Abstract:
A method and apparatus for providing clearances in the edges of a denture to accommodate the adjacent anatomical landmarks utilizes a molding compound dispenser in which a sufficient quantity of molding compound to cover the edges of an impression tray is maintained in a limited temperature range above melting temperature. The dispenser may be provided with heat exchange surfaces that can be selectively covered by the practitioners hand to control the amount of heat loss to the surroundings. Manual pressure may be used to deposit the compound onto the edges.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to denture fabrication processes, and more particularly to a process and apparatus for applying thermoplastic impression compound molding onto custom denture impression trays.  
           [0003]    2. Description of the Prior Art  
           [0004]    The process of fabricating dentures entails a variety of steps each necessary to properly conform the denture interface to the unique muscular and skeletal architecture of the patient. It is a time consuming, elaborate process. In accordance with the currently prevailing practice a customized denture impression tray is first fabricated, generally conforming with the maxillary or mandibular structure of the patient, with the peripheral edges of the tray shortened or reduced to accommodate molding compound impressions of the adjacent anatomical landmarks. These molding compound impressions are made in a border molding bead applied onto the peripheral edges of the tray which is repositioned into the patient while the compound is still soft. The patient is then asked to perform various movements of the facial musculature while the border molding compound is setting to provide in the impression appropriate clearances within the buccal, labial and/or lingual folds. When these borders are thus defined the rest of the tray is filled with impression material to provide a full impression of the denture interface.  
           [0005]    In this process the desired border molding compound is one of several thermoplastic compounds having a setting temperature just somewhat higher than the patient&#39;s body temperature. The minimal difference between a tolerable temperature, i.e., temperature at which tissue is injured, and the temperature within the patient&#39;s oral cavity leaves little working range. Within this marginal temperature range the molding compound must be applied as a ribbon or bead along the denture impression tray borders, and thereafter fitted to the patient while still soft. Of course, the bead would frequently set up along the ribbon length and the incidence of improper impression because of already hardened molding compound was both an unwanted and an anticipated event. The practitioner was therefore expected to re-heat the molding compound ‘stick’ and thereafter parts of the ribbon over an open flame, each time with some concern over excessive temperature levels that may injure the patient. The necessary experience and skill in this border molding process, together with the attention required, have substantially affected the cost of dentures and any simplification techniques would greatly enhance both the quality of the product and its cost. Amongst the simplifications the task directed at maintaining the border molding pliable while it is applied is the most cumbersome and therefore most likely to benefit from any improvement.  
           [0006]    In the past various devices have been developed which in one way or another eject heated compounds in the course of dentistry. Examples of such devices are described in U.S. Pat. No. 3,522,654 to Schoelz and U.S. Pat. No. 3,614,389 to Malisza, both directed to electrically heated dental wax dispensers, and U.S. Pat. No. 4,265,618 to Herskovitz et al. describing endodontic syringes for dispensing thermoplastic material. While suitable for the purposes intended, each of the foregoing dispenses thermoplastic matter in quantities that are less than those that can pose substantial risk of burn injury, or that dispense molten substances onto those body elements that are generally resistive to heat.  
           [0007]    Concurrent with these developments, those engaged in the adhesives art have also developed a variety of devices that in one manner or another melt matter for use as a glue or adhesive. Examples of such devices include those taught in U.S. Pat. No. 5,026,187 to Belanger et al., U.S. Pat. No. 5,462,206 to Kwasic, U.S. Pat. No. 4,773,566 to Hougland, U.S. Pat. No. 3,744,921 to Weller et al. and others. Again, while wholly suitable for the purposes intended, each of the foregoing dispensers seeks to maximize the best transfer to the thermoplastic material, and considerations of tissue injury are attended primarily by protective structure and shielding.  
           [0008]    Also devised in the part are molten adhesive dispensers which include automatic cut-outs, like those taught in U.S. Pat. No. 4,546,235 to Kolter and U.S. Pat. No. 6,105,824 to Singleton, and those provided with temperature controllers like those shown in U.S. Pat. No. 3,665,158 to Froedge and U.S. Pat. No. 4,816,642 to Dennison. Each of these, while well suited for its purpose, lacks the requisite control and heat exchange configuration that is required for dispensing matter within the limited temperature range bounded by normal body temperature and the temperatures associated with tissue injury.  
