GB2183362A - Mirror for ascertaining parallelism for dental purposes - Google Patents

Abstract

A device is provided for ascertaining the parallelism of flanks of rows of teeth which are to be prepared when working to produce a dental prosthesis. The device comprises a mirror 12 having a concave face 13 mirrored at 14 towards its concave side. The contour of the mirror is oval and larger than the arc of the row of teeth to be observed and is narrower at its broadest part than the maximum width to which the corner of the mouth can be stretched. The concave surface is part of a paraboloid of revolution such that the rays of light emanating from the row of teeth parallel with the geometrical longitudinal axis are thrown on the eye as a converging beam. <IMAGE>

Description

SPECIFICATION Device for ascertaining parallelism for dental purposes The invention relates to a device according to the preamble to the Main Claim.

One ofthe most important prerequisitesforthe later problem-free insertion and removal of the completed replacementdenture is the paralleleism ofthe prepared abutmentteeth.

In the case of a simple pre-moulded crown, partial crown or moulded filling, non-parallel preparation surfaces can lead to considerable difficulty with insertion.

It goes without saying that these difficulties are quite substantially increased with the increase or number of rigidly interconnected abutment teeth for fixed or removable dentures.

In this connection the word "parallelism" is frequently used in the dental literature. However, it does not mean a parallelism in the mathematical sense. Instead, it means that the prepared tooth surfaces have no undercuts and that-where a plurality of abutmentteeth are involved -the direction of withdrawal ofthe replacement denture is the sameforall abutment teeth. This avoidsjamming.

The fact that the preparation faces aligned parallel in this sense impose exacting demands on manual skill and visual efficiency of the practitioner emerges from the fact that in the past numerous attempts have been madeto achieve parallelism by mechanical aids or at least to facilitate their manufacture.

Preference to parallelism in the above-mentioned sense are to befound in the article by Scherentitled "Parallelityfrom APEX-June 1970". In thatarticle,wax models are recommended and furthermore the use of steel plates is described. Then, measurements are recommended which are inappropriate to practical requirements. Neither the device nor the method succeeded in becoming established and have remained the state ofthe art but onlyon paper.

Described in the Dental Review, No. 12/1966, page 484, is an "Aid to parallel configuration This is a spirit level-type device. In this particular instance, the spirit level was connected to a terminal strip through a lockable ball and socket joint. Theterminal stripwasthen applied in the rear hand support zone oftheangle piece. Then, the grinding cylinder had to be applied to a reference surface of a tooth stump and the spirit level adjusted accordingly. For preparation, the mechanic was intended always to hold his grinding device so that the bubble ofthe spirit level was always in the middle. This is naturally more an artistic arrangement than a modus operandi which can be carried out in practice.It is not reliably possible accurately and simultaneously to view two remote reference points while at the same time moving the handpiece. Furthermore, the entire preset reference plane is no longer true ifthe patient moves his head somewhat.

Described in the Journal of Prosthetic Dentistry - January 1969 - pages 79 to 85, is a parallelometer which is astounding in its complexity but which is completely inviable in practice. For example, the question arises here of whether such an instrument is in fact sterilisable as it has so many joints, screwthreads, etc.

The magazine "Zahnarztliche Praxis" (Dental Practice) - No.5/1985-pages 195 and 196, draws attention to thefactofhow important is parallel alignment ofthe tooth stumps. In the left-hand column on page 195,first paragraph, the surgical implants are described forthe application of future processes andtheirdisadvant- ages are outlined. Then, a bettersolution is proposed, although this also works with a model. Special transfer plates of metal are required. Instead, devices made from titanium alloys are used which have special holders.

The parallel orientation ofthe root stumps is carried out on the model in the laboratory. The special case of surgical implantation cannot readily be used for conventional preparation technique in the mouth in orderto produce crowns and bridges. This latest solution, too, is complicated and expensive.

The problem on which the invention is based is to indicate a device according to the preamble to the Main Claim by which it is possible cheaply and accuratelyforthe userto establish whetherthe surfaces are "par- allel" in the sense mentioned atthe outset.

According to the invention, this problem is resolved by the features shown in the characterising part ofthe Main Claim. The term "oval" must not be understood here in its mathematical sense. Instead, it is intended to mean what can be normally regarded as "oval" in that it may, for example, also include an ellipse.

The form which rows of teeth may adopt can be derived from series of investigations. Viewed from the dental aspect, the curve in the upper jaw is oval while in the lower jaw it is more parabolic. It is also known from series of tests how broad the arcs are at their broadest point and how long they are. In this respect, there is no need to pay much attention to the bite of small children because it is only in very rare cases thattheir mouths require prosthesis. Nevertheless, if it is desired to carry out such dental work, then mirrors are provided for various sizes of bite, one single size of mirror being capable of coping with 90% of patients.

