Abstract
A syringe for medical use, the syringe having a selectively retractable needle and a multifunctional needle holder and retainer ring assembly including a needle holder with a head section having an inwardly tapering frustoconical section of the fluid pathway and a rearwardly facing annular surface surrounding the rear opening, and an elastomeric retainer ring with a substantially unexpanded collar extending rearwardly beyond the rearwardly facing annular surface to increase needle pushout force, reduce activation force and improve fluid flow through the syringe. Annular projections inside the retainer ring and on the outer surface of the head section of the needle holder are also disclosed.
Claims
1. In a syringe having a barrel; a rearwardly biased, forwardly projecting needle; a needle retraction assembly slidably seated inside a forward portion of the barrel; and a plunger assembly slidably engaging an inside wall of the barrel, the improvement comprising: a multifunctional needle holder and retainer ring assembly provided as part of the needle retraction assembly, the multifunctional needle holder and retainer ring assembly further comprising a needle holder with an elongate front section, a larger diameter head section, and a fluid pathway extending through the needle holder in coaxial alignment with the needle; and an elastomeric retainer ring disposed between the needle holder and the barrel; wherein the head section further comprises a rear opening into an inwardly tapering frustoconical section of the fluid pathway, and a rearwardly facing annular surface surrounding the rear opening, and wherein a circumferentially extending collar portion of the retainer ring extends rearwardly behind the rearwardly facing annular surface.
2. The syringe of claim 1 wherein the frustoconical section has a wall surface inclined at an acute angle ranging from about 40 to about 50 from the longitudinal axis of the needle holder.
3. The syringe of claim 2 wherein the wall surface is inclined at an acute angle of about 45 from the longitudinal axis of the needle holder.
4. The syringe of claim 1 wherein a line lying in the wall surface of the frustoconical section and intersecting the longitudinal axis of the needle holder also intersects the rearwardly extending portion of the retainer ring at a point proximal to the rear end of the retainer ring.
5. The syringe of claim 1 wherein the rearwardly facing annular surface has a width that is approximately equal to the distance by which the circumferentially extending collar portion of the retainer ring extends rearwardly beyond the rearwardly facing annular surface.
6. The syringe of claim 1 wherein the rear opening has a radius and the rearwardly extending collar portion of the retainer ring extends behind the rear end of the needle holder by a longitudinal distance that is less than the radius of the rear opening.
7. The syringe of claim 1 wherein the head section has a first radius, the rear opening has a second radius, and the rearwardly extending portion of the retainer ring is substantially unexpanded and extends behind the rear end of the needle holder by a longitudinal distance that is substantially equal to the difference between the first and second radius.
8. The syringe of claim 1 wherein the head section of the needle holder has a head section with an outwardly facing surface, the retainer ring has an inwardly facing surface contactinig the outwardly facing surface of the head section of the needle holder, and wherein at least one of the inwardly facing surface of the retainer ring and the outwardly facing surface of the head section of the needle holder comprises an annular projection that is desirably configured to enhance frictional interference between the head section of the needle holder and the retainer ring while still permitting relative sliding movement between them during needle retraction.
9. The syringe of claim 8 wherein the inwardly facing surface of the retainer ring comprises an annular projection that is configured to slidably engage the outwardly facing wall of the head section of the needle holder.
10. The syringe of claim 8 wherein the inwardly facing surface of the retainer ring comprises an annular projection having a cross-section generally configured as a truncated prism with oppositely inclined proximal and distal surfaces and a substantially flat, inwardly facing surface disposed between the oppositely inclined proximal and distal surfaces.
11. The syringe of claim 8 wherein the outwardly facing surface of the head section comprises an outwardly facing annular projection, the annular projection further comprising a forwardly inclined rear surface, a front surface transverse to the longitudinal axis through the needle holder, and a substantially flat, outwardly facing surface disposed between the rear and front surfaces.
12. In a syringe comprising a barrel, a selectively retractable needle, a plunger assembly slidably engaging the barrel, and a needle retraction assembly, the improvement comprising: a multifunctional needle holder and retainer ring assembly, the multifunctional needle holder and retainer ring assembly further comprising: a needle holder having a head section with an outwardly facing surface, a retainer ring disposed between the barrel and the needle holder, and having a radially expanded, inwardly facing surface portion contacting the outwardly facing surface of the needle holder; wherein at least one of the inwardly facing surface portion of the retainer ring and the outwardly facing surface of a head section of the needle holder comprises an annular projection that is desirably configured to enhance frictional interference between the head section of the needle holder and the retainer ring while still permitting relative sliding movement between them during needle retraction.
