WO2012042547A1

WO2012042547A1 - Rapid palatal expander with anti-return tooth

Info

Publication number: WO2012042547A1
Application number: PCT/IT2011/000079
Authority: WO
Inventors: Marco Valle
Original assignee: Lancer Italia S.R.I
Priority date: 2010-09-30
Filing date: 2011-03-18
Publication date: 2012-04-05
Also published as: ITMI20101788A1; IT1401989B1

Abstract

A rapid palatal expander includes a double -screw jack having a cylindrical head with two diametric holes that intersect at 90° for insertion of a starting key. Two screws threaded clockwise and anticlockwise extend from the two bases of the head, engaged with two respective nut screw blocks joined, by palatal arms, to metal crowns cemented to the molars of the upper semi -arches. Two pins pass through the blocks from opposite sides in relation to the double screw and guide them in opposite directions during rotation of the screw jack. A circumferential tooth projects from the cylindrical surface of the head of the screw jack, exerting pressure against the pins to prevent any undesired rotation that might reduce the degree of expansion achieved.

Description

Rapid palatal expander with anti-return tooth

Field of application of the invention

The present invention relates to the field of orthodontic appliances and in particular to a rapid palatal expander fitted with an anti-return tooth. Review of the known art

The Rapid Palatal Expander (RPE) is a therapy used in orthodontics to correct malocclusion of the mouth due to insufficient transversal growth of the upper jaw. The therapy is carried out by temporary application to the mouth of an orthopaedic device called a "rapid palatal expander" (or disjunctor) that opens the palatine suture and consequently widens the palate to create more space between the teeth and allow the mouth to close properly. RPE therapy is generally used on very young children when there is not yet fusion of the palatine suture; once synostosis has been completed, however, surgical help would be needed to give an RPE.

Though several types of rapid palatal expanders exist (Haas, McNamara and others) they all have the same basic structure comprising:

- a double-screw jack with a cylindrical head from whose bases two threaded bars project in opposite directions, their ends being engaged in two nut screw blocks. In the head are two diametric holes intersecting at 90° for insertion of a key fo r rotating the head by a one-quarter turn at a time; - two pins sliding in holes in the nut screw blocks, on opposite sides in relation to the head, to guide translation of the blocks in opposite directions while the screw jack is rotating;

- at least two metal crowns, called molar or premolar bands, each fitted onto a tooth of the upper dental semi-arch and cemented to said tooth;

- rigid palatal arms that join the screw jack to a respective molar band, said arms being shaped to place the blocks close to the palatal mucous on the sides opposite to the palatine suture and transversal to it.

Actuating the screw jack in the sense of moving the nut screw blocks farther from each other, determines the pressure exerted by the palatal arms against the respective arches and consequent opening of the palatine suture causing a transversal expansion of the upper jaw.

While the palatal expander is in place, the jack is subjected to continuous stress, mainly due to mastication, and this tends to cause the jack head to rotate in the direction opposite to that which causes expansion. In that case the screw blocks come closer together thus reducing pressure on the upper dental semi-arches. To avoid this happening palatal expanders have been devised fitted with anti-return devices.

Subject of patent application WO 2006/124680 A1 is a palatal expander wherein there are four radial protuberances, set at 90° one from another, on the head of the screw jack acting as arms to activate the expander. There are therefore no diametric holes in the head and the protuberances are all inclined in the same direction so that each one has a concavity by means of which the patient can make the jack work in the correct direction of rotation. The expander has two elastic slats that respectively project from the two guides on the screw blocks towards the head of the jack and which, when the expander is working, come in contact with the protuberances at their convex surface. The torque applied causes the slats to bend so that the teeth project beyond them. If the head of the jack rotates in the opposite direction, the teeth make contact with the slats on the side of the concavity. In that case a much higher torque would be needed to bend the slats compared with that required only for the tendency of the expander to slacken while in action. The slats therefore oppose rotation of the head of the jack in the direction opposite to the operative direction, acting as an anti-return device.

Subject of patent application US 2003/0207225 A1 is a palatal expander wherein two circular flanges on the head of the jack radiate beyond the head and threaded bars. On the edge of the flanges are triangular notches with differently inclined sides. There is a pawl on the expander which projects from a guide on the screw blocks towards the head of the jack so that the pawl is slightly pressed against the edge of the flanges. The pawl is made of a material sufficiently elastic to allow it to bend, while the expander is functioning, and maintain contact with the edge of the flanges. The notches and the pawl together constitute a free-wheel mechanism that stops the head of the jack from rotating in the non-activating direction.

