Implant Surgical Anatomy

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Review

Practical Application of Anatomy for the DentalImplant Surgeon

Gary Greenstein,* John Cavallaro,* and Dennis Tarnow*

A proficient knowledge of oral anatomy is needed to provideeffective implant dentistry. This article addresses basic ana-tomic structures relevant to the dental implantologist. Pertinentmuscles, blood supply, foramen, and nerve innervations thatmay be encountered during implant procedures are reviewed.Caution must be exercised when performing surgery in certainregions of the mouth. Furthermore, numerous suggestions areprovided regarding the practical application of anatomy tofacilitate successful implant therapy. J Periodontol 2008;79:1833-1846.

KEY WORDS

Anatomy; dental implants.

The study of anatomy familiarizes

the implant surgeon with normaland atypical oral structures.

Knowledge of oral structures and ordi-nary anatomic variations, which usuallydiffer with respect to size and shape,enhance patient evaluations and facili-tate precise surgical procedures. A thor-ough understanding of anatomy providesthe implant surgeon with the confidenceto resector augment tissues in an attemptto restore form, function, esthetics, andhealth. This article reviews the practical

application of basic anatomy to implanttherapy. It does not attempt to discussevery blood vessel, nerve, and musclefound within the oral cavity, but rather itfocuses on structures routinely encoun-tered, which are critically important toplanning and executing dental implantsurgery.

MANDIBULAR STRUCTURES

Mandibular ForamenThe location of the mandibular foramenmay vary based on race and ethnicity,andthis can affectthe success of block in-jections.1,2 Among adult cadaveric man-dibles, the foramen was found inferior tothe occlusal plane, at its level, or aboveit 75%, 22.5%, and 2.5% of the time, re-spectively.1 In another study,2 the figureswere 29.4%, 47.1%, and 23.5%, respec-tively. Therefore, according to these in-vestigations, 2.5% to 23.5% of blockinjections given at the level of occlusionwould be ineffective. Accordingly, it is

* Department of Periodontology and Implant Dentistry, New York University College ofDentistry, New York, NY.

Private practice, Freehold, NJ. Private practice, Brooklyn, NY. Private practice, New York, NY.

doi: 10.1902/jop.2008.080086

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advisable to inject patients 6 to 10 mm superior to theocclusal plane,3 which usually accounts for anatomicvariations.Thedistancetothemandibularforamenas-sessed on cadavers revealed that it is within the reachof a short needle (needle length= 21 mm).4 Therefore,short needles can be used to attain anesthesia in the

mandible. If there are symptoms of a good block injec-tion, but the patient is still symptomatic, infiltrate thelingual aspect of the molar teeth, because there maybe additional innervation from C2and C3(cutaneouscoli nerve of the cervical plexus).5

Inferior Alveolar CanalThe trigeminal nerve, the fifth cranial nerve, has threemain branches: ophthalmic, maxillary, and mandibu-lar.6 The mandibular nerve gives rise to the inferior al-veolar nerve (IAN). It enters the mandibular canal onthe medial surface of the ramus by the lingula. The ca-

nalis;

3.4 mm wide, and the nerveis;

2.2 mm thick.7

Within the canal there is a nerve, an artery, a vein, andlymphatic vessels. The artery lies parallel to the nerveas it traverses anteriorly, but its position varies with re-spect to being superior or inferior to the nerve withinthe mandibularcanal.7Therefore,itispossibletoinad-vertently penetrate into the mandibular canal andinduce neurologic damage without provoking hemor-rhaging and vice versa.

When developing an osteotomy over the mandibu-larcanal, corticalbone ispenetrated first,and theprep-aration terminates within softer cancellous bone. The

mandibular canal usually has cortical bone around it,which may provide some resistance to drilling. How-ever, clinicians should not rely on tactile feedback tosignal the canal is about to be penetrated, because atwist drill can enter the canal with little warning. Con-versely, when traversing from more to less mineralizedregions of the posterior mandible during osteotomydevelopment, a sudden decrease in resistance maygive an erroneous impression that the canal has beenbreached. Accordingly, there is no substitute for pre-cise radiometrics, safety devices (e.g., drill stops),and a plan for attaining specific implant lengths in thisregion of the mouth.

The IAN may present in different anatomic configu-rations. The nerve may lower gently as it proceeds an-teriorly,ortherecanbeasharpdeclineorthenervecandrape downward in catenary fashion (curled as hang-ingbetweentwopoints).8TheIANcrossesfromthelin-gualtothebuccalsideofthemandibleandoften,bythefirstmolar,itislocatedmidwaybetweenthebuccalandlingual cortical plates of bone.9 Usually, the IAN di-vides into the mental and incisive nerves in the premo-larmolarregion.10Themental nerve emergesfrom themental canal, and anterior to the mental foramen themandibular canal is referred to as the incisive canal.11

Implant placement buccal or lingual to the IAN is a

risky maneuver and should not be attempted withoutthe aid of computed tomography (CT).

The mandibular canal bifurcates in the inferiorsuperior or mediallateral plane in;1% of patients.12

A bifurcated canalmay manifestmorethanone mentalforamen and, the bifurcation may not be seen on pan-

oramicor periapical films. Theundetected presence ofa bifurcated mandibular canal can result in an incor-rect estimation of available bone superior to the man-dibular canal.

