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Instrument Set andEquipment

173

In this chapter, we examine the armamentarium for dental local anesthesia, mainly needles, cartridges, syringes, and other instruments.

Needles

Needles enable the local anesthetic solution to pass from the cartridge to the tissues surrounding the tip. Current dental needles are disposable (singletipped for use in cartridge- type syringes (ISO 7885:2010). They were introduced in 1959 (Dobbs1965) and the early 1960s (Bedrock et al.1999; Pogrel2009), and are recommended for use in dentistry by the Council on Dental Materials and Devices of the American Dental Association (ADA) (Alling and Christopher1974). This type of needle has the advantage that it resolves some of the older problems (Alling and Christopher1974), namely, loss of sharpness and barbing owing to repeated use, breakage of the needle owing to metal fatigue, and cross- infection resulting from the needle being used in more than one patient.

Needles come in individual wrappers and are sealed with protective caps that maintain the sterility ensured during manufacture using ethylene oxide or gamma irradiation (Oikarinen and Perkki1975a; Council on Dental Materials1986).

Needles are made of flexible stainless steel (18/8 type) to prevent them from breaking if bent. According to the manufacturer, the metal contains up to 17 compounds, the most important being iron, chrome, and nickel (Oikarinen and Perkki1975a). It is important to note that the manufacturing technique and the composition can affect the characteristics of rigidity and deflection (Robinson etal.1984; Van der Bijl and Rossouw1996).

Modern disposable dental needles of all lengths and gauges are very resistant to breakage resulting from traction or bending and easily exceed safety standards (Oikarinen and Perkki 1975a; Cooley and Robinson 1979; Robinson

use) and double-

etal.1984; Van der Bijl and Rossouw1996; Tomas etal.2000). Nonetheless, it is important to note the following: (i) the finest 30G needles are weaker than thicker gauges (27 and 25G) (Oikarinen and Perkki1975a; Robinson etal.1984; Tomas etal.2000; Pietruszka etal.1986; Bhatia and Bounds1998; Zelster etal.2002) and (ii) the hub is the weakest part of the needle and the point where most break (Pietruszka etal.1986; Bhatia and Bounds1998; Zelster etal.2002).

Parts ofa Needle

Modern double- tipped disposable needles comprise the following parts (Figure11.1).

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).

Local Anesthesia in Dentistry: A Locoregional Approach, First Edition. Jesús Calatayud and Mana Saraghi. © 2024 John Wiley & Sons Ltd. Published 2024 by John Wiley & Sons Ltd. Companion website: www.wiley.com/go/Calatayud/local

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Anterior sheath

needle

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174

Posterior sheath

Multibevel front tip

Adapter

needle

Figure11.1 Parts of a disposable double- tip needle.

Source:Redrawn with modifications from Jastak etal. (1995).

Shaft Hub

Posterior

● 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).

Figure11.2 Multibevel tip of double- tip disposable needles.

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).

Figure11.3 Lumen of the needles with an irregular surface.

Source: Drawn from Oikarinen and Perkki (1975a).

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Needles 175

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Protective Sheath

This is the sheath that keeps the needle sterile and sealed. It is formed by two protective caps:

● The anterior cap, which covers the front, or active, part

of the needle and is fitted to the socket or adapter.

● The posterior cap, which covers the short posterior nee-

dle and is fitted to the socket or adapter. At the same time, it overlies the anterior cap so that it is the first part to be removed when the seal is broken. The needle can be screwed into the anterior part or needle adapter of the syringe, while the anterior part of the needle remains covered and is protected.

Where the protective caps join, at the level of the adapter, there is a label showing the gauge, length, manufacturer, and expiry date of the needle (Meechan2002).

Lengths andGauges

As we shall see, length and gauge (G) play a key role in the selection of needles for the various anesthetic techniques.

Table11.1 shows the different lengths of needle in the active (anterior) part, which is that running from the hub to the tip. As we can see, there are three lengths – long, short, and extrashort– although each varies depending on the manufacturer. However, the most widely used lengths at present are shown.

The gauges used during the first half of the twentieth century were thick (20 or 23G), although after the Second World War new needles appeared. These were made of stainless steel and were much more resistant and flexible,

Table11.1 Lengths ofneedles.

