CA1087801A - Endosseous plastic implant and method - Google Patents

Abstract

ABSTRACT
Nontoxic polymeric plastic medical implants for endosteal and periosteal applications such as filling bone defects, replacing entire bony parts, and tooth replacement either immediately after extraction or subsequent to healing, and a method of fabricating such implants to produce a porous surface having a predetermined pore size, pore depth, and degree of porosity. The method of fabrication of the porous portion of the implant involves adding sodium chloride crystals or other nontoxic leachable substance of controlled particle size corresponding to the desired pore size to a powdered polymer-liquid monomer mixture in relative amounts corresponding to the desired degree of porosity. After heat poly-merization without an initiator, followed by abrasive removal of the resulting surface skin, the salt is leached from the plastic to provide said porosity. To obtain bone ingrowth in the case of intraosseous implantation, the pore size must be 200-400 microns, whereas pore size for soft tissue ingrowth should be 50-150 microns.

Description

The present invention relates to prosth~sis ma~exials for artificial bones or teeth and par-ticulaxly to a plastic implant and a method for fabricating such a plastic implant to have a controlled sur~ace porosity, with pore size depending 5 upon the nature of tissue ingrowth desired. -In the related~fields of dental implantoloyy and ortho-pedic and surgical repair and replacement, numerous implan~
materials have been u~ed with varying degrees of success.
Metallic implants and prosthes~s, ~or example, have been found to have drawbacks due to chemical and elec~rolytic reaction~, body rejection, toxic response, metal fatlgue and failure, inter-ference with healing, and inabili~ty to bond with bone. More recently, ceramics and polymers have been used with improved results insofar as bodily reac~ion is concernedi but problems ~-still remain in~developing permanent fibrous connecti~e tissue and/or bony attachment (,fixation) at the implant sitè.
~ Providing an implant with a porous~sur~face at itC; inter~
face with bone or other tissue has been recognized as promoting ` ~ `
a firm union between the implant and the ad~acent membrane in :
which it is em:edded. For example,-U.S. Patent No. 3,628,248 issued December 21, 1971 to E. A. Kroder et al. describes a procedure for pre~ ring a tooth root replica implant from a ~mixture of polymethyl meth~crylate beads and potsssium chloride granules in a weight ratio of approximately 4~ The powder mixture~is combined with liquid monomer and a small amount of benzoyl peroxide (to initiate polymerization) in a mold for self-curing. After removal from the mold, the root implant is !
placed in boiling water for about 1 minute to extract a portion -of the pQtassium chloride~ thereby producing porosity.
As pointed out, however, in U.S. Patent No. 3,713,860, .~

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issued January 30, 1973 -to A. Auskern, residues of additives to methyl methacrylate monomer, such as inhibitor, catalyst, or promoter, may cause tissue reactions at the implant site, and the benzoyl peroxide initiator of Kroder is known to be toxic to human tissues. The Auskern patent discloses a porous ceramic aluminum oxide bone substitute that is impregnated with pure methyl methacrylate (MMA) monomer. The monomer is then polymer-ized by gamma radiation from a suitable source, such as cobalt-60.
If not pr~viously shaped to size, the plastic-reinforced ceramic body is then machined, and the areas w~e-!bonte-o~-~ tissue ingrowth is desired are exposed to a suitable solvent (e.g.acetone) in an ultrasonic bath for 15 to 30 minutes to dissolve the polymer plastic to a preferred depth in the range of 100 to 400 microns.
According to Auskern, connective and bone tissue will grow into the pores and be firmly bonded to the ceramic if the porosity of the ceramic is in the range of 30-50 percent by volume, with the greater number of pores being in the range of 75 to 150 microns in diameter. On the other hand, a paper pub-lished by the present applicant in The New York ~ournal of `~
Dentistry, Vol~. 42, No. 10, pp. 331-341, December, 1972, suggested that the optimum porosity size for attachment between the perio-steum and a polymer plastic tooth implant might all in the range of 150-450 microns.
Another investigator has suggested a prosthesis material useful for artificial bones or teeth in the form of a heat-con- ~
solidated body composed of an integrated mixture of discrete `
microcrystals of calcium phosphate and a refractory compound such as aluminum oxide (see U.S. Patent No. 3,787,900 issued ;
January 29, 1974 to T. D. McGee). In using this material for dental implants/ McGee suggests providing porosity for the base ~0~7~a~
of the teeth, where they fit in the alveolar sockets, by adding a volatile material such as naphthalen~ crystals to the dry in-gredients at the root end o the implant molding before pressing and firing. In one example, forty percent naphthalene crystals were incorporated into one half of a cylindrical specimen to pro-duce 200-500 micron pores in one end after firing. The specimen ~ `
was implanted into the jawbone of a dog. A very strong bond, believed to be mineralized ~one, developed between the implant and the jaw after 10 weeks.
- 10 Considerable work on dental implant material comprising an acrylic polymer mixed with organic bone has been done by M. Hodosh. See, or example, U.S. Patents No. 3,609,867 and 3,789,029 In the latter patent, Hodosh describes a process for obtaining increased porosity near the outer surace of the polymer ; 15 implant, where such ~ porosity is most beneficial, by adding a ., . - . .
measured amount of N-tributyl phosphate (a blowing agent) to a . mixture of polymethacrylate, grated bone, and a foaming agent.
The total mixture is polymerized in a mold at 300 F for 30 ~r minutes. After removal from the mold, the processed implant ~; 20 has an outer skin that must be removed by suitable procedures, such as sand-blasting, to expose the porosity caused by the ,. , N-tributyl phosphate. According to Hodosh, when the prosthesis is implanted, tissues penetrate the pores to effect a secure fibrous interlock between the implant and the surrounding perio-dontal membrane. With such a blowing agent no `predictable orsta~dard pore size can be achieved, however.
Despite the progress in ceramic and polymer plastic implants and prosthes~ outlined above!, there still exists a need for a plastic implant having standardized pore size (e.g., a desired hole, any dimension, uniformly distributed, every time) ~ 4 ':

