Australian Dental Journal

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1 Australian Dental Journal The official journal of the Australian Dental Association REVIEW Australian Dental Journal 2011; 56: doi: /j x Applied anatomy of the pterygomandibular space: improving the 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 the inferior alveolar nerve block. In addition to the inferior alveolar and lingual nerves, other structures in this space are of particular significance for local anaesthesia, including the inferior alveolar vessels, the sphenomandibular ligament and the interpterygoid 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 to improved success rates, and provide safer and more effective anaesthesia. The direct technique for the inferior alveolar nerve block is used frequently by most clinicians in Australia and this review evaluates its anatomical rationale and provides possible 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 used in dental clinical practice and, considering its importance for mandibular anaesthesia, it is essential that the anatomical rationale for this technique is well understood. The relationships of structures in the pterygomandibular space have significant bearing on the effectiveness of the IANB, as well as its safety. Failure of mandibular anaesthesia and associated safety concerns are common problems, 1 with as many as 20% of IANBs reported to result in ineffective anaesthesia. 2 It has been suggested that many of these failures are associated with vascular damage and or variations in the anatomical pattern of the relevant nerves and surrounding fibrous tissue. This review examines published research concerning the location, size and overall relationships of structures in the pterygomandibular space, and highlights the need for clinicians to have a thorough understanding of the relevant anatomy so that IANBs can be delivered as safely and as effectively as possible. It builds on the excellent description of the applied anatomy of the pterygomandibular space by Barker and Davies, 3 as well as a series of published papers by Shields. 4 6 Scope of the review The literature selected for this review has been limited to work published in English from the 20th century onwards. Standard anatomical textbooks as well as keyword searches using the online PubMed database have been used. PubMed search terms included most anatomical terms relating to anatomy of the pterygomandibular space, as well as local mandibular anaesthesia and its possible complications. Further relevant papers were identified by examination of the reference lists of the useful articles found. The aims of this review are to summarize and critically evaluate the existing literature on what is currently known about the contents and relationships of structures in the pterygomandibular space, including the inferior alveolar nerve (IAN), vein and artery and the sphenomandibular ligament. General anatomy of the pterygomandibular space The pterygomandibular space is a small fascial-lined cleft containing mostly loose areolar tissue. 5 It is bounded medially and inferiorly by the medial pterygoid muscle 7 and laterally by the medial surface of the mandibular 112 ª 2011 Australian Dental Association

2 Applied anatomy of the pterygomandibular space Fig 1. Diagrammatic representation of a transverse section of the right 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 = tendon of temporalis; L = lingula). The needle is shown passing through the buccinator muscle, B, and into the pterygomandibular space where it is 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 a neurovascular bundle, which occupies a spooned-out depression on the medial surface of the ramus. The LN is located anterior and medial to the IAN. buccinator muscle. Once in the pterygomandibular space, the aim of the technique is to deposit local anaesthetic solution at a level just superior to the tip of the lingula (Figs 1 and 2). Diffusion of local anaesthetic solution from the needle tip to the IAN anaesthetizes the nerve just prior to it entering the mandibular foramen. The lingual nerve lies medial and anterior to the IAN and it can be anaesthetized during an IANB. This is achieved by withdrawing the needle and swinging the barrel of the syringe toward the dental midline. Several intraoral landmarks can be used to guide the clinician when administering an IANB. Firstly, when the mouth is wide open, the pterygotemporal depression represents the injection site. It is situated between the raised ridge of mucosa overlying the pterygomandibular raphe medially and the mucosa that overlies the anterior border of the mandibular ramus laterally. The intraoral landmark laterally is the ridge produced by the tendon of temporalis and the medial landmark is referred to as the pterygomandibular fold (Fig 3). The level at which the needle should reach the bone just superior to the lingula is indicated by the maximum concavity on the anterior surface of the mandibular ramus, an area known as the coronoid notch. 