Anatomia, Histologia, Embryologia
ORIGINAL ARTICLE
The Inferior Alveolar Nerve of the Horse: Course and Anatomical Relationship with Mandibular Cheek Teeth I. Iacopetti1*, M. Faughnan2, S. Bono2, B. Cozzi3 and C. Facchini1,2 Addresses of authors: 1 Department of Animal Medicine, Production and Health, University of Padova, Viale dell’Universita 16 -Agripolis-, 35020 Legnaro, Padova, Italy; 2 Veterinary Equine Practitioner, Padova, Italy; 3 Department of Comparative Biomedicine and Food Safety, University of Padova, Viale dell’Universita 16 -Agripolis-, 35020 Legnaro, Padova, Italy
*Correspondence: Tel.: +39 049 8272950; fax: +39 049 8272954; e-mail: [email protected] With 4 figures Received March 2014; accepted for publication July 2014 doi: 10.1111/ahe.12144
Summary The precise location of neurovascular structures within the relatively long mandibular canal of the horse is of paramount importance in surgical procedures of the area. The inferior alveolar nerve (IAN) enters the mandibular canal on the medial (lingual) surface of the mandible and innervates all the mandibular teeth. During its course, the nerve moves laterally, crossing the roots of the inferior cheek teeth. However, the exact anatomical relationships occurring between the IAN and the roots of the equine mandibular cheek teeth have not been described in detail. In this study, the mandibles of 40 horses were examined with CT scans and then used for bilateral detailed anatomical dissection, to assess the path of the IAN and its relationship to the roots of the lower cheek teeth. The data obtained show that the equine IAN is located ventral to the apices of the molar teeth (311/411, 310/410, 309/409 according to the Triadan numerical system). At the level of PM4 (308/408), the IAN is located on the lingual side of the roots and coronally to its apices. At the level of PM3 (307/407), the IAN is then found on the lingual side of the roots but in proximity to the apices. In 2 of 40 horses (=5%), the IAN moves towards the lingual side between the mesial and the distal root of PM4. Our observations are valuable for planning a surgical approach to the ventral side of the mandible in the horse and to avoid potential post-operative complications.
Introduction A detailed knowledge of the precise location of the mandibular canal is essential to prevent injury to its neurovascular structure during various veterinary dental procedures, such as exodontia, endodontics, orthopaedic and oncologic surgery. The mandibular canal begins at the mandibular foramen ventral to the insertion of the temporalis muscle on the medial (lingual) surface of the mandibular ramus. This canal runs rostrally through the mandibular body till its termination at the mental foramina on the lateral (vestibular) surface of the mandible approximately
© 2014 Blackwell Verlag GmbH Anat. Histol. Embryol. 44 (2015) 333–337
3–4 cm rostral to PM2 (306/406 according to the Triadan numerical system). The purely sensory inferior alveolar nerve (IAN, nervus alveolaris inferior) enters the mandibular canal and innervates all the lower mandibular teeth as well as the gum and periodontium (for a description of the nerve in the horse, see Wissdorf et al., 1998). After emerging from the mental foramen as mental nerve (n. mentalis), it then provides sensory innervation to all tissue located rostrally, while a branch coming from the inferior dental plexus inside the mandibular canal (ramus alveolaris inferior rostralis) reaches the canine and incisors teeth (Wissdorf et al., 1998; Nomina Anatomica Veterinaria, 2012). During its course
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in the mandibular canal, it moves from the medial to the lateral side, crossing the roots of the mandibular cheek teeth before emerging from the mental foramen. Radiographically, in an oblique projection, the mandibular canal appears as a radiolucent linear image along the ventral aspect of the mandible. However, the precise relationship between the mandibular canal and the dental roots cannot be revealed by radiographs. In the last few years, computerized tomography (CT) has been increasingly used in equine dentistry as a valuable diagnostic tool. CT provides detailed cross-sectional images of tissues with bony and soft tissue contrast, thus