Length

Type Millimeters Inches

Long 41 15/8

38 1½

35 1 ⁄

30 1 ⁄

Short 25 1

20 ¾

Extra-

short 12 ½

10 —

8 ⁄

The most common gauges used in dentistry are shown in bold.

therefore the gauge could be reduced to 25G (Harrison1948). Each gauge has an external diameter (gauge) and an internal diameter (lumen): the smaller the gauge number, the thicker the external and internal diameters of the needle (Table11.2). The main gauges used at present are 25, 27, and 30G. However, by far the most widely used is 27G (Alling and Christopher1974).

Needles: Critical Aspects

Needles are subject to limitations that should be clarified to facilitate appropriate choice and use.

Table11.2 Needle gauges.

International Gauge External diameter Internal diameter

gauge France Millimeters Inches Millimeters Inches

20G 90/100 0.90 0.360 — — 22G 70/100 0.70 0.280 — — 23G 60/100 0.60 0.024 0.30 0.012 25G 50/100 0.50 0.020 0.25 0.010 26G 45/100 0.45 0.018 0.25 0.010 27G 40/100 0.40 0.016 0.20 0.008 28G 35/100 0.35 0.014 0.20 0.008 30G 30/100 0.30 0.012 0.15 0.006 32G 26/100 0.26 0.010 — —

The most common gauges used in dentistry are shown in bold. Source: Data from Oikarinen and Perkki (1975a), Council on Dental Materials and Devices (1978), Trapp and Davies (1980), Lehtinen (1983), Jastak etal. (1995), Meechan (2002), Malamed (2004), Gaudy and Arreto (2005), and ISO 7885 (2010).

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176

Aspiration andGauge

Poiseuille’s law describes the relationship between diameter and resistance to flow of a liquid in a tube, intravenous catheter, or needle. The resistance to flow is inversely related to the radius to the fourth power. For example, if the internal diameter of the needle (lumen) is halved, the resistance to the flow of liquid inside the needle increases 16- fold (Wittrock and Fischer1968). Furthermore, as blood transports formed elements (red cells, lipoproteins, etc.) (Guyton 1976), it is three to four times more viscous and dense than water, with the result that resistance is greater.

Both invitro studies (Smith1968a,1968b; Wittrock and Fischer 1968; Cooley and Robinson 1979; Piesold etal.1998) and clinical studies (Cohen etal.1969; Watson and Colman 1976; Trapp and Davies 1980; Brownbill etal.1987) have shown that blood can be aspirated with fine, narrow gauges (30G) and thick gauges (25G). However, the narrowest gauges (30G) have a slow and reduced aspiration flow rate, since they must overcome higher flow pressure (Smith 1968a, 1968b; Cooley and Robinson1979; Piesold etal.1998). In addition, the evaluation of a true positive aspiration requires a sufficient volume of aspirated blood such that a color change is noted in the cartridge (Watson and Colman 1976). For this reason, it is not advisable to use 30G needles and we should use

27and 25G needles to ensure that aspirated blood is present in sufficient quantities when a positive aspiration does indeed occur and can therefore be seen by the provider

(Cooley and Robinson1979; Piesold etal.1998).

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 fac-

tor (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).

Table11.3 Clinical trials that compare thepain caused by

dental injection withneedles ofvarious gauges.

Statistically significant

Nonsignificant results

Reference Gauge Reference Gauge

Fuller etal. (1979) 30G, 27G, 25G Ram etal.

Mollen etal. (1981) 27G, 25G Ghasemi etal.

Lehtinen (1983) 30G, 27G Brownbill etal. (1987) 30G, 25G Carr and Horton (2001) 27G, 25G Flanagan etal. (2007) 30G, 27G, 25G

The gauges that produce the least pain are shown in bold.

results

(2007)

(2014)

30G, 27G

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 nee-

dle 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.

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Figure11.4 Tip of nondeflecting needles (Truject®)

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vs.conventional needles.

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).

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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).

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:

Criteria forthe Selection ofNeedles

Given the above, we can list practical criteria for selecting needles, depending on the technique and specific needs, as follows:

● Long 25G needles (the thickest) should be used for deep

insertions in regional block, e.g. mandibular block, for several reasons:

○ This is the gauge that is least likely to deflect during

deep insertions.

○ Aspiration is successful with 25G needles and in deep

insertions positive aspirations are common.

