WO2008110322A1

WO2008110322A1 - Safety mechanism for syringes, mounting method and safety syringe provided with the safety mechanism

Info

Publication number: WO2008110322A1
Application number: PCT/EP2008/001882
Authority: WO
Inventors: Sergio Restelli
Original assignee: Sergio Restelli
Priority date: 2007-03-13
Filing date: 2008-03-10
Publication date: 2008-09-18
Also published as: ITMI20070500A1

Abstract

A safety mechanism (200) which can be applied to a syringe (100) to make it safe is described, comprising a needle-holder contrasting element (8) suitable to be made integral to the head of the syringe body (1), and a sleeve (5) which can be mounted by sliding on the syringe body (1) to pass from a retracted utilization position to an extended safety position where the needle (2) of the syringe is covered. The sleeve comprises at least one elastic tab (57) inclined towards the inside to block the contrasting element (8) when the sleeve is in the extended safety position. The contrasting element (8) comprises at least one cam surface (86) interacting with the elastic tab (57) to bend it towards the outside, following a relative rotation of the contrasting element (8) with respect to the sleeve (5) so as to allow the insertion of the contrasting element (8) into the sleeve (5).

Description

SAFETY MECHANISM FOR SYRINGES, MOUNTING METHOD AND SAFETY SYRINGE PROVIDED WITH THE SAFETY MECHANISM

DESCRIPTION

The present invention relates to the field of the safety syringes provided with a special safety mechanism so as to avoid needle stick accidents and to prevent the reuse of the syringe after the injection.

Various types of safety syringes are known, provided with a protection sleeve which slides on the syringe body so as to pass from a retracted position, where the needle is exposed to permit the injection, to an extended position where the needle is completely covered, so preventing the re-use of the syringe and acting as a protection against needle stick accidents.

Such a safety mechanism for syringes is described by WO2004/045685 in the name of the same applicant. This safety mechanism comprises a sleeve provided with a pair of tabs inclined towards the inside of the sleeve and intended to abut against a contrasting element integral to the head of the syringe body, when the sleeve is in its extended position covering the needle. In this way the sleeve cannot return to its retracted position.

The mounting of the syringe body with the contrasting element inside the protection sleeve is somewhat complex due to the presence of said tabs inclined towards the inside. In fact, the contrasting element abuts against the tabs with the risk of breaking them. For this reason, special tools must be provided during the assembly, which can bend the tabs of the sleeve towards the outside, so that said tabs do not abut against the contrasting element of the syringe body.

Moreover, during the assembly, the syringe body must be completely inserted inside the sleeve, without using the plunger, which would otherwise release the safety mechanism. For this purpose, a special tool must be provided, such as a punch, which can push the syringe body into the sleeve.

Therefore, the automatic assembly of said safety mechanism is difficult to implement. Furthermore, said mechanism cannot be mounted on a standard syringe, which must be modified, in particular by removing the rear gripping tabs.

Object of the present invention is to eliminate the drawbacks of the prior art, by providing a safety mechanism, which can be mounted automatically on a standard syringe, in order to make it a single-use, safety syringe.

A further object of the present invention is to provide said safety mechanism which is also economical, easy to manufacture and easy to mount.

Said objects are reached according to the invention with the safety mechanism, the mounting method and relative safety syringe whose characteristics are defined in the attached independent claims 1 , 13 and 14, respectively.

Advantageous embodiments of the invention are defined in the dependent claims.

The safety mechanism according to the invention can be applied to a standard syringe in order to make it safe. The safety mechanism comprises:

- a contrasting element suitable to be made integral to the head of the syringe body, and - a sleeve to be mounted by sliding on said syringe body in order to pass from a retracted utilization position where the needle of the syringe is uncovered to an extended safety position where the needle is covered.

The sleeve comprises at least one elastic tab inclined towards the inside, suitable to abut with said contrasting element,when the sleeve is in the extended safety position, preventing a successive withdrawal of the sleeve.

The contrasting element further comprises at least one cam surface interacting with said elastic tab to bend it towards the outside, following a relative rotation of the contrasting element with respect to the sleeve, so as to allow the insertion of the contrasting element into the sleeve.

The advantages of the safety mechanism according to the invention, which permit an easy and handy mounting of said mechanism on the syringe, appear evident.