eliminating the problem of overlap. The resulting images represent two dimensional transverse slices of objects with different degrees of thickness, providing much more detail of the normal anatomy of equine mandibular cheek teeth, including, infundibular and pulpar anatomy as well as detailed information regarding possible dental pathology (Dacre et al., 2008; Simhofer et al., 2008; Windley et al., 2009a,b). Several reports discuss the value of CT in documenting conditions of the equine skull (Tietje et al., 1996; Morrow et al., 2000; Walker et al., 2002; Lischer et al., 2005; Quinn et al., 2005; Puchalski, 2006; Simhofer, 2010); however, to our knowledge, no information is available about the appearance and course of the IAN and its relationship with the roots of the mandibular cheek teeth. Several post-operative complications are associated with procedures involving mandibular cheek teeth, including dental extractions performed by either intraoral or repulsion techniques, or in the case of endodontic procedures (Butson et al., 1997; Dixon et al., 2000; Schramme et al., 2000; Earley, 2010; Facchini et al., 2014). However, to the best of our knowledge, no cases of associated IAN lesions have been reported in the literature. Eventual damages to the IAN could lead to altered sensitivity in the corresponding area. This specific occurrence is well known in the human dental surgery (Worthington, 2004; Strauss et al., 2006; Hillerup, 2007), but may be underestimated in equine medicine possibly due to the difficulties in pin-pointing the precise cause of the symptomatology.
(a)
The aim of this study is to determine the exact course of the equine IAN in relation to the mandibular cheek teeth roots. To this effect, dissection of a series of the equine skulls (40) was carried out, and CT scans were performed. Given the close relationship between the nerve and the roots of some mandibular teeth, any lesions to the IAN and its resulting consequences should be examined in greater depth. It is our hope that our data will provide useful topographic information and therefore shed light on the risk of possible iatrogenic damages to the nerve during surgery on the apices of the mandibular cheek teeth.
Materials and Methods Dissection techniques This study was performed at the campus of the School of Agriculture and Veterinary Medicine at the University of Padova (Italy). Forty equine mandibles were collected during necropsy procedures performed for the purpose of post-mortem diagnosis. The horses were all adults (6– 15 years) of both genders and various breeds. The mandibles were dissected to eliminate the cortical part of the bone until the periostium was reached and subsequently removed with a surgical blade. An oscillating saw with a blade diameter of 5 cm was used to create a rectangular box-like section laterally, extending for the whole length of the mandibular cheek tooth arcade until approximately 1 cm from the ventral margin of the mandibular body (Fig. 1a). The periosteum was then removed with a surgical scalpel and a hammer (Fig. 1b), and the cheek teeth were exposed. Care was then taken to not dissect beyond the cortical tissue so as to avoid damages to the underlying structures, especially the IAN. After the box-like section had been exposed, all mineralized material and spongy tissue were then carefully removed (Fig. 1c), exposing the tooth roots and the IAN (see below). This technique was applied to both mandibles, to check the symmetrical course of the IAN. The course of the nerve could then be clearly delineated in relation to the roots of the mandibular teeth (Fig. 2a).
(b)
(c)
Fig. 1. Dissection of the mandible. (a) rectangular box-like section in the lateral side of the mandible; (b) removal of the periosteum with a sharp scalpel and a hammer; and (c) Careful removal of the all mineralized material with a scalpel revealing spongy bone.
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Fig. 2. Course of the IAN in the mandible. (a) exposure of the roots of the cheek teeth and the inferior alveolar nerve; (b) detailed relationship between IAN course and the root apex of PM3–4 and M1; and (c) detailed relationship between IAN course and the root apex of PM3–4 and M1–2. The position of PM4 is reported in all the images for reference.