           [0009]    Those in the art will appreciate that normal heat transfer exchanges between body temperature to the temperatures of the ambient environment will occur with some expedience, particularly if the surface-to-volume ratio of the heated mass is high. These heat transfer conditions, and the maximum temperature limits of any matter that will contact human tissue, require high volumes of stored molten compound for quick, large quantity dispensing of the border bead. A method and structure adapted to these narrow constraints is extensively sought, and it is one such process and structure that is disclosed herein.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, it is the general purpose and object of the present invention to provide a dispenser useful in heating and dispensing thermoplastic molding compound limited in temperature range.  
           [0011]    Other objects of the invention are to provide a process for dispensing thermoplastic molding compound in large quantities.  
           [0012]    Further objects of the invention are to provide a dispensing structure useful in heating large quantities of thermoplastic molding compound within a limited temperature range.  
           [0013]    Yet additional objects of the invention are to provide a method and structure for applying molten border molding material onto the edges of a custom denture impression tray that is limited in temperature range.  
           [0014]    Briefly, these and other objects are accomplished within the present invention by providing a thermoplastic molding compound dispenser useful in heating hardened compound in cylindrical cartridges to a limited temperature above melting. More precisely, the dispenser is provided with a cylindrical heating chamber dimensioned for intimate surface contact with the exterior of the cartridge and surrounded by an electrical heating element. This heating chamber is aligned along the longitudinal axis of an elongate dispenser body terminating in a dispensing nozzle at its forward, and a cartridge receiving opening at the rearward end.  
           [0015]    Preferably the heating element is positioned close to the forward end of the heating cavity, in series with a temperature control circuit provided with a sensor or thermocouple adjacent the nozzle. A set of radially extending hoops is attached to the body, rearward of the sensor, both to provide finger engagements in the course of dispensing and to serve as sufficiently large heat exchange surfaces for effective temperature control. Furthermore, a generally planar, lateral body extension rearward of the hoops provides the other manipulative structure while also accommodating heat exchange. The presence of such enlarged heat exchange surfaces allows for the closely regulated temperature control accommodating the narrow range of controlled heat ranges of the molding compound.  
           [0016]    Preferably, the cavity volume heated by the electric element is sufficient to store the full quantity of the molding compound necessary for the complete border molding ribbon. Concurrently, the surface areas of the hoops and the handle structure selected for sufficient heat loss to overcome any gain due to the heat internally stored, limits any heat excursions beyond those set in the temperature control circuit. In this manner the surface-to-volume characteristics of the dispenser structure are useful in minimizing the potential of an unwanted temperature increase in the molten molding compound while also providing manipulative convenience.  
           [0017]    The foregoing dispenser can then be utilized to lay a complete ribbon of molding compound in a single application, extending over the whole of the peripheral edge of the dental casting form which can thereafter be immediately positioned on the maxillary or mandibular surfaces of the patient. Once thus positioned, a well-known sequence of muscular manipulations can then be used to conform the border moldings to a set shape that accommodates the muscular surface distortions. In this manner the process of forming and constructing dentures is rendered convenient, allowing for a much more precise accommodation of the anatomical landmarks and much more comfortable denture.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a perspective illustration of an inventive dispenser structure useful in applying thermoplastic border molding to the edges of a dental casting tray;  
         [0019]    [0019]FIG. 2 is a side view in partial section, of the inventive dispenser illustrated in FIG. 1;  
         [0020]    [0020]FIG. 3 is a circuit diagram of a temperature control circuit useful with the present invention;  
         [0021]    [0021]FIG. 4 is a graphical illustration of the temperature profiles based on surface-to-volume ratios along the longitudinal axis of the inventive dispenser;  
         [0022]    [0022]FIG. 5 is a flow chart of a sequence of steps in accordance with the inventive process disclosed herein;  
         [0023]    [0023]FIG. 6 is a perspective illustration of a custom denture impression tray having a peripheral bead of molding compound deposited thereon in accordance with the inventive process described herein; and  
         [0024]    [0024]FIG. 7 is a further perspective illustration of an alternative implementation of a dispenser structure in accordance with the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    As shown if FIGS.  1 - 4  the inventive thermoplastic molding compound dispenser, generally designated by the numeral  10 , includes an elongate, generally cylindrical body  11  provided with a nozzle insert  12  at the forward end communicating with a central cavity  14  extending through the body. Cavity  14  is circular in section conformed for a serial, intimate surface contact, receipt of cylindrical cartridges or &#39;sticks&#39;  15  of hardened thermoplastic molding compound insertable into the cavity at the rearward end of body  11 . Preferably each cartridge  15  comprises a thermoplastic material structure like that sold under the mark or style “Kerr Impression Compounds” by the KERR CORPORATION, 28200 Wick Road, Romulus, Mich. 48174-2600. In commercial practice these cylindrical cartridges or &#39;sticks&#39;  15  are color-coded in accordance with their melting or softening temperature and are available in melting ranges of 122-124 degrees Fahrenheit, 128-130 degrees Fahrenheit and 132-133 degrees Fahrenheit. From these ranges the practitioner selects the appropriate temperature range based on the size of the task, the temperature of the surroundings and the sensitivity of the patient.  
