3 SELF-ASPIRATING DENTAL SYRINGE

Luca Martinelli

The self-aspirating dental syringe

Publication no. 3 - 31st October 2012

1. THE SELF-ASPIRATING SYRINGE

The self-aspirating syringe is so called because the aspiration takes place "automatically", without any need for retracting the plunger.

There are at least two types of self-aspirating syringes, a standard and a "disk" model.

1.1 Standard self-aspirating syringe

 

Pic. 1

1.2 Disk self-aspirating syringe

The structure is identical to the previous one, with the difference that the disk is integral with the vial-holder cylinder, but completely independent with respect of the plunger (It has a through hole that allows the plunger to slide inside it).

 Example of a type of self-aspirating syringe with disk.

Pic. 2

1.3 Loading

The loading is substantially identical to the side-loading syringes except that the vial holder cylinder is to be retracted in order to have the space necessary to insert the tubular vial.

Hold the syringe and retract the plunger;

Pic. 3

Insert the tubular vial into the barrel of the syringe;

Pic. 4

Pic. 5 

When the plunger is retracted, to allow the tubular vial to enter into the barrel, the vial holder cylinder is placed at a rearmost position with respect to the tubular vial, while the plunger is completely out of it (hidden in the handle of the syringe).

Finally, the plunger is released and with it, automatically, also the vial holder cylinder.

When the plunger of the syringe is released the vial holder cylinder (1-Pic. 6) touches the tubular vial (2-Pic. 6) holding it into the barrel.

Pic. 6

The plunger of the syringe (1-Pic. 7) move downwards, until contact with the rubber plunger of the tubular vial (2-Pic. 7).

Pic. 7

Please note that the vial holder cylinder (3-Pic. 7) can not be pushed beyond its natural travel when it is separated from the plunger of the syringe and has no reference points which allow its descent (pressure) in an independent and separate way.

Then insert the needle as usual.

1.4 Operating principle of both "Standard and Thumb-disk" syringes

1.4.1 Physical operating principle

The self-aspiration occurs by variation of tubular vial internal pressure; this takes place when the tubular vial, pushed by the plunger of the syringe or by the vial holder cylinder, opposes the self-aspirating tube located inside the barrel of the syringe.

2 OPERATION

When the diaphragm (2 - Pic. 8) of the tubular vial (5 - Pic. 8) opposes the tube of the syringe (3 - Pic. 8) it undergoes a bending (2 - Pig 8) inwards, this causes an increase in pressure inside the tubular vial (7 - Pic. 8); its release (9 - Pic. 8) causes a drop in pressure inside the tubular vial, and this creates simultaneously an aspiration towards its inside (9 - Pic 8).

Pic.8

Detail of tubular vial in contact with the tube 

Pic.9

The physical operating principle of the two types of self-aspirating syringes (standard and thumb disc) is identical, the difference is the possibility of using two different activation modes.

1.4.3 "Standard" self-aspirating syringe

In "standard" self-aspirating syringes the aspiration is only operated by the plunger of the syringe.

Take as a reference the illustrations in Fig. 8, when pushed, the plunger of the syringe also pushes the rubber plunger of the tubular vial, which in turn pushes the anaesthetic towards the needle, this operation pushes all the tubular vial towards the tube of the syringe.

The diaphragm of the tubular vial thus opposes the tube and undergoes a pressure inwards the tubular vials, increasing its inner pressure; when released, the plunger causes a drop in internal pressure that simultaneously originates an aspiration.

5 Disk self-aspirating syringe

The disc is used when an aspiration is required even before pushing the plunger of the anaesthetic.

The physical operating principle is the same, this is simply and accessory, different mode of activating the self-aspiration.

The vial holder cylinder is integral with the disk (Pic. 10), if the disk is pressed (Pic. 11), the perimeter of the vial holder presses on the perimeter of the tubular vial (Pic. 12) which will be pushed downwards, causing in turn a contrast between the diaphragm and the tube, resulting in the self-aspiration.

Disc/vial holder cylinder (one-piece instrument)

Pic. 10

Pic. 11

               Disc pulled upwards            Disc lowered on the 
                                                          tubular vial
Pic. 12

Finally, in the standard self-aspirating syringe is the plunger that, by pressing on the rubber plunger of the tubular vial, activates the self-aspirating mechanism; in the disk syringe the self-aspiration can also be activated by the disk that, being integral with the vial holder cylinder, presses on the glass edge of the tubular vial (so not on the rubber plunger of the tubular vial) thus pushing and activating the self-aspiration.