1 An alternate guideline for determining the correct height of ramus. Posteriorly, parotid glandular tissue curves medially around the back of the mandibular ramus to form a posterior border, while anteriorly the buccinator and superior constrictor muscles come together to form a fibrous junction, the pterygomandibular raphe. Of particular importance to local anaesthesia, the pterygomandibular space contains the IAN, artery and vein, the lingual nerve (LN), the nerve to mylohyoid, the sphenomandibular ligament and fascia (Fig 1). Direct technique for the inferior alveolar nerve block and its anatomical rationale Numerous techniques have been suggested to obtain mandibular anaesthesia. The direct approach, also known as the direct thrust technique, remains one of the most commonly used. 1 In addition to this technique, other alternatives for anaesthetizing the IAN include the indirect technique, 8 the anterior injection technique, 9 the Gow-Gates method 10 and the Akinosi- Vazirani closed-mouth block approach. 11,12 This review will concentrate on the direct IANB, which is the most frequently used technique in many parts of the world, including Australia. The direct IANB technique involves the insertion of a needle into the pterygomandibular space by piercing the Fig 2. Photograph of a skull with simulated maximum opening of the mouth. A string has been attached to indicate where the pterygomandibular raphe would normally be located. This structure attaches to the pterygoid hamulus superiorly and descends to the inner aspect of the mandible near the most posterior molar. The pterygomandibular fold refers to the fold of mucosal tissue that overlies the pterygomandibular raphe and the needle should always be inserted lateral to the fold. The barrel of the syringe usually needs to be positioned over the contralateral premolars so that the needle tip can contact bone just superior to the lingula at the appropriate depth of needle insertion, approximately mm in adults. The thumb or another finger can be used to palpate the coronoid notch, as seen in the photograph, to assist in establishing the correct height of needle insertion. (L = lingula; PMR = pterygomandibular raphe; H = pterygoid hamulus; CN = coronoid notch.) ª 2011 Australian Dental Association 113

3 JN Khoury et al. ideal needle placement and angulation, such as the degree of ramal flaring and the height and width of the mandibular ramus. 5 Fig 3. Intraoral photograph of the right side of the oral cavity showing 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 and underlying buccinator muscle before entering the pterygomandibular space. The height is at the level of the coronoid notch, the most concave region on the anterior border of the mandibular ramus. Approximate depth of needle penetration required in most adult patients is about mm. (CN = coronoid notch; PTD = pterygotemporal depression; PMF = pterygomandibular fold.) entry for the IANB includes inserting the needle approximately 1 cm above the lower occlusal plane when the mouth is fully open. 13 Other landmarks include locating a level midway between the upper and lower dental arches when the mouth is wide open and visualizing the apex of the buccal pad of fibrous tissue that forms an apex close to the pterygomandibular fold. 3 The buccal pad is a submucosal fibrous band separating the buccinator muscle from the overlying oral mucosa 3 and it should not be confused with the buccal pad of fat which is an area of adipose tissue between the buccinator muscle and masseter muscle. The appropriate horizontal angulation of the syringe to enable the needle to reach bone without damaging nearby structures varies between individuals. The degree of ramal flaring, morphology of the internal oblique ridge, morphology of the lingula, dental arch shape and alignment of teeth can influence horizontal needle angulation. Generally, as a guide, the syringe barrel should be over the premolars on the contralateral side. 5 This angulation can be modified if bone has not been contacted by the needle tip at an appropriate insertion depth of around mm. 14 Once the correct needle position and angulation have been determined, the needle is then withdrawn one or two millimetres and aspiration performed before injection. Figure 4 shows the appearance of key intraoral landmarks for an IANB in different individuals. In addition to intraoral landmarks, some authors have emphasized the importance of extraoral landmarks in evaluating Specific anatomical features of the pterygomandibular space Anatomical information regarding the general contents and relationships of structures in the pterygomandibular space is relatively consistent in the literature, providing a basic framework upon which the clinician can reflect when administering an IANB. However, the reporting of more specific details about the anatomy of the pterygomandibular space lacks consistency and can be confusing due to varying terminology in texts and publications. The following sections highlight the extent of variation in descriptions in the literature. Medial surface of the mandibular ramus The surface anatomy of the pterygomandibular space shows predictable patterns which can guide the clinician when administering IANBs. The medial surface of the mandibular ramus exhibits a number of relevant features for determining the required depth of needle insertion. As the inferior alveolar neurovascular bundle approaches the mandibular foramen, it lies lateral to the sphenomandibular ligament in the confines of a spooned-out depression on the medial aspect of the ramus, referred to as the sulcus colli (Fig 5). 