Denio et al.13 evaluated cadavers to determine howclosetheIANwastotheapicesofmandibularposteriorteeth.Themeandistancetothesecondmolar,firstmo-lar, and premolars was 3.7, 6.9, and 4.7 mm, respec-tively. Similarly, Littner et al.14 reported the upperborder of the mandibular canal was located 3.5 to5.4 mm below the root apices of first and second mo-lars. Other investigators15 found that the canal was of-

tenclosetotheinferiorborderofthemandible.Itisalsopossible for the mandibular canal to be adjacent to theapex of the mandibular molar (Fig. 1). Therefore, withregard to developing osteotomies over the inferior al-veolar nerve, it should be recognized that mean dis-tances between apices of teeth and the nerve canalreported in articles may not apply to anyparticular pa-tient. Hence, to avoid untoward sequelae in the poste-rior mandible, the location of the nerve needs to beverified before an osteotomy is created.

With regard to radiographs, Denio et al.13 reportedthat in 28% of patients the mandibular canal could not

be clearly identified in the second premolar and firstmolar regions on periapical radiographs. Therefore,if the inferior alveolar canal cannot be seen on a peri-apical film, it is recommended to obtain a panoramicfilm and adjust distances for radiographic distortion.If it still cannot be detected, a CT scan is needed.Osteotomies should not be developed in the posterior

Figure 1.The mandibular canal is adjacent to the apex of the mandibularfirst molar. Arrow points to mandibular canal abutting alveolus of

extracted tooth #30.

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mandibleuntilthepositionoftheinferioralveolarcanalis established.

Several additional facts about radiographs shouldbe considered. The angulation of the periapical filmcan affect the perceived location of the canal with re-spect to the bone crest.16 For instance, if the x-ray

beam is perpendicular to the canal, but not the film,elongation occurs, and the canal appears further fromthe crest than it really is. Conversely, when the x-raybeam is perpendicular to the film, but is not parallelto the canal, foreshortening happens. Table 1 listsmean linear radiographic errors with respect to differ-entx-raytechniqueswhenlocatingthemandibularca-nal.17 The numbers in the table represent mean errorsandcanbeincorrectbyevenlargeramounts.Thesein-accuraciesneedtobetakenintoaccountwhencreatingan osteotomy in sensitive areas. To avoid misinterpre-tation of linear measurements on radiographs, clini-

cians can use markers of known dimension whentaking an x-ray (e.g., 5-mm-diameter ball bearing).16

Mental Foramen and NerveCommonly, three nerve branches of the mental nerveemerge from the mental foramen (each ;1 mm in di-ameter).11 They supply innervation to the skin of themental foraminal area, the lower lip, chin, mucousmembranes, and the gingiva until the second premo-lar. Occasionally, the mental nerve emerges from thebuccal plate of bone and reenters the alveolar bone toprovide innervation for the incisor teeth.18

Thelocationofthementalforamendiffersinthehor-izontal and vertical planes, and these variations maybe related to race.19-21 For example, horizontally theforamen is often found flanked by theapices of premo-lars in white individuals19 and next to the apex of thesecond mandibular premolar among Chinese sub-jects.20 Atypically, the foramen may be situated bythe canine or the first molar.20,21 In these situations,theincisivecanalstartswherethementalnerveemergesfrom the mandible.

Thepositionoftheforamenalsovariesintheverticalplane.19Pertinently,itwasreportedthatinthefirstpre-molar area of 936 patients, the foramen was situatedcoronal to the apex in 38.6% of cases, at the apex in15.4% of cases, and apical to the apex in 46.0% ofcases.Theforamenslocation,inrelationtothesecondpremolar,was coronal to theapex in 24.5% of cases,atthe apex in 13.9% of cases, and apical to the apex in61.6%ofcases.Thus,cautionmustbeexercised,espe-cially when placing immediate implants in the premo-lararea,becausein25%to38%ofcasestheforamenislocated coronal to the bicuspids apex.19

Theanterior loop of the mental foramen refers to theIAN when it courses inferiorly and anteriorly to the fo-ramen and then loops back to emerge from the fora-

men22 (Fig. 2). Detecting and quantifying the size of

the anterior loop was done by using diverse diagnosticmethods:panoramicfilmsofpatients,panoramicfilmsofmarkers in dried skullsand cadaver mandibles, peri-apical films of cadaver jaws, CT scans of patients, andsurgical cadaver dissections.23Conflictingresults withregard to the size and prevalence of the anterior loopmay be due to different criteria used to characterizethe anterior loop and dissimilar diagnostictechniques.

Surgical dissection furnished the best evidence forvalidating thepresence of theanterior loop of themen-tal foramen.23 Furthermore, comparing radiographicanddissectiondatafromthesameindividualssuppliedinsightas to the dependability of radiographs to detectthe anterior loop.24-26 In this regard, Mardinger et al.24

reported that anatomically identified loops often didnot show on periapicalfilms, and radiographs supplieda 40% false-positive finding when related to their cor-responding cadaver dissections. In another study,25

loop dimensions varied from 0.0 to 7.5 mm on periap-

ical radiographs and from 0 to 1.0 mm between

Table 1.

Distortion on Radiographs17

Type of Radiograph Mean (mm; range) %

Periapical 1.9 (0 to 5) 14

Panoramic 3.0 (0.5 to 7.5) 23

CT scan 0.2 (0.0 to 0.5) 1.8

Figure 2.Anterior loop of the mental foramen (arrow). The inferior alveolar nervecourses beneath and mesial to the foramen and then loops back toemerge from the foramen.

