khoury et al-2011-australian dental journal

R E V I E WAustralian Dental Journal 2011; 56: 112–121

doi: 10.1111/j.1834-7819.2011.01312.x

Applied anatomy of the pterygomandibular space: improvingthe success of inferior alveolar nerve blocks

JN Khoury,* S Mihailidis,* M Ghabriel,� G Townsend*

*School of Dentistry, The University of Adelaide, South Australia.�Discipline of Anatomy and Pathology, School of Medical Sciences, The University of Adelaide, South Australia.

ABSTRACT

A thorough knowledge of the anatomy of the pterygomandibular space is essential for the successful administration of theinferior alveolar nerve block. In addition to the inferior alveolar and lingual nerves, other structures in this space are ofparticular significance for local anaesthesia, including the inferior alveolar vessels, the sphenomandibular ligament and theinterpterygoid fascia. These structures can all potentially have an impact on the effectiveness of local anaesthesia in this area.Greater understanding of the nature and extent of variation in intraoral landmarks and underlying structures should lead toimproved success rates, and provide safer and more effective anaesthesia. The direct technique for the inferior alveolar nerveblock is used frequently by most clinicians in Australia and this review evaluates its anatomical rationale and providespossible explanations for anaesthetic failures.

Keywords: Inferior alveolar nerve block, dental anaesthesia, mandibular nerve, sphenomandibular ligament, lingual nerve.

Abbreviations and acronyms: IAA = inferior alveolar artery; IAN = inferior alveolar nerve; IANB = inferior alveolar nerve block;IAV = inferior alveolar vein; LN = lingual nerve; PVP = pterygoid venous plexus.

(Accepted for publication 6 September 2010.)

INTRODUCTION

The inferior alveolar nerve block (IANB) is widely usedin dental clinical practice and, considering its impor-tance for mandibular anaesthesia, it is essential that theanatomical rationale for this technique is well under-stood. The relationships of structures in the pterygo-mandibular space have significant bearing on theeffectiveness of the IANB, as well as its safety. Failureof mandibular anaesthesia and associated safety con-cerns are common problems,1 with as many as 20% ofIANBs reported to result in ineffective anaesthesia.2 Ithas been suggested that many of these failures areassociated with vascular damage and ⁄ or variations inthe anatomical pattern of the relevant nerves andsurrounding fibrous tissue. This review examines pub-lished research concerning the location, size and overallrelationships of structures in the pterygomandibularspace, and highlights the need for clinicians to have athorough understanding of the relevant anatomy so thatIANBs can be delivered as safely and as effectively aspossible. It builds on the excellent description of theapplied anatomy of the pterygomandibular space byBarker and Davies,3 as well as a series of publishedpapers by Shields.4–6

Scope of the review

The literature selected for this review has been limitedto work published in English from the 20th centuryonwards. Standard anatomical textbooks as well askeyword searches using the online PubMed databasehave been used. PubMed search terms included mostanatomical terms relating to anatomy of the pterygo-mandibular space, as well as local mandibular anaes-thesia and its possible complications. Further relevantpapers were identified by examination of the referencelists of the useful articles found. The aims of this revieware to summarize and critically evaluate the existingliterature on what is currently known about thecontents and relationships of structures in the pterygo-mandibular space, including the inferior alveolar nerve(IAN), vein and artery and the sphenomandibularligament.

General anatomy of the pterygomandibular space

The pterygomandibular space is a small fascial-lined cleftcontaining mostly loose areolar tissue.5 It is boundedmedially and inferiorly by the medial pterygoid muscle7

and laterally by the medial surface of the mandibular

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Australian Dental JournalThe official journal of the Australian Dental Association

ramus. Posteriorly, parotid glandular tissue curvesmedially around the back of the mandibular ramusto form a posterior border, while anteriorly the bucci-nator and superior constrictor muscles come togetherto form a fibrous junction, the pterygomandibularraphe. Of particular importance to local anaesthesia,the pterygomandibular space contains the IAN,artery and vein, the lingual nerve (LN), the nerve tomylohyoid, the sphenomandibular ligament and fascia(Fig 1).