Course of the Inferior Alveolar Nerve in Horses
(b)
(a)
CT scans The entire mandible body was analysed from the ramus to the mental foramen before dissection. Scan acquisition data were 120 kV, 175 mA and 2.9 s. of rotation time, and slice thickness was 3 mm. A bony acquisition algorithm was used to improve details of the structure. The mandibular CT images were obtained using a 3rd generation CT (Tomscan LX, Philips, Amsterdam, The Netherlands). The mandibles were positioned with the ventral border and the mandibular canal as close as possible to the horizontal plane in relation to the tomography table. The axial slices were made as perpendicular as possible to the mandibular canal to obtain high-quality images. Results Bilateral comparison of the progression of the IAN showed that the nerve maintains a symmetrical course in the hemimandibles of all the subjects examined. The anatomical course of the IAN did not vary with age. Anatomical results In all cases examined, the IAN penetrates the cancellous bone on the medial side of the ramus and runs immediately in the central portion of the mandible maintaining a ventral course in relation to the root apices of (a)
(c)
the mandibular molar teeth M3–1 (Triadan 311/411-309/ 409). In 95% (38 horses) of the subjects examined, the nerve changes direction between the mesial root of M1 and the distal root of PM4 (Triadan 308/408) and then runs close to the lingual cortex of the mandible. At the level of PM4, the nerve is thus localized both lingually and coronally to the apices of this tooth (Fig. 2b). In the remaining 5% of the subjects examined (2 horses), a variation was found in the course of the nerve at this site. Precisely, the IAN runs lingually and coronally as described above, but passes between the distal and the mesial roots of PM4 instead. The nerve continues rostrally on the lingual aspect of the root of PM3 returning to a position running at the same level of the apices (Fig. 2c). The nerve continues rostrally and then moves to the vestibular side of the mandible upon reaching the roots of PM2 (Triadan 306/406). Subsequently, part of the nerve continues its course towards canine and incisor teeth, while the external branches emerge from the mental foramen rostral to PM2. CT scans Analysis of CT scan data confirmed what was observed by direct visual dissection. The nerve runs in a specific canal on the lingual side of the mandible, close to the inner (b)
Fig. 3. CT images of the mandible including the canal for the IAN. (a) duplication of the branches of IAN in the mandibular canal (red stars) in relation to the root of PM3 and (b) location of mandibular canal (red star) in relation to the root of PM2.
© 2014 Blackwell Verlag GmbH Anat. Histol. Embryol. 44 (2015) 333–337
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limit of the cortical. Using this technique, we also recognized a number of variations in the course of the nerve that would be otherwise impossible to detect, such as duplication of the nerve branches (Fig. 3 a, b). The two canals of the duplicate nerve run one above the other, but not side by side. Discussion The direct visualization of the course of the equine IAN through the removal of the bone cortical and periosteal of the mandible bone in a rectangular box-like section was a satisfactory and economical mean to thoroughly follow both the course and exact location of the nerve and its relationship with the roots of the mandibular cheek teeth. The use of CT confirmed the data obtained with anatomical dissection and provided more accurate and detailed information with potential clinical value for equine practitioners. The path of the IAN in the horse and its relationship to the roots of the mandibular cheek teeth has occasional importance for surgery in the area. However, the anatomical images obtained from this study show a close proximity between the nerve and the roots of some specific mandibular cheek teeth. Our observations, summarized in Fig. 4, show that the course of the IAN is located in the central portion of the mandibular bone and runs ventrally to the root apices of the molars (M1–3). Consequently, the risk of damaging the nerve in surgical procedures involving the roots of the molar teeth is rather low, as the general consensus is to use a lateral or ventro-lateral surgical approach to these structures. During its rostral course, at the level of PM4, the IAN then runs lingually to the distal and mesial roots and is found coronally to the apices of both roots. Consequently, surgical procedures involving a ventro-lateral approach to the roots of PM4 should be relatively free of risks of accidental iatrogenic damage of the IAN. Available references show that the percentage of apical infections involving PM4 is 9.5% (Dixon et al., 2000). Both PM2 and PM3 are the most frequent mandibular cheek teeth that are affected by apical infections according to the literature (Dixon et al., 2000). Specifically, the percentage of apical infections involving PM3 and PM2 is 34.5 and 31%, respectively (Dixon et al., 2000). Our observations show that the IAN runs indeed very close to the roots apices of PM3 and PM2, with potential negative consequences in the case of various local surgical procedures such as endodontia, dental repulsion, fractures, apicectomy, etc. The variability of the pathway of the IAN, together with the narrowness of the mandibular body and the ves-
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Fig. 4. Schematic representation of variations in the course of the IAN in the mandible. Right side – blue line, commonest route of the nerve (95% of the subjects). Left side – green line, route of the nerve in 5% of the cases.
