BACKGROUND OF THE INVENTION Sintered ceramic powders are increasingly used for the fabrication of precision parts.

A composition (often referred to as"feedstock") is provided including a ceramic precursor in powder form (generally a metal or mineral oxide) as a suspension, slurry or colloid in a binder. Many binders are known to one skilled in the art, see for example U. S. 4,923, 652; U. S. 5, 284, 695; U. S. 5,641, 920; U. S. 5,977, 230; U. S. 6, 048,954 ; U. S. 6,070, 107; U. S.

6,197, 843 or U. S. 6,429, 285.

Ceramic precursor powders are relatively homogenous in size. Ceramic precursor powders generally have an average particle size of 0.5 micron to 40 micron. Increasingly smaller particle size powders, having an average particle size of 10 nm to 100 nm, are used.

In a method known as ceramic injection molding (CIM) the composition is injected into a mold using standard injection molding techniques to give a"green part". In a debinding step, a large proportion of the binder is removed to form a"brown part". Finally the part is sintered (most often in an oven) at temperatures of from about 800°C to about 2000°C.

During sintering, the individual powder particles fuse to form the final part. During sintering the part shrinks: the green part is typically 10% to 30% larger than the final part.

In order to improve mechanical properties the sintering can be done under pressure, especially under high-isostatic pressure (HIP). Oxygen-sensitive powders are sintered in an inert environment.

Perhaps the greatest disadvantage of CIM is that certain shapes, especially shapes with internal volumes or cavities are difficult to fashion. Another disadvantage is the expensive and time-consuming mold-making step. The cost of making a mold is not significant when a large number of identical parts need be made but makes CIM an

unsuitable technique for custom manufacture. Increasingly, in such fields as reconstructive surgery and dental prosthetics there is a demand for custom-made parts.

An alternative to CIM is free form manufacturing (FFM) using a ceramic precursor containing composition. Although the composition is often a liquid or slurry, in some cases a powder composition is used. In FFM a part is made"from the ground up"by sequentially dispensing composition layers one on top of another, see for example, U. S. 6,376, 148; U. S.

6, 238,614 ; U. S. 6,228, 437; U. S. 6,066, 285; U. S. 6,117, 612; U. S. 6,046, 426; U. S. 5,059, 266; U. S. 5,204, 055 or U. S. 6,206, 672. A review of the state of the art of FFM can be found in U. S. 6,376, 148, which is herein incorporated by reference for all purposes as if fully set forth herein.

In a first planning step, a model of the desired part is"sliced"to produce a plurality of layers that when stacked together give the part. Typically CAD/CAM software slices a virtual model of the desired part and divides it into a plurality of layers. Thereafter, a dispensing device is used to dispense a first layer of composition. The layer is"sintered", most often with a laser. Unlike real sintering as described hereinabove, in the first sintering step of FFM binder is fused. For clarity, the first"sintering"step is hereinbelow referred to as fusing. Where heated and subsequently solidified, the binder sticks together and rigidly associates ceramic precursor particles together.

Using selective laser sintering only selected areas of a given layer are bound allowing complex shapes to be fashioned. One alternative process to the selective laser sintering is electrophotographic binder powder deposition as taught in U. S. 6,376, 148.

After the first layer is dispensed and fused, a succeeding layer is dispensed on the first layer and fused. This process continues layer by layer until all the layers have been dispensed and appropriate areas fused, giving a"green part". Excess composition, that is composition deposited in the layers and not fused, is removed. A debinding step is performed to remove not-fused binder, giving a"brown part". The manner in which debinding is performed depends on the nature of the binder and includes moderate heating (e. g. 50°C to 400°C) to vaporize or liquefy the binder, or removal of the binder with a solvent.

After debinding, the"brown part"is sintered to give the final part. As in CIM process, the incipient part shrinks during sintering.

Spatial control of layer dispensing, and in the case of"selective sintering"the fusing steps, are typically performed using CAD/CAM software and hardware interfaced with a

FFM device. Thus FFM is exceptionally applicable in making prototypes, limited editions and custom parts and objects.

Two disadvantages of FFM using sinterable ceramic particles are shrinkage of the part during sintering and the high porosity of the final fabricated part.

As noted hereinabove, sintering leads to a roughly 10-30% reduction of volume of the "green part". Shrinkage is a problem when it is desired to make two or more closely fitting parts. Even a slight differential in the magnitude of shrinkage may cause the two parts to fit improperly.

Porosity of parts is a result of the shape of the particles and the presence of binder in the composition. Even when dry particles are optimally packed, large interstitial spaces remain between particles. The addition of binder to a composition increases the porosity even further as the binder occupies a significant volume. Although sintering reduces the empty volume as evidenced by the shrinkage of the parts, parts made by FFM using sinterable ceramic particles remain porous.

The greatest negative effect of the porosity is structural weakness. Even under moderate mechanical stress such parts can break. Another negative effect is that the parts may develop virtually undetectable microfractures. These microfractures may cause the part to unpredictably and suddenly fail. As a result, FFM using sinterable particles is most often used for producing production tools (such as molds) or to fashion parts for models.

One of the fields where the disadvantages of powder fabrication techniques are most felt is in the field of dental prosthetics. In U. S. 5,852, 766 is disclosed a method for preparing dental prosthetics using CIM. As stated above, CIM is too expensive for the wide spread preparation of custom parts.

One of the most common procedures that a dentist is requested to perform is restoration of a tooth following endodontic treatment. The result of such a procedure is schematically depicted in Figure 1. In Figure 1, a tooth remnant 24 has lost a clinical crown slightly above a gum line 42. Tooth remnant 24 has an endontontically treated main root canal 22. Root canal 22 houses a dental post 12. Dental post 12 is firmly embedded in root canal 22 by means of threading on shank 14 of post 12, by cementation or other method.

Retentive head 18 of post 12 protrudes through the damaged tooth line 44 and constitutes the basis for the restoration of the damaged tooth. A matrix 46 is filled with a core material 48. After preparation of a core, an artificial crown 50 covers the core, completing the restoration of tooth remnant 24.