Subject of patent application US 2007/0275341 A1 is a palatal expander in which a helical spring presses the pawl against the head of the jack. The spring is placed in a housing connected to a guide on the nut screw blocks and occupies a radial position in relation to the head of the jack. The pawl partially fits into one of the diametric holes in the head for insertion of the starting key thus preventing any rotation by the head. When activating the expander the key is inserted in the hole in the head occluded by the pawl, then pressed out. The head of the jack is therefore free to rotate and during its rotation the spring presses the tip of the pawl against the lateral surface of the head. Activation having been completed, the tip of the pawl is aligned with another hole in the head into which it snaps once again preventing any rotation. Subject of patent US 5,281 ,133 is a palatal expander wherein a belt is fixed by its ends to the two guides on the nut screw blocks and is therefore pressed against the head of the jack. On the belt is a rounded protuberance that penetrates into one of the holes in the head for insertion of the starting key. Belt elasticity is such that the torque needed to get the protuberance out of the hole causing the head of the jack to rotate, is greater than that needed for the tendency of the expander to slacken while operative. The belt therefore opposes any undesired rotation by the head of the jack.

There are also palatal expanders in which the head of the jack has four longitudinal lobes each between one hole and the next, and a slightly arched metal slat fixed transversally to the two guides of the nut screw blocks so that the concavity faces towards the head of the jack. The slat is so bent that, during activation, the lobes press against it so thus further accentuating the arch. As the torque needed to bend the slat is greater than that for the tendency of the expander to slacken while operative, the slat prevents any undesired rotation by the head of the jack.

In comparison with the structure of the palatal expander of above, the described anti-return devices are more or less complex additional components that increase the difficulties of making the expander, difficulties that are then reflected in higher production costs.

Further, whenever the jack is at its maximum length before the desired expansion has taken place, dentists encounter difficulties over disassembling the jack on the spot to replace pins and the double screw with longer ones. The main difficulty consists in fitting the head of the jack in a notch in the wall of the guide pins, there usually being a notch in present palatal expanders to prevent the pins coming away from the screw blocks. Dentists therefore have to remove the whole palatal expander and put in a new one. This obviously involves making a new cast of the patient's upper arch and using it to make new molar bands and the palatal arms of the new expander causing delays and extra costs.

Purpose of the invention

Purpose of the present invention is therefore to overcome the above drawbacks and propose a rapid expander provided with anti-return means that do not require any additional components.

Summary of the invention

In order to achieve this purpose, subject of the invention is a rapid palatal expander comprising:

- a screw jack with a cylindrical head extending from the bases of which are two respective bars fixed to the head, threaded clockwise and anticlockwise that engage two respective nut screw blocks moving them in opposite directions during rotation of the head;

- means engaged in the nut screw blocks to guide them in said opposite directions;

- palatal arms that join said nut screw blocks to crowns anchoring them to respective upper dental semi-arches, said arms being shaped so that the screw jack is close to the palatal mucous, substantially in the centre of the palate, rotation of the head of the screw jack in a previously set direction thereby causing reciprocal moving away of the two nut screw blocks and consequent expansion of the palate;

- means for anti-return to prevent undesired rotation of the head of the screw jack in the direction opposite to the previous one;

wherein, according to the invention, the anti-return means include a circumferential tooth projecting from the cylindrical surface of the head of the screw jack to exert pressure against said guide means, as described in claim 1 .

Further characteristics of the present invention considered innovative are described in the dependent claims. \ According to one aspect of the invention the circumferential tooth is discontinuous at two diametric holes, in the cylindrical head of the screw jack, intersecting at 90°.

According to another aspect of the invention the profile of the circumferential tooth is triangular.

According to one aspect of the invention, the means for guiding the nut screw blocks include two pins engaged in holes in said nut screw blocks situated on opposite sides in relation to the holes that engage the threaded bars.

According to one aspect of the invention in each pin there is a notch into which the head of the jack is partly fitted, to prevent the pins from emerging from the respective holes in the nut screw blocks, the circumferential tooth exerting pressure against the inner wall of the notch.

According to another aspect of the invention the notches extend beyond the head of the jack as far as the opposite end of the pins.

According to another aspect of the invention, at least two toroidal rings of resilient material are fitted on the two threaded bars close to the head of the screw jack, each ring being compressed between the bar, onto which it is fitted, and the wall beyond the notch in the adjacent pin to prevent said pin from accidentally slipping.