13. The syringe of claim 12 wherein the inwardly facing surface portion of the retainer ring further comprises an annular projection facing radially inward that is configured to slidably engage the outwardly facing surface of the head section of the needle holder.
14. The syringe of claim 12 wherein the annular projection of the inside surface of the retainer ring has a cross-section generally configured as a truncated prism with oppositely inclined proximal and distal surfaces and a substantially flat, inwardly facing surface disposed between the oppositely inclined proximal and distal surfaces.
15. The syringe of claim 12 wherein the outwardly facing surface of the head section comprises an outwardly facing annular projection, the annular projection further comprising a forwardly inclined rear surface, a front surface transverse to the longitudinal axis through the needle holder, and a substantially flat, outwardly facing surface disposed between the rear and front surfaces.
16. A method for increasing the push-out force without increasing the activation force before and during an injection administered using with a syringe having a barrel; a rearwardly biased, forwardly projecting needle; a needle retraction assembly slidably seated inside a forward portion of the barrel; and a plunger assembly slidably engaging an inside wall of the barrel, comprising: providing as part of the needle retraction assembly a multifunctional needle holder and retainer ring assembly further comprising: a needle holder with an elongate front section, a larger diameter head section, and a fluid pathway extending through the needle holder in coaxial alignment with the needle; and an elastomeric retainer ring disposed between the needle holder and the barrel; wherein the head section further comprises a rear opening into an inwardly tapering frustoconical section of the fluid pathway, and a rearwardly facing annular surface surrounding the rear opening, and wherein a substantially unexpanded, circumferentially extending portion of the retainer ring extends rearwardly behind a portion of the rearwardly facing annular surface; and administering an injection with the syringe.
17. A syringe comprising a retractable needle and a cooperatively configured needle holder and retainer ring assembly wherein: the retainer ring is elastomeric and comprises an axially extending, inwardly facing wall having an inside diameter; the needle holder has a head section having an outside diameter greater than the inside diameter of the retainer ring that is inserted into a forwardly facing, radially expanded portion of the retainer ring; and a substantially unexpanded portion of the retainer ring extends rearwardly behind the head section of the needle holder.
18. A syringe comprising a retractable needle and a cooperatively configured needle holder and retainer ring assembly wherein: the needle holder further comprises a head section having a first axial length and an outside diameter; the retainer ring is elastomeric and further comprises an inwardly facing wall having a second axial length and an inside diameter that is less than the outside diameter of the needle holder; the first axial length is less than the second axial length; and wherein the head section of the needle holder is inserted into a forwardly facing, radially expanded first portion of the elastomeric retainer ring and an unexpanded second portion of the elastomeric retainer ring extends rearwardly behind the inserted head section of the needle holder.
19. A syringe having a retractable needle and a needle holder and retainer ring assembly comprising a needle holder and an elastomeric retainer ring wherein: needle holder further comprises a head section with an outside diameter; the elastomeric retainer ring has an unexpanded inside diameter less than the outside diameter; the head section is inserted into a front portion of the elastomeric retainer ring that is expanded to accommodate the head section; and an unexpanded rear portion of the elastomeric retainer ring extends rearwardly behind the head section by a distance that is less than half the diameter of the head section.