In general, two types of dental post and core systems are known in the art:"active"or screw-in type systems and"passive"type systems. Active dental post and core systems mechanically engage the walls of the root canal and tooth dentin. Passive dental post and core systems are bonded in endodontically treated teeth utilizing cements or other adhesives.

In U. S. 5,915, 970, included by way of reference as if fully set forth herein, is found a survey of dental posts and dental post and core systems.

Although it is desirable that the endodontic treatment be permanent, it is sometimes necessary as a result of peri-apical diseases, incomplete or inadequate root canal treatments, and dental post failures, to perform endodontic re-treatment. According to Cohen S. and <BR> <BR> Burns R. "Pathways of the pulp"pp. 794,799-801, endodontic re-treatment is necessary in up to 34 percent of the cases.

The first step in endodontic re-treatment is removal of crown 22, the core and dental post 12. Removal of dental post 12 from root canal 10 is most often done by drilling-out dental post 12 using a special high-speed carbide bur. Drilling-out dental posts, especially ceramic posts, is difficult and results in the loss of tooth material. As a result, dental post removal is considered a complex and difficult procedure that may be traumatic to the patient.

Such a traumatic experience helps reinforce the anti-dentist stereotype typified, for example, in such films as"Little Shop of Horrors". Removal of a dental post even when performed by a highly skilled dentist, can be risky and can lead to damage to remaining tooth and even to tooth extraction. Missing tooth can be reconstructed, but tooth reconstruction is a long, painful and expensive procedure.

In U. S. 4,178, 688, U. S. 5,951, 286 and in U. S. 6,280, 197 are described devices for the removal of dental posts. A trephine drill bit, being substantially a hollow cylindrical body, with an internal diameter roughly equal to that of the dental post to be removed and saw teeth around the terminal rim, is used to drill-out an amount of tooth tissue flush with the dental post. In U. S. 4, 178, 688 enough tooth is removed to allow easy extraction of the dental post.

In U. S. 5,951, 286, a circular groove around the top of the dental post allows the dental post to be engaged with pliers for extraction or in the case of a threaded dental post allows the use of a lever, such as a screwdriver, to extract the threaded dental post. In U. S. 6,280, 197 the drill bit automatically engages and rotates the dental post inside the tooth, allowing the extraction of the dental post. Use of these devices necessarily removes a considerable amount of tooth tissue, weakening the tooth. Further, the use of pliers in U. S. 5,951, 286 or the dental post

rotation of U. S. 6,280, 197 cause stress to the tooth, which may lead to cracking or splitting of the tooth. In such a case, the whole tooth must be extracted.

In U. S. 5,173, 049 is described a method of loosening a dental post for removal by introducing a liquid in the proximity of the dental post and evaporating the liquid with the help of a laser. The resulting cavitation jiggles the dental post, breaking any adhesive around the dental post, loosening it. Once the entire adhesive is broken, the dental post is easily extracted. This procedure involves laser cutting of the dentin, resulting in tooth structure loss.

This method is inefficient, weakens any remaining tooth structure and can cause tooth-cracking stress.

Ultrasonic vibrations have also been employed to aid in the removal of dental posts, by loosening the bound between adhesive and the dental post itself. The heat and vibrations produced often damage the remaining tooth material.

Removable dental posts have been described in the art. French Patent 8, 515,527 and U. S. 5,326, 263 both disclose tapered dental posts. The tapering is designed to ease dental post extraction. The purpose of these dental posts is to allow the temporary attachment of a dental prosthetic for a relatively short time. As such, the removable dental posts are further configured to be easily removable. For example, whereas permanently installed dental posts are often configured with reliefs, threads or grooves on the dental post shank, the removable dental posts have a smooth outer surface, thus are rendered unsuitable for use as permanent dental posts.

In German patent application DE 3901640 (also known as French patent 2,626, 167 or British patent GB 2,214, 087), a dental post made of composite material having a central filament to which surrounding fibers and surrounding resin are not bound is disclosed. The dental post of DE 3901640 is configured to permanently attach a dental prosthetic. When it is desired to remove the dental post, the tip of the central filament is exposed and used to draw out the central filament. The resulting channel is used as a guide for a drill, allowing the remaining resin and fibers making up the dental post to be reamed with only minimal damage to surrounding tooth. Unfortunately, in many cases, during the act of drawing out the central fiber becomes stuck or tears, precluding extraction. Further, such a dental post is necessarily made of composite material limiting the ability to make a dental post according to the teachings of DE 3901640 of other commonly used, and often preferable, materials such as metal or ceramic.

It would be highly advantageous to have a method for making parts using FFM that retains the advantages of the prior art methods and yet allows for making parts that are more structurally sound. It would also be advantageous to have a dental post that is configured both for permanent installation and for simple removal when and if the need arises, without damaging remaining tooth tissue.

SUMMARY OF THE INVENTION The objective of making parts using FFM that retains the advantages of the prior art methods and yet allows for making parts that are more structurally sound is achieved by the FFM method provided by the present invention. The objective of an easily removable permanently installed dental post is achieved by the dental post provided by the present invention.

There is provided according to the teachings of the present invention a method that is substantially a modification of the prior art FFM method based on using sinterable powders having different average sized. Powders having a large average size are used primarily to define the shape and structure of an article being made while the small average size particles are used primarily to fill the interstitial spaces between the large sized particles. Thus there is provided a method of producing an article by free form manufacturing by a. creating a three-dimensional model of the article; b. generating a plurality of slices of the three dimensional model, the slices generated so that when the slices are stacked the three-dimensional model is substantially reproduced ; and c. producing the article as the plurality of slices in a stacked state by sequentially reproducing each slice of the plurality of slices as a corresponding layer in a stacked state, by for a given layer i. dispensing a first composition, the first composition including a first sinterable powder having a first average particle size, to make a structure layer, the structure layer substantially recreating a respective slice; and ii. onto the structure layer dispensing a second composition, the second composition including a second sinterable powder having a second average particle size wherein the second average particle size is smaller than the first average particle size, to make a diffusion layer.

Subsequent processing analogous to processing in prior art FFM leads to formation of the article. Specifically, after all the layers have been dispensed, most often sequentially and the incipient part debound and sintered, the final part results.