The term "clearfield of view" is a terminus technicus. At a distance of 100 mm and subjectto normal visual acuity, children see quite sharply. It is well-known that the increasing age, the long-sightedness of the aged may mean 500 mm or more. In optics, when designing instruments such as, for example, telescopes, cam- eras, etc., the premise adopted is that of a standard clear field of view of 250 mm. Since hereinafterthe clear field of view is indicated as variable, it would be readily possible to indicate the form of mirror according to the invention also for users who suffer markedly from the sight problems linked with advancing years.

The mirror according to the invention is an object which can be manufactured very cheaply and which is self-exaplanatory. It does not burden the patient, requires no special storage place, is hygienic and easily sterilised, even together with other instruments, hardly occupies any space in the autoclave and can be used not only for looking at the teeth but can also be used to reflect the necessary light, which was not so far possible with any other instrument employed for parallelism purposes. The mirror is suitable for the upper jawjustas much as it isforthe lowerjaw. No assembly nor dismantling is required. It can also be used for depressing thy tongue and provides a large area image ofthe pharyngeal space to permitoflarge-area comparisons.The eye is very sensitive to parallelisms ofthis kind and to the unskilled it is also possibleeasily to establish whetherthere is parallelsim or non-parallelism.

The features according to Claim 2 provide a formula which is sufficient both forthe conditions ofthe characterising part of the Main Claim and which also can be used if, for example, the mirror or its moulded shape or injection moulded shape is to be produced by rotational methods, for example byturning operations.

Thefeatures according to Claim 3 provide aformula which is adequate both forthe features ofthecharacterising part ofthe Main Claim and which can also be used ifthe mirror or its injection moulded form isto be produced on a numerically controlled machine such as, for example, a CNC machine. Where these mach- ines are concerned,the premise adopted is a plane which in the present case is the x"-y" reference plane. The two reference lines indicate that this formula has evolved from the preceding one, via double co-ordinates transformation.

Should second degree reflections occur on thefront of the mirror,then these arevoided by thefeature according to Claim 4.

Byvirtue of the features according to Claim 5, the mirror becomes smallerand it is no longer possibleto use itto gain access to the pharyngeal spacewhich is not of interest with regard to inspecting teeth.

The features of Claim 6 make it possible if necessary to use the middle portion to depress the tongue and the large radii mean thatthe riskofinjuringthe corners of the mouth orthe pharyngeal space is nullified.

By reason ofthe features according to Claim 7, there is no need to intervene in the mirrorandthe bowl portion can possibly be used to press away a lip which may be behind it.

The user can employthefeatures according to Claim 8to hold the mirroratthe correctangle ofinclination.

If the mirror is used frequently and/or if one has an eye to angular positions, than an experienced user need not use the aiming device at all. For this reason, it is ideal ifthe aiming device can be made removable, so that many users can work initially with the aiming device and then, if possible, may also omit it. Handling the mirrorgives one afeeling forthe correct hold just as, for example, Press photographers will in course oftime have a feeling forthe correct angle of inclination ofthe camera without needing to lookthroughtheviewfinder.

The features according to Claim 9 make aiming simple. The aiming device need only be provided once,and furthermore it provides the least disturbance centrally.

Byvirtue ofthefeatures of Claim 10, an easily manufactured and non-disturbing aiming device will be provided.

Aiming is very easy if one uses a standard or reference shape according to Claim 11.

Even for unskilled users,the features of Claim 12 make it possible to keep the eye atthe distance on which calculations are based. If the stem is removable, then forthe firstfew uses, it can be made long enough for practice purposes. However, the length makes the device somewhat bulky both to use and also to store and also to disinfect.

The features according to Claim 13 mean thatthe device is cheaper and less bulky after a very short practice time or after none at all.

Fora userwith a normal three-dimensional feel, a stem with the features according to Claim 14will be entirely satisfactory.

The features according to Claim 15 improve the sighting facilities.

By reason ofthefeatures according to Claim 16,there is no need to provide any special visually standing out surface, which would be possible, for instance, by providing a coloured spot. Instead, it is possible to use as a fixing member a dark-coloured screw with a machine thread and then the end face will automatically have the qualities of a visually standing out surface.

The features according to Claim 17 mean thatthe mirror is both unbreakable and also inexpensive and yet is sufficiently thickthat its edges will not cut. If the mirror were made from very thin material, then every possible endeavour could be appliedto the rounding of the edges and the mirrorwould nevertheless still be capable of cutting.

