CA1157299A - Method for the manufacture of dentures and device for carrying out the method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

For the manufacture of a denture, or artificial teeth, the prepared tooth stump is reproduced as a working model, and the substitute piece, suitably conformed to the working model, is shaped by hand. In order to facilitate the dentist's work, and at the same time attain great accuracy, it is provided that reproduction of the tooth stump is accomplished electro-optically in such a manner that reproduction-information signals corresponding to the tooth stump are generated and are converted in an electronic com-puter into electrical and/or magnetic control singals with the aid of which a milling machine automatically mills the working model out of a block of material; and/or that the positive substitute piece is milled by the milling machine from a block of material in accordance with control signals which are derived from reproduction-information sign-als which are electro-optically derived from the hand-shaped substitute or replacement mode. The device for carrying out the method is characterized by an optical reproduction-photographing device for sweeping over the portion of the tooth in question; furthermore, by an electronic computer for sensing and evaluating the reproduction-information signals generated by a reproducing device, a milling machine, and/or if necessary, a storage apparatus.

Description

115~2~9 ~ he invention relates to a method for the manu-factuxe of ~rtificial teeth or dentures in which the prepared stu~p of a tooth or teeth is reproduced in a working model, and the substitute piece, adapted by hand to correspond to the working model is formed, and also relates to a device for carr~ing out the method.

Numerous methods of the above-cited type are known for the manufacture of artificial teeth. The ability to pro-duce an exactly-seated crown, as an artificial tooth, depends on control of the copying technique. The three most common methods are copying, taking an impression with silicone or rubber compositIon, or with hydrocolloid, as well as with a copper ri~n~ and moldin~ material. The dentist's work and education are made difficult by the fact that he must uncon-ditionally command a number of methods for taking an impres-sion. Also, for taking an impression, the elastic material, for example, the gums, must ~e drawin back sufficiently that the la`ter-prepared teeth or tooth stumps will be grasped by it. Retraction of the soft part requires a not-significant expenditure of la~or.

Furthermore, in cases in which several crowns are required in the front area, it is important to prepare the teeth and then provide them with a well-seated temporary substitute. With the aforesaid measures, several sittings and impressions are necessary.

A new or impression method is a method using cop-per rin~ and molding material. The drawback o~ the materialutil~zed cons~sts ~n the fact th~t, with it, an impression of the ent~Xe row o~ teeth c~nnot be taken at the same time, an impress~on o~ unde~neath parts cannot be taken, and the fai~rly waxm molding materi~a~l can lead to irritation of the pul~, Por~at~on ~nd ~nsertion o~ the ring must be done with extraord~nar~ caution in order that t~e soft part will not be ~' t 1 .572~

damaged. In the case of multiple supportin~ (or pillar~
teeth, takin~ an impression i5 very time consumin~.

~ second method is performed with rubber composi-tion material. In the last fifty years, three types of elastic or impression material on a rubber base have been developed w~th polysulfide, silicone, and polyether rubber.
Silicone rubBer and polyether rubber do indeed have a better color and a more agreeable taste and are more acceptable from the esthet~c point of vie~r but they are not utilized as fre~uentl~ as polysulfide ruh~er.

~ith the use of elastic-rubber materials, the ex-penditur~ of time is great. However, the principal drawback is that the retraction of the soft parts and complete dryness of the area in question are necessary, since the molding material can be shifted out of position very easily.

In a series of experiments by Ferguson and Strode with rub~er-composition lmpressions, 10% of 250 impressions has inclusions of air below the surface, which, after the pressure is released, cause expanding depressions in ~he matrix and protrusions on tfie crown. Premature removal of pressure is a frequent cause of failures. The time between the beginning of mixing and the taking of the im-pression should amount to at least ten, better fifteen, minutes. Here, however, errors and false measurements can take place ver~ easil~.