and with no toxicity. ln addition, ~here is a need for a Variety of porous plastic implant designs adapted for use in widely different jawbone configurations where no teeth exist tfor example, narrow ridge implants, implants near maxillary sinuses, and implants in regions of inadequate alveolar height~.
In particular, there is a need for a method o~ fab..icating ~` such an implant that can be performed quickly and easily by a dental or orthopetic surgeon or technician during an operating situation when the allowable time between the removal of a tooth 10 or other surgical procedure and the fitting of the completed :::
implant is measured in minutes. ;
: It is an object of the present invention to provide a : simple method for duplicating any hard tissue in the body (i.e., bones, teeth or cartilage) and to permanently affix the implant to its adjacent tissues.
It is another object of the present invention to provide a simple method for fabricating a cast polymer plastic implant having that surface portion intended to be exposed to an endos-seous or connective tissue environment formed with a predeter-mined optimal pore size for encouraging either osteogenesis orconnective tissue ingrowth from the host bone or so~t tissues . into the implant matexial~
It is another object of this invention to pro~ide a process ~or fabricating a polymethyl methacryla~e implant struc-ture without using any toxic additives, or any toxic or otherwisedangerous materials to create the desired degree~ si.~ and depth of implant porosity. ;;
Another object of the invention is to provide a molded plastic structure in any desired shape by a.procedure that is rapid simple and inexpensive, and that requires only the usual -~
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equip~ent a~ailable to a dental or orthopedic surgeon.
These and other objects are achieved by the method of the present invention, which provides a cast polymethyl metha-crylate implant having an integrally cast exposed porous surface portion or portions with a predetermined pore size or sizes in the range of from about 50 microns to about 400 microns and a controlled degree or porosity such that upon insertion into an implant site, such suxface portions will be particularly and selectively receptive to the development of the deisred type of ~ibrous connective attachment ~e.g., either "soft" connective tissue or "hard" bony tissue) to the contiguous host tissue of ..
khe implant siteO
For example, the method of the present inVention will provide a tooth root implant having a predetermined porosity in the areas exposed to alveolar and gingival environments. The surface of the implant tooth root, whether the implant is a replica of an extracted root or is a preformed shape to suit the site of a ~ssing tooth, should have a pore size specifically adapted to promote ingrowth and adhesion of periodontal membrane tissues in the portion within the jawbone socket and adapted to encourage ingrowth of soft tissue at and below the gum line.
On the other hand, implants intended to be inserted into direct contact with, or to replace, bony parts o the body should have a larger pore si2e ~at least 150 micrans) that has been found essential to promote hard bone cell ingrowth into the in-traosseal portion of the implant.
Such an implant is prepared by adding a completely non-toxic, water soluble, crystalline material (e.g. sodium chloride or sugar) to a powdered acrylic polymer, the volume ratio of crystalline material to polymer being equal to the desired porosity .. ~ :.:.... , . . . . . .... ,, :

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of the finished implant. The particle size o~ the crystalline material, which preferably i5 sodium chloride, should be pre-selected to be the same as the desired pore size of the implant material.
S It is important that the acrylic monomer have no added promoter or catalyst which cou}d cau~e a toxic reaction at the implant site. The above mixture is then moistened to a suitable ~ consistency with liquid monomer and poured into a mold that has ;~ been suitably prepared by a technlque as described in detail below. The mixture may fill the mold or, alternatively, the mixture may be poured around a previously placed reinforcing member of metal, hard nonporous acrylic plastic, or other suit-able material that may even include a processed natural root of ; a previously extracted tooth for which the implant is to be a replacement. Also, instead of having a reinforcing core, the mixture of salt and acrylic polymer-monomer can be reinforced by adding carbon fibers.
If the imp}ant is intended to be a replacement for a tooth root, a pin of either hard plastic or metal may be incorp-orated in the mold for securing a subsequently placed tooth cap.
~lternatively, the tooth crown portion may be molded integrally ~ !
with the root by filling the top portion of the mold with pure acrylic polymer-monomer mixture to form a hard non-porous tooth crown portion.
After the mold is filled, the plastic mixture is poly-merized by placing the mold flas]c in boiling water for a suitable time (usually about 10 minutes), as is well known in the art. ; ;
~he solidified implant is then removed from the mold.
During the polymerizing process a thin plastic skin forms over the portion composed of salt and plastic mixture, thereby . . .

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sealing the salt crystals inside the surface~ This skin must be removed, preferably by sandblasting, to expose the surface salt crystals. The salt is then leached out by placing the re-plica in boiling water again for about 4 to 15 minutes. ~his simultaneously sterilizes the replica, which is then ready for immediate insertion into the previously prepared implant site.
As indicated above, an important part of the method of the present invention is the proper preparation of the mold ; surface in those regions of the replica implant that are to have a porous surface. It has been found that without such preparation, the surface region of the polymerized salt-plastic mixture has a deficiency of salt crystals, thereby reducing or eliminating the porosity in the precise location where maximum porosity is desired. According to the method of the present invention, ~15 therefore, a preliminary coating of the leachable salt or sugar ;crystals is attached selectively to the portions of the mold surface prior to filling the mold with the plastic mixture.
The procedure for attaching the crystals to the mold surface may be accomplished by any one of several alternative methods that also include coating the mo~d with a conventional release agent to facilitate removing the inished replica from the mold. A well-known release agent used in dentistry is sold under the name "Mar va-oil", but any similar preparation may be used instead.
Usually, a thin coating of the release agent alone will be applied to the entire inside surface of the mold, but in one method, salt crystals having a particle size equal to the desired pore size can be mixed with the release agent until it is satura-ted and the resulting mlxtur~ applied to those regions of the mold where a porous surface of the replica is desired. When the 8.

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release agent driesi, a d~nse layer of salt crystals will be attached to the selected portions of the moldi sux~ace. Thie mold is then ready for the plastic casting steps described previou81y.
In another method of mold preparation, the entire inner surface of the mold is coated with a release agent. An acetate cement, such as "Duco" brand sold by E.I. du Pont de Nemours & Co.
(Inc.), is thinned with a solvent such as acetone or methyl ethyl ketone to increase its flowability and is then painted over the release agent in the region where por~sity is desired. Salt crystals of desired size are sprinkled onto the thinned glue while it is still tacky. When the cement dries, a dense coating of :, - - .
salt crystals of the desired size will be attached to the selected areas of the mold, ready for the plastic mixture to be cast.
Instead of mixing the salt in an acetate cement or pre-mixing it in the release agent, the crystals can simply be sprink led on the release agent (ox on a cement coating on top of the release agent) immediately after it has been coated on the mold , . , ~
surface and while it is still tacky. This technique is particu-larly useful when the entire surface of the replica is to be , :, porous.
Another method of attaching leachable crystals to the ; mold surace comprises coating the inside of the mold with mineral i;
oil, preparing a saturated solution of sodium chloride in hot water, coating the oil layer with the saturated saline solution, ; j .
25 and placing the mold in an oven to evaporate the water. It has ~
~ , heen found that if salt crystals of the desired size are used to make the saline solution and ~f the solution is so saturated that crystals of that size remain undissolved, then the additional ~;
crystals formed when the water evaporates are also gènerally of ~ ;
that same size. ~Although the mechanism by which this occurs is .
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not fully understood, it is believed that the excess crystals in the solution may serve as "models" ~or other crystals that precipitate from the solutiorl as the water evaporates. Regard-less of the reason, this method also results in a dense layer of salt crystals attached to the mineral oil coating, which acts as a release agent for the subsequently cast plastic mixture.
. With any of the preceding mold preparation techniques, :
a cast polymethyl methacrylate implant can be prepared which has high surface porosity of a predetermined pore size for creating the most effective bond with the surrounding host tissues while permitting the interior portion of the replica body to be made with reduced porosity or even nonporous for increased strength, as required by a given implant application.
In accordance with the foregoing, more particularly, there is provided:
A method of fabricating a plastic endosseous implant ;1 comprising the steps of:
~;i (a) preparing a replica mold corresponding to the ~; desired shape of the endosseous implant, ~ 20 (b) attaching to the interior surface of the mold a i layer of water-soluble crystals of a particle size correspond-,, ing to a first pore size desired in a surface region of the implant, (c) introducing acrylic polymer and monomer mixture `

to fill said mold, (d) curing the material in said mold to form the , implant, (e) removing said implant from said mold, and (f) leaching said water-soluble crystals from said implant.

