Abstract:
A lockable safety shield assembly for a prefillable syringe is provided. The design of the lockable safety shield assembly enhances pharmaceutical manufacturers&#39; ease of assembling the various components as part of its filling or processing of the prefillable syringes in normal practice, while at the same time minimizes difficulties in mating parts made from different materials. A tube is placed around the outside surface of the syringe barrel and affixed thereto. A collar is provided on the tube adjacent the distal end of the syringe barrel. A safety shield is axially slidable over the tube between a retracted position, wherein the distal end of the piercing element associated with the prefillable syringe is exposed, and an extended position, wherein the safety shield is locked to the collar to protectively cover the distal end of the piercing element. The safety shield includes locking structure configured so that the shield can be easily fitted over the tube. The locking structure includes at least one deflectable arm provided on the body of the shield. The deflectable arm includes a proximal end deflectable towards the interior of the shield. A stop member is provided on the interior of a shield in spaced relation to the proximal end of the deflectable arm. A ring is axially slidable over the shield to deflect the arm towards the interior of the shield to activate the locking structure. The safety shield is slid distally by an end user such that the collar is lockingly retained between the stop member and the proximal end of the deflectable arm. The collar, the stop member, and the deflectable arm may be configured to provide tactile as well as audible indication of locking to the end user.

Description:
[0001]    This application is a continuation of U.S. patent application Ser. No. 08/941,448. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to a lockable safety shield assembly for a prefillable syringe, and more particularly, to a lockable safety shield assembly for a prefillable syringe which is easily attached to a prefillable syringe and which can be subsequently activated by an end-user to form a locking mechanism for the shield.  
         BACKGROUND  
         [0003]    As is known in the art, syringes are medical delivery devices utilizable to administer a medicament to a patient. Syringes are normally provided in prefillable form, wherein a set dosage of medicament can be filled into the syringe by a pharmaceutical manufacturer for distribution to the end user, or they are empty and intended to be filled by the end user from a vial or other source of medicament at the time administration of the medicament is desired.  
           [0004]    Syringes typically include a barrel portion adapted to retain the medicament. The distal end of the barrel is normally configured to mate with a conventional piercing element, such as a pointed needle cannula made of steel or like material or a blunt ended cannula formed of plastic, to deliver the medicament contained in the barrel. A plunger rod is inserted through the open distal end of the syringe barrel and, through its engagement with an elastomeric or rubber-like stopper element fitted in a fluid-tight manner within the interior of the barrel, a user can apply manual force to the plunger to deliver the medicament through the piercing element.  
           [0005]    More typically the case with pointed needle cannulae, accidental needle sticks that occur after the needle cannula has been used generally pose a greater health risk. To avoid such accidents, many prior art hypodermic syringes include a rigid cylindrical safety shield telescoped over the syringe barrel. The prior art safety shield can be slid from a proximal position where the needle cannula is exposed for use, to a distal position where the safety shield protectively surrounds the needle cannula.  
           [0006]    Most prior art hypodermic syringes with cylindrical safety shields telescoped over the syringe barrel include structure for locking the safety shield in its distal position. Examples of such structures are found, for instance, in U.S. Pat. Nos. 5,342,309 and 5,385,555 to Hausser and U.S. Pat. No. 5,304,149 to Morigi.  
           [0007]    While generally suitable for the tasks employed, further improvements are still being sought. For instance, where the syringe is prefillable by a pharmaceutical manufacturer, it would be beneficial to incorporate improvements into the locking structure employed with the safety shields to better facilitate a pharmaceutical manufacturer&#39;s processing of the various components associated with the syringe barrel and safety shield. These improvements should also facilitate the end user&#39;s normal use of the prefillable syringe and easy activation of the safety shield once safe disposal of the syringe is desired. Moreover, most of the prior art approaches focus their efforts on employing a plastic collar element on some distal portion of the syringe barrel, with the shield locking onto the collar. While a valid approach, because many of the prefilled syringes currently in use are made of glass, it can be difficult to assemble the plastic collar element onto the syringe barrel. Accordingly, it would be useful to design a structure whereby the collar were incorporated as part of the shield structure itself.  