         [0026]    Each cartridge  15  is manually advanced, by thumb pressure, into cavity  14  until the first one bottoms out at the nozzle insert  12 . In this placement the forward portion of the cartridge is aligned within the interior of a helical heating element  21  connected to an electrical outlet EO on one end and to a temperature controller  50  on the other end. Controller  50 , in a manner described more precisely hereinbelow, then completes the other part of the circuit between the heating element  21  and the outlet EO.  
         [0027]    More precisely, a temperature sensor  22  positioned within the body  11  in thermal communication with the cavity  14 , proximate the nozzle  12 , is useful to provide the actual temperature of the cartridge  15  within the cavity to be then compared against the temperature selected by the practitioner by way of a manual selection input through a potentiometer  51  on the controller  50 . Of course, the temperature selected through the potentiometer  51  will be in accordance with one of the several temperature ranges of the cartridge  14  and a plurality of markings  58 - 1  through  58 - 3  is provided adjacent the potentiometer to facilitate the selection.  
         [0028]    In the implementation shown in FIG. 3 sensor  22  may be one of several types of temperature sensing devices, illustrated herein by way of a thermistor connected in a divider circuit across a resistor  23  to ground to form a divider circuit with its division point tied to the negative input of an operational amplifier  55 . The other amplifier input, in turn, may receive the wiper signal of the potentiometer  51  through which the practitioner selects the temperature, with the amplifier  55  then summing the difference therebetween. This difference signal D 55  is then useful to gate a silicon controlled rectifier SCR  56  in circuit between the filament  21  and the AC electrical outlet EO, controlling the power-on pulse duration across the heater. In this manner a substantially linear feedback control arrangement is devised controlling the compound temperature next to nozzle  12  to a temperature difference that is in an inverse ratio to the gain of the operational amplifier  55  selected by the ratio of a feedback resistor  57  and input resistors  56  between potentiometer  51  and thermistor  22 .  
         [0029]    To provide the requisite control authority and precision an opposite, heat loss, path is effected by way of planar structures extending from the body  11  that also serve as manipulative surfaces. More precisely, extending laterally from body  11 , at a longitudinal location spaced rearwardly from the location of the heating element  21 , are two opposed hoops  16  and  17  each dimensioned to receive the index and/or middle finger of the practitioner. Even further rearwardly is a planar projection  18  that is useful as a handle to be grasped by the palm when the fingers are inserted into the hoops. Once thus grasped the projecting portion of the last cartridge  15  can be pushed by the thumb to dispense the molten compound out of the nozzle  12 .  
         [0030]    In the foregoing implementation both the opposed hoops  16  and  17  and the surface of the handle  18  serve as effective heat exchange mechanisms. Those skilled in the art will appreciate that any heat exchange will follow the functions of the cube of the temperature difference for convective exchange and the fourth power of the difference for radiative exchange. An examplary temperature distribution profile TP 1  is shown in FIG. 4 along the length of body  11 , with the surfaces exposed to ambient exchange, against a profile TP 2  corresponding to the temperature profile when the surfaces of hoops  16  and  17  and of handle  18  are within the palm of the practitioner. Thus the manipulative surfaces are cooperatively useful with the temperature control to modulate the heat loss and therefore the control loop error. By selecting a control temperature CT in potentiometer  51  that is close to the melting temperature MT of stick  14  this manual shielding of the exchange surfaces will be useful to an experienced practitioner in the process described below.  