According to another aspect of the invention, the lengths of threaded bars onto which the rings are fitted, have no threading.

Advantages of the invention

The main advantage of the invention compared with the known art lies in there being no need for any additional anti-return means, said circumferential tooth forming an integral part of the screw jack head and, as such, being made during the same machine tool process.

In the form already produced where the notches extend as far as one end of the pins, assembly of the expander, being simpler, may be done in situ so that it can be further, and more easily, lengthened. In this way, whenever the jack reaches is maximum expansion, but even more is needed, dentists can reuse the molar bands, palatal arms and blocks of the palatal expander to be replaced, and assemble a new palatal expander themselves so shortening the times and reducing the costs of the operation.

Short description of the drawings

Further purposes and advantages of the present invention will be made clearer from the following detailed description of an example realised and from the attached drawings provided for purely explanatory reasons which in no way limit the invention, wherein:

Figure 1 is a perspective view from below of a palatal expander according to the present invention, fitted to the upper dental arch of a patient.

Figure 2 is an exploded perspective of the palatal expander in Figure 1 , before being fitted.

Figure 3 is a view from above of the central body of a double-screw jack forming part of the expander in Figure 1.

Figure 4 is a central cross section of the operative head of the jack in Figure 3.

Figure 5 is a side view of an element in Figure 2.

Figure 6 is a longitudinal section according to plane A-A in Figure 5. Figure 7 is a view from above of another element of Figure 2.

Figure 8 is a partial enlargement of the palatal expander in Figure 1 . Figure 9 is a cross section through plane B-B in Figure 8

Figure 10 is a variation realised of the palatal expander in Figure 8. Detailed description of some preferred forms of realising the invention In the following description identical parts that appear in different figures may be marked with the same symbols. In describing a figure reference may be made to parts not expressly shown in that figure but in preceding figures. Scales and proportions of the various parts illustrated do not necessarily represent reality.

Figure 1 shows a rapid palatal expander 1 fixed to the upper dental arch_of a patient. The expander is supported by two metal crowns 2, 3, called molar bands, fitted onto opposite molars in the two semi-arches and fixed to them with cement, for example, glass ionomer cement. The two molar bands 2, 3 are welded at one end to two respective arched bars 4, 5, 6, 7 called palatal arms, the other end being joined at the side to two respective nut screw blocks 8, 9 of a double screw jack 10. The pairs of arms 4, 5 and 6, 7 are shaped so that the nut screw blocks 8, 9 are close to the palatal mucous on the side opposite the palatine suture. The double-screw jack 10 has a cylindrical head 1 1 from the bases of which two cylindrical bars, 12, 13 extend; the bars of equal length threaded clockwise and anticlockwise are fixed to the head and respectively engaged in the two nut screw blocks 8, 9 at both ends. The ends of two cylindrical pins 14, 15 emerge from respective holes (hatched) in the nut screw blocks 8, 9 cut on opposite sides in relation to a central threaded hole for screwing to the threaded bars 12, 13. Pins 14, 15 guide translation of the nut screw blocks 8, 9 while the head 1 1 of the jack 10 is turning.

The pins 14, 15 are fitted at their ends with devices (not shown in the figures) to lock them in the position of maximum aperture to prevent disassembly of the palatal expander 1 when in action.

Instead of being welded to the metal crowns 2, 3 the palatal arms 4, 5 and 6, 7 can be joined to bands (not shown in the figures) that extend from crowns 2, 3 to rest on the palatal surface of the enamel of the teeth adjacent to said crowns.

Blocks 8, 9 can be covered with respective resin shields (not shown in the figures) that lightly touch the palatine mucous, to improve the shape of the palatine vault while the expander 1 is being worn.

Each of these parts of the palatal expander 1 is made of ISO 5832-9 stainless steel possessing good mechanical characteristics due to the presence of nitrogen and manganese. Alternatively other types of biomedical steel can be used such as ISO 5832-1 D and ISO 5832-1 E possessing inferior mechanical characteristics but also being less costly than ISO 5832-9 steel. The palatal expander can also be made of titanium, a material combining high biocompatibility with the advantage of not producing allergies and of having no adverse effect on food flavours.