3 Superiorly, the sulcus colli begins as a shallow depression but it becomes progressively more pronounced as it travels inferiorly until it eventually leads into the mandibular foramen. Just anterior to the sulcus colli, on the medial aspect of the ramus, is a crest of thickened bone (Fig 5). 3 It has been suggested that the IAN lies along the anterior border of the sulcus colli for at least 10 mm above the lingula. 15 However, no research has been published to verify this. If the nerve does descend via this path, it may be partially protected from oncoming needles by a crest of thickened bone which bulges anteriorly in front of the nerve. Considering that the ideal level of injection is just superior to the lingula, this crest of thickened bone is the structure that the needle tip should contact before withdrawal and aspiration. This would allow for deposition of local anaesthetic in close proximity to the IAN, yet ensuring the safety of important structures from iatrogenic trauma. The IAN is also guarded anteriorly by the lingula as it nears the mandibular foramen (Fig 5). The lingula is a projection of bone to which the sphenomandibular ligament attaches and this structure can provide some protection to the IAN from oncoming needles. 4 In contrast, the LN is quite bare with no bony protection, exposing it to an increased risk of direct contact during needle insertion 114 ª 2011 Australian Dental Association

4 Applied anatomy of the pterygomandibular space Fig 4. Four representative intraoral photographs of the right side of the oral cavity showing the key intraoral landmarks observed and palpated when administering an IANB. (CN = coronoid notch; PTD = pterygotemporal depression; PMF = pterygomandibular fold.) The dotted line indicates 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. due to its anteromedial position to the IAN. It also tends to be stretched when the mouth is wide open. These characteristics may explain why the LN is more likely to experience neurosensory disturbances following an IANB than the IAN. The ability to precisely position the needle close to the IAN during an IANB hinges on a number of factors and is generally difficult to evaluate while performing the procedure. Variations in mandibular size and shape, relative position of the mandibular foramen to the lingula and the required depth of soft tissue penetration add to the uncertainty about whether the needle is close enough to the IAN to achieve adequate anaesthesia. 16 Fig 5. Medial surface of the right mandibular ramus showing some landmarks relevant to IANBs. A crest of thickened bone lies slightly superior to the lingula and it represents the area where needle contact should be made on insertion, as it lies close to the inferior alveolar neurovascular bundle but minimizes the risk of damage to structures in the bundle. Although needle contact with the lingula may produce satisfactory anaesthesia, it is likely that needle withdrawal after initial bony contact will cause local anaesthetic solution to be deposited medial 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.) Fascial relationships The pterygomandibular space is a cleft, lined at its anterior, posterior, superior, inferior and medial boundaries by various fasciae. 3 The medial wall of the space is covered by the interpterygoid fascia (Fig 6) which lies on the lateral surface of the medial pterygoid muscle. 4 This fascia has a complex shape as it attaches superiorly to the base of the skull and lines the medial surface of the lateral pterygoid muscle, then descends onto the medial surface of the ramus, attaching to it just superior to the insertion of the medial pterygoid muscle. 3 Posteriorly, the interpterygoid fascia bridges the gap between the two pterygoid muscles, involving attachment of the fascia to the entire posterior border of the mandibular ramus all the way up to the level of ª 2011 Australian Dental Association 115

5 JN Khoury et al. nature and structure of fascia within the region represents a gap in current anatomical knowledge. There is a very close relationship between the sphenomandibular and stylomandibular ligaments and the adjacent interpterygoid fascia, leading some to suggest that the former may represent localized thickenings of the latter. 3 Others have observed how the sphenomandibular ligament can be separated in blunt dissection from the adjacent fascia, 18 leading them to consider that they are separate structures, with the interpterygoid fascia forming an intervening layer between the sphenomandibular ligament and the medial pterygoid muscle. To date, no histological evaluation of these tissues has been published to precisely specify the nature of this relationship. Anatomy of the sphenomandibular ligament The sphenomandibular ligament is a band of fibrous tissue that connects the lingula on the mandible to the spine of sphenoid on the skull base (Fig 7). The shape, length, thickness and nature of attachment of this ligament varies considerably between individuals. Garg and Townsend 18 dissected seven cadavers and found Fig 6. Transverse section of the right mandibular ramus at the level of the lingula showing the IAN located just behind the tip of the lingula, anterior to the veins and artery. The thickening of the fibrous tissue medial to the neurovascular bundle represents the sphenomandibular ligament. During an IANB, the ideal position to deposit local anaesthetic solution is