GB2030868A - Bite characteristics of teeth - Google Patents

Abstract

Apparatus for evaluating bite characteristics of teeth, comprises an impression member with at least one layer of an elasto-plastic memory material that retains a bite impression, the memory material being optically isotropic or uniformly anisotropic and exhibiting as the result of bite conditions mechanical stress birefringence observable by polaroid light. The memory material may be a polymer, e.g. nylon or polyethylene. A polariscopic instrument arranged to examine the impression member has an illuminating white or monochromatic light source, a first linear or circular polarising filter arranged between the light source and a clamp for the impression member to be examined, and a second linear or circular polarising filter arranged to pass light transmitted through or reflected by the member to a magnifying eyepiece, the clamp being constructed to hold the member such that a bite impression in the member is not disturbed.

Description

SPECIFICATION Apparatus for evaluation of the bite characteristics of a set of teeth This invention relates to apparatus for the evaluation of the bite characteristics of sets of teeth.

Both for medical, and especially dental scientific purposes, as well as in the day-to-day practice of doctors and especially dentists, it is often extremely desirable to ascertain the true characteristics of the bite in order to make correct diagnosis and provide appropriate treatment.

To obtain accurate information about the bite, it is not sufficient to simply X-ray the set of teeth to be examined, which consists essentially of the two opposed jaws with the teeth and/or prostheses enclosed therein. Nor is it enough to take an impression thereof in a mass which starts out plastic and subsequently hardens. These known examination methods indeed provide certain data about the structure and characteristics of the set of teeth.

However, they provide only relatively coarse indications concerning the actual occlusion characteristics of opposed teeth or rows of teeth, and no indication at all of the real pressure or contact forces which exist between opposed teeth. Yet, as previously stated, it is extremely important to obtain the most precise information of at least qualitative but preferably also quantitative nature, not only with respect to the occlusion, i.e. the normal closed position of the teeth, but also with respect to the mastication, i.e. the characteristics prevailing during chewing, and also with respect to incision, that is the characteristics which prevail during the cutting phase of biting.

For such gnathologic investigations of teeth arches, there is known a method and apparatus utilizing a piezoelectric transducer which is inserted between the teeth to be examined and then subjected to biting pressure. Thereupon the pressuresensitive transducer produces an electrical signal which can be displayed or recorded by an instrument which has been previously calibrated to regis teethe force applied to the transducer.

However, these known methods and instruments are capable only of measuring the total force applied and give no information about the distribution of the contact forces between the two arches. It is, for example, not possible to mount several such transducers in different locations along an arch and then obtain an effective measurement of the prevailing contact forces. This is because both the piezoelectric transducers and the two arches are comparatively rigid. Therefore a first transducer may absorb the entire mastication force, while a second one remains completely unloaded or loaded only to an insignificant degree.

Moreover, even if the tooth arches were to deform in somewhat elastic manner in response to chewing force application to piezoelectric transducers applied at several points, this would still not provide a reasonably accurate representation of the actual characteristics, either with respect to occlusion, or with respectto mastication, orincisional biting.

It is also apparent that these disadvantages cannot be overcome by reducing the rigidity of such transducers, as for example, by enclosing them within an elastic envelope of rubber or the like. This is because such an arrangement is capable of measuring only localised forces and with relatively great imprecision, whereas it is not possible to ascertain the true bite- or mastication force characteristics at all actual contact points of a set of teeth. Furthermore, this known arrangement is not capable of differentiating between occlusion and mastication, and the results obtained with this known instrumentation also do not provide any indication as to the mechanicalgeometric relationships between opposing cusps.

This known instrumentation also does not provide results which take into account the various effects of mouth movement during opening and closing of the arches which accompany food intake and therefore provides no information about the dynamic characteristics.

To obtain at least qualitatively useful data, dental clinics currently use carbon paper or other pressure-released indicating media, which is placed in strip form between the arches of a set of teeth and which, after biting, and transfer or some indicating media to the teeth, provides certain qualitative information about the prevailing contact forces and the mechanical-geometric relationship between the cusps of opposed natural teeth and/or prostheses.

