BACKGROUND ART An assembly of this kind is described in prior patent specification WO 90/07912. The assembly described has a disposable plastics syringe tip which is connected via an adaptor to a handpiece body to which air and water are supplie. The tip can be discarded after use to avoid spread of infection between patients. The adaptor is selected for the particular handpiece body to permit use of a tip having a standardised connection. It is also possible to connect a disposable plastics syringe tip directly (i. e. without an adaptor) to a dedicated handpiece body.

Whilst this known arrangement provides an effective means of preventing transfer of infection via material retained within the tip, there is the possibility of infected material from one patient gaining access beyond the tip to the handpiece body or adaptor from where it can be passed on to the next patient through a fresh tip.

An object of the present invention is to provide means-to avoid such transfer of infection due to access of infected material beyond the tip.

DISCLOSURE OF THE INVENTION According to the invention therefore there is provided a dental syringe assembly comprising a handpiece body adapted to be connected to sources of air and water, and a tip arranged to be connected to the handpiece body for directing flow of air and water from the handpiece body into the patients mouth, characterised by the provision of a non-return valve arranged to be connected between the tip and the handpiece body.

With this arrangement, the valve can help prevent draw back along the tip from the open end thereof of contaminated fluids and can also block access of infected material beyond the tip into the handpiece body and thereby help prevent transmission of infection between patients.

In the case where the syringe assembly is arranged for separate delivery of air and water, the non-return valve is preferably applied to the water delivery, although alternatively or additionally, the valve may be applied to the air delivery.

In a particularly preferred embodiment, the tip has separate air and water passages which may be arranged coaxially or alongside each other and these are connected separately to air and water inlets at the handpiece body, and in this case the non return valve is preferably applied to the water passage only.

The non-return valve may be of any suitable form. In a preferred embodiment, the valve has a spring loaded valve member, such as a ball,

which is urged into sealing engagement with a valve seat, fluid pressure during normal operation being sufficient to move the member off the seat to permit passage of fluid.

Other kinds of non-return valve can also be used. For example, in an alternative embodiment, the valve is of the constriction kind whereby valve parts, such as silicon rubber reeds, tend to collapse or are urged into sealing contact with each other but move apart in normal operation to permit passage of fluid.

In the case where the valve is applied to water delivery which is separated from air delivery, the valve may incorporate an outlet flow nozzle to direct flow into the water passage of the tip and help prevent communication, and hence transfer of material, between the air and water passages.

This nozzle may be such as to extend into the water passage of the tip and help form a seal therewith. The nozzle may be tapered to form a tight wedging fit within the tip passage and this may contribute to or establish the seal. Alternatively or additionally there may be an O-ring seat or other sealing arrangement to fit around the tip passage, for example an O-ring seal concentrically around the said nozzle whereby a tubular part of the tip defining an inlet to the water passage fits tightly between the nozzle and the O-ring seal.

In one embodiment, the valve may form part of an adaptor used

between the handpiece body and the tip such adaptor having one end arranged to be screwed or otherwise tightly and securely mated with the particular handpiece, and an opposite end having means for detachable engagement with the tip.

As described in the above mentioned prior patent specification, the tip may be disposable and the means on the adaptor for detachable engagement with the tip may be adapted for use with a standardised tip.

The means on the adaptor for detachable engagement with the tip may comprise a bayonet connection, although other connections may also be used.

In an alternative embodiment, and especially in the case where the tip is detachably connected directly to a dedicated handpiece, the valve may form part of or be incorporated in the handpiece.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described further by way of example only and with reference to the accompanying drawings in which:- Figs. 1 & 2 are diagrammatic sectional views of alternative forms of syringe assemblies according to the invention; and Fig. 3 is a detail of a modified version of the arrangement of Fig. 2.

BEST MODE OF CARRYING OUT THE INVENTION Referring to Fig. 1 this shows a three way dental syringe assembly having a handpiece body 1, and a tip 2.

The body 1 comprises a tubular structure connected at one end 3 to a flexible coaxial hose arrangement 4 with inner and outer conduits respectively connected to pressurised sources of water and air (not shown).

These conduits are in communication with respective passageways running through the tubular structure to air and water outlets at the opposite end 4.

This opposite end 4 is of the form of an internally threaded cap.

Two levers or buttons 5,6 are mounted on the body and operate flow control mechanisms. When one lever or button 5 is pressed this allows flow of water through the respective passageway in the tubular structure 1. When the other lever or button 6 is pressed this allows flow of air through the other passageway in the tubular structure 1.

The tip 2 is a moulded disposable plastic structure having inner and outer coaxially arranged tubes 7,8. At one end 9 the tip 2 has an angled section.

At the other end 10 the tip has an enlarged outer sleeve 11 in communication with the outer tube 8. The inner tube 7 extends within this sleeve 11. The sleeve 11 has opposite side bayonet slots 12.

The tip 2 is connected detachably to the body 1 by means of an adaptor 13. This adaptor 13 comprises a cylindrical adaptor body 14 secured within an outer cylindrical sleeve 15. The adaptor body 14 has a central through passageway 16.