3Q A thi~rd known metfiod is performed with reversible h~drocollQid Th~$ ~mpressian material has been utilized s~nce a~out 1~25~ It ~ec~mes flui~d upon being heated and rQturn$ to ~ts ~el st~t~ afte~ coolin~ Qff. The principal draw~ha~ck ~ h~drocoll~d ~ the nece$si~ of makin~ or cast-in~ of the ~mpres~Qn immed~atel~, I ~ 5~29~

Since hydrocolloid does ~ot displace the gums, retraction of the soft part and meticulous drying are indispensable for accuracy of reproduction o~ the marginal area. The di~ficul~
ties and the expenditure of time re~uired here are well understood.

The following guidelines must be followed in order to guarantee accuracy of the impression: The spoon material must first be tempered in order that it will assume a certain jelly-like consistency. In that way, the viscosity will be increased, and temperature shock to the teeth will be reduced.
In addlt~on, the impression spoon must have a layer of material three mm. tfiick occlusally and laterally, in order that sufficient accuracy will be assured.
Aside from the unpleasant odor of the rubber composition material, thi~s method is correspondingly difficult, time-consuming and complicated. At least five minutes are required for an impression o~ the entire row of teeth. In order to avoid alterations of form, tAe model must be cast within fifteen to forty minutes. The difficulties of the known methods for manufacturing artificial teeth, which are not conceiva~le without the impression method, are recognized.

However, still further difficulties arise in con-nection with temporary measures. Every patient is concerned about h~s appearance after preparation for the crown has been completed. The old rule that a temporary or provisional substitute does not have to be aS beautiful as the permanent 3Q substitute is plainly ~alse~ ~ell-made and meticulously-fitted subst~tute shapes fulf~ll a functional and an estethic purpose.

The main p~o~lem of an~ provisional substitute shape is Qpt~al protect~on fo~ the tooth and the tissue surroundin~ ~t ~or the time until the final substitute is finished. The temporary substitute offers ~he dentist the possi~ility of making it exactly clear how the final sub-stitute must appear wIth regard to its shape and size as well as color, to a certain extent. Besides, the temporary sub-sti~ute makes i~ possi~le to modi~y the provisional arrange-ment before the final substitute is manufactured. Al.so, the provisional substitu~e protects the pulp from chemical, ther-mic, and mechanical influences. One of the older methods, still in use today, is the alginate technique for manufactur-ing a temporary acrylate su~stitute.

Even with the temporary substitute, at least oneimpression must first he taken for producing the working model of the prepared stump of the tooth; then a substitute model must be prepared by hand from wax or the like, and the temporary substitute must be modeled with its aid.

Therefore., the problém of the invention is the improvement of the known method for the manufacture of den-tures with the aforementioned features and measures, as wellas the creation of a device for carrying out such a method, with whl~ch the dentist~s work is facilitated and at the same time greater accuracy is attained.

According to the invention, this problem is solved by the fact that reproduction of the stump of the tooth is accomplished electro-optically ~n such a manner that repro-duction-information si~nals corresponding to the stump of the tooth are generated and are converted in an electronic computer into electrical and~or ma.gnetic control signals, with the a;`d o~ which a millin~ machine automatically mills the working ~odel out of a ~lock of materialj and in such ~ manner that the psSitive xeplacement piece is milled out of a b.lock o~ mater~al ~n acc~rdance with control signals whi~c~ are dex~ved ~rom repr~duct~on-inform~tion signals which are der~ved electro-opticall~ from the hand-formed sub-- 4 ~

