Editor: Allan G. Farman, BDS, PhD (odont.), DSc (odont.), Diplomate of the American Board of Oral and Maxillofacial Radiology, Professor of Radiology and Imaging Sciences, Department of Surgical and Hospital Dentistry, The University of Louisville School of Dentistry, Louisville, KY . Contributor: Dr. C.J. NortjØ, BChD, PhD, DSc, Professor and Chairman of Oral and Maxillofacial Radiology, Tygerberg, South Africa, President-Elect of the International Association of Dentomaxillofacial Radiology. Featured Article: Panoramic radiographic appearance of the mandibular canal in health and in disease Volume 2, Issue 2 US $6.00 Panoramic radiographic appearance of the mandibular canal in health and in disease The mandibular canal is of particular importance to the dentist and dental specialist as it carries both the dental division of the trigeminal nerve and the innervation for the lower lip. The trigeminal nerve enters the inner surface of the mandibular ramus at the mandibular foramen, in the vicinity of a bony eminence, the lingula. This is a fact learned in study of anatomy and reinforced by the everyday necessity of locating an inferior dental block injection for local analgesia required in many dental procedures. What is not so well understood is that normal is a range and that variations do occur in which there may be more than one nerve entry point a factor that might account for failed anesthesia in at least a small percentage of patients. Such variations have been described both during studies of macerated mandibles from cadavers and also from the study of panoramic radiographs. Panoramic radiographs may also help find the position of the mental foramen, through which the nerve supply to the lower lip passes. Failure to protect the mental foramen can lead to permanent loss of normal sensation in the lower lip. The panoramic radiographic positioning of the mental foramen and the mandibular canal has been used as an indication of bone loss following dental extractions. A comprehensive study of variations in the mandibular canal in patients who had not suffered mandibular pathoses or trauma By Dr. Allan G. Farman in collaboration with Dr. C.J. NortjØ found that the mandibular canals are usually, but not invariably, bilaterally symmetrical, and that the majority of hemimandibles contain only one major canal. 1 The position of the canal varies with respect to the apices of the tooth roots and the lower border of the mandible. They can be classified as high (Type I close to the apices of the teeth), intermediate (Type II) or low (Type III close to the lower cortex of the mandible) varieties. 2 The proportions of types varies with the investigation perhaps indicating a geographic or ethnic variability. 1,2 Neither study showed a gender difference with respect to the positioning of the canal. There were almost equal numbers of high and low canals in a South African study with few intermediate canals. 1 In a Greek study there were few high canals and almost equal proportions of intermediate and low canals. 2 The Greek study also found asymmetry in canal positioning in almost one in five of those studies; whereas the South African study found this to occur in less than one in a hundred. 1,2 It can be concluded that in a single panoramic radiograph the mandibular canal should not be used as a set reference point for assessment of bone loss following extractions. To make such an assessment requires sequential panoramic radiographs on a given patient. Supplemental mandibular canals large enough to be seen on panoramic radiography are rare but are occasionally present, the most common being duplicate canals commencing from a single mandibular foramen, and the least
Transcript
Editor:
Allan G. Farman, BDS, PhD(odont.), DSc (odont.),Diplomate of theAmerican Board of Oraland MaxillofacialRadiology, Professor ofRadiology and ImagingSciences, Department ofSurgical and HospitalDentistry, The University ofLouisville School ofDentistry, Louisville, KY.
Contributor:
Dr. C.J. Nortjé, BChD, PhD, DSc,Professor and Chairman ofOral and MaxillofacialRadiology, Tygerberg, SouthAfrica, President-Elect of theInternational Association ofDentomaxillofacial Radiology.
Featured Ar ticle:
Panoramic radiographicappearance of themandibular canal in healthand in disease
Volume 2, Issue 2 US $6.00
Panoramic radiographic appearance of themandibular canal in health and in disease
The mandibular canal is ofparticular importance to thedentist and dental specialist as itcarries both the dental division ofthe trigeminal nerve and theinnervation for the lower lip. Thetrigeminal nerve enters the innersurface of the mandibular ramus atthe mandibular foramen, in thevicinity of a bony eminence, thelingula. This is a fact learned instudy of anatomy and reinforcedby the everyday necessity oflocating an inferior dental blockinjection for local analgesiarequired in many dentalprocedures. What is not so wellunderstood is that normal is arange and that variations do occurin which there may be more thanone nerve entry point � a factorthat might account for failedanesthesia in at least a smallpercentage of patients. Suchvariations have been describedboth during studies of maceratedmandibles from cadavers and alsofrom the study of panoramicradiographs. Panoramicradiographs may also help find theposition of the mental foramen,through which the nerve supply tothe lower lip passes. Failure toprotect the mental foramen canlead to permanent loss of normalsensation in the lower lip. Thepanoramic radiographicpositioning of the mental foramenand the mandibular canal hasbeen used as an indication of boneloss following dental extractions.
