO-TiO.sub.2 -ZrO.sub.2 -MgCl.sub.2 ceramic material using a dental crown 
substrate as the extruder head. The coronal substrate had been shaped from 
yttrium-containing zirconia to a standard molar tooth configuration. The 
forming conditions were the same as in Example 3. The forming resulted in 
an artificial dental crown having a hydroxyapatite coat layer of 1 to 2 mm 
thick joined to the coronal substrate surface. The bond strength between 
the substrate and the coat layer was 350 MPa. It was observed that the 
coat layer had an average grain size of 1.0 .mu.m and the grains had been 
distorted and oriented. 
EXAMPLE 5 
Dental Root 
Hydroxyapatite prepared by a wet process having a Ca/P atom ratio of 1.67 
and a specific surface area of 80 m.sup.2 /g in BET was subjected to 
unidirectional pressing under a pressure of 50 kg/cm.sup.2, and then to 
cold isostatic pressing (CIP) under a pressure of 2,900 kg/cm.sup.2. It 
was then calcined for two hours at 1,000.degree. C. in air and then HIP 
sintered for two hours at 1,000.degree. C. and 2,000 atm. in argon gas, 
obtaining a sintered hollow cylinder closed at one end, but open at the 
other end. This sintered cylinder serving as a cover or sleeve had a wall 
thickness of 2 mm, an average grain size of 0.6 .mu.m, and a coefficient 
of thermal expansion of 12.times.10.sup.-6. 
A solid columnar piece of titanium with a coefficient of thermal expansion 
of 8.4.times.10.sup.-6 serving as a root substrate was fitted in the 
hollow sintered cylinder as shown in FIG. 1. The titanium piece had a 
diameter of 2 mm and a height of 18 mm and a plurality of tabs on the 
outer surface (see FIG. 4). The assembly was covered with zirconia ceramic 
powder having a high melting point (which does not melt at 700.degree. C. 
or higher) and an average particle size of 0.3 .mu.m, and further enclosed 
in a silica glass envelope as shown in FIG. 2. Superplastic forming was 
effected by an HIP process under the conditions: a temperature of 
900.degree. C., a pressure of 60 MPa, and a time of 30 minutes. 
At the end of HIP forming, the glass envelope was broken and the zirconia 
powder was brushed off. There was obtained an artificial dental root 
having a hydroxyapatite coat layer of 1.9 mm thick joined to the root 
substrate surface. The bond strength between the substrate and the coat 
layer was 100 MPa, as represented by the peel strength which was measured 
by withdrawing the root substrate from the joined root substrate coat 
assembly secured in a jig. The coat layer had a surface roughness Rmax of 
100 .mu.m and a flexural strength of 450 MPa. 
EXAMPLE 6 
Dental Crown 
A sintered hollow cylinder of hydroxyapatite was obtained by the same 
procedure as in Example 6. 
The sintered hollow cylinder serving as a crown cover was fitted over a 
coronal substrate of titanium. The assembly was subjected to superplastic 
forming by a HIP process under the same conditions as in Example 5. There 
was obtained an artificial dental crown whose physical properties were 
approximately equal to those of the root in Example 5. 
Using the same substrate, cover, and ceramic powder as in Examples 5 and 6, 
the assembly was subjected to superplastic forming by a HP process under 
approximately the same conditions as in the HIP process. Approximately 
equivalent results were obtained. 
Biological affinity tests were performed on the dental root samples of 
Examples 1, 3 and 5. 
The dental roots were implanted in holes of 3.times.4.times.6 mm formed in 
the jaw bone of male adult rabbits weighing 2.5 to 2.8 kg. Polished, but 
non-decalsified specimens were prepared after the lapse of six weeks from 
the operation. An SEM photomicrograph was taken on the interface between 
the root and the newly grown bone. It was found in all examples that the 
newly grown bone completely adhered to the root (implant) and penetrated 
and filled into pores therein, proving that the roots had high biological 
affinity. 
A high resolution image was taken under a TEM on the interface between the 
artificial dental root and the matrix bone after 24 weeks. The sequence of 
bone cells was identical on both the root and matrix bone sides, with 
little or no distinct boundary observed therebetween. 
There has been described a living hard tissue replacement or implant which 
is prepared from calcium phosphate base ceramic material by superplastic 
forming. Replacements of desired configuration, surface nature, and 
dimensional precision can be prepared simply by exchanging a mold used in 
the forming process. Although the need for tailoring an implant to 
particular configuration and dimensions corresponding to an individual 
patient imposed many problems to implants with respect to molding and 
shaping, the present invention enables the preparation of an implant 
meeting a particular patient's site by forming the implant in a particular 
mold conforming to the site. Replacements of complex shape can be readily 
prepared while their surface nature can be a mirror or rough surface 
through a proper choice of the mold. Strength is not lost by processing. 
Particularly when the living hard tissue replacements or implants are 
formed from calcium fluorophosphate, there are obtained artificial dental 
roots which are fully resistant against dental caries and acids. 
There has also been described a composite body in which a calcium phosphate 
base ceramic material is firmly joined to a heterogeneous material by 
superplastic forming. The firm joint can be achieved at relatively low 
temperatures. 
The living hard tissue replacement and composite body need not be profile 
finished by machining. They are thus free from flaws and defects which 
cause to create cracks during repetitive use. It is possible to prepare a 
replacement or composite body having an outer surface with complex 
irregularities which cannot be reproduced by machining. 
In particular, the composite body preparation method using an HIP process 
allows pressure to be evenly applied to the constituent materials to 
provide an increased bonding force, avoiding any gaps being left at the 
interface between the substrate and the coat and preventing exfoliation at 
the interface during repetitive use. 
Since no exfoliation occurs with an increasing thickness of a calcium 
phosphate base ceramic material layer, the coat layer can have any desired 
thickness. The coat layer may be either as thin as about 1 .mu.m or as 
thick as about 10 mm. The coat layer thickness may be selected over the 
wide range depending on the intended use. In addition, the coat layer can 
be applied to a uniform thickness without a substantial variation and to a 
complex configuration with ease. The variation in quality among composite 
products is minimized. 
The composite products can be controlled to any desired surface roughness 
through a choice of the particle size of ceramic powder to be filled 
around the calcium phosphate base ceramic material/heterogeneous material 
assembly during HIP or HP forming. 
A composite body can be prepared from a substrate and a cover by shaping 
only the substrate to a precise configuration, fitting the ceramic cover 
of a roughly conforming shape over the substrate, and subjecting the 
assembly to superplastic forming. Steps of shaping, bonding, and surface 
treatment can be concurrently performed by a single forming step, with the 
benefits of eliminated need for machining and increased efficiency of 
working and manufacture. A plurality of composite bodies can be produced 
at the same time so that a cost reduction is expectable from potential 
mass production. 
The method for preparing a composite body according to the invention 
ensures that living hard tissue replacements having improved mechanical 
strength, biological affinity, and formability and well suited as 
artificial dental roots and crowns are produced at a low cost. 
Although some preferred embodiments have been described, many modifications 
and variations may be made thereto in the light of the above teachings. It 
is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.