The present invention relates to dental tools, and in particular tools for use in placing filling material into teeth and, in particular, root canals of teeth. Traditionally, no single tool has been available for placing root filling material such as, for example, calcium hydroxide powder or MTA (Mineral Trioxide Aggregate), into dental root canals effectively. In particular, no single tool has been available for placing calcium hydroxide powder into children's root canals. As shown in figs. l(a) and (b), while mature (i.e. adult) teeth 1 are formed with a closed apex 2 at the root, and a flared root canal 3, immature (i.e. children's) teeth 5 have an open apex 6 at the root (where periapical tissue is exposed) and a wide and parallel root canal 7. Root canal treatment is required in children in the event of a child traumatising their teeth, most commonly the incisors. Such trauma could, for example, be due to a blow to the tooth leading to fractures and luxations or combinations of both. If left untreated and undiagnosed infections may grow and lead to an abscess - an acute and destructive process that results in the localised collection of pus sacs in the oral cavity, which would lead to rotting. Therefore root canal treatment is required in which a paediatric dentist can repair and save the traumatised tooth. The procedure involves the difficult technique of placing calcium hydroxide powder into the open apex of a root canal in a child's tooth. This is usually done by cleaning out the tooth (from the crown end), placing calcium hydroxide powder up into the root canal so as to form a root end closure at the apex of the tooth against which a GP (Gutta Percha) filling can be condensed without the possibility of sealant or GP going through the apex into the periapical tissues. Filling root canals is a difficult procedure in that access to the root canal is awkward. It is particularly difficult in children due to the smaller size of a child's mouth. WO 01/67980 Al describes a dental device for dispensing a slurry-like dental compound, such as MTA, into a tooth cavity, for example a root canal. This device is designed for retrograde filling of a root end-canal which has been exposed by surgery through gum and bone. In mature (adult) teeth, the apex of the tooth root is normally closed. Prior to use of the device disclosed in WO 01/67980 Al the root of the tooth must first be re-sectioned (flattened) using a surgical bur and a root end-canal is then created to a certain depth using a cutting tool. The device of WO 01/67980 Al comprises a delivery tool with a dispensing tip supporting a plastic tubing sleeve. Hydrated MTA is pressed manually (by a user) into the empty sleeve to fill the sleeve. The free end of the sleeve is then placed around the entry to the prepared root canal to be filled and downward pressure must then be applied manually (by the user) to the dispensing tip so as to move the tip relative to the sleeve. This movement forces the hydrated MTA down into the root canal. This device is not designed for pushing MTA up a root canal towards the apex. It is an aim of the present invention to avoid or minimise one or more of the foregoing disadvantages. According to the present invention there is provided an apparatus for dispensing root filling material into a dental root canal, the apparatus comprising: a housing provided with a loading chamber for holding root filling material; a guide sleeve mounted to the housing and having one end protruding therefrom, the guide sleeve being in communication with the loading chamber for receiving root filling material therefrom; a movable rod, disposed at least partially within the housing, for dispensing root filling material from the protruding end of the guide sleeve; and actuator means for moving the rod so as to cause root filling material to be dispensed from the protruding end of the guide sleeve. At least a portion of the movable rod may be disposed within the guide sleeve. The movable rod is preferably formed and arranged for sliding movement in a longitudinal direction through the bore of the guide sleeve in use of the apparatus. Desirably, the cross-sectional shape of the rod and the guide sleeve may be substantially the same and the cross-sectional diameter of the rod is preferably close to, but slightly less than, the cross-sectional diameter of the guide sleeve. This means that the rod will be particularly effective in pushing the root filling material out of the guide sleeve, into the root canal to be filled. Conveniently the guide sleeve may be transparent in order to allow visualisation of material within the guide sleeve. Preferably, the actuator means comprises a rack and pinion arrangement. The actuator means may conveniently further include user control means, which may conveniently be in the form of a sliding switch, for controlling the operation of the rack and pinion arrangement. The rack and pinion arrangement preferably comprises a first rack fixed to the housing, a