LOCAL ANAESTHETIC TECHNIQUE

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. 

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 the maxilla, 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 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 nerve is 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.




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. The lingual 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.


One of dentistry's most difficult challenges is consistently anaesthetizing 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 advantages and disadvantages for this technique are listed in the table below.

Advantages	Disadvantages
Practitioner acceptance Faster onset than higher blocks Bony landmark	Area of injection is vascular; 10 -15% chance of positive aspiration Unlikely to anaesthetize accessory nerves Unlikely to anaesthetize 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.
   
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.
   
   
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 anaesthetized.
  

The Gow-Gates Mandibular Block

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

Advantages	Disadvantages
Perceptible end point (bone) Fewer blood vessels at this level, therefore less chance of positive aspiration Long buccal nerve anaesthesia likely Possible longer duration of anaesthesia Less chance of anaesthetizing accessory nerves	Mouth wide open Must use extraoral landmarks, which may increase the difficulty of this procedure

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 

Ask the patient to open his or her mouth wide.



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



Move the finger or thumb superiorly approximately 10 mm.



Rotate the finger or thumb to parallel an imaginary line from the ipsilateral corner of the mouth to the tragal notch of the ear. 



Insert the needle at a point between the palpating fingernail and the pterygomandibular raphe at the middle aspect of the fingernail.



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



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.


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.)


Once bone is contacted, withdraw the needle tip 1 mm to prevent injecting into the periosteum, which would be painful.


Aspirate.


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 anaesthetized.
  
  

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.

Advantages	Disadvantages
Can be used for patients with trismus Can be used for patients with a strong gag reflex Mouth is closed, so injection may be less threatening to patient Possibly less pain, because tissues are relaxed Good for macroglossic patients	Difficult to visualize depth of injection Difficult in patients with widely flaring ramus Difficult in patients with pronounced zygomatic ridge or internal oblique ridge

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 15^o to 20^o. 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 millimeters) 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.
   
   
   
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.
   
   
   
   
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 visualizing 7 mm of needle remaining outside the tissue (if using a long needle).
   
   
   
   
   
8. Aspirate.
   
   
   
9. Inject a full cartridge.
   
   

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 No soft tissue (lip or tongue) anaesthesia Can operate bilaterally in the mandible Can anaesthetize a "hot" tooth Good approach for accessory innervation High success rate	Short duration of anaesthesia Must limit volume due to increased vascularity in the cancellous bone Difficult access to posterior mandible Anatomical limitations Some patients experience palpitations Cannot use in areas of periodontal disease

Technique

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

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 No soft tissue anaesthesia Works well for "hot" teeth Good approach for accessory innervation High success rate	Patient may experience post-operative pain There is a transient decrease in pulpal blood flow to the tooth Cannot be used in areas of periodontal disease Pressure is required to inject into the PDL space Multiple injections are required for multi-rooted teeth (one injection per root) May not work on long roots (e.g., cuspids)

Technique

1. Anaesthetize 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.
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 the jet 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 anaesthetize 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 pk_a - ph factors and tissue ph factors
  
• needle-to-jaw size discrepancy
  
• needle deflection
  
• volume factors
  
• skeletal and neuroanatomic variations
  
• local anaesthetic or vasoconstrictor degradation
  
• unco-operative patients
  

Local anaesthetic pK_a - 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 pK_a of the local anaesthetic, the pH of the local anaesthetic and the pH of the tissue in which the anaesthetic is being deposited. 

The pK_a 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 anaesthic 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 pK_a 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 pK_a values. The extreme example in this case is procaine (Novocain), which has a pK_avalue 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 pK_a of the local anaesthetic, the longer its onset of action due to the fewer lipid soluble particles initially available to cross the nerve sheath. More simply put, higher pK_a equates to decreased potency.

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 pK_a of the local anaesthetic; that is, it shifts the equilibrium toward 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 anaesthetize a "hot" tooth or when trying to anaesthetize an area of severe periodontal disease.

The local anaesthetic itself can cause another pH problem. Local anaesthetics with a vasoconstrictor contain  sodium metabisulphite. This  is quite acidic, 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 sodium metabisulphite 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. 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

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 a procedure 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 anaesthetize 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 sagittal direction. 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.


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 minimize the onset of anaesthesia.

A fifth factor to consider is the actual volume of the local anaesthetic. Some patients require more than one cartridge of local anaesthetic to anaesthetize 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 maximizes the volume and saturates the pterygomandibular 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 parallel to 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 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 the submandibular 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 mostly on the stability of the vasoconstrictor. For this reason, sodium metabisulphite is used as a preservative or stabilizer 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 maximize 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 utilized. 

Unco-operative 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 visualize the landmarks necessary to achieve 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, conscious sedation 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 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 sterilizing solution and the solution seeps into the cartridge, the sterilizing solution can cause a burning sensation upon injection. 

The likelihood of a burning sensation can be minimized by using fresh anaesthetics with little 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.