1 1 572~9 stitute model. sy means of the novel invention it is pos-sihle to reproduce the tooth stump with the adjacent maxil-lary areas, electro-optically; ~hat is, to photo~raph them, to film them, or to reproduce -them on a Roentgen plate.
Reproduction information signals can be generated from such a reproduction, which corresponds to the stump of the tooth with the adjacent areas, a photoyraph or an X-ray picture being scanned, optically, for example. Manifeastly, the distance from line to line permits extraordinarily great accuracy. Throu~h the use of the measures according to the invention, difficulties no longer arise in making reproduc-tions of the stump of the tooth accurate to one ~ . When the reproduction-information signals have been suitable generated by the corresponding scanner, they are entered into an electronic computer, in which they are converted into electrical and/or magnetic control signals. It is not neces-sary here to go into detail into the construction of the - electronic computer, for which the current state of the art it is possible to encode, process, and decode suitable infor-mation signals. In this way, electromagnetic, electrical,or magnetic control signals can be obtained which are fed into a suitable control aggregate in a milling machine, which, controlled by these control signals, automatically mills out the desired working model from a block of material.
For this, an abrasion-proof synthetic material, but also gypsum, a resin, or another desired material can be used, as long as the millin~-away is assured. Consequently, a very precise ~orking model can be produced in astonishing fashion without the cum~e~some and partially inaccurate re-production pracess.

~ lso, a pos~tiye substutute piece c~n ~e pro-duced simultaneousl~y, alternatively, or additionally through the tea~ch~ng o~ tfie inYention ~thout the impression method.
The mill~n~ ~ch~ne m~lls the pQsitiVe substitute piece--that is, e~ther the temporary substitute piece or a bridge or the like, from a block of suitahl~ material in accordance with contrvl signals, which are similarly obtained as described above. ~rhey are derived from reproduction-lnformation signals which, ag~in, are derived electro-optically, for example, by photographing, filming, X~ray photo~raphing or the like, from the hand-shaped substitute model. It is especially suitable if, in accordance with the invention, the electro-optical reproduction of the stump o~ the tooth and/or the substitute model is accomplished photogrammetrically or by means of Roentgen photographs, and if the reproduction-information signals are derived from a paper photo, an optical film, a Roentgen plate,or a video tape.

The most varied kinds of Roentgen apparatus for re-producing teeth and jawbones are known. For example, if apanorama Roentgen apparatus is improved and equipped in such a manner that the teeth appear on the Roentgen plate viewed not only fxom one side ~ut also are photographed from below, from the other side, and overlapping, on a plurality of Roen-tgen plates, then tfie method of known photogrammetry can beused, so that the Roentgen photographs, subdivided into optionally-fine raster or coordinates, can be converted into such fine reproduction-information signals that therewith a very precise spatial description of the tooth or jawbone in question is at hand.

In the same way, however, one can reproduce the stump of the tooth or the prepared denture spatially by means of photograpfiic pictures or by means of films. For this, the known method of photo~rammetry is utilized. As is known, this is a method techni~ue according to which photographic photograms can be pxoduced, altered geometrically or structu-rally, and evaluated grap~lcally or numeric~lly. The funda mental methods for photoq~a~metry were developed almost ex-clus~vely~ ~n connecti~n ~th the treatment of geodetic meas-uring~tecfin~cal prohlems~

~ 6 -~\~ ., .
~, ~ :L5729g !

Photo~rammetry has significant meaninc3 for topo-graphy, ~e~ial photograph measurements being principally em-ployed. In known manner, attention is paid to longitudinal and transverse overlapping of numerous photoyraphs made one after another in succession of a flyiny body.

If this known method is utilized in dental technique, the prepared denture can ~e reproduced in astonishing fashion with extraordinary accuracy, ~er~ rapidly, and without burden-some treatment of the patient.