tibular course of the nerve, is among the potential factors that could lead to post-surgical complications of the equine mandible. To this effect, CT scans may be a valuable diagnostic and pre-operative tool to avoid possible damages to the IAN in the area of the premolar tooth roots, as is also shown in the dog (Martinez et al., 2009). Interestingly, the topography of the equine IAN is similar to the human (Gowgiel, 1992). This comparative anatomical similarity merits further study among mammalian species, especially considering the ample variation of both food type and mastication among the various species. © 2014 Blackwell Verlag GmbH Anat. Histol. Embryol. 44 (2015) 333–337
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To the best of our knowledge, injuries to the IAN have never been reported in the literature as a complication of equine dental or surgical procedures including extractions, apicectomy and endodontic treatments. However, the potential risks of damaging the IAN close to the apex of the roots of PM2-4 should be taken into account, especially considering the anatomical variations of its course in proximity to the roots of PM4. Acknowledgements The Authors wish to thank prof. Alessandro Zotti of the Department of Animal Medicine, Production and Health of the University of Padova for providing the CT scans, and Mr. Emanuele Zanetti of the Department of Comparative Biomedicine and Food Science of the University of Padova for his skillful technical assistance. References Butson, R. J., J. Robinson, M. C. Schramme, and S. A. May, 1997: Endodontic therapy for periapical infection in ten equine cheek teeth: preliminary results, Proceedings of 5th World Veterinary Dental Congress, 1997 Birmingham, England. Dacre, I. T., S. Kempson, and P. M. Dixon, 2008: Pathological studies of cheek teeth apical infections in the horse: 4 aetiopathological findings in 41 apically infected mandibular cheek teeth. Vet. J. 178, 341–351. Dixon, P. M., W. H. Tremaine, K. Pickls, L. Kuhns, C. Hawe, J. McCann, B. C. McGorum, D. I. Railton, and S. Brammer, 2000: Equine dental disease, Part 4: a long-term study of 400 cases: apical infections of cheek teeth. Equine Vet. J. 32, 182–194. Earley, E. T., 2010: How to identify potential complications associated with cheek teeth extraction in the horse. AAEP Proceedings 2010 vol. 10, 435–480 Baltimore, MA, USA. Facchini, C., S. Bono, M. Faughnan, and I. Iacopetti, 2014: Endodontic treatment of a mandibular premolar abscess in a horse: a case study. Equine Vet. Educ. 26, 194–199. Gowgiel, J. M., 1992: The position and course of the mandibular canal. J. Oral Implantol. 4, 383–385. Hillerup, S., 2007: Iatrogenic injury to oral branches of the trigeminal nerve: records of 449 cases. Clin. Oral Investig. 11, 133–142. Lischer, C. J., U. Walliser, P. Witzmann, E. M. Wehrli, and S. Ohlerth, 2005: Fractures of the paracondylar process in four horses: advantages of CT imaging. Equine Vet. J. 37, 483–487. Martinez, L. A. V., M. A. Gioso, C. M. Villegas Lobos, and A. C. B. Fonseca Pinto, 2009: Localization of the mandibular canal in Brachycephalic dogs using computed tomography. J. Vet. Dent. 26, 156–163.