According to a feature of the present invention, the first sinterable powder and the second sinterable powder are independently selected from the group of sinterable powders consisting of ceramic, composite, metal, organic and cermet powders. Specifically such powders include, but are not limited to, zirconium oxide, yttrium oxide, hafnium oxide, aluminum oxide, titanium oxide, magnesium oxide, silicon oxide, cerium oxide, boron oxide, calcium oxide, potassium oxide, vanadium oxide, sodium oxide, lanthanum oxide, boron oxide, iron oxide, titanium carbonitride and combinations thereof. Further such powders include, but are not limited to, titanium and titanium alloys.

According to a feature of the present invention the first sinterable powder has an average particle size of between about 0.5 micron and about 40 micron, more preferably between about 0.5 micron and about 10 micron.

According to feature of the present invention the second sinterable powder has an average particle size of less than about 100 nanometer, more preferably less than about 30 nanometer.

According to a feature of the present invention the structure layer is between about 10 micron and about 250 micron thick, more preferably between about 10 micron and about 30 micron thick.

According to a feature of the present invention, the amount of the second powder dispensed to form a given diffusion layer is between about 100% and about 120%, more preferably between about 100% and about 105%, of the average of volumes of interstitial spaces in a structure layer above and a structure layer below that diffusion layer.

There is also provided according to the teachings of the present invention an article made of a powder using FFM, characterized in that the powder includes a population of particles having a first average particle size and a population of particles having a second average particle size wherein the second average particle size is smaller than the first average particle size. According to a feature of the present invention, the powder having the second average particle size is substantially homogeneously distributed throughout the article.

According to a further feature of the present invention, the concentration of the powder having the second average particle size at a surface of the article is lower than a concentration of the same inside the article. In such a way the inner parts of the article are strong while the

outer surfaces of the product are porous allowing, for example, good adhesion of the article to other similar articles.

There is also provided according to the teachings of the present invention, an improved method of FFM for fashioning an article composed of a plurality of closely fitting parts, being generally a method of producing an article made of a first part and a second part, the second part being separated from contact with the first part by a seam, by: a. creating a three-dimensional model of the article ; b. generating a plurality of slices of the three dimensional model, the slices generated so that when the slices are stacked the three-dimensional model is substantially recreated; and c. recreating the model as a plurality of slices in a stacked state by sequentially reproducing each slice of the plurality of slices as a corresponding layer in a stacked state (similarly to prior art FFM methods), by for a layer i. in areas of a layer corresponding to an actual incipient part, dispensing a first composition including a sinterable powder; and ii. in areas of a layer corresponding to a seam between the parts making up the article, dispensing a second, non sinterable, composition. By non-sinterable composition is meant a composition that does not sinter under conditions at which the sinterable powder of the first composition sinters. Generally, the non-sinterable composition does not sinter at a temperature lower than or substantially equal to the temperature at which the sinterable powder of the first composition sinters.

According to a feature of the present invention, the second composition includes a powder that is sinterable at a temperature substantially higher than the sintering temperature of the sinterable powder of the first composition. According to a further feature of the present invention the first composition includes a binder and the second composition is substantially composed of that binder. According to a further feature of the present invention, the seams define a weak point between any two parts. After the whole article is made, the two parts can be separated by breaking at a weak point. This allows for the construction of temporary and easily removable support elements.

Herein by powder precursor is meant the sinterable ceramic precursor powder used in the art of CIM or FFM. Such ceramic precursor powders are typically powders of oxides of zirconium, yttrium, hafnium, aluminum, titanium, magnesium, silicon, cerium, boron,

calcium, potassium, vanadium, sodium, lanthanum, boron and iron. Also included are powders of such materials as titanium carbonitride.

Although, for the sake of clarity, the discussion hereinbelow refers explicitly to making ceramic objects using sinterable ceramic precursor powders, it is important to note that the scope of the invention includes making metal objects using sinterable metal precursor powders (metal or metal alloy powders). organic, cermet or composite powders as well. One skilled in the art is well aware of the many types of metal precursor powders that can be used.

A typical list of suitable powders can be found in U. S. 5,338, 508. Titanium and titanium alloys powders are the preferred sinterable metal precursor powders of the present invention.

Herein by binder is meant the term as used in the art. Typical binders have been described in, for example, U. S. 4, 923, 652; U. S. 5,284, 695; U. S. 5,641, 920; U. S. 5,977, 230; U. S. 6,048, 954; U. S. 6,070, 107; U. S. 6,197, 843 and U. S. 6,429, 285.

There is also provided according to the teachings of the present invention a dental post.

The dental post of the present invention is configured as a member for the attachment of a dental prosthetic to a tooth and is configured as prior art dental posts, but characterized by having at least one permanent cavity.

According to a feature of the present invention, the at least one cavity is of a length equal to the shank of the dental post. According to a feature of the present invention the cavity is substantially centered in the shank.

According to a further feature of the present invention, the cross section of the cavity is substantially circular.

According to a further feature of the present invention, the cross section of the cavity is substantially not circular (acircular). and can be, amongst others, rectangular, cross-shaped, oval, ellipse, square, pentagonal, hexagonal and star-shaped.

According to a still further feature of the present invention, the outer surface of the shank can be threaded.

According to a feature of the present invention, the at least one cavity is open at the top end of the shank and is of a length so as to terminate at least 0.2 mm and no more than 3 mm from the bottom end of the shank.

According to a further feature of the present invention, the at least one cavity is open at the top end and at the bottom end of the shank.

According to a feature of the present invention, the at least one cavity is substantially conical in shape and wherein the base of the conical-shaped cavity is open at the top end of the shank.

According to a feature of the present invention, the at least one cavity is at least partially filled with a radio-opaque material. According to a further feature of the present invention, the radio-opaque material is substantially softer than a material from which the member is fashioned.

According to a further feature of the present invention, the outer surface of the shank has features such as protrusions, serrations, grooves and threads.