A comprehensive study ofvariations in the mandibular canalin patients who had not sufferedmandibular pathoses or trauma
By Dr. Allan G. Farman incollaboration with Dr. C.J. Nortjé
found that the mandibular canalsare usually, but not invariably,bilaterally symmetrical, and thatthe majority of hemimandiblescontain only one major canal.1 Theposition of the canal varies withrespect to the apices of the toothroots and the lower border of themandible. They can be classifiedas high (Type I � close to theapices of the teeth), intermediate(Type II) or low (Type III � close tothe lower cortex of the mandible)varieties.2 The proportions of typesvaries with the investigationperhaps indicating a geographicor ethnic variability.1,2 Neither studyshowed a gender difference withrespect to the positioning of thecanal. There were almost equalnumbers of high and low canals ina South African study with fewintermediate canals.1 In a Greekstudy there were few high canalsand almost equal proportions ofintermediate and low canals.2 TheGreek study also found asymmetryin canal positioning in almost onein five of those studies; whereasthe South African study found thisto occur in less than one in ahundred.1,2 It can be concludedthat in a single panoramicradiograph the mandibular canalshould not be used as a setreference point for assessment ofbone loss following extractions. Tomake such an assessment requiressequential panoramic radiographson a given patient.
Supplemental mandibularcanals large enough to be seen onpanoramic radiography are rarebut are occasionally present, themost common being duplicatecanals commencing from a singlemandibular foramen, and the least
“ Primary lesions developing within themandibular canal are frequently neuralor vascular in origin.”
common arising from two separateforamina [Fig. 1 & 2].1-3 Suchduplicate canals are found in only0.5 to 1.0 % of studied adultpopulations.1,2,4 They are sometimestermed �bifid canals.�2,4 That suchbilateral canals are a reality ratherthan a projection artifact has beenproven both by anatomicaldissection (Carter and Keen 1971)5
and also by computed tomography(Quattrone et al, 1989)6. Whether thecontents are neural, neurovascularor simply vascular is a contentiouspoint. If nerves were present in thetwo canals, this might account forsome failure to achieve localanesthesia when applying blockinjections.
On occasion, appearance ofduplicated mental foramina is alsoobserved [Fig. 3]. Such trueduplication needs to bedistinguished from the separatedepictions of the mental canal atits origin from the mandibular canalcentral within bone, and at its exitfrom the facial cortex of themandible.
It is possible that bifid canalsrepresent a minor expression ofstructural twinning. Very rarely, themandible may evidence augnathus,a variant of paragnathus.7 Such acase, subsequently treatedsuccessfully by surgeons Davis andBreytenbach in Cape Town, SouthAfrica, is illustrated in Fig. 4. In thiscase, unilateral duplication of themandible was accompanied byduplication of the mandibularcanal � and also of the dentition forthat jaw quadrant.
Pathological conditions ofthe mandibleThe effects of pathologicalconditions of the mandible on thepanoramic appearance of themandibular canal was first reportedby the author of the present report,
a quarter of a century ago.8
It was found that variousdisease processes canaffect the panoramicradiographic appearanceof the mandibular canal in avariety of ways. Localizedloss of the canal corticalbone was found withchronic apicalperiodontitis, chronicpericorontitis, advancedchronic destructiveperiodontitis (in patientshaving a high mandibularcanal), and rarely also withvery large Stafne�s bonecavities. Generalized loss ofthe canal�s cortical bonewas usually indicative ofsevere infection oraggressive neoplasia, andwas found in associationwith rarefying osteomyelitis,invasive squamous cellcarcinoma, multiplemyeloma, osteogenicsarcoma and occasionallywith ameloblastoma.Displacement of the canalsuggested a benign cysticor neoplastic process, andwas found with largeradicular cysts, residualdental cysts, dentigerouscysts and the cemento-ossifying fibroma amongother benign conditions.