Figure 2 shows an exploded view of the palatal expander 1 without the molar bands 2, 3 and with the palatal arms 4, 5 and 6, 7 before being bent. Blocks 8 and 9 are made in a single piece, centrally to the two respective arms 4, 5 and 6, 7 aligned longitudinally. Three holes 20, 21 , 22 are cut through each block 8 and 9 transversally to the arms. Hole 20, between holes 21 and 22, is threaded. Pins 14 and 15 are inserted in holes 21 and 22 in the two blocks 8, 9. The middle holes 20 in the two blocks 8, 9 are threaded clockwise and anticlockwise to screw in the ends of threaded bars 12, 13. Two notches are respectively made in a central position in the cylindrical wall of pins 14 and 15. Pins 14, 15 are inserted into the respective holes 21 and 22 with notches 23 and 24 facing each other.

Figure 3 shows the head 1 1 of the screw jack 10 with the threaded bars 12, 13 that extend from its two bases. Head 1 1 and bars 12, 13 are made by turning from a single piece. The diameter of head 1 1 is obviously greater than that of the threaded bars 12, 13. There is a transversal cut 12a in threaded bar 12 at its free end so that a screwdriver can be used to screw bar 13 to block 9 while the expander

I is being assembled. A circumferential tooth 27, discontinuous at orthogonal diametric holes 25, 26 (Figure 4) projects centrally from the lateral surface of the head 10. As an example, the ratio between the height of tooth 27 and the radius of the bases of head 1 1 of jack 10 is about 1/10.

Figure 4 shows a central cross section of the head 1 1 of jack 10 at the crest of tooth 27, where it will be seen that two diametric holes 25, 26 pass through the head 1 1 , said holes being reciprocally orthogonal and their axes intersecting at a common centre. These holes serve for insertion of a starting key (not shown in the figures) to rotate the head

I I of jack 10 by a quarter turn in the desired direction. Figure 5 shows the nut screw block 8 joined to the palatal arms 4 and 5, aligned along the longitudinal axis, between their two ends. The figure also shows that holes 20, 21 and 22 have substantially the same diameter and are at an equal distance transversally to the longitudinal axis of arms 4, 5. The description given for block 8 also applies to block 9.

Figure 6 shows a section of the ensemble in Figure 5 along a longitudinal plane through the centres of aligned holes 20, 21 and 22 where it will be seen that the middle hole 20 is equidistant from holes 21 and 22.

Figure 7 shows the pin 14 (or 15) with a rectangular notch 23 (24) at its centre. In holes 21 (22), pin 14 (25) is used to link the two blocks 8 and 9 while not preventing them from translating. Notch 23 (24) is delimitated by a longitudinal wall 30 joined orthogonally, at the two sides, to two transversal walls 31 , 32.

Figure 8 shows a configuration close to the maximum extension made possible by screw jack 10. In that configuration the ends of pins 14, 15 emerge for a short length from holes 21 , 22 in blocks 8, 9, as do the ends of the threaded bars 12, 13 from the threaded holes 20 into which they are screwed. Head 1 1 partly fits into notches 23, 24 with both holes 25, 26 accessible. In the same configuration two opposite arches, of the four in which the circumferential tooth 27 is divided by holes 25, 26, are in contact with the longitudinal walls 30 of notches 23, 24. The distance between guide pins 14, 15 is such that the arches of tooth 27 exert pressure against the longitudinal walls 30 of notches 23, 24. The longitudinal walls 30 of notches 23, 24 are slightly longer than the height of the cylindrical head 1 1. The double screw of jack 10 is formed of the cylindrical head 1 1 with the bars 12, 13 threaded clockwise and anticlockwise aligned as from each base of the head. Pins 14, 15 are parallel to the double screw, which is equidistant from both, and whose axis is coplanar with the axis of both. Operatively speaking, starting from a configuration in which blocks 8, 9 are at the minimum distance from the head 1 1 , the palatal expander 1 is activated once a day using the key provided to make the head 1 1 do a quarter turn. Direction of rotation, namely that in which the nut screw blocks 8, 9 withdraw one from another, is shown by the arrows marked on blocks 8, 9 in Figure 1. The fact that the head 1 1 is partly fitted into notches 23, 24 prevents the pins 14, 15 from emerging through the holes 21 , 22 in both blocks. When the head 11 is turned to the position seen in Figure 8, the friction set up, between the above arches of circumferential tooth 27 and the longitudinal walls 30 of notches 23, 24, is such as to prevent an undesired relaxation of the thrust exerted by screw jack 10 against the upper dental semi-arches on account of stresses to which the palatal expander 1 is continually subjected, especially during mastication.