There is also provided according to the teachings of the present invention a method of preparing a tooth root canal for attachment of a dental prosthesis by: a. providing a dental post having a cavity, the cavity opening at the top end of the dental post where the dental post is made of a radio-transparent first material; b. at least partially filling the cavity with a radio-opaque second material; and c. fitting the dental post into the tooth root canal; where the second material is softer than said first material. According to a feature of the present invention, the filling (b) precedes the fitting (c). According to another feature of the present invention, the fitting (c) precedes the filling (b).

According to a feature of the present invention, the second radio opaque material is fluid (e. g. liquid, gel, suspension, slurry, cream) According to still further feature of the present invention, filling includes injection the second material into the cavity, for example, with the help of a needle and syringe.

There is also provided according to the teachings of the present invention a method for attaching a dental post into a tooth root canal by: a) placing an amount of a light-curable adhesive in the tooth root canal, the light-curable adhesive configured to cure upon exposure to light having a first wavelength; b) fitting a dental post having a hollow cavity into the tooth root canal, the dental post made of a first material, the first material being transparent to the first wavelength of light; c) putting a light source into the hollow cavity; and d) activating the light source within the hollow cavity. According to a feature of the present invention, the light source includes a light-guiding fiber.

There is also provided according to the teachings of the present invention a method for removing a dental post from a tooth root canal by a) providing a dental post having a shank and a cavity in the shank; b) exposing a cross section of the cavity; c) inserting a part

of a removal device into the cross section of the cavity; and d) activating the removal device, effecting removal of the dental post.

According to a feature of the present invention, the part of the removal device inserted is an abrasive device (such as a bur or drill) and the activation leads to reaming of the shank, effecting erosion and subsequent removal of the dental post.

According to a feature of the present invention, the shank of the provided dental post is threaded and the part of the removal device inserted is configured to engage surfaces of the cavity upon rotation of the inserted part of the removal device, effecting rotation and subsequent removal of the dental post. Clearly, when this feature is applied, it is advantageous that the cross section of the cavity be acircular.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 (prior art) is a schematic depiction of a cross section of a post and core within a tooth; FIGS. 2 depict a cross section of a dental prosthetic manufactured in accordance with the method of the present invention from the side (A) and from the top (B); FIGS. 3 depict a sealed, unidirectional valve manufactured in accordance with a method of the present invention in perspective (A) and in cross section (B); FIGS. 4A-4C are schematic depictions of archetypal shanks with cavities of dental posts of the present invention; FIGS. 5A-5C are schematic depictions of threaded dental posts of the present invention having acircular cavities; FIGS. 6A-6C are schematic depictions of dental posts of the present invention having retentive heads; FIGS. 7A-7B depict removal of a dental post of the present invention by reaming; and FIGS. 8A-8B depict removal of a threaded dental post of the present invention by rotation using a removal tool.

DETAILED DESCRIPTION OF THE INVENTION The present invention is of objects made by improved FFM using sinterable ceramic particles as well as the methods for making the objects. The principles and use of the FFM

method of the present invention may be better understood with reference to the drawings and description. Before turning to details of the present invention, it should be appreciated that the FFM method of the present invention provides two sets of features. each of which may be used alone, or which may be combined to provide a particularly useful method.

The first feature of the FFM method of the present invention relates to objects made by FFM that have a low porosity and great strength, as well as a method for making the objects. In general, this is achieved by using at least two ceramic precursor powders differing in average particle size. A first ceramic precursor powder, the structure powder, has relatively average large particle size (from about 0.5 micron to about 40 micron) and includes the greatest mass of the part. A second ceramic precursor powder, the diffusion powder, has a relatively small average particle size (less than about 100 nm).

The second feature of the FFM method of the present invention relates to multi part objects where the parts fit closely, as well as a method for making the multi part objects.

According to the present invention during the planning step of FFM, slices of the multi part object when assembled from the plurality of parts are prepared. During the actual deposition of the layers corresponding to the slices, the seam areas of each layer are filled not with ceramic-powder containing composition but only with a non-sinterable filling material.

During or after the production steps, the filling material between two parts disappears, leaving the multi part object made of closely fitting pieces in an assembled state. Since the different parts were fused and sintered together, shrinkage is virtually identical and the parts fit together very well.

The present invention is also of an innovative dental post, which allows for secure attachment of a dental prosthetic but which can also be easily removed when necessary with little or no damage to the tooth from which the post is removed. This is achieved by providing a cavity in the shank of the dental post, the cavity extending from the top end of the shank towards the bottom (apical) end of the shank. The cavity can either serve as a guide for reaming the post out of the tooth or to engage a removal instrument such as a screwdriver or Allen (hex) wrench.

In a preferred embodiment, the dental post of the present invention is made in accordance with the teachings of the FFM method the present invention.

Low porosity parts made by ceramic powder FFM

As stated hereinabove, both shrinkage and the porosity of a part produced by powder fabrication arises inherently from the use of a composition of powder in binder. The FFM method of the present invention reduces the interstitial volume of a part, increasing its density and strength.

According to the first feature of the FFM method of the present invention are defined two distinct ceramic precursor powders. A first ceramic precursor powder, the structure powder, has relatively average large particle size (from about 0.5 micron to about 40 micron).

A second ceramic precursor powder, the diffusion powder, has a relatively small average particle size (less than about 100 nm). The two powders are dispensed in distinct layers, a structure layer of structure powder and diffusion layer of diffusion powder. Most preferred is that between any two diffusion layers is dispensed a structure layer. During sintering, the diffusion powder particles diffuse into the interstitial spaces of the structure layer. The diffusion properties of the nanometer size particles (diffusion particles) are higher by 3 to 5 orders of magnitudes than the diffusion properties of the micron size particles (structure particles).

The relative amounts of the two powders are chosen so that the volume of the diffusion powder is dependent on the total volume of interstitial spaces found in the structure layers after sintering. One skilled in the art is able to determine the exact value of the interstitial spaces of any given structure powder. Most easily, an object of some shape is dispensed, debound and sintered in the usual way from structure powder. After sintering, it is a simple matter to calculate the volume of interstitial spaces in the object from density measurements.