20. A syringe having a barrel, a plunger assembly, a retractable needle and a cooperatively configured needle holder and retainer ring assembly comprising: a head section of the needle holder further comprising a rear opening into a coaxially aligned fluid flow path communicating with the retractable needle and a frustoconical, inwardly facing wall surface located proximally to the rear opening; and an elastomeric retainer ring having a first longitudinally extending portion that is expanded radially and surrounds the head section of the needle holder and a second longitudinally extending collar portion that is substantially unexpanded radially and extends rearwardly behind the head section of the needle holder; wherein the rearwardly extending collar portion of the elastomeric retainer ring cooperates with the frustoconical, inwardly facing wall surface to reduce a thumb force required to advance the plunger assembly inside the barrel and to reduce turbulence in a fluid entering the rear opening into the fluid flow path during an injection, and to reduce the retention of any air bubble in fluid disposed around the opening during use of the syringe.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The apparatus of the invention is further described and explained in relation to the following drawings wherein:
[0029] FIG. 1 is a longitudinal cross-sectional view through the front end of a medical syringe configured according to one embodiment of the invention and having an installed needle and needle retraction mechanism, an elastomeric retainer ring, and a slidably engaged plunger assembly (partially broken away) with a plug blocking access into a needle retraction cavity;
[0030] FIG. 2 is a longitudinal cross-sectional view of the apparatus of FIG. 1 with the plunger assembly broken away and the retainer ring moved forwardly relative to the head of the needle holder and the barrel as needle retraction is initiated by forward movement of the plunger assembly (indicated diagrammatically by arrow 52;
[0031] FIG. 3 is an enlarged cross-sectional detail view taken from FIG. 1;
[0032] FIG. 4 is an enlarged cross-sectional detail view taken from FIG. 3;
[0033] FIG. 5 is a rear perspective view of a needle holder suitable for use in a syringe configured according to the embodiment of FIG. 1;
[0034] FIG. 6 is an enlarged longitudinal cross-sectional view of the needle holder of
[0035] FIG. 5;
[0036] FIG. 7 is a rear perspective view of a retainer ring suitable for use in one embodiment of the syringe of FIG. 1;
[0037] FIG. 8 is a longitudinal cross-sectional view through the retainer ring of FIG. 7;
[0038] FIG. 9 is an enlarged detail view as in FIG. 3 of an alternate embodiment of the invention wherein the retainer ring is configured differently from the embodiment shown in FIG. 3;
[0039] FIG. 10 is an enlarged detail view as in FIG. 3 of another alternate embodiment of the invention wherein the retainer ring and the outer surface of the head of the needle holder are cooperatively configured differently from the embodiment shown in FIG. 3;
[0040] FIG. 11 is a longitudinal cross-sectional view of an alternate embodiment of the syringe as in FIG. 1 wherein the front end of the handle of the plunger assembly is stepped rearwardly to cause one side of the front end of the plunger handle to concentrate a thumb force applied to the plunger on one side of the retainer ring; and
[0041] FIG. 12 is a longitudinal cross-sectional view of the apparatus as in FIG. 11 wherein the front end of the handle of the plunger assembly is inclined rearwardly to cause one side of the front end of the plunger handle to concentrate a thumb force applied to the plunger on one side of the retainer ring.
[0042] It should be appreciated that the accompanying drawings are not drawn to scale.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Referring to syringe 10 as shown in FIG. 1, the rear portion of barrel 12 and plunger 40 of syringe 10 are broken away to permit enlargement of the elements disposed at the front of syringe 10. Referring to FIGS. 1, 5 and 6, inclined shoulder 74 (FIG. 6) of needle holder 14 is seated inside a nose portion of barrel 12, with front tip 17 of needle holder 14 projecting forwardly through an opening in the distal end of the nose portion. Needle 15 has a forwardly projecting beveled tip and a rear portion that is secured by conventional means in fixed relation to a variable diameter axial bore 32 through the needle holder 14. The proximal end of needle holder 14 has a head section 18 with a larger diameter outer wall 76 that forms a forwardly facing annular shoulder. The annular shoulder abuts against the proximal end of compressed coiled spring 25 when spring 25 is seated inside the nose portion of barrel 12 around an elongate section of needle holder 14 that extends between inclined shoulder 74 and the distal end of outer wall 76 of head section 18. In this configuration, when coiled spring 25 is compressed during assembly of syringe 10, it biases needle holder 14 rearwardly relative to the nose portion of barrel 12.
[0044] Although plunger 40 as shown in FIG. 1 is made with plunger seal 44 that can slide rearwardly to a point where it abuts forwardly facing annular shoulder 46 of plunger 40 as plunger tip 50 is moved forward relative to barrel 12 by the application of thumb force during an injection. By contrast, syringe 100 seen in FIG. 11 is made with a differently configured plunger tip 104 and a plunger seal 106 that is held in place between two annular shoulders and cannot slide axially relative to the plunger handle during an injection. The multifunctional needle holder and retainer ring of the invention is nevertheless applicable to syringes having either plunger configuration.