Figure 9 shows a cross section of the head 1 1 of pins 14, 15 in Figure 8 along plane B-B passing by the crest of tooth 2. The figure brings out the fact that, with each increase of palatal expansion, the head 1 1 must necessarily rotate by a quarter turn so that, on completing the rotation, a new hole in head 1 1 is accessible for continued activity.

The head 1 1 is not exclusively in contact with guide pins 14, 15 during the brief moment of rotation and never when rotation is completed (and this also facilitates activation). Therefore, when active, there is always pressure between the arches of circumferential tooth 27 and the walls 30 of notches 23, 24.

Figure 10 illustrates a variant of the palatal expander in Figure 1. The figure shows a palatal expander where, for simplicity, the nut screw blocks as well as the arms and anchoring crowns have been omitted. The palatal expander includes a screw jack 33 and a cylindrical head 35 from whose bases two cylindrical bars 36, 37 of equal length extend, fixed to the head 35, threaded clockwise and anticlockwise and respectively engaged in two nut screw blocks (not seen in the figure). The diameter of head 35 is greater than that of the threaded bars 36, 37 and is identical to that of head 1 1 of the expander 1 . The threaded bars 36, 37 are longer than threaded bars 12, 13 and are without threading for a short length 36a, 37a starting from the respective sides of the head 35. In the nut screw blocks are three aligned holes, the central one being for screwing in the respective threaded bars 36, 37 and the other two for insertion of pins 38, 39 that guide translation of the nut screw blocks and prevent them from rotating. In the cylindrical wall of pins 38, 39 are two respective rectangular notches 40, 41 that extend from substantially central positions to opposite ends. Each notch 40, 41 is delimitated by a longitudinal wall 42 joined orthogonally, at one side, to a transversal wall 43. Pins 38, 39 are mounted with the longitudinal walls 42 of notches 40, 41 parallel and reciprocally facing a central section at least as long as the height of head 35 which can therefore be partially included between the notches, both holes 25, 26 being accessible. In a configuration such as this two opposite arches, of the four arches into which the circumferential tooth 27 is divided, are in contact with the longitudinal walls 42 of notches 40, 41. Distance between pins 38, 39 is such that the above arches of said tooth 27 exert pressure against the longitudinal walls 42 of notches 40, 41. Two pairs of silicon toroidal rings 44, 45, also known ad O-rings, are respectively fitted across the threaded bars 36, 37 at non-threaded sections 36a, 37a. Thickness of the O-rings is such that the pair of O-rings 44 is compressed between threaded bar 36 and pin 39, externally to notch 41. Similarly, the pair of O-rings 45 is slightly compressed between bar 37 and pin 38, externally to notch 40.

Exactly as in the case of palatal expander 1 , the nut screw blocks in which the threaded bars 36, 37 are engaged, are joined to palatal arms fixed to molar bands cemented to the teeth of the patient's upper arch. When the expander is in use the head 35 of the screw jack occupies the position described in Figure 10. In that position, the friction set up between the arches of tooth 27 and the longitudinal walls 42 of notches 40, 41 is such as to prevent undesired rotation of head 35 tending to reduce the degree of expansion achieved. Further, the friction created, by compression of the pairs of O-rings 44, respectively, between bar 37a and pin 39 in the part beyond notch 41 , and by compression of the pair of O-rings 45, respectively, between bar 37a and pin 38 in the part beyond the notch 40, prevents pins 38, 39 from emerging through the holes in both nut screw blocks.

The palatal expansion described in Figure 10 is applied, for example, in cases where the palatal expander used on a patient has reached its maximum and greater expansion is required. When that happens, dentists usually remove the whole palatal expander, disassemble it and recover the molar bands, palatal arms and the blocks. The new expander is then assembled, this being done as follows:

1 . Screw the threaded arm 37 into the corresponding nut screw block as far as the section 37a without threading;

2. Screw the threaded arm 36 into the other nut screw block as far as the section 37a without threading;

3. If necessary slightly unscrew one or both nut screw blocks until holes 21 and 22 are aligned;

4. Press hard (because of the presence of O-rings 44 and 45 and because of friction with the tooth 27) to insert pin 38 in holes 22 on the side of notch 40 until the transversal wall 43 of notch 40 is close to the head 35;

5. Press the pin 39 into holes 21 on the side of notch 41 , until the transversal wall 43 of notch 41 is close to the head 35;

6. Rotate head 35 of the screw jack using the starting key of the expander in the direction opposite to that indicated by the arrows on the blocks, to make the blocks move away one from another until the molar bands can once again be fitted onto the patient's teeth. Differently from the case of palatal expander 1 , notches 40, 41 of pins 38, 39 extending as far as one end of pins 38, 39, means that the palatal expander described in Figure 1 1 can be assembled by dentists without any need for special equipment.