This method is relatively practical in that it is simple and quick and thereby suitable for clinical use. However, it does not provide any quantitative data and has only low accuracy even in a qualitative sense.

Methods of examination through impression of arches into plastic materials, such as wax plates or plastic which later hardens, which have previously been briefly mentioned, not only are unable to provide data for contact forces but also are very difficult to interpret with respect to relationships between opposed cusps. Moreover these methods are not capable of providing practical records of observations made.

The present invention has the primary object of improving upon known apparatus and instruments of the types previously described while overcoming their disadvantages, and to provide apparatus and instrumentation capable of effecting qualitative and quantative gnathologic examination and determination of occlusion, mastication and incision over the entire set of teeth. A further desired capability is that of providing differentiating data with respect to occlusion and mastication, and also to provide information concerning the mechanical-geometric relationship between opposed cusps and the dynamic characteristics during mastication. Finally, it is desired that the results be not only obtained in simple manner but also be capable of being recorded.

According to the present invention, apparatus for evaluation of the bite characteristics of a set of teeth comprise an impression member having at least one layer of an elastoplastic memory material that will retain, at least temporarily, a bite impression; the elastoplastic memory material being optically isotropic or substantially uniformly anisotropic and exhibiting as the result of bite conditions mechanical stress birefringence observable by polarised light Reference heretofore and hereinafter to an elastoplastic memory material is meant to be a material that responds to an impression imparted through bite conditions by retaining same as unchanged as possible for at least a predetermined period of time, thereby making possible its analysis and, if desired, its recording.

In reference heretofore or hereinafter to a material which is "optically isotropic, and exhibiting mechanical birefringence", what is meant is a material which is initially not birefringent by virtue of being optically isotropic, but which becomes birefrigent under stress.

As is well known, berefringence normally occurs only in optically an isotropic media, such as crystals with non-cubic lattices. These split an incident beam into two component beams, which generally have different group orientations and different group and phase velocities, both obeying the conventional laws of refraction. If the media are non-absorbing, then both component beams are polarised linearly and mutually perpendicularly.

However, in addition to the above-described natural birefringence of optically and an isotropic crystals, there also exists what is referred to as deformation or stress birefringence, which is produced by external mechanical input or load in normally isotropic media. Such an optically isotropic material subjected to pressure or tension, may then behave like a crystal having a single optical axis, that axis being oriented in the direction of the pressure or tension.

The internal stresses created by loading give rise to the phenomenon which is referred to here as mechanical birefringence. The varying internal stresses can provide an indication of the applied load, provided the material being used has been suitably calibrated.

In order to obtain not only the desired effect of mechanical stress birefringence, but in order to also preserve the phenomena produced by loading for at least a time period sufficient for examination and, if desired, for recording, there is provided in accordance with the invention a material which is not completely elastic. The material in question can therefore be said to exhibit elasto-plastic memory.

Preferably the invention utilises an elasto-plastic material which is flexible and foil-like. It is particularly preferred to use such foil of extremely uniform initial thickness of between 0.1 and 0.3 mm.

Particularly suitable materials are polymers, among which polyamids such as nylon or polyethylene are preferred. However other materials can also be used.

Also an elasto-plastic memory material that is substantially uniformly an isotropic but which exhibits mechanical stress birefringence can also be used.

If a two-sided impression is desired, then a single layer of elasto-plastic memory material is preferably used. This yields an impression of one arch on the upper surface of the layer and an impression of the opposite arch on the lower layer surface. This suffices to enable not only quantative determination of the contact forces, but also their correlation with the mechanical-geometric properties of the respective teeth. For example it is possible to discern that the contact force produced at a given opposed pair of teeth is produced by a cusp of the upper tooth engaging a bow-shaped configuration of cusps in the opposed lower tooth, and the like.

On the other hand, if for any reason separate examination of the upper and lower arches is desired, then two layers which are readily separable afterthe impression has been taken are preferably used.