There is also provided:

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An endosseous plastic implant having an exposed porous surface portion ~dapted to promote only soft connective tissue - ingrowth from both surrounding epidermal and osseous environ-ments, said surface portion having preselected pore size ex-clusively in the range of from about 50 to about 150 microns, a porosity in the range 50-75% by volume, and depth o~ porous portion being not less than 2 m.m., and wherein said plastic material consists of essentially pure polymethyl methacrylate and containing no polymerizing catalyst.
Further details regarding the replica implants and their method of fabrication will be provided in the following `~` description of the preferred embodiments in conjunction with the accompanying drawings.
Fig. 1 is an enlarged side elevation of a composite hard plastic crown and porous plastic root tooth implant in a lower jaw of normal alveolar bone height, showing the mandibular bone in cross-section;
Fig. 2 is a view similar to Fig. 1 but showing a plas- ;;
tic tooth implant cemented into the alveolar bone socket;
Fig. 3 is a view similar to Fig. 1 but showing an implant with a hard plastic core in a porous plastic root;
Fig. 4 is similar to Fig. 3 but showing an implant with a hard plastic root coated with porous plastic;
Fig. 5A is a view similar to Fig. 4 but showing a tooth implant formed from a natural extracted tooth with its root coated with porous plas tic; : ~ ~
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Fig. 5s is an enlarged view of the alveolar environment within the circle of Fig. 5A;
Fig. 6 is an enlarged side elevation of a natural mandi-bular tooth in a jaw having an inadequate alveolar bone height;
S Fig. 7 is a view similar to Fig. 6 but showing the same tooth following extraction, coating of the root with porous plastic, reinsertion, and subsequent stimulation of co~mective tissue ~ ingrowth to create a normal alveolar height;
: Fig . 8 is a view similar to Fig~ 7 but showlng a plastic replica tooth implant having a hard plastic crown and porous plastic root set into a built-up alveolar ridge of synthetic bone;
Fig. 9A is a view similar to Fig. 8 but showing a porous plastic tooth root implant having a hard plastic core;
., Fig. 9B is a to~ view o~ the embodiment o~ Fig. 9A;
Fig. 10 is a side elevation of in situ replacement by porous plastic implant material of a bony periodontal defect in order to build up lost alveolar h0ight;
~ Fig. 11 is a side elevation of a multiple tooth replace-`~ ment by means of a synthetic alveolar ridge implant in a lower - 20 jawbone;
Fig. 12 is a cross-section of Fig. 11 taken in ~he di-rection of arrows 12-1~;
Fig. 13 through 18 illustrate the steps of an alternate method o~ correcting for inade~uate alveolar bone height, in , .
which Fig. 13 shows a natural tooth ln situ;
Fig. 14 shows the mandibular socket of Fig. 13 after extraction of the tooth;
Fig. 15 shows a combination drill and tap in position to prepare a deeper, threaded tooth support socket;
Fiy. 16 shows the drill/tap completing the new socket;
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Fig. 17 shows the prepared thxeaded socket after the combination drill/tap has been backed out;
Fig. 18 shows a prefabricated threaded porous plastic tooth support post inserted into the prepared socket, ready to 5 receive a hard plastic tooth crown; ¦
Fig. 19 shows a set of prefabricated threaded porous ~
plastic tooth support po0ts in a graduated range of sizes for I -use in differ~nt regions of the mouth and in different sizes of jawbone; -" 10 Fig. 20 shows a threaded plastic type of tooth implant n situ with the tooth crown in place;

Fig. 21 is a side view in partial section. of the ; ~
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threaded support post of the t.ooth implant of FI~. 20;
~i Fig. 22 is a bottom view of the threaded support post of Fig. 21;
Fig. 23A, B, and C show prefabricated threaded root implants in a range of sizes to suit different bone ridge forma-tions;
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Fig. 24 is a side elevation o a hard-plastic screw post set in a porous plastic root implant for receiving a tooth cap;

~; Fig. 25 shows a temporary screw plug fitted in the porous plastic root implant of Fig. 24;

Fig. 26 is a top view of the implant of Fig. 25;

Fig. 27 is a side view of a blade-type implant for use , 25 in molar replacement9 Fig. 28 is an end view of the implant of Fig. 27;

Fig. 29 is a top view of~the implant of Fig. 27;

~ Fig. 30 is a side view of a blade-type implant for use I in regions where there is less bone available than for the ;~

implant of Fig. 27;

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Fig. 31 is a side vlew of another blade~type implant for use in front tooth replacement;
Fig, 32 is a side view o~ a blade-type implant de~ign for use in replacing an upper tooth near a maxillary sinus:
Fig. 33 is an end view of ~he implant of Fig. 32;
Fig. 34 is a bottom view of the implan~ of Fig. 32;
Figs. 35A and 35B are progressively smaller-sized ;~ versions of the implant of Fig. 32 for use in smaller jaws with smaller maxillary sinuses;
Fig. 36 is a side elevation of another form of blade-type tooth support implant;
Fig. 37 is a frontelevation of the implant of Fig. 36;
Fig. 38 is a top view of the implant of Fig. 36;
Fig. 39 is a side elevation o~ a conical form of tooth support implant;
Fig. 40 is a front elevation of the implant of Fig. 39;
Fig. 41 is a top view of the implant of Fig. 39;
Fig. 42 is a side elevation o~ still another blade-type tooth support implant having a flared lower portion;
Fig. 43 is an end view of the tooth support implant of Fig. 42;
Fig. 44 is a top view of the tooth support implant o~
Fig. 42;
~' Fig. 45 is a side view of a lower jawbone having a diseased or otherwis~. defective section replaced by an inter~
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locking porous plastic implant;
Fig. 46A is a side view of a lower jawbone of which a major portion has been replaced by a replica plastic implant;
Pig. 46B is an enlarged sectional view of the mortise and tenon joint between the hos~ bone and the implant of Fig. 45A;
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Fig. 47 illustra~es an alternative form of attachment of a plastic mandibular defect replacement to the host bone by means of acrylic screws a~d plates;
Fig. 48 is a top view of a mandible having plastic con-dylar head implants at each hinge joint;
Fig. 43 is a side elevation of a cast polymer composite joint prosthesis having a solid plastic ball joint head, a small pore porous plastic midbody, and a large pore porous plastic fixation pin;
Fig. 50 is an alternate embodiment of a composite plastic joint prosthesis having ~ large bone ingrowth holes in the fixation pin;
;l Fig. 51 shows a joint prosthesis inserted in the ad-. -joining end of a limb bone;
Fig. 52 shows a hard plastic ball for use as a condylar head in a joint prosthesis;
Fig. 53 is a photomicrograph at approximately 3X of a chlorine map of a sample of porous plastic having pore sizes of 100-150 microns;
- 20 Fig. 54 is a photomicrograph at 40 X of a sample of " porous plastic having pore sizes of 75-150 microns;
Fig. 55 is a photomicrograph at 25 X of a sample of porous plastic having pore sizes of 300-400 microns;
In khe detailed discussion that follows, various ap-plications of the invention to dental implant design will be -l described, foIlowed by examples of bone implants and prosthetic devices, and concluding with a description of alternative methods for making the previously described implant structures and for emplacing them in the intended body location. In the drawings, the same or similar features will be identified by the same ref-~.