Direct technique for the inferior alveolar nerve blockand its anatomical rationale

Numerous techniques have been suggested to obtainmandibular anaesthesia. The direct approach, alsoknown as the direct thrust technique, remains one ofthe most commonly used.1 In addition to this tech-nique, other alternatives for anaesthetizing the IANinclude the indirect technique,8 the anterior injectiontechnique,9 the Gow-Gates method10 and the Akinosi-Vazirani closed-mouth block approach.11,12 This re-view will concentrate on the direct IANB, which is themost frequently used technique in many parts of theworld, including Australia.

The direct IANB technique involves the insertion of aneedle into the pterygomandibular space by piercing the

buccinator muscle. Once in the pterygomandibularspace, the aim of the technique is to deposit localanaesthetic solution at a level just superior to the tip ofthe lingula (Figs 1 and 2). Diffusion of local anaestheticsolution from the needle tip to the IAN anaesthetizes thenerve just prior to it entering the mandibular foramen.The lingual nerve lies medial and anterior to the IANand it can be anaesthetized during an IANB. This isachieved by withdrawing the needle and swinging thebarrel of the syringe toward the dental midline.

Several intraoral landmarks can be used to guide theclinician when administering an IANB. Firstly, whenthe mouth is wide open, the pterygotemporal depres-sion represents the injection site. It is situated betweenthe raised ridge of mucosa overlying the pterygoman-dibular raphe medially and the mucosa that overlies theanterior border of the mandibular ramus laterally.The intraoral landmark laterally is the ridge producedby the tendon of temporalis and the medial landmark isreferred to as the pterygomandibular fold (Fig 3). Thelevel at which the needle should reach the bone justsuperior to the lingula is indicated by the maximumconcavity on the anterior surface of the mandibularramus, an area known as the coronoid notch.1 Analternate guideline for determining the correct height of

Fig 1. Diagrammatic representation of a transverse section of theright mandibular ramus at the level at which an IANB would be given.(M = masseter; R = ramus; IAN = inferior alveolar nerve; IAV =

inferior alveolar vein; IAA = inferior alveolar artery; SML =sphenomandibular ligament; MP = medial pterygoid muscle; LN =lingual nerve; B = buccinator; PMR = pterygomandibular raphe;SCM= superior constrictor muscle; P = parotid gland; TT = tendonof temporalis; L = lingula). The needle is shown passing through thebuccinator muscle, B, and into the pterygomandibular space where itis directed to an area of bone just superior to the lingula, L. The IAN,

IAV and IAA are wrapped together by a fibrous sheath, in aneurovascular bundle, which occupies a spooned-out depression onthe medial surface of the ramus. The LN is located anterior and medial

to the IAN.

Fig 2. Photograph of a skull with simulated maximum opening of themouth. A string has been attached to indicate where the pterygo-

mandibular raphe would normally be located. This structure attachesto the pterygoid hamulus superiorly and descends to the inner aspectof the mandible near the most posterior molar. The pterygomandib-

ular fold refers to the fold of mucosal tissue that overlies thepterygomandibular raphe and the needle should always be insertedlateral to the fold. The barrel of the syringe usually needs to be

positioned over the contralateral premolars so that the needle tip cancontact bone just superior to the lingula at the appropriate depth ofneedle insertion, approximately 20–25 mm in adults. The thumb oranother finger can be used to palpate the coronoid notch, as seen in the

photograph, to assist in establishing the correct height of needleinsertion. (L = lingula; PMR = pterygomandibular raphe;

H = pterygoid hamulus; CN = coronoid notch.)

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Applied anatomy of the pterygomandibular space

entry for the IANB includes inserting the needleapproximately 1 cm above the lower occlusal planewhen the mouth is fully open.13 Other landmarksinclude locating a level midway between the upper andlower dental arches when the mouth is wide open andvisualizing the apex of the buccal pad of fibrous tissuethat forms an apex close to the pterygomandibularfold.3 The buccal pad is a submucosal fibrous bandseparating the buccinator muscle from the overlyingoral mucosa3 and it should not be confused with thebuccal pad of fat which is an area of adipose tissuebetween the buccinator muscle and masseter muscle.