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cadaver specimens; usually radiographs overesti-mated the extent of the anterior loop.25 In a differentinvestigation,26 the anterior loop was identified on 27%(six/22) of the panoramic films and 35% (eight/22)of the cadavers. The dimensions of anterior loops inpanoramic radiographs varied from 0.5 to 3 mm,

and cadaver specimensmanifestedanterior loopsthatranged from 0.11 to 3.31 mm.However, the investiga-tors noted that 50% of the x-rays were interpreted in-correctly, and the presence of a loop was not verifiedby surgical dissection. Furthermore, 62% of the surgi-cally detected loops were not found radiographically.In general, it can be concluded that many false-posi-tiveand false-negativefindings occurwhen identifyingthe anterior loop with x-rays.

Thus, when there is concern with respect to the lo-cation of the mental nerve, it should be exposed toidentify its position before implant insertion. First, de-

termine on theradiographwhere themental foramen islocated. If it is in the premolar region, create a verticalreleasing incision on the mesial aspect of the canineand reflect the flap past the mucogingival junction.Then use wet gauze to elevate the flap apically andexpose the roof of the mental foramen.23 The gauzeshields thenerve from being damaged, andthe perios-tealelevatorcanbeusedtopushthegauzeapically.Todetermine how much bone is available for implant in-sertion, measure the distance between the alveolarcrest andthe coronal aspectof theforamen with a peri-odontal probe (Fig. 3). The chosen implant length

should provide a safety margin of 2 mm from thenerve.23,27 This measurement minus 2 mm can alsobe used to safely insert an implant mesial, above, ordistal to the mental foramen up to the mesial half ofthe first molar area.27

When measurements are taken from the crest of thebone to theroof of theforamen to determinethe appro-priate implant length in the foraminal area, there issome additional safety room based upon three ana-tomic considerations: the foramen coincides with thebuccal plate and the osteotomy will be developed lin-gual to the foramen and its contents; the foramen iscone shaped, with the widest part of the funnel onthe buccal aspect; and the nerve emerges from a pathinferior to the foramen.27 The mental nerve comes outof the mental canal, which is angled upward at ;50(range, 11 to 70) from the mandibular canal (Fig.4).28Therefore, it should be noted that the inferioralve-olar nerve is lateral and apical to the mental foramen.

If it is desired to place an implant, which is largerthan the safety distance determined above, anteriortothementalforamen,aCTscanisnecessarytodeter-mine whether an anterior loop is present, or it is neces-sary to probe within the foramen to ascertain if there isan anterior loop. In this regard, a curved probe (e.g.,

Nabers 2N probe) can be gently placed into the fora-

mentoassessifitsdistalaspectisopen.Ifitisnotopen,then the nerve entered on the mesial side, and this de-notes that an anterior loop is present (Fig. 5). The me-sial side of the foramen is consistently patent, becauseat this site the anterior loop emerges from the bone orthe incisal nerve proceeds anteriorly. Note that the pa-tency on the mesial aspect of the foramen leading tothe incisal region and an anterior loop feel similar, anditis notpossibleto distinguish between thesetwo struc-tures.27 It must be emphasized that probing into themental foramen should be done very gently; other-wise, neurologic damage can be done to the nerve.

Furthermore, even if the presence of an anterior loop

Figure 3.To determine how much bone is available for implant insertion over themental foramen, measure the distance from the alveolar crest to thecoronal aspect of the foramen (arrow) with a periodontal probe.

Figure 4.CT scan. The mental nerve emerges from the mental canal. The mentalcanal is angled upward at;50(range, 11to 70).


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is corroborated by probing, its length is still unknown.Therefore, as a general guideline, if there is an anteriorloop of the mental foramen and a CT scan is not avail-able to determine its dimensions, and it is desired toplace an implant deeper than the safety measurementon the mesial aspect of the foramen, it is prudent toplace the distal aspect of the implant 6 mm anteriorto the mental foramen to avoid damaging the loopwhen drilling the osteotomy.28

Mandibular Incisive CanalNumerous investigations29-32 reported that there is atrue incisive canal mesial to the mental foramen,which is a continuation of the mandibular canal. It hasalso been noted that the incisive canal may appearas a maze of intertrabecular spaces, which includeneurovascular bundles.33 The incisive nerve suppliesinnervation to teeth (first bicuspid, canine, and lateraland central incisors).

The incisive canal is typically found in the middlethird of the mandible (in 86% of cases).30 It usually nar-rows as it approaches the midline and only reaches themidline18%ofthetime.30Thenerve usuallyterminatesapical to the lateral incisor and sometimes apical to the

central incisor.30 The incisive canals width is 1.8 0.5

mm, and it was found in 96% of as-sessed cadavers.30However, whenthe appearance of the incisive ca-nal on panoramic radiographswas evaluated, Jacobs et al.34 re-ported that it was seen only on

15% of the films (n= 545). In con-trast, it was observed on 93% ofCT scans.

In the interforaminal area, aslong as the mental foramen andthe anterior loop of the mental fo-ramen (if present) are avoided,implants can usually be insertedwithout too much thought given tothe presence of the incisive canal.However, if there is an unusuallylarge incisive nerve canal, a patient

can experience discomfort duringosteotomy development preclud-ing implant placement35 or experi-ence postoperative pain requiringimplant removal.36 Consequently,consideration should be given tothe size of the incisive canal beforeplacing implants deeply in theinterforaminal area.

