Abstract:
The system and method disclosed herein prevent a patient from interrupting the vacuum created by the vacuum line attached to the saliva ejector so that suction continues to be present even if the patient clamps his or her lips tightly about the shaft of the saliva ejector. In the illustrated embodiment, the backflow prevention mechanism includes a sleeve positionable on the shaft of the saliva ejector and engageable by the patient&#39;s lips. The sleeve includes at least one conduit that remains open to allow air to enter the patient&#39;s mouth even when the patient&#39;s lips are tightly closed. The air entering the patient&#39;s mouth via the at least one conduit enables the vacuum line to continue to draw bodily fluids and contaminants from the patient&#39;s mouth.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This application is directed to dental accessories, and, more particularly, is directed to an apparatus and method for preventing backflow in saliva ejectors used for removing accumulated liquids and other materials from the mouth of a patient during dental procedures. 
     Description of the Related Art 
     A saliva ejector is an apparatus used by a dentist or a dental hygienist during dental procedures to evacuate saliva, blood and other debris from a patient&#39;s mouth. The saliva ejector includes a suction line and a low-volume saliva ejector tip. The saliva ejector tip is inserted into the patient&#39;s mouth with the ejector tip in the region of the material to be removed. Suction is applied via a suction line to evacuate the material from the patient&#39;s mouth and to discharge the evacuated material to a waste container. In accordance with acceptable dental practice, the saliva ejector tip is a single use device that is replaced after each patient. In contrast, the suction line may be replaced or cleaned only after the suction line has seen used with several patients. Unfortunately, this practice can result in fluids from a previous dental patient or from multiple previous patients remaining in the suction line when the saliva ejector is used on a subsequent patient. 
     A common dental practice is to have a patient close the patient&#39;s lips around the low-volume saliva ejector tip and expectorate to help evacuate the mouth. Closing the lips around the ejector tip can cause a decrease in the magnitude of the vacuum line negative pressure. The decrease in magnitude may allow the previously evacuated fluid remaining in the suction line to flow backwards into the patient&#39;s mouth. This backflow of fluid can carry oral contaminants such as blood, viruses, bacteria, and fungi from prior patients. Studies have found that oral bacteria can survive inside of the suction line. Furthermore, diseases, such as influenza, strep, and hepatitis B, can be passed to subsequent patients if material remaining in the suction line backflows into the patient&#39;s mouth. 
     In order to decrease the possibility of the backflow of fluids into a dental patient&#39;s mouth, dentists and hygienists routinely instruct patients not to close their lips around the saliva ejector tips so as to prevent a decrease in the negative magnitude of the vacuum line pressure. However, humans tend to close their lips around a straw when sucking, and the request to keep the lips apart during sucking is counter to this behavior. Moreover, even if a patient succeeds in overcoming the tendency to close his or her lips around the ejector tip, the saliva ejector tip may still get wedged in a position in the patient&#39;s mouth—for instance in the cheek folds—to block the end of the tip and cause a decrease in the negative magnitude of the vacuum line pressure, which can result in backflow. 
     Another approach for decreasing the chance of oral contaminants being passed from patient to patient is to clean or change the suction line after every patient. However, this approach is time consuming and is cost prohibitive. Furthermore, if the suction line is rinsed thoroughly with a germ killing rinse, any remaining rinse residue in the suction line may backflow into the mouth of a subsequent patient. This circumstance would be unpleasant for the dental patient or possibly dangerous depending on the caustic nature of the rinse. 