just above the tip of the lingula, as it allows the needle tip to be in close proximity to the nerve, without directly contacting it and risking damage. (SML = sphenomandibular ligament; IAN = inferior alveolar nerve; IAA = inferior alveolar artery; IAV = inferior alveolar vein; L = lingula; IPF = interpterygoid fascia.) the condylar neck. 15 This fascia, sometimes referred to as temporopterygoid fascia, becomes very thin anteriorly and forms the anterior boundary of the pterygomandibular space by bridging the gap between the anterior border of the medial pterygoid muscle and the fascia overlying the tendinous insertions of the temporalis muscle. All these fascial linings closely adapt to the structures that create the borders of the pterygomandibular space (i.e. medial pterygoid muscle, parotid gland). Their presence has been recognized as a potential barrier to diffusion of local anaesthetic solution that is deposited outside this pouch-like network, thus increasing the probability of inadequate anaesthesia. 3,9,17 The structure and attachments of fascia in the pterygomandibular space have been reported in numerous publications but no methodology or sampling characteristics have been provided to indicate how such descriptions were generated. Hence, the true Fig 7. Photograph of the pterygomandibular space on the left side from a medial view. The medial pterygoid muscle and tongue have been removed to expose the fibrous tissue that forms the sphenomandibular ligament and associated fascia. A needle has been inserted through the buccinator muscle and into the pterygomandibular 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.) 116 ª 2011 Australian Dental Association

6 Applied anatomy of the pterygomandibular space that the sphenomandibular ligament ranged in shape from a thin band that descended for a short distance from the spine of the sphenoid to a broad bi-concave ligament with prominent insertions. Similarly, Shiozaki et al. 19 observed considerable variation in 40 Japanese cadavers, with some sphenomandibular ligaments attaching to the medial aspect of the mandibular ramus anterior and posterior to the lingula, in addition to their direct attachment to this structure. Due to its density and shape, the sphenomandibular ligament has the potential to impede diffusion of local anaesthetic solution to the IAN if the tip of the needle is placed too far medially in relation to the ligament. 3 In vivo diffusion studies involving radiographic analysis of local anaesthetics mixed with contrasting medium have found that local anaesthetic solution diffuses easily through the loose connective tissue of the pterygomandibular space if it is introduced directly into the space. 9,17 However, deposition of local anaesthetic in a location where it is separated from the IAN by the sphenomandibular ligament or other fibrous tissue may impede diffusion. The direct IANB technique has been illustrated and described as involving insertion of the needle until it comes into contact with the lingula. Some anatomical studies have found cases where the ligament attaches to the superior border of the lingula. 18 This may increase the possibility that the needle tip could arrive at a position that is medial to the ligament, especially if bony contact of the needle tip is at, medial or inferior to the apex of the lingula. In such cases, diffusion of local anaesthetic would need to occur through the ligament or around it to produce its desired effect. To avoid this, it is recommended that the level of needle contact with bone should be slightly superior to the 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 innervates mandibular teeth which may be relevant when attempting an IANB. As the posterior division of the mandibular nerve descends and approaches the mandibular foramen, it gives off the nerve to mylohyoid which often follows an antero-inferior course on the medial aspect of the mandibular ramus. 23 In some cases, however, part of the course of this nerve may involve an intra-osseous component. 24,25 Anatomical variabilities such as this, or variation in the height at which the nerve to mylohyoid branches off the IAN, may ultimately influence whether this nerve is anaesthetized during an IANB. This is relevant for local anaesthesia as the nerve to mylohyoid can provide accessory innervation to mandibular teeth ,26 It has also been reported to innervate the chin and tip of the tongue in some individuals. 27 Bennett and Townsend 28 when analysing six human cadavers reported that the average distance between the mandibular foramen and the branching point of the nerve to mylohyoid was 13.4 mm, ranging from 3.9 to 27.0 mm, while Wilson et al. 22 reported after observing 37 human cadavers an average branching distance of 14.7 mm, ranging from 5.0 to 23.0 mm above the mandibular foramen. The dental relevance of these observations is that the greater the distance between the point at which the nerve to mylohyoid branches off the IAN and the location where the local anaesthetic solution is deposited, the greater the likelihood that the nerve to mylohyoid may not be fully anaesthetized, leading to potential failure in achieving anaesthesia. In addition to the height of the branching point, there may be physical barriers that separate the nerve to mylohyoid from the area where local anaesthetic solution is deposited during an IANB. The nerve to mylohyoid travels behind the sphenomandibular ligament at its attachment to the lingula. 