         SUMMARY OF THE INVENTION  
         [0008]    A lockable safety shield assembly for prefillable syringes is provided. The lockable safety shield is designed to facilitate a pharmaceutical manufacturer&#39;s assembly of the various components associated with the lockable safety shield onto the syringe in conjunction with how the syringes are normally processed by the pharmaceutical manufacturer. The locking structure associated with the safety shield of the present invention may be designed such that it will not interfere with easy assembly by the pharmaceutical manufacturer, but which can be easily activated by an end user to facilitate safe covering of the distal end of a piercing element once the prefillable syringe has been used.  
           [0009]    The assembly includes a tube mounted about the outside surface of the syringe barrel. The tube can be locked, for instance, to the flange provided about the proximal end of the syringe barrel. The tube features a collar positioned adjacent the distal end of the syringe barrel. The collar can be configured as a flat, disk-like structure formed in an annular manner about the outside of the tube.  
           [0010]    A safety shield is coaxially mounted over the tube. The safety shield is axially movable between a retracted position, wherein the distal tip of a piercing element affixable to the barrel is exposed, and an extended position, wherein the distal tip of the piercing element will be covered. The safety shield includes a proximal end, a distal end, and a circumferential sidewall therebetween. The circumferential sidewall has an outside surface and an inside surface. Particularly, the shield is provided with user-activatable locking structure to secure the shield in its extended position respective of the syringe.  
           [0011]    In one configuration, the locking structure includes at least one deflectable arm incorporated as part of the structure of the sidewall. The deflectable arm includes a distal end mounted with the circumferential sidewall, and a proximal end which is deflectable towards the interior of the shield. At least one stop member is mounted to the interior surface of the shield and spaced a distance from the proximal end of the deflectable arm to define a gap between the stop member and the proximal end of the deflectable arm. The deflectable arm has a free position, wherein the outside surface of the arm is substantially co-planar with the outside surface of the circumferential sidewall, and a locking position, wherein the deflectable arm is urged towards the interior of the shield so that the proximal end of the deflectable arm is positioned towards the interior of the shield.  
           [0012]    Means are provided on the shield to activate the locking structure. In one configuration, a circumferential ring is axially slidable about the outside surface of the shield between a first position, wherein the ring is spaced away from the deflectable arm, and a second position, wherein the ring is positioned against the at least one deflectable arm to secure the arm in the locking position. The ring may be secured to the distal end of the shield by a user-severable connection. The user-severable connection may entail, for instance, one or more frangible sections of material provided between the ring and the distal end of the shield. A pair of spaced abutments may be provided on the outside surface of the deflectable arm to retain the ring in its second position.  
           [0013]    In practice, the safety shield assembly is typically preassembled by the syringe manufacturer and supplied to the pharmaceutical manufacturer in a sterile state, ready for attachement to the prefillable syringe once it has been filled with a desired drug.  
           [0014]    After the syringe has been filled with the drug and otherwise subjected to various processing operations, such as various particulate inspection operations, labelling operations, plunger rod assembly operations, or the like, the safety shield assembly can be easily fitted to the syringe. The tube is fitted about the syringe barrel and clipped onto the flange of the syringe barrel. In the preassembled state, the shield located respective of the tube such that the shield will be placed in a retracted position vis-a-vis the piercing element. Various structures associated with the tube may be provided with sloped surfaces to ensure smooth placement of the safety shield assembly over the syringe flange.  
           [0015]    Subsequent to placing the shield assembly onto the syringe barrel, the locking structure must be activated. The locking structure is activated by detaching the ring from its user-severable connection, and sliding the ring proximally so that it is engaged between the abutments provided on the deflectable arm. If desired, the device may be shipped by a pharmaceutical manufacturer to an end user with the ring connected to the shield by its user-severable connection, so that the end-user himself activates the locking structure. Alternately and preferably, the pharmaceutical manufacturer activates the locking structure as part of the assembly process, such that the filled syringe with safety shield assembly is shipped to an end-user with the locking structure already activated.  
           [0016]    After the syringe has been used to administer an injection, the shield is slid distally over the tube. The collar will be captured between the proximal end of the deflectable arm and the stop member in order to lock the shield in its extended position. In a preferred embodiment, the deflectable arm, the collar, and the stop member may be configured to provide audible as well as tactile indication to the end user that the shield has been securely locked with the collar.  
