Anterior Part

This is the active part of the needle, which penetrates the tissues and is in turn made up of the following:

• The shaft, or shank. This is the external part, which is characterized by being very polished and smooth (Van der Bijl 1995), as well as being covered by a thin layer of silicone to ensure that the shaft passes easily through the tissue with the least resistance, thus making the insertion less painful (Winther and Petersen 1979; Van der Bijl 1995). The silicone layer also helps to prevent oxidation of the metallic surface (Van der Bijl 1995).
• Tip. Current needle tips are tribevel with an eccentric bevel on one side, a main bevel, and two secondary bevels on the beveled surface of the main bevel (Figure 11.2). This modern concept is based on multibevel tips or scalpel points ensure the best possible edge and reduce the force of penetration, thus decreasing pain and injury in the mucosa and tissues (Winther and Petersen 1979; Lehtinen and Oksala 1979). The bevel angles are also shallow (9–12°) to reduce deflection as the needle crosses the tissues (Aldous 1968; Robinson et al. 1984; Stacy and Hajjar 1994; Meechan 2002).

It is interesting to observe that once injected, the anesthetic solution spreads around the tip of the needle in an oval, with approximately equal quantities on both sides of the bevel.

Middle Part

This part is in turn composed of two parts, the hub and the socket (adapter).

• The hub separates the anterior part of the needle from the posterior part. It is the weakest part and the point where the needle usually breaks, therefore the needle must never be completely inserted into the tissues up to the hub (Pietruszka et al. 1986; Bhatia and Bounds 1998; Zelster et al. 2002).
• The socket, or adapter, is the point where the needle is attached to the mouth of a cartridge‐type syringe. Plastic sockets lack an internal thread. The thread is created by screwing the socket onto the threaded mouth of the syringe. Metallic sockets are already threaded. Many manufacturers place a triangle or arrow or some other mark on the socket to help the dentist align the needle with the bevel.

Posterior Part

This is a shorter needle (17–25 mm) that perforates the diaphragm of the cartridge and is located inside the syringe. Its tip is beveled at a steeper angle (15–55°) (Meechan 2002). Manufacturers sometimes make the back part of the needle too short, in which case it does not perforate the diaphragm of the cartridge, or too long, thus leaving anesthetic solution inside the cartridge because the plunger cannot reach the final stage owing to its contact with the posterior part of the excessively long needle.

Of note, the lumen of the needle has a rough surface (Oikarinen and Perkki 1975a) (Figure 11.3) and a larger lumen does not make injections less painful (McPherson et al. 2015).

Pain and Gauge

Most dentists think that smaller calibers (30 and 27G) cause less pain during insertion and injection (Smith 1968a; Cooley and Robinson 1979; Mollen et al. 1981; Van der Bijl 1995). However, clinical studies indicate that there is little difference in the perception of pain; six clinical trials revealed no statistically significant differences in the pain produced by different gauges of needle (30, 27, and 25G) and a further two trials show that the finest 30G needle produces less pain than a 27G needle (Table 11.3). In any case, the differences are negligible (even if they are statistically significant, as in the latter two trials they were of minimal clinical relevance). With respect to needles, the truly important factors involved in injection pain are as follows:

1. Design of the tip. This is possibly the most important factor (Lehtinen and Oksala 1979) and the reason why modern needles are multibevel (Lehtinen and Oksala 1979; Winther and Petersen 1979).
2. The fine layer of silicone covering the surface of the shaft of the needle reduces resistance during insertion into tissue (Winther and Petersen 1979; Van der Bijl 1995).

As we shall see in Chapter 13, possibly the most important individual factor involved in pain during insertion and injection is the skill/technique of the dentist (Mollen et al. 1981; Saloum et al. 2000; Goodell et al. 2000; Ram and Peretz 2003; Nusstein and Beck 2003).