Said mounting can be performed either manually or automatically and without the help of special tools. In fact, it is sufficient to apply the contrasting element to the head of the syringe body and then insert and rotate the syringe body with the contrasting element inside the sleeve to bend the tabs of the sleeve towards the outside and to permit the further insertion of the syringe body inside the sleeve. Alternatively, the contrasting element can be inserted into the sleeve until it is locked by the sleeve tabs. Then the syringe body is pushed inside until it is integral to the contrasting element.

Further features and advantages of the present invention will become more apparent from the following detailed description of an exemplary but non-limiting embodiment thereof, as illustrated in the accompanying drawings, in which:

Fig. 1 is an exploded axonometric view of the safety mechanism according to the invention and of a traditional syringe;

Fig. 2 is an enlarged perspective view of a contrasting element forming part of the safety mechanism of Fig. 1 ;

Fig. 3 is an axial, cross-sectional view according to the plane IH-IH of Fig. 2;

Fig. 4 is an enlarged, partially broken off perspective view of a protection sleeve forming part of the safety mechanism of Fig. 1 ;

Fig. 5 is an axial, cross-sectional view according to the plane V-V of Fig. 4; Fig. 6 is perspective, partially broken off view of a first mounting step of the safety mechanism according to the invention, where the spring has been omitted for clarity sake;

Fig. 7 is a perspective view of a syringe ready for use, where the safety mechanism according to the invention has been mounted and the spring has been omitted; and Fig. 8 is a perspective view of the safety syringe of Fig. 7, where the needle is safe after the use.

The safety mechanism according to the invention, which can be applied to a syringe to make it singe-use and safe, is hereunder described with the help of the Figures.

Fig. 1 illustrates a traditional syringe, indicated as a whole with the reference number 100, and a safety mechanism composed of a group of components indicated as a whole with the reference number 200.

The syringe 100 is a standard syringe generally available on the market and comprises a syringe body 1 , an injection needle 2, a plunger 4 and a stem 41. The syringe body 1 is cylindrical, internally hollow, and defines a cylindrical chamber 10. The rear end of the body 1 is open towards the outside and has a rectangular flange 11 composed of two tabs projecting radially towards the outside, in opposite directions, defining finger-grip means for the user.

The front end of the body 1 ends with a head, open towards the outside, with a substantially cylindrical or truncated-cone shaped tang 12, having a much smaller diameter than the body 1 , so as to define a shoulder 13 on the front part of the syringe body 1.

The injection needle (standard) is mounted or incorporated into a hollow needle- holder element 20, having a cylindrical or truncated-cone shape and with an axial chamber 21 suitable to house the tang 12 of the head of the syringe body. The needle- holder element 20, of the standard type, has a collar 22 projecting towards the outside.

The plunger 4 can slide hermetically into the chamber 10 of the syringe body 1. The plunger 4 is mounted on the head 40 of the stem 41. The stem 41 has a cross-shaped transversal section and ends at the rear with a discoid flange 42 providing an abutment surface for the user's fingers during the injection.

Near the rear flange 42, around the stem 41 , a driving crown or disc 43 can be provided. A safety tab (not illustrated) can be provided between the rear flange 42 and the driving crown 43, which can be removed by the operator to allow the injection. The driving crown 43 has empty spaces 43' in circular sectors with a center angle of 90°, arranged diametrically opposite each other.

The driving crown 43 can be produced as an integral part of the stem 41 or of the rear flange 42 of the stem 41. Should the stem 41 of the traditional needle 100 not be provided with the driving crown 43, said driving crown 43 can be made as a separate element forming part of the group of components of the safety mechanism 200 and, therefore, can be mounted on the stem 41.

The group of components of the safety mechanism 200 comprises a contrasting element 8, a spring 7 (optional) and a protection sleeve 5. As illustrated also in Figs. 2 and 3, the contrasting element 8 has a cylindrical or truncated-cone shaped body 80, internally hollow, with an axial cavity 81 having a diameter substantially equal to the external diameter of the front part of the syringe body 1 so that it can be applied to it by pressing. The contrasting element 8, frontally and axially, has a cylindrical or truncated-cone shaped tang 82, with a smaller diameter than the body 80, so as to generate a shoulder 84. An annular seat is present between the tang 82 and the front part of the cylindrical body 80. The tang 82 is hollow and has an internal screw thread 83. The thread 83 of the contrasting element is suitable to engage, by means of screwing, with the collar 22 of the needle-holder element 20 of the standard needle.