In a further embodiment of the invention, a separator is placed between the two layers. This separator is preferably of elastic or deformable material and has a thickness of 0.5 to 3 mm. This has the further advantage that the presence of the elastic separator layer can simulate the presence of food between the two arches.

A polariscopic instrument in accordance with another feature of the present invention for the examination of apparatus in accordance with the present invention that has been subjected to bite conditions includes an illuminating white or monochromatic light source, a first linear or circular polarising filter arranged between the light source and a clamp for the impression member to be examined, and a second linear or circular polarising filter arranged to pass light transmitted through or reflected by the impression member to a magnifying eyepiece, the clamp being constructed to hold the impression member in such a manner that a bite impression in the member is not substantially disturbed.

The above and other features if the present invention are illustrated, by way of example, on the accompanying drawings, wherein: Fig. 1 is a diagrammatic, cross-sectional view of an impression member arranged between a set of teeth of which the bite characteristics are to be evaluated; Fig. 2 is a plan view of the impression member, viewed from the direction of arrow II in Fig. 1; Fig. 3 a is a diagrammatic elevation of an instru mentforanalysing the impressed member by light transmission; Fig. 3 b is a diagrammatic elevation of a similar instrument to that of Fig. 3 a, but employing light reflection; Fig. 4 is a detail plan view of the impression produced in the member by a tooth or cusp; Fig. 5 is a very diagrammatic sketch of a scissors clamp; Fig. 6 is a perspective view of a particular shape of impression member; ; Fig. 7 is a perspective view of another form of impression member in position in a holder; Fig. 8 is a section of another embodiment of an impression member; and Fig. 9 is a section of a further embodiment of an impression member.

Fig. 1 shows an impression member in the form of a sandwich of two parallel strips 1 and 2 and an interposed separation layer 3. The strips 1 and 2 are each formed of a flexible foil having a very uniform thickness of between 0.1 and 0.3 mm and preferably 0.15 mm.

The foil material is not completely elastic, i.e. it is elasto-plastic, is optically isotropic or uniformly anisotropic and can exhibit mechanical stress birefringence. The material may be a transparent polymer; a polyamid, particularly nylon; or polyethylene.

The separator layer3 may be of an elastic or deformable material, e.g. rubber and may have a thickness of 0.5 to 3 mm, particularly 1.8 mm.

For a gnathological examination of occlusion, mastication and incision the three layer composite impression member is interposed between upper and lower arches 4 and 5, of sets of teeth or prostheses or dentures, and the two arches are pressed or bit together. Pressure is maintained on the impression member for a period ranging from 10 to 60 seconds. Thereafter the impressed impression member is removed for analysis by an instrument such as those illustrated by Figs. 3a and 3b.

In each case the instrument has a white or monochrome light source 9, a glass diffuser plate 10, two linear or circular polarising filters 11 and 12 and, to hold the strips 1 and 2, a scissors clamp holder 15 that is capable of tilting one strip with respect to the other. The clamp 15 is shown in Figs. 3b and 5 to hold the strips 1 and 2 only at one end or edge thereof. This is to prevent the clamp from affecting the teeth impressions or introducing additional stresses into the strips. An eyepiece or magnifying lens 13 is shown to be attached to a stand 16 and is arranged above one filter 12. The eyepiece is normally a lens having magnification of 3 to 10 times.

A camera 14 may be pivotally attached to the stand 16, either above or in place of the lens 13, to record analysis results.

Fig. 2 shows, in a highly diagrammatic manner, the impressions 17 made in the upper strip 1 by a set of upper teeth 7, the impressions appearing to the naked eye as uniform points. Fig. 4 shows, again in a highly diagrammatic manner, the appearance of an impression 17 when viewed through an analysis instrument. The irregular areas of differing hatchings in the diagram are intended to represent different colours (isochromatics). These colours, when calibrated for a particular foil material, enable a quantative determination to be made of contact biting forces.

Additionally, the tooth impressions 17 reveal mechanical-geometric relationships between upper and lower teeth, for example they make it possible to determine at which locations and therefore at which cusps, there occur contact forces between teeth under examination.

