TECHNICAL FIELD

The present invention aims to provide a method for the orthodontic casting of gnathostatic models of dental arches and an instrumental system for carrying out said method.

Gnathostatic models of dental arches are one of the most important orthognathodontic instruments in the diagnostic-prognostic procedure required for the development of an adequate therapeutic plan. Such models should reproduce the clinical situation under examination and permit the monitoring of subsequent variations in the anatomical structures considered as well as their spatial and functional relationships. The main feature of gnathostatic models is the support of the anatomical portion by a manufactured art portion or plaster base which enables these models to be arranged on a support plane and brought together, thereby simply imitating the occlusion relationships. BACKGROUND ART To date, various methods have been developed for the creation of such models which may be schematically defined as cast-making and squaring techniques. Some of

these techniques are easy to use and, apart from the faithful reproduction of the dental arches, serve merely to produce generic model bases which can be aligned in occlusion when arranged on a support plane. Other more complex techniques adopt a system for truing the models in their plaster bases in order to obtain better control over occlusion, some possibility of subsequent comparison, easy laboratory processing and filing, functionality in clinical practice and presentability. Yet other techniques attempt to relate the models to data obtained from facial photographs or cephalometric tracings derived from laterolateral teleradiograms of the cranium. Finally, we know of techniques in which the jaws are biometrically related to the face to produce orientated models, though these techniques are unable to prevent the modification of the perimeters of the plaster of the plaster-cast bases. None of the techniques proposed to date can produce gnathostatic models which reproduce the individual orientation of the dental arches in occlusion vis-a-vis an anatomicocranial reference plane such as the Frankfurt or Camper plane while employing systematic truing inside the cast to prevent variations in the perimeters of the plaster bases of such models.

Hence, these techniques do not permit the use of plaster models for the reliable, standardised, repeatable and easily comparable reproduction of the exact relative

position of the dental arches in occlusion such as to permit any clinical observation of the occlusal relationships and individual craniofacial lie in the arches themselves. DISCLOSURE OF INVENTION

The aim of the present invention is the creation of a method for the casting of gnathostatic models of the dental arches which eliminates the disadvantages of the known cast-making techniques specified above. The present invention achieves its stated aim since it employs a method for casting gnathostatic models of the dental arches characterised by including a first phase in which the inclination of a patient's upper dental arch is compared to an anatomicocranial reference plane on the same patient, a second phase in which a first model of the said upper arch is positioned in a first moulding base of a caster at an inclination to a basal plane on said moulding base that is identical to said inclination measured and has a relative truing position, a third phase in which a first cast enveloping said first model is poured into said first moulding base, a fourth phase in which a second model of the said patient's lower dental arch is positioned in a second moulding base of said caster in a position of relative occlusion vis-a-vis said first model, and a fifth phase in which a second cast enveloping the second model is poured into said second moulding base. The present invention also relates to an instrumental

system for casting gnathostatic models of dental arches characterised by including means for measuring the inclination of a patient's upper dental arch vis-a-vis a cranial reference plane on the same patient; a caster consisting of a first moulding base for a cast of a first model of said upper arch and a second moulding base for a cast of a second model of a lower dental arch of said patient and orientator-truer means for positioning said first model on said first moulding base at an inclination to a basal plane on the said moulding base that is identical to said inclination of the said upper dental arch vis-a-vis the previously measured reference plane in a relatively trued position. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention we now append a description of one preferred form of the system which is purely indicative, not in . any way restrictive and relates to the drawings enclosed herewith, in which: figure 1 is a top plan view of a facebow forming part of an instrumental system for cast-making on the basis of the specifications of the present invention; figure 2 shows a lateral elevation of the figure 1 facebow; figure 3 shows a lateral elevation of a bite fork forming part of the system invented; figure 4 gives a top plan view of the fork in figure 3; figure 5 gives a top plan view of an orientator-truer

assembly forming part of the system invented; figure 6 shows a lateral elevation of the figure 5 assembly; figure 7 shows a lateral elevation of a twin-based caster forming part of the system invented; figure 8 gives a top plan view of an upper moulding base on the caster shown in figure 7; figure 9 shows a lateral elevation of a lower moulding base on the caster shown in figure 7; figure 10 gives a top plan view of the orientator-truer assembly that holds the bite fork and is mounted on the upper moulding base; figure 11 shows a lateral elevation of the orientator-truer assembly that holds the bite fork and is mounted on the upper moulding base; figure 12 shows a front elevation of a support that can be used in one variant of the system invented; figure 13 shows a lateral elevation of the orientator-truer assembly mounted on the figure 12 support.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention aims to provide a method for the orthodontic casting of gnathostatic models of dental arches and an instrumental system for carrying out said method. The system consists of numerous interactive components which can be deployed as described below and exemplified by figures 1-9.