Once the volume of the interstitial spaces of the structure layers is known, the amount of diffusion powder deposited can be determined. An amount of diffusion powder that is significantly less than the volume of interstitial spaces in the structure layers will yield a more porous and weaker part. An amount of diffusion powder that is significantly more than the volume of interstitial spaces in the structure layers will yield a stronger part which is somewhat inhomogeneous as there will be diffusion layers where not all the diffusion powder has diffused into the structure layers.

It is most preferable that the volume of diffusion powder dispensed between any two structure layers be approximately equal or higher than (within 20%, preferably within 5%) the volume of interstitial spaces in the structure layers.

Consider a diffusion layer that is to be dispensed between two structure layers.

Approximately half of the diffusion powder of the layer shall diffuse to the lower structure layer and half to the upper structure layer. Further, on the average the diffusion layer is to fill up to half way into each of the structure layers. Thus the simplest manner of calculating the volume of the interstitial spaces needed to be filled by any single diffusion layer is to take half the sum of the thickness of the layers multiplied by the percent of a structure layer that is interstitial spaces. If a diffusion layer is to be sandwiched between two 10 micron thick structure layers, the preferred amount of diffusion powder dispensed is to fill preferably between 100% and 120%, more preferably between 100% and 105% of the interstitial spaces of 5 microns in the lower structure layer, and 5 microns in the upper layer. More simply said, the amount of diffusion powder dispensed to make a diffusion layer is between about 100% and about 120%, more preferably between about 100% and about 105%, of the average of the volumes of interstitial space in a structure layer above and below the diffusion layer.

More simply said, the amount of the diffusion powder dispensed to make a diffusion layer is roughly the average of the volumes of interstitial space in the structure layers above and below that diffusion layer.

In a preferred embodiment of the first feature of the FFM method of the present invention, between any two structure layers (generally of a structure composition made by mixing a structure powder with binder) is dispensed a diffusion layer (composed of a diffusion composition including diffusion powder optionally combined with binder).

According to the preferred embodiment of the first feature of the FFM method of the present invention, after aplanning step of slicing a model of an object to be made and then determining the size and shape of each corresponding layer to be dispensed, a structure layer is dispensed as described in the art. To make a structure layer, a structure composition made of structure powder in binder is dispensed, as known to one skilled in the art.

After dispensing, the structure layer is fused or selectively fused, e. g. , by selective laser sintering, in the usual way.

Onto the structure layer is dispensed a diffusion layer. To make a diffusion layer, a diffusion composition made up of diffusion powder is dispensed. Optionally, the diffusion composition includes binder. In many cases, especially when the diffusion layer is very thin, the diffusion composition does not include binder. In a preferred embodiment the diffusion layer does not undergo a fusing step.

After the diffusion layer is dispensed, a further structure layer is dispensed onto the diffusion layer and fused. The above process is repeated until all layers have been dispensed and the"green part"is made.

An important design consideration when using the method of the present invention is whether the layers or parts of layers that correspond to outer surfaces of the part to be made, for example the first and last layers of a part being manufactured, are a diffusion layer or a structure layer. In cases where it is desirable that the faces be porous, e. g. to enhance inter-part adhesion, it is preferred that the layers or parts of layers dispensed be of the first (structure) composition. If there is no need for porosity on the part faces, it is preferred that the layers or parts of layers dispensed include the second (diffusion) composition.

As known in the art, the completed"green part"undergoes a debinding step to remove binder to make a"brown part". The"brown part"is subsequently sintered. During the sintering process, the diffusion layer particles diffuse into the interstitial volume of the structure layers. When sintering actually occurs, the porosity of the part decreases and the density of the part increases. The strength of the part is increased as a result of an increase in contact points between particles.

According to the preferred embodiment of the first feature of the FFM method of the present invention, a structure layer is as thick as possible. The thicker each layer is, the fewer structure layers are needed to describe that part and as a consequence, the faster a part of a given size is made. That said, there are many factors that influence the maximum thickness of a layer according to FFM, such as but not limited to the efficacy of the fusing step, the flow rate, the particle size and viscosity of the composition. Thus, although the structure layer can be deposited at any thickness, it is generally preferred that the structure layer be greater than about 10 micron, preferably greater than about 25 micron. Experience shows that generally a layer thicker than 250 microns is not practical.

According to the preferred embodiment of the first feature of the FFM method of the present invention, since the amount of diffusion powder dispensed to make a diffusion layer is dependent on the volume of interstitial space of an adjacent structure layers, the thickness of a diffusion layer is determined by the thickness of the adjacent structure layers. For example, a diffusion layer is deposited between two 25 micron thick structure layers containing a structure powder known to have 20% interstitial spaces. The average thickness of the upper and lower layer is 25 microns. Since, during sintering, the diffusion powder migrates to both layers, the amount of diffusion powder necessary is 20% of a 25 micron

thick layer, that is about 5.0 microns thick. In some case it may be desirable to deposit less than a maximal amount of diffusion powder. Also, in some cases, it may be desirable to deposit somewhat more than the maximal amount of diffusion powder. The exact proportion of the diffusion powder to structure powder is also dependent on factors including but not limited to the relative average radii, radius homogeneity and the amount of binder of each of the two powders and desired material properties. Clearly sintering under pressure or HIP can be combined with the teachings of the present invention to gain additional preferred properties.

Generally, the binder used in the structure composition and the diffusion composition is chosen to be removable under substantially the same conditions. Most often the two binders are chosen to be substantially identical, although differences in binder may be dictated as a result of the particle size difference. Clearly the compositions of the present invention also optionally include other additives of which one skilled in the art is well aware.

The diffusion powder and the structure powder used in making a single part are typically of substantially the same chemical composition, but no less preferably of different chemical compositions. For example, when making parts for dental uses, the diffusion powder can be aluminum oxide while the structure powder can be zirconium oxide.

Both the diffusion powder and the structure powder used in making a single part typically have a homogenous chemical composition, but no less preferably have a heterogeneous chemical composition. For example, when making parts for dental uses, both the diffusion powder and the structure powder can be mixtures of aluminum oxide powder mixed with zirconium oxide powder.

Generally the composition of the powders is chosen so as to achieve certain desired properties. For example it is exceptionally advantageous in dental applications (vide iyzfra) to select a structure powder and a diffusion powder, both powders of the zirconium oxide alloy 3Y-TZP.