Benign lesions withinthe mandibular canalPrimary lesions developingwithin the mandibular canalare frequently neural orvascular in origin. Benignneoplasm within the canalwill tend to widen the canaland cause superior andinferior displacement of thecanal as the lesion expands.Especially with slow growing
Fig. 1. Examples of�bifid,� or �duplicate,�mandibular canals.Such canals have beenconfirmed in variousstudies both usinganatomical dissectionand by computedtomography.
Fig. 4. Augnathus (avariant of paragnathus)demonstrating anextreme form ofduplication of themandibular canal. (Casetreated by Professors D.Davis and M.Breytenbach, CapeTown, South Africa.)
lesions the cortical plate of thecanal will remain intact. Fig. 5illustrates a case of neurilemmomaarising within the mandibular canal.This is a homogeneously radiolucentlesion that has caused dilation of thecanal in the site of the tumor. Thenormal canal blends with the lesionboth mesially and distally with thecortical plate expanding toencompass the lesion. Certainly, notall neuilemmonas of the mandibleare associated with dilation of themandibular canal, especially if theyare situated in the premolar oranterior regions.10 However, dilation ofthe mandibular canal, when present,does suggest a lesion epicenterwithin the canal.
Shapiro et al (1984) investigatedthe maxillofacial radiographicmanifestations of neurofibromatosis(von Recklinghausen�s disease), acondition affecting one in 3000 livebirths in which those affected areprone to the development of benignneural tumors, neurofibromas.11 Theyfound that 72 % of the 22 subjectsstudied had oral or maxillofacialradiological signs of the diseasesuch as widened mandibular canals(6 cases) or enlarged mandibularforamina (6 cases including two whoalso had widened canals). Lee et al.(1996) found that six of 10 patientswith neurofibromatosis showedenlargement of the mandibularforamen.12
Malignant lesions within themandibular canalPrimary malignancies arising withinthe mandibular canal are extremelyrare.13 When they do arise they willreflect a tissue of origin from the siteconcerned; i.e. neural, vascular,fibrous or smooth muscle. Fig. 6illustrates a case of primaryleiomyosarcoma arising in the leftmandibular body and causingdestruction of the canal outline. The
Fig. 6. Leiomyosarcoma(malignant neoplasm ofsmooth muscle),epicentered on themandibular canal, withdestruction of thecanal�s cortical outlines.
Fig. 7. Radicular cyst arising from the grosslydecayed left mandibular first permanent molartooth. Pressure developing within the cyst due toan osmotic gradient causes growth of the lesionand displacement of adjacent structures includingthe mandibular canal.
Fig. 8. Large dentigerous cyst associatedwith the crown of a horizontally positionedunerupted third molar tooth in the right sideof the mandible. The right mandibular canalis displaced downwards in comparison withthe ipsilateral canal.
Fig. 9. Ameloblastomain the right mandibularbody. The lesionresulted in resorptionof the apices of thesuperjacent teeth �but in downwarddisplacement of theintact subjacentmandibular canal.
Fig. 10. Codontodownwthe manpermanshows ecrown bresembcyst. Thedisplaceborder o
Fig. 11. Cementoblastoof the mandibular firstmolar tooth displacingthe roots of the seconpremolar and permansecond molar teeth. Tmandibular canal hasalso been displaceddownwards.
Fig. 12. Squamous cell-carcinoma invadingthe left mandibularbody and ramus �and eroding themandibular canalcortices. The lesionoriginated peripherallyto bone and hence is�saucer-shaped.�
young male patient evidencedparesthesia of the left side of thelower lip. No other site of diseasewas found so this is presumed aprimary lesion. Not allmalignancies cause destructionof the canal outline. Extranodalnon-Hodgkin�s lymphoma hasbeen reported to causeenlargement of the canal notunlike that described for benigntumors.14,15 Metastases affectingthe mandibular canal site arealso rare, but certainly morecommon than primarymalignancies.