18 Consequently, the density and shape of this structure may prevent effective diffusion of local anaesthetic during an IANB. Similarly, if part of the course of the nerve to mylohyoid is encompassed by bone, which has been reported in the literature, then this will also act as a potential barrier. 29 Relationship of structures within the inferior alveolar neurovascular bundle Typically, major nerves and their branches are accompanied by an artery and vein. This is also true for the nerves within the pterygomandibular space, such as the IAN. 30 Anatomical descriptions of the pterygomandibular space have been published but accounts often neglect to mention how the IAN and associated blood vessels are arranged within their neurovascular bundle. Of the few descriptions reported, a number of patterns have been identified, but they lack consistency and in some cases are directly conflicting. These reports also do not provide a standardized height in the superoinferior plane at which these structural relationships were analysed, leading to possible variations in the descriptions as the IAN, inferior alveolar artery (IAA) and inferior alveolar vein (IAV) arise from different regions within the infratemporal fossa before converging inferiorly to form a neurovascular bundle. The presence of an IAN, IAA and IAV are not disputed, providing an important and essential neurovascular supply to the mandibular teeth. The IAA arises from the maxillary artery which branches off the external carotid artery in the vicinity of the mandibular condylar neck. 3 As it travels inferiorly, it assumes a path close to the IAN. The degree to which the IAA transverses the pterygomandibular space from its origin to its eventual path alongside the IAN depends on ª 2011 Australian Dental Association 117

7 JN Khoury et al. whether the maxillary artery follows a path that is superficial or deep to the lateral pterygoid muscle. Independent of this, the IAV exits the mandibular foramen, acting as a tributary to the pterygoid venous plexus (PVP) which is closely associated with the lateral pterygoid muscle. The specifics of exactly how each of these structures (IAN, IAA and IAV) interact together along their path toward the mandibular foramen have not been described clearly. Barker and Davies 3 suggested that the IAN is relatively anterior while the inferior alveolar vasculature is more posterior, with the IAV being closest to the bone. Their explanation for this arrangement relates to the path taken by these structures from their origin superiorly to the mandibular foramen inferiorly. For example, the IAN and lingual nerves separate from each other on the deep surface of the lateral pterygoid muscle where they each enter the pterygomandibular space along the lateral surface of the medial pterygoid muscle, and this is relatively more anterior than where the IAV feeds into the PVP. 3 Similarly, Sicher and Dubrul 8 and Murphy and Grundy 14 reported that the inferior alveolar vasculature was generally placed more lateroposteriorly and closer to the bone than the nerve, which was always located more anteriorly. However, it is important to note that neither of these publications provide information on sampling methods or sample size. There are numerous other reports that agree with the observations of Murphy and Grundy, 14 Barker and Davies, 3 and Sicher and Dubrul. 8 However, when most authors make reference to or illustrate the relationships of the IAN, IAA and IAV, the inferior alveolar vessels are coupled together. 7,13,15,20,30 32 In each of these examples, the IAN is always represented as being anterior to the blood vessels. Hence, while these descriptions may be consistent with earlier reports, they are less specific and provide no details about how such information was obtained. In contrast to the preceding reports, there have been other descriptions of different relationships between the IAN, IAA and IAV. For example, Wadu et al. 33 suggested that the course of the IAN was closer to the mandible, with the artery and vein being placed more medially. Cousins and Bridenbaugh 34 similarly suggested that the IAN was closer to the mandible and lateral to the IAA and IAV. Another variation in the description of this relationship was an observation by Malamed 1 that the IAA was positioned more anteriorly compared with the IAN. Roda and Blanton, 35 though maintaining that the IAA and IAV are very close to the bone when compared to the IAN, reported a number of possible relationships with their respective frequencies. Although no descriptions of methodology or sampling characteristics are provided, their review article suggested that the IAN was anterior to the blood vessels in 70% of cases while in 20% of cases, the IAN was medial to the blood vessels. The blood vessels were anterior to the IAN in 10% of cases. A more recent study involving 56 specimens has demonstrated similar findings, with the inferior alveolar blood vessels tending to be posterior, posterolateral or posteromedial to the IAN in most cases. 36 Figure 6 shows an example of a typical arrangement of the IAN and associated vessels. Potential anatomical causes for failure of anaesthesia Anaesthetic failures occur frequently with IANBs, even with experienced clinicians. There are many reasons why this may occur. The two major factors being poor operator technique and anatomical variation. 