The oldest cavity filling materials presently being used are the "silver" fillings which are made by mixing a powdered metal with mercury to form a metallic material. This material remains formable and can be easily worked to fill a cavity drilled in a tooth by a dentist before permanently hardening. Generally, such materials are relatively easy to work, with even without assistance to the dentist from another person. A dentist can use his spatula to take a quantity of powdered metal and mix it with mercury to form a relatively hard, rigid mixture. This material may be loaded into an applicator which includes a cylindrical chamber and a plunger for depositing the material into a cavity in a tooth which has been drilled.
Because of the nature of alloy filling materials, they are easily formulated and loaded into the applicator using only one hand. Likewise, once loaded into the applicator, the material may be applied to the cavity to be filled using a single hand. This leaves the other hand of the dentist free to retract the surrounding tissues as well as keep the field clear of saliva and other extraneous materials which otherwise may introduce impurities into the filling, changing its mechanical and physical properties. Thus the dentist is able to simultaneously prepare and apply filling material with one hand while keeping the field clear with the other.
Recent years have seen a dramatic increase in the use of light-curable composite filling material. These filling materials, typically comprising silica or quartz mixed in a resin base, are typically sold to dentists in the form of tubes or screw and thread syringes. The composite materials used to fill cavities in the posterior teeth must withstand greater compressive force as compared to front teeth fillings. Composite filling materials used for posterior filling require greater compressive strength and are therefore more highly filled. This decreases their viscosity making necessary a greater mechanical advantage in order to dispense.
As discussed above, prior art dental materials such as alloy or silver are made from relatively soft and workable materials which are loaded into an applicator and combine to over time form a permanently hardened filling. In contrast, because modern light-cured composite material must be prepared and packaged in their final form and as such are of relatively low viscosity it is necessary to use two hands to force the material from the tube. Likewise, in the case of screw type syringe packages, the syringe must be operated by rotation of the plunger within the syringe cylinder. Rotation is necessary because of the relatively non-viscous nature of the composite material and the force which must be exerted in order to force it from the syringe cylinder. In particular, the mechanical advantage provided by the screw and thread operation of the syringe results in generating relatively great pressure, thus forcing a relatively hard and non-viscous material out from the syringe. Nevertheless, despite the mechanical advantage provided by the screw and thread of the syringe it is necessary to use two hands in order to force the same to turn and to advance composite material from the syringe.
In view of these limitations, it is customary for the dentist to have an assistant who operates the syringe while the dentist keeps the field clear with his hands. After a sufficient amount of material has been extruded by the assistant, the dentist may take the material from a place where it has been deposited (or directly from the syringe), using, for example, a spatula or the like.
As can be seen from the above discussion, it is thus necessary to have twice the manpower present when the composite material is to be introduced into the drilled out cavity. Clearly the same involves additional cost.
While the composite materials presently being used thus offer many advantages, they suffer from the drawback of being relatively inconvenient to use and expensive in terms of the labor required.