In accordance with the invention, lt is suitableif the electri~cal and~or magnetic control signals are stored intermediately. The evalutated reproduction-information signals, which can then be used directly, to control the milling machine or indirectly after the intermediate storage, for re-selection, make possi~le the placing for spatial ima-ges with greates accuracy in the smallest space. In passing, so to speak, the dentist can hold his patient's denture fast spatially and very accurately, feed the prepared signals to a storage device, and then recall them at any time. Also, the economic si~nificance for dental-technlcal laboratories is recognized. Expense for material is smaller through the lack of many molding materials, less working force is used for modeling, and yet the dentists achieves substantially better accuracy so tHat a set of teeth produced by him--a bridge extendin~ over a number of teeth, for example--can be accurately and immediately fi-tted into the patient's prepared dentrue and cemented in.
3~
~ccordin~ to the ~nvent~on, it is of further ad-vanta~e ~f the xeproduct~on-~nfor~ation signals can be ev~luated ~o~ optic~l-y~ual ~epxoduction on a video display app~atu~. B~ mean$~ of techni~ues kno~n in the field of electron~c~s~, ~t ~s pos~s~file to process the reproduction-info~mation signals ohtained throu~h the aforementioned ~ 1 57~99 !

measure into electrical and/or magnetic data, which c~n be optically visible on a video display apparatus without any-thin further. These measures facilitate the preparation of the patient' 5 denture for the dentist, because without ex-pense of material for molding materials, and without tediousen~agements disagreeable to the patient, for hand-grinding between en~agements, he can always ascertain and check the re~l state of affairs from all perspective, so that he can dispense with inspection with mirrors in the so-far customary manner as his sole aid. Th~s method is especially useful when a number of teeth next to one another have to be pre-pared. Then he can set up various enlargements on the video display apparatus and check the parallel positioning, for example, which up to the present time he has been able to do only by eye measurement. The known method was inaccurate and time-consuming and in simple fashion becomes more ac-curate and quicker by means of the new method.

Furthermore, the invention is advantageous because various reproductions are shown simultaneo`usly, optically visible, on the video display apparatus for purposes of comparison. Operating according to this method is especially serviceable for protecting the substance of the tooth, in order that irreparable failures will not originate as a re-sult of the substance of the tooth being eroded in the wrongpositions. For example, the ideal state of a prepared tooth or portion of a denture can be preprogrammed and stored in a computer. The dentist can deduce this ideal state from a photograph of the actual state of affairs if he allows for the desired ~rind-of~ of 1.8 or ~.8 mm., or any other amount o~ grind-off necessary~ Then, in the processing in between time$, checks can alway~s ~e made ~ Qn a monitor, the actual state and the ideal theoretical state being projected one a~ove the other on the projectiQn scxeen, so that the dentist immedi~teIy~reco~n1ze at which points tooth substances must ~ 8 --~ ~ S~299 he removed and at which points not. Regulations of the denture can ~e promoted In this ~ay, and a denture of twenty teeth i$ ~ossible simultaneo~sly with greatest accuracy ~nd w~thout dan~er.

The device ~or carrying out the method is charac-terized by an optical reproducing ~BM device for sweepin~ over the tooth parts concerned, and further by an electronic computer for sensing and evaluating ~he reproduction informa-tion signals ~enerated b~y the reproducin~ device; by a mill-ing machine, and~or, if necessary, by a storage apparatus.
The use o~ these known apparatuses in this sequence and for the aforement~oned purpose is surprising. The dentist can make use of a generall~-known technique and, with economical-ly-replaceable means, acfiieve results whose equal has been sought for up to the present time w-;th regard to rapidity, accuracy, agreeable treatment of the patient, etc.

It is of special advantage if, according to the invention, the optical reproductive device has a holder in the form of an impression spoon for a plurality of lenses, to whic~ pfioto-conductive ca~les are secured, at the other end of which cables a source of light and a recording device are mounted with which an electronic sensing device is con-nected; and if a computer for evaluating and/or storing the reproduction-information signals is connected. The other alternative for the reproducing device--namely, the Roentgen panorama photographing apparatu~--has been described above, it has to be supplemented in such a manner that the tooth part in question c~n be viewed from at least three sides.
Only suitable motor driye~ need be pxovided with the aid~
of which Yarious Roentgen photog~aphs, mutually overlappin~
and d~ected ~rom all s~des Q~ th~ s~ace onto t~e stump Q~ the toot~, ~re produced~