Lingual Foramen andLateral CanalsVascular canals are often present

in the midline and lateral to themidline of the mandible. Gahleitner et al.37 foundone to five vascular canals per patient. The meandiameter of the midline canals was 0.7 mm (range,0.4 to 1.5 mm); the lateral canals in the premolararea were slightly smaller (mean, 0.6 mm). The lin-gual foramen was detected in 99% of the mandibleswhen evaluating skull dissections.38 However, the fo-ramen was only found on 49% of the periapical filmsbecause the angulation of the x-ray beam affected itsimage.

The lingual foramen harbors an artery that corre-

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artery.42 The sublingual artery is found above themylohyoid muscle and is the major nutrient vessel inthe floor of the mouth.42 The submental artery fre-quently traverses inferiorly to the mylohyoid musclebut was noted to pierce through the mylohyoid musclein 41% of dissected cadavers.43 Hofschneider et al.41

also reported that the sublingual and submental arter-ies may course anteriorly in close proximity to the lin-gual plate, and branches of these blood vessels enteraccessory foramina along the lingual cortex (Fig. 6).Inadvertent penetration through the lingual corticalplate into the floor of the mouth while preparing an os-teotomycancausearterialtrauma,therebyresultingindevelopment of a sublingual or submandibular hema-toma.Itwasmentionedthatseveringanartery2mmindiameter with a probable blood flow of 0.2 ml per beat(70beatsperminute)canresultin420mlbloodlossin30 minutes.44 This quantity of hemorrhage can cause

swelling, and the tongue may be pressed superiorlyand posteriorly, blocking the airway, and causing up-per airway distress.45,46 The patient requires aggres-sive medical and possibly surgical management if anairway crisis develops.

It is also possible to induce hemorrhaging when el-evating a flap if a vessel entering an accessory canal issevered. On the lingual aspect, proper flap elevationand visualization where the osteotomy is being devel-oped helps to avoid accidental perforations. Bleedingfromthefloorofthemouthisfirstmanagedbypressureand then ligation of severed blood vessels.

Submandibular and Sublingual Fossae

Thesubmandibularfossaisadepressiononthemedialsurface of the mandible inferior to the mylohyoid line,anditcontainsthesubmandibulargland. 47Thesublin-gualglandisfoundinthesublingualfossa.48Thisfossais a shallow depression on the medial surface of the

mandible on both sides of the mental spine, superiorto the mylohyoid line. The submandibular and sublin-gualfossaemustbepalpatedpriortoosteotomydevel-opment; if there is a large undercut, the lingual bonyplate can be perforated inadvertently, resulting inhemorrhaging. Lingual concavities with a depth of6 mm were reported in 2.4% of assessed jaws (n= 212;CTscanswereused).43 Ifthereisalargeundercut,anin-strumentcanbeplacedintoandparalleltotheundercutto visualize and measure the extent of the depression. ACT scan with radiopaque markers provides the most ac-curate information. Pertinently, the angulation thatthe

implant is placed needs to accommodate the undercutto remain in bone during osteotomy preparation.

The Lingual and Mylohyoid NervesThe mandibular branch of the trigeminal nerve givesrise to the lingual nerve.49This nerve provides sensoryinnervation to the mucous membranes of the anteriortwo-thirds of the tongue and the lingual tissues. At thetime of implant surgery in the posterior mandible, thelingual nerve can be injured if the lingual flap is not re-flected cautiously. The lingual nerve is usually located3 mm apical to the osseous crest and 2 mm horizon-

tally from the lingual cortical plate in the flap.50

How-ever, in15% to20% of cases, the nerve may besituatedat or above the crest of bone, lingual to the mandibularthird molars.51 In addition, 22% of the time the lingualnerve may contact the lingual cortical plate.50 To cir-cumvent lingual nerve injury, the elevator should beused to protect the nerve in the flap, and the tissueshould be managed gently to preclude causing a tran-sient pressure-traction injury. It is recommended thatlingual, vertical releasing incisions be avoided. Fur-thermore, incisions distal to the second molar shouldbe made on the buccal aspect of the ridge to provideadditional room for safety, because the lingual nervemay be lying over the retromolar ridge.51

The mylohyoid nerve is a branch of the inferior alve-olar nerve.52 It arises just prior to where the IAN entersthe mandibular foramen. On the deep surface of the ra-mus, it moves down in a groove to reach and innervatethe mylohyoid muscle and the anterior belly of the di-gastric muscle. This nerve may also contribute to an in-ability to attain complete anesthesia due to accessorysensory innervation to the anterior and posterior man-dibularteeth.52,53 Inpatientswhoexperiencediscomfortdespite signs of a good block injection, additional infil-tration on the lingual aspect in the posterior region

may help to attain more profound anesthesia.53

Figure 6.Blood vessels entering the lingual cortex of the mandibular anteriorteeth. Arrows point to vascular channels in the lingual cortical plate

of bone.