The appropriate horizontal angulation of thesyringe to enable the needle to reach bone withoutdamaging nearby structures varies between individuals.The degree of ramal flaring, morphology of the internaloblique ridge, morphology of the lingula, dental archshape and alignment of teeth can influence horizontalneedle angulation. Generally, as a guide, the syringebarrel should be over the premolars on the contralateralside.5 This angulation can be modified if bone has notbeen contacted by the needle tip at an appropriateinsertion depth of around 20–25 mm.14 Once thecorrect needle position and angulation have beendetermined, the needle is then withdrawn one or twomillimetres and aspiration performed before injection.Figure 4 shows the appearance of key intraoral land-marks for an IANB in different individuals. In additionto intraoral landmarks, some authors have emphasizedthe importance of extraoral landmarks in evaluating

ideal needle placement and angulation, such as thedegree of ramal flaring and the height and width of themandibular ramus.5

Specific anatomical features of the pterygomandibularspace

Anatomical information regarding the general contentsand relationships of structures in the pterygomandibu-lar space is relatively consistent in the literature,providing a basic framework upon which the cliniciancan reflect when administering an IANB. However, thereporting of more specific details about the anatomy ofthe pterygomandibular space lacks consistency and canbe confusing due to varying terminology in texts andpublications. The following sections highlight theextent of variation in descriptions in the literature.

Medial surface of the mandibular ramus

The surface anatomy of the pterygomandibular spaceshows predictable patterns which can guide the clini-cian when administering IANBs. The medial surface ofthe mandibular ramus exhibits a number of relevantfeatures for determining the required depth of needleinsertion. As the inferior alveolar neurovascular bundleapproaches the mandibular foramen, it lies lateral tothe sphenomandibular ligament in the confines of aspooned-out depression on the medial aspect of theramus, referred to as the sulcus colli (Fig 5).3 Superi-orly, the sulcus colli begins as a shallow depression butit becomes progressively more pronounced as it travelsinferiorly until it eventually leads into the mandibularforamen. Just anterior to the sulcus colli, on the medialaspect of the ramus, is a crest of thickened bone(Fig 5).3

It has been suggested that the IAN lies along theanterior border of the sulcus colli for at least 10 mmabove the lingula.15 However, no research has beenpublished to verify this. If the nerve does descend viathis path, it may be partially protected from oncomingneedles by a crest of thickened bone which bulgesanteriorly in front of the nerve. Considering that theideal level of injection is just superior to the lingula, thiscrest of thickened bone is the structure that the needletip should contact before withdrawal and aspiration.This would allow for deposition of local anaesthetic inclose proximity to the IAN, yet ensuring the safety ofimportant structures from iatrogenic trauma. The IANis also guarded anteriorly by the lingula as it nears themandibular foramen (Fig 5). The lingula is a projectionof bone to which the sphenomandibular ligamentattaches and this structure can provide some protectionto the IAN from oncoming needles.4 In contrast, the LNis quite bare with no bony protection, exposing it to anincreased risk of direct contact during needle insertion

Fig 3. Intraoral photograph of the right side of the oral cavityshowing key anatomical landmarks observed when giving an IANB.The site for needle penetration is the pterygotemporal depression,which is outlined. The needle travels through the oral mucosa andunderlying buccinator muscle before entering the pterygomandibularspace. The height is at the level of the coronoid notch, the mostconcave region on the anterior border of the mandibular ramus.Approximate depth of needle penetration required in most adultpatients is about 20–25 mm. (CN = coronoid notch; PTD =pterygotemporal depression; PMF = pterygomandibular fold.)


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due to its anteromedial position to the IAN. It alsotends to be stretched when the mouth is wide open.These characteristics may explain why the LN is morelikely to experience neurosensory disturbances follow-ing an IANB than the IAN.

The ability to precisely position the needle close tothe IAN during an IANB hinges on a number of factorsand is generally difficult to evaluate while performingthe procedure. Variations in mandibular size andshape, relative position of the mandibular foramen tothe lingula and the required depth of soft tissuepenetration add to the uncertainty about whether theneedle is close enough to the IAN to achieve adequateanaesthesia.16

Fascial relationships

The pterygomandibular space is a cleft, lined at itsanterior, posterior, superior, inferior and medialboundaries by various fasciae.3 The medial wall of thespace is covered by the interpterygoid fascia (Fig 6)which lies on the lateral surface of the medial pterygoidmuscle.4 This fascia has a complex shape as it attachessuperiorly to the base of the skull and lines the medialsurface of the lateral pterygoid muscle, then descendsonto the medial surface of the ramus, attaching to it justsuperior to the insertion of the medial pterygoidmuscle.3 Posteriorly, the interpterygoid fascia bridgesthe gap between the two pterygoid muscles, involvingattachment of the fascia to the entire posterior borderof the mandibular ramus all the way up to the level of