     It has been recognized in the art that cross-contamination between patients, for example, dental patients, can occur when evacuators (suctioning devices) attached to vacuum lines are used to remove such bodily fluids and contaminants. Various articles, guidelines and studies have addressed the potential for such cross-contamination including, for example:  Possibility of Cross - Contamination Between Dental Patients by Means of the Saliva Ejector , C. M. Watson, R. L. S. Whitehouse, JADA, Vol. 124, April 1993 ; Backflow in Low - Volume Suction Lines: The Impact of Pressure Changes , G. Mann, T. Campbell and J. Crawford, JADA, Vol. 127, May 1996 ; Cross - Contamination Potential of Saliva Ejectors Used in Dentistry , J. Bargeau, et al., Journal of Hospital Infection, 1998: 40:303-11; and  Guidelines for Infection Control in the Dental Health Care Setting— 2003, Center for Disease Control, 2003, all of which are herein incorporated by reference to the extent not inconsistent with the present disclosure. Such cross-contamination can occur as vacuumed bodily fluids, contaminants or both can backflow from the vacuum line into the patient&#39;s mouth, body or both. Backflow can occur when flow is interrupted when a patient closes his or her mouth over the vacuum tip, thereby stopping flow or even overpowering the negative vacuum pressure by sucking on the tip. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, a need exists for a system and method that prevents backflow from a vacuum line attached to a saliva ejector, thus preventing vacuumed bodily fluids, contaminants or both from entering the patient&#39;s mouth via the saliva ejector. The system and method disclosed herein prevent a patient from interrupting the vacuum created by the vacuum line attached to the saliva ejector so that suction continues to be present even if the patient clamps his or her lips tightly about the shaft of the saliva ejector. Accordingly, the system and method accommodate the natural tendency of a patient to close his or her lips during the suction procedure. In the illustrated embodiment, the backflow prevention mechanism includes a sleeve positionable on the shaft of the saliva ejector and engageable by the patient&#39;s lips. The sleeve includes at least one conduit that remains open to allow air to enter the patient&#39;s mouth even when the patient&#39;s lips are tightly closed. The air entering the patient&#39;s mouth via the at least one conduit enables the vacuum line to continue to draw bodily fluids and contaminants from the patient&#39;s mouth. 
     One aspect of the embodiments disclosed herein is a backflow preventer for a saliva ejector wherein the saliva ejector includes a shaft having a distal end and a proximal end, wherein the distal end is configured to receive fluids, and wherein the proximal end connectable to a vacuum line. The backflow preventer comprises a hollow cylindrical shaft extending longitudinally between a first end surface and a second end surface. The hollow cylindrical shaft has an inner surface and an outer surface with a shaft wall defined between the inner surface and the outer surface. The inner surface is configured to fit over and frictionally engage the shaft of the saliva ejector. At least one longitudinal bore extends through the shaft wall between the first end surface and the second surface to provide a passageway for air. 
     In certain embodiments of the backflow preventer, the inner surface of the hollow cylindrical shaft has a shape approximating an ellipse. The ellipse has a major axis and a minor axis, wherein the major axis has a length greater than an outside diameter of the shaft of the saliva ejector, and the minor axis has a length less than the outside diameter of the shaft of the saliva ejector. The hollow cylindrical shaft is deformable by pressure applied across the major axis to temporarily decrease the length of the major axis of the ellipse while increasing the length of the minor axis of the ellipse such that the length of the minor axis is greater than the outside diameter of the shaft of the saliva ejector to enable the hollow cylindrical shaft to be positioned on the shaft of the saliva ejector. 
     In certain embodiments of the backflow preventer, the inner surface of the hollow cylindrical shaft of the backflow preventer includes a first indentation aligned with a first end of the minor axis and a second indention aligned with a second end of the minor axis. The indentions reduce the thickness of the wall of the hollow cylindrical shaft at the first and second ends of the minor axis to facilitate the outward deformation of the wall. 
     In certain embodiments of the backflow preventer, the inner surface of the hollow cylindrical shaft has a primary shape approximating a circle. The circle has a diameter greater than an outside diameter of the shaft of the saliva ejector. The inner surface further includes a plurality of protrusions extending inward from the primary shape of the inner surface. The protrusions have innermost surfaces that define an inner circle having a diameter less than the outside diameter of the shaft of the saliva ejector. The innermost surfaces frictionally engage the shaft of the saliva ejector when the backflow hollow cylindrical shaft is positioned on the saliva ejector. 