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Course of the Inferior Alveolar Nerve in Horses
Morrow, K. L., R. D. Park, T. L. Spurgeon, T. S. Stashak, and B. Arceneaux, 2000: Computed tomographic imaging of the equine head. Vet. Radiol. Ultrasound. 41, 491–497. Puchalski, S. M., 2006: Computed tomographic and ultrasonographic examination of equine dental structures: normal and abnormal findings. AAEP Focus meeting 2006 Indianapolis USA Quinn, G. C., W. H. Tramaine, and J. G. Lane, 2005: Supernumerary cheek teeth (n = 24): clinical features, diagnosis, treatment and outcome in 15 horses. Equine Vet. J. 37, 505–509. Schramme, M. C., J. C Boswell, J. Robinson, S. A. May, R. K. Smith, D. Platt, and J. Schumacher, 2000: Endodontic therapy of periapical infection of the cheek teeth: a study of 19 horses. AAEP Proceedings, 2000. 46, 113–116 San Antonio, TX, USA. Simhofer, H., 2010: Advantages and limits of computed tomography in the diagnosis of equine dental disease. BEVA Proceedings, 2010. 32–33 Birmingham, England. Simhofer, H., C. Stoian, and K. Zetner, 2008: A long-term study of apicectomy and endodontic treatment of apically infected cheek teeth in 12 horses. Vet. J. 178, 411–418. Strauss, E. R., V. B. Ziccardi, and M. N. Janal, 2006: Outcome assessment of inferior alveolar nerve microsurgery: a retrospective review. J. Oral Maxillofac. Surg. 64, 1767–1770. Tietje, S. M., M. Becker, and G. B€ ockerhoff, 1996: Computed tomographic evaluation of head disease in the horse: 15 cases. Equine Vet. J. 28, 98–105. Walker, A. M., D. C. Sellon, C. J. Cornelisse, M. T. Hines, C. A. Ragle, N. Cohen, and H. C. Schott II, 2002: Temporohyoid osteoarthropathy in 33 horses (1993–2000). J. Vet. Int. Med. 16, 697–703. Windley, Z., R. Weller, W. H. Tremaine, and J. D. Perkins, 2009a: Two-and three-dimensional computed tomographic anatomy of the enamel, infundibulae and pulp of 126 equine cheek teeth. Part 1: findings in teeth without macroscopic occlusal or computed tomographic lesions. Equine Vet. J. 41, 433–440. Windley, Z., R. Weller, W. H. Tremaine, and J. D. Perkins, 2009b: Two-and three-dimensional computed tomographic anatomy of the enamel, infundibulae and pulp of 126 equine cheek teeth. Part 2: findings in teeth without macroscopic occlusal or computed tomographic lesions. Equine Vet. J. 41, 441–447. Wissdorf, H., H. Gerhards, and G. Huskamp, 1998:Praxisorientierte anatomie des pferdes. Schaper Alfed Verlag, Hannover, pp.1–628. World Association of Veterinary Anatomists, International Committee on Veterinary Gross Anatomical Nomenclature, 2012: Nomina Anatomica Veterinaria, 5th edn. Hannover, Germany: The Editorial Committee. pp. 1–160. Worthington, P., 2004: Injury to the inferior alveolar nerve during implant placement: a formula for protection of the patient and clinician. Int. J. Oral Maxillofac. Implants 19, 731–734.