second rack movable relative to the housing, and a pinion mounted between and in operational engagement with the two racks. The movable rod may conveniently be attached at one end to a distal end of the movable rack, with the other end of the rod pointing towards the protruding end of the guide sleeve. The slidable switch may conveniently be attached to the pinion such that movement of the switch by a user causes the pinion to move along the fixed rack which, in turn, causes the movable rack to move the rod towards or away from the protruding end of the guide sleeve (the direction of movement being dependent on the direction in which the user slides the switch). It will be appreciated that an advantage of using one fixed and one movable rack is that for any distance, d, travelled by the pinion, the distance travelled by the movable rack (and hence the rod attached thereto) is twice that of the pinion. This provides the device with an extended reach useful when filling large (i.e. long) root canals. Preferably, the length of the rod is sufficiently long that the rod can be moved to an extended position (using the actuator means) in which the rod extends through and protrudes beyond the protruding end of the guide sleeve. This has the advantage that the rod can thus be used to pack the dispensed root filling material up the root canal. Distance indicator makers may also be provided on the rod so as to allow the user to see the distance the rod has been extended from the end of the guide sleeve and into the root canal. Advantageously the actuator may be formed and arranged so as to enable the rod to be moved between said extended position and a retracted position in which the free end of the rod is disposed within the loading chamber, or on the opposite side of the loading chamber to the protruding end of the guide sleeve. By moving the rod to its retracted position, this allows filling material in the loading chamber to flow freely into the guide sleeve (with which the chamber is in communication). The user positions the apparatus so that the protruding end of the guide sleeve is disposed adjacent to a root canal to be filled. By then moving the rod towards its extended position, the rod pushes the material in the guide sleeve out of the protruding end of the guide sleeve into the patient's root canal. By continuing to move the rod to its extended position the dispensed material is packed firmly up into the patient's root canal. It will be appreciated that the guide sleeve can easily be re-loaded with more filling material by simply retracting the rod so that the free end thereof moves back far enough to allow more filling material to flow into the guide sleeve from the loading chamber, and then moving the rod forward again to dispense more material into the root canal. The loading chamber is preferably disposed proximal to an end of the housing from which the guide sleeve protrudes. The loading chamber may conveniently be detachable from the housing to allow for easy loading and re-filling of the chamber with root filling material. For example, the loading chamber may be detachably mounted in a complementary recess provided therefor on one side of the housing so as to be visible to a user. Optionally, the loading chamber may comprise one or more transparent walls for enabling a user to view the contents of the loading chamber, in use of the apparatus. The guide sleeve may have one (open) end mounted within or adjacent to the loading chamber, by virtue of which filling material in the loading chamber can move freely into the guide sleeve for dispensing from the protruding end thereof. Alternatively, the guide sleeve may extend through the loading chamber and a section of the guide sleeve which passes through the loading chamber may be provided with an aperture for allowing filling material in the chamber to move freely into the guide sleeve. The protruding end of the guide sleeve may conveniently by bent at an angle to a main axis of the housing. This provides a more ergonomic design which is easier to use, reducing strain on the user's wrist. In this embodiment it will be appreciated that the movable rod must be flexible enough to allow it to manoeuvre through the bend in the sleeve. The apparatus is preferably provided in the form of a hand tool. Most conveniently, the apparatus may be provided in the form of a disposable hand tool. A preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Fig. l(a) is a cross-sectional side view of an adult tooth; Fig.l(b) is a cross-sectional side view of a child's tooth; Fig.2 is a perspective view of a hand-held tool according to one embodiment of the invention; Fig.3 is a perspective view of various components of the tool of Fig.2; Figs. 4(a) to (c) are schematic plan views of a "double stroke" rack and pinion arrangement; Fig.5 is an exploded view of the tool of Fig.2 illustrating how the tool is assembled from its various components; Fig. 6 is a detail perspective view of a front end portion of the tool of Fig.2, showing a detachable loading chamber; Fig.7 is a detail perspective view of the front end portion of the tool of Fig.6, with the loading chamber detached; Fig.8 is a detail perspective view of the front end portion of the tool of Figs. 2, 6 and 7, showing a packer rod protruding through a dispensing sleeve of the tool; and Fig.9 illustrates the tool of Fig. 2 in use on a dummy patient.