16 Other potential reasons for anaesthetic failure include psychological issues where patient fears and anxieties lead to either exaggerated or imagined pain and discomfort, or where acute localized infections within the pterygomandibular space or distal branches of the IAN reduce the effectiveness of local anaesthetic. 37 Apart from the nerve to mylohyoid, other nerves may also provide accessory innervation to mandibular teeth, potentially leading to failure of anaesthesia. Barker and Lockett 38 observed canals in the rami of mandibles which led to the apices of lower posterior molars, particularly third molars. Ossenberg 39 suggested that sensory nerves, most likely branches of the long buccal nerve, may travel through many of these retromolar foramina. As the long buccal nerve arises from the anterior division of the mandibular nerve, direct IANBs will not anaesthetize these branches. In these situations, a Gow-Gates block may be used as local anaesthetic is deposited in a much higher location within the pterygomandibular space, where anaesthesia of the IAN, lingual nerve and buccal nerves can be obtained with a single injection. 1 Tong 40 has also reported a case of a patient who presented for removal of an impacted lower molar in whom the great auricular nerve, a branch of the cervical plexus, appeared to provide additional innervation to the region around the angle of the mandible. Bifid mandibular canals have the potential to increase the difficulty of achieving adequate anaesthesia using the IANB technique. 1,16 Embryologically, the development of mandibular bone through intramembranous ossification occurs around the IAN. Consequently, alterations in the anatomy of this nerve and or its communications with other nerves will be reflected in mandibular bony development. 16 The prevalence of this anatomical variation varies between 0.35% 41 to almost 1% of the population. 42 Usually diagnosed by a panoramic radiograph, there are a number of different patterns that may present. The type suggested to be the most problematic for IANBs is where there are two independent mandibular foramina with a portion of the IAN entering both simultaneously. 1 This form of 118 ª 2011 Australian Dental Association

8 Applied anatomy of the pterygomandibular space variation is also known as a Type 4 bifid canal according to the classification outlined by Langlais et al. 42 Mandibular prognathism is another anatomical variation that can complicate IANBs. Prognathic mandibles generally have a lingula that is positioned higher than the coronoid notch, making it more difficult for the operator to insert the needle at the correct height. 15 The difference in height between the lingula and coronoid notch may be as much as 1 cm. In these cases, needle insertion above normal is indicated. The effects of needle deflection during insertion into the pterygomandibular space have been suggested to lead to reduced effectiveness of IANBs. 43 The degree to which a needle deflects relates to the density of the medium through which it is inserted, the gauge of the needle 44 and the nature and degree of taper of the needle s bevel. 1 Many studies to date have been conducted to evaluate these effects in an attempt to determine whether they are clinically significant. In vitro studies have shown that needles have a tendency to deflect toward the non-bevelled side during insertion into media of homogenous density This has led some to suggest that the bevel should be orientated away from the ramus to guide the needle toward the bone on insertion, thus reducing the likelihood of over-insertion of the needle. 45 However, in vivo research has found no significant differences between the effectiveness of direct IANBs when administered with the bevel away from the ramus compared with the bevel toward the bone. 47 Anatomically, the density of the tissue within the pterygomandibular space is mostly loose areolar tissue, which lacks dense fibrous elements. 5 Hence, if an IANB is executed correctly, it is likely that needle deflection would be minimal, especially with needles of larger diameter. More recently, a new technique of needle insertion has been suggested which involves rotating the needle while it is inserted. 48 This is in an attempt to negate any potential needle deflection by preventing the needle s bevel from being on any particular side for the duration of needle insertion. 48 In vitro research has indicated that this method can reduce deflection. 43 However, an in vivo study has not shown this technique to be clinically superior with respect to the level of anaesthesia attained in individuals with irreversible pulpitis. 