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ORIGINAL ARTICLE
The Inferior Alveolar Nerve of the Horse: Course and Anatomical Relationship with Mandibular Cheek Teeth I. Iacopetti1*, M. Faughnan2, S. Bono2, B. Cozzi3 and C. Facchini1,2 Addresses of authors: 1 Department of Animal Medicine, Production and Health, University of Padova, Viale dell’Universita 16 -Agripolis-, 35020 Legnaro, Padova, Italy; 2 Veterinary Equine Practitioner, Padova, Italy; 3 Department of Comparative Biomedicine and Food Safety, University of Padova, Viale dell’Universita 16 -Agripolis-, 35020 Legnaro, Padova, Italy
*Correspondence: Tel.: +39 049 8272950; fax: +39 049 8272954; e-mail: [email protected] With 4 figures Received March 2014; accepted for publication July 2014 doi: 10.1111/ahe.12144
Summary The precise location of neurovascular structures within the relatively long mandibular canal of the horse is of paramount importance in surgical procedures of the area. The inferior alveolar nerve (IAN) enters the mandibular canal on the medial (lingual) surface of the mandible and innervates all the mandibular teeth. During its course, the nerve moves laterally, crossing the roots of the inferior cheek teeth. However, the exact anatomical relationships occurring between the IAN and the roots of the equine mandibular cheek teeth have not been described in detail. In this study, the mandibles of 40 horses were examined with CT scans and then used for bilateral detailed anatomical dissection, to assess the path of the IAN and its relationship to the roots of the lower cheek teeth. The data obtained show that the equine IAN is located ventral to the apices of the molar teeth (311/411, 310/410, 309/409 according to the Triadan numerical system). At the level of PM4 (308/408), the IAN is located on the lingual side of the roots and coronally to its apices. At the level of PM3 (307/407), the IAN is then found on the lingual side of the roots but in proximity to the apices. In 2 of 40 horses (=5%), the IAN moves towards the lingual side between the mesial and the distal root of PM4. Our observations are valuable for planning a surgical approach to the ventral side of the mandible in the horse and to avoid potential post-operative complications.
Introduction A detailed knowledge of the precise location of the mandibular canal is essential to prevent injury to its neurovascular structure during various veterinary dental procedures, such as exodontia, endodontics, orthopaedic and oncologic surgery. The mandibular canal begins at the mandibular foramen ventral to the insertion of the temporalis muscle on the medial (lingual) surface of the mandibular ramus. This canal runs rostrally through the mandibular body till its termination at the mental foramina on the lateral (vestibular) surface of the mandible approximately
© 2014 Blackwell Verlag GmbH Anat. Histol. Embryol. 44 (2015) 333–337
3–4 cm rostral to PM2 (306/406 according to the Triadan numerical system). The purely sensory inferior alveolar nerve (IAN, nervus alveolaris inferior) enters the mandibular canal and innervates all the lower mandibular teeth as well as the gum and periodontium (for a description of the nerve in the horse, see Wissdorf et al., 1998). After emerging from the mental foramen as mental nerve (n. mentalis), it then provides sensory innervation to all tissue located rostrally, while a branch coming from the inferior dental plexus inside the mandibular canal (ramus alveolaris inferior rostralis) reaches the canine and incisors teeth (Wissdorf et al., 1998; Nomina Anatomica Veterinaria, 2012). During its course
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in the mandibular canal, it moves from the medial to the lateral side, crossing the roots of the mandibular cheek teeth before emerging from the mental foramen. Radiographically, in an oblique projection, the mandibular canal appears as a radiolucent linear image along the ventral aspect of the mandible. However, the precise relationship between the mandibular canal and the dental roots cannot be revealed by radiographs. In the last few years, computerized tomography (CT) has been increasingly used in equine dentistry as a valuable diagnostic tool. CT provides detailed cross-sectional images of tissues with bony and soft tissue contrast, thus eliminating the problem of overlap. The resulting images represent two dimensional transverse slices of objects with different degrees of thickness, providing much more detail of the normal anatomy of equine mandibular cheek teeth, including, infundibular and pulpar anatomy as well as detailed information regarding possible dental pathology (Dacre et al., 2008; Simhofer et al., 2008; Windley et al., 2009a,b). Several reports discuss the value of CT in documenting conditions of the equine skull (Tietje et al., 1996; Morrow et al., 2000; Walker et al., 2002; Lischer et al., 2005; Quinn et al., 2005; Puchalski, 2006; Simhofer, 2010); however, to our knowledge, no information is available about the appearance and course of the IAN and its relationship with the roots of the mandibular cheek teeth. Several post-operative complications are associated with procedures involving mandibular cheek teeth, including dental extractions performed by either intraoral or repulsion techniques, or in the case of endodontic procedures (Butson et al., 1997; Dixon et al., 2000; Schramme et al., 2000; Earley, 2010; Facchini et al., 2014). However, to the best of our knowledge, no cases of associated IAN lesions have been reported in the literature. Eventual damages to the IAN could lead to altered sensitivity in the corresponding area. This specific occurrence is well known in the human dental surgery (Worthington, 2004; Strauss et al., 2006; Hillerup, 2007), but may be underestimated in equine medicine possibly due to the difficulties in pin-pointing the precise cause of the symptomatology.