Benign lesions peripheralto the mandibular canalSlow growing benign cysts andtumors peripheral to themandibular canal are likely tocause gradual displacement ofthe canal rather than resorptionof the canal cortices. Examplesof such conditions are illustratedin Fig. 7-11. When a homogeneousradiolucency is associated withexpansion of the apicalperiodontal ligament space of anon-vital root canal, and thelesion is large enough to causedisplacement of the mandibularcanal, the most likely diagnosis isa radicular cyst [Fig. 7.]. If ahomogeneous radiolucencysurrounds the crown of anunerupted tooth and is attachedto the tooth at the enamel-cemental junction. The mostlikely diagnosis is a dentigerouscyst. It should be cautioned thata variety of other conditions canenvelope the crown of a tooth;hence, histopathologicalconfirmation is required. Largedentigerous cysts can alsocause the displacement of theaffected tooth and if itapproaches the mandibularcanal displacement, is to be
expected [Fig. 8.]. Benign tumorscan also cause canaldisplacement. The mostcommon benign odontogenicneoplasm is the ameloblastoma[Fig. 9.] and this can either causedisplacement or resorption ofthe canal, or can in some casessimply camouflage the canal byaddition of septae and �soapbubble� trabecular patterns. Fig.10 is a detail from a panoramicradiograph of a patient having acalcifying epithelialodontogenic tumor. The lesionhas displaced an adjacenttooth and there is dilatation orinvasion of the follicle spaceresembling a dentigerous cyst.This highlights the importance ofhistopathological evaluation oftissue from supposeddentigerous cysts. Thedisplaced tooth has alsoresulted in displacement of themandibular canal in this case.
Fig. 11 illustrates the detail ofa cementoblastoma that hasdisplaced the mandibular canaltowards the lower cortex of themandible. The features of thiscondition are entirely benign.16
While this particular case wasexcised in its entirety, it issometimes possible toendodontically treat anaffected tooth and then simplysurgically excise the lesion.
Malignant lesionsperipheral to themandibular canalSevere infections, such assuppurative osteomyelitis, andmalignant neoplasms are notinfrequently associated with anirregular erosion or lysis of theaffected jaw � and themandibular canal is not sparedin this process.
The most common
alcifying epithelialgenic tumor causingard displacement ofdibularn firstent molar whichnvelopment of they a radiolucency
ling a dentigerous mandibular canal isd towards the lowerf the mandible.
ma
denthe
“ Slow growing benign cysts and tumors peripheral to themandibular canal are likely to cause gradual displacementof the canal rather than resorption of the canal cortices.”
malignancy affecting the oralcavity is squamous cell carcinomaarising in the oral mucosa. Thelesion can secondarily invadeadjacent bone [Fig. 12]. Lesionsarising within bone generally have a�brandy glass� appearance whenthey erode outwards. Incomparison, lesions arisingperipherally, such as invadingsquamous cell carcinoma,produce a �saucerized�appearance. The mandibular canalmight be thought of as a �highwayfor metastases� hence, erosion ofthis structure can be viewed as anegative factor regardingprognosis. A study of gingivalcarcinoma found no statisticaldifference between the diagnosticaccuracy of panoramicradiographs and computedtomography for the determinationof the supero-inferior invasion ofthe mandible.17
The most common malignancyof bone is myeloma. This conditiontends to occur in late middle ageand in the elderly with �punched-out� radiolucencies often beingfound in many bones, but showing aparticular predilection to thecalvarium. An example of a lyticlesion forming centrally within themandible is illustrated in Fig. 13. Thisparticular lesion has not spared themandibular canal and has resultedin a pathologic fracture.
Less common malignancies ofthe jaws include the osteogenicsarcoma and the chondro-sarcoma. Both of these conditionscause lysis of normal bone,including the cortices of themandibular canal when the lowerjaw is affected. Both can alsodemonstrate abnormal boneformation including floccules or�sunburst� appearances. A�sunburst� appearance isdemonstrated in the osteogenic
sarcoma illustrated in Fig. 14 wheretrabeculations of abnormal newbone are superimposed on thebasic lytic lesion. This case alsodemonstrates a �floating tooth�where the bone supporting a leftmandibular molar has beendestroyed and growth of the lesionhas elevated the tooth.
Lesions obscuring themandibular canalSome conditions can obscure theappearance of the mandibularcanal through producing acomplex trabecular pattern thatcamouflages the canal. Conditionsthat cause this effect includebenign tumors such as theodontogenic myxoma,hamartomas such as intraosseoushemangiomas, and the familial�fibro-osseous� condition,cherubism [Fig.15.] Cherubism is adominantly inherited condition thatis usually bilateral andpredominantly affects both sidesof the mandible. Other conditionsthat may obscure the mandibularcanal are those in which densebone is deposited. Such conditionsinclude osteopetrosis, late stagefibrous dysplasia [Fig. 16.] and floridosseous dysplasia [Fig. 17.] Fibrousdysplasia generally arises in youngindividuals and causes expansionof the jaw unilaterally and typicallydoes not cross the midline.Sclerosis generally occurs by earlyadulthood. Florid osseous dysplasiamost frequently is found in middleage women of African extraction.