Deflection of the Needle and Gauge

Deep linear insertions (e.g. regional block, such as mandibular block) with cartridge‐type syringes and the palm‐thumb grasp (e.g. traditional technique) cause the needle to deflect as it advances owing to the quantity of tissue taken up by the lumen (Jeske and Boshart 1985). This deflection is toward the tip, that is, the side opposite the bevel (Cooley and Robinson 1979; Hochman and Friedman 2000). The factors affecting this deflection are as follows:

1. Gauge. The thicker the gauge, the more rigid the needle is and the less likely it is to deflect (Aldous 1968; Robinson et al. 1984; Jeske and Boshart 1985; Van der Bijl and Rossouw 1996; Hochman and Friedman 2000), therefore 25G needles (the thickest) are recommended for regional block by specification no. 54 of the Council on Dental Materials, Instruments and Equipment of the ADA (Council on Dental Materials 1986), since these are the needles that are least likely to deflect.
2. Length. The longer the needle is, the more likely it is to deflect (Aldous 1968; Van der Bijl 1995).
3. Bevel. The greater the angle is, the more likely it is to deflect (Aldous 1968); this is why current bevels have shallow angles (9–18°) and the steep angles of previous versions are no longer used.
4. Characteristics of the metal used and manufacturing techniques. Needles of the same gauge and length from different manufacturers deflect to different degrees (Robinson et al. 1984; Van der Bijl and Rossouw 1996).

The currently marketed fine 28G Truject^® needle is almost nondeflective during deep linear insertions owing to the design of the tip, which is centered on the longitudinal axis of the needle (Figure 11.4). This decreases the central area by 75%, thus reducing the amount of tissue taken up in the lumen as the needle advances (Jeske and Boshart 1985). The point of traditional needles is situated eccentrically; if it is placed more centrally, the needle is less likely to deflect (Aldous 1968).

Lesions Caused by a Barbed Needle

Repeated use in the same patient leads to loss of the edge at the tip in 80% of needles, thus causing tip of the needle to bend or barb (Oikarinen and Perkki 1975a). Barbing can be caused accidentally during preparation of the syringe (Jastak et al. 1995) – the most unusual cause – and during injection, when the needle meets the bone (Dentists’ Desk 1983; Jastak et al. 1995; Malamed 2004).

When a barbed needle is removed and/or inserted at another site (especially if the tip of the needle is deflected outwards), it can damage the muscles (causing trismus), the nerve stems (causing long‐lasting paresthesia), and the vessels (causing hemorrhage) (Stacy and Hajjar 1994).

It is difficult to see a barbed needle, although it is easily observed by wiping a sterile gauze across the tip, which catches in the material (Dentists’ Desk 1983; Jastak et al. 1995). A barbed needle should be discarded and a new one selected; hence the recommendation to change the needle after two to four injections (Dentists’ Desk 1983; Jastak et al. 1995; Malamed 2004).

Breakage of Needles

One of the main causes of needle breakage is the use of unsuitable needles in truncal block, owing to the depth of the insertion. It is important to note the following causes:

1. The use of short needles that are inserted deep into the soft tissues as far as the hub, which is the weakest part, where the needle usually breaks (Pietruszka et al. 1986; Bhatia and Bounds 1998; Zelster et al. 2002). Around 50% of breakages in the dentist’s office occur in this situation (Annex 37).
2. The use of fine‐gauge needles (30G), which are the weakest and have proven to be more fragile in vitro (Oikarinen and Perkki 1975a; Robinson et al. 1984). Around 60% of breakages in mandibular block involve needles of this gauge (Annex 37).

In conclusion, in regional block techniques such as mandibular block, it is not recommended to use short needles, fine‐gauge needles (e.g. 30G), or short and fine‐gauge needles, all of which account for more than 75% of cases of breakage (Annex 37).

Anterior Part or Needle Adapter

This part comprises the diaphragm, a fine latex membrane through which the needle penetrates, and the aluminum cap, which is generally silver in color and surrounds and holds the diaphragm to the needle adapter (Figure 11.5). The aluminum must not come into contact with the local anesthetic solution, since this can speed up the degradation of sympathomimetic vasoconstrictors such as epinephrine (Milano et al. 1982).

Neck

The needle adapter is joined to the body by a narrowing of the glass tube, where 0.1 ml of anesthetic solution is trapped and therefore cannot be injected (Cannell et al. 1975).

Cylindrical Body

The body is made of transparent glass in order to see blood clearly in positive aspirations and the volume injected via the movement of the plunger. The cylinder is the body of the syringe. As the plunger advances down the syringe pushed by the piston of the syringe, it can inject the solution via the needle.

Posterior Part

The posterior part contains the plunger or rubber stopper that is inside the cylindrical tube. This rubber stopper does not reach the end of the tube; if it does reach the end, then this could indicate that the solution is contaminated. The harpoon of the piston is attached to the rubber stopper and thus enables aspiration. Its main function is to move along the cylindrical tube pushed by the piston to inject the anesthetic solution.