○ The injection is no more painful than with smaller

gauges (27G or 30G), as shown in clinical trials.

● Short 27G needles should be used for infiltrative tech-

niques that require little depth but successful aspiration, for two reasons:

○ In these cases, a short needle is more comfortable than

a long one.

○ Aspiration is successful if a vessel is punctured.

● Short or extra- short 30G needles are recommended in

the following cases:

○ When it is not necessary to aspirate, since their ability

to reveal truly positive aspirations is very poor (see Chapter13).

○ When it is not necessary to inject deep into the tissue,

since the needle deflects considerably more than with other gauges and is at greater risk of breakage.

Cartridges

The cartridge is a cylindrical tube that encloses the anesthetic solution between an opening sealed by a rubber diaphragm at the anterior part and a plunger, or plug, at the

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Needle

Diaphragm

Does not

ach the

end

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178

adapter

Aluminum

cap

Figure11.5 Parts of a cartridge.

Body BackNeck

Plunger

posterior part (Figure 11.5). Cartridges are disposable (acartridge can only be used for one patient) and have several advantages:

1) They guarantee the sterility of the anesthetic solution.

2) They are easy and quick to load and clean. Modern car-

tridges do not require the dentist to break the ampule, draw up local anesthetic, and expel the air, therefore the dentist’s skin does not come into contact with the solution, which may spill during these maneuvers.

3) They prevent contact with the metal of the syringe.

Acidic solutions with epinephrine and other sympathomimetic vasoconstrictors interact with metals to release metal ions (nickel, zinc, copper) that irritate tissues and cause pain on injection, especially copper ions (Lundqvist etal.1948).

Cartridges were introduced in 1920 by the army surgeon Harvey S. Cook, who compared them to a rifle cartridge, in the sense that one cartridge per patient is loaded and “fired” (Dobbs 1965). The Cook-

Waite company subsequently introduced the cartridge with the commercial name “Carpule”, which became so popular that many professionals today use the term “carpule” for all cartridges (Nevin and Putterbaugh1949).

Cartridge volume is worthy of comment. Initially, cartridges contained 1, 2, and 2.5 ml (Nevin and Putterbaugh1949). The 2- ml cartridges contained 1.8 ml of anesthetic solution, with 0.2 ml taken up by the plunger. From 1950 onward, the standard 1.8- ml cartridge became widely used (Gruber 1950). At present, use of these cartridges is standard practice throughout the world (Dentists’ Desk1983; Malamed2004). They measure 63–65

mm in length and 8–9 mm in diameter (Meechan2002). The cartridges make it possible to inject up to 1.7 ml of anesthetic solution since the other 0.1 ml is trapped in the neck of the cartridge between the diaphragm and the plunger (Cannell etal.1975).

Cartridges containing 2 and 2.2 ml are currently availa-

ble, but only in certain countries, such as the United

Kingdom and Australia. They are used less frequently because, being longer, holding the syringe is difficult when the dentist has small hands, thus hampering the aspiration maneuver.

The tube is made of one of two materials: (i) glass, which is better quality, with the result that most anesthetic solutions come in this type of container, and (ii) plastic, which is only used in some countries and is worse, for the following reasons:

1) The plunger does not move as easily or as smoothly

against plastic as against glass (Jastak et al. 1995; Malamed2004).

2) Plastic is less transparent, thus making it more difficult

to evaluate positive aspirations (Jastak etal.1995).

3) Spills and leaks of solution are more common during

injection than with glass cartridges (Malamed2004).

4) Resistance to pressure is half that of glass cartridges,

therefore they are not recommended for the periodontal ligament technique (Table18.2, Chapter18). Plastic cartridges have the advantage that they do not break under pressure, but the tube becomes deformed and the anesthetic solution leaks out (Meechan etal.1990).

Of particular interest is the fact that the diaphragm and the plunger contain small quantities of latex, therefore latex allergens may be present in the solution (Brown etal.2002). However, to date there have been no reports of allergy to latex via local anesthetic cartridges (Shojaei and Haas2002).

Parts ofa Cartridge

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

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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 resist-

ance 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 sys-

tem 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 differ-

ent contents.

Storage ofCartridges

To ensure optimal performance with good preservation of the active ingredients, cartridges should be stored

following a series of norms that can be divided in two groups: those that apply to all cartridges and those that also apply to cartridges containing sympathomimetic vasoconstrictors.