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ence number in each figure for convenience.
The need for a tooth implant may arise from a varietyof caus~s, and each situa~ion may require a diferent implant structure. In addition, the type of tooth being replaced (e.g.
5 molar, bicuspid or incisor) and its location in the mouth will ;
affect the required shape of the implant support or root, both -~because each tooth encounters a different stress and because the alveolar ridge at the ~ron~ of the mouth is normally narrower than at the rear. The location of the maxillary sinus cavities in the upper jaw may also affect the shape of the implant support. Con-sequently, the present invention contemplates a range of sizes, shapes, and constructions, as illustrated by the following draw-ings of tooth implant embodlments. A11 of the di~erent designs incorporate,however/ the underlying concept of pore size and porosity depending upon the type of tissue ingrowth that is desired.
Figs. 1 through 5 illustrate several embodiments for use when replacing an existing tooth in a soc~cet having normal alveolar bone height. In these dra~ings, as well as in all subsequent 20 drawings representing in situ conditions, the anatomical details ~;
are rendered in a formalized schematic manner for simplicity.
Referring to Fig. 1, a plastic replica tooth implant 101 ~ `
comprises a hard pla~stic tooth crown 102 integrally molded with a porous plastic tooth root 103. The term replica will be used to refer to a tooth implant or other plastic implant structure that has the exact size and shape of the tooth or bone that it ;~
replaces. Such a replica will normally be cast in a mold that has been prepared using the original tooth or bone as the model for the mold. In this case the ~ooth replica 101 has been set in an alveolar cavity 104 of the lower jawbone or madible 105. ~ ~`

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The ~awbone is composed of a cortical or outer bcne portion 106 and an inner spongy medullary portion 107. The example of Fig. 1 illustrates a~m~ble having a normal alveolar height; i.e., the tooth socket extends w~ll up near the top of the root portion of the implant. Above the jawbone lies the aermis 108 with the ~; outer Govering of the gums or gingiva~ 109 extending up to the top of the p~rous root portion of the implant.
Fig. 1 illustrates the situation after the imp~ant has -become fully attached to the alveolar socke~ by means of a perio-, dontal membrane 110 and to the gingiva by an epithelial attachment 111. In addition, hard bone ingrowth ].12 has occurred through two holes 113 and 114 drilled in the root portion of the replica, each hole;being approximately 2 to 3 millimeters in diameter.
Finally, fibrous tissue ingrowth 115, that constitutes an exten-sion of the periodontal membrane, has occurred between the alveolar bone and the intermediate root portion of the tooth replica. As a consequence, the tooth implant is firmly attached both to the underlying jawbone and to the overlying gingiva in a manner com-pletely simulating the environment of a natural healthy tooth.
Fig. 2 illustrates a replica 101 that is the same as the replica of Fig. 1. In this case the replica has been cemented , , into the laveolar cavity by a suitable cement, such as methyl polyacrylate (carboxylate) cement, glass-ceramic cement, or a cement known as Kodak 9-10 manuaatured by the Eastman Kodak Company. In this embodiment the cement coating in the root portion of the replica provides an initial strong attachment to the jawbone, which is then supplemented over a period of weeks by ingrowth of periodontal tissue as described above.
The next two figures, 3 and 4, illustrate two embodi-ments of a replica tooth implant formed by a two-stage technique.

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In the~e embodiments, the root portion is cast first and inserted in the alveolar socket. In the embodiment of Fig. 3 the root portion comprises a hard plastic insert 117 that extends, either partially or almost fully, to the bottom of the root and is surrounded by porous plastic 118 ~s illustrated. The possible range of lengths of the hard plastic insert 117 is shown between the dash line 119 and solid line 120 on the drawing. After the root portion has been installed for a period of several weeks;
so that connective ti~sue has had a chance to develop between the lower root and the alveolar socket and between the gingiva~
and the upper root, a preformed acrylic crowm 121 can be attached to the upper portion 122 of the hard plastic insert that extends ~`
- above the porous plastic root by any conventional cementing ~ `
; technique. In this way the implant support or root is protected by the adjacent teeth against the normal stresses exerted on a tooth crown during the critical period while the attachment membranes are forming betwe0n the root and the surxounding socket and gingiva. ~f desired, any conventional cro~n material other ~
than acrylic, for example a porcelain jacket or acrylic veneers, can be used for the finished tooth.
In Fig. 3, the hard plastic pin 122 provides rein~orce-ment in the upper portion of what is primarily a porous plastic root. In the embodiment of Fig. 4, on the other hand, a hard plastic root replica 123 is coated with a relatively thin porous plastic outer covering 124 to provide a surface that will promote ~ -i!ngrowth of the periodontal membrane and the connective tissue at the yingival contact area. The embodiment o Fig. 4 is like the Fig. 3 embodiment in that the tooth crown 121 is cemented to the exposed mounting post of the hard plastic root 123 after the root has become fixed in the alveolar socket. This process normally , - 17. ~ ~

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takes only a few weeks at the most~
Occasionally in cases of gum disease, a so~nd natural tooth will become loose in its socket. In such a situation, it may be possible to use the tooth itself as the base for a dental implant~ Figs, SA and SB show a natural tooth 125 that has been extracted, the root portion 126 etched with phosphoric acid, a thin coating 127 of porous plastic applied to the root portion, and the tooth reinserted in its socket~ When this procedure is accompan~édJand followed by the proper treatment for alleviating the underlying problems of the gum disease, the result is a re-creation o~ ~he necess~ry periodontal membrane ingrowth in the lower root portion and epithelial attachment of the gingiva with the upper portion of the porous plastic coating to produce firm fixation of the tooth in its socket and a return to normal and healthy periodontal conditions.
As stated ab~ve, the tooth implant embodiment of Figs.
l and 2 is formed as an exact replica of a natural tooth by using the latter, immediately~following the extraction, as a pattern for a plaster mold fashioned by conventional flasking techniques.
After the mold has set, the root portion is filled with a flowable mixture o acrylic plastic and leachable crystals of predetermined ; size. The crown portion of the mold is then filled with pure acrylic polymer/monomer mixture and the entire flask placed in boiling water until the replica casting has hardened. The cast replica is then removed from the mold and the crystals leached rom the root, thereby creating the desired porosity. The details of this procedure are described in the final portion of the specification.
The two-step procedure for the implant of Fig. 3 differs from the one-step implants above in that a plaster mold of only 18.
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-the root of the extracted tooth is made in the first step.
hard (solid) plastic pin is placed in the mold before filling th~ remainder with the mixture o~ plastic and leachable crystals.
The lower end of the pin serves ~o relnforce the root 9tructure, : 5 while -the upper end extends above the root to act as a mounting post for the preformed crown that is installed in the secona step of the procedure.
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In making the embodiment of Fig. 5A, the tooth root, after it has been etched with phosphoric acid, is coated with ;~
wax and then pressed back into its socket. In this way, the wax coating takes an exact snug impression of the tooth socket. Then the plaster mold is made in the usual way. The tooth is removed from the mold, dipped in boiling water to remove the wax, and . .
replaced in the mold. The space previously occupied by the wax is then filled with a mixture of polymer, monomer, and leachable crystals, which is then processed to form the porous plastic coat-ing. Instead of, or in addition to, premixing the leachable ; crystals with the polymer-monomer mixture, the crystals can be attached to the in~èrior surface of the mold before the mixture ~` ~
is added, thereby increasing the porosity at the surface.
The embodiment of Fig. 4 is made in the same way as that of Fig. 5A e~cept that the etched root of the natural tooth may be used as a pattern to make a hard plastic replica, formed with a crown mounting post at the top, that becomes the core of the implant. Alternatively, the hard plastic core may be selected from an assortment of prefabricated root shapes and sizes. This embodiment provides a structural strength comparable to the .. . .
natural tooth by using a maximum amount of hard plastic in the root portion, and can be used when the natural tooth is not suit~
able tbecause of decay, fracture, or other defects).