Fig.2 illustrates a disposable hand tool 10 for use in carrying and dispensing root filling material, such as calcium hydroxide powder, into the root canal of a child's tooth. The tool comprises a housing 12 within which a rack and pinion arrangement (not seen in Fig.2) is housed, for controlling sliding movement of a rod (also not seen in Fig.2), hereinafter referred to as the "packer rod" 30, in a guide sleeve 14 mounted in the housing but protruding from an operational end 11 thereof. A user may control movement of the pinion (so as to, in turn, control movement of the rod) by means of a sliding switch 16 protruding through a slot 18 provided therefor in an upper side 13 of the housing. A transparent detachable loading chamber 20 for carrying root filling material is snap- fitted into a complementary recess 21 provided therefor in an underside 15 of the housing, proximal to the operational end 11 of the tool 10. Fig.3 illustrates some assembled parts of the tool 10 prior to being inserted into the housing 12. Fig.5 shows these parts, together with other parts of the tool 10, prior to being assembled together to form the tool 10. When assembled, as seen in Fig.3, the sliding switch 16 is physically attached to the pinion 22 of the rack and pinion arrangement. This arrangement consists of a first rack (not seen in Figs.3 and 5) which is fixed to an interior surface of a side wall 17 of the housing (the housing itself is formed of two elongate parts 12a, 12b as shown in Fig.5), a second rack 24 which is movable relative to the housing, and the pinion 22 which is mounted between the two racks in toothed engagement with each rack. A distal end 25 of the movable rack 24 (hereinafter referred to as the "free rack") is attached to one end of the packer rod 30 which (when the tool is assembled as shown in Fig.2) extends through the bore of the guide sleeve 14 which, as seen in Figs. 3 and 5, extends through the housing 12 and protrudes from the operational end 11 of the housing. The protruding portion 19 of the sleeve 14 has a smooth bend provided therein, as seen in the drawings. The sleeve 14 is fixed to the housing. The sleeve 14 is mounted in the housing 12 such that the sleeve extends through the interior of the detachable loading chamber 20. The portion of the sleeve 14 which is disposed in the chamber 20 is provided with an aperture 40 (see Fig.6) via which filling material loaded in the chamber 20 can flow/move freely into the bore of the sleeve 14. Figs. 4(a), (b) and (c) illustrate the operation of the rack and pinion arrangement. This has a "double stroke" design which allows the user's input movement to be doubled. The system is controlled by the user directing the movement of the pinion 22 (via the sliding switch 16). When the user slides the switch 16 so as to move the pinion from a Position 1, shown in Fig. 4(a), to a Position 2, shown in Fig. 4(b), the distance travelled by the pinion is d. This is the distance the user moves their finger to move the pinion. In comparison, the free rack 24 travels distance 2d from Position 1 to Position 2. The maximum range of movement 4d required from the free rack 24 is achieved by moving the pinion by a distance 2d (i.e. half as much). The system works by making the pinion 22 effectively travel with double velocity. While the pinion rotates to move the free rack 24, the pinion is also moving relative to the fixed rack 26. Hence the distance travelled by the free rack 24 is twice that of the pinion. For use on children's root canal treatment a suitable maximum range of movement of the free rack (and hence of the rod) is 60mm (i.e. 4d = 60mm). The length of the rod 30 must obviously designed so that this maximum range of movement of the free rack extends the free end of the rod from the protruding end of the sleeve 14 by a desired (predetermined) maximum distance. The way in which the tool 10 is used to dispense and pack root filling material into a child's root canal will now be described. First the transparent load chamber 20, detached from the housing 12, is loaded with the root filling material (in this case calcium hydroxide powder, but in other cases other filling materials such as, for example, MTA, may be used). With the packer rod 30 in a retracted position, in which the free end of the rod is disposed on the opposite side of the sleeve aperture 40 to the protruding portion 19 of the sleeve 14, the chamber 20 is then clipped/snap- fitted back on to the housing 12. Relying on gravity, the material drops into the bore of the sleeve 14 via the aperture 40 provided in the sleeve. The tool 10 is