           [0017]    In a preferred embodiment, at least two deflectable arms are provided. The two deflectable arms are located on diametrically opposite sides of the shield. Correspondingly, two stop members are provided to interact with the two deflectable arms. When viewed along the central axis of the shield, the stop members can be circumferentially aligned with a respective proximal end of the deflectable arms, or they can be circumferentially offset from a respective proximal end.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention will now be described in greater detail by way of reference to the appended drawings, wherein:  
         [0019]    [0019]FIG. 1 depicts, in perspective view, a lockable safety shield assembly of the present invention mounted to a syringe;  
         [0020]    [0020]FIG. 2 is an exploded perspective view of the lockable safety shield assembly of FIG. 1;  
         [0021]    [0021]FIG. 3 is a cut-away view of the lockable safety shield of the present invention being mounted over the tube and the syringe barrel;  
         [0022]    [0022]FIG. 4 is a cross-sectional view of the lockable safety shield assembly of the present invention subsequent to mounting to the syringe;  
         [0023]    [0023]FIG. 5 is a cross-sectional view of the lockable safety shield assembly of the present invention after an end user has activated the locking mechanism;  
         [0024]    [0024]FIG. 6 is a cross-sectional view of the lockable safety shield assembly of the present invention after it has been urged in a distal direction by an end-user to protectively cover the needle cannula;  
         [0025]    [0025]FIG. 7 is partial cross-sectional view of the lockable safety shield assembly of the present invention illustrating activation of the locking mechanism;  
         [0026]    [0026]FIG. 8 is a partial cross-sectional view of the lockable safety shield assembly of the present invention interacting with the collar located on the tube;  
         [0027]    [0027]FIG. 9 is an end elevational view of the lockable safety shield assembly in accordance with the present invention; and  
         [0028]    [0028]FIG. 10 is a partial cross-sectional view of a preferred manner of configuring the collar in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    A convention utilized throughout this description is that the term “distal” refers to the direction furthest from a practitioner, while the term “proximal” refers to the direction closest to a practitioner.  
         [0030]    Turning now to the drawings, wherein like numerals denote like components, FIGS. 1-4 depict one embodiment of a lockable safety shield assembly  30  in accordance with the present invention. The lockable safety shield assembly  30  in accordance with the present invention can be provided in conjunction with a prefillable syringe  10 . As illustrated in FIGS. 1 and 2, prefillable syringe  10  includes a barrel  16  characterized by a proximal end  14 , a distal end  12 , and a hub  24  provided adjacent the distal end. A flange  18  may be provided about the proximal end, with a plunger rod  20  inserted through proximal end  14 . Plunger rod  20  is attached to a stopper  22  used for urging fluid through barrel  16 . Fluid is emitted or aspirated via a fluid path, structured at distal end  12  of the syringe, that communicates with interior portions of barrel  16 . For instance, if the pre-fillable syringe is configured as a luer lock syringe, the fluid path is configured as a luer tip (not shown) provided at distal end  12 . Here, pre-fillable syringe  10  is illustrated with a fluid path configured as a piercing element  26 , such as a pointed needle cannula. Piercing element  26  is characterized by a distal end  28 .  
         [0031]    The lockable safety shield assembly  30  in accordance with the present invention includes a tube  200  which is mounted about barrel  16  of the syringe. The tube  200  features a proximal end  204 , a distal end  202 , and a circumferential sidewall  206  therebetween. A flange structure  208  is provided adjacent proximal end  204  of the tube. As best seen in FIG. 3, flange structure  208  includes a lip  210  sized to lockingly engage flange  18  of the syringe barrel when tube  200  is placed over barrel  16 . Adjacent distal end  202  of the tube there is provided a collar  32 . Collar  32  can be formed as a disk-like structure characterized by an edge  34  and a side portion  36 . Collar  32  has a thickness “e” and is characterized by a maximum diameter “g”. Collar  32  may be provided as part of the structure of tube  200 . Alternately, as the skilled artisan will appreciate, collar  32  may be separately supplied and retained to the tube by friction fit, adhesives, mechanical means, or like structure. If desired, edge  34  can display one or more raised nubs  39  having grooves  40 . Grooves  40  can be aligned with complementary rails (not shown) provided on an interior portion of safety shield  42  for aligning the safety shield during the assembly process.  