One end portion 17 of the passageway 16 is enlarged and contains

a valve ball 18 which is urged by a coil spring 19 into sealing engagement with a valve seat 20 at the junction between the enlarged end portion 17 and the adjacent small diameter portion 21 of the passageway 16.

The coil spring 19 is held captive within the enlarged end portion 17 of the passageway 16 by means of an inserted tube 22 having a central tapered nozzle 23 directed away from the ball 18. Concentrically around the nozzie 23 at the free end of the enlarged end portion 17 of the passageway 16 there is mounted an 0-ring seal 24. A further, smaller 0- ring seal 25 is set into opposite free end 21 of the passageway 16.

The adaptor body 14 has a further passageway 26 extending therethrough alongside the central passageway 16. This further passageway 26 opens at one end 27 adjacent the narrower end of the central passageway 16. At its opposite end 28, this passageway 26 opens to one side of the adaptor body 14.

The end part of the adaptor body containing the enlarged end portion 17 of the passageway 16 is within the confines of the sleeve 15.

The opposite end part projects beyond the sleeve 15 and has an outer peripheral screw thread 29. The adaptor body 14 has an outer flange 30 which retainingly abuts an inner flange 31 at one end of the sleeve 15. At the opposite end of the sleeve 15 there are opposed inwardly directed bayonet pins 32.

An annular space is defined between the sleeve 15 and the end part

of the adaptor body 14 containing the enlarged end portion 17 of the central passageway 16. This annular space is bounded at its inner end by an O-ring seal 33 mounted on the flange 30. The side-opening end 28 of the side passageway 26 opens into the annular space.

The projecting end of the adaptor body 14 having the outer screw thread 29 is screwed tightly into the internally threaded cap end 4 of the handpiece body 1. The central water passageway outlet engages the O-ring seal 25 to form a water tight sealed connection with the central passageway 16 in the adaptor body 14. The outer coaxial air passageway connects with the side passageway 26 in the adaptor body 14.

The enlarged end 10 of the tip 2 is inserted within the annular space and is held in position by engagement of the bayonet slots 12 with the bayonet pins 32. The free edges of the outer sleeve 11 bear against the outer O-ring seal 33, and the inner tube 7 fits tightly on the tapered nozzle 23 and is engaged on its outer periphery by the O-ring seal 24.

The inner tube 7 is thereby connected with the central passageway 16 and the outer tube 8 is connected with the side passageway 26 in a secure manner and with water-tight sealing between the air and water passageways.

When the lever or button 6 is actuated to admit pressurised air this flows through the side passageway 26 in the adaptor body 14 into the outer tube 8 of the tip 2 to exit from the free end thereof. In this position the

ball 18 is pressed tightly against the valve seat 20 ensuring that the inner tube 7 in the tip is sealed relative to the water passageway 16 in the handpiece body 14.

When the lever or button 5 is actuated to admit pressurised water this flows through the central passageway 16 in the adaptor body 14 and displaces the ball 18 off its valve seat 20 to permit flow into the inner tube 7 of the tip to exit from the free end thereof. When this lever or button 5 is released to cut off flow of pressurised water the valve ball 18 is immediately spring urged onto its valve seat 20 to seal the tip 2 relative to the handpiece body 1 and thereby prevent reverse flow of any infected matter.

After use, the tip 2 can be removed from the adaptor 13, by disengaging the bayonet fixing, and discarded. Subsequently a fresh tip can be used with the adaptor.

The adaptor 13 is selected to fit the particular handpiece body 1 at the screwed end. At its opposite end it is adapted for use with a standardised disposable tip 2. Different adapters can be provided for use with different handpiece bodies 1, such adaptors differing with regard to the size or dimensions or mode of attachment of the projecting end of the valve body but having the same size, dimensions and structure at the other end.

Fig. 2 shows an alternative embodiment which differs essentially only with regard to the valve member. Other parts are constructed in like

manner to Fig. 1 and the same reference numerals are used for these.

In place of the ball 18, the spring 19 and the nozzle 23, there is a reed structure 34 held in place by a cylindrical structure 35. The reed structure 34 has two silicone rubber strips or flaps shaped to lie against each other at the end facing the tip 2 to define a closed tapering passage.

When water pressure is applied to the central passageway 16 the strips or flaps move apart to permit passage of water. When the pressure is turned off, the flaps collapse against each other and prevent backwards flow from the tip 2.

As shown in Fig. 2, there is no nozzle for sealing engagement with the inner tube 7 of the tip 2.

With the modification of Fig. 3, a nozzle 36 for this purpose is shown, such nozzle being integral with the tube 35.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only. Thus, for example, although the foregoing description is directed to an arrangement in which the tip 2 is connected to the handpiece body 1 via an adaptor 13 it is to be understood that the tip may be directly connected to a dedicated handpiece in which case the releasable connection for the tip 2 and the one-way valve arrangement will form part of or be incorporated in or attached to the handpiece body 1, for example essentially by integrating the structure shown in the drawings with the handpiece body 1 rather than having the detachable connection established with the screwthread 29.