         [0031]    As shown in FIGS. 5 and 6 the sequence of steps rendered possible by the dispenser  10 , shown generally as process  500 , commences with step  501  in which an impression tray  111  is formed to a general conformation with the maxillary or mandibular architecture of the patient. The peripheral edge  112  of the tray is reduced in height to provide clearance for the changing anatomy in the various folds within which the eventual denture is to be placed. Once the tray is thus formed sticks  15  are selected and the temperature corresponding thereto is set by way of the setting of potentiometer  51 , both in step  502 . In step  503  the selected sticks  15  are then serially inserted into cavity  14  with the last stick  15  exposed at the rearward end of body  11 . The temperature controller  50  then heats the internally captured stick material to melting temperature and maintains it thereat as heat is given off the surfaces. In step  504  the practitioner grasps the dispenser  10  and by pressing on the exposed stick end applies a bead of molten compound  114  onto the peripheral edge  112  of the tray. In the course of this application the heat exchange rate is limited by the hand of the practitioner and the bead is therefore slightly warmer as result thereof. With the bead thus deposited the tray  111  is inserted in position adjacent the mandibular or maxillar anatomical structures in step  505  and the patient is then exercised through the necessary movements. In the course of these movements the anatomical landmarks of the patient deform the bead to provide the necessary clearances. Once the bead compound hardens the tray is removed in step  506  and impression compound is applied to the tray for the complete denture impression. Then in step  507  the tray is repositioned once more into the patient and a full impression is made.  
         [0032]    Thus the inventive process facilitates a dental impression without the repeated instances of reheating that were the rule in the past. As result accuracy of the denture interface is improved while the process is also simplified. Great economies in the cost of a denture are therefore realized in a process that has also been rendered more accurate.  
         [0033]    While the foregoing is achieved by way of the control circuit set out in FIGS.  1 - 4 , other dispenser implementations are equally useful. Thus as shown in FIG. 7 a molding compound dispensing system generally designated by the numeral  710  once again includes an elongate body dispenser  711  connected to across a controller assembly  750  again to the electrical outlet EO. Like numbered parts functioning in the manner previously described, the dispenser body  711  is provided with the central cavity  14  conformed to receive in a sequence cartridges  15  advanced towards a forward nozzle  12  by thumb pressure. At the forward end cavity  14  is once more surrounded by a helical heating filament  21  in circuit with the controller  750  which may be one of the several commercially available controllers exemplified by the microprocessor implemented controller sold under the model no., mark or style CN491A by Omega Engineering, Inc., One Omega Drive, Stamford, Conn. 06907-0047. Controllers of this type are generally adapted to accommodate a wide variety of input devices and are also conformed to provide linear output to the filament  21  either by current level, voltage level or pulse width. In the implementation herein the controller  750  receives the temperature information by way of a thermocouple  722  mounted, as hereinbefore set out, adjacent nozzle  12 .  
         [0034]    Controller  750 , in accordance with the commercial implementation thereof, is provided with a numerical temperature display  752  positioned adjacent a selector switch  753  through which the display mode is selected between a temperature readout of the temperature sensed by the thermocouple  722  or the temperature control input effected manually by an up switch  755  and a down switch  756 . In a manner known in the art controller  750  modulates either the voltage or the current applied to the heating filament  21 .  
         [0035]    Those in the art will appreciate that other control techniques can be utilized in the course of practicing the present invention. For example, an on-off technique can be effected by providing sufficient control authority (heat gain vs. heat loss), which are respectively effected by the thermal capacity or wattage of the filament  21  and the heat exchange or heat loss capacity to the ambient environment across the body  711 . The first is achieved by providing sufficient current rating in the filament while the second is a function of the surface area to effect the heat exchange.  
         [0036]    Accordingly, body  711  may be convolved along its exterior to form a plurality of grooves  731  which may also aid the function of a grasping surface in the course of use. As before described, the shielding effect of the practitioner&#39;s palm over these heat exchange convolutions will tend to raise the effective steady state temperature of the compound during its application to the edges  112  of tray  111 . In this manner manual skills can be acquired in the course of practicing the invention that will further enhance the comfort and quality of the denture produced.  
         [0037]    Obviously, many modifications and variations can be effected without departing from the spirit of the invention instantly disclosed. It is therefore intended that the scope of the invention be determined solely by the claims appended hereto.