         [0032]    For purposes of clarity, FIGS. 1-6 depict collar  32  as having a rectangular cross-section, with side portion  36  displaying relatively flat walls that are directly linked to edge  34 . If so desired, collar  32  can be configured in this manner to construct a lockable safety shield assembly in accordance with the present invention. However, FIG. 10 depicts a preferred way for configuring the collar (here designated by the numeral  32   a ) so to provide better audible as well as tactile indication that shield  42  has been locked to the collar, as well as to enhance locking action between the shield and the collar, as will be further explained.  
         [0033]    Referring then to FIG. 10, as before, collar  32   a  includes an edge  34  and a side portion  36 . However, here, side portion  36  is not completely flat. Rather, when viewed in cross-section, edge  34  and side portion  36  are linked by a locking tooth  34   a . Locking tooth  34   a  features a distally-facing canted edge  36   a  and a proximally-facing canted edge  36   b . Each of canted edges  36   a ,  36   b  of the locking tooth terminate at a shelf  37  that intersects with side portion  36 . As before, if desired, grooves (not shown) can be provided on edge  34 , for the purposes previously explained. As before, collar  32   a  includes a thickness “e” and a maximum diameter “g”. Locking tooth  34   a  includes a maximum width “q” which need not be equal to thickness “e” displayed by collar  32   a.    
         [0034]    Safety shield  42  is characterized by a proximal end  44 , a distal end  45 , and a circumferential sidewall  46  therebetween. Circumferential sidewall  46  includes an outside surface  48  and an inside surface  49 . Outside surface  48  is characterized by a diameter “N”. Shield  42  is slidable both over tube  200  and collar  32 , and thus over syringe barrel  16 , between a retracted position (FIGS. 1, 4 and  5 ), wherein distal tip  28  of piercing element  26  is exposed, and an extended position (FIG. 6), wherein the distal tip of the piercing element is protectively covered.  
         [0035]    Lockable safety shield assembly  30  in accordance with the present invention includes user-activatable locking structure. A feature of the lockable safety shield assembly in accordance with the present invention is that the locking structure can be implemented such that it is not “activated” during assembly of the components, making it easier to assemble the components to syringe  10 . Subsequent to assembly of the components, the locking structure can be “activated”, putting the safety shield assembly in a ready-to-use state.  
         [0036]    Making reference to FIGS. 1-3 and  7 - 9 , shield  42  features at least one deflectable arm  50 . Arm  50  is preferably provided as part of the structure of the shield. Arm  50 , which may be molded as part of the shield or thereafter created from the shield via various cutting processes, includes a distal end  54  and a proximal end  52 . Distal end  54  is resiliently affixed to sidewall  46  such as by a living hinge arrangement, while proximal end  52  is not attached to the sidewall and is configured for free movement towards an interior portion of shield  42 . In a preferred embodiment, at least two deflectable arms  50  are provided on shield  42 , with these two arms  50  located on diametrically-opposite sides of shield  42 . Arms  50  are thus configured for a free position (FIGS. 1 and 4), wherein the surface of the arms are substantially co-planar with circumferential sidewall  46  of the shield, and a locking position (FIGS. 5, 7, and  8 ), wherein proximal ends  52  of arms  50  are urged towards the interior of the shield.  
         [0037]    As best reflected by FIGS. 7 and 8, in a preferred configuration, proximal ends  52  of the deflectable arms are not straight. Rather, to best make use of the features of collar  32   a  (FIG. 10), proximal ends  52  are preferably canted in a manner such that when the deflectable arms are positioned towards the interior of the shield, the proximal ends will be oriented to lockingly cooperate with distally-facing canted edge  36   a  of collar  32   a , as will be explained hereinbelow.  