Although one skilled in the art can make the ceramic powders with the properties necessary for implementing the teachings of the present invention, ceramic powders useful in implementing the present invention are commercially available, for example, from Tosoh Corporation (Japan), Mitsubishi Mining Cement Co. (Japan) or Nanophase Technologies (United States).

Close-fit multi-part object

It is often desirable to produce an object composed of two or more closely fitting parts. Due to the fact that parts produced from sinterable powders shrink during the sintering process, it is very difficult to make two parts separately with confidence that the parts shall subsequently fit sufficiently well together.

The second feature of the FFM method of the present invention is a modified version of FFM used so as to produce multi-part objects where the different parts fit closely. During the planning stage (where slices of a model of the object are generated) the object, in a state where the object is assembled from the component parts, is divided into layers in the usual way. Seams between two parts are designated as such. During any given dispensing step, non-sinterable filler is dispensed in areas of a given layer corresponding to incipient seams. It is important to note that by non-sinterable filler is preferably meant a material having a volume so as to define a seam, but does not sinter during the sintering step where the"brown part"becomes the finally produced object. The filler is can be substantially powder-less binder. The width of such a seam is typically between 10 microns and 250 microns. When the two parts of an object being fashioned according to the method herein are destined to be glued together by adhesive (for example, in the fashioning of a dental prosthetic), the seam is preferably between about 50 microns and about 150 microns wide. During the debinding step, such filler can disappear along with the binder. In such a way, areas in the"green part" which were incipient seams become actual seams in the"brown part". As is clear to one skilled in the art, since the layers making up all of the parts of a single object undergo all steps of the production process together it is certain that the final produced parts fit together with high precision.

As noted hereinabove, despite the fact that discussion above has been focussed, for the sake of clarity, exclusively on ceramic powders, it is clear to one skilled in the art that the present invention is applicable also for FFM using other sinterable powders such as sinterable metal, organic, cermet or composite powders. Specifically, the FFM method of the present invention is exceptionally useful when applied to the manufacture of objects from powders of titanium and titanium alloys.

In certain cases when the second feature of the FFM method of the present invention is implemented, it may be necessary to add temporary supporting-elements to ensure that incipient parts of one object remain properly oriented and without touching or shifting during all production steps. Until sintered, an object is susceptible to warping. This is overcome by adding a temporary supporting-element, planned and dispensed along with the object. The

incipient object and the supporting element are substantially separated at a seam as described hereinabove or connected at the seam and with a weak point. After sintering, the supporting element and the object itself are separated, the supporting element being discarded. An example of the use of a temporary supporting-element is depicted in Figures 2.

In Figure 2A is depicted a cross section of a model of an assembled three-part dental prosthetic 3 made up of a post 5, a core 7 and a cap 9. Evident are also temporary supporting elements 11 attached to cap 9 with weak points 13. In Figure 2B is depicted a slice 15 of model 3 at the dashed line. When a layer corresponding to slice 15 is deposited, the shaded areas of slice 15, corresponding to post 5, core 7 and cap 9 are deposited as sinterable FFM composition while the non-shaded seam areas, 17 and 19 are deposited as non-sinterable filler material. As stated above the width of seam areas such as 17 and 19 for dental applications is generally between 50 and 150 micron. Note that the relative sizes of the various features in Figure 2 have been exaggerated for clarity.

The second feature of the FFM method of the present invention as described immediately hereinabove can also be used to define spaces and chambers in an object. Thus, objects having internal channels and pathways may be produced. Multi-part objects may be fashioned wherein at least one part is inextricably confined within another. For example a sealed, unidirectional valve 23 (see Figure 3) can be manufactured with ease. Liquid entering through inlet 25 pushes ball 27 against stop 29, allowing liquid to pass through outlet 21. In contrast, liquid entering through outlet 21 pushes ball 27 against lip 23 of inlet 25, sealing inlet 25 and preventing passage of liquid.

Although the second feature of the FFM method present invention can be implemented using prior art powder compositions, it is clear that due to the reduced shrinkage and greater strength, it is most preferred to produce a multi part object according to the second aspect of the present invention using the method and compositions of the first aspect of the present invention.

Another example for the use of the two feature of the FFM method of the present invention is in the production of dental prosthetic 3, depicted in Figure 2. In such a prosthetic, the individual parts must fit perfectly, preferably with no more than an about 0.05 to about 0.15 mm space between post 5 and core 7, and about 0.05 to about 0.15 mm between core 7 and cap 9. Such spacing is known to be ideal for good mutual adhesion. The prior art does not teach of a reproducible method to cheaply produce these three parts to such a tolerance, especially not as custom made parts.

As is clear to one skilled in the art, as in other FFM methods, parts produced according to the method of the present invention have stepped (rough) outer surfaces. In the specific case of dental prosthetics a stepped surface is preferred. As a consequence of the fact that the surface area for adhesion between parts is increased, the adhesion line has improved properties. In cases where the stepped structure of a produced part is insufficient, the part can be planned to have extra serrations or equivalent features.

Another multi-part object that can be made using the second feature of the FFM method of the present invention is a dental prosthetic featuring a dental post of the present invention. The dental post of the present invention is suitable for the permit attachment of a dental prosthetic to a tooth remnant. However, when it is necessary to remove the prosthetic after fitting, part of the prosthetic structure is removed, exposing the head of the post, allowing easy removal of the post and prosthetic, as described hereinbelow.

In Figures 4, three archetypal shanks of dental posts of the present invention are depicted. Figure 4A depicts a straight walled shank with a parallel-walled cavity 10,4B a tapering shank (substantially a truncated cone shape) with a parallel-walled cavity 10, and Figure 4B a straight-walled shank with a tapering (not parallel-walled) cavity 10. In Figures 4,12 indicates the bottom (apical) end of the shank, the end which is embedded inside a tooth when the dental post is fitted while 14 indicates the opposite (top) end of the shank. It is seen that the cavity defines a volume that is in communication with the surroundings through the top end of the shank.

