Syringe having a retractable needle

A medical device for delivering a medicament to a patient, comprising:

• a barrel having a forward end and a rear end and defining a reservoir within which the medicament may be contained;
• a needle cannula having a forward tip and a rear end;
• a needle cannula hub connected to said rear end of said needle cannula and providing fluid communication of said needle cannula with said reservoir, said needle cannula hub being selectively movable between an initial position in which said forward tip of said needle cannula is exposed, and a retracted position in which said forward tip of said needle cannula is disposed in said device;
• a barrel hub member coupled to said forward end of said barrel over said needle cannula hub and dimensioned to provide retractable telescoping movement of said needle cannula hub with respect to said barrel hub member to allow said needle cannula hub to move to said retracted position;
• means for releasably securing said needle cannula hub at said barrel forward end when said needle cannula hub is in said first position said releasably securing means comprising a retaining member having a forward face and a rear end defining a bore therethrough and positioned between said forward end of said barrel and said barrel hub member;
• urging means disposed between said barrel hub member and said needle cannula hub for urging said needle cannula hub to said retracted position when said releasably securing means is activated;
• and
• a plunger rod having a hollow first end supporting a stopper and positioned in said reservoir, and a second end having a thumb pad for receiving medicament delivery pressure for causing said plunger rod to move within said reservoir to cause the medicament to be expelled from said reservoir, said plunger rod comprising cutting means for cutting away a portion of said stopper when said stopper is fully inserted into said barrel, said cutting means further comprising means for activating said releasably securing means after said stopper portion is cut away, said activating means comprising means for engaging said bore and elastically radially expanding said bore to a size sufficient to release said needle cannula hub from said releasably securing means, for allowing said urging means to move said needle cannula hub to said retracted position through said hollow first end of said plunger rod.