     In certain embodiments of the backflow preventer, a metallic wire is embedded in the wall of the backflow preventer. The metallic wire retains a shape when bent such that when the backflow preventer is bent to a selected shape the metallic wire inhibits the backflow preventer from returning to a previous shape. 
     Another aspect of the embodiments disclosed herein is a method for preventing backflow through a saliva ejector when a patient&#39;s lips are closed onto the saliva ejector. The method comprises positioning a backflow preventer onto a shaft of the saliva ejector. The backflow preventer includes at least one airflow passage between a first end surface and a second end surface. The method further comprises positioning the saliva ejector in the patient&#39;s oral cavity. The backflow preventer is positioned on the saliva ejector at a location between the patient&#39;s lips such that when the patient&#39;s lips are closed, the first end surface of the backflow preventer is exposed to the ambient atmosphere outside the patient&#39;s lips and the second end surface of the backflow preventer is located within the oral cavity of the patient. The at least one airflow passage enables air to pass from the ambient atmosphere into the oral cavity of the patient. 
     In certain embodiments, the method further comprises bending a portion of the saliva ejector and the backflow preventer from an original longitudinal shape to a selected arcuate shape. The backflow preventer includes an embedded metallic wire which inhibits the backflow prevent from returning to the original longitudinal shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other aspects of this disclosure are described in detail below in connection with the accompanying drawing figures in which: 
         FIG. 1  illustrates a perspective view of a saliva ejector onto which the backflow preventer disclosed herein can be installed; 
         FIG. 2  illustrates a distal end elevational view of the saliva ejector of  FIG. 1 ; 
         FIG. 3  illustrates a cross-sectional elevational view taken from the proximal end of the saliva ejector along the line  3 - 3  of  FIG. 1 ; 
         FIG. 4  illustrates a pictorial perspective view of the saliva ejector of  FIG. 1  with the proximal end of the saliva ejector attached to a vacuum line and with a distal end portion of the saliva ejector inserted into the mouth of a patient, the patient&#39;s lips shown in the preferred open position; 
         FIG. 5  illustrates the pictorial perspective view of  FIG. 4  with the patient&#39;s lips closed around the shaft of the saliva ejector; 
         FIG. 6  illustrates a perspective view of a first embodiment of a backflow preventer, the embodiment including an internal shape-retention wire shown in hidden lines; 
         FIG. 7  illustrates a distal end view of the backflow preventer of  FIG. 6 ; 
         FIG. 8  illustrates the distal end view of the backflow preventer of  FIG. 7  with the relaxed configuration of  FIG. 7  shown in solid lines and with the squeezed (vertically stretched) configuration shown in phantom lines; 
         FIG. 9  illustrates a perspective view of the backflow preventer of  FIGS. 6-8  attached to the shaft of the saliva ejector of  FIGS. 1-3 ; 
         FIG. 10  illustrates a cross-sectional elevational view of the saliva ejector and the backflow preventer of  FIG. 9  taken from the proximal end of the saliva ejector along the line  10 - 10  of  FIG. 9 ; 
         FIG. 11  illustrates a pictorial perspective view of the backflow preventer and saliva ejector of  FIGS. 9 and 10  with a distal end portion (not shown) of the saliva ejector in the patient&#39;s mouth and with the backflow preventer between the patient&#39;s lips; 
         FIG. 12  illustrates a perspective view of a second embodiment of a backflow preventer, the second embodiment including a shape-retention wire shown in hidden lines; 
         FIG. 13  illustrates a distal end view of the backflow preventer of  FIG. 12 ; 
         FIG. 14  illustrates a perspective view of the backflow preventer of  FIGS. 12 and 13  installed on the saliva ejector of  FIGS. 1-3 ; 
         FIG. 15  illustrates a cross-sectional elevational view of the saliva ejector and the backflow preventer of  FIG. 14  taken from the proximal end of the saliva ejector along the line  15 - 15  of  FIG. 14 ; and 
         FIG. 16  illustrates a pictorial perspective view of the backflow preventer and saliva ejector of  FIG. 14  with a distal end portion (not shown) of the saliva ejector positioned in a patient&#39;s mouth and with the backflow preventer between the patient&#39;s lips. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An anti-suction saliva ejector is disclosed in the attached drawings and is described below. The embodiment is disclosed for illustration of the anti-suction saliva ejector and is not limiting except as defined in the appended claims. 