In this way, when the contrasting element 8 is applied to the head of the syringe body 1, the tang 12 of the syringe body is placed axially inside the tang 82 of the contrasting element, leaving an annular cavity between the external surface of the tang 12 of the syringe body and the internal surface of the tang 82 of the contrasting element, so as to generate a so-called Luer cone on the head of the syringe body so that the needle-holder 20 of the injection needle 2 can be screwed on. Therefore, the needle-holder 20 of the standard needle remains confined inside the contrasting element 8, which also acts as a supporting element for the needle 2.

The needle-holder 20 can be pre-mounted in the contrasting element 8 and, afterwards, the contrasting element 8 is applied to the head of the syringe body. Alternatively, the needle-holder 20 can be pre-mounted on the head 12 of the syringe body and, afterwards, the contrasting element 8 is applied to the syringe body.

On the upper side of the body 80 of the contrasting element there are two slots 85, diametrically opposite each other. On one side of each slot there is a tapered surface 86, which joins the external surface of the cylindrical body 80 of the contrasting element.

On the rear side of the body 80 of the contrasting element there are two slots 87 arranged diametrically opposite each other and spaced at an angle of 90° compared to the slots 85.

As better illustrated in Figs. 4 and 5, the group of components of the safety mechanism 200 comprises a needle-protection element indicated with the reference number 5 and having the shape of a sleeve. The sleeve 5 has a substantially cylindrical hollow body 50, with an axial chamber 51 open both at the front and at the rear. The front part 52 of the sleeve 5 has a smaller diameter ending in an annular collar 59 (Figs. 7 and 8) which projects towards the inside. The internal diameter of the body 50 of the sleeve is slightly greater than the external diameter of the body 80 of the contrasting element 8, so that the sleeve 5 can slide axially on the body 80 of the contrasting element 8, when the contrasting element 8 is applied to the head of the syringe body 1.

A flange 53 is provided near the rear part of the body 50 of the sleeve, forming two rigid flaps projecting radially towards the outside, in diametrically opposite directions, so as to generate a finger-grip for the user. The rear flange 53 of the sleeve 5 is substantially equal to the rear flange 11 of the syringe body 1 of a standard syringe.

Two parallel walls 54 are mounted on the rear flange 53 of the sleeve which project backwards in a longitudinal direction. The walls 54 are spaced at a distance between them sufficient to form a space in which the rear flange 1 1 of the syringe body can be housed.

On each wall 54 there is a flexible longitudinal tab 55 inclined towards the inside and ending with a front abutting surface 55'. In other words, the tab 55 is inclined towards the sleeve axis and towards the sleeve chamber 51. Each flexible tab 55 is made by means of a "U"-shaped cut 56 in the respective wall 54. In this way, each flexible tab 55 can be bent towards the outside and return elastically to its original position, inclined towards the inside.

On the rear part of the sleeve body 50, there is a second pair of flexible longitudinal tabs 57 arranged diametrically opposite each other and aligned to the first pair of tabs

55. Each tab 57 is inclined towards the inside and ends with a rear abutting surface

57' opposite the abutting surface 55' of the first tab. The second tabs 57 are also made by means of a "U"-shaped cut 58 in the sleeve body 50. The tabs 57 can be bent towards the outside and return elastically to their original position, inclined towards the inside. Finally, the group of components of the safety mechanism 200 optionally comprises a helical spring 7 to make the safety mechanism 200 automatic. The spring 7 is intended to be housed in the annular seat 88 of the contrasting element 8, around the tang 82, and inside the front part 52 of the sleeve body. In fact, the external diameter of the spring 7 is slightly less than the internal diameter of the front part 51 of the sleeve 5 and the internal diameter of the spring 7 is slightly greater than the external diameter of the tang 82 of the contrasting element.

The mounting of the group of components of the safety mechanism 200 on the syringe 100 is described hereunder, with reference to Fig. 1.

The contrasting element 8 is applied by pressure to the head of the syringe body 1 , in such a way that the tang 12 of the syringe is placed axially inside the tang 82 of the contrasting element and the shoulder 13 of the syringe abuts against the shoulder 84 of the contrasting element. This forced coupling makes the contrasting element 8 integral part of the syringe body 1.

The plunger 4 is mounted on the head of the stem 41 and inserted into the chamber 10 of the syringe body.

The spring 7 is inserted into the sleeve 5, from the rear end of the sleeve 5, so that one end of the spring abuts against the collar 59 of the front part 52 of the sleeve.