The length of the shank of a typical dental post of the present invention is between about 6 mm and about 25 mm.

The diameter of a shank of a typical dental post of the present invention is between about 0.5 mm and about 3 mm. For a typical tapered-shank dental post of the present invention the diameter of a shank at the top end of the shank is between about 0.5 mm and about 3 mm, whereas the diameter of a shank at the bottom (apical) end is between about 0.5 mm and about 2.0 mm.

The length of a cavity of a dental post of the present invention can range from being very shallow (substantially only being a dimple) to being a channel with a length from the top end of the shank extending through the apical end of the shank (as in Figures 4A and 4B).

Only in limited application is it advantageous that the cavity be only a dimple. Preferably, the length of the cavity is from about 15% to 100% of the length of the shank.

As noted hereinabove, a cavity of a dental post of the present invention is either parallel-walled (Figures 4A and 4B) or tapering (Figure 4C). The cross section of a cavity of the dental post of the present invention can be any shape, and is chosen for ease of manufacture and dependent on the expected method of removal. When it is expected that removal of the dental post be by reaming where the cavity acts as a guide (vide infi-a), the cross section of the cavity is advantageously, but not necessarily, circular (Figures 4).

When it is planned that fitting and removal of the dental post be performed by screwing with the help of a threaded shank, the cross section of the cavity is most preferably not circular (acircular) and can have any shape which is useful for engaging a tool such as a screw driver, Allen wrench or the such. As is clear to one skilled in the art, such shapes include, but are not limited to, rectangular (suitable for a standard screwdriver), cross-shaped (suitable for a Phillips screwdriver), oval, ellipse, square, pentagonal, hexagonal (suitable for an Allen wrench) and star-shaped. In Figure 5A is depicted a dental post of the present invention having a cavity 10a with a slot-shaped (rectangular) cross section suitable for engaging a standard screwdriver. In Figure 5B is depicted a dental post of the present invention having a cavity 10b with a hexagonal cross section suitable for engaging a Allen wrench. In Figure 5C is depicted a dental post of the present invention having a cavity 10c with a cross-shaped cross section suitable for engaging a Philips screwdriver.

The inner dimensions of a cavity 10 at the top end of a shank of a dental post of the present invention can range from between about 0.1 to about 0.8 mm.

The shank of a dental post of the present invention can be smooth. Preferably, like prior art dental posts, the outer surface of the shank of a dental post of the present invention is not smooth. When a dental post is designed to be attached in a tooth using an adhesive, the shank surface advantageously has reliefs, patterns, threads or serrations to improve dental post retentivety. When a dental post is designed to be screwed into and out of a tooth, threads are fashioned on the shank, for example, threads 16 in Figures 5. Other features that can be added to the surface of a shank of a dental post of the present invention are grooves and the such to prevent rotation or to act as vents to allow air and liquid to escape during fitting (as taught in U. S. 5,073, 112).

A first general embodiment of the dental post of the present invention consists substantially only of a shank, the shank having at least one cavity, as depicted in Figures 4 and Figures 5. A second general embodiment of the dental post of the present invention consists, in addition to a shank, a retentive head 18, see Figures 6. When a dental post of the

second general embodiment of the present invention is installed in a tooth, retentive head 18 is not inserted into the root canal, but rather protrudes therefrom so as to form a convenient base for the attachment of other parts of a dental prosthesis, namely a core. It is most advantageous that a retentive head be of a size and shape so as to act as a core and allow direct attachment of a crown without necessitating a separate core. In the art many and varied shapes of retentive heads are known.

In Figures 6 are depicted three dental posts of the present invention with retentive heads.

In Figure 6A is depicted a dental post with a box-shaped retentive head 18a. In the dental post depicted in Figure 6A parallel-walled circular cross-section cavity 10 extends from the top of head 18a through apical end 12 of shank 14.

In Figure 6B is depicted a dental post with a cubic retentive head 18b. In the dental post depicted in Figure 6B parallel-walled hexagonal cross-section cavity 10 extends from the top of head 18b through apical end 12 of tapered shank 14, shank 14 having a rounded-cone shape.

In Figure 6C is depicted a dental post with a cylindrical retentive head 18c. In the dental post depicted in Figure 6C not-parallel-walled circular cross-section cavity 10 extends from the top of head 18c to roughly 2/3 the length of shank 14. In the dental post depicted in Figure 6C, cavity 10 does not extend to apical end 12 of rounded-cone shaped shank 14.

It is clear to one skilled in the art that the dimensions and surface features of a dental post of the present invention are determined by the material and method used to fashion the dental post and by required performance parameters, such as strength, flexibility and retentivity.

A dental post of the present invention can be fashioned from any of the materials known in the art to be useful for dental posts. Included are medical-grade metals, preferably, pure titanium and alloys of titanium (such as titanium 6AL-4VD), stainless steel, cobalt chromium, nickel chromium, gold, platinum-iridium alloys, palladium and palladium alloys.

The methods use in fashioning a metal dental post of the present invention is preferably fashioned by methods such as FFM, metal injection molding, extrusion, centrifugal casting, precision casting, machining and combinations thereof.

Included materials are also ceramics, especially ceramics made from oxides such as zirconium, yttrium, hafnium, aluminum, titanium, magnesium, silicon, cerium, boron, calcium, potassium, vanadium, sodium, lanthanum, and iron oxides. A preferable ceramic

material for fashioning a dental post of the present invention is a combination of about 0. 20% to about 90% aluminum oxide with a zirconium oxide alloy 3Y-TZP. As is known to one skilled in the art, such ceramic materials have superior mechanical properties and are commonly used in medical applications. The exact ratio of components of the ceramic materials is determined by the relative importance of the opposing requirements of strength and aesthetics.

The methods use in fashioning a ceramic dental post of the present invention is preferably fashioned by methods such as free-forming, ceramic injection molding, extrusion, centrifugal casting machining and combinations thereof.