       FIGS. 1, 2 and 3  illustrate an example of a typical saliva ejector  100 . As illustrated, the saliva ejector comprises a hollow cylindrical shaft  102  having an open proximal end  104  and having a suction head  106  at the opposite distal end. As used herein, “proximal” refers to the end of the saliva ejector closest to the dentist, the dental hygienist or other dental practitioner (not shown) handling the saliva ejector, and “distal” refers to the opposing end, which is inserted into the mount of a patient (see  FIGS. 4 and 5  discussed below). 
     The shaft  102  of the saliva ejector  100  has a typical outside diameter of approximately 6.5 millimeters (slightly more than ¼ inch). The proximal end  104  of the saliva ejector is attachable to a vacuum line  110  in a conventional manner. For example, the vacuum line can comprise medical grade tubing having an inside diameter of approximately ¼ inch so that the vacuum line fits snugly onto the shaft. The vacuum line is coupled to a vacuum source (not shown). The vacuum source applies a vacuum to the saliva ejector via the vacuum line and receives the bodily fluids, rinse water and other materials suctioned from a patient&#39;s mouth. 
     The suction head  106  at the distal end of the hollow shaft  102  typically comprises an enlarged cap  120 . The cap has a collar  122  that attaches the cap to the shaft. The cap also has an inlet portion  124 . The inlet portion can have many different configurations. In the illustrated saliva ejector, the inlet portion includes a plurality of longitudinal slots  126  that extend from the distal end of the cap to the collar. An internal support ring  128  keeps the material of the cap surrounding the slots spaced apart to prevent the slots from closing. Fluids and contaminants pass through the slots and are drawn through the hollow shaft to the vacuum line  110 . 
     The hollow shaft  102  of the typical saliva ejector  100  is flexible and can be bent to a desired configuration. The shaft can comprise medical grade thermoplastic or other suitable material. Such a material tends to return to an original shape after being bent (e.g., to the generally straight shape illustrated in  FIGS. 1-3 ). Accordingly, many saliva ejectors include a longitudinal shape-retention wire (e.g., a copper wire)  130  (shown in hidden lines in the figures) that retains a shape into which it is bent. 
     As shown in  FIG. 4 , while performing a dental procedure, a dental practitioner (e.g., a dentist, a dental hygienist, or the like) frequently inserts the saliva ejector  100  into the mouth of a patient  140  to remove bodily fluids and contaminated material. The saliva ejector can remain in the original straight configuration and be held in the mouth by the dental practitioner during the suction procedure as shown in  FIG. 4 . Alternatively, the saliva ejector may be bent into a hook-like configuration so that the saliva ejector can remain in the mouth for an extended period without being held. Often, the insertion of the saliva ejector is accompanied by spraying of water (not shown) into the mouth to dislodge contaminated material so that the material and the water can be suctioned from the mouth. 
     Although the patient  140  should be instructed to keep his or her mouth open during the suction procedure as shown in  FIG. 4 , the appropriate instructions are often not provided, or, if provided, the instructions are ignored by the patient. Thus, many, if not most, patients close their lips around the shaft of the saliva ejector  100  as illustrated in  FIG. 5 . As discussed above, when a patient closes his or her lips around the shaft of the saliva ejector, a vacuum quickly forms in the patient&#39;s mouth, which counteracts the vacuum provided by the vacuum line  110 . Backflow can occur, which can result into bodily fluids and other materials in the vacuum line flowing from the vacuum line into the patient&#39;s mouth. Such fluids or other materials can be from a previous patient, can be residual cleaning fluids from attempts to clean the vacuum line after a previous patient procedure or can be a combination of both. Such a backflow of material is undesirable at best and can be dangerous, as discussed above. 