         [0038]    At least one stop member  70  is provided on inside surface  49  of the shield in an area adjacent proximal end  44 . In a preferred embodiment, two stop members  70  are provided, each corresponding to a respective one of two deflectable arms  50  (see FIG. 9). Stop member  70  includes a distally-facing stop surface  72 . Stop surface  72  is preferably spaced from proximal end  52  of the deflectable arm a distance “b” as measured along central axis “z” of the shield (see FIG. 3).  
         [0039]    As best illustrated in FIGS. 7 and 8, in a preferred configuration, stop surface  72  is preferably not straight. Rather, to best make use of the features of collar  32   a  (FIG. 10), stop surface  72  is preferably canted in a manner such that it will lockingly cooperate with proximally-facing canted edge  36   b  of collar  32   a , as will be explained hereinbelow.  
         [0040]    Stop member  70  also includes a proximally-facing sloped surface  74 . Proximally-facing sloped surface  74  facilitates sliding of shield  42  over collar  32  when the shield is assembled to tube  200 . Stop member  70  can be located such that stop surfaces  72  are circumferentially aligned with respective proximal ends  52  of deflectable arms  50 , when viewed along central axis “Z” of the shield. Alternately, if desired, stop members  70  can be placed on interior surface  49  of the shield such that they are circumferentially offset from respective proximal ends  52  of the deflectable arms, when viewed along central axis “Z” of the shield.  
         [0041]    Shield  42  includes structure permitting activation of the deflectable arms  50  into their locked position (FIGS. 5, 6,  7 ,  8 ). A respective pair of distal and proximal abutments  58 ,  56  are provided on deflectable arms  50 . Distal abutment  58 , which includes a distally-facing sloped surface  60 , is spaced from proximal abutment  56  by a distance “a” that defines a gap  59 . As seen in FIGS. 3 and 4, in a preferred embodiment where two diametrically opposite deflectable arms are included, the diametrically opposite pair of gaps  59  define a diameter “M” slightly greater than diameter “N” defined by outside surface  48  of she shield when the arms  50  are in their free position. The effect is that gap  59  is configured so that it is elevated from the level of surrounding outside surface  48  of cylindrical sidewall  46  prior to activation of deflectable arm  50 .  
         [0042]    A circumferential ring  80  having an inside wall  82  is disposed for coaxial movement with circumferential sidewall  46  of the shield. Inside wall  82  has a diameter “P” at least equal to, if not greater than, diameter “N” defined by the outside surface of shield  42 . Ring  80  has a width “d” at least equal to, if not slightly less than, the width of gap  59  as defined by distance “a”. To simplify later assembly, shield  42  is typically configured with ring  80 . Ring  80  can be retained to shield  42 , for instance, by a user-severable connection. That is to say, shield  42  and ring  80  are preferably supplied to the pharmaceutical manufacturer as a single unit. For example, ring  80  can be supplied by the component manufacturer such that it is secured to the shield by a user-severable connection such as a threaded connection. Preferably, and as is disclosed herein, ring  82  is secured to distal end  45  of the shield by one or more user-severable connections  84  configured as thinned, frangible sections of material between ring  80  and proximal end  45  of the shield, thereby permitting easy removal of the ring from the shield for further assembly operations. This type of structure also permits the component manufacturer to easily form ring  80  and shield  42  as a single molding.  
         [0043]    Shield  42  further includes structure for retaining shield  42  in the retracted position without being inadvertently “over-retracted” with respect to syringe  10  and, hence, inadvertently removed from syringe  10  in the proximal direction. As seen in FIG. 3, one or more distal stop members  76  can be provided on interior surface  49  of the shield adjacent distal end  45 . Distal stop members  76  are configured to engage a distally-facing side portion  36  of the collar to prevent shield  42  from being over-retracted in a proximal direction. If desired, one or more tertiary stop members  78  can be provided on shield  42 , each spaced from a respective distal stop member  76  by a distance “c” approximately equal to thickness “e” of collar  32 . Hence, collar  32  can be releasably retained between distal stop members  76  and tertiary stop members  78  when the shield is in its retracted position. If desired, tertiary stop member  78  can be provided with a proximally-facing sloped surface  79  which, like proximally-facing sloped surface  74  associated with stop members  70 , assists in the assembly of shield  42  about collar  32 .  