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Turning nex~ to Figs. 6 through 9, these Eigures illus-trate various implant techniques and embodiments ~or use in si-tuations where the alveolar height is abnormally low. Fig. 6 depicts a natural tooth 128 ln situ in the alveolar caviky 129 of a mandible 130, having cortical bone portion l31 and inner spongy bone portion 132. In Fig. 6 the height o~ the alveolar bone is abnormally low, causing the gums (gingiva~ 133 and the underlying dermis 134 to recede, the~eby exposing intexmediate root portion 135 between the tooth crown portion 136 and the lower root portion 137 that is held in the socket by periodontal - membrane 138. This is an atrophied condition of the periodontal environment that causes gradual deterioration and ultimate loss o~ the natural tooth, foll~w~d by a strong tendency for alveolar resorption unless appropriate dental countermeasures are under-taken promptly.
Assuming that the atrophy of the jawbone and resulting damage to the natural tooth has progressed to the point where extraction of the tooth is required, Figs~ 7-9 illustrate three approachea to the problem of rebuilding the alveolar ridge and fixating a replacement tooth. In Fig. 7 r the natural tooth has been removed, treated in exactly the same way as the embodiment of Fig. 5, and replaced in its socket. The porous plastic coating 139 is strongly bonded to the etched root 1~0 and, in turn, pro-vides a superior surace ~or encouraging the ingrowth of perio-dontal membrane cel}s in the alveolar socket.
At the time that the tooth is extracted, the dermis isalso undercut along each side of the jawbone, as indicated by lines 141 and 142. Then after the treated tooth has been re-~ :.
placed in its socket, the dermis and overlying gingiva are pulled up to the normal gum line of the tooth and sutured in place.

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Within a ~ew weeks, new periodontal membrane tissue 143 will grow upward from the jawbone and attach firmly to the porous plastic surface on the previously exposed intermediate portion of the tooth roo~. Concurrently, the gingiva will form an epithelial attachment to the upper margin of the porous plastic coating. Thus, the reparative approach taken with this embodi ment is to encourage the formation and attachment of fibrous connective tissue to the previously exposed root portion of the natural tooth. After a period of natural growth and adaptation, a desired normal gingival condition results, as shown by gingiva 132 disposed at the base o crown portion 144, with the root portion properly embedded in alveolar cavity 129 with fibrous connective attachments 138 in alveolar cavity 139 and new connec~
tive tissue 143 creating the e~ect of a normal alveolar ridge.
Fig. 8 shows an embodiment of a tooth implant 145, of the same type as that of Fig. 1, that has been inserted in alveolar cavity 129 to replace the natural tooth shown in Fig. 6. This embodiment is formed as an exact replica of the natural tooth by using the latter as a model for a plaster mold fashioned by con-ventional flasking techniques, just as described in connection with Fig. 6. ~rhus, repll~ca embodiment 145 of Fig. 8 comprises3a hard (i~e. Solid~ cast polymer crown 146 integrally formed with a porous plastic root portion 147. In this case, however, the alveolar ridge has been built up artificially at the time o `
inserting the implant by packing synthetic bone 148 around the exposed intermediate portion of the root.
The synthetic bone used to form the alveolar ridge in ;~
this and following examples preferably is composed of large pore cast porous polymer (e.g. polymethyl methacrylate) having pore sizes in the range of 200-400 microns. Other conventional 21.

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synthetic bone ma-terials can be used, however, such as porous ceramic titanium dioxide (TiO2), glass ceramic, calcium phosphate (Ca3 (PO4)2), or biodegradable polymer (lactic acid).
After -the dermis has been undercut and the gingiva pulled up and sutured in place, new connective tissue attachments 149 and 150 will occur between the implant and the synthetic alveolar ~idge and between the synthetic ridge and the conti~uous dermis, respectively.
Referring to Figs. 9A and 9B, the two-step procedure ~o described in connection with Figs. 3 and 4 can also be used in the s3iltuation of inadequa~e alveolar height. The example of Figs. 9A and 9B uses a replica root embodiment similar to that of Fig. 3, with a built~up alveolar ridge of synthetic bone, as in th~e example of Fig. 8. The replicà root includes a solid plastic insert 151, having an exposed crown mounting post 152 and a reinforcing root poxtion 153 surrounded by a porous plastic jacket 154. Two holes 155 and 156 are provided for additional fixation by means of hard bone ingrowth.
As in the case of the one step implant, the dermis is undercut and the~gingiva pulled up around the upper margin o the porous plastic ~;acket, where it is held in place by sutures 157.
In some cases, as shown by Fog. 10, alveolar bone re-- sorption will create a trough 158 around the natural tooth 159.
This situation can be treated in two ways. In the first procedure, porous synthetic bone~material 159 (preferably polymethyl metha-crylate with 200-400 micron pores) is packed around the inter-mediate portions of roots 160 and 161 to replace the lost alveolar bone. The second procedure is used when the tooth is loose and 30 includas the additional steps of first extracting the tooth, ~ `

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etching the root, sealing the end of the root canal, and re-placing the -tooth in its socket before packing the synthetic bone material around it. The root etching procedure produces a roughened surEace that promotes a good attachment to the perio-dontal membrane. If necessary, a porous plastic coating can beadded, according to the previously described techniques. As in the procedure illustrated by Figs. 7 - 9, the dermis should be undercut and the gingiva raised to the former gum line and su-` tured until connective tissue ingrowth has occurred.
The previous examples have all dealt with ~re~placement of a single existing tooth by using the extracted tooth, either as a pattern for a cast plastic replica or, by suitable treatment, as its own replacement. Figs. 11 and 12 illustrate a procedure for replacing a group of from two to five teeth by means of a multiple implant in two steps or stages. This procedure can be used both in the situation where all the replaced teeth are missing and there is very little alveolar bone height or where the group of teeth are all periodontally involved and must be removed.
In the first stage, an ar`tific~al alveolar ridge 162 of porous plastic is cast, using a wax impression or similar material .. .: , as a pattern for a plaster mold by conventional teqhni~ues.
Solid plaskic inserts 163 may be cast integrally with the arti-ficial ridge or may be cemented in place subsequently, as desired.
The entire ridge structure is then~placed in mating contact with mandible 164, (having alveolar canal 165) and the previously undercut gums 166 sutured in place around the inserts, leaving crown mounting posts 167 exposed.
Because the gingiva will not form an epithelial attach-` 30 ment with the smooth surface of the inserts, the exposed por~ions : ~ .