       FIG. 6  illustrates a perspective view of a backflow preventer  200  in accordance with a first embodiment. The backflow preventer solves the backflow problem discussed above.  FIG. 7  illustrates an end view of the backflow preventer. The backflow preventer comprises a hollow cylinder  210  having a wall  212  with a generally elliptical cross-sectional profile. The cylinder may be referred to as an elliptic cylinder wherein the wall has an outer surface  214  and an inner surface  216 . In the illustrated embodiment, the cylinder has a length L ( FIG. 6 ) from a first end surface  220  to a second end surface  222 . In the illustrated embodiment, the length L is approximately 1 inch. The length L can vary. For example, in alternative embodiments, the length L can be as short as 0.5 inch and can be as long as 4 inches depending on the application. For example, a shorter length can be used for a pediatric patient, and a longer length can be used when the saliva ejector  100  is likely to be moved to multiple locations within a patient&#39;s mouth (e.g., during a typical tooth cleaning procedure). In the illustrated embodiment, the backflow preventer is extruded from a medical grade thermoplastic elastomer, such as, for example, the material used in medical tubing and the like. 
     As illustrated in  FIG. 7 , the outer surface  214  of the generally elliptical cross-sectional profile of the cylinder wall  212  of the cylinder  210  of the backflow preventer  200  has an outer major axis (horizontal in  FIG. 7 ) with a length L1 of approximately 0.38 inch and has an outer minor axis (vertical in  FIG. 7 ) with a length L2 of approximately 0.34 inch; however, the profile is modified as described below. The primary thickness of the wall of the cylinder between the outer surface and the inner surface  216  is approximately 0.05 inch; however, the thickness of the wall varies at specific locations as described below. The primary thickness of the wall can also differ for different embodiments. For example, the wall thickness of 0.05 inch may be suitable for most applications; however, a greater wall thickness can be advantageous in other applications. The generally elliptical cross-sectional profile of the inner surface  216  of the cylinder wall has an inner major axis (horizontal in  FIG. 7 ) with a length L3 of approximately 0.28 inch and has an inner minor axis (vertical in  FIG. 7 ) with a length L4 of approximately 0.24 inch. The foregoing dimensions of the inner and outer axes are referred to herein as the “resting dimensions” of the backflow preventer. 
     As further illustrated in  FIG. 7 , the inner surface  216  of the wall  212  of the cylinder  210  includes a first inner arcuate notch  230  and a second inner arcuate notch  232  positioned at opposite ends of the inner major axis. Each inner arcuate notch has a radius of approximately 0.025 inch; and each inner arcuate notch includes a pair of fillets to the inner surface, each fillet having a radius of approximately 0.1 inch. 
     As further illustrated in  FIG. 7 , the outer surface  214  of the wall  212  of the cylinder  210  includes a first arcuate protrusion  240  and a second arcuate protrusion  242  at opposite ends of the outer major axis. Each protrusion has a radius of approximately 0.025 inch; and each protrusion includes a pair of fillets to the outer surface, each fillet having a radius of approximately 0.1 inch. As illustrated the protrusions on the outer surface are aligned with the inner arcuate notches. 
     The outer surface  214  of the wall  212  further includes a first outer arcuate notch  250  and a second outer arcuate notch  252  at opposite ends of the outer minor axis. Each outer arcuate notch has a radius of approximately 0.025 inch; and each outer arcuate notch includes a pair of fillets to the outer surface, each fillet having a radius of approximately 0.025 inch. The two outer arcuate notches cause the wall to be thinner between the inner surface  216  and the notches. As described below, the two outer arcuate notches provide a hinging effect that is used to enable the backflow preventer to be installed easily onto the shaft  110  of the saliva ejector  100  shown in  FIGS. 1-3 . 