         [0044]    Operation of a device will now be explained.  
         [0045]    As seen in FIG. 3, means can be fitted to the syringe adjacent the distal end to provide a barrier to the fluid path leading to the medicament which will be contained within barrel  16 . The barrier is normally fitted by the syringe manufacturer and here, can be fitted to the syringe barrel in a process subsequent to placement of collar  32 . If the fluid path is structured as a luer tip, for instance, a luer tip cap can be fitted. Here, where a piercing element  26  is configured on the syringe, a well-known needle shield  29  can be fitted about piercing element  26  so that a proximal-end of needle shield  29  rests against collar  32 . Collar  32  should be configured such that maximum diameter “g” is at least slightly greater than a maximum diameter “f” displayed by needle shield  29 , such that needle shield  29  will not interfere between operation of collar  32  and shield  42 .  
         [0046]    Preferably, safety shield assembly  30  is preassembled by the component manufacturer and presented to the pharmaceutical company in a sterile state, ready to be fit to the syringe once filled with the drug. Syringe  10  is supplied to the pharmaceutical manufacturer in a sterile state, ready to be filled with a desired drug and otherwise processed in a well-known manner. For instance, various filling operations, particulate inspection operations, labelling operations, plunger rod assembly operations may be conducted on the syringe.  
         [0047]    As a first step in assembling the safety shield assembly, shield  42  is fitted to tube  200 . Proximally-facing sloped surfaces  74  and, if provided,  79 , assist shield  42  to smoothly surpass edge  34  of the collar. Shield  42  is slid over tube  200  in a proximal direction, such that collar  32  rests against distal stop members  76 . If tertiary stop members  78  are provided, collar  32  will be releasably retained between distal stop members  76  and tertiary members  78 . After the various filling and processing operations have been conducted on the syringe, tube  200  is slid over syringe barrel  16  such that flange  18  of the syringe is captured by lip structure  210  of the tube. As the shield is held to the tube in a retracted position, with needle shield  29  affixed to piercing element  26  will project beyond the distal end  202  of the tube. Syringe  10 , together with lockable safety shield assembly  30 , can be shipped by the pharmaceutical manufacturer to an end-user in a ready-to-use state.  
         [0048]    [0048]FIG. 4 is representative of safety shield  42  of the present invention mounted about syringe  10  prior to activation of locking structure, while FIG. 5 is representative of the same construction after activation of the locking structure. Preferably, a pharmaceutical manufacturer will ship the prefillable syringe to the end-user in the state shown in FIG. 5, such that the locking structure is pre-activated. One way to do this is that the locking structure may be activated by the component manufacturer, as a final step in the preassembly of the safety shield assembly  30 . For instance, the step of sliding ring  80  towards gap  59  can be effected as a final step in an assembly motion during which shield  42  is fitted to tube  200 . In this vein, it will also be realized that because the component manufacturer preferably supplies the ring and shield to pharmaceutical manufacturer as a preassembled, single unit, the later assembly operations by the pharmaceutical manufacturer are simplified, in that the pharmaceutical manufacturer need only be concerned with sorting, orienting, distributing and then assembling a single unit about the syringe. Alternately, of course, it will be realized that the activation step can be done by by the pharmaceutical manufacturer itself, as part of the operation in fitting the safety shield assembly to the syringe.  
         [0049]    Nonetheless, FIG. 5 illustrates activation of the locking structure, whether by the component manufacturer, by the pharmaceutical manufacturer or by an end-user. The user-severable connections  84  connecting ring  80  to distal end  45  of the shield have been severed, and ring  80  has been slid proximally with respect to shield  42 . Aided by distally-facing sloped surface  60  associated with distal abutment  58 , inside surface  82  of the ring glides along outside surface  48  of the shield, over distally-facing sloped surface  60 , and is secured in gap  59  defined between distal abutment  58  and proximal abutment  56 . Because inside diameter “P” of ring  80  is smaller than diameter “M” between diametrically-opposed gaps  59 , ring  80  exerts an inwardly-directed force upon deflectable arms  50 . Deflectable arms  50  are thus deflected about their distal ends  54 , such that proximal ends  52  are pushed towards the interior of shield  42 . As a deflectable arm  50  is urged inwardly (see particularly FIGS. 7 and 8), proximal end  52  of a given deflectable arm and stop surface  72  of a respective stop element  70  define a gap  100  between them. Gap  100  is designed to “capture” collar  32  when the shield is slid distally to cover piercing element  26 . Where the collar has a flat side portion  36 , then gap  100  should have a width “b” at least equal, if not slightly greater than, thickness “e” of the collar to securely retain collar  32  in gap  100 . If, as depicted in FIG. 10, the collar is configured with locking tooth  34   a , then width “b” should be chosen so as to accommodate the maximum width “q” associated with locking tooth  34   a , so that locking tooth  34   a  is securely retained in gap  100 .  