1~572~9 The other em~odiment with the purel~ optical photogr~phy or with films, can be constructed particulaxly advanta~eously and easily ~y inserting photoconductive cahles likewise ~ade of known ~ibrous material. With their aid, it is possi~le without an~thin~ further to bring the lenses into their place in such an area near to the teeth to be photographed ~hat the entire upper or lower denture is photo-graphed, for example, with one photograph. Then it is ad-vantageous if an extinguishing device is connected to the optical reproducing device. It serves as a timer and wipes out various photographs in the manner of known photogrammetry.
Here, also, the ima~es are appropriately arranged to be over-lapping.

By means of the measures in accordance with the invention, the more or less defective natural tooth crowns can be built up again in their original form, or occasionally can even ~e produced in a functionally still more favourably form. With the technique described, not only can the work-ing model be produced, but also the temproary substitute can be built up rapidly and accurately. The latter serves to be placed upon the stump of the tooth as a protection against chemical and physical in~luences, against acids and sugar, heat and cold, for example.

The metal jacket crown finds employment as a pro-tective crown, as a replacement crown, or as a supporting element. Here also the avorable e~fect of the measures in accordance ~ith the inVentiQn becomes evident: prepara-t~on o~ the tooth is accomplished in known manner by remov-ing at least Q~4 to Q.5 mm. ~r~m the tooth as coverin~
thicknesss of the croWn, ~lternati~ely, jacket crowns made of poxcelain ~r plastic re~uiXe a step-b~-step preparation of the tooth. In an~ case~ the stulnp o~ the tooth is pre-pared, and then xeproduced ~ the steps in accordance withthe inyention. The replacement piece, as the positive, is - lQ ~

modeled by hand on the workin~ model from well-modelable wax. W~en the substi-tute piece i5 finished, it likewise can ~e electro-optically detected and s-tored, possibly in three dimensional form, wi-th great accuxacy, so that a mill-ing machine can mill the temporary substitute, or even thereplacement piece, from wax with -the aid of control signals, which are previously supplied to the milling machine. Their situation is governed in accordance with the pxepared den-ture portion, but the latter, ~y means of the above-described measures, is already present data-wide in the form of coded control signals stored on, for example, a magnetic tape or punched tape.

Further advantages, features, and possiblities Of applicati~on of the instant invention will be apparent from the ensuing description in connection with the drawings.

F~g. 1 shows, perspectively, a working model with a bridge placed thereon;

Fig. 2 shows, likely perspectively and schematically, a reproducing dev~ce with photo-conductive cables;

Fig. 3 shows, schematically, the arrangement of the entire device;

Fi~. 4 shows, perspectively, a ground-off stump of a tooth;

Fig. 5 shows, in perspective, a modeled replacement piece made of wax and made, of course, for another tooth stump, not for that according to Fig. 4, and;

Fig. 6 shows, in perspective representation, the placing of a rather large long bridge on a w~rking model in special parallel construction.

1157~9 in special parallel cons-truction~
To make clear the device for manufacturing artifi-- lla -1 ~S72~9 cial teeth, a preferred embodiment will be described helow with the aid of the phot~graphic method according to phols-gram~etry.

Figure 1 shows, in perspective, a working model 1 o~ a lower jaw on which a bridge 2 is placed at front right.

Figure 2 xepr~sents, in perspective, the optical reproducing device, designated generally as 3, which has a mounting 4 i~n the form of an i~pression spoon, which fits directly over the lower jaw according to Figure 1. By means of a spoon handle 5, shown broken off, this support 4 is turned upside down over the row of teeth, with the adjacent areas. At fixed distances over all surfaces, a large number of schematically-represented lenses 6 are secured to the support 4, o~ whic~ only four lenses 6 on the horseshoe~shaped surface and three lenses on the U-shaped adjacent surface are represented i~n the representation of Figure 2. However, it goes without saying tfiat suitahle mountings for lenses 6 are provided from the ins~de--that is, from the roof o~
the mouth--as well as on tHe rear side. The lenses are optically prepared in such a manner that mutually-overlap-ping images of the jaw area with teeth situated directly in the vicinity are formed, so tha~ thus the lens 6 produces a picture which overlaps, at least partially with that of the lens 6'. This rule holds good also for all the other lenses.