Based on the description given of a preferred example of realization of the present invention, some changes can obviously be made by experts in the field without thereby departing from the sphere of the invention as will be clear from the following claims.

Claims

1. A rapid palatal expander including:

- a screw jack (10) with a cylindrical head (1 1 ) extending from the bases of which are two respective bars (12, 13) fixed to the head (1 1 ), threaded clockwise and anticlockwise, that engage two respective nut screw blocks (8, 9) moving them in opposite directions during rotation of the head (1 1);

- means (14, 15) engaged in the nut screw blocks (8, 9) to guide them in said opposite directions;

- palatal arms (4, 6) that join said nut screw blocks (8, 9) to crowns (2, 3) anchoring them to respective upper dental semi-arches, said arms (4, 5, 6, 7) being shaped so that the screw jack is close to the palatal mucous, substantially in the centre of the palate, rotation of the head (1 1 ) of screw jack (10) in a previously set direction thereby causing reciprocal moving away of the two nut screw blocks (8, 9) and consequent expansion of the palate;

- means for anti-return to prevent undesired rotation of the head (1 1 ) of the screw jack (10) in the direction opposite to the previous one, characterized in that said anti-return means include a circumferential tooth (27) projecting from the cylindrical surface of the head (1 1 ) of screw jack (10) to exert pressure against said guide means (14,15).

2. Rapid palatal expander as in claim 1 , characterized in that there is a break in the circumferential tooth (27) at the position of two diametric holes (25, 26), in the cylindrical head (1 1) of screw jack (10), that intersect one another at 90°.

3. Rapid palatal expander as in claim 1 , characterized in that the circumferential tooth (27) has a triangular profile.

4. Rapid palatal expander as in claim 1 , characterized in that said guide means include two pins (14, 15) engaged in holes (21 , 22) of the nut screw blocks (8, 9) situated on opposite sides in relation to the holes (20) that engage said threaded bars (12, 13).

5. Rapid palatal expander as in claim 1 , characterized in that in each guide pin (14, 15) there is a notch (23, 24) in which the head (1 1 ) of the screw jack (10) partially fits to prevent the pins (14, 15) emerging from respective holes (21 , 22) in the nut screw blocks (8, 9) the circumferential tooth (27) exerting pressure against the internal wall of the notch (23, 24).

6. Rapid palatal expander as in claim 5, characterized in that said notches (40, 41 ) extend beyond the head (35) of the screw jack (33) as far as the opposite end of the pins (38, 39).

7. Rapid palatal expander as in claim 5, characterized in that at least two toroidal rings (44, 45) of resilient material, preferably silicon, are respectively fitted onto the threaded bars (36, 37) close to the head (35) of the screw jack (33), each ring being compressed between the bar, onto which it is fitted, and the wall beyond the notch (40, 41 ) of the adjacent pin (38, 39) to prevent said undesired slippage of said pin.

8. Rapid palatal expander as in claim 7, characterized in that two respective pairs of toroidal rings (44, 45) of resilient material, are fitted onto said threaded bars (36, 37) close to the head (35) of the screw jack (33).

9. Rapid palatal expander as in claims 7 or 8, characterized in that there is no threading (36a, 37a) on a section of said threaded bars (36, 37) close to the head (35) onto which said toroidal rings (44, 45) are fitted.

10. A screw jack (10) for use in a rapid palatal expander, said screw jack having a cylindrical head (1 1 ) from the bases of which two respective bars (12, 13) extend, fixed to the head (11 ) threaded clockwise and anticlockwise to engage two respective nut screw blocks (8, 9) guided in opposite directions during rotation of the head (1 1 ), characterized in that it includes a circumferential tooth (27) that projects from the cylindrical surface of the head (1 1 ) to exert pressure against two guide pins (14, 15) of said nut screw blocks (8, 9) to prevent undesired rotation of the head (1 1 ).

1 1 . A bar (14) utilizable as a guide pin in a rapid palatal expander to guide, in a pair of bars (14, 15) laid parallel, two nut screw blocks (8, 9) drawn in opposite directions by a screw jack (10) having a cylindrical head (1 1 ), characterized in that it includes a central notch (23) into which the head (1 1 ) of the screw jack (10) can be partially fitted so preventing translation of the bar (14).