Fig 5. Medial surface of the right mandibular ramus showing somelandmarks relevant to IANBs. A crest of thickened bone lies slightlysuperior to the lingula and it represents the area where needle contactshould be made on insertion, as it lies close to the inferior alveolarneurovascular bundle but minimizes the risk of damage to structuresin the bundle. Although needle contact with the lingula may producesatisfactory anaesthesia, it is likely that needle withdrawal after initialbony contact will cause local anaesthetic solution to be depositedmedial to the sphenomandibular ligament and, hence, reduce its

effectiveness. (CN = coronoid notch; Li = lingula; SC = sulcus colli;GNM = groove for nerve to mylohyoid; CB = crest of thickened

bone; MN = mandibular notch.)

Fig 4. Four representative intraoral photographs of the right side of the oral cavity showing the key intraoral landmarks observed and palpatedwhen administering an IANB. (CN = coronoid notch; PTD = pterygotemporal depression; PMF = pterygomandibular fold.) The dotted lineindicates the location of the PTD and the curved outline represents the level of the CN, which is the most concave area on the anterior border of the

ramus. CN can be palpated to assist in establishing correct height of needle penetration.



the condylar neck.15 This fascia, sometimes referred toas temporopterygoid fascia, becomes very thin anteri-orly and forms the anterior boundary of the pterygo-mandibular space by bridging the gap between theanterior border of the medial pterygoid muscle and thefascia overlying the tendinous insertions of the tempo-ralis muscle. All these fascial linings closely adapt to thestructures that create the borders of the pterygoman-dibular space (i.e. medial pterygoid muscle, parotidgland). Their presence has been recognized as apotential barrier to diffusion of local anaestheticsolution that is deposited outside this pouch-likenetwork, thus increasing the probability of inadequateanaesthesia.3,9,17

The structure and attachments of fascia in thepterygomandibular space have been reported in numer-ous publications but no methodology or samplingcharacteristics have been provided to indicate howsuch descriptions were generated. Hence, the true

nature and structure of fascia within the regionrepresents a gap in current anatomical knowledge.

There is a very close relationship between thesphenomandibular and stylomandibular ligaments andthe adjacent interpterygoid fascia, leading some tosuggest that the former may represent localized thick-enings of the latter.3 Others have observed how thesphenomandibular ligament can be separated in bluntdissection from the adjacent fascia,18 leading them toconsider that they are separate structures, with theinterpterygoid fascia forming an intervening layerbetween the sphenomandibular ligament and themedial pterygoid muscle. To date, no histologicalevaluation of these tissues has been published toprecisely specify the nature of this relationship.

Anatomy of the sphenomandibular ligament

The sphenomandibular ligament is a band of fibroustissue that connects the lingula on the mandible to thespine of sphenoid on the skull base (Fig 7). The shape,length, thickness and nature of attachment of thisligament varies considerably between individuals. Gargand Townsend18 dissected seven cadavers and found

Fig 6. Transverse section of the right mandibular ramus at the level ofthe lingula showing the IAN located just behind the tip of the lingula,anterior to the veins and artery. The thickening of the fibrous tissuemedial to the neurovascular bundle represents the sphenomandibular

ligament. During an IANB, the ideal position to deposit localanaesthetic solution is just above the tip of the lingula, as it allows the

needle tip to be in close proximity to the nerve, without directlycontacting it and risking damage. (SML = sphenomandibular

ligament; IAN = inferior alveolar nerve; IAA = inferior alveolarartery; IAV = inferior alveolar vein; L = lingula; IPF = interp-

terygoid fascia.)

Fig 7. Photograph of the pterygomandibular space on the left sidefrom a medial view. The medial pterygoid muscle and tongue have

been removed to expose the fibrous tissue that forms thesphenomandibular ligament and associated fascia. A needle has beeninserted through the buccinator muscle and into the pterygomandib-

ular space to indicate where an IANB would be administered.Note that the mouth is closed, which would not be the case when

a direct IANB is given to a patient. (LN = lingual nerve; P = palate;PH = pterygoid hamulus; LPP = lateral pterygoid plate; SML =sphenomandibular ligament; NM = nerve to mylohyoid; R = ramus;

36 = lower left first molar.)