In particular, the system consists of: a facebow (1)

(figs. 1-2) used to identify a craniofacial reference plane " " on the patient"; a caster (2) (figs. 7-8-9) used to create bases of gnathostatic models; a bite-fork (3) (figs. 3-4) for the measurement of inclinometric relationships between the patient's upper dental arch and the above-mentioned reference plane " ", and an orientator-truer assembly (4) (figs. 5-6) that is used for the transfer of the above-mentioned inclinometric relationships onto the caster (2) in a relative truing position.

In figures 1 and 2, facebow (1) consists essentially of two flat arched arms (5, 6 ) at right and left respectively which are hinged together at one end by a pin (7) and have auricular olives (8) at their opposite ends. A rotating support rod (9) extending upwards at right angles to the arms (5,6) is mounted on pin (7) . An adjustable rod (12) equipped with a sliding longitudinal toggle (13) controlled by screw 10 is attached by the same knurled screw 10 and nut 11 to the top (9') of rod (9) . One end of the rod (12) bears a nasal support (14) . Arm (5) is fitted with a bowl spirit level (15) . Arm (6) bears an indicator (19) with a tip (20) that rests on the median plane of arms 5 and 6 that slides along the arm by means of a screw (16) which engages a longitudinal toggle (17) and a knurled nut (18).

In figures 3 and 4, bite fork (3) incorporates an essentially C-shaped section (24) and a straight handle (25).

Fork 3 includes an impression face (26) on which, in use, a thermoplastic impression paste (31) shown by the dotted lines on figures 3 and 4 is laid along the section (24). On the same face (26) at the level of handle (25), a slide (27) for a sliding indicator (28) with a tip (29) facing section (24) of fork (3) is attached in the median position. Indicator (28), whose purpose will be clarified later can be fixed in any desired position by knurled screw (30). The lower face of fork (3) bears longitudinal slide (34) which is used to attach fork (3) to orientator-truer assembly (4). For this purpose a threaded stalk (35) set at right angles to the plane of fork (3) to which a knurled locking nut (36) is screwed protrudes from slide (34).

A bowl spirit level (37) with a screwed clamp (38) into which the handle can be inserted and secured can be mounted on handle (25) of fork (3). Bowl (39) of spirit level (37) incorporates a practical Cartesian grid (40) with ordinates parallel to one longitudinal axis of fork (3) and originating from the centre of the bowl i.e. from the point where the air bubble (41) will lie when fork (3) lies along a perfectly horizontal plane. For practical purposes the grid (40) is calibrated in degrees (°) so that by reading the coordinates of the equilibrium point of the bubble on the grid itself we can identify the inclination of fork (3) vis-a-vis a horizontal plane around the median longitudinal axis and

around an axis transversing fork (3). Assuming that the radius of the ball to which the bowl belongs is approximately 57.34 mm, each degree of inclination of each of these two axes corresponds to a 1 mm shift of the bubble.

The bubble itself is small, only 1-2 mm in diameter to permit an accurate reading of the inclination. Turning now to figures 5 and 6, the orientator-truer assembly (4), hereinafter called "assembly 4" for brevity, incorporates an L-shaped arm (44) with a parasagittal hinge (45) and a frontal hinge (46) at each end on the inside and the outside respectively. The axes (a and b) of these hinges lie at right angles to each other and meet at point 0. Parasagittal hinge (45) consists of two plates (47,48) hinged together by means of pin (49) with axis (a). Plate (47) is rigidly attached to arm (44) and has an arched top (50) defining a cylindrical surface coaxial to pin (49) and incorporating a goniometric scale (51) with a central zero. The upper surface of plate (48) holds an indicator (52) with a hand designed to move over the scale (51). Rigidly attached to plate (48) is a square-section rod (53) which has plate (54) rigidly secured to one end and lying parallel to the plane identified by axes "a" and "b" when indicator (52) is the zero position. Plate (54) is symmetrical to plane β set at right angles to axis "a" and containing axis "b". It is approximately C shaped and equipped with a seat