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,

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180

with minimum and maximum values of 15–30 °C

°F) (Hondrum et al. 1993). High temperatures

(59–86

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), although with new manufacturing and packaging techniques, we can extend the optimal time point until 18 months.

Studies of stored samples that have not yet reached their expiry date show that the drugs are well preserved and adhere to the minimum levels set by the United States Pharmacopeia (USP) (Kirchhoefer et al. 1986a, 1986b; Smith1991). Nevertheless, as time passes, it is important to bear in mind the following:

● Anesthetics become more painful as their pH decreases

(Oikarinen etal.1975b; Moorthy etal.1984; Crose 1991), since the sulfites become sulfates and release protons (Hondrun and Ezell1996).

● Solutions lose potency as a result of the decrease in the

concentration of sympathomimetic vasoconstrictor (epinephrine): as time passes, the solution will contain lower quantities of antioxidant (sulfite), which protects it (Smith 1991; Hondrun and Ezell 1996), even if minimum levels are maintained (Smith1991).

Problems Affecting Cartridges

Dental local anesthetic cartridges may be subject to some of the following problems:

1) Small bubbles (<1–2 mm). These are the remnants of

nitrogen used during manufacture to eliminate air and oxygen from anesthetic solutions containing sympathomimetic vasoconstrictors and thus extend the half- life of the catecholamines (Milano etal. 1982; Thoma and Struve1986). These bubbles are harmless, and the cartridges can be used without risk for the patient (Dentists’ Desk1983; Jastak etal.1995; Malamed2004).

2) Signs indicating that the cartridge has been frozen,

namely, large bubbles (>2 mm, the main sign), extruded plunger, and suspended particles. When a cartridge has been frozen and then thawed, suspended particles may remain inside (Hondrum etal.1993). These are formed by the residue of lubricants such as silicone or paraffin

(Cooley and Lubow 1981). Furthermore, the extruded plunger may compromise sterilization of the solution, therefore these cartridges must be withdrawn (Dentists’ Desk1983; Jastak etal.1995; Malamed2004). Of interest, lidocaine with epinephrine solution freezes at –3 °C (Hondrum etal.1993).

3) Extruded plunger. This situation is the result of a

plunger being frozen (see above) or of contamination of the anesthetic solution. The contamination may have been caused by chemical vapors (Chasteen etal.1988) or by submerging the cartridge in disinfectant solution (Shannon and Feller1972; Shannon and Wescottt1974). In both cases, the contaminant enters the cartridge via the semipermeable membranes, that is, the diaphragm of the needle adapter or, even more likely, via the rubber stopper (Fyhr and Brodin1987; Passon etal. 1992). In any case, the cartridges should not be used owing to the risk of injecting the patient with solution contaminated by chemical products.

4) Corroded/rusted aluminum cap. When the corrosion is

white in color, it usually means that the cartridge has been placed in a quaternary ammonium disinfectant, which tends to cause an electrolytic reaction (Dentists’ Desk1983; Jastak etal.1995; Malamed2004). When the corrosion is red in color, it means that in the box where cartridge was stored another cartridge has broken and its liquids have rusted the metal of the cap (Dentists’ Desk1983; Malamed2004). The cartridge should be disposed of in both cases.

5) Abnormal appearance of the anesthetic solution inside

the cartridge (Jastak etal.1995). The cartridge should be disposed of in the following cases:

○ Yellow, brown, or dark- brown color. This is caused by

oxidation of epinephrine resulting from the formation of melanins and other inactive compounds. While not toxic, the solution lacks vasoconstrictive activity (Smith1920).

○ Particles and sediments from lubricant residue on the

plunger (silicone, paraffin, glycerin) after freezing (Cooley and Lubow1981).

○ Milky color due to contamination by chemical vapors

(Chasteen etal.1988; Passon etal.1992) or bacterial contamination (Meechan2002).

Degradation ofDrugs inthe Cartridge

The half- life of a drug is the time between its manufacture (manufacture and packaging) and the point where its biological activity falls below 90%. The physical and safety properties of the drug are maintained throughout this period (Hondrum etal.1993). The life of local anesthetic solutions is determined by sympathomimetic vasoconstrictors since

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local anesthetic is very resistant to degradation (Hondrum et al. 1993). Below, we discuss the degradation of various compounds of local anesthetic solutions inside cartridges.