~hoto-conductive cables 7 lead from the lenses 6, 6I thxough a collecti~ye cable 8 to a device, represented schematicallY In Fi~ure 3 and designated ~enerall~ at 9, xn which a se~i ~efle.ctor suX~ace 10 permits passage-through o$ a poXt~on o~ ~ays 11 and a poxt~on of ra~s 12. The li~ht souxce 13 generates xa~s 12 of l~ght, which pass through the se~i~reflector sur~ace lQ and assure i~llumination of the pert~nent portion of the toot~.w~th the aid of the lenses 6, 6'. The light c~mes ~rom the illuminated tooth po~tions through the photo-conductive cable 7 from the de~ice 9 into the photo-apparatus 14, whose shutter is actuated by a relea~se device 15. In this way photographic reproductions originate, which are represented schematically at 16 in ~ig. 3. They are sensed hy the scanning device 17 to gen-erate reproduction-information signals, which then are sup-plied to the electronic computer 18. In the latter, the reproduction-information signals correpsonding to the tooth stump in ~uestion are converted and arrive as electrical or magnetiC control signals, either directly over the path 19 or indirectly, after being stored, over the path 20, in themagnetic tape 21 or the punched tape 22 and then, over the path 23, in an automatic milling machine 24.

Solely by way of illustration, Fig. 4 shows, in perspective, a prepared tooth stump which has a liabally-ground-in cervical shoulder.

In Fig. 5 is shown a wax positive modeled for another form of tooth stump. A plaster-of-paris container is formed around the latter in known manner; the way is burned out; and then metal, preferably gold, is filled into produce an identlcally-shaped molded article, as shown in Fig. 5.

I~ accordance with the in~ention, milling from wax or a temporary sufistitute can take place without any-thing further with the a~d o~ the electronically-controlled mill~n~ machi~ne, wh~ch rece~ves the control signals from a co~putex ax ~r~m a ~to~ge dey~ce into which they have heen ~ed ~n the ahaye-de~scribed mannex, namely, b~ Roentgen phot~g~aph~ or h~Y photog~a~etr~.

~nally, Fig. 6 sh~w~, ~n perspective, a working ~ 13 ~

1~5729~

model 1 in which th~ bridge 2 is to be set in place ~irst.
Two tooth stumps 50 and 51 will be recognized, which had to be prepa~red previously, precisely parallel, in correspond-ance to the dashed lines and in correspondence to the arrow 52. This parallel arrangement of the posts is nece-ssary in order to make possible the incorporatlon of this multi-member bridge. The greater the number of posts, the more dif~icult it is to attain parallelity of all anchors.
In accordance with the invention, however, this parallelity lQ is attained rapidly and very accurately by the above-described measures, through the fact that the reproduction~information signals for an optically-visi~le reproduction are evaluated on a video display apparatus. With suitablP enlaryement, the parallelit~ can be checked at any time and very accurate-ly during the preparation. In order to protect the substanceof the tooth.and not to damage the pulp, the exact thickness of material which.is required to set the bridge 2 in place can be removed from the tooth b~ the simultaneous represen-tation, for comparison purposes, of a previously-given theo-2a retical photograph with the actual photograph.

Work in the treatment room can be carried outrapidly and precesely by use of the measures in accordance with this invention. All that is needed there is the re-producing dev~ce 3, the photo-conductive cable 7, the device 9, the light source 13, and the photo apparatus 14; also possible the release (or extin~uishing) device 15.

