Abstract:
A safety syringe adapted consists of an outer casing that supports a conventional hypodermic needle and an inner casing which supports a blunt cannula that is disposed inside the inner lumen of the hypodermic needle. The inner casing is keyed to the outer casing by one or more pins that engage helical tracks formed along the inner surface of the outer casing. Rotation of the inner casing relative to the outer casing advances the inner casing so that the blunt cannula is moved from a retracted position within the needle to an extended position in which the blunt end of the cannula extends beyond the tip of the needle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to medical equipment and more particularly to hypodermic syringes. 
     As is well known, hypodermic syringes are used in the medical and veterinary fields to inject substances into humans and animals. One problem presented by the use of hypodermic syringes is protecting those working with the syringes from accidental needle-stick wounds. This is particularly important with a hypodermic syringe that has been used because, following the injection, the needle may be contaminated and spread infections such as hepatitis and HIV. 
     Numerous efforts have been undertaken to provide means for shielding the sharp tip of the needle of a hypodermic syringe before and after use. One category of needle shields comprises a sheath that surrounds the external surface of the needle after use. One example of a sheath type device is disclosed in U.S. Pat. No. 4,631,057 to Mitchell. The shielded needle of Mitchell comprises a syringe having an external needle guard mounted to the body of the syringe. The needle guard is placed in a retracted position to expose the needle for use, then slid forward to an extended position over the sharp end of the needle after use. U.S. Pat. No. 4,425,120 to Sampson et al. also discloses a needle guard comprising a sheath that surrounds the body of the syringe that is advanced to cover the needle after use. A common disadvantage of sheath-type safety needles is that the sheath typically covers all or substantially all of the body of the syringe and, therefore, the body of the syringe cannot be manipulated directly by the medical professional when inserting the needle. Consequently, there is substantial “play” between the surface being held by the medical professional and the needle, which makes needle insertion more difficult to control. 
     A second broad category of safety needles comprise the so-called “stylet-type” safety needles in which a blunting stylet or cannula is advanced through the inner lumen of the needle. U.S. Pat. No. 1,087,845 to Stevens discloses a needle in which an inner blunt cannula is extended within the inner lumen of a sharp needle by means of a thumbscrew operating a miniature rack and pinion. U.S. Pat. No. 1,527,291 to Zorraquin discloses a needle with a blunt probe that is spring loaded into its extended position. The blunt tip springs forward when the needle has passed through the wall or membrane being penetrated, thereby giving the surgeon an indication when the needle has passed through the membrane. U.S. Pat. No. 1,867,624 to Hoffman discloses a biopsy needle consisting of an inner and outer cannula. U.S. Pat. No. 2,623,521 to Shaw discloses an indicating stylet needle with a blunt probe that is spring loaded into its extended position in a manner similar to Hoffman. U.S. Pat. No. 5,009,642 to Sahi et al. discloses a syringe in which the blunting stylet is spring-loaded and snaps forward into the extended position automatically when the plunger is fully depressed. All of the above patents have relatively complex and expensive mechanisms and, in the case of the spring-loaded stylets, present the possibility of accidental deployment of the blunting stylet when trying to withdraw medicine from an ampoule. What is needed, therefore, is a safety syringe that is free from the risk of accidental self-deployment that is simple to operate and inexpensive to manufacture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures in which like references designate like elements and, in which: 
         FIG. 1  is a perspective view of a safety syringe incorporating features of the present invention; 
         FIG. 2  is an exploded perspective view of the safety syringe of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the safety needle of  FIG. 1  shown with the syringe plunger withdrawn; 
         FIG. 4  is a cross-sectional view of the safety needle of  FIG. 1  shown with the syringe plunger fully depressed; 
         FIG. 5  is a cross-sectional view of the safety needle of  FIG. 1  shown with the blunting cannula advanced; 
         FIG. 6  is a partial cross-sectional view of a needle carrier incorporating features of the present invention; 
         FIG. 7  is an enlarged view of a portion of  FIG. 6 ; 
         FIG. 8  is an alternative embodiment of the enlarged portion of  FIG. 6 ; 
         FIG. 9  is an exploded perspective view of an alternative embodiment of a safety syringe incorporating features of the present invention; 
         FIG. 10  is a partial cutaway view of an alternative embodiment of a syringe barrel incorporating features of the present invention; 
     
    
    