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The Long Buccal Nerve

The buccal nerve is a branch of the mandibular nervethat is derived from the trigeminal nerve, and it beginshigh in the infratemporal fossa.54 It transmits sensoryinnervation to the buccal gingiva and mucosa of thecheek from the retromolar area to the second premo-

lar. It courses between thetwo heads of thelateral pter-ygoidmuscle,underneaththetendonofthetemporalismuscle, and then under the masseter muscle to con-nect with the buccal branches of the facial nerve onthesurfaceof thebuccinatormuscle.An anatomic var-iation of the long buccal nerve, called Turners varia-tion, consists of the nerve emerging from a foramenin the retromolar fossa. When this variation exists,trauma in this region can cause paresthesia to the ad-jacent gingiva and mucosa.55

Muscles Attached to the MandibleThere are 26 muscles attached to the mandible.56

There are two single muscles (orbicularis oris andplatysma) and 12 pairs of bilateral muscles, whicharelistedinalphabeticorder:anteriorbellyofdigastric,buccinator, depressor anguli oris, depressor labii infe-rioris, genioglossus, geniohyoid, masseter, mentalis,mylohyoid, lateral pterygoid, medial pterygoid, andtemporalis. Several of these muscles are of particularconcern to the implant surgeon.

Mentalis MuscleThementalismuscleisapairedsmallmusclethatorig-inates in the incisive fossa of the mandible and insertsinto the integument of the chin.57 The muscle fibers

pass in an inferior direction, and upon contraction,they elevate the lower lip. When a flap is raised in thisregion, the entire mentalis muscle should not be re-leased off from the mental protuberances, becausethe muscle may fail to reattach well.58 This can resultin an appearance referred to as a witchs chin (doublechin).58 Full-thickness replaced flaps, which do notreachtheinferiorborder,usuallydonotaffectfacialap-pearance. However, vestibular incisions that sharplydissect this muscle require special suturing (i.e., themuscle layer and then the overlying soft tissues).

Mylohyoid MuscleTwo flat mylohyoid muscles form a sling inferior to thetongue, supporting the floor of the mouth.59 Their or-igin is the mylohyoid line on the medial aspect of themandible, which extends from the symphysis to thelast molar. They insert on the body of the hyoid boneandoverliethedigastricmuscles.Thismuscleisanim-portantanatomicbarrierseparatingthesublingualandsubmandibular spaces. The submandibular fossa isbelow the mylohyoid muscle, and the sublingual fossais superior to the muscle.

Manipulation of the muscle should be performedonly to fulfill clearly defined objectives. In this regard,

sometimes there are situations, such as guided bone

regeneration procedures (GBR), when it is desirableto advance a flap a large distance to achieve primaryclosure. To achieve this, besides flap elevation onthe buccal aspect, it may be necessary to partially dis-lodge the mylohyoid muscle fromits origin to facilitatelingual flap advancement. First, the lingual flap is ele-

vated to the mylohyoid muscle, and wet gauze can bepressed apically with a periosteal elevator or the oper-ators finger to achieve blunt displacement of the mus-cle. Subsequent to surgery, the partially displacedmuscle reattaches without untoward sequelae.

Genial Tubercles (genioglossus

and geniohyoid muscles)Thegenialtuberclesaresmall,bonyelevationslocatedon the lingual surface of the mandible. They are foundon either side of the midline close to the inferior borderofthemandibleandserveasthepointofinsertionofthegeniohyoid and genioglossus muscles.60 There aretwosuperior andtwo inferior tubercles. Thegenioglos-sus originates from the superior genial tubercles, andthe geniohyoid originates from the inferior genial tu-bercles. The lingual foramen may be found in the mid-dle of the tubercles. The average height of the superiorgenial tubercleis 6.17mm, and its widthis 7.01mm.61

If there is advanced bone resorption in the mandibularanterior region, theheightof thesuperior tubercle maycoincide with or be higher than the superior level of theridge(Fig.7).Whenelevating flaps forsurgicalaccess,

Figure 7.A)Because of osseous resorption of the alveolar ridge, the genialtubercle area (arrow) is now superior to the alveolar ridge.B)Panoramic x-ray demonstrating genial tubercle area is superior to

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the genioglossus muscle should not be completely re-flected off from the tubercles because the tongue mayretract to the posterior part of the throat and obstructthe airway.58

Depressor Anguli Oris and Depressor

Labii InferiorusTwo muscles that overlie the mental foramen need tobe displaced when exposing the roof of the foramen:depressor anguli oris (triangularis) and depressor labiiinferioris (quadratus labii inferioris).62 Once the flap iselevated past the mucogingival junction, these mus-cles can be released by using wet gauze to push backthe flap. The wet gauze is used to protect the mentalnerve. Reflection of these muscles does not result inuntoward sequelae.

Buccinator and Orbicularis Oris MusclesThe submucosa is strongly attached to the buccinator

muscle in the cheek region and the orbicularis oris inthe lip area.63 When a surgical procedure is done ad-jacent to one of these muscles, such as GBR, a soft tis-sue flap often needs to be advanced to attain primaryclosure. In this regard, it may be necessary to createan incision that provides periosteal fenestration andpenetrates several millimeters into the submucosa,thereby incising one or both of these muscles to facil-itate coronal positioning of the flap.

Masseter MuscleThe masseter muscle consists of two portions: super-ficial and deep.64 The superficial part arises from thezygomatic arch and zygomatic process of the max-illa.64 It inserts into the angle and lower half of the lat-eral surface of the ramus of the mandible. The deepportion arises from the zygomatic arch and inserts intothe upper half of the ramus and into the lateral surfaceof the coronoid process. When the mandibular ramusareais usedas a donor sitefor bonegrafting (i.e., blockgraft), part of the masseter muscle is released from theramus when the periosteum is elevated in this region.