At this point, the group composed of the syringe body 1 together with the contrasting element 8 integral thereto, is inserted into the sleeve 5 from the rear end of the sleeve so that the other end of the spring 7 abuts against the shoulder 84 of the contrasting element and is housed in the annular seat 88 of the contrasting element.

Then, slightly forcing the contrasting element 8 inside the sleeve, the front edge of the body 80 of the contrasting element pushes the first pair of tabs 55, present in the sleeve walls 54, towards the outside. Then the body 80 of the contrasting element goes beyond the first pair of tabs 55, which return elastically to their original position. In this situation, the syringe body can no longer be removed from the sleeve since the rear edge of the contrasting element would abut against the abutting surface 55' of the first pair of tabs 55. However, once the first pair of tabs 55 has been passed, the axial insertion of the syringe body 1 is blocked when the front edge of the body 80 of the contrasting element abuts against the abutting surface 57' of the second pair of tabs 57.

At this point, as shown in Fig. 6, the body 1 is rotated in the direction of the arrow Fl with respect to the sleeve 5 or the sleeve 5 can be rotated in the direction of the arrow F2 that is opposite to the direction of the arrow Fl . As a result, the abutting surface 57' of each tab 57 abuts within the respective slot 85 of the contrasting element.

Continuing to rotate the syringe body 1 with respect to the sleeve 5, the tabs 57 of the sleeve slide on the respective tapered surfaces 86 of the contrasting element. As a result, the tapered surfaces 86 act as cams for the tabs 57 and, therefore, the tabs 57 of the sleeve are pushed towards the outside by the tapered surfaces 86, until the abutting surface 57' of the tabs no longer abuts with the edge of the body 80 of the contrasting element.

At this point, the syringe body 1 can be pushed axially inside the sleeve 5, until the rear flange 1 1 of the syringe body abuts against the rear flange 53 of the sleeve, between the rear walls 54 of the sleeve. It should be noted that the lateral edge of the rear flange 1 1 of the syringe body pushes the first pair of tabs 55 of the sleeve towards the outside. Then, when the rear flange 1 1 goes over the tabs 55, the tabs 55 return elastically to their original position (inclined towards the inside) and the abutting surface 55' of the tabs 55 abuts against the rear flange 1 1 of the syringe body, holding the syringe body 1 inside the sleeve 5.

In this situation, the spring 7 is completely compressed in the head of the sleeve 5 and any axial movement whatsoever of the syringe body 1 with respect to the sleeve 5 is therefore prevented. In fact, the rear flange 1 1 of the syringe body is confined between the rear flange 53 of the sleeve and the abutting surface 55' of the tabs 55.

At this point, as shown in Fig. 7, the head of the syringe body is located near the head 52 of the sleeve and the needle-holder can be mounted on the tang 12 of the head of the syringe body. Then the safety mechanism 200 is mounted on the syringe 100 that is ready for use. It is evident from the detailed description that the various elements of the safety mechanism 200 can easily be mounted either manually or automatically on the syringe 100 without the help of special tools to bend the tabs 57 towards the outside or to insert the syringe body 1 inside the sleeve 5.

The operation of the syringe 100 provided with the safety mechanism 200 according to the invention is described hereunder.

Initially, the plunger 4 is pushed to its end of stroke with the tabs 55 of the sleeve passing through the spaces 43' of the driving crown of the stem, so as not to trigger the safety mechanism of the sleeve 5. The spaces 43' of the driving crown of the stem also permit the discharge of the air from the inside of the syringe body, before drawing in the preparation to be injected.

Then the needle 2 is placed inside the vial containing the preparation to be injected and the plunger 4 is drawn back in order to allow the preparation to enter the syringe body 1.

Evidently, the syringe 1 can also be pre-filled.

At this point, the syringe is ready for the injection. When, during the injection, the stem 4 arrives at its end of stroke inside the chamber 10 of the syringe body, the driving crown 43 of the stem 41 comes into contact with the first pair of flexible rear tabs 55 of the sleeve walls 54, causing the bending of said tabs 55 towards the outside.

As a result, the rear flange 1 1 of the syringe body disengages from the abutting surface 55' of the rear tabs 55. Consequently, the axial movement of the sleeve 5 with respect to the syringe body 1 is no longer prevented. Therefore, due to the action of the spring 7 that is released, the sleeve 5 can move axially forward with respect to the syringe body 1 and/or the syringe body can move axially backward with respect to the sleeve 5.