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between the top of the artiicial ridge and the base of the mounting posts should be covered with a suitable material 168 such as nylon velour or dacron mesh. Alternatively, these por-tions of the inserts 163 can be provided with porous surfaces (50-150 microns pore size) by the methods of this invention.
Conversely, it should be mentioned that gingival attachment in any of the other described examples can be obtained through th0 use of nylon velour or dacron mesh, if desired~
A~ter the implant has been in place for several weeks so that firm attachments have formed between the mandible and the large pore artificial ridge ~hard bone ingrowth) and between ` the gingiva and the velour, mesh or small pore insert surface (soft tissue ling~wth), a one-piece group of solid plastic crowns 169 can be cemented onto the mounting posts. It will be apprec-iated, o course, that the crowns can be made of any other suit-able conventional dental material and also that the artificial ridge can be made o any o~ the synthetic bone materials previous-~ ly described. Furthermore, the artificial ridge building tech-- nique can be used alone without insert teeth to provide support for a~denture.
Instead of building up an artific~al alveolar ridge to provide sufficient socket de~th for proper support of implant teeth, the sockets themsel~es can be deepened by the procedure illustrated by Flys. 1 through 17 and a prefabricated implant root installed as shown in Fig. 18. In Fig. 13, a natural tooth 170 is in an abnormally low alveolar socket 171 of a jawbone 172 ;
having an alveolar canal 173. According to the illustrated pro-cedure, the tooth is extracted (Fig. 14), and a combination drill and tap 174 mounted in a slow speed handpiece 175 is inserted ~-into the alveolar socket (Fig. 15), run down to the desired ' ' ' ~` 24.
:, ;

., . , . ; . . ; . . . . --depth ~Fig. 16), and then ba~ked out leaving a deepenad, threaded socket 176 (Fig. 17). A mating threaded plastic insert 177 (Fig. 18) having a suitable porous sur~ace is then threaded into ~ ~
the socket. The insert has a suitable crown mounting post 178 '~' for subsequent attachment of a hard plastic crown 179 (shown in , dashed lines~.
Since the threaded plas'cic insert i9 sized to match the drilled and tapped hole, it is possible to provide an assortment ` of preselected insert sizes, as shown in Fig. 19, to correspond ,~ 10 to similar sized taps for a full range of tooth and alveolar site sizes. The threaded inserts will simplify the job of prepar-ing a tooth implant in many cases when there is an existing tooth , ,~ as well as in those situations where the tooth is missing,. ` ~ ' :,1 Additional embodiments of threaded porous plastic inserts ' '' are shown in Figs. 20 through 23. In Fig. 20, a threaded insert 180 o$ porous plastic is shown in place in a jawbone 181 sur-mounted by dermi~ 182 and gingiva 183. A~solid plastic crown 184 .. :
is cemented ~nto mounting post 185 at the top of the insert. If desired, the mounting post portion of the insert can be made of ~ ' ' 20 integrally~ molded solid plastic instead of porous plastic. Not ;,;";' ,'~ onIy does this combination provide added strength but also the smooth surface of the solid plastic post will inhibit formation of epithelial attachment for the gingiva~above the upper margin of the porous plastic cylindrical portions of the insert. ~' ' 25 Features of this embodiment include a flared lip 186 -at the top of the threaded portion that provides additional anchoring after the natural bone has grown in above it, as shown ~, in Fig~ 20, Another feature is an axial center hole 187 (best '' ,~ seen in Figs. 21 and 22) that also produces impraved anchorage ', 30 by providing space for the ingrowth of hard bony cells and 25.
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additional surface area for periodontal membxane atkachment.
Eigs. 23A, B, and C illustrat a range of sizes for this type of lmplant in which the diameters vary inversely with length so that the surface area will remain approximately constant.
Thus, the different sizes are adapted for use in different regions of the jaw, but all provide similar anchoring surface.
; In the embodiment of Figs. 24 through 26, a tapered large pore porous plastic implant 188 having two transverse bone ancho-ring holes 189 and 190 is provided with tapered threaded hole 191 for receiving a mating threaded hard plastic mounting post support 192. Insert 188 is set into alveolar bone 193 so that its top surface is slightly below the top of the alveolar ridge, thereby encouraging the natural bone to grow over the implant, firmly locking it in place. The threaded plastic insert is ce-mented in the threaded hole with its mounting post 196 extending above the gingiva to receive a crown 197 (shown in dashed lines~
'1: : ' .
aXter the implant has become securely attached to the jawbone by hard bony ingrowth. Alternat~vel~y,~. the i.mplant can be made `
of small pore(50-150 micron3 porous plastic~ In that case, it `~
will become attached to the jawbone by means of a softer perio-; dontal membrane, which provides a shock absorbing mediumA
In situations where there axe no neighboring teeth to shield the mountin~ po~t during the period that the implant is becoming attached to the host bone, a temporary hard plastic or metal plug 198 having a top slot 199 can be screwed into the threaded hole, with its top surface ~lush with or below the surrounding gingiva 195. Ater the implant is set, the plug is removed and the permanent insert screwed in and cemented in place.
The porous plastic material used with the present 26.

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invention is also suitable fox making blade-type implants, and Figs. 27 through 44 illustrate a number of diferent embod~ments.
In Figs. 27 - 29 a wide plate implant 200 of small pore plastic has an integrally cast solid plastic mounting post insert 201.
The implant site is prepared by cutting a thin slot in the jaw-bone 202 deep enough so that the top of the implant blade will be below the top of the alveolar ridge. Dermis 203 and gingiva 204 are then su~ured snugly against the tapsred collar 205 to ;
"
allow the gingiva to become attached to it5 porous surface. Sub- ;
sequently, a crown 206 (shown in dashed lines) can be cemented!
onto mounting post 207 Blade-type implant 200 has four bone ingxowth holes 208 and is particularly suited by shape and size or use in the rear of the lower jaw. Smaller and narrower blade type implants 209 ;
(Fig. 30) and 210 (Fig. 31) are suitable for bicuspid and incisor supports, respectively.
"I
In the upper jaw (maxilla), the presence of the maxillary -~sinus cavities create a problem in finding sufficient bone to support a tooth implant. This problem is solved by the cùrved-~;; 20 blade designs o Figs, 32 - 34, in which a blade-type implant 211 has a curved inner edge 212 to con~orm to the curve of a maxillary sinus cavity 213. This type of implant can also come in a range of sizes as shown by implants 214 and 214~ of Figs.
35A and 35B, respectively.
Still ~urther implant shapes for crown mounting post !
supports are shown in Flgs. 36 through 44. Implant 215 (Figs.
36 - 38) has an arrowhead shape, implant 216 (Figs. 39 - 41) has ~
a conical shape, and implant 217 (Figs. 42 - 44) has a ish-tail ~ -~, ,.
shape.