MAXILLARY STRUCTURES

Thickness of the Gingiva and Palatal MucosaThe thickness of the gingival and palatal epithelium is;0.3 mm.65 The gingiva is supported by a laminapropria (firm connective tissue), whereas palatal epi-thelium is sustained by a lamina propria and submu-cosa. Average gingival thickness ranges from 0.53to 2.62 mm (mean, 1.56 mm),66 and palatal widthvaries from 2.0 to 3.7 mm, with a mean of 2.8 mm.67

The best location for harvesting a connective tissuegraft is in the maxillary caninepremolar region.68

Thin grafts may be garnered several millimeters awayfromthegingivalmargin,andthickergraftscanbehar-vestedfurther away from thegingival marginwherethe

submucosa is wider.68 The thickest grafts can be ob-

tainedinthetuberosityregion(i.e.,5mm).68Thewidthof the palatal tissue can be estimated by sounding thebone with a periodontal probe or a needle that has anendodontic stopper. Graft height is limited by the po-sition of the greater palatine artery, whose location issubsequently discussed.

Nasopalatine ForamenThe nasopalatine foramen is also referred to as the in-cisive foramen (Fig. 8).69 Upon flap reflection withinthe foramen, two lateral canals are noticeable, whicharecalled incisive canals or foramina of Stenson. Theytransmit the anterior branches of the descendingpalatine vessels and the nasopalatine nerves. Occa-sionally, one to four canals may be present.69 Thenasopalatine foramen is;4.6 mm wide and is located;7.4 mm from the labial surface of an unresorbedridge.69 The nasopalatine canal (mean length, 8.1mm) exits the incisive foramen. A large incisive canalmay be an obstacle to implant placementin thecentralincisor region. When a large canal was present, Artziet al.70 displaced its contents (moved it over withoutelimination) and placed an implant. In contrast,Rosenquist and Nystrom71 enucleated the canal, in-serted a bone graft, and subsequently placed an im-plant. It is also often possible to angle an implantand avoid the canal.

When performing surgery in the nasopalatine area,some clinicians create a crestal incision labiallyaround the incisive papilla to avoid transecting thecontents of the nasopalatine canal.72 An incision

through the canal region does not usually have a det-rimental affect; however, it occasionally results insome numbness of the anterior palatal tissue.

Infraorbital ForamenThe infraorbital nerve and blood vessels emerge fromthe infraorbital foramen. The foramen is usually lo-cated directly under the pupil of the eye on the inferiorportion of the infraorbital ridge, and it can be palpated

Figure 8.

Incisive foramen exposed (nasopalatine canal [arrow]).


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through the skin of the cheek. The infraorbital nerve isfound5mmbelowtheinferiorportionoftheinfraorbitalridge,73 and it can be injured during surgery. It is a sig-nificant landmark, and intraoral flap elevation shouldcease several millimeters inferior to it. The averageheight of the maxillary sinus is 36 to 45 mm;74 there-

fore, a lateral window extending;15 mm from the al-veolar ridge crest usually avoids encroaching on theinfraorbital nerve. However, if advanced resorptionof the maxilla transpired, vigilance needs to be exer-cised when elevating a flap to avoid damaging the in-fraorbital nerve.

Greater Palatine ForamenThe posterior maxilla needs to be treated cautiously inthe region of the greater palatine foramen. The greaterpalatine artery and nerve emerge from the foramenand traverse the palate anteriorly. The foramen wasfound opposite the third molar in 86% of cases, be-tween the second and third molar in 13% of cases,and opposite the second molar in 1% of cases.75 Otherinvestigators76 noted that the foramen was detectedby the third molar in 55% of cases, between the secondand third molar in 19% of cases, opposite the secondmolar in 12% of cases, and distal to the third molar in14% of cases.

The foramenis located halfway betweentheosseouscrest and the median raphe. Wang et al.77 reported amean distance of 16 mm from the center of the greaterpalatine foramen to the mid-sagittal plane of the hardpalate. Severing the palatal artery close to the foramen

can present a problem, because it can retract into thebone, which precludes ligating it. The precise locationof theforamen can be determined prior to flap elevationby sounding the bone with an anesthetic needle.

Blood Supply in the MaxillaThe internal maxillary artery (maxillary artery) arisesfrom the external carotid artery behind the neck of themandible and provides branches to several regions ofthe face: mandibular, pterygoid, and pterygopala-tine.78 Surgery in the maxilla can involve arteries inthe pterygopalatine region: descending palatine ar-tery, sphenopalatine artery terminal branch, infraor-bital artery, posterior superior alveolar artery, andthe artery of pterygoid canal.