[0045] Referring again to FIG. 1, elastomeric retainer ring 24 is made with an inside diameter that is less than the outside diameter of head section 18 and the front portion of elastomeric retainer ring 24 is desirably expanded during insertion of head section 18 during assembly so to facilitate insertion of axially extending outer wall 76 of head section 18 of needle holder 14. Following insertion of head section 18 into the front portion of elastomeric retainer ring 24 and after retainer ring 24 is disposed between head section 18 of needle holder 14 and inside wall section 26 of barrel 12, elastomeric retainer ring 24 exerts sufficient holding force against head section 18 of needle holder 14 to resist relative motion or fluid leakage around retainer ring 24 during use of syringe 10 prior to needle retraction. Other performance benefits as discussed herein are also achieved through use of the structurally modified multifunctional needle holder and retainer ring assembly of the invention. As used herein, the term multifunctional needle holder and retainer ring assembly (or simply needle holder and retainer ring assembly as used below) is understood to include the installed combination of needle holder 14 and retainer ring 24 in a syringe 10 of the invention. This cooperatively configured combination of elements is discussed in greater detail below in relation to FIGS. 1-10 of the drawings.
[0046] Another feature of the present invention is disclosed and described in relation to FIGS. 1, 5 and 6. Proximal end 34 of head section 18 comprises an opening into a proximal portion of variable diameter bore 32 of needle holder 14 that further comprises a frustoconical section having a wall surface 20 disposed in the proximal end portion of head section 18 of needle holder 14. The inside diameter of frustoconical wall surface 20 at proximal end 34 is desirably greater than the inside diameter of rear openings of the needle holders of similarly made conventional syringes. It is also sufficiently less than the outside diameter of head section 18 at proximal end 34 of needle holder 14 that proximal end 34 of head section 18 can withstand compressive forces exerted on it by elastomeric retainer ring 24 without deformation or collapse. This feature produces a relatively narrow, rearwardly facing annular surface around the rear opening at proximal end 34 of needle holder 14.
[0047] Frustoconical interior wall surface 20 of head section 18 tapers radially inward forwardly of proximal end 34 and forms a proximal end of fluid flow path 64 through needle holder 14 that is more conducive to receiving a smooth flow of liquid from interior chamber 22 of barrel 12 (FIG. 1) into needle holder 14, thereby contributing to a reduction in the thumb force needed for a user of syringe 10 to advance plunger 40 inside barrel 12 during an injection. The diameter of bore 32 defining fluid flow path 64 decreases at a substantially constant rate until a point is reached where the constant convergence of wall surface 20 ends and the inside diameter of axial bore 32 is differently configured to accommodate the insertion and secure attachment of needle 15 inside needle holder 14. This needle holder configuration provides an inwardly tapering fluid flow path 64 for a medicinal fluid pushed by a plunger assembly from barrel 12 into needle 15 during an injection using medical syringe 10.
[0048] The combination of a narrower rearwardly facing annular surface extending around and a wider, rear opening at proximal end 34 of needle holder 14 cooperate to reduce the accumulation or hang-up of small air bubbles in the space identified by arrow 29 (visible in FIIGS. 9, 10, 11-12) around the opening at proximal end 34 of needle holder 14. In one embodiment of the invention, the frustoconical section inside the rear opening of head section 18 of needle holder 14 has a wall surface 20 inclined at an acute angle ranging from about 40 to about 50, and preferably about 45, from the longitudinal axis 102 (visible in FIGS. 9-10) of needle holder 14. In another embodiment of the invention, line 100 (visible in FIGS. 4, 9-10) lying in wall surface 20 of the frustoconical section and intersecting longitudinal axis 102 of the needle holder also intersects the rearwardly extending portion of the retainer ring tangentially at a point closely adjacent to the rear end of the retainer ring.
[0049] By narrowing the width of proximal end surface 34 while maintaining a constant outside diameter for head section 18 of needle holder 14 and widening the opening defined by the proximal edge of interior surface section 20 of needle holder 14, a smoother flow profile into fluid flow path 64 (FIG. 3) is established and less transverse surface area is provided at proximal end 34 of needle holder 14 on which air bubbles can otherwise accumulate behind needle holder 14. Such air bubbles sometimes concern patients or clinicians, or cause such persons to perceive that an incomplete dose is being administered through syringe 12. Also, in contrast to conventional retractable syringes, because the annular surfaces of proximal end 34 of needle holder 14 and proximal end 28 of retainer ring 24 are not transversely aligned, the proximal ends of needle holder 18 and retainer ring 24 cannot collectively provide a wider contiguous surface upon which air bubbles can accumulate around the rear opening of the needle holder prior to or during an injection.