(55) at the centre for slide (34) of fork (3) (figure 11). At each of its free ends and facing frontal hinge (46) plate (54) has stops (56) to limit the axial movement of fork (3) . Frontal hinge (46) consists of two plates (60, 61) hinged together by pin (62) with axis "b". Plate (60) is rigidly attached to arm (44) with an indicator plus a hand (63) which moves over a goniometric scale (64) of the upper arcuated rim (65) of plate (61) in exactly the same way as described for parasagittal hinge (45).

Plate 61 carries a pair of rigidly secured parallel downward-facing studs (66) needed for mounting on the moulding base of a caster as described below. Studs (66) are equipped with axial stops (66') to indicate their axial assembly position. When indicator (63) points to zero, the plane identified by axes "a" and "b" is at right angles to the stud axes. Rod (53), arm (44) and studs (66) are positioned so that on mounting onto the moulding base, point 0 at the intersection of axes "a" and "b" lies at the centre of the caster (2).

Hinges (45) and (46) are both equipped with locking screws (67) that lock the two plates forming the hinge itself into a selected angular position. Locking systems (67) are conveniently made up of a screw (68) built into one of the plates of each pair and in sliding engagement with an arched toggle (69) on the other plate and a tightening bolt (70). Figure 7 shows caster (2) consisting of a moulding base

(75) for the creation of a support cast for the models of the upper dental arch (hereinafter designated "upper base 75" for brevity) and a moulding base 76 for the creation of a support cast for the models of the lower dental arch (hereinafter designated "lower base 76" for brevity) .

Bases (75) and (76) (figures 8 and 9) are both irregular heptagons in shape and symmetrical to a vertical longitudinal plane. Each base consists of an anterolateral portion (75') and (76') defining four sides of the heptagon and a posterolateral portion (75") and (76") defining the remaining three sides. Sections (75') (75") and (76') (76") are joined by bolts (81) which tighten the respective flanges (77) at their touching ends. Flanges (77) are practically supplied with engagement systems (78) e.g. a raised portion on one flange matched by a seat on the other which ensure that the flanges are correctly positioned. Moulding bases (75) and (76) are outlined by their respectively upper and lower flat basal surfaces (79) and (80) which form the support bases for caster (2). A square-section rod (84) protrudes from posterolateral section (75") of upper base (75) and is also delimited by the upper basal surface (79), one end of which carries a clamp (85) with locking screw (86) and cylindrical seats (87) whose axes are at right angles to upper basal surface (79). In precisely the same way, a rod (88) protrudes from the

posterolateral section (76") of lower base (76), its outer edge delimited by lower basal surface (80) which has two studs (89) identical to assembly (4) studs (66) at one end. These studs engage seats (87) on clamp (85) of the upper base (75). Studs (89) correspond to two axial stops (90) which mesh at the front into clamp (85) to ensure that bases (75) and (76) are correctly spaced. The complete caster (2) consisting of the connected bases (75) and (76) is shown in figure 7. We shall now describe the casting method for which the present invention was designed and the operation of the various parts of the instrumental system. As a preliminary to the method, models of the patient's upper (M) and lower (not shown) dental arches are made in the conventional manner and the relative occlusion position of the two arches is recorded using occlusion wax.

Subsequently, a median line identified by clinical observation of the patient is traced onto model (M) of the upper dental arch. Model (M) is then carefully trued and pressed into thermoplastic paste (31) which is placed on section (24) of fork (3) and heated to malleability. The position of the model vis-a-vis the fork is established by the point at which the median line on the model meets the longitudinal axis of the fork (3) and in anteroposterior direction by using indicator 28 to set a preselected anterior limit to ensure that once the operation is completed the occluded