GREATER PALATINE ARTERY

The descending palatine artery emerges from thegreater palatine foramen and traverses anteriorly in agroove on the medial side of the hard palate to the inci-sive canal.79 The end branch of the artery enters theincisive canal to anastomose with the nasopalatinebranch of the sphenopalatine artery. Monnet-Cortiet al.80 reported that the distance from the gingivalmargin to the greater palatine artery ranged from

12.07 2.9 mm in the canine area to 14.7 2.9 mm

at the mid-palatal aspect of the second molar level.With regard to the greater palatine artery, it is prudentto assess the height of the palatal vault to establishthe extent to which a surgical procedure can be per-formed (e.g., harvestinga connectivetissuegraft)with-out damaging the artery. It is advantageous to leave

2 mm between the artery and the end of the surgical in-cision.81 Based upon the shape of the palatal vault, it ispossible to estimate how far the palatine artery is fromthe cemento-enamel junction: low vault (flat) = 7 mm,average palate = 12 mm, and high vault (U-shaped) =17 mm.81 The mean palatal vault height for malesand females is 14.9 and 12.7 mm, respectively.81

When performing a connective tissue graft, a split-thickness palatal flap, andso forth,the surgeon shouldbe ready to manage accidental injury to the greaterpalatine artery. If the artery is deemed to be close tothe site of surgery, it may be advantageous to place

deep sutures to lasso and ligate the greater palatineartery distal to the surgical site prior to initiating ther-apy. If the artery is damaged, this step may precludehemorrhaging. To manage bleeding from a damagedblood vessel, apply pressure, and clamp the palatalflap where the incision was made with a hemostat. Ifthe bleeding vessel is visible, ligate it, or apply electriccautery. Additional deep sutures are needed if thebleeding vessel is not visible.

SPHENOPALATINE ARTERY

The sphenopalatine artery emerges from the spheno-

palatineforamenandenters thebackpartof thesuperiormeatus of the nose.79,82 It gives rise to the posterior andmedial lateral nasal branches. The former spreads for-ward over the conchae and anastomoses with nasalbranches of the descending palatine and ethmoidal ar-teries.Theposteriormedialnasalbranchessupplybloodto the posteromedial and posterior wall of the maxillarysinus.When doing a sinus lift, caution must be exercisedtoavoiddamagingthesevesselsiftheprocedureisbeingextended to the posterior wall of the sinus.

INFRAORBITAL AND POSTERIOR SUPERIOR

ALVEOLAR ARTERY

The infraorbital artery provides branches to the ante-rior part of the sinus. These vessels anastomose withvessels of the posterior superior alveolar artery withinthe buccal plate of bone (intraosseous artery) and inthe buccal tissues (extraosseous artery). The intraos-seous artery is


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hemorrhaging blood vessel (Fig. 9). On occasion, it isnecessary to continue preparing the window, despitethe bleeding, until the membrane is elevated and thebone can be clamped. The assistant should hold thesuction tipnext to thebleeding site to preserve good vis-ibility. When the intraosseous artery is severed on the

mesial aspect of the lateral window, it probably is alsoinjured on the distal aspect. Therefore, the clinicianmay need to deal with bleeding on both sides of the lat-eral window. If a CT scan reveals the presence of an in-traosseousarterybeforestartingthelateralwindow,anditispossibletocircumventit,developthelateralwindowinferior to the artery, and elevate the membrane inter-nally in a superior direction. However, reasonable ac-cess must be achieved, because a poorly locatedlateral window can lead to a non-optimal sinus lift out-come.Anothertechniquethatcanbeusedtoisolatetheartery and not injure it during creation of the lateral win-

dow uses a piezosurgery unit (ultrasonic) (Fig. 10).84

Anterior Nasal SpineUnder the nose, in the midline, at the lower margin ofthe anterior aperture, there is a sharp bony processformed by the forward elongation of the maxillae thatis referred to as theanterior nasal spine (Fig. 11).85 Itisused as a landmark when elevating a large flap in thepremaxilla in preparation for flap advancement. Caremust be exercised when extending flap elevation be-yond this point because the tissue is thin, and it is pos-sible to penetrate through the tissue into the nose. Thebony rim of the nares is referred to as the piriform rim.

Maxillary SinusThe maxillary sinus (antrum of Highmore) is pyrami-dal in shape and is the largest paranasal sinus. Typicalaverage dimensions ofthe sinus are height, 36to45mm,width, 25 to 35 mm, and length, 38 to 45 mm.74 The

Figure 9.If the lateral window for a sinus lift is created and an intraosseousartery hemorrhages, move the membrane laterally and compress thebone with a mosquito hemostat to occlude the hemorrhaging bloodvessel. Arrow points to location where hemostat is occluding the blood

vessel.

Figure 10.Piezosurgery was used to create a lateral window and isolate theintraosseous artery without inducing hemorrhaging.

Figure 11.Anterior nasal spine (arrow). It is located under the nose, in the midline,at the lower margin of the anterior aperture. It is a sharp bony processformed by the forward elongation of the maxilla.

Figure 12.CT scan. The ostium (arrow) is the opening from the maxillary sinus

into the middle meatus of the nose.


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ostium is the opening from the sinus to the middle me-atus of the nose (Fig. 12). It is situated on the superioraspect of the medial wall of the maxillary sinus abovethe first molar. The mean distance from the most infe-rior point of the antral floor to the ostium is 28.5 mm.86

Thus, when performing a sinus lift, the sinus should not

be overfilledwith graftmaterial beyond 15 mm to avoidpotentially blocking the ostium and causing sinusitis.