The photogxaph.y or ~ilm ox Roentgen plate desi~ned 16 in Fig~ 3 can then ~e eyaluated~ encoded, and fed into ~ ll~n~ machine.~ ~.hich th.en ~ills the desixed working model or ~he denture fxom the b.lock Q~ m~ater~al provided.

ThQ ~nvent~on ~lso make.s storage or archiYing pos-sible ~n ~.yex~ space-sav~n~ ~nner~ so that all data which a dent~st re~uires relati`ng to his patients are available 1 15~29~

for retr~eyal at any time. For example~ in thi~ ~a~ it is possihle to make an archive (or record~ of the young~ undam-aged bite of a patient b~ means of photo~rammetr~ and then later make a denture if necessary. Also, such a comprehen-sive ~rchive is of great significance in criminolog~.

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Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for the manufacture of dentures in which the prepared tooth stump is reproduced in a working model, and a denture suitably conformed to the working model is formed, comprising means forming an optical reproducing device for sweeping over a tooth or tooth stump and provid-ing an electrical output signal of three dimensional surface information coordinates, a computer for receiving said out-put signal, and a milling machine operated and controlled by said computer for forming a three dimensional reproduction of said tooth or tooth stump in accordance with said surface information coordinate signals.

2. The apparatus of claim 1, in which said optical-recording means includes a holder in the form of an impression spoon and a plurality of lenses in said spoon, a photo-con-ductive cable having terminal ends associated with said lenses, a light source, a means for applying light from said source to the other end of said cable, means collecting the images from said lenses through said cable and applying the same to a photographic camera device for forming a film image thereon, and a scanning device for scanning said image and generating said surface information coordinate signals to be applied to said computer.

3. The apparatus of claim 1, in which said light apply-ing means and said collecting means is a half-silvered mirror.

4. The apparatus of claim 2, in which said lenses are positioned to form mutually partially overlapping images.

5. A method useful in the manufacture of dentures in which a prepared tooth stump is reproduced in a working model and a final denture or a temporary replacement piece is suit-ably conformed to the working model, including the steps of:
providing electro-optically produced three-dimensional sur-face information corresponding to the tooth stump, converting said electro-optically produced information corresponding to tooth stump by a computer into coordinate control signals, and operating a milling machine automatically from said con-trol signals to reproduce the working model of the stump from a block of material.

6. The method of claim 5, further including the steps of forming a replacement piece from a working model by providing electro-optically produced surface information corresponding to the surface of the working model, converting said electro-optically produced information corresponding to the working model into coordinate control signals by the computer, and operating the milling machine automatically from said coordi-nate control signals of the working model to produce a re-placement piece from a block of material.

7. The method of claim 6, in which said replacement piece is a temporary replacement piece.

8. The method of claim 6, in which said replacement piece is a permanent denture.

9. The method according to claim 6, in which the working model is reproduced from surface information derived from a permanent record of surface information of the patient's original tooth configuration.

10. The method of claim 9, in which the surface infor-mation is derived from Roentgen photography.

11. The method according to claim 5, 6 or 7, in which a permanent reproducible record is made of the control signals applied to the milling machine.

12. The method of claim 5, 6 or 7, further including the step of providing a video display of the surface informa-tion corresponding to the working model for the purpose of comparison.

13. The method for the manufacture of dentures in which a prepared tooth stump is reproduced in a working model, and a denture or replacement piece is suitably conformed to the working model, comprising the steps of photogrammetrically deriving surface information of the desired tooth configura-tion, converting said photogrammetrically supplied surface information to control signals for the operation of a milling machine, and milling the denture or replacement piece from a block of material in accordance with said control signals.

14. The method according to claim 12, in which said photogrammetrically reproduced information is retained in a permanent record such as a magnetic or a video tape.

15. The method according to claim 12, in which said photogrammetrically reproduced information is obtained from a permanent record of the patient's original teeth.

16. The method of claim 12, further including the step of providing a video display of the photogrammetrically pro-duced surface information for the purpose of comparison.