The plunger may be solid, which is the most usual case, so that it can attach to the harpoon or the plunger support system. Less often, it is hollow to house a special system such as the blades of the Uniject^® syringe (Meechan 2002).

Other Elements

• Silicone lubricant on the interior surface of the cylinder enables the plunger to slide smoothly along the glass tube. This lubricant was previously paraffin (wax) or glycerin. Both compounds could harden with time or with low ambient temperatures, thus creating resistance to the movement of the plunger or making it move along in fits and starts, especially at the beginning of the maneuver (blocked or sticky stopper).
• There is a transparent security foil on the surface of the cylinder. The foil serves the following purposes:
  • To prevent pieces of glass falling into the patient’s mouth by limiting uncontrolled shattering of the cartridge if it breaks owing to excessive pressure during injection (e.g. during the periodontal ligament technique) (Rawson and Orr III 1985) or if it is cracked/split because of damage during transport (see Chapter 21).
  • To facilitate administration of the exact volume of anesthesia by means of a line along the axis of the cartridge that acts as a volume indicator (graduated scale).
  • To provide information on the name of the anesthetic solution and vasoconstrictor, concentrations, commercial name, lot number, and date of expiry.
• Content color code. This code may be a colored ring around the cylinder or the color of the rubber stopper or the aluminum cap at the needle adapter. A ring code system is used in the United States but not in Europe, where manufacturers have their own codes, therefore different brands can use the same colors on cartridges with different contents.

Norms for All Cartridges

1. Store in their original packaging (Passon et al. 1992). Cartridges are not airtight compartments and may be contaminated by chemical vapors (Chasteen et al. 1988; Passon et al. 1992) via penetration of the rubber part of the diaphragm or the plunger (Passon et al. 1992); however, the package is completely closed and sealed.

   Packages come in two formats: vacuum packed cans with 50 cartridges, which are rarely used today, and, more frequently, packages with five blister packs each containing 10 perfectly sealed and closed cartridges (the packages may also contain 10 blister packs).

2. Store in a dry place, since humidity tends to deteriorate both the packages and the cartridges.
3. Return damaged or deteriorated packages because the cartridges may be cracked or split and thus carry a risk of breakage during injection and/or loss of stability of the anesthetic solution (Meechan 2002; Malamed 2004). Cartridges should also be returned if the aluminum cap of the diaphragm in the needle adapter is dented or damaged, since the underlying glass may also be broken (Malamed 2004).

Cartridges should never be stored in the following ways:

• Submerged in disinfectant, since this can penetrate the cartridge and contaminate the anesthetic solution, leading to painful injections and long‐term paresthesia (Shannon and Feller 1972; Shannon and Wescottt 1974).
• Together with products that release chemical vapors, such as resin solvents, since these too can penetrate the cartridge (Chasteen et al. 1988; Passon et al. 1992).

Norms for Cartridges Containing Catecholamines

Sympathomimetic vasoconstrictors such as epinephrine, norepinephrine, and levonordefrin are all very sensitive to degradation, therefore the expiry date appears on the cartridge (Hondrum et al. 1993). In addition to the abovementioned norms, these cartridges are subject to additional storage conditions:

1. Darkness, given that light speeds up oxidation of the vasoconstrictors, especially epinephrine. By light, we mean daylight (Gerke et al. 1977; Thoma and Struve 1986), fluorescent light in rooms (Hondrun and Ezell 1996), and ultraviolet light (Ciarlone and Fry 1980).
2. The recommended storage temperature is 20–22 °C (68–72 °F), which is equivalent to ambient temperature, with minimum and maximum values of 15–30 °C (59–86 °F) (Hondrum et al. 1993). High temperatures speed up oxidation of the catecholamines and therefore their degradation (Fry and Ciarlone 1980; Kelly and Dalm 1985; Thoma and Struve 1986), thus cartridges should never be kept in cartridge warmers for long periods.
3. Packages indicating that more than half of the lifetime of the drug has passed, that is 18 months, should be rejected before the expiry date (Dentists’ Desk 1983; Jastak et al. 1995), since the duration of the solution is 3 years. Solutions previously lasted for a shorter period when they were in good condition (Gerke et al. 1977
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