Local Anesthesia

Local anesthetics undergo hydrolysis, although they are very resistant to degradation and can last up to 6

years in storage at extreme temperatures without deteriorating (Hondrum etal.1993).

Sympathomimetic Vasoconstrictors (Epinephrine)

Unlike local anesthetics, catecholamines are very vulnerable to degradation, therefore the active half-

life and expiry date of the solutions are marked on the cartridges (Hondrum etal.1993). These drugs degrade in three ways:

● Oxidation (the main route). Oxidation is the loss of elec-

trons by an atom with addition of oxygen or removal of hydrogen in the organic molecules (Hondrum et al. 1993). The oxidative pathway of epinephrine in aqueous solutions is shown in Table11.4. Oxidation has various causes: oxygen (Milano etal. 1982: Hondrum et al. 1993), increases in pH (Fyhr and Brodin 1987), given that above pH 6 the solution degrades in a few hours (de Jong and Cullen 1963), increases in temperature (Gerke etal. 1977; Fry and Ciarlone1980; Thoma and Struve1986), and sunlight or fluorescent/ultraviolet light (Gerke et al. 1977; Ciarlone and Fry1980; Thoma and Struve1986; Hondrum etal.1993).

● Racemization. Racemization is the conversion from the

levo/levoisomer form, which is 15–20 times more potent, to the dextro/dextroisomer form to create a mixture of the two (racemic mixture) that has half the vasoconstrictive potency (see Chapter 6). The main cause of this transformation is the fall in pH below pH

2 (Milano etal.1982). Other, less important causes are increases in temperature and light (Hondrum etal.1993).

Table11.4 Oxidative pathway ofepinephrine inaqueous

solutions.

Epinephrine

↓

Leucoadrenochrome

↓

Adrenochrome (red)

↓

Adrenolutin (yellow or brown)

↓

Melanins (Brown) nonvasoactive

Source: Data from: Milano etal. (1982), Kirchhoefer etal. (1986b).

● Degradation by sulfites (sulfonation). Sulfites protect

epinephrine and vasoconstrictive catecholamines from oxidation, but they also produce anaerobic degradation of these substances (Hajratwala 1975; Fyhr and Brodin1987). However, fortunately, the process is very slow (Milano etal.1982; Fyhr and Brodin1987; Hondrum etal.1993). Epinephrine becomes epinephrine sulfonic acid via this pathway. The main causes of this process are time and, to a much lesser extent, soluble aluminum, which speeds up the reaction (Milano etal.1982).

To better preserve sympathomimetic vasoconstrictors inside the cartridge, the options available are as follows: (i)degasification with nitrogen to eliminate oxygen during the manufacture of anesthetic solutions (Milano etal.1982; Thoma and Struve 1986), (ii) maintaining an acid pH (2.7–5.5) (USP38 2015), because this helps to keep vasoconstrictive amines stable, and (iii) adding sulfites as antioxidants, since this extends the active life of these vasoconstrictors by preventing the oxygen that enters the cartridge from inactivating the amine through oxidation (Milano etal.1982; Klein1983; Hondrum etal.1993) and helps to maintain an acidic pH.

Sulfites

On oxidation, sulfites become sulfates and release two protons that tend to reduce the pH of local anesthetic solutions. This transformation is caused by oxygen entering the cartridge and increased temperature and light (Hondrum and Ezell1991,1996).

Syringes

The syringe is the instrument that contains the cartridge. The needle through which the anesthetic is injected is also screwed to the syringe. Today, standard dental syringes are made of metal (chrome or stainless steel), thus making them robust, longaccording to the manufacturer and enable use of local anesthetic cartridges and aspiration before injection. They are used with one hand and are designed for a thumb- palm or palm- thumb grasp (Figure 11.6) and linear insertion (Council on Dental Materials and Devices 1974, 1978). This is the most widely used dental syringe today, therefore we call it the conventional cartridge- type syringe.

Parts ofa Cartridge- type Syringe

Anterior Part or Needle Adapter

This is the part where the disposable double- tipped needle is screwed onto the adapter (Figure 11.7).

lasting, and sterilizable. They vary

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