The features according to Claim 18 provide an optimum value because then the patient is less required to open his mouth. For example, if oneweretoichoose an angle of around 30", then the patient would haveto open his mouth substantially more. In itself, one oughttothinkthata mirror held flat provides the least compulsion to open the mouth. This is nottrue.

The features of Claim 19 provide an optimum value insofar as here the patient is the least required to open his mouth and also viewing is very simple in that one sights parallel with the plane passing through the top of the curve ofthe rowofteeth.

The invention will now be described hereinafter with reference to preferred embodiments shown in the accompanying drawings, in which: Figure 7 is a front view of the device according to the invention, together with a centimetre ruler; Figure2 shows a rear view ofthe device according to the invention, together with a centimetre ruler; Figure3is a sectiontaken onthe line 3-3 in Figure 1; Figure 4 is a section taken on the line 4-4 in Figure 1,the device according to the invention being in the position of use, forthe case of a 45' mirror; Figure 5 is a plan view of a standard arc of lowerjawteeth; Figure 6 is a view showing how a paraboloid lies in relation to the row of teeth for a 45" mirror;; Figure 7is a viewsimilarto that in Figure 6 butfora 300 mirror; Figure 8 is a view of a row ofteeth having a reference plane which is translatorily and rotatorily displaced in respect of Figure 6 but with no change to the surface ofthe mirror,and Figure9is a representation ofthe height lines according to Figure 8 and the associated formulafordetermining the contours ofthe mirror.

A mirror 11 comprises a 3 mm thick layer 12 of synthetic plastics material which is injection moulded and so has the necessary hardness and inherent rigidity. Furthermore, it consists of a material which even under sterilisation temperatures will notdistortwithin the meaning of the surface quality of mirrors, nor become undulating, shrink or suffer such changes.

The layer 12 is of equal thickness all over, which satisfies these properties and also meets production needs. The coating 12 has a concave front surface 13 to which a mirrored surface 14 is applied so that inthe direction of view in Figure 1, a concave mirror results. In terms of its geometrical central plane 16, the mirror 11 is symmetrical. Therefore its in Figure 1 left and right-hand edges 17,18 are symmetrical and extend according to an oval which is cut off at the bottom with a straight centre zone 19. The centre zone 19 passes with large radii 21,22 into the edges 17, 18. Adjacent the upper curves 23 ofthe oval is a thumb-sized bowl 24 having upwardly a rectangularform ofwhich the edges are rounded. The bowl 24 does not need to have any other optical properties. Therefore, it can be completely flat.Nor does it matter if it continues the concavity which isto be discussed hereinafter.

According to Figures 1,2 and 4, there is atthe distance of 1 cm above the middle zone 19 a bore 26 inthe geometircal central plane 16. Resting on the back 27 of the coating 12 is the head 28 of a machine screw of which the shortthreaded shank 29 transverses the coating 12 to be screwed rigidly into a threaded hole 31 in a metal rod 32. The rod 32 has a recumbent U-shaped profile. Its middle portion 33 is straight. Its left-hand end portion 34 is angled-overtowards the 1.30 p.m. direction through an angle of 45" and incorporates threaded hole 31. The threaded shank 29 has an end face 36 which is dark in colour by reason ofthematerial from which the threaded shank 29 is manufactured.

The middle zone 33 is at an angle of450tothetangent of that portion ofthe mirrorsurface 14which isthere.

The right-hand end portion 37 ofthe rod 32 is bentoververtically upwardly, being 8 mm short in proportion to the length ofthe middle zone and is rounded off as a safeguard againstthe risk of injury. The right-hand end zone 37 has a circular hole 38 of which the geometrical central axis 39 meets the centre ofthe endface36.

The geometrical central axis 39 lies in the geometrical central plane 16. It is atan angle of to the tangent on the mirror surface 14 atthat point.

The row ofteeth 41 shown in Figure 4 is shown in plan view in Figure 5. It extends from the rear molars42to the front incisors 43. It is the row of teeth in the lowerjawwhich is shown and which extends like a halfoval.

According to Figure 4, rays of light 44,46,47 extend parallel upwardly from the row of teeth 41. Since the mirror surface 14 is a portion of a specific paraboloid as yet to be explained and which occupies a specific location, the rays of light 44,46,47 are thrown rightwards as a beam of light 48 which, in the view according to Figure 4, has an upper limit 49 and a lower limit 51,the plane ofthe lower limit5l falling into the plane 52 which is at a right-angletothe plane ofthe drawing in Figure4and which is constituted bythetop ofthe row ofteeth 41. The row ofteeth 41 shown in Figure 5 is ofthe dimensions indicated therein because this isa statistically ascertained mean.