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that the sphenomandibular ligament ranged in shapefrom a thin band that descended for a short distancefrom the spine of the sphenoid to a broad bi-concaveligament with prominent insertions. Similarly, Shiozakiet al.19 observed considerable variation in 40 Japanesecadavers, with some sphenomandibular ligamentsattaching to the medial aspect of the mandibular ramusanterior and posterior to the lingula, in addition to theirdirect attachment to this structure.

Due to its density and shape, the sphenomandibularligament has the potential to impede diffusion of localanaesthetic solution to the IAN if the tip of the needle isplaced too far medially in relation to the ligament.3

In vivo diffusion studies involving radiographic analysisof local anaesthetics mixed with contrasting mediumhave found that local anaesthetic solution diffuseseasily through the loose connective tissue of thepterygomandibular space if it is introduced directlyinto the space.9,17 However, deposition of local anaes-thetic in a location where it is separated from the IANby the sphenomandibular ligament or other fibroustissue may impede diffusion. The direct IANB techniquehas been illustrated and described as involving insertionof the needle until it comes into contact with thelingula. Some anatomical studies have found caseswhere the ligament attaches to the superior border ofthe lingula.18 This may increase the possibility that theneedle tip could arrive at a position that is medial to theligament, especially if bony contact of the needle tip isat, medial or inferior to the apex of the lingula. In suchcases, diffusion of local anaesthetic would need to occurthrough the ligament or around it to produce its desiredeffect. To avoid this, it is recommended that the level ofneedle contact with bone should be slightly superior tothe lingula.

Accessory innervation from the nerve to mylohyoid

The nerve to mylohyoid is primarily motor in nature,but it may contain a sensory component that innervatesmandibular teeth20–23 which may be relevant whenattempting an IANB. As the posterior division of themandibular nerve descends and approaches the man-dibular foramen, it gives off the nerve to mylohyoidwhich often follows an antero-inferior course on themedial aspect of the mandibular ramus.23 In somecases, however, part of the course of this nerve mayinvolve an intra-osseous component.24,25 Anatomicalvariabilities such as this, or variation in the height atwhich the nerve to mylohyoid branches off the IAN,may ultimately influence whether this nerve is anaes-thetized during an IANB. This is relevant for localanaesthesia as the nerve to mylohyoid can provideaccessory innervation to mandibular teeth.20–23,26 It hasalso been reported to innervate the chin and tip of thetongue in some individuals.27 Bennett and Townsend28

when analysing six human cadavers reported that theaverage distance between the mandibular foramen andthe branching point of the nerve to mylohyoid was13.4 mm, ranging from 3.9 to 27.0 mm, while Wilsonet al.22 reported after observing 37 human cadavers anaverage branching distance of 14.7 mm, ranging from5.0 to 23.0 mm above the mandibular foramen.The dental relevance of these observations is that thegreater the distance between the point at whichthe nerve to mylohyoid branches off the IAN and thelocation where the local anaesthetic solution is depos-ited, the greater the likelihood that the nerve tomylohyoid may not be fully anaesthetized, leading topotential failure in achieving anaesthesia.

In addition to the height of the branching point, theremay be physical barriers that separate the nerve tomylohyoid from the area where local anaestheticsolution is deposited during an IANB. The nerve tomylohyoid travels behind the sphenomandibular liga-ment at its attachment to the lingula.18 Consequently,the density and shape of this structure may preventeffective diffusion of local anaesthetic during an IANB.Similarly, if part of the course of the nerve tomylohyoid is encompassed by bone, which has beenreported in the literature, then this will also act as apotential barrier.29

Relationship of structures within the inferior alveolarneurovascular bundle

Typically, major nerves and their branches are accom-panied by an artery and vein. This is also true for thenerves within the pterygomandibular space, such as theIAN.30 Anatomical descriptions of the pterygomandib-ular space have been published but accounts oftenneglect to mention how the IAN and associated bloodvessels are arranged within their neurovascular bundle.Of the few descriptions reported, a number of patternshave been identified, but they lack consistency and insome cases are directly conflicting. These reports alsodo not provide a standardized height in the supero-inferior plane at which these structural relationshipswere analysed, leading to possible variations in thedescriptions as the IAN, inferior alveolar artery (IAA)and inferior alveolar vein (IAV) arise from differentregions within the infratemporal fossa before converg-ing inferiorly to form a neurovascular bundle.

