p318 local anaesthetic techniquanestezia locala

7/28/2019 P318 Local Anaesthetic Techniquanestezia locala

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Local Anaesthetic Technique

Author: David A. Isen Hon. B. Sc., D.D.S., F.A.D.I.

Neuroanatomical Considerations

For dental anaesthesia, the neuroanatomical focus is the fifth cranial nerve, also known as

the trigeminal nerve. This nerve has three divisions - the ophthalmic division (V1), the

maxillary division (V2) and the mandibular division (V3). The maxillary dentition receives

innervation from V2, and the mandibular dentition receives innervation from V3.

RD Online Dental Education www.rdsurgery.co.uk 1

After reading this article, the

reader should be able to:

Explain the anatomy and

nerve supply of the teeth.

discuss factors different

local anaesthetic

techniques.

recognize reasons for

failure of anaesthesia.
http://www.rdsurgery.co.uk/http://www.rdsurgery.co.uk/


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The maxillary nerve enters the pterygopalatine fossa and branches into three major

sections: the ganglionic branches, the zygomatic nerve and the posterior superior alveolar

nerve.

The ganglionic branches travel to the pterygopalatine ganglion, which in turn sends

sensory, parasympathetic and sympathetic fibres back to the maxillary nerve.

The zygomatic nerve enters the orbit and travels along the lateral wall. It bifurcates into

two terminal branches, the zygomaticofacial nerve, which supplies sensation to the cheek,

and the zygomaticotemporal nerve, which supplies sensation to the temple area. There is

also a parasympathetic component to the lacrimal gland.

The posterior superior alveolar nerve travels inferiorly on the infratemporal surface of themaxilla, entering the maxillary sinus and eventually terminating in sensory branches for the

maxillary molars and their surrounding buccal gingiva, with the possible exception of the



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mesiobuccal root of the first molar.

As the maxillary nerve continues, it enters the infraorbital groove and becomes the

infraorbital nerve. This nerve gives rise to the middle and anterior superior alveolar nerves.

The middle superior alveolar nerve supplies sensation to the mesiobuccal root of the

maxillary first molar, the premolars and the associated buccal gingival. However, this nerveis not present in all people; if the nerve is absent, these areas are innervated by the

posterior and anterior superior alveolar nerves. The main areas of sensory innervation for

the anterior superior alveolar nerve are the cuspid, and central and lateral incisors and the

buccal gingiva in that area.

The infraorbital nerve continues and eventually passes through the infraorbital foramen

onto the face, supplying the lower eyelid, the side of the nose and the upper lip.



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The mandibular nerve leaves the base of the skull through foramen ovale. The first branch

from the main trunk is the nervous spinosis, which runs superiorly through the foramen

spinosum to supply the meninges. The next branch is the first motor nerve, which supplies

the medial pterygoid muscle. Inferior to that branch, the mandibular nerve splits into an

anterior trunk and a posterior trunk. The anterior trunk is both sensory and motor. The

sensory trunk is the long buccal nerve, which supplies the buccal soft tissue distal to the

first molar. The motor component supplies the masseter, temporal and lateral pterygoid

muscles. The posterior trunk sends off the auriculotemporal nerve that gives sensory

perception to the side of the head and scalp and sends twigs to the external auditory

meatus, the tympanic membrane and the temporomandibular joint. The posterior trunk

then almost immediately divides into the lingual nerve and the inferior alveolar nerve. Thelingual nerve supplies the anterior two-thirds of the tongue and the lingual surface of the

mandibular gingiva. The mandibular nerve sends a branch to the mylohyoid muscle and

the anterior belly of the digastric muscle and then enters the mandibular canal. This nerve

gives sensation to the mandible, the buccal gingiva anterior to the first molar, the lower lip

and the pulps of all the mandibular teeth in that quadrant.



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One of dentistry's most difficult challenges is consistently anaesthetising the mandibular

dentition. A conventional mandibular block has a failure rate of at least 15% to 20%. There

are a number of possible reasons for this phenomenon, one of which is accessory

innervation (see "The Reasons For Incomplete Anaesthesia", below).

Dental injection techniques include the inferior alveolar nerve block, the Gow-Gates

mandibular block, the Vazirani-Akinosi closed mouth mandibular block, intraosseous

injections, periodontal ligament injections and various adjunctive techniques.