(a)
The aim of this study is to determine the exact course of the equine IAN in relation to the mandibular cheek teeth roots. To this effect, dissection of a series of the equine skulls (40) was carried out, and CT scans were performed. Given the close relationship between the nerve and the roots of some mandibular teeth, any lesions to the IAN and its resulting consequences should be examined in greater depth. It is our hope that our data will provide useful topographic information and therefore shed light on the risk of possible iatrogenic damages to the nerve during surgery on the apices of the mandibular cheek teeth.
Materials and Methods Dissection techniques This study was performed at the campus of the School of Agriculture and Veterinary Medicine at the University of Padova (Italy). Forty equine mandibles were collected during necropsy procedures performed for the purpose of post-mortem diagnosis. The horses were all adults (6– 15 years) of both genders and various breeds. The mandibles were dissected to eliminate the cortical part of the bone until the periostium was reached and subsequently removed with a surgical blade. An oscillating saw with a blade diameter of 5 cm was used to create a rectangular box-like section laterally, extending for the whole length of the mandibular cheek tooth arcade until approximately 1 cm from the ventral margin of the mandibular body (Fig. 1a). The periosteum was then removed with a surgical scalpel and a hammer (Fig. 1b), and the cheek teeth were exposed. Care was then taken to not dissect beyond the cortical tissue so as to avoid damages to the underlying structures, especially the IAN. After the box-like section had been exposed, all mineralized material and spongy tissue were then carefully removed (Fig. 1c), exposing the tooth roots and the IAN (see below). This technique was applied to both mandibles, to check the symmetrical course of the IAN. The course of the nerve could then be clearly delineated in relation to the roots of the mandibular teeth (Fig. 2a).
(b)
(c)
Fig. 1. Dissection of the mandible. (a) rectangular box-like section in the lateral side of the mandible; (b) removal of the periosteum with a sharp scalpel and a hammer; and (c) Careful removal of the all mineralized material with a scalpel revealing spongy bone.
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Fig. 2. Course of the IAN in the mandible. (a) exposure of the roots of the cheek teeth and the inferior alveolar nerve; (b) detailed relationship between IAN course and the root apex of PM3–4 and M1; and (c) detailed relationship between IAN course and the root apex of PM3–4 and M1–2. The position of PM4 is reported in all the images for reference.
Course of the Inferior Alveolar Nerve in Horses
(b)
(a)
CT scans The entire mandible body was analysed from the ramus to the mental foramen before dissection. Scan acquisition data were 120 kV, 175 mA and 2.9 s. of rotation time, and slice thickness was 3 mm. A bony acquisition algorithm was used to improve details of the structure. The mandibular CT images were obtained using a 3rd generation CT (Tomscan LX, Philips, Amsterdam, The Netherlands). The mandibles were positioned with the ventral border and the mandibular canal as close as possible to the horizontal plane in relation to the tomography table. The axial slices were made as perpendicular as possible to the mandibular canal to obtain high-quality images. Results Bilateral comparison of the progression of the IAN showed that