Further included materials are composite materials, especially where the resin of the composite material includes thermoplastics such as polyethylene, polypropylene, polysulfone, polycarbonate, polyimide, epoxy-based materials, polyester, polyolefin, acrylic, methacrylic monomer, polyolefin, polyurethane, styrene and mixtures thereof. In addition to the resins, the composite materials preferable include fibers, chopped fibers, powders and mixtures thereof made of quartz, glass, borosilicate glass, lithium aluminum silicate, barium aluminum silicate, strontium, zinc glass, colloidal silica, zirconia, ceramic materials, Kevlar (g (DuPont, USA), carbon and graphite.

The methods use in fashioning a composite dental post of the present invention is preferably fashioned by methods such as FFM, pultrusion, filament winding, braiding, injection molding, resin transfer molding, autoclave cure, press molding machining and combinations thereof.

Manufacture of the dental post of the present invention, irrespective of the material used, is most preferably performed using either injection molding or FFM.

For example, when manufactured using injection molding a dental post of the present invention can be directly fashioned including head and shank surface features as well as cavities.

Also for example, when manufactures using FFM, a dental post of the present invention is directly and accurately fashioned, including head and shank surface features as well as cavities. Specifically, as each succeeding powder layer is fused, areas of powdered layers in locations and areas destined ultimately to correspond to the cavity are not fused.

Upon completion of assembly and fusing of the entire dental post, non-fused powder is removed, for example by suction.

Clearly most advantageously, a dental post of the present invention is fashioned using the FFM method of the present invention.

The method of manufacture of a dental post of the present invention having a retentive head is dependent on the material from which the dental post itself is made. For virtually all materials used, it is possible to fashion a large dummy from which the shank and retentive head are machined in the usual way. When the dental post is made of cast or molded materials, or using FFM, it is advantageous to fashion the shank and the retentive head directly.

It is clear to one skilled in the art that fitting and use of a dental post of the present invention is substantially identical to fitting and use of a prior art dental post and needs not be discussed hereinfurther. As noted hereinabove, fitting can be accomplished by a variety of adhesives or by mechanical methods such as screwing the dental post into the tooth itself.

A first embodiment of removal of a dental post of the present invention is depicted in Figures 7. In Figure 7A is depicted a dental post 20 of the present invention embedded in a root canal 22 of a tooth remnant 24. In Figure 7A, the crown and core attached to post 20 have been removed, for example by cutting or sawing. Exposed is a cross section 26 (shaded for clarity) of dental post 20 and a cross section 28 of cavity 10. In Figure 7B, exposed cavity cross section 28 is used as a guide for a bur 29 attached to dental drill 30 (or other such device) to ream out post material, as well as adhesive remnants without damaging any tooth tissue.

A second embodiment of removal of a dental post of the present invention is depicted in Figures 8. In Figure 8A is depicted a threaded dental post 32 of the present invention embedded in a root canal 22 of a tooth remnant 24. In Figure 8A, the crown and core attached to post 32 have been removed, for example by cutting or sawing. Exposed is a cross section 26 (shaded for clarity) of dental post 32 and a hexagonal cross section 28 of cavity 10. Also seen are threads 16 on the shank of dental post 32.

In Figure 8B, exposed hexagonal cavity cross section 28 is engaged with a removal tool, in Figure 8B an Allen (hex) wrench 34. The tip of wrench 34 is inserted through exposed cavity cross section 28 and rotated so as to engage the inner walls of cavity 10 causing rotation of dental post 32. Due to the action of threads 16 during rotation, dental post 16 is extracted from root canal 22. Release of threads 16 and dental post 16 from adhesive present can be performed using moderate vibration of dental post 16 with the help of an

ultrasonically vibrating probe tip inserted into cavity 10 through exposed cavity cross section 26.

It is clear to one skilled in the art that one advantage of a dental post of the present invention arises from the length of the cavity. Wherever the shank of a post is cut, a cross section of the cavity is exposed that is suitable for simple removal of the dental post in its entirety.

It cannot be guaranteed that the dentist who installed a dental post of the present is the one who removes the same dental post. Further, it is possible that the dental records of a patient not be available when it is necessary to remove the post. It is thus preferable that there be a way to mark the cavity of a dental post of the present invention. A dentist preparing a patient for removal of a dental prosthesis can interrogate the mark and accordingly plan removal of the dental post of the present invention.

One method of marking a dental post of the present invention is to fashion the dental post from a material or to impregnate the dental post with a material that is radio-opaque, as is well-known to one skilled in the art. By radio opaque is meant that the material appears distinctly in a dental imaging device, most commonly an imaging device based on the projection and detection of X-rays. Advantageously, the dental post itself is radio-transparent, and the cavity is filled with a material that is both radio opaque and does not interfere with the removal function of the cavity. Such materials are well known in the art and include suspensions or slurries of radio-opaque substance in relative soft matrices such as gum, rubber or resins. The cavity can be filled with the radio-opaque material before or after fitting of the dental post in the tooth of the patient. When filling is performed after fitting, the radio-opaque material is preferably substantially liquid and introduced into the cavity in the usual way, for example with the help of a syringe and needle. One skilled in the art is well acquainted with radio-opaque materials suitable for use as described hereinabove.

The use of light curable adhesives in attaching dental posts is known in the art, see for example U. S. 5,073, 112 or U. S. 6, 282, 013. A light-curable adhesive is put in a prepared tooth, and a dental post, fashioned from a transparent or light-directing material, is put in place and excess adhesive removed. A light source is used to illuminate the adhesive through the dental post. This is a time-consuming process and may lead to unevenly cured adhesive.

The cavity of a dental post of the present invention can be used to overcome this disadvantage. Light-curable adhesive and a transparent dental post of the present invention are installed in a usual way. A light source of proper dimensions, such as a fiber optic guiding

light from a dental laser or other source of light is then inserted into the cavity. It is clear to one skilled in the art that curing of light curable adhesive in this fashion is preferable to light curing through a prior art transparent or light-directing dental post.

It is important to note that included in the term"light-curable adhesive"are also the popular"dual-cure adhesives", that is adhesive mixtures which only partially or of which only one of a plurality of components actually cures under illumination.

After the light source is removed, the cavity can be filled with radio-opaque material as described hereinabove.

The present invention is not limited to the embodiments described herein but also relates to all kinds of modifications thereof, insofar as they are within the scope of the claims.