The arch series of impressions 17 also reproduces the overall force distribution along the entire arch. In the described embodiment, the elastic separator layer 3 is used to simulate biting conditions which prevail during average food intake.

Corresponding impressions for the teeth 8 of the lower arch 5 exist in the lower strip 2, the separation layer having been removed after impression of the composite impression member. Using appropriate alignment means, e.g. accurately positioned holes 18 in both strips, the two strips may be superimposed and viewed simultaneously, thus providing information aboutthe relationship between opposed teeth in the upper and lower arches.

As will be recognised, the apparatus of this invention may be used for determining the contact forces of teeth, implant supports (providing a capability of making adjustments to achieve relatively uniform loadings for the support) and for prosthetic work.

The determination of prevailing contact forces, using apparatus in accordance with the invention, for prosthetic work can avert problems and make possible the achievement of substantial uniformity of forces for occlusion or mastication or incision.

Additionally, apparatus in accordance with the invention can be used for the creation of official records, and particularly to supplement fingerprints or other personal identifying indicia. The results obtained by means of the invention do not change over extended periods of time, or at least do not change to such an extent as to preclude identification of a specific person, unless that person has had major prosthetic work performed in the interim.

The phenomena which are employed in the practice of the present invention can be described as follows.

The materials which are suitable for use as strips 1 and 2 have been found to have certain specific characteristics. In particular, when a thin strip of a suitable material is subjected to localised stress, as is the case if opposed teeth or arches bear down upon or bite it, corresponding localised strain results. This strain can be visually observed by use of instruments such as those illustrated in Figs. 3a and 3b. The strip manifests so-called "fringes" surrounding the area under stress. This is sometimes referred to as the photoelastic effect, and the materials exhibiting this effect are correspondingly referred to as photoelastic materials.

The present invention not only utilises this photoelastic effect as such, but also utilises a specific development of this effect. In certain materials, and in physical configurations of such materials, the strain which results from applied stress varies more or less proportionally with stress over an initial range of applied stresses, but then ceases to do so when a certain threshold is exceeded. Beyond this threshold, the proportional relationship no longer holds and the material is not completely relieved of the strains produced therein, even after the stress ceases to be applied. This retention of strain even after removal of the stress may be referred to as "photo-elastic memory". It is this residual strain attributable to photo-elastic memory that is employed by the present invention.

The strips 1 and 2 previously discussed constitute what may be called photo-elastic memory tapes.

When subjected to the bite of a set of teeth, as previously explained, the local stresses set up by contact points between opposed teeth give rise to corresponding local strains in the photo-elastic memory tape or tapes.

The type of material and its thickness, as well as other parameters discussed below, are so selected that, under practical bite conditions, a sufficient degree of local stress and resulting strain is produced to cause the material to develop corresponding residual photo-elastic memory. The time span during which the bite conditions, and therefore pressure, need to be maintained may range from even as little as two seconds up to one minute, depending on the type and thickness of the material used.

After removal of the photo-elastic memory tape or tapes from between the teeth or arches under examination, the residual photo-elastic memory is then made visible by means of the previously described instruments.

It will be recognised that these instruments are, per se, somewhat analogous to known polariscopes, of a transmission type for Fig. 3a and of a reflection type for Fig. 3b. However, heretofore polariscopes have not been used to examine teeth bites and also they have not been used to render visible the residual photo-elastic memory of teeth bites in photo-elastic memory tapes.

Moreover, the instrument construction utilising scissors clamps 15 has not been used before. This construction makes it possible to take strips 1 and 2, containing the respective photo-elastic memory of the forces resulting from bite impressions of the upper and lower arches, and simultaneously observe both strips in the same geometric relationship that prevailed when the strips were bitten by sets of teeth under examination.

For example, if a cusp in a tooth forming part of one arch fits perfectly within a corresponding fossa of an opposed tooth in the other arch, then the localised permanent strains set up during biting will be very uniform and give rise to corresponding uniform visible fringe patterns. On the other hand, if there is local misalignment, or some unintended projection on one side or the other, this will set up more intensive and concentrated local permanent strains in the memory tape, which will manifest themselves as local distortions and intensifications of the fringing effect With experience in observing these visual effects, it becomes possible to interpret them quickly and accurately from the stand-point of the corrections to the bite which they indicate to be required.