     SUMMARY OF THE INVENTION 
     The present invention comprises a safety syringe adapted to inject substances into humans and animals. According to an embodiment of the present invention, the safety syringe has an outer casing that supports a conventional slant-cut hypodermic needle and an inner casing which supports a blunt cannula that is disposed inside the inner lumen of the hypodermic needle. The inner casing is keyed to the outer casing by one or more pins that engage helical tracks formed along the inner surface of the outer casing. Rotation of the inner casing relative to the outer casing advances the inner casing so that the blunt cannula is moved from a retracted position within the needle to an extended position in which the blunt end of the cannula extends beyond the tip of the needle. In operation, fluid intended for injection into the patient can be drawn into the inner casing in a manner similar to a conventional syringe by withdrawing the plunger, which is disposed within the inner casing. Because the needle is rigidly attached to the outer casing, the outer casing can be handled by skilled personnel as they would handle an ordinary hypodermic needle. The fluid contained within the inner casing is injected into the patient by depressing the plunger in a conventional manner. Once the plunger is fully depressed, the inner casing is rotated relative to the outer casing to advance the blunting cannula to its extended position thus blunting the tip of the needle. The inner casing may be manipulated directly or may be manipulated by means of keying the plunger to the inner casing so that twisting the plunger causes the inner casing to rotate. A shear pin or other frangible element between the plunger and inner casing may be provided to prevent the cannula from being retracted once it has been fully extended. 
     DETAILED DESCRIPTION 
     The drawing figures are intended to illustrate the general manner of construction and are not necessarily to scale. In the detailed description and in the drawing figures, specific illustrative examples are shown and herein described in detail. It should be understood, however, that the drawing figures and detailed description are not intended to limit the invention to the particular form disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein and for setting forth the best mode for carrying out the invention. 
     With reference to the figures and in particular  FIGS. 1-5 , safety syringe  10  generally comprises a needle carrier  12 , a hollow hypodermic needle  14 , a syringe barrel  16  and a plunger  18 . In the illustrative embodiment, needle carrier  12  is in the form of a hollow plastic cylinder formed of a high-strength plastic such as Polyamide (nylon), Polyphenylene Oxide (PPO), Acrylonitrile Butadiene Styrene (ABS) or Polyethylene (PE) having appropriate graduations  20  so that the amount of liquid in the safety syringe  10  can be determined. Needle carrier  12  has an end wall  22  to which hypodermic needle  14  is attached in a conventional manner. Central cavity  24  is generally cylindrical and is open at the end opposite end wall  22 . In the illustrative embodiment, syringe barrel  16  is a hollow plastic cylinder also formed of a high-strength plastic such as nylon, PPO, ABS or PE having an end wall  26  to which a blunting cannula  28  is attached in a conventional manner. 
     Needle  14  is mounted coaxially with the central cavity  24  of needle carrier  12  and blunting cannula  28  is mounted coaxially to syringe barrel  16 . The interior diameter  30  of central cavity  24  and the outer diameter  32  of syringe barrel  16  are sized to allow syringe barrel  16  to be inserted into needle carrier  12 . Thus, when assembled, syringe barrel  16  is disposed within needle carrier  12  with blunting cannula  28  disposed within the central lumen  34  of needle  14 . Syringe barrel  16  includes a pin or other track engaging member  36  which engages an angled slot, which is formed on the cylindrical surface defining central cavity  24  and therefore forms a helical track  38  formed in needle carrier  12 . This couples needle carrier  12  to syringe barrel  16  in a manner the function of which will be explained more fully hereinafter. 
     Plunger  18  comprises a stem portion  40 , a piston  42 , a thumbpad  44  and a flange  46 . The stem portion  40 , thumbpad  44  and first engaging member or flange  46  of plunger  18  are preferably formed of a high-strength plastic such as nylon, PPO, ABS or PE. The interior cavity  48  of syringe barrel  16  has a substantially cylindrical cross section, an open proximal end and a distal end having a distal wall where the distal wall of syringe barrel  16  has an opening adapted to receive blunting cannula  28 . The piston  42  is preferably formed of an elastomeric compound such as polyurethane, buna or silicone to enable it to sealingly engage the interior cavity  48  of syringe barrel  16 . Flange  46  is generally circular in cross-section except for a flat  52  which engages a corresponding flat  54  formed in the second engaging member or collar  56  of syringe barrel  16  when plunger  18  is fully depressed. 