One ofthe possible ways of ascertaining theform ofthe mirrorsurface 14 underspecificconditions of use is explained with reference to Figure 6. Figure 6 shows in cartesian co-ordinates and X-and the Z-axis.The Y-axis passes through the zero and is at a right-angletothe plane ofthe drawing. Naturally, one could equally well adopt as a premise a view in polar co-ordinates.

Figure 6 shows thetrace of paraboloid 1 1 y2 Z=500 X2+50G in the XZ plane. In the Z-axiswe see a geometrical longitudinal axis 53 which is the axis of rotation in respect ofthe paraboloid. The rowofteeth 41 has its plane 52 parallel withtheXY plane. The clearfield of view of 250 mm is shown on the left in the drawing and ends atthe left-hand end ofthe rowofteeth 41 .The mirrorface 14 is atan angle of450.This450angle is atangent angle and is applied wherethe mirrorface l4cutsthe plane of the clear field of of 250 mm. Forthis embodiment, it has been assumedthatan almost 250 mm long rod 56 is provided which is 1 mm thick so that the mirrorface 14 is 1 mm higherthanthe plane 52. On the leftin Figure 6, this in itself negligible amount of 1 mm is shown. The rod 56 has on the right an annulardiaphragm 57 which is art a distance of 20 to 25 mm from the eye 58. In this position the eye 58 is as shown in the drawing 125 mm down from the zero point in the Z-axis. If one wants to see the entire row of teeth 41, then the projected height of the mirrorface 14 is a total of 42.8 mm. This is a measure of howfarthe patient must open his mouth. Shown in the drawing above the rowofteeth 41 are the vertically upwardly emerging light beams which arethrown at the eye 58 as a bundle of beams. This view also shows that the user is capable of observing parallelism or non-parallelism on teeth in the row 41. The vast majority of persons will be ableto open the mouth by around 43 mm.

A view according to Figure 6 will also easily show the associated method of manufacture. The Z-axis is used as an axis of rotation whetherfor manufacturing the mirror surface 14 itself, for example by grinding, or for producing an injection mould forthe surface layer 12. The indicated formula shows howthetool must be guided. The number 500 is natu rally twice the assumed clear field of view of 250 mm. lfthe basis adopted is some otherclearfluid of view, then theformula can easily be amended accordingly.

It is also possible to envisage angles other than 45". A solution for a mirror which is to be used at an angle of 300 is shown in Figure 7. By virtue of the detailed explanation in Figure 6, this Figure 7 speaksfor itselfand requires no further explanation. Attention should merely be drawn to thefactthatthis drawing shows thatfor the patient it is increasingly less favourable the more one diverges from the 45" angle, because then he must open the mouth by around 51 mm, which many cannot manage. Figure 7 showsthatthe in itselfinitially evident assumption that a mirror which is held flat provides better results is inaccurate.

The general formula for a paraboloid in this manner of representation is:

andR= radius of curvature at the apex of the pa rabola.

Figure8 shows in turn the rowofteeth 41, of which its plane 52 and the mirrorface 14which is held atan angle a. To determine it, the premise adopted here is a cartesian system of co-ordinates having its origin in the bottom left area at around X = 0. TheX-axis is shown in the drawing. TheZ-axis sat a rightangleto itand starts at its origin. The plane of the drawing in Figure 8 is therefore the X-Z plane. TheY-axis is at a right-angle to the plane of the drawing and likewise starts at the origin. The X-Y planethus defined is the reference plane which may, for example, be the tabletop of an NCN machine.The resultthen is the valueZwhich must be used for guiding the tool, based on the indicated formula, the number "500" being twice the clearfield of view which is assumed to be 250 mm. The number 1000 is the quadruple thereof. The values X, Y and Z should be used in millimetres and a is the angle of inclination ofthe mirrorsurface atorigin, i.e. the angle of inclination ofthe X-Y plane in this representation.

Figure9 shows the height lines to be obtained from this formula. Figure9 shows an oval band 59 which is the projection ofthe row of teeth 51 on the mirror surface 14. From this band 59, a distance should be maintained such as is regarded as necessary having regard to the fact that the mirror 11 must also be placed in the mouth ofthe patient, with a resultant gain in the edges 17,18 ofthe mirror 1.

The invention is open to numerous variations: After sometime and with a fairsense of space, the user will no longer require the aiming device so that it can be omitted.

If the aiming device is provided, then it oughtto be made removable in the interests of bettersterilisation.