The presence of an IAN, IAA and IAV are notdisputed, providing an important and essential neuro-vascular supply to the mandibular teeth. The IAA arisesfrom the maxillary artery which branches off theexternal carotid artery in the vicinity of the mandibularcondylar neck.3 As it travels inferiorly, it assumes apath close to the IAN. The degree to which the IAAtransverses the pterygomandibular space from its originto its eventual path alongside the IAN depends on



whether the maxillary artery follows a path that issuperficial or deep to the lateral pterygoid muscle.Independent of this, the IAV exits the mandibularforamen, acting as a tributary to the pterygoid venousplexus (PVP) which is closely associated with the lateralpterygoid muscle. The specifics of exactly how each ofthese structures (IAN, IAA and IAV) interact togetheralong their path toward the mandibular foramen havenot been described clearly. Barker and Davies3 sug-gested that the IAN is relatively anterior while theinferior alveolar vasculature is more posterior, with theIAV being closest to the bone. Their explanation forthis arrangement relates to the path taken by thesestructures from their origin superiorly to the mandib-ular foramen inferiorly. For example, the IAN andlingual nerves separate from each other on the deepsurface of the lateral pterygoid muscle where they eachenter the pterygomandibular space along the lateralsurface of the medial pterygoid muscle, and this isrelatively more anterior than where the IAV feeds intothe PVP.3 Similarly, Sicher and Dubrul8 and Murphyand Grundy14 reported that the inferior alveolarvasculature was generally placed more lateroposteriorlyand closer to the bone than the nerve, which wasalways located more anteriorly. However, it is impor-tant to note that neither of these publications provideinformation on sampling methods or sample size.

There are numerous other reports that agree with theobservations of Murphy and Grundy,14 Barker andDavies,3 and Sicher and Dubrul.8 However, when mostauthors make reference to or illustrate the relationshipsof the IAN, IAA and IAV, the inferior alveolar vesselsare coupled together.7,13,15,20,30–32 In each of theseexamples, the IAN is always represented as beinganterior to the blood vessels. Hence, while thesedescriptions may be consistent with earlier reports,they are less specific and provide no details about howsuch information was obtained.

In contrast to the preceding reports, there have beenother descriptions of different relationships between theIAN, IAA and IAV. For example, Wadu et al.33

suggested that the course of the IAN was closer to themandible, with the artery and vein being placed moremedially. Cousins and Bridenbaugh34 similarly sug-gested that the IAN was closer to the mandible andlateral to the IAA and IAV. Another variation in thedescription of this relationship was an observation byMalamed1 that the IAA was positioned more anteriorlycompared with the IAN. Roda and Blanton,35 thoughmaintaining that the IAA and IAV are very close to thebone when compared to the IAN, reported a number ofpossible relationships with their respective frequencies.Although no descriptions of methodology or samplingcharacteristics are provided, their review article sug-gested that the IAN was anterior to the blood vessels in70% of cases while in 20% of cases, the IAN was

medial to the blood vessels. The blood vessels wereanterior to the IAN in 10% of cases. A more recentstudy involving 56 specimens has demonstrated similarfindings, with the inferior alveolar blood vessels tendingto be posterior, posterolateral or posteromedial to theIAN in most cases.36 Figure 6 shows an example of atypical arrangement of the IAN and associated vessels.

Potential anatomical causes for failure of anaesthesia

Anaesthetic failures occur frequently with IANBs, evenwith experienced clinicians. There are many reasonswhy this may occur. The two major factors being pooroperator technique and anatomical variation.16 Otherpotential reasons for anaesthetic failure include psy-chological issues where patient fears and anxieties leadto either exaggerated or imagined pain and discomfort,or where acute localized infections within the pterygo-mandibular space or distal branches of the IAN reducethe effectiveness of local anaesthetic.37