     The wall  212  of the cylinder  210  further includes a plurality of longitudinal bores  260  that extend from the first end surface  220  to the second end surface  222 . In the illustrated embodiment, each longitudinal bore has a diameter of approximately 0.02 inch. In alternative embodiments, the bores may have different diameters. In the illustrated embodiment, four longitudinal bores are provided; and the four bores are spaced apart from the respective major and minor axes by approximately 45 degrees. Preferably, each bore is positioned approximately midway between the outer surface  214  and the inner surface  216 . For example, in the illustrated embodiment, the bores are positioned on an elliptical curve  262  (shown in phantom) having a major axis with a length of approximately 0.37 inch and having a minor axis with a length of approximately 0.33 inch. 
     As further shown in  FIGS. 6 and 7 , the backflow preventer  200  can also include an encapsulated wire  270  (shown in hidden lines) in the wall  212  of the cylinder. The encapsulated wire extends generally between the first end  220  and the second end  222  of the cylinder; however, in the illustrated embodiment, the encapsulated wire is molded into the wall of the backflow preventer and is not exposed at either end of the backflow preventer. In one embodiment, the wire has a diameter of approximately 0.01 inch (e.g., approximately 30 AWG (American wire gauge)) and comprises copper or other suitable metal. The location of the encapsulated wire within the wall is not critical. In the illustrated embodiment, the wire is located approximately midway between the outer surface  214  and the inner surface  216  and is located approximately midway between one of the longitudinal bores  260  and the arcuate protrusion  240 . The encapsulated wire functions as a shape-retention wire so that the backflow preventer will remain bent if the dental practitioner bends the backflow preventer and the saliva ejector  100  as part of a dental procedure. 
     The inner arcuate notches  230 ,  232 , the outer arcuate notches  250 ,  252  and the protrusions  240 ,  242  on the inner surface  214  and the outer surface  216  of the wall  212  of the cylinder  210  enable the dental practitioner to attach the backflow preventer  200  to the shaft  102  of the saliva ejector  100  as shown in  FIG. 9  and to move the backflow preventer to a desired position on the shaft of the saliva ejector. 
     In the illustrated embodiment, the inner minor axis length L4 of the wall  212  of the cylinder  210  is approximately 0.24 inch, which is slightly less than the outer diameter of the shaft  102  of the saliva ejector  100 . Thus, the backflow preventer cannot be easily inserted onto the shaft of the saliva ejector in view of the resting dimensions of the backflow preventer. As illustrated in  FIG. 8 , by applying pressure to the first protrusion  240  and the second protrusion  242  of the backflow preventer, the dental practitioner causes the wall of the cylinder to be squeezed inwardly along the inner major axis L3 and the outer major axis L1 to shorten the two major axes. The shortening along the major axes is represented by the shortening of the inner major axis from the length L3 to the length L3′. The hinging effect of the thinner thicknesses of the wall between the inner surface  216  of the wall and each of the outer arcuate notches  250 ,  252  causes a portion of the wall proximate to the outer arcuate notches to bulge outwardly, which causes the lengths of the minor axes to increase while the pressure is applied. The lengthening of the minor axes is represented in  FIG. 8  by the increase in the length of the inner minor axis from L4 to L4′. Accordingly, the notches facilitate the deformation of the wall of the cylinder outward along the minor axes and inward along the major axes as illustrated by the modified inner surface  216 ′ and the modified outer surface  214 ′, which are shown in phantom lines in  FIG. 8 . The increase in length of the inner minor axis from L4 to L4′ is sufficient to cause the inner minor axis length to temporarily exceed the 6.5-millimeter outer diameter of the shaft of the saliva ejector. Although the applied pressure also causes the inner major axis length to decrease from L3 to L3′, the original inner major axis length (resting dimension) of approximately 0.28 inch is sufficient so that the reduced length L3′ while the pressure is applied remains greater than the outer diameter of the shaft of the saliva ejector. Thus, the dental practitioner is able to easily slide the backflow preventer onto the shaft of the saliva ejector and to move the backflow preventer to a desired location on the shaft as shown in  FIGS. 9 and 10 . 