It should be noted that the sleeve 5 moves automatically thanks to the action of the spring without the operator having to intervene manually in any way. Obviously, if the spring 7 is not provided, the syringe body can be moved manually by the operator with respect to the sleeve. In fact, the user can easily hold the rear flange 1 1 of the syringe body with one hand and the sleeve body with the other hand.

In case the spring 7 is provided, the telescopic movement of the sleeve 5 and of the syringe 100 is controlled and adjusted by the user's hand. In fact, the user keeps the index and middle finger on the flange 53 of the sleeve and the thumb on the rear flange 42 of the stem of the plunger, accompanying the movement of the sleeve 5 and of the syringe 100 towards the safety position and adjusting the speed of the movement of the sleeve 5 with respect to the syringe 1.

As illustrated in Fig. 8, at the end of the sleeve stroke, the body 80 of the contrasting element 8 is held between the two pairs of tabs 55 and 57. In this situation, the needle 2 is completely protected by the sleeve 50, so avoiding needle stick accidents and the re-use of the syringe.

In particular, the abutting surface 55' of the first pair of tabs engages inside the respective spaces 87 of the rear edge of the body of the contrasting element and the abutting surface 57' of the second pair of tabs abuts against the front edge of the body 80 of the contrasting element. In this way, any axial movement or rotation of the sleeve 5 with respect to the syringe body 1 is prevented.

The fact that the abutting surface 55' of the first pair of tabs 55 is engaged in the spaces 87 of the contrasting element, prevents any rotation whatsoever of the contrasting element 8 with respect to the sleeve 5. In fact, a rotation of the contrasting element 8 with respect to the sleeve 5 could cause a sliding of the second pair of tabs 57 on the cam surface 86 of the contrasting element and, therefore, disengage the tabs 57 from the contrasting element and permit new insertion of the syringe body 1 into the sleeve 5.

It should be noted that when the injection is finished, the contrasting element 8 will slide forward, pushed by the spring or through a manual action, until it is confined between the two pairs of tabs 55 and 57. Since the needle-holder 20 is screwed into the thread 83 of the contrasting element, it is impossible for it to move out of the protection sleeve 5 and, therefore, to cause needle stick accidents.

By pulling the sleeve 50 forward, the contrasting element 8 can be removed from the syringe body 1 , but the needle 2 remains trapped, together with the contrasting element 8, inside the sleeve 5.

It should be noted that the spring 7, if present, always remains protected inside the sleeve 5, even when the sleeve 5 is in its extended safety position.

It should also be noted that, in the present invention, a standard type syringe 100, composed of a body 1 , of a plunger 4 with a stem 41 and of a needle 2 with a needle- holder 20, is made safe with the addition of only two elements forming part of the safety mechanism 200, i.e. the sleeve 5 and the contrasting element 8. Moreover, the group of components 200 may optionally comprise also a spring 7 so as to make the safety mechanism 200 automatic.

Naturally, a person skilled in the art may apply to the embodiment of the present invention, many modifications and variations, all of which, however, are included within the scope of protection of the invention as defined by the following claims.

Claims

1. A safety mechanism (200) to be mounted on a syringe (100) to make it safe, comprising: - a contrasting element (8) suitable to be made integral to the head of the syringe body (1), and

- a sleeve (5) to be mounted by sliding on said syringe body (1) so as to pass from a retracted utilization position where the needle (2) of the syringe is uncovered to an extended safety position where the needle (2) is covered, said sleeve comprising at least one elastic tab (57) inclined towards the inside, suitable to abut with said contrasting element (8) when the sleeve is in the extended safety position, so preventing a successive backward movement thereof, characterized in that said contrasting element (8) further comprises at least one cam surface (86) interacting with said elastic tab (57) to bend it towards the outside, following a relative rotation of the contrasting element (8) with respect to the sleeve (5), so as to allow the insertion of the contrasting element (8) into the sleeve (5).

2. A safety mechanism (200) according to claim 1 , characterized in that said contrasting element (8) comprises engaging means (83) suitable to engage with a needle-holder (20) so as to block the needle-holder (20) in the contrasting element (8).

3. A safety mechanism (200) according to claim 1 or 2, characterized in that said contrasting element (8) comprises a cylindrical body (80) having at least one slot (85) in its front side, said cam surface (86), having the shape of a tapered surface which joints the external lateral surface of the body (80) of the contrasting element, being provided laterally to said slot (85).