arches will lie in a median anteroposterior position in the caster (2). Usefully, indicator (28) can be made in such a way that the distance between indicator tips (29) and the end of fork (3), when indicator (28) is in its most retracted position, is identical to the maximum antero-posterior dimension of the moulding bases. In that situation, the advance of the indicator to indicate the anterior limit of the impression of the model in the thermoplastic paste is equal to half the difference between the above-mentioned maximum anteroposterior dimension of the models brought together in occlusion in the relative position recorded, as can be easily measured. The impression has to be made in the thermoplastic paste in such a way as to ensure that the greatest possible number of cuspids come into contact with the impression surface (26) on the fork in order to create a smooth, stable model surface on the fork itself, the upper surface of which meets the occlusal plane of the model. Facebow (1) is used to identify the anatomical reference plane a on the patient's head. If, for example, the Camper plane passing through the nasal tragus and ala is selected for the reference plane, it can be easily identified by applying facebow (1) to the patient's head in such a way that the auricular olives (8) engage the outer ear, the nasal support (14), appropriately adjusted meets the upper nose and the tip (20) of indicator (19), also appropriately adjusted, comes into

contact with the tip of the nasal ala (figure 2) . Once the reference plane coinciding with the median plane of facebow (1) is identified, the patient's head is moved until the reference plane is perfectly horizontal as can be easily verified using spirit level (15).

At this point, assuming that spirit level (37) has been pointed on the fork (3), fork (3) and its impression are introduced into the patient's mouth, care being taken to fit the impression onto the teeth of the patient's upper arch so that the teeth barely touch surface (26) on the fork (3), as happened when the impression was prepared using the model. The orientation of the patient's upper dental arch vis-a-vis reference plane a and set horizontally is established by graduated spirit level (37) on which the measurements of the inclination angles around a longitudinal and a transversal axis can be read. For this purpose, it is useful to mark the position of the bubble on the bowl using an erasable vetrographic pen in order to read the measurements after fork (3) has been extracted from the patient's mouth.

Once spirit level (37) has been removed, fork (3) is mounted on orientator-truer assembly (4) with impression face (26) turned downwards, as described earlier. Assembly (4) is then attached to upper base (75) of caster (2) which is turned upside down with its basal surface (79) resting on a casting surface (figures 10

and 11). When mounted on assembly (4), fork (3) acquires a position which ensures that the median and central transversal axes of impression face (26) coincide respectively with axis (b) of the frontal hinge and axis (a) of the parasagittal hinge, when indicators (52) and (63) are set in zero position. As a result, the combined movement of hinges (45) and (46) will orientate impression face (26) on fork (3) in the space around point 0, at the intersection of the two hinge axes which coincides with the geometric centre of the impression face. In its turn, when assembly (4) is connected to base (75) point 0 assumes a central position inside caster (2), as described earlier. At this point parasagittal (45) and frontal (46) hinges on assembly (4) can be regulated by loosening their respective locking screws (67) in order to reproduce the orientation of the fork (3) measured earlier using spirit level (37). To do this, it is enough to tilt the fork (3) so that the indicators (52) and (63) on the hinges indicate the coordinates (ordinates and abscissas respectively) to be read on grid (40) of spirit level (37) on scales (51) and (64). Locking screws (67) are then used to lock the hinges into the position described. Fork (3) is now suspended in a central position over upper moulding base (75) and presents the same inclination vis-a-vis the anatomical reference plane (actually the Camper plane) as basal plane (79) of

moulding base (75), when the fork (3) is introduced into the patient's mouth and adapted to fit onto the teeth of the patient's upper arch by the impression (31) in thermoplastic paste, which has been made as described earlier.

Once fork (3) has been systematically trued and oriented, the model of the upper arch is attached to it, naturally also upside down, so that it fits into the impression on fork (3). Model M therefore presents the same inclination vis-a-vis basal plane (79) as the patient's upper arch to the Camper plane. The model is attached to the fork (3) quite simply using a rubber band. At this point the cast of the upper model (M) can be prepared. Ideally, upper moulding base (75) is immersed in a pile of wet plaster until the plaster envelops the model: any surplus plaster around the model is easily removed by hand. The cast thus obtained has preselected geometrical characteristics defined by the shape of the moulding base and the model (M) will be trued and orientated vis-a-vis the basal surface of the cast. Once the upper cast has been poured, fork (3) and orientator-truer assembly (4) are removed very easily by removing the rubber band that secures it to the model ( ^M ) (which is by that time incorporated into the cast) and by loosening clamp (85) on base (75). The procedure for the creation of the lower cast is as follows. Once the upper cast has hardened, the previously

prepared occlusion wax is moulded onto model M of the upper dental arch. The lower model, obviously upside down, is then placed on the occlusion wax so that it fits onto the relative impression. The lower model is now inclined vis-a-vis the basal plane of the upper base just as the patient's lower arch is, in occlusion, vis-a-vis the Camper plane. The lower model can therefore be attached provisionally to the lower cast by a rubber band or something similar. Finally, the lower moulding base (76) is attached to base (75): by inserting the studs (89) of the former into the clamp (85) of the latter, the two bases are clamped together and the caster is overturned onto a flat surface for the pouring of the lower cast, exactly as described for the upper cast.