Apart from the nerve to mylohyoid, other nerves mayalso provide accessory innervation to mandibular teeth,potentially leading to failure of anaesthesia. Barker andLockett38 observed canals in the rami of mandibleswhich led to the apices of lower posterior molars,particularly third molars. Ossenberg39 suggested thatsensory nerves, most likely branches of the long buccalnerve, may travel through many of these retromolarforamina. As the long buccal nerve arises from theanterior division of the mandibular nerve, direct IANBswill not anaesthetize these branches. In these situations,a Gow-Gates block may be used as local anaesthetic isdeposited in a much higher location within thepterygomandibular space, where anaesthesia of theIAN, lingual nerve and buccal nerves can be obtainedwith a single injection.1 Tong40 has also reported a caseof a patient who presented for removal of an impactedlower molar in whom the great auricular nerve, abranch of the cervical plexus, appeared to provideadditional innervation to the region around the angle ofthe mandible.

Bifid mandibular canals have the potential to increasethe difficulty of achieving adequate anaesthesia usingthe IANB technique.1,16 Embryologically, the develop-ment of mandibular bone through intramembranousossification occurs around the IAN. Consequently,alterations in the anatomy of this nerve and ⁄ or itscommunications with other nerves will be reflected inmandibular bony development.16 The prevalence of thisanatomical variation varies between 0.35%41 to almost1% of the population.42 Usually diagnosed by apanoramic radiograph, there are a number of differentpatterns that may present. The type suggested to be themost problematic for IANBs is where there are twoindependent mandibular foramina with a portion ofthe IAN entering both simultaneously.1 This form of


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variation is also known as a Type 4 bifid canalaccording to the classification outlined by Langlaiset al.42

Mandibular prognathism is another anatomical var-iation that can complicate IANBs. Prognathic mandiblesgenerally have a lingula that is positioned higher thanthe coronoid notch, making it more difficult for theoperator to insert the needle at the correct height.15 Thedifference in height between the lingula and coronoidnotch may be as much as 1 cm. In these cases, needleinsertion above normal is indicated.

The effects of needle deflection during insertion intothe pterygomandibular space have been suggested tolead to reduced effectiveness of IANBs.43 The degree towhich a needle deflects relates to the density of themedium through which it is inserted, the gauge ofthe needle44 and the nature and degree of taper of theneedle’s bevel.1 Many studies to date have beenconducted to evaluate these effects in an attemptto determine whether they are clinically significant.In vitro studies have shown that needles have atendency to deflect toward the non-bevelled side duringinsertion into media of homogenous density.44–46 Thishas led some to suggest that the bevel should beorientated away from the ramus to guide the needletoward the bone on insertion, thus reducing thelikelihood of over-insertion of the needle.45 However,in vivo research has found no significant differencesbetween the effectiveness of direct IANBs when admin-istered with the bevel away from the ramus comparedwith the bevel toward the bone.47 Anatomically, thedensity of the tissue within the pterygomandibularspace is mostly loose areolar tissue, which lacks densefibrous elements.5 Hence, if an IANB is executedcorrectly, it is likely that needle deflection would beminimal, especially with needles of larger diameter.

More recently, a new technique of needle insertionhas been suggested which involves rotating the needlewhile it is inserted.48 This is in an attempt to negate anypotential needle deflection by preventing the needle’sbevel from being on any particular side for the durationof needle insertion.48 In vitro research has indicatedthat this method can reduce deflection.43 However, anin vivo study has not shown this technique to beclinically superior with respect to the level of anaesthe-sia attained in individuals with irreversible pulpitis.49

Further research is required to more accurately assesswhether this technique has clinical advantages.

Failure of anaesthesia can prove challenging for theclinician to understand. If an IANB has failed, it isessential that the operator carefully evaluates his ⁄ hertechnique as well as common anatomical variations todetermine what may have contributed to the problem.If the cause(s) are not accurately identified, this maylead to multiple IANBs that continue to fail. Not onlydoes this damage more tissue than necessary, placing

the patient at increased risk of trismus, but it mayreduce patient confidence in the operator’s abilities andreinforce negative stereotypes of oral health profession-als.