[0050] In another preferred embodiment of the invention that is described in relation to FIGS. 1, 3 and 4, syringe 10 having a selectively retractable needle 15 that also comprises a multifunctional needle holder and retainer ring assembly is disclosed in which unexpanded proximal end 28 of retainer ring 24 extends rearwardly past proximal end 34 of needle holder 14 for a short distance that is preferably not significantly greater than the radial width of the annular surface at proximal end 34 of needle holder 14. This rearward extension of retainer ring 24 relative to needle holder 14 forms a relatively short, axially extending, radially unexpanded annular collar 69 that presses radially inward around proximal end 34 of retainer ring 24. Also, during assembly of syringe 10, inwardly projecting annular bump 72 inside barrel 12 can force annular collar 69 formed by the radially unexpanded, rearwardly extending proximal end 28 of retainer ring 24 radially inward behind the outer edge of the annular surface at proximal end 34 of needle holder 14. These factors cooperate to cause proximal end 28 of retainer ring 24 to squeeze inwardly behind outer wall 76 (FIG. 6) at proximal end 34 and thereby reduces the likelihood of rearward slippage of needle holder 14 relative to retainer ring 24 without increasing the activation force required to push retainer ring 24 forwardly off outer wall 76 during needle retraction following an injection.
[0051] Where the new multifunctional needle holder and retainer ring assembly of the invention is desired to be retrofitted into a existing syringe barrel configuration of an otherwise similarly configured prior art syringe without barrel redesign or retooling, the axial length of retainer ring 24 is retained and the axial length of head section 18 of needle holder 14 is reduced relative to retainer ring 24 to establish substantially unexpanded annular collar 69 (FIGS. 3-4). Proximal end 34 of head section 18 is then disposed in a position slightly forward of proximal end 28 of retainer ring 24 when retainer ring 24 is satisfactorily installed between the outside wall of head section 18 and inside wall section 26 of barrel 12. At the same time, the distal, forwardly facing surface of retainer ring 24 is desirably substantially even with the distal, forwardly facing annular surface of head section 18 of needle holder 14. Said differently, when retainer ring 24 is slightly compressed between head section 18 and inside wall section 26 of barrel 12 during installation, retainer ring 24 desirably has an axial length that exceeds the axial length of head section 18 of needle holder 14 so that proximal end 28 of retainer ring 24 forms a short, rearwardly extending collar 69 around proximal end 34 of needle holder 14.
[0052] In one embodiment of the invention, proximal end 28 of retainer ring 24 extends behind proximal end 34 of needle holder 14 by a distance not significantly wider than the width of the rearwardly facing annular surface at proximal end 34 of needle holder 14. In another embodiment of the invention, a line 100 lying in frustoconical wall section 20 of needle holder 14 and intersecting the longitudinal axis of needle holder 14 and fluid pathway 64 will also preferably tangentially intersect the rearwardly extending, unexpanded portion of the retainer ring 24 at a point closely adjacent to proximal end 28 of retainer ring 24. In each of these preferred embodiments, retainer ring 24 is desirably cooperatively configured together with needle holder 14 so that the rear opening into liquid flow path 64 (FIG. 3) has an inwardly tapered frustoconical wall surface 20 as discussed above.
[0053] In another embodiment of the invention that is disclosed and described in relation to FIGS. 1, 3-4 and 7-8, syringe 10 has a selectively retractable needle 15 and also comprises an assembly of multifunctional needle holder 14 and retainer ring 24. Retainer ring 24 desirably further comprises inwardly facing surface sections 58 separated by an inwardly facing annular projection 66 that in this embodiment is configured as a trapezoidal prism having oppositely inclined proximal and distal surfaces extending between one of inwardly facing surface sections 58 and the corresponding side of a substantially flat inner surface 66 disposed between them. In FIGS. 3 and 4, which are progressively enlarged to better depict the facing and contacting surfaces between retainer ring 24 and outer wall 76 (FIG. 6) of head section 18 of needle holder 14, it is observed by comparison with the cross-sectional view of retainer ring 24 in FIG. 8 that body 68 of retainer ring 24 is compressed radially from both the barrel side and the needle holder side during installation of retainer ring 24 between inside wall section 26 of barrel 12 and the outer wall of head section 18 of needle holder 14. This compression enhances the holding force that limits relative axial movement of needle holder 14 relative to retainer ring 24 and of retainer ring 24 relative to inside wall section 26 of barrel 12, and also reduces the likelihood of fluid leakage past retainer ring 24. Distorted air pockets 70 that are best seen in FIG. 4 are also believed to enhance the holding force exerted by retainer ring 24 relative to head section 18 and inside wall section 26 of barrel 12.