The Inferior Alveolar Nerve Block

The inferior alveolar nerve block is the most widely used technique for blocking the

hemimandible. However, as mentioned above, due to neuroanatomical and skeletal

variations, there is a failure rate of 15% to 20% in achieving complete anaesthesia. The



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advantages and disadvantages for this technique are listed in the table below.

Advantages Disadvantages

Practitioner acceptance Area of injection is vascular; 10 -15%chance of positive aspiration

Faster onset than higher blocks Unlikely to anaesthetise accessory nerves

Bony landmark Unlikely to anaesthetise long buccal nerve

Difficult to see landmarks in some patients

(e.g., macroglossia)

The landmarks for this injection are as follows:

the coronoid notch (the greatest depression on the anterior border of the ramus), also

called the external oblique ridge

the internal oblique ridge

the pterygomandibular raphe

the pterygotemporal depression

the contralateral mandibular bicuspids

Technique

1. Palpate the anterior ramus border at the coronoid notch.



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2. Slide the finger or thumb posteriorly and medially until a ridge of bone is palpated.

This is the internal oblique ridge.

3. Insert the needle into soft tissue in the pterygotemporal depression, which is halfway

between the palpating finger or thumb and the pterygomandibular raphe.

4. Approximate the height of the injection by the middle of the palpating fingernail or

thumbnail.

5. Ensure that the barrel of the syringe is located over the contralateral mandibular

bicuspids.



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6. Insert until bone is contacted, and then withdraw ~1 mm. The depth of insertion for

the average-sized adult is approximately 25 mm.

7. Aspirate.

8. Inject a full cartridge.

Onset and duration

Onset for hard tissue anaesthesia is 3 to 4 minutes.

Duration for hard tissue anaesthesia is 40 minutes to 4 hours, depending on the type

of local anaesthetic used and whether a vasoconstrictor is used.

It is unlikely that the long buccal nerve will be anaesthetised.

The Gow-Gates Mandibular Block

In 1973, Dr. George Gow-Gates published an article describing an alternative techniquefor blocking the mandible. The advantages and disadvantages of this technique are listed



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in the table below.


Perceptible end point (bone) Mouth wide open

Fewer blood vessels at this level, therefore

less chance of positive aspiration

Must use extraoral landmarks, which may

increase the difficulty of this procedure

Long buccal nerve anaesthesia likely

Possible longer duration of anaesthesia

Less chance of anaesthetising accessory

nerves


10 mm above the coronoid notch


the pterygomandibular raphe

the neck of the condyle

the contralateral mandibular bicuspids

an imaginary line from the corner of the mouth to the tragal notch of the ear

(extraorally).

Technique

1. Ask the patient to open his or her mouth wide.

2. Palpate the coronoid notch and slide the finger or thumb to rest on the internal

oblique ridge.



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3. Move the finger or thumb superiorly approximately 10 mm.

4. Rotate the finger or thumb to parallel an imaginary line from the ipsilateral corner of

the mouth to the tragal notch of the ear.

5. Insert the needle at a point between the palpating fingernail and the

pterygomandibular raphe at the middle aspect of the fingernail.

6. Ensure that the barrel of the syringe is located over the contralateral bicuspids.



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7. As the injection proceeds, ensure that the angle of the needle and syringe is parallel

to the imaginary line from the corner of the mouth to the tragus of the ear.

8. Insert until bone is contacted (at the neck of the condyle), which should occur at a

depth of approximately 25 mm. (Note: This is not a deeper injection, because the

patient's mouth is open wide and, as a result, the condyle has translocated anteriorly

to provide a target.)



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9. Once bone is contacted, withdraw the needle tip 1 mm to prevent injecting into the

periosteum, which would be painful.

10. Aspirate.


Onset and duration

Onset for hard tissue anaesthesia is 4 to 12 minutes, with the anterior areas taking

the longest amount of time.

The long buccal nerve will likely be anaesthetised.

The Vazirani-Akinosi Closed Mouth Mandibular Block

In 1960, S. Vazirani published a paper describing a closed mouth mandibular block;

however, it was not until 1977, when J.O. Akinosi published a paper on this approach, that

the technique gained popularity. The advantages and disadvantages of this technique are

listed in the table below.



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Can be used for patients with trismus Difficult to visualise depth of injection

Can be used for patients with a strong gag

reflex

Difficult in patients with widely flaring

ramusMouth is closed, so injection may be less

threatening to patient

Difficult in patients with pronounced

zygomatic ridge or internal oblique ridge

Possibly less pain, because tissues are

relaxed

Good for macroglossic patients


the maxillary buccal mucogingival line or root apices of the maxillary teeth

the coronoid notch


the occlusal plane

Technique

1. Prepare the needle and syringe by bending the needle approximately 15o to 20o.

This bend accommodates for the flare of the ramus. Do not bend the needle more

than once when preparing.

2. Ask the patient to slightly open (a few millimetres) his or her mouth and execute a

lateral excursion toward the side that is being injected.

3. Palpate the coronoid notch and slide the finger or thumb to rest on the internal

oblique ridge.



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4. Move the finger or thumb superiorly approximately 10 mm.

5. Insert the needle tip between the finger and maxilla at the height of the maxillary

buccal mucogingival line. Orient the bend of the needle such that the needle looks as

though it is going laterally in the direction of the ear lobe on the injection side. The

needle remains parallel to the occlusal plane.



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6. After the needle has been inserted 5 mm, remove the palpating finger or thumb and

use it to reflect the maxillary lip to enhance vision.

7. Inject to the final depth of approximately 28 mm for the average-sized adult, therefore

visualising 7 mm of needle remaining outside the tissue (if using a long needle).

8. Aspirate.




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Onset and duration

Onset for hard tissue anaesthesia is 3 to 4 minutes

There is an increased possibility of obtaining long buccal nerve anaesthesia as compared

to the inferior alveolar nerve block.

Intraosseous Injections

With intraosseous injections, the local anaesthetic solution is deposited directly into the

cancellous bone surrounding the teeth being treated. These techniques can be considered

if one of the primary nerve blocks has failed. Early techniques for delivering the local

anaesthetic into the cancellous bone used a round bur to perforate the cortical plate, with

the drug then being injected through this hole. Over the past 20 years, new and more

effective devices have been introduced into the marketplace. Two of the more common

products are Stabident and the X-tip. Each of these products uses a different technique,

and the practitioner is encouraged to follow specific instructions.


Immediate onset of anaesthesia Short duration of anaesthesia

No soft tissue (lip or tongue) anaesthesia Must limit volume due to increased

vascularity in the cancellous bone

Can operate bilaterally in the mandible Difficult access to posterior mandible

Can anaesthetise a "hot" tooth Anatomical limitations

Good approach for accessory innervation Some patients experience palpitations

High success rate Cannot use in areas of periodontal disease

Technique



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1. Follow the specific instructions supplied with the delivery system.

2. Anaesthetise the soft tissue to ensure that the perforation of the cortical plate is

painless. Inject an infiltration of 0.2 mL to 0.3 mL of local anaesthetic into the buccal

fold near the area to be perforated.

3. Take a radiograph to ensure that there is enough bone at the perforation site so that

the periodontal ligament space or root surfaces will not be violated.

4. Perforate the bone using whichever device has been chosen. The site of perforation

is on the attached gingiva approximately 1 mm to 2 mm coronally to the mucogingival

line.

5. Negotiate the needle through the perforated bone into the cancellous space and

slowly inject 0.9 mL of local anaesthetic. This volume provides pulpal anaesthesia for

the teeth on either side of the perforation. The injection should be done slowly, over

about 45 seconds per 0.9 mL, to avoid palpitations as much as possible.

Do not exceed one cartridge of intraosseous anaesthetic per appointment.

Anatomical limitations include inadequate bony space between the teeth, a cortical plate of

bone that is too thick to perforate, a low-lying maxillary sinus and a horizontally impacted

third molar. In addition, the technique cannot be used between central incisors due to the

lack of cancellous bone.

This technique should not be used on patients with cardiac disease.

Onset and duration

The onset of anaesthesia is immediate.

Duration for pulpal anaesthesia is 20 to 30 minutes if a vasoconstrictor is used and

significantly less than that if a vasoconstrictor is not used.



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Periodontal Ligament Injection

In the periodontal ligament (PDL) injection, local anaesthetic is injected with pressure into

the PDL space. A number of devices are available to facilitate this type of injection by

providing the necessary pressure; however, this technique can be done with a standard

syringe. If using a standard syringe, the practitioner can express three-quarters of the

volume within the local anaesthetic cartridge to lessen the pressure that has to be pushed

against and to decrease the chance that the glass cartridge will break.


Immediate onset of anaesthesia Patient may experience post-operative

pain

No soft tissue anaesthesia There is a transient decrease in pulpal

blood flow to the tooth

Works well for "hot" teeth Cannot be used in areas of periodontal

disease

Good approach for accessory innervation Pressure is required to inject into the PDLspace

High success rate Multiple injections are required for multi-

rooted teeth (one injection per root)

May not work on long roots (e.g., cuspids)

Technique

1. Anaesthetise the soft tissue to allow for a comfortable PDL injection. Inject an

infiltration of 0.2 mL to 0.3 mL of local anaesthetic into the buccal fold adjacent to the

desired tooth.

2. Embed the needle into the PDL space.

3. Inject 0.2 mL per root.



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4. Allow 10 seconds to pass to allow back pressure to dissipate and ensure that local

anaesthetic does not leak into the mouth upon removal of the needle.

Onset and duration

The onset of anaesthesia is immediate.

The duration of pulpal anaesthesia is highly variable and somewhat unpredictable.

Adjunctive Techniques

Other techniques and devices have been used and reported to provide some level of

either soft tissue or hard tissue anaesthesia.

Electronic dental anaesthesia is a technique wherein electrodes are fixed to locations on

the patient's face, and the patient is given controls that can send stimuli from one

electrode to the other. The theory is similar to that behind TENS (transcutaneous electric

nerve stimulation). The electrical signal seems to decrease the patient's ability to perceive

pain. Although these devices are no longer marketed, some dentists have reported

success with them in situations where light anaesthesia is required (e.g., deep scaling).

Also now available are ultrasonic scalers, through which the patient controls a low-intensity

DC current that goes through the scaler tip to the tooth. This stimulus may be able to block

the perception of mild pain. Further evaluation of these devices is required.

Another device used by some practitioners is thejet injector, of which different models are

available. They can expel the local anaesthetic with such force and in such a fine stream

that it can penetrate soft tissue without a needle. The disadvantage is that only enough

volume can be expressed to anaesthetise the soft tissue, and they may therefore be used

for topical anaesthesia but not for pulpal anaesthesia.

Reasons for Incomplete Anaesthesia

The reasons for incomplete local anaesthesia are as follows:

local anaesthetic pka - ph factors and tissue ph factors

needle-to-jaw size discrepancy



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needle deflection

volume factors

skeletal and neuroanatomic variations

local anaesthetic or vasoconstrictor degradation

uncooperative patients

Local anaesthetic pKa - pH factors and tissue pH factors

When a local anaesthetic is injected into tissue, two particles are in equilibrium: a lipophilic

(lipid-soluble) neutral particle and a positively charged hydrophilic (water-soluble) particle.

Initially, it is advantageous to have the greatest proportion possible of lipophilic particles,

because these particles can pass through the lipid membrane of the nerve. Once inside

the nerve, a new equilibrium is established, and a new set of hydrophilic particles form.

These hydrophilic, charged molecules work to stop the action potential inside the nerve.

The practitioner can influence the ratio of lipophilic molecules to hydrophilic particles to

decrease the onset of anaesthesia. Three factors can affect this equilibrium: the pKa of the

local anaesthetic, the pH of the local anaesthetic and the pH of the tissue in which the

anaesthetic is being deposited.

The pKa of a local anaesthetic is defined as the pH at which half of the local anaesthetic

particles in equilibrium are neutral (lipophilic) and half are charged (hydrophilic). For

example, if a local anaesthetic had a pH of 7.4 and was injected into normal tissue, which

also has a pH of 7.4, there would be equal amounts of both types of particles. The

anaesthetic would therefore be likely to have a relatively short onset of action due to the

large initial proportion (50%) of lipophilic molecules able to cross the lipid nerve

membrane. Unfortunately, all local anaesthetics have pKa values higher than 7.4. As a

result, the injection of a local anaesthetic shifts the equilibrium toward the hydrophilic

molecules, with proportionately fewer available lipophilic particles. Practitioners are forced

to live with the onset times that result from these greater-than-7.4 pKa values. The

extreme example in this case is procaine (Novocain), which has a pKa value of 9.1. This

value results in a very long onset of action time, which is one of the poor qualities of ester

local anaesthetics that have led to their depopularization as injectable local anaesthetics in

dentistry. Therefore, the general rule of thumb is that the higher the pKa of the local

anaesthetic, the longer its onset of action due to the fewer lipid soluble particles initiallyavailable to cross the nerve sheath. More simply put, higher pKa equates to decreased

potency.



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A factor that dentists can influence is pH. There are two separate issues with respect to

pH: the pH of the tissues where the local anaesthetic is being injected and the pH of the

local anaesthetic itself. As mentioned above, normal tissue pH is 7.4, but if there is an

infection in the area of injection, the pH will be lower (in the acidic range). The effect of this

infection is similar to the high pKa of the local anaesthetic; that is, it shifts the equilibriumtoward the charged hydrophilic side of the equation and thereby lessens the initial amount

of lipophilic particles available. This equilibrium, in turn, increases the time to onset of

anaesthesia. If the infection is severe and the pH of the tissue therefore quite low, few

lipophilic particles will be available, and the local anaesthetic might not work at all. Most

dentists have experienced this failure of anaesthesia when attempting to anaesthetise a

"hot" tooth or when trying to anaesthetise an area of severe periodontal disease.

The local anaesthetic itself can cause another pH problem. Local anaesthetics with a

vasoconstrictor contain the preservative sodium metabisulphite. This preservative is quiteacidic, and in high concentrations it can lower the overall pH of the local anaesthetic

solution to 4 or 5. The higher the concentration of the vasoconstrictor, the more

preservative is required and the lower the pH. Thus, the solution injected into the tissues

can be quite acidic.

Consider the following example: A practitioner attempts a mandibular block using a local

anaesthetic with 1:100,000 epinephrine. While the practitioner is working on a tooth, the

patient feels pain. The practitioner administers another block with the same solution, but

the patient still perceives pain. If the practitioner gives yet a third block, the pH in the

pterygomandibular triangle will be so acidic that the equilibrium will be shifted well away

from the lipophilic particles and there will be no opportunity for local anaesthetic molecules

to cross into the nerve. A block will never be achieved in this situation regardless of how

much vasoconstrictor-containing local anaesthesia is administered. It is recommended that

if, after two attempts at a block, there is still incomplete anaesthesia, the practitioner try a

vasoconstrictor-free solution injected into a slightly different location in the

pterygomandibular triangle. This injection should increase the pH in the area and possibly

even buffer it somewhat, because a "plain" solution has a more basic pH. There should

then be enough lipophilic particles to cross the lipid nerve membrane.

Needle-to-jaw size discrepancy

In dental practice, two popular lengths of needles are available for routine injections. The

short needle is approximately 25 mm or one inch long, and the long needle measures

approximately 35 mm or 1 5/8 inches long.

Short needles cannot be recommended for mandibular block injections in adult patients.

The depth required for a mandibular block for the average-sized adult is 25 mm. Thus, to

reach the injection end point with a short needle, the practitioner must inject to the hub.



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This practice could cause complications in the unlikely event of needle breakage. Also, it is

easier to lose one's orientation and angulation, which could mislocate the injection.

Furthermore, if the patient is larger than average, the final depth will not be achieved

unless the practitioner pushes the needle into the tissues beyond the hub. If the

practitioner is performing a Vazirani-Akinosi mandibular block, which has an average depth

of 25 mm to 27 mm, it becomes even more difficult to achieve the final depth.

Long needles afford the practitioner the ability to observe the length of needle that is

remaining outside the tissues once the final depth has been achieved. For the average-

sized adult, the practitioner would observe 10 mm of needle remaining outside the tissues

once the final position has been attained using a long needle for the conventional

mandibular nerve block. Simply put, long needles may increase success rates in achieving

mandibular blocks.

Needle deflection

When a needle is inserted into tissue, it deflects due to the density of the tissue pushing

against the bevel of the needle. The deeper the needle is inserted and the thinner the

needle (the higher the gauge), the more the needle deflects. The deflection occurs such

that the needle is pushed away from the bevel. A study by Aldous first demonstrated this

phenomenon. Using a tissue medium of hydrocolloid and hot dogs, Aldous demonstrated

that a 30-gauge needle inserted to a depth of 25 mm would deflect 4 mm, a 27-gauge

needle would deflect 2 mm and a 25-gauge needle would deflect 1 mm. Repeat studies by

other scientists using human tissue and radiography have yielded similar results. Because

a 4-mm deflection is enough to mislocate any block injection, there is valid reason for

using more stable, lower-gauge needles.

The orientation of the bevel is important not only with respect to needle deflection. The

practitioner may wish to know where the bevel is once the needle has been inserted into

tissue. For example, when infiltrating, it is customary to face the bevel toward bone to

avoid scraping the periosteum. Also, when performing a Vazirani-Akinosi block, the

practitioner may wish to face the bevel toward the patient's midline to have the needle

deflect laterally, toward the nerve. There are needles on the market that have markings on

the hub, indicating the position of the bevel.

Volume factors



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Dentists usually rely on one cartridge of local anaesthetic to provide profound anaesthesia

to most areas. Nonetheless, a number of factors can contribute to inadequate volume of

local anaesthesia and the resulting need to inject more than one cartridge.

The first factor is time. When a mandibular block is given, the practitioner must wait 3 to 4

minutes to allow the anaesthetic to completely bathe the nerve, thus totally blocking it. If aprocedure is commenced before the time required for complete anaesthesia, the patient

will experience discomfort, as the full volume of anaesthetic will not have had a chance to

anaesthetise the whole thickness of the nerve.

Second, there is an anatomical structure that can physically stop the local anaesthetic

from travelling to the inferior alveolar nerve. If local anaesthetic is deposited too far

medially away from the inferior alveolar nerve, it is blocked from travelling laterally by the

sphenomandibular ligament and its associated fascia. This ligament runs from the

sphenoid process to the lingula, and attached to it is a fascia that fans out in a sagittaldirection. Local anaesthetic cannot cross this barrier, and it is therefore crucial to inject

lateral to the ligament. Otherwise, the patient will experience incomplete anaesthesia or

maybe even no anaesthesia at all.



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Another anatomical factor to consider is the vasculature. If the local anaesthetic is

deposited into a vessel, no anaesthesia is obtained. It is recommended to use a wider-

lumen (lower-gauge) needle to increase the likelihood of success in obtaining a positive

aspiration. For example, a 25-gauge needle offers a much more reliable indicator of

positive aspiration than does a 30-gauge needle, which offers a very poor indicator of

positive aspiration.

A fourth factor, also anatomical, is the thickness of the nerve. The inferior alveolar nerve,

at the level of the conventional mandibular nerve block, is thinner than the core mandibular

nerve, which is approximated in the Gow-Gates block. This thicker nerve requires a longer

onset time for complete infiltration; the conventional mandibular nerve block takes 3 to 4

minutes to complete anaesthesia, compared to the 10 to 12 minutes for the Gow-Gates

block. The other important reason for the longer onset time is simply the longer distance

the drug has to travel in a Gow-Gates versus a standard block. The practitioner could

consider an intraosseous or PDL injection to minimise the onset of anaesthesia.

A fifth factor to consider is the actual volume of the local anaesthetic. Some patients



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require more than one cartridge of local anaesthetic to anaesthetise the mandible.

Accessory innervation (see below under "Skeletal and neuroanatomic variations"), thicker

nerves and larger patients may necessitate more anaesthetic. For such patients, a

practitioner may decide to give two cartridges of local anaesthesia in slightly different

locations - for example, one in the location of the conventional block, and one in the area

of the Gow-Gates block. The extra dose maximises the volume and saturates thepterygomandibular space with anaesthetic.

Skeletal and neuroanatomic variations

A variety of anatomical variances can lead to a missed block if not considered in

landmarking. Skeletal factors, such as class of occlusion and the width of the ramus,

change the location of the lingula relative to the intraoral landmarks. In addition, a ramus

that flares widely from the midline requires the syringe to be located more over the

contralateral molars when blocking the hemi-mandible, while a ramus that is more parallelto the mid-sagittal plane requires the syringe to be more over the contralateral cuspids.

Another crucial skeletal anatomical variant is the width of the internal oblique ridge. It is on

this ridge that the practitioner's finger must rest for all mandibular block procedures,

including the conventional, the Vazirani-Akinosi and the Gow-Gates. If the patient has an

exceedingly wide internal oblique ridge and the practitioner's finger is not resting on this

ridge of bone, it is very difficult to negotiate the needle past this bony ridge to approach the

inferior alveolar nerve. This nerve is located on the medial aspect of the ramus behind the

large ridge. Palpating a wide inferior alveolar ridge is also cause to rotate the syringe more

posteriorly, toward the contralateral molars.

A final skeletal anatomical factor is the position of the mandibular foramen. The location of

this foramen can vary both in its anterior - posterior position and its inferior - superior

position. Blocks given more superiorly, for example, the Gow-Gates block, may in part be

more successful due to the increased chance of being superior to this foramen. Therefore,

the local anaesthetic is not being deposited inferior to where the nerve enters the mandible

(which would result in incomplete anaesthesia).

Dissection studies have shown that both the mylohyoid nerve and the mandibular nerve

can send accessory nerves through various locations in the pterygomandibular triangle.

These accessory nerves can enter the mandible in various lingual locations on the ramus

or on the alveolar ridge. The mandibular nerve has been shown to send accessory nerves

that can enter the mandible through foramina in the retromolar area on the coronoid

process. The mylohyoid nerve can send branches through foramina located anywhere on

the lingual aspect of the mandible and thus directly supply accessory innervation to any of

the mandibular teeth. Either type of accessory innervation could cause a patient to

experience incomplete anaesthesia with a conventional mandibular nerve block.

Correcting the lack of complete anaesthesia is possible through a number of different



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techniques. First, a Gow-Gates block can be given; because this block is more superior in

the pterygomandibular triangle, it is more likely to be superior to the location of where the

accessory nerve leaves the core nerve. Second, 0.4 mL to 0.5 mL of local anaesthetic can

be injected into the retromolar area or lingual to the tooth being treated. This lingual

injection would occur on the vertical wall of the mandible in the area of the unattached

gingiva. The practitioner should be careful to avoid the floor of the mouth, where thesubmandibular salivary gland exists.

Local anaesthetic or vasoconstrictor degradation

All local anaesthetic cartridges have an expiry date on their label. This date tells the

practitioner the product's shelf life from the time of manufacturing to the time when a

certain number of the anaesthetic or vasoconstrictor molecules have degraded to a degree

that the product may be less effective. Local anaesthetic molecules are relatively stable

and degrade very slowly. As a result, the shelf life of a local anaesthetic depends mostlyon the stability of the vasoconstrictor. For this reason, sodium metabisulphite is used as a

preservative or stabiliser for the vasoconstrictor molecule. A number of factors can lead to

the premature breakdown of an anaesthetic and the vasoconstrictor within a cartridge,

including extreme temperatures, excessive light and oxygen exposure. To maximise the

shelf life of the contents inside the cartridge, the local anaesthetic molecule should be

stored at room temperature away from sunlight and room light. Dental offices are unlikely

to experience temperature extremes, but consideration should be given to how the local

anaesthetic was delivered to the office. Local anaesthetics can easily freeze or overheat if

left in a delivery truck during seasonal extremes. These temperature variations can lead to

the premature degradation of the molecules in the cartridge.

Autoclaving or repeatedly using cartridge warmers will decrease the shelf life of the

contents of the local anaesthetic cartridge.

Local anaesthetics should not be purchased for stockpiling in such amounts that the stale

date arrives before the solution can be utilised.

Uncooperative patients

Incomplete anaesthesia is not only frustrating for the practitioner but is also uncomfortable

at best or devastating at worst for the patient. Many dental-phobic patients report a prior

dental visit in which they experienced pain. When these patients next attend a dental

office, they do so with great trepidation. It can be very difficult for them to walk through the

front door of the dental office, let alone open their mouths wide to allow for dental

treatment. For this reason, profound anaesthesia can be difficult to obtain with dental-

phobic patients. Many of these patients may have had other reasons for incomplete

anaesthesia, and now, to compound the problem, they are unwilling to open their mouths

wide enough for the practitioner to be able to visualise the landmarks necessary to achieve



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a successful injection.

In such situations, the practitioner must strive to elicit the patient's co-operation through

reassurance and explanation. For example, the practitioner could say, "Please lift your

chin up and open your mouth wide. That will really help the anaesthetic to work." If the

patient's anxiety is strong enough that it impedes their ability to co-operate, conscioussedation such as nitrous oxide and oxygen may be considered.

Other Issues

Needle length and gauge

The three standard dental needle lengths are long (~35 mm), short (~25 mm) and ultra-

short (~12 mm). The exact measurements vary slightly. In general, it is suggested that long

needles should be used for deeper injections such as blocks in the mandible to improve

accuracy (see "Needle-To-Jaw Size Discrepancy", above, under "Reasons for Incomplete

Anaesthesia"). Short needles can be used elsewhere, and ultra-short needles may be

useful for a PDL injection.

The three standard dental needle gauges, or thicknesses, are 25-gauge, 27-gauge and

30-gauge. The choice depends on two main factors. First, the thicker the needle, the more

stable it is and the less it deflects when pushed into tissue; therefore, a practitioner may

decide to use thicker needles on heavier-set individuals. Second, neither 27-gauge nor 30-

gauge needles are reliable aspirators of blood;

therefore, whenever the practitioner is injecting into an area where there is the possibility

of entering a blood vessel, a 25-gauge needle should be used. The patient will not be able



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to discern the difference between the prick of a 25-, 27- or 30-gauge needle. One needle

will not hurt more than another. The key to reducing pain during injection, regardless of the

needle gauge, is to inject slowly.

Burning on injection

A burning sensation on injection may occur for two reasons. First, local anaesthetics with a

vasoconstrictor are acidic because of the preservative required for the vasoconstrictor.

This acidity can cause the anaesthetic to burn when it is injected into tissues. As the

cartridge ages and approaches the expiry date, the vasoconstrictor begins to break down,

resulting in even a lower pH and therefore even more burning on injection. Second, if

cartridges are immersed in sterilising solution and the solution seeps into the cartridge, the

sterilising solution can cause a burning sensation upon injection.

The likelihood of a burning sensation can be minimised by using fresh anaesthetics withlittle or no vasoconstrictor and by injecting slowly.

Cartridge warmers

Cartridge warmers are used with the hope that increasing the temperature of the local

anaesthetic will decrease the amount of pain felt by the patient during the injection. There

is no scientific evidence that warming a local anaesthetic cartridge from room temperature

(the temperature of the anaesthetic while stored) to body temperature changes the amount

of discomfort experienced by the patient. In fact, even if the anaesthetic is warmed, it will

approach the temperature of the needle (room temperature) as it is pushed through and

into the tissues. As well, repeatedly heating or overheating the cartridge results in

degradation of the vasoconstrictor, thereby decreasing the shelf life of the product,

decreasing the duration of local anaesthesia and, in the case of overheating, causing more

pain during injection.

Summary

Injecting local anaesthetics can become routine for dental practitioners because of the

high efficacy and wide safety margin of these products. Nonetheless, there are instances

when these drugs do not work or when they must be used with caution. This section has

attempted to highlight important issues about local anaesthetic use to aid practitioners in

making their local anaesthesia practice as effective and as safe as possible.

References

Akinosi JO. A new approach to the mandibular nerve block. Brit J Oral Surg1977-78;15:83-87.



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Aldous JA. Needle deflection: A factor in the administration of local anesthetics. JADA

1968;77(3):602-4.

Davidson M. Bevel-oriented mandibular injections: Needle deflection can be beneficial.

Gen Dent 1989;36(3):410-12.

Gow-Gates G. Mandibular conduction anesthesia: A new technique using extraoral

landmarks. Oral Surg 1973 Sept.

Hochman M, Friedman M. In vitro study of needle deflection: A linear insertion technique

versus a bi-directional rotation insertion technique. Quintessence Int 2000 Jan:33-39.

Kaufmann E, Weinstein P, Milgrom P. Difficulties in achieving local anesthesia. JADA 1984

108:205-8.

Roda R, Blanton P. The anatomy of local anesthesia. Quintessence Int 1994;25(1):27-38.

Vazirani S. Closed mouth mandibular nerve block: A new technique. Dent Digest 1960

66:10-13.

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