Once the plaster is completely hardened, the two cast models are removed from caster (2) by uncoupling anterolateral sections (75') or (76') from posterolateral section (75") or (76") of bases (75) and (76) . On figures 12 and 13 number (94) indicates an adjustment support for assembly (4). As will be clarified later, if this support is used, spirit level (37) can be replaced by a simple conventional spirit level (37') which has no goniometric grid. Support (94) incorporates a base plate (95), a pair of uprights (96) and an upper cross-beam (97) which supports orientator-truer assembly (4). It is also supplied with a pair of seats (98) for studs (66) and a

clamp (99) which locks the studs.

Plate (95) is supplied with three adjustable feet (100) each of which incorporates a screw screwed into the plate with knurled head (101) for manual adjustment and a spirit level (102) to verify the plate lie is horizontal.

Support (94) is used as follows.

The initial phases of the method are as described earlier up to the measurement of the inclination of the patient's upper dental arch using spirit level (37'). The latter has no graduated scale, so that the operator can only mark the balance point of the bubble with a vetrographic pen but cannot read off the inclination angles directly. Subsequently, fork (3) is mounted on the orientator-truer assembly (4) as described. Assembly (4) is secured in reverse position (i.e. with studs (66) and impression face (26) on fork (3) facing upwards) on support (94) (figure 13). It is secured by adjusting clamp (99) after using spirit level (102) to check that the plate is horizontal and after using screws (100) to adjust any plate tilt. If hinges (45) and (46) are in "zero" position i.e. if fork (3) lies on the plane identified by hinge axes "a" and "b", that plane will be parallel to plate (95).

On completion of the above, the assembly (4) hinges can be adjusted to reproduce the precise inclination of fork (3) that corresponds to the effective position of the

patient's upper arch vis-a-vis the Camper reference plane (which is assumed to be horizontal) to ensure that the bubble equilibrium point coincides with the position recorded earlier. Once the hinges are locked into this position, assembly (4) can be removed from support (94) and mounted on upper moulding base (75). Thereafter the upper cast can be poured as described earlier. All subsequent phases are exactly as described earlier. Any examination of the method used to make gnathostatic models following the dictates of the present invention and the device for carrying out the method will reveal its obvious advantages. Above all, the method makes it possible to make gnathostatic models that reproduce the individual orientation of occluded dental arches vis-a-vis an anatomicocranial reference plane, while simultaneously providing for the systematic truing of the anatomical portion of the models inside the art portion or cast, thereby ensuring that geometrical characteristics are constant. The models obtained in this way are particularly useful, since by simply arranging them in occlusion on a support plane on one of their bases (e.g. the posterior or one of the lateral bases) the operator can quickly identify by clinical observation both the occlusal relationships and the individual craniofacial lie of the dental arches.

In addition, since the geometrical characteristics of the cast perimeter remain constant, it is easy to

compare a sequence of clinical situations in a single case or different cases.

The identification of the orientation of the patient's upper dental arch vis-a-vis the reference plane is easily performed without any physical contact between the impression fork and the anatomical facebow. Hence, the application of the facebow to the patient's head requires very little pressure on the rest points, since there is no need for any external pressure on the facebow and the operation does not inconvenience the patient in any way. This itself is an important factor, since most orthodontic work is on children. The gnathostatic models obtained in this way offer concrete individual information on basal lie and overjet, vertical lie and overbite, the inclination of the occlusal plane as well as the absence or presence of orientation asymmetries.

Finally, it is clear that the method and instrumental system described can be supplied with modifications and variants that remain within the protection of the present invention. In particular, the Frankfurt plane can replace the Camper plane as a reference plane. In addition inclinometric relationships may be deduced from a cephalometric tracing rather than being measured directly on the patient, using a facebow. Finally, moulding bases of any other shape may be used instead of the one described.