Moreover, by calibrating the memory tapes in advance with known forces, quantitative information concerning the local forces can be derived by "counting fringes" and measuring fringe areas. In this respect the procedure is analogous to that conventionally employed with polariscopes.

Moreover, bytilting the tapes 1 and 2 progressively toward or away from each other, a visual effect can be observed which is strikingly lifelike and representative of the changes which actually take place during movement of the arches toward or way from each other. This is capable of providing valuable clinical insights into the relationships prevailing in the set of teeth under examination.

Preferably, the polarising filters 11 and 12 are so mounted as to tilt jointly with memory tapes 1 and 2, respectively, said filters being attached for example to the scissors clamps so as to be also displaced by movement of scissors clamps 15.

It is generaliy desirable to apply, to any surface of a memory tape 1 or 2 which is exposed to contact with outside objects, e.g. the teeth to be examined, a protective layer (sheet or coating) of material which does not exhibit the photo-elastic memory effect to any substantial degree. This has the purpose of preventing scratches and other unintended deformations from being introduced into the memory tape itself and ensures that the tape will only be "activated" by intended bite forces.

If a single tape 1 or 2 is used, then the protective coating is preferably present on both exposed faces of the tape.

If a sandwich of tapes 1 and 2 with a separator 3 is used, then the protective coating may be present only on the exposed sides of tapes 1 and 2, respectively.

The material of the protective layer may be polyvinyl chloride and its thickness between 0.01 mm and 1 mm.

The separator layer 3, as previously noted, may be of rubber, or it may be of foam or sponge or putty- or wax-like non-elastic material. Its thickness preferably ranges from about 0.3 to 3 mm.

It will also be understood that, although the photo-elastic memory tapes are preferably isotropic before being subjected to the bite, they may also be initially anisotropic, provided this condition is sub stantiallyuniform overthetape. In such a case the pattern when visualised will appear against a generally uniformly coloured background.

A photo-elastic memory tape in accordance with the present invention may also be provided with a coating or foil of carbon-like material. By so doing, the technique of the present invention may, in effect, be combined with prior known techniques which, as discussed above, use carbon paper to determine certain aspects of bites. In such case, the carbon-like foil used may also serve as the protective layer for the memory tape which is mentioned above. If this carbon foil is opaque, it must be stripped off after impression and before the memory tape is used for visualisation. However, a thin coating of carbon-like material can also be applied, which is sufficiently light-transmissive so that no after-treatment is needed before visual utilisation, the carbon-like layer can be removed by solvents, water or other liquids.

The carbon-like material could be any type of pressure-release indicating media such as a transferable paint of any colour which leaves a trace of colour on the teeth when they are brought into contact with the coated memory tape.

Furthermore, as shown in Fig. 8, a reflective layer 23, such as a layer of paint, can be applied to the memory tape 24 and a carbon or like layer 25 applied on top of the reflective layer 23. Such an impression member can then be analysed by means of a reflection polariscope without removing the carbon layer 25.

The memory tapes are shaped to cover the area of the arches to be examined. Preferably, a single tape is shaped to fit the entire arch and Fig. 6 shows a suitable tape shape 20. Fig. 7 shows another tape shape 21 and also shows the tape 21 inserted in a holder 22 by means of which the tape can be conve niently inserted and held in place between the arches to be examined. It will be noted that holder 22 is so sized that the memory tape 21 is slightly bowed. This has been found desirable for a good fit between the arches and to provide clearance for the tongue if used in a human mouth.

It is also possible to superimpose on the memory tape itself a printed grid of lines for convenient geometric reference of the visual phenomena which will eventually be visualised by its use. It is also possible to superimpose other relevant information, such as a dental chart.

It is contemplated that the visualisation and analysis of memory tapes in accordance with the present invention will normally be performed after removal from between the arches to be examined.