As mentioned earlier, the same porous plastic implant 27.

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material used for tooth implants can also be used for bone im-plants, bone replacement, and joint prostheses. The primary difference is in pore size, since hard bone ingrowth requires pore sizes of at leaqt 200 400 microns. Referring to Figs.
s 45 - 48, there are shown various techniques for mandibular repair that are equally useful for bone ~epair in other parts of the body. In Fig. 5 a porous plastic mandibular replacement section 218 is locke~ into mandible 219 by dovetails 220 and 221. It - wilI be appreciated that the dovetail joints are actually a relatively tight fit and additionally may be cemented in place for adequate fixation until bone ingrowth can ocaur.
A large mandibular segment 222 (Figs. 46A and B) is joined to the host hinge bone section 223 by a morkise and tenon 224 and 225. As before, the joint can be cemented for stabiliza-tion until bone ingrowth takes place. In Fig. 47 a bone insert226 is held in place by acrylic screws 227 through acrylic plates 228 and also by cement, if desired. In Fig. 48 plastic condylar heads 229 have been implanted at each hinge of mandible 230.
The polymethyl methacrylate material described herein -~
.
is also useful for making hip joint or other joint prostheses.
In Fig. 49, a prosthesis of conventional form has a hard, sclid plastLc head 231, a small pore (50-150 microns) plastic midbody 232, and a large pore (200-400 microns~ plastic ixation pin 233.
The small pore midbody provides a surface for ligament attach ment, and the large pore pin provides a surface for hard bony attachment when the pin is inserted in the end of a leg bone 234, for example (see Fig. 51). An alternate joint prosthesis of conventional design is shown in Fig. 50 with a smooth hard non-porous condylar head 235, a small pore midbody 236 and a large pore fixation pin 237 having large bone ingrowth holes 238. In ~`
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Fig. 52 a hard plastic ball 239 has a porous plastic cover 240.
It should be understood that in reEerring to porous plastic portions, whether small pore or large pore, the porosity -~can either extend all the way through the portion or be only in a surface layer, depending on the strength requi~ements of the ~ , implant or prosthesis.
An important aspe~t of the present invention lies in the selection of proper pore size for the desired type of tissue attachment, whether soft co~nective tissue or hard bone tissue.
Slnce the desixed attachment of the replica embodiments of Figs.
l - 9 is by relatively soft ibrous connective attachments 24 and ,., ;.: ,;
28, the pore size of the sheath portion is preselected in the ~; range from about 50 microns to about 150 microns, while the degree of porosity is p~eferably in the range from about 50 percent to about 75 percent by volume.
~ In recent tests on rats conducted under the direction I ~` of the applicant, it has been discovered that the choice of pore ' size 1s critical in determining the type of cell ingrowth that ~il1 oca~ur in porous polymethyl methacrylate ~hereinafter called ~ -porous PMM) material. In these tests, two series of small samp~
, :
les o porous PMM having pore size~ predominantly in the range~
i~ 50-150 microns and 200-400 microns, respectively, were implanted in rats~in ~our different site environments; subcutaneous, intra-muscular, intracerebral and intraosseous. ~he subcutaneous im~
25~ plants wére retrieved at lntervals ~p to 70 days, but the lmplants in the other sites were kept in place a maximum o only 30 days.
In each of~the three non-osseous sites the sur~Qunding tissues were flrmly attached to the implants of both pore sizes, but there was no evidence of either cartilage development (chon-drogenesis) or bone~ development (osteogenesis) at any of the non-~ , , '' :' osseous implant sites. At none of the sites was any .infectionor any other pathological symptom noted. At the intraosseous sites, on the other hand, there was found in all cases a repara-tive bone formation adjacent to the implanted spec.imens.: Within the pores of the material, however, bone cells were found only in the large-pore (200-400 microns) sp~cimens. In the small-pore (50-150 microns) specimens only soft tissue ingrowth was evident. These tests indicate, therefore, that the type of con-nection between bone and an intraosseous implant can be controlled ~` lO by preselecting the pore size of the implant material in the reg-ions of the implant surface that are contiguous to the host bone.
Referring again to the embodiment of Figs. l - 9, there-fore, by preselecting the surface pore size of the entire root ~ portion of the implant to be in the range rom about 50 microns : 15 to about 150 microns, the growth of soft connective tissue into . ~ .
both the upper root surfaces and the intraosseal root surfaces ~.:
can be assured. ;
The method of fabrication of the implant tooth replica .:
in each case is as follows~
(1) the natural tooth is extracted;
(2) the natural tooth is used, by well-known conven-tional dental laboratory techniques (e.g. by flasking) to form a replica mold;

(3) a conventional and well-known mold release agent is applied in the mold;

(4) a foundation for the porous portion of the replica root section is prepared by:
(a) determining the desired porosity or porosity ; range, and degree of porosity (b) selecting water soluble salt crystals having - .

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;:
~ crystal sizes corresponding to the aforesaid pore sizes, ;~ (c) attaching a layer of said salt crystals to selected portions of said release agent coating on the inside of the mold, (d) mixing acrylic polymer and monomer with said :
` salt composition in volume proportions corresponding to said desired degree of porosity to provide a molding compositlon of the required volume,

(5) introducing said composition into the root portion ; 10 of said mold;

(6) adding additional acrylic polymer and monomer to `i fill said mold, including sald crown portion;

(7) heat polymerizating the ingredients in said mold;

(8) extracting the casting from said mold;

(9) sandblasting or otherwise removing the surface skin from the xoot portion of the casting to expose the salt par~
;~ ticles;

(10) boillng the casting in water for about four to fifteen minutes to leach out sald salt to leave pores and voids ln the place thereof~ and

11) pressing said porous casting into the extraction ~; socket ~ormerly oc~upie~ by the natural tooth.
The salt crystals may be any water solu~}e salt. Sodium chloride is preferred because of price and ease of removal, and ~; 25 e~specially because it is completely non-toxic and, in fact, residual amounts in the center portion of the root may benefit the hèaling proaess.~ On the other hand, any leaching crystaline materlal capable~of;more or less precise size classification may be used as long as it is no~-*oxic and has chemical compati- ``
bility with tbe plastic employed. The leaching medium may be 31.
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other than water, likewise depending on its compatibility with the plastic and its non-toxicity.
As examples of the type of pores and porosity that are obtained by the above described casting techniques, micro-photographs of samples of polymethyl methacrylate materials made with three diffexent sizes of salt crystals are shown in Figs. ~
53 - 55 Fig. 53 is a so-called chlorine scan at approximately ~ -3X power of a sample of sodium chloride crystals having a size range of 100-150 microns. The photograph was made before leaching out the salt crystals, which show up as light spots on the dark background.
Fig. 54 shows a sample, after leaching, at 40X power that was made with salt crystals having a size range of about 75-150 microns. Fig. 55 shows a similar sample at 25X power that was made with salt crystals in the size range o 300-400 `~
microns. All samples were made with equal volumes of salt crystals and polymer.
In view of polymèthylmethacrylate's proven adaptability or implants it is recognized as preferred or the instant inven-tion at the present time. Recognizing, however, the rapid ad-vance o scientiic knowledge and expertise, it may well be a very short time that another, readily available, material may be used or the purposes o~ the present invention and perhaps even with greater advantage.
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Claims (40)

WHAT IS CLAIMED IS:

1. A method of fabricating a plastic endosseous implant comprising the steps of:
(a) preparing a replica mold corresponding to the desired shape of the endosseous implant, (b) attaching to the interior surface of the mold a layer of water-soluble crystals of a particle size corresponding to a first pore size desired in a surface region of the implant, (c) introducing acrylic polymer and monomer mixture to fill said mold, (d) curing the material in said mold to form the implant, (e) removing said implant from said mold, and (f) leaching said water-soluble crystals from said implant.