         [0050]    [0050]FIG. 6 is representative of shield  42  in its extended position to protectively cover distal tip  28  of the piercing element. After needle shield  29  has been removed and an injection administered in the usual manner, shield  42  is grasped by an end-user and slid distally with respect to syringe  10 . Collar  32 , which is fixed to tube  200 , will thus be urged towards proximal end  44  of the shield. Owing to the resiliency of deflectable arms  50 , edge  34  of the collar will push arms  50  in an outward direction from the interior of the shield and thus will pass along the length of arms  50 . Edge  34  will eventually deflect past proximal end  52 , causing the arm to re-deflect in an inward direction toward the interior of the shield. An audible indication, such as a clicking sound, will be heard to indicate activation of the safety mechanism. Thus, collar  32  will come to rest in gap  100 , with the collar sandwiched between proximal end  52  and stop surface  72 . Collar  32  is thus lockingly held between stop surface  72  and proximal end  52  of the arm, arresting further movement of the shield in either proximal or distal directions respective of syringe  10 . Accordingly, shield  42  is retained in its extended position with respect to piercing element  26 , protectively covering distal tip  28  against inadvertent touch contact.  
         [0051]    As previously explained, in a preferred configuration (FIGS. 7, 8 and  10 ), deflectable arms  50 , stop members  70  and collar  32   a  are configured to provide good audible as well as tactile indication that shield  42  has been locked to collar  32   a . As edge  34  of collar  32   a  clears proximal end  52  of the deflectable arm, locking tooth  34   a  enters gap  100 . As soon as the locking tooth enters gap  100 , the resilient characteristics of the deflectable arm cause the arm to re-deflect in an inward direction toward the interior of the shield. At this moment, an interior portion  49   a  of deflectable arm  50  that is located adjacent proximal end  52  is thrust against shelf  37  of collar  32   a . As soon as interior portion  49   a  is thrust against shelf  37 , tactile as well as audible sensations are transmitted to the user, signaling that shield  42  has been locked to collar  32   a . Owing to the canted orientations of both proximal end  52  of the deflectable arms and stop surface  72  of the stop members  70 , each of the proximal ends  52  and stop surfaces  72  are lockingly retained against their respective distally facing canted edge  36   a  and proximally-facing canted edge  36   b , such that tooth  34   a  is securely locked within gap  100 . Thus, by a simple distal movement, shield  42  is self-locking with collar  34   a.    
         [0052]    The various components can be formed from suitable materials, such as medical-grade plastics or the like. They may be injection molded in manners well-known to the skilled artisan, resulting in reliable and cost-effective production of the locking shield assembly. The locking shield assembly of the present invention can be adapted with prefillable syringes of various dimensions and capacities. Various enhancements can be provided to permit easy assembly. For instance, sections of the sidewall adjacent the proximal end of the shield can be thinned, or slits can be incorporated in an area of the shield between the proximal end of the shield and the proximal edge of the deflectable arms, all to enhance the easier sliding of the shield proximally over the collar. Of course, the dimensions for the various components can be selected such that when the shield is in its retracted position, the distal tip of the piercing element is exposable for an injection, and when the shield in its extended position, the distal tip of the piercing element will be protectively covered to enable the end user to safely discard the syringe.  
         [0053]    It will be appreciated and understood by those skilled in the art that further and additional revisions to the invention may be devised without departing from the spirit and scope of the appended claims, the invention not being limited to the specific embodiments shown.