However, it is also possibleto perform such analysis in situ by opening the arches and using a reflectiontype polariscope instrument For this purpose, also, there is preferably used an impression member such as shown in Fig. 8, the reflective layer 23 providing the reflectivity required by the reflection-type polariscope instrument. The use of a carbon layer 25 is optional insofar as the polariscope analysis itself is concerned.

To obtain a permanent record of the impressions made in the memory tape by the bite the tape itself may be preserved and its impression pattern visualised anew as and when needed by means of a polariscopic instrument. Alternatively, a photograph may be taken ofthevisualised pattern, as previously described. The visualised pattern can also be recorded by xerography, as indicated in Fig. 9, wherein a memory tape 26 has a reflective coating 27 applied to it. It is then placed upon the bed 28 of a xerographic machine with the tape 26 toward the bed, and with a polarising filter 29 between the tape 26 and the bed 28. The filter 29 may be linearly polarising but is preferably circularly polarised.

When tape 26 is illuminated during the initial stage of the xerographic process, the strain pattern therein will become visualised and simultaneously xerographically recorded. Other optical recording techniques may be used in corresponding manner.

The visualised fringe image can also be magnified and projected onto a screen. It can then be photographed from the screen and may also be further magnified from the screen by using an eyepiece or microscope. Suitable screens may be movie screens, those of profile projectors, and the like.

Claims (44)

1. Apparatusforthe evaluation of the bite characteristics of a set of teeth comprising an impression member having at least one layer of an elasto-plastic memory material that will retain, at least temporarily, a bite impression; the elastoplastic memory material being optically isotropic or substantially uniformly anisotropic and exhibiting as the result of bite conditions mechanical stress birefringence observable by polaroid light.

2. Apparatus as claimed in claim 1, wherein the material is of uniform thickness and is between 0.1 and 0.3 mm in thickness.

3. Apparatus as claimed in claim 2, wherein the material is a flexible and foil-like tape.

4. Apparatus as claimed in any of claims 1 to 3, wherein the elasto-plastic memory material is a polymer.

5. Apparatus as claimed in any of claims 1 to 4, wherein the elasto-plastic memory material is transparent.

6. Apparatus as claimed in claim 4 or claim 5, wherein the polymer is a polyamid or is polyethylene.

7. Apparatus as claimed in claim 6, wherein the polyamid is nylon.

8. Apparatus as claimed in any of the preceding claims and comprising two layers of photo-plastic memory tape.

9. Apparatus as claimed in claim 8, wherein a separator layer is interposed between the two layers of memory tape.

10. Apparatus as claimed in claim 9, wherein the separator layer is of elastic or deformable material.

11. Apparatus as claimed in claim 10, wherein the separator layer material is soft or spongy.

12. Apparatus as claimed in claim 11,wherein the separator material is of rubber, or foam, or sponge material or is of a putty- or wax-like material.

13. Apparatus as claimed in any of claims 9 to 12, wherein the separator layer is between 0.5 and 3.0 mm. in thickness.

14. Apparatus as claimed in any of the preceding claims, wherein at least an exposed face of the memory tape is covered by a protective layer or coating of a material that coes not exhibit substantial mechanical stress birefringence.

15. Apparatus as claimed in claim 14 wherein the protective layer or coating is polyvinyl chloride of between 0.01 and 1 mm in thickness.

16. Apparatus as claimed in any of claims 1 to 13, wherein an exposed face of the memory tape is covered by a pressure-released indicating medium.

17. Apparatus as claimed in claim 16, wherein the medium is carbon-like, or of transferable paint or foil.

18. Apparatus as claimed in claims 14 and 16, wherein the pressure-release indicating medium also serves as the protective coating or layer.

19. Apparatus as claimed in any of claims 15 to 18, wherein the pressure-release medium is removable from the memory tape and may be opaque.

20. Apparatus as claimed in any of claims 15to 18, wherein the pressure-release medium is suffi ciently transparent to permit polarised light examination of the underlying memory tape.

21. Apparatus as claimed in claim 16, wherein light reflective material is present between the pressure-release medium and the memory tape.

