A replaceable needle assembly is provided for a retractable, prefilled syringe comprising a barrel having a mounting member and a plunger, whereby the retractable needle can be replaced by a user without affecting the retraction mechanism. The replaceable needle assembly comprises a needle fitted to a retractable needle mount and a needle retainer. The needle mount is removable, screw-threadedly mountable to the mounting member of the barrel. The needle retainer comprises a plurality of fingers that releasably engage the retractable needle to thereby prevent inadvertent retraction of the retractable needle before engagement by the plunger. The plunger comprises a plunger seal comprising an outer member and an inner member that engages the needle for spring-driven retraction. At the end of retraction, a lock is formed between the plunger and barrel to prevent re-use of the syringe.一種可縮回預先注滿的注射器之可替換的針頭組件一具有安裝元件的針筒和一活塞，藉此使用者可以替換可縮回的針頭不影響縮回機構，可替換的針頭組件包括一針頭套於一可縮回的針頭底座和一針頭固定件，針頭底座可移動地螺接安裝於針筒的安裝元件，針頭固定件包括複數個指部釋放地卡掣可縮回的針頭藉以防止可縮回的針頭在活塞卡掣前不慎的縮回，活塞封套包括一內部元件和一外部元件卡掣針頭為了彈力驅動縮回，在縮回的端部，活塞和針筒之間該活塞封套包括一可釋放連結的內部元件和外部元件。