2. The method of claim 1 wherein said water-soluble crystals are sodium chloride.

3. The method of claim 1 wherein step (b) comprises painting the root portion of the mold with a layer of acetate cement and sprinkling said crystals on the cement while it is still tacky.

4. The method of claim 3 wherein said layer of acetate cement is peeled from the surface of the replica to expose the water-soluble crystals before said leaching step.

5. In the method of claim 1, said leaching step being carried out in boiling water for about 4 to 15 minutes.

6. The method of claim 1 comprising the additional steps before step (c) of preparing a mixture of acrylic polymer and monomer with water-soluble crystals of a particle size corresponding to a second pore size desired in a subsurface region of said implant and in relative proportions corresponding to the degree of porosity desired therein and introducing said mixture into the mold.

7. The method of claim 1 comprising, between steps (b) and (c), inserting a reinforcing pin in said mold so as to be concealed wholly within said implant when cured.

8. The method of claim 1 wherein a freshly ex-tracted tooth is used as a master in preparing a replica mold in step (a) corresponding to at least the root portion of said tooth so that the implant molded therefrom is a replica of said root portion, and comprising the further steps of coating the plastic root portion with cement and inserting the root portion into the socket of the freshly extracted tooth before the cement hardens for providing initial fixation of the replica implant in the socket.

9. The method of claim 8 wherein the cement comprises methyl polyacrylate (carboxylate).

10. The method of claim 8 wherein the cement comprises a glass-ceramic cement.

11. The method of claim 1 wherein step (b) comprises coating the interior of the mold with a release agent and sprinkling said crystals on the release agent before it hardens.

12. The method of claim 1 wherein step (b) comprises mixing said crystals with a release agent and painting the mold interior with said mixture.

13. The method of claim 1 wherein step (b) comprises coating the mold interior with a release agent, pre-paring a saturated aqueous solution of said crystals, coating the layer of release agent with said saturated solution, and heating the mold to evaportate the water from said aqueous solution.

14. The method of claim 13 wherein said release agent comprises mineral oil.

15. An endosseous plastic implant having an exposed porous surface portion adapted to promote only soft connective tissue ingrowth from both surrounding epidermal and osseous environments, said surface portion having preselected pore size exclusively in the range of from about 50 to about 150 microns, a porosity in the range 50-75% by volume, and depth of porous portion being not less than 2 m.m., and wherein said plastic material consists of essentially pure polymethyl meth-acrylate and containing no polymerizing catalyst.

16. The implant of claim 15 wherein said porous surface portion comprises the root portion of a tooth replica.

17. The implant of claim 15 comprising another exposed surface portion adapted to promote hard bony ingrowth from a contiguous osseous environment, said other surface portion having preselected pore sizes exclusively in the range of about 200 to about 400 microns.

18. An implant according to claim 17 comprising a joint prosthesis having a condylar head composed of solid plastic, a midbody joined to the head and comprising said first-mentioned porous surface portion, and a fixation pin extending from the midbody and comprising said another porous surface portion, the pore size of said midbody being adapted to permit soft tissue ingrowth for ligament attachment, and the pore size of said fixation pin being adapted to permit hard bony cell ingrowth for endosseous fixation when the prosthesis is implanted in a bone of a living being.

19. An implant according to claim 18 wherein the fixation pin includes transverse bone ingrowth holes.

20. An implant according to claim 19 comprising a hip joint prosthesis.

21. An implant according to claim 19 comprising a mandibular hinge prosthesis.

22. An endosseous non-toxic polymer plastic implant according to claim 17, said other surface portion having a porosity in the range from about 50 percent to about 75 per-cent, and a depth of porosity of at least 2 millimeters.

23. An implant according to claim 22 wherein said other porous surface portion is formed with a dovetail for making a dovetail joint with the bone implant site.

24. An implant according to claim 22 wherein said other porous surface portion is formed with a tenon for making a mortise and tenon joint with the bone implant site.

25. An implant according to claim 16 comprising a support for a tooth crown, said support having a threaded portion adapted to screw into a tapped alveolar socket, the porous surface portion of the implant including said threaded portion, and a crown mounting post at one end of said implant.

26. An implant according to claim 25 wherein said threaded portion is tapered from adjacent the mounting post to a reduced diameter at the other end of the implant.

27. An implant according to claim 25 wherein said porous portion extends through the threaded portion of the implant.

28. An implant according to claim 25 wherein said threaded portion has a constant mean diameter.

29. An implant according to claim 25 comprising an axial center hole extending inward from said other end of the threaded portion to provide space for the ingrowth of hard bony cells.

30. An implant according to claim 25 comprising a flared lip at one end of the threaded portion between the threaded portion and the crown mounting post.

31. An implant according to claim 30 comprising an intermediate cylindrical porous portion between said flared lip and said crown mounting post.

32. An implant according to claim 16 comprising a tapered foundation member for an artificial tooth crown support, the foundation member having a tapered threaded hole extending axially inward from a large diameter end, said hole being adapted to receive a mating tapered threaded crown support.

33. An implant according to claim 32 wherein the foundation member is composed of porous plastic having a pore size from 50 - 150 microns.

34. An implant according to claim 16 comprising a support for a tooth crown, said support having a blade portion substantially wider in one dimension than in the cross dimension, said blade portion comprising the porous surface portion of the implant, and a crown mounting post extending from one side of the blade portion.

35. An implant according to claim 33 comprising a tapered plastic collar at the junction of the blade portion and the crown mounting post, said collar having a porous surface adapted to promote an epithelial attachment to surrounding gingiva when the support is implanted into a jawbone of a living being.

36. An implant according to claim 35 wherein the blade portion has a plurality of bone ingrowth holes.

37. An implant according to claim 36 wherein the side of the blade portion opposite said one side is curved convexly, the curvature being adapted to conform to the curvature maxillary sinus when the blade portion is implanted in the upper jaw of a living being.

38. An implant according to claim 35 wherein said blade portion has an arrowhead shape.

39. An implant according to claim 36 wherein said blade portion has a fishtail shape.

40. An implant according to claim 16 comprising a support for a tooth crown, said support having a conically shaped portion comprising the porous surface portion of the implant and a crown mounting post extending axially from the larger diameter end of the conical portion.