     With particular reference to  FIGS. 3-5 , safety syringe  10  is assembled by inserting syringe barrel  16  into needle carrier  12  until track engaging member  36  engages helical track  38 . In the illustrative embodiment, both syringe barrel  16  and needle carrier  12  are formed of moderately flexible plastic material such as PE, which enables the mating parts to deform to allow track engaging member to snap in place within helical track  38  thus retaining syringe barrel  16  to needle carrier  12 . With track engaging member  36  thus engaged in helical track  38 , blunting cannula  28  is disposed within needle  14  with the distal end  58  of blunting cannula  28  recessed within the distal end  60  of needle  14 . 
     To inject medication into the patient, plunger  18  is depressed in a conventional manner to expel the liquid from interior cavity  48  of syringe barrel  16  through the interior lumen  62  of blunting cannula  28  and through the central lumen  34  of needle  14 . Once plunger  18  is fully depressed, the flat  52  of flange  46  engages the flat  54  formed in collar  56 . This action keys plunger  18  to syringe barrel  16 . Plunger  18  is then rotated through an appropriate arc, preferably between 45 degrees and 270 degrees and most preferably about 90 degrees. Syringe barrel  16  being keyed to plunger  18  moves through the same arc. Track engaging member  36  engaged in helical track  38  converts rotational motion of syringe barrel  16  into linear motion. The helical track  38  preferably has a helical pitch of between 30 degrees and 60 degrees. Accordingly, twisting plunger  18  advances syringe barrel  16  within needle carrier  12  until, as shown in  FIG. 5 , the blunt distal end  58  of blunting cannula  28  extends beyond the sharp distal end  60  of needle  14 . This effectively blunts the distal end  60  of needle  14 . 
     With reference to  FIG. 6 , helical track  38  formed in needle carrier  12  may include detent means  64  adapted to retain syringe barrel  16  in the position with blunting cannula  28  fully advanced. Detent means  64  may comprise a counter-bore  64   a  adapted to receive track engaging member  36  as shown in  FIG. 7 . Alternatively, detent means  64  may comprise a plurality of lock members  64   b  as shown in  FIG. 8 . 
       FIG. 9  shows an alternative embodiment of a safety syringe  910  comprising a needle carrier  912 , hypodermic needle  914 , syringe barrel  916  and plunger  918 . In the illustrative embodiment of  FIG. 9 , needle carrier  912 , includes a pin or other track engaging member  936  that engages a corresponding track  938  formed on the outer surface of syringe barrel  916 . Plunger  918  includes a piston  942  that sealingly engages the interior cavity  948  of syringe barrel  916 . Plunger  918  includes a thumbpad  944  and a first engaging member or flange  946 . Flange  946  further includes a tab  952  that engages a corresponding recess  954  formed in second engaging member or collar  956  of syringe barrel  916 . Tab  952  and recess  954  cooperate to key plunger  918  to syringe barrel  916  in a manner similar to the embodiment of  FIGS. 1-5 , however, tab  952  may be designed to fail by breaking off at a predetermined torque such that once blunting cannula  928  has been moved to its fully extended position, tab  952  breaks off rendering it difficult or impossible to retract blunting cannula  928  from its fully extended position. 
     Although certain illustrative embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the invention. For example, although in the illustrative embodiment of  FIGS. 1-9 , the syringe barrel and needle carrier are formed of plastic materials capable of deforming to allow the safety syringe to be assembled, a track  1032  may be provided that extends to the open end  1034  of syringe barrel  1016 . Additionally, although in the illustrative embodiment of  FIG. 9 , tab  952  comprised a frangible element as shown in the illustrative embodiment of  FIG. 10 , second engaging member or collar  1056  of syringe barrel  1016  may include a frangible portion, such as an undercut portion  1050  designed to break off rendering it difficult or impossible to retract the blunting cannula once fully extended. Finally, although track  38  is shown as a recessed portion in the embodiment of  FIGS. 6-10 , for ease of manufacturing, track  38  may comprise a slot that extends through the exterior wall of needle carrier  12 . Accordingly, it is intended that the invention should be limited only to the extent required by the appended claims and the rules and principles of applicable law. Additionally, as used herein, unless otherwise specifically defined, the terms “substantially” or “generally” when used with mathematical concepts or measurements mean within ±10 degrees of angle or within 10 percent of the measurement, whichever is greater.