Oral surgery

Local anesthesia

Oral surgery

Local anesthesia

Terminology

• Anesthesia =• Analgesia =

• Local anesthesia• General anesthesia• Sedation == general analgesia

• Paraesthesia

• Pain threshold

Local anesthesia

• Methods for inducing local anesthesia:• Mechanical trauma• Low temperature• Anoxia• Chemical irritant…….euginol• Neurolytic agent ……….alcohol• Chemical agent

Ideal properties of a local anesthetic agent

Mandatory• Potent and

reliable• Reversible• Minimal

toxicity(MOS)• Rapid onset• Acceptable

duration• Non irritating• Stable in solution• Easily

metabolized

Preferable• Minimal

allergisity• Adequate shelf

life• Surface

anesthesia• Easy to sterile

Indications of local anesthesia

• Diagnosis• Reduce or eliminate pain

during dental treatment

Contraindications of local anesthesia

Systemic• Uncooperative

patient like child • Uncontrolled

hemorrhagic patient

• Allergic patient to local anesthesia

Local• Patient received

radiotherapy• Acute infection at

injection site • Vascular

abnormality at operation site

Contraindications of local anesthesia

RelativeAdvanced liver diseaseThyrotoxic goiter Uncontrolled cardiovascular

disease

Advantages of local anesthesia

Simple technique Minimal

equipment Transportable Minimal

contraindication Hemorrhage

could be controlled by vasoconstrictor

No airway impairment

Minimal postoperative care

No need for anesthetist

Duration could be controlled

Co-operative patient simplify the work

Disadvantages of local anesthesia

• Difficult to achieve co-operation• Mechanical obstruction by large

tongue or limited mouth opening• Failure due to anatomical variation or

incomplete anesthesia• Prolonged parasthesia• Spread of acute infection

Mode of action of local anesthesia

• Prevention of generation and or conduction of nerve impulse

• Act as chemical roadblock between source of impulse and brain

Sensory neuron

Afferent neuron

Mantle fiber

core fiber

Structure of nerve fiber

Nerve bundle of different fibers separated from other bundles by dens fibrous tissue

Generation of nerve impulse

Resting membrane potential (RMP)

(-70) mv due to impermeability of membrane to sodium ion

Concentration of sodium ion is more in the extra cellular area

- 70 mv

--70+35

Na++

K+

Generation of action potential

Intracellular axoplasm

Extra cellular

Potassium efflux

Repolarization

0.7 second

Sodium influx

Depolarization

0.3 second

Threshold level (firing level)

Impulse propagationtransmittion of nerve impulse

along the neuron

• Movement of the impulse along the axon:

Saltatory movement (jumping)Creep conduction

Mode of action of local anesthesia

Local anesthetics interfere with propagation of the action potential by blocking the increase in sodium permeability during depolarization.

Depolarization inhibitedFiring level not reached Action potential inhibited

Theories of action of local anesthesia

Acetylcholine theoryCalcium displacement theorySurface charge theoryMembrane expansion theorySpecific receptor theory

Nerve Blockade Theories

Membrane expansion theoryAgent molecules must be lipid

soluble Membrane is “disordered”Channel changes occurBenzocaine as example for this

theory

Extra cellular

Intracellular axoplasm Membrane expansion theory

Nerve Blockade Theories

Specific Receptor Theory–Anesthetic agent receptor in channel

–Accessed from intracellular side

Intracellular axoplasm

Extra cellular

Specific receptor theory

H W

• Type of nerve fiber and their characteristic feature

• Acupuncture as anesthesia in dentistry

Structure of local anesthesia

Ester:

Amide:

Example:

Exception: Benzocaine, which lacks a substituted amino group

R —COO—R —N

R —NHCO—R —N

1 2R

R3

4

21R

R3

4

H N— —COO—(CH ) —N2 2 2

C H2 5

C H2 5

R — Lipophilic aromatic residue.

R — Aliphatic intermediate connector.

R , R — Alkyl groups, occasionally H. Constitute with N the hydrophilic terminus.

1

2

3 4

PH and local anesthesiaDissociation constant PKa:

• pH at which 50% of drug present in free base form and 50% in cationic form (water soluble)

• Most local anesthetic Pka(7-9)How can local anesthesia cross nerve membrane?

• Local anesthetic solution prepared as weak acid form at 4-5 pH to prevent precipitation of free base in neutral solution, thus it present as cataionic form that must converted to base form to be lipid soluble and cross cell membrane

Buffering capacity

• Local anesthetic agent after injection and with function of plasma buffering will dissociate into free base form and cataionic form

• Free base form cross the cell membrane

• After crossing the membrane an other dissociation occur and cataionc form resulted will bind the receptor

Onset (induction time)

Time from injection of local anesthesia to the sign of adequate surgical anesthesia achievedFactors affect onset (induction time)

1. Concentration2. pH3. PKa4. Anatomical barrier5. Lipid solubility

Mixing pen

Recovery

• Time from early sign appeared to the complete loss of all effects of drug occur

• This results from reduced concentration of drug with no binding with the receptors

Recovery time

Concentration gradient depleted by:• Dilution by interstitial fluid• Action of capillary and lymph• Absorption by other tissue• Hydrolysis of ester

Extraanuronal

Diffusion from intranuronal tissue (mantle fiber)

Duration

• Time from induction to complete recovery from local anesthesia

• It depend on:

1. Protein binding

2. Vasoactivity

3. concentration

4. Vascularity of the site

Recovery slower than induction ?

Protein binding capacity

Principle of reinjection

Profound anesthesia:

• Increase concentration gradient to mantle fiber then to the core fiber

Failure ??1. Edema

2. Localized hemorrhage

3. Clot formation

4. Reduced pH (poor buffer capacity)

5. hypernatremia

After reinjection in prolonged procedure 2 situation may occur

Progression of local anestheticfunction

• Dull pain• Temprature • Sharp pain• Touch • Deep pressure• Proprioception• Motor function

H. W.

• Why infected area presents a poor site for action of local anesthesia