then positioned by the user into the patient's mouth in a position for accessing the root canal (from the crown end of the tooth). The user then moves the switch forward (towards the protruding portion 19 of the sleeve) so as to cause the rack and pinion arrangement to move the rod 30 towards the operational end 11 of the tool 10, so as to push the filling material out of the protruding portion 19 of the sleeve 14 into the root canal to be filled. Once the material is dispensed into the root canal in this manner, the user continues to move the switch in the same direction so as to extend the rod through and beyond the protruding end 19 of the sleeve so that the rod 30 acts as a packer to push the dispensed filling material up the root canal, towards the open apex of the root. Additionally, the user can use the rod to apply further pressure to pack the dispensed material more tightly in the root canal by simply pushing the whole tool forward (the user keeping their finger pressing forward on the switch 16 so that the rod cannot retract) so as to apply further pressure with the free end of the extended rod 30. Reloading for repeated application is achieved by pulling the rod back (by the user moving the sliding switch 16 in the opposite direction) sufficiently to allow more material to drop into the sleeve from the loading chamber. The whole tool 10 will be disposed of once the root canal treatment has been completed. It will be appreciated that the double stroke design of the rack and pinion arrangement enables maximum extension of the packer rod to be achieved with a much smaller movement of the user's finger on the switch 16. This reduces wrist strain on the user. Moreover, by designing the packer rod 30 to be suitably long the tool is able to easily pack material up relatively long root canals. Additionally, the rod 30 can be used to apply pressure to pack the material tightly in the root canal. Thus the tool 10 has the advantage of being a single device capable of carrying and dispensing the filling material into the root canal and also packing the material in the canal. Additionally, the tool is self-reloading (until the material in the chamber 20 runs out) for repeated application of material, simply by the user manipulating the sliding switch 16. Moreover, the tool can be operated by a single hand 35 of the user, freeing up the user's other hand 36 to carry out different tasks such as holding a mirror 45 to view the tooth being treated, as shown in Fig. 9. The tool is ergonomic, fitting comfortably into the user's hand and requiring only the user's finger (most conveniently the index finger) on the sliding switch 16 to manipulate the movement of the rod 30 so as to dispense and pack filling material. Moreover, the shape of the tool fits easily and comfortably into the patient's mouth, the tool having a particularly slimline elongate from making it easy to use in small mouths such as children's mouths. The transparent loading chamber makes it easy for the user to see the amount of filling material available for application/re-application. If the sleeve 14 is also made of a transparent material it is also possible for the user to judge the distance the material has been moved towards/into the root canal. The tool 10 is designed to be disposable, the various components of the tool being made in a manner and of materials appropriate for a disposable device. In the above-described example, for instance, the two parts 12a and 12b of the housing are made of ABS and PVC respectively, the sleeve 14 is made of PVC, the pinion 22 is made of (rigid) nylon, the free and fixed racks and the switch are made of Acetal, the load chamber is made of clear Polycarbonate and the rod 30 is made of Polyethylene. (It will be appreciated that the rod must be sufficiently flexible to allow it to slide through the bent portion 19 of the sleeve 14.) The two-part housing, the free rack, the switch and the load chamber are conveniently made using injection moulding. The sleeve 14 and rod 30 are conveniently formed by extrusion. It will be appreciated that although the device described above is particularly suited for root canal treatment on children it could equally be used for treating adult teeth. It will though be appreciated that the size of the tool 10 and its component parts will be determined to suit the main intended application of the device. For example, where the device is intended for use on children's teeth the device will generally be designed to be smaller in size than a device designed for use on adult teeth. In the above described embodiment, intended primarily for use on children's teeth, the diameter of the packer rod 30 is designed to be in the range of 0.9- 1.0mm while the diameter of the sleeve 14 is designed to be approximately 0.2 - 0.3mm wider than the rod, to allow for smooth sliding movement of the rod in the sleeve. Also, these diameters keep the sleeve and rod sufficiently small to be easily disposed and manoeuvred in a child's mouth.