49 Further research is required to more accurately assess whether this technique has clinical advantages. Failure of anaesthesia can prove challenging for the clinician to understand. If an IANB has failed, it is essential that the operator carefully evaluates his her technique as well as common anatomical variations to determine what may have contributed to the problem. If the cause(s) are not accurately identified, this may lead to multiple IANBs that continue to fail. Not only does this damage more tissue than necessary, placing the patient at increased risk of trismus, but it may reduce patient confidence in the operator s abilities and reinforce negative stereotypes of oral health professionals. Research methods and their relative usefulness Gross dissection has been the most common method of examining the pterygomandibular space and it provides arguably the most useful insights into how soft tissue structures relate to the osteology of the skull in three dimensions. Anatomical studies of the sphenomandibular ligament and relationships of the IAN to the IAA and IAV(s) are often conducted in this way. Advantages of gross dissection are that it allows for qualitative analysis of how structures relate to each other as they travel supero-inferiorly, anteroposteriorly and mediolaterally. Clear weaknesses of this approach are that it disturbs superficial structures in the area of interest, it may distort the exact relationships of nerves and their related blood vessels, it cannot be performed on living subjects, and it does not lend itself to quantitative analysis. Transverse sectioning of anatomical material can also be performed and provide useful data. Such transverse sections can be viewed macroscopically or prepared for histological interpretation. Advantages of this approach are that it does not disturb the anatomical patterns in a transverse plane, thus making it ideal for analysing the relationships of the IAN to the IAA and IAV. Similarly, if histological sections are prepared, they provide much greater detail regarding the structures depicted, such as the number of IAVs and the number of IAN fascicules and the nature of connective tissues. Also, quantitative analyses can be performed when anatomical material is prepared using this method, such as determining precise distances between specific structures. A disadvantage is that this method does not provide a three-dimensional view of structures. Osteological features of the mandible have been studied on numerous occasions using both qualitative and quantitative approaches in various populations. Although there are obvious limitations in what can be extrapolated from osteological research, these studies are powerful and involve large sample sizes, in some cases over 300 specimens. 13 As IANBs require recognition of bony landmarks as part of the execution of the technique, osteological studies can provide useful data regarding how mandibular anaesthesia could be made more effective. They also provide insights into why IANBs may fail sometimes, such as due to nerves travelling in accessory foramina. Radiographic and computerized tomographic (CT) methods have also been used to analyse the pterygomandibular space. 9,17 Radiographs involve a twodimensional representation of three-dimensional ª 2011 Australian Dental Association 119

9 JN Khoury et al. structures, thus making them useful in identifying bony relationships in a plane that is perpendicular to the X-ray source. Panoramic radiographs have been used to identify bifid inferior alveolar canals and these studies are extensive, involving retrospective analysis of thousands of radiographs. 41 Radiographs have also been used to follow the diffusion patterns of local anaesthetic mixed with radiopaque contrasting media when administered as IANBs. 17 More recently, CT imaging has been used to analyse the dynamics of local anaesthetic diffusion. 9 This research has included acceptable sample sizes but more of these studies are needed to add to what is currently known about diffusion paths in the pterygomandibular space. SUMMARY AND CLINICAL TIPS Just as an understanding of the basic anatomy of the pterygomandibular space promotes safe and effective anaesthesia, improved knowledge about less explored regions and relationships should make the administration of IANBs even safer and more effective. Considering that this is the main technique for achieving mandibular anaesthesia in many parts of the world, it is essential that clinicians are familiar with the relevant anatomy and understand how anatomical variations can lead to anaesthetic failures. Based on this review of the anatomy of the pterygomandibular space, three key points underpin the basis of a successful IANB technique: (1) the rationale of the direct IANB is based on being able to reliably position the needle tip above the tip of the lingula by paying attention to the entry point, the level of injection and the angulation of the syringe. The entry point is the pterygotemporal depression located between the pterygomandibular fold medially and the coronoid notch laterally. Placing a cotton bud onto the pterygotemporal depression can assist in defining this structure as the tissue in this area is less dense than the structures on either side; (2) the level of injection can be gauged by palpating the coronoid notch, and also keeping the needle parallel to and about 1 cm above the lower occlusal plane. The syringe should be angulated over the premolar teeth on the contralateral side, but angulation will vary from patient to patient according to various anatomical factors; and (3) bone should always be contacted with a direct IANB at the appropriate depth of approximately mm. Following this, the needle should be withdrawn 1 2 mm and aspiration performed prior to injection. ACKNOWLEDGEMENTS This review has arisen from research funded by the Australian Dental Research Foundation. 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