     After positioning the backflow preventer  200  on the saliva ejector  100 , the pressure applied to the protrusions  240 ,  242  is released so that the backflow preventer relaxes toward the original shape; however, the complete relaxation of the backflow preventer is precluded because the outer diameter of the shaft  102  of the saliva ejector is greater than the original length L4 of the inner minor axis. Thus, the inner surface  216  of the backflow preventer engages the shaft as shown in the end view of  FIG. 10 . The engagement of the inner surface of the backflow preventer with the shaft provides sufficient friction with respect to the shaft to inhibit longitudinal movement of backflow preventer along the shaft so that the backflow preventer remains where positioned by the dental practitioner. The backflow preventer may be repositioned by applying pressure to the protrusions as before or by applying sufficient pressure overcome the frictional engagement of the inner wall with the shaft. 
     After installing the backflow preventer  200  onto the saliva ejector  100  as shown in  FIGS. 9 and 10 , the vacuum line  110  is attached to the shaft  102  of the saliva ejector in a conventional manner, as described above. The backflow preventer is positioned on the shaft of the saliva ejector so that when the saliva ejector is placed in the mouth of the patient  140 , the backflow preventer is located between the lips of the patient as shown in  FIG. 11 . Thus, even if the patient closes his or her lips onto the shaft of the saliva ejector as shown in  FIG. 11 , the backflow preventer will be positioned between the lips with one end surface (e.g., first end surface  220 ) exposed outside the patient&#39;s lips and with the other surface (e.g., the second end surface  222 ) positioned within the patient&#39;s mouth. Thus, the plurality of longitudinal bores  260  provide open conduits for the passage of air from outside the patient&#39;s lips to inside the patient&#39;s mouth even if the lips are closed tightly around the outer surface  214  of the backflow preventer. Additional air flow can also be provided in the cavities created between outer wall of the shaft of the saliva ejector and the first and second inner arcuate notches  230 ,  232 . 
     If the initial positioning of the backflow preventer  200  on the saliva ejector  100  is not correct, the backflow preventer is easily repositioned by applying pressure to the protrusions  240 ,  242  to temporarily increase the inner minor axis length L4, by moving the backflow preventer while the pressure is applied, and then by releasing the pressure to again engage the outer surface of the shaft  102  of the saliva ejector. The backflow preventer may also be repositioned in a similar manner if the location of the saliva ejector is changed during the procedure, and the original positioning of the backflow preventer is no longer correct. 
     As discussed above, the shaft  102  of the saliva ejector  100  may be bent to a hook-like shape or other suitable shape to allow the saliva ejector to remain within the mouth of the patient  140  for a longer duration. If the backflow preventer  200  is positioned on the portion of the shaft to be bent, the backflow preventer is sufficiently flexible to also be bent. As further discussed above, the backflow preventer can also include the encapsulated wire  270  to assist in retaining the bent shape of the backflow preventer so that the resilient thermoplastic material of the backflow preventer does not tend to straighten, which could result in modifying the desired hook-like shape of the shaft of the saliva ejector. 
     After the backflow preventer  200  is installed on the saliva ejector  100  and the saliva ejector is connected to the vacuum line  110 , the saliva ejector is positioned (and repositioned, if necessary) in the mouth of the patient  140  with the backflow preventer between the patient&#39;s lips as shown in  FIG. 11 . During the dental procedures, the saliva ejector is operated by the dental practitioner in a conventional manner by selectively activating the vacuum source when evacuation is needed. Even if the patient closes his or her lips over the saliva ejector, the backflow preventer between the patient&#39;s lips maintains open air passages via the longitudinal bores  260  that prevent a vacuum from forming within the oral cavity of the patient&#39;s mouth. Thus, vacuum applied via the vacuum line continues to remove fluids and other material from the mouth without any occurrence of backflow. In general, the first end (distal end) surface  222  of the backflow preventer is located a sufficient distance from the portion of the patient&#39;s mouth where the saliva and other debris collect so that the saliva and other debris do not enter the longitudinal bores during the evacuation process. 