The maxillary sinus is surrounded by six walls.87 1)The anterior wall contains the infraorbital nerve andblood vessels to the anterior teeth. The infraorbital ar-tery gives off the anterior superior alveolar arteriesthat supply the sinus mucosa in the anterior sectionof the sinus. 2) The superior wall is very thin andmakes up the orbital floor. A bony ridge containsthe infraorbital canal with the nerve and blood vessels.3) The posterior wall corresponds to the pterygomax-illary region, which separates the antrum from the

pterygopalatine fossa. It contains the posterior supe-rior alveolar nerve and blood vessels, including thepterygoid plexus of veins and internal maxillary ar-tery. 4) The medial wall separates the sinus from thenasal fossa. The maxillary ostium (around first molararea) drainsinto the middlemeatus of thenasal cavity.5) The sinus floor may extend between the roots ofthe maxillary molars. The floor may be 10 mm be-low the floor of the nasal cavity. 6) The lateral wallforms the posterior maxillary and zygomatic process.This wall provides accessfor the sinus graft procedure.

The medial wall derives its arterial supply from na-

sal mucosal vasculature. This comes from branchesof the sphenopalatine artery: posterior lateral nasaland posterior septal branches. The frontal, lateral,and inferior walls derive their arterial supply fromthe osseous vasculature (infraorbital, facial, and pal-atine arteries). The medial sinus wall drains throughthe sphenopalatine vein. All other veins drain throughthe pterygomaxillary plexus. Innervation is providedby nasal mucosa nerves and the superior alveolarand infraorbital nerves.

Septa (Underwoods clefts) have been located in31.7% of the maxillary sinuses in the premolar area,and they usually do not compartmentalize the an-trum.88 However, they frequently get larger as theyproceed medially. Therefore, during a sinus lift, mem-brane elevation over partial septa should proceed lat-erally to medially, because elevation attemptedanteriorly to posteriorly is more prone to create a per-foration.To accommodatelargeor multiple septa dur-ing a sinus lift, more than one lateral window can becreated as part of the antral opening.88 In addition,septa are a concern if an osteotome sinus floor eleva-tion procedure is planned because it is difficult to in-fracture the subantral floor under them.

There are several other issues of interest regarding

themanagement of themaxillary sinus area. If diagnos-

tic imaging indicates that the inferior wall (alveolarridge) or the lateral wall ofthe sinus has a bonyfenestra-tion, a split-thickness flap needs to be developed overthese defects to avoid tearing the Schneiderian mem-branewhentheflapiselevated(Fig.13).Subsequently,aspartofthemembranerelease,theresidualtissueover

thebonedefectsmustbepushedintothesinus,becausethe sinus membrane cannot be separated from the softtissue that was lodged in the osseous defects.

During a lateral window preparation, if a tear in theSchneiderian membrane occurs and it is a relativelysmall defect, the opening can be patched with a colla-gen barrier.89 However, when a tear occurs along theperiphery of the window and it is difficult to reengagethe membrane, before the tear elongates, extend theosteotomy several millimeters in bone away from theoriginal site. Remove the bone over the membraneto attain better visibility and accessibility, and re-

engage the membrane where it is not torn (Fig. 14).The normal width of the Schneiderian membrane is

generally 0.3 to 0.8 mm.90 However, it can appear

Figure 14.Perforation of the membrane along the periphery of the lateralwindow. To reengage the membrane and avoid tearing of the

membrane, more bone is removed to expose more membrane (arrows).

Figure 13.CT scan. There is a fenestration (arrow) in the inferior wall of the sinus.When a sinus lift is done, after a split-thickness flap is elevated, thetissue in the fenestration is pushed into the sinus because themembrane and the tissue are fused.


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thicker if there is chronic inflammation resulting inhyperplasia. If the membrane is very thick, considerobtaining an ear, nose, and throat consult before pro-ceeding with implant placement or a sinus lift.

Nerve Innervation in the Maxilla

The sensory nerves of the palate are branches of themaxillary nerve.91 The greater palatine nerve inner-vates the gingiva, mucous membranes, and most ofthe glands of the hard palate.91 The nasopalatinenerve supplies the mucous membranes of the anteriorhard palate. The lesser palatine nerves supply the softpalate.91 The infraorbital nerve innervates the mu-cosa of the maxillary sinus; the maxillary incisors, ca-nine, and premolars; the maxillary gingiva; theinferioreyelid and conjunctiva; part of the nose; and the supe-rior lip.92 The posterior superior nerve supplies thegingiva and mucous membranes in the posterior max-

illa, sinus, and molar teeth.91

CONCLUSIONS

Familiarity with the anatomic structures pertaining todental implantology is critically important. Preplan-ning and review of anatomy before surgical proce-dures can help to avoid problems. However, certainanatomic structures may be problematic with respectto treatment planning. In this regard, many of theshortcomings of two-dimensional radiography fortreatment planning can be eliminated with the useof three-dimensional imaging. In particular, if themandibular or mental nerves position is not clearor if it is unclear how much bone is present for implantplacement, a CT scan should be ordered. Similarly,CT scans are an important diagnostic aid in predeter-mining the dimensions of the maxillary sinus andthe presence of unexpected findings (e.g., septa, tu-mors, and intraosseous arteries). A general rule tofollow is, if you are wondering if you need a CT scan,order one. Finally and importantly, proper trainingshould be obtained to provide advanced surgicalprocedures.

ACKNOWLEDGMENT

The authors report no conflicts of interest related tothis review.

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Correspondence: Dr. Gary Greenstein, 900 W. Main St.,Freehold, NJ 07728. Fax: 732/780-7798; e-mail: [email protected].

Submitted February 10, 2008; accepted for publicationMarch 21, 2008.


Implant Surgical Anatomy

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