[0054] Referring to FIG. 9, another embodiment of the invention is disclosed in which a differently configured retainer ring 80 is installed in combination with needle holder 14 to produce a differently configured multifunctional needle holder and retainer ring assembly. In this embodiment, retainer ring 80 is again lengthened axially or, alternatively, head section 18 of needle holder 14 is shortened relative to retainer ring 80 to again produce a short annular collar having a proximal end 86 that extends rearwardly of proximal end 34 of head section 18. As discussed above in relation to retainer ring 24, the portion of retainer ring 80 extending rearwardly behind head section 18 of needle holder 14 can also compress inwardly behind proximal end 34 of head section 18 of needle holder 14 to increase the axial holding force between them. In this embodiment, however, inwardly facing surface 84 of retainer ring 80 lacks the inwardly facing annular projection as previously shown and described in relation to FIGS. 7-8.
[0055] Referring to FIG. 10, another embodiment of the invention is disclosed in which retainer ring 80 is installed in combination with needle holder 94 having head section 98 to produce another differently configured multifunctional needle holder and retainer ring assembly. In this embodiment, retainer ring 80 is again lengthened axially or, alternatively, head section 98 of needle holder 94 is shortened relative to retainer ring 80 to again produce a short annular collar having a proximal end 86 that extends rearwardly of proximal end 97 of head section 98. As discussed above in relation to retainer ring 24, retainer ring 80 can also compress inwardly behind proximal end 97 of head section 98 of needle holder 94 to increase the axial holding force between them. In this embodiment, however, the outer wall surface of head section 98 comprises an outwardly facing annular projection 96 having an inclined surface on the proximal side that allows the substantially cylindrical inside surface of retainer ring 80 to slide up and over annular projection 96 during assembly and also during needle retraction following use of syringe 10 as discussed below in relation to FIGS. 1-2 and 11-12. Prior to and during use of syringe 10, however, a forwardly facing square shoulder on the distal side of annular projection 96 will restrict any rearward movement of retainer ring 80 relative to head section 98 of needle holder 94 as might otherwise be experienced, for example, during an instance of needle push-through..
[0056] Referring again to FIG. 1, during an injection using syringe 10, front end 50 of plunger tip 42 is advanced inside barrel 12, forcing medicinal fluid disposed inside variable volume chamber 22 (not shown) forwardly into the opening disposed at proximal end 34 of needle holder 14, and then through needle holder 14 and needle 15. When front end 50 contacts proximal end 28 of retainer ring 24, the continued application of thumb force to plunger assembly 40 initiates three concurrent movements: Plunger seal 44 begins moving rearwardly toward forwardly facing shoulder 46; retainer ring 24 begins moving forwardly along the interfaces with head section 18 of needle holder 14, eventually pushing retainer ring 24 off of head section 18 and into annular space 30; and plunger plug 48 begins moving rearwardly toward a coaxially aligned needle retraction cavity disposed inside the plunger handle behind plunger plug 48.
[0057] Referring to FIG. 2, parts of the plunger assembly are removed for illustrative purposes, arrows 52 depict the direction of movement of retainer ring 24 and arrow 54 depicts the direction of movement of needle holder 14. Retainer ring 24 is shown in a position where it has almost entirely disengaged from needle holder 14, and compressed spring 25 is beginning to expand rearwardly, driving needle holder 14 and needle 15 rearwardly toward the coaxially aligned needle retraction cavity.
[0058] Referring again to syringe 10 in FIG. 1, it is seen that front end 50 of plunger tip 42 of plunger assembly 40 is circular and is transverse to the longitudinal axis of syringe 10. Referring to syringe 100 in FIG. 11, it is seen that front end 104 is stepped back axially on one side of plunger 102, so that one arcuate segment of front end 104 will contact retainer ring 24 before the remainder does, thereby initially concentrating the thumb force on one side of retainer ring 24 to assist in sliding it forwardly relative to needle holder 14. Referring to syringe 200 in FIG. 12, it is seen that front end 204 is inclined rearwardly on one side of plunger 202, so that one arcuate segment of front end 204 will contact retainer ring 24 before the remainder does, again initially concentrating the thumb force on a relatively short arcuate segment on one side of retainer ring 24, after which the thumb forces is progressively distributed around the remainder of proximal end 28 of retainer ring 24 to assist in sliding retainer ring 24 forwardly relative to needle holder 14.
[0059] Other benefits, advantages, embodiments and modifications of the present invention will likely become apparent to those of ordinary skill in the art upon reading this disclosure, and it is intended that the scope of the invention be limited only by the broadest interpretation of the specification and claims to which the inventors and applicants are legally entitled.