22. Apparatus as claimed in any of claims 1 to 15, wherein one face of the memory tape is covered with a material that is at least light reflecting on the side facing said tape.

23. Apparatus as claimed in claim 21 or 22, wherein the light reflecting material is an aluminium coating.

24. Apparatus as claimed in any of the previous claims, wherein the impression member is of a size and shape to enable the bite characteristics of an entire set of teeth to be obtained.

25. Apparatus as claimed in any of the previous claims, wherein the impression member is held in a holder.

26. Apparatus as claimed in claim 25, wherein the holder is so shaped and sized with respect to the impression member that the latter is bowed away from the plane of the holder when held therein.

27. A polariscopic instrument arranged to examine apparatus as claimed in any of claims 1 to 26 and having been subjected to bite conditions, wherein the instrument includes an illuminating white or monochromatic light source, a first linear or circular polarising filter arranged between the light source and a clamp for the impression member to be examined, and a second linear or circular polarising filter arranged to pass light transmitted through or reflected by the impression member to a magnifying eyepiece, the clamp being constructed to hold the impression member in such a manner that a bite impression in the member is not substantially disturbed.

28. An instrument as claimed in claim 27, wherein the clamp is constructed so as not to introduce additional stresses in the impression member.

29. An instrument as claimed in claim 27 or claim 28, wherein the clamp has two parts each to hold one layer of elasto-plastic memory material.

30. An instrument as claimed in claim 29, wherein one part of the clamp istiltablewith respect to the other part.

31. An instrument as claimed in claim 30, wherein the clamp is a scissors clamp.

32. An instrument as claimed in claim 30 or claim 31, wherein the polarising filters of the instrument are mounted to be tiltable jointly with the clamp part.

33. A method of permanently recording the mechanical stress birefrigence patterns in apparatus as claimed in any of claims 1 to 26 and having been subjected to bite conditions, wherein a photograph is taken of the visualisation of the birefringence patterns in any of claims 27 to 31.

34. A method of permanently recording the mechanical stress birefringence patterns in apparatus as claimed in claim 22 or claim 23 and having been subjected to bite conditions, wherein a polarising filter is placed against the other face of the memory tape, the tape is illuminated through the filter and the resulting birefringence patterns photographed or recorded on a xerographic or like copier.

35. A method as claimed in claim 34, wherein the xerographic or like copier is used to provide the illumination.

36. Apparatus for evaluation of the bite characteristics of a set of teeth substantially as described with reference to, or as shown by Figs. 1,2,4,6,7,8 or 9 of the Drawings.

37. A polariscopic instrument arranged to examine apparatus as claimed in any of claims 1 to 26, or claim 36, and substantially as described with reference to or as shown by Figs. 3a or 3b, of 5 of the Drawings.

38. A method of permanently recording mechan ical stress birefrigence patterns in apparatus as claimed in any of claims 1 to 26, or claim 36, and substantially as described with reference to Figs. 3a or3b or9.

39. A method of evaluating the bite characteristics of a set of teeth comprising the steps of introducing apparatus as claimed in any of claims 1 to 26 or claim 36 between an opposed pair of arches of teeth, causing the opposed arches to approach one another and bring pressure to bear on the impression member producing bite conditions, maintaining said pressure for a given period of time and thereafter observing mechanical stress birefringence patterns produced in the member as the result of the bite conditions by means of polarised light

40. A method as claimed in claim 39, wherein the period ranges from 2 to 60 seconds.

41. A method as claimed in claim 39 or claim 40, wherein the impressed member is removed from between the arches and observed by means of a polariscopic instrument as claimed in any of claims 27 to 32, or claim 37.

42. A method as claimed in claim 39 or claim 40, wherein the impressed member is observed in situ by means of fully opening the arches, illuminating the member with polarised light and observing light reflected from the memberthrough a polarising filter.

43. A method as claimed in any of claims 39 to 42, wherein the elasto-plastic memory material has been calibrated with respect to its birefringent properties by the application of known loads to enable quantitative assessment of bite forces to be obtained.

44. A method of evaluating the bite characteristics of a set of teeth substantially as herein described.