Research methods and their relative usefulness

Gross dissection has been the most common method ofexamining the pterygomandibular space and it providesarguably the most useful insights into how soft tissuestructures relate to the osteology of the skull in threedimensions. Anatomical studies of the sphenomandib-ular ligament and relationships of the IAN to the IAAand IAV(s) are often conducted in this way. Advantagesof gross dissection are that it allows for qualitativeanalysis of how structures relate to each other as theytravel supero-inferiorly, anteroposteriorly and medio-laterally. Clear weaknesses of this approach are that itdisturbs superficial structures in the area of interest, itmay distort the exact relationships of nerves and theirrelated blood vessels, it cannot be performed on livingsubjects, and it does not lend itself to quantitativeanalysis.

Transverse sectioning of anatomical material can alsobe performed and provide useful data. Such transversesections can be viewed macroscopically or prepared forhistological interpretation. Advantages of this approachare that it does not disturb the anatomical patterns in atransverse plane, thus making it ideal for analysing therelationships of the IAN to the IAA and IAV. Similarly,if histological sections are prepared, they provide muchgreater detail regarding the structures depicted, such asthe number of IAVs and the number of IAN fasciculesand the nature of connective tissues. Also, quantitativeanalyses can be performed when anatomical material isprepared using this method, such as determining precisedistances between specific structures. A disadvantage isthat this method does not provide a three-dimensionalview of structures.

Osteological features of the mandible have beenstudied on numerous occasions using both qualitativeand quantitative approaches in various populations.Although there are obvious limitations in what can beextrapolated from osteological research, these studiesare powerful and involve large sample sizes, in somecases over 300 specimens.13 As IANBs require recog-nition of bony landmarks as part of the execution of thetechnique, osteological studies can provide useful dataregarding how mandibular anaesthesia could be mademore effective. They also provide insights into whyIANBs may fail sometimes, such as due to nervestravelling in accessory foramina.

Radiographic and computerized tomographic (CT)methods have also been used to analyse the pterygo-mandibular space.9,17 Radiographs involve a two-dimensional representation of three-dimensional



structures, thus making them useful in identifying bonyrelationships in a plane that is perpendicular to theX-ray source. Panoramic radiographs have been used toidentify bifid inferior alveolar canals and these studiesare extensive, involving retrospective analysis ofthousands of radiographs.41 Radiographs have alsobeen used to follow the diffusion patterns of localanaesthetic mixed with radiopaque contrasting mediawhen administered as IANBs.17 More recently, CTimaging has been used to analyse the dynamics of localanaesthetic diffusion.9 This research has includedacceptable sample sizes but more of these studies areneeded to add to what is currently known aboutdiffusion paths in the pterygomandibular space.

SUMMARY AND CLINICAL TIPS

Just as an understanding of the basic anatomy of thepterygomandibular space promotes safe and effectiveanaesthesia, improved knowledge about less exploredregions and relationships should make the administra-tion of IANBs even safer and more effective. Consid-ering that this is the main technique for achievingmandibular anaesthesia in many parts of the world, it isessential that clinicians are familiar with the relevantanatomy and understand how anatomical variationscan lead to anaesthetic failures.

Based on this review of the anatomy of the pterygo-mandibular space, three key points underpin the basisof a successful IANB technique: (1) the rationale of thedirect IANB is based on being able to reliably positionthe needle tip above the tip of the lingula by payingattention to the entry point, the level of injection andthe angulation of the syringe. The entry point is thepterygotemporal depression located between thepterygomandibular fold medially and the coronoidnotch laterally. Placing a cotton bud onto the pterygo-temporal depression can assist in defining this structureas the tissue in this area is less dense than the structureson either side; (2) the level of injection can be gaugedby palpating the coronoid notch, and also keeping theneedle parallel to and about 1 cm above the lowerocclusal plane. The syringe should be angulated overthe premolar teeth on the contralateral side, butangulation will vary from patient to patient accordingto various anatomical factors; and (3) bone shouldalways be contacted with a direct IANB at theappropriate depth of approximately 20–25 mm. Fol-lowing this, the needle should be withdrawn 1–2 mmand aspiration performed prior to injection.

ACKNOWLEDGEMENTS

This review has arisen from research funded by theAustralian Dental Research Foundation. The assistanceof the Discipline of Anatomy and Pathology, The

University of Adelaide, and Victor Marino, School ofDentistry, The University of Adelaide, is greatly appre-ciated. The dissection shown in Fig 7 was performed byDr Zac Morse.

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Address for correspondence:Professor Grant C Townsend

School of DentistryThe University of Adelaide

Adelaide SA 5005Email: [email protected]



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