It is possible also to provide only halfthe mirror 11, i.e. the right-hand or left-hand half according to Figure 9, in factwhen, for instance, the mouth is too narrow, whetherthis isfrom birth or a condition acquired by injury.

The coating 12 can also be constructed as a light guide, e.g. at its edge or in its middle zone accordingto Figure 9, where no teeth can be seen anyway. In this way, it is possible also to project light into the mouth. It would then be appropriate to connect the bowl 24 by available techniques to a fibre optic arrangement.

If it is desired to practice with the mirror, then this can be easily done by, for example, drilling a numberof parallel holes into a plank. Theirparalellism is then, of course, guaranteed. Hold the mirror so that itis possible to look parallel into all the holes.

Claims (25)

1. Device for ascertaining the parallelism of flanks of rows of teeth which are to be prepared when working to produce a dental prosthesis, characterised in that it comprises a mirror having the following features: a) the mirror has a concave face and is mirrored towards its concave side; b) the contour ofthe mirror is oval and larger than the arc of the row of teeth to be observed and is narrower and its broadest partthan the maximum width to which the corner of the mouth can be stretched;; c) a concave surface is a portion from such a paraboloid of revolution wherein,fromtheselected angleof the mirror in the mouth and the eye ofthe observer both in the geometrical longitudinal axis of the paraboloid of revolution and also the eye within the "clearfield of view", the rays of light emanating from the row ofteeth parallel with the geometrical longitudinal axis are thrown on the eye as a converging beam.

2. Device according to Claim 1, characterized in that for a clear field of view, the concave surface follows the formula

x, y, z, so representing millimetres, z being the geometrical longitudinal axis, x extending parallel with and above the plane ofthetop edge ofthe row ofteeth, a being the angle of inclination ofthe mirrorto the plane ofthetop edge ofthe rowofteeth and so being the clearfield of (reference field of view).

3. Device according to Claim 1, characterized in thatfora clear field of view S,, the concave surface follows the formula

wherein x", y", z" and Scare represented in millimetres, the x" axis in side view ofthe row of teeth starting from the rear bottom edge ofthe concave surface, in the central plane thereof and forming there the tangent ontheconcave surface, the axis being at a right-angletothex"axis and at the sametime passing throughthe geometrical longitudinal axis, they" axis being at a right-angletothex"axis and z" axis and a being theangle between the x" axis and the planeformed bythetop edge oftheteeth.

4. Device according to Claim 1, characterized in that the mirror is mirrored on its concave surface.

5. Device according to Claim 1, characterised inthatthe arcofthe oval which when the mirror is used is located farthest inside the mouth is cut off.

6. Device according to Claim 5, characterised in that the cut-off is straight in its middle zone and merges with a large radius into the contours ofthe mirror.

7. Device according to Claim 1, characterised in that the arc of the oval which is farthest outside in the mouth when the mirror is used merges into a bowl shape.

8. Device according to Claim 1, characterised in that an aiming device is provided which indicatesthe angle of inclination of the mirror.

9. Device according to Claim 8, characterised in that the aiming device is provided in the centre of the mirror.

10. Device according to Claim 8 and 9, characterised in thatthe aiming device comprises a straight stem having a reference shape at its free end.

11. Device according to Claim 10, characterised in thatthe reference shape is a hole, and plane of which is art a right-angleto the stem.

12. Device according to Claim 10, characterised in that the stem is so long as the clear field of view.

13. Device according to Claim 10, characterised in that the stem is substantially shorterthanthe mirror.

14. Device according to Claim 13,characterised in that the stem is 0.5 to 5 cm long and is preferably 2.2cm + 50% long.

15. Device according to Claim 8, characterised in that provided on the mirror is a small optically standingout surface which is part of the aiming device.

16. Device according to Claim 15, characterised in that the surface is the end face of a fixing member.

17. Device according to Claim 1, characterised in that the mirror consists of a several millimetres thick synthetic plastics material having a non-cutting rounded shape at its edges.

18. Device according to Claim 1, characterised inthatthe angle is 45" + 20".

19. Device according to Claim 18, characterised in thatthe angle is 45".

20. Device according to Claim 10, characterised in thatthe reference shape is a notch.

21. Device according to Claim 10, characterised in thatthe aiming device is removably mounted on the mirror.

22. Device according to Claim 1, characterised in that the back ofthe mirror is mirrored.

23. Device according to Claim 1, characterised in that the mirror consists of metal.

24. Device according to Claim 1, characterised in that the mirror is of inorganic glass.

25. A device as claimed in claim 1, substantially as described herein with reference to the accompanying drawings.