Concluding RemarksIt is sometimes believed that thespecial anatomic structures of thejaws � especially the teeth � makethe radiologic interpretation ofdisease entities affecting thejawbones particularly difficult asthey hinder comparison with
lesions of a similar nature found inbones elsewhere in the body.18 Theconverse can be the case if theaffects on these very structures areused as clues to discovering thenature of the condition. Themandibular canal is usually clearlydepicted in the panoramic dentalradiograph. The dentists familiaritywith the normal range for itsanatomy � and the ways in which itcan be affected by variousdisease entities, should place thedentist at an advantage indetection and interpretation of thenormal, versus disease. It should bekept in mind that while somedisease entities produceconsistent features that might helpradiologic differentiation, others(e.g. ameloblastoma) showvariable or non-specific changes.Nevertheless, in combination withthe other well-describedradiologic features of these lesions,interpretation of changesconcerning the canal as shown onpanoramic radiography may wellassist in deriving a more accuratedifferential diagnosis list. Inparticular, canal displacement isalmost invariably a feature ofbenign lesions, whereas extensiveloss of the canal cortical plate isusually a feature of severe infectionor aggressive neoplasia.
Fig. 13. Myeloma:the lesion hasdestroyed thecortices of themandibular canaland also resultedin a pathologicalfracture of the jaw.
“ A study of gingival carcinoma found no statistical difference between thediagnostic accuracy of panoramic radiographs and computed tomographyfor the determination of the supero-inferior invasion of the mandible.”
Variations in the normal anatomy of theinferior dental (mandibular) canal: aretrospective study of panoramicradiographs from 3612 routine dentalpatients. Brit J Oral Surg 1977; 15:55-63.
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7. Farman AG, Escobar V. Duplication of theoral and maxillofacial structures.Quintessence 1986;17:731-737.
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9. Xie Q, Wolf J, Tilvis R, Ainamo A. Resorptionof the mandibular canal wall in edentulousaged population. J Prosthet Dent1997;77:596-600.
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11. Shapiro SD, Abramovitch K, Van Dis ML,Skoczylas LJ, Langlais RP, Jorgenson RJ,Young RS, Riccardi VM. Neurofibromatosis:oral and radiographic manifestations. OralSurg Oral Med Oral Pathol 1984;58:493-498.
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13. Farman AG, Kay S. Leiomyosarcoma of theoral cavity. Oral Surg 1977;43:402-409.
14. Yamada T, Kitagawa Y, Ogasawara T,Yamamoto S, Ishii Y, Urasaki Y. Enlargementof mandibular canal without hypesthesiacaused by extranodal non-Hodgkin�slymphoma. Oral Surg Oral Med Oral PatholOral Radiol Endod 2000;89:388-392.
15. Bertolotto M, Cecchini G, Martinoli C,Perrone R, Garlaschi G. Primary lymphomaof the mandible with diffuse widening ofthe mandibular canal. Eur Radiol 1996;6:637-639.
17. Nakayama E, Yoshiura K, Yuasa K, Tabata O,Araki K, Kanda S, Ozeki S, Shinohara M.Detection of bone invasion by gingivalcarcinoma of the mandible: a comparisonof intraoral and panoramic radiography andcomputed tomography. DentomaxillofacRadiol 1999;28:351-356.
Fig. 14. Osteogenic sarcomaof the left mandible. Thelytic phase of the lesion hasdestroyed the outline of themandibular canal. Note the�sunburst� appearance ofnew bone formation that isconsidered a classic, butnot invariable, feature ofthe condition.
Fig. 15. Cherubism:The trabecularpatterns within thebilateral lesions ofthe mandibleobscure the outlinesof the mandibularcanals in theaffected areas.
Fig. 16. Fibrous dysplasia (late phase):The �frosted glass� trabeculationsthat develop in the latter stages offibrous dysplasia have reduced theclarity of the mandibular canal.
Fig. 17. While most lesions offlorid osseous dysplasia occurabove the mandibular canal,large lesions combined withfactors of panoramic projectiongeometry conspire to obscurethe mandibular canals.