4. A safety mechanism (200) according to any one of the previous claims, characterized in that said contrasting element (8) comprises anti-rotation means (87) which can engage with said sleeve to prevent the rotation of the sleeve (5) with respect to the contrasting element (8) when the sleeve is in the extended safety position.

5. A safety mechanism (200) according to claim 4, characterized in that said anti- rotation means of the contrasting element (8) comprise at least one space (87) made in the rear side of the cylindrical body (80) of the contrasting element and suitable to be engaged by a respective elastic tab (55) made in said sleeve (5) and projecting towards the inside.

6. A safety mechanism (200) according to any one of claims 3 to 5, characterized in that said contrasting element (8) comprises a pair of slots (85) made in the front side of the cylindrical body in diametrically opposite directions and a pair of spaces (87) made in the rear side of the cylindrical body in diametrically opposite directions and distanced at an angle of 90° compared with the slots (85).

7. A safety mechanism (200) according to any one of claims 3 to 6, characterized in that said contrasting element (8) comprises a tang (82) with a smaller diameter, connected to the cylindrical body (80) by means of a shoulder so as to define a radial seat (88).

8. A safety mechanism (200) according to claim 7, characterized in that said tang (82) of the contrasting element comprises an internal threading (83) suitable to engage with the collar of a needle-holder (20).

9. A safety mechanism (200) according to any of the previous claims, characterized in that said sleeve (5) comprises:

- a rear flange (53) projecting radially from the sleeve body,

- two rigid longitudinal walls (54) projecting backwards from said rear flange (53) parallel to each other so as to define a space suitable to house the rear flange (1 1) of the syringe body (1),

- a first pair of longitudinal elastic tabs (55) made in said rigid rear walls (54) and bent towards the inside, and

- a second pair of longitudinal elastic tabs (57) made in the body of said sleeve and bent towards the inside.

10. A safety mechanism (200) according to claim 9, characterized in that it comprises a spring (7) intended to be compressed between said contrasting element (8) and a front collar (59) of said protection sleeve (50).

1 1. A safety mechanism (200) according to claim 10, characterized in that the plunger (4) of the syringe comprises a driving crown (43) to operate said first pair of longitudinal elastic tabs (55) of the protection sleeve so that said sleeve moves forward due to the action of said spring (7).

12. A safety mechanism (200) according to claim 1 1 , characterized in that said driving crown (43) comprises empty spaces (43') suitable to permit the passage of said first pair of longitudinal tabs (55) of the protection sleeve and the discharge of the air contained into the syringe body (1) before aspirating the preparation to be injected.

13. Method for mounting a safety mechanism (200) according to any one of the previous claims on a syringe (100), comprising the following steps:

- application of the contrasting element (8) to the head of the syringe body (1) so that to make it integral to the syringe body, - axial insertion of the syringe body (1) with the contrasting element (8) into the protection sleeve (5) and rotation of the syringe body with the contrasting element (8) with respect to the sleeve so that the elastic tabs (57) inclined towards the inside of the sleeve bend towards the outside permitting the further axial insertion of the contrasting element (8), - further axial insertion of the syringe body until the rear flange (1 1) of the syringe body is blocked by elastic tabs (55) inclined towards the inside of the sleeve (5) so as to hold the syringe body (1) inside the sleeve (5).

14. A safety syringe (100) comprising: - a syringe body (1), internally hollow and open at the front and rear ends,

- a plunger (4) which slides inside the syringe body (1) with an injection stroke extending from a retracted filling position to an extended position where the syringe is emptied, said plunger (4) being provided in the rear with a stem (41) which can be activated manually and brought outside the syringe body by means of the rear end (42) of said stem,

- an injection needle (2) supported by a needle-holder (20), which can be engaged with the front end (13) of the syringe body (1),

- a contrasting element (8) integral to the head of the syringe body (1), and

- a sleeve (5) mounted by sliding on said syringe body (1) to pass from a retracted utilization position where the needle (2) of the syringe is uncovered to an extended safety position where the needle (2) is covered, said sleeve comprising at least one elastic tab (57) inclined towards the inside, suitable to abut with said contrasting element (8) when the sleeve is in the extended safety position, characterized in that said contrasting element (8) further comprises at least one cam surface (86) interacting with said elastic tab (57) of the sleeve to bend it towards the outside, following a relative rotation of the contrasting element (8) with respect to the sleeve (5), so as to allow the insertion of the contrasting element (8) into the sleeve (5).