       FIG. 12  illustrates a perspective view of a backflow preventer  300  in accordance with a second embodiment. The structure of the second backflow preventer is similar to the structure of the previously described backflow preventer and operates in a similar manner. The second backflow preventer comprises a hollow cylinder  302 , which can have a length from a first end surface  304  to a second end surface  306  that can be varied in accordance with the application, as discussed above. In the illustrated embodiment, the length can vary from approximately 0.5 inch to approximately 4 inches. 
     Unlike the previously described backflow preventer  200  with the elliptical cross-sectional profile, the second backflow preventer  300  of  FIG. 11  has a generally circular profile as shown in  FIG. 13 . An outer surface  310  of the hollow cylinder  302  has an outer diameter in a range of approximately 0.28 inch to approximately 0.3 inch. The second backflow preventer has an inner surface  312  having a primary inner diameter of approximately 0.26 inch; however, the inner surface includes a plurality of arcuate crush ribs (or inward protrusions)  314 . In the illustrated embodiment, six crush ribs are positioned at approximately equal angular distances around the inner surface of the backflow preventer (e.g., at approximately 60-degree intervals. The crush ribs extend inwardly from the inner surface by a distance of approximately 0.01 to 0.02 inch such that the inner surface has a secondary inner diameter (represented by a circle  316  in phantom) of slightly less than 0.25 inch. Accordingly, the selected secondary inner diameter provides a snug fit against the outer surface of the shaft  102  of the saliva ejector  100  when positioned on the saliva ejector as shown in  FIGS. 14 and 15 . The arcuate crush ribs provide longitudinal contact surfaces where the innermost surfaces of the crush ribs are tangential to the outer surface of the shaft. Accordingly, the crush ribs provide sufficient frictional engagement with the outer surface of the shaft so that the alternative backflow preventer will remain where positioned; however, the frictional engagement is easily overcome by applying moderate force to slide the backflow preventer to a desired location on the shaft as described above. 
     The second backflow preventer  300  of  FIGS. 12 and 13  includes a plurality of longitudinal bores  320  which extend from the first end surface  304  to the second end surface  306  of the hollow cylinder  302 . In the illustrated embodiment, six longitudinal bores are positioned equally distantly around the cylinder. Each bore is generally positioned midway between adjacent crush ribs  314  and is generally positioned midway between the outer surface  310  and the primary inner surface  312 . In the illustrated embodiment, each of the longitudinal bores has a diameter of approximately 0.01 inch. The diameter is sufficiently large to allow air to flow therethrough to preclude the formation of a vacuum as described above. The diameter is sufficiently small to inhibit the outward flow of saliva and other fluids which may be close to the second end surface of the backflow preventer. For example, the surface tension of the saliva is sufficiently high to prevent the saliva from entering the small diameter longitudinal bores. When positioned on the shaft  102  of the saliva ejector  100  as shown in  FIGS. 14 and 15 , the spacing between the primary inner surface and the shaft between adjacent crush ribs also provides longitudinal pathways for additional air flow into a patient&#39;s mouth. 
     The alternative backflow preventer  300  of  FIGS. 13 and 13  may also include an encapsulated wire  330 , which operates as described above to assist in retaining a bent shape of the combined backflow preventer and saliva ejector  100 . 
     As illustrated in  FIG. 16 , the second backflow preventer  300  and the saliva ejector  100  operate in a similar manner to the previously described combination of the first backflow preventer  200  and the saliva ejector. When the second backflow preventer and the saliva ejector are positioned in the mouth of the patient  140 , the second backflow preventer is positioned (or repositioned if necessary) on the saliva ejector so that the second backflow preventer is positioned between the patient&#39;s lips as shown in  FIG. 16  with the first (distal) end surface  304  in the patient&#39;s oral cavity and with the second (proximal) end surface  306  exposed to the ambient atmosphere. Thus, even if the patient&#39;s lips are closed around the outer surface of the backflow preventer, air will pass through the longitudinal bores  320  to prevent a vacuum from forming within the oral cavity. 
     One skilled in art will appreciate that the foregoing embodiments are illustrative of the present invention. The present invention can be advantageously incorporated into alternative embodiments while remaining within the spirit and scope of the present invention, as defined by the appended claims.