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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.
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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.
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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.
Advantages Disadvantages
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
The landmarks for this injection are as follows:
10 mm above the coronoid notch
the internal oblique ridge
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.
11. Inject a full cartridge.
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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Advantages Disadvantages
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 landmarks for this injection are as follows:
the maxillary buccal mucogingival line or root apices of the maxillary teeth
the coronoid notch
the internal oblique ridge
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.
9. Inject a full cartridge.
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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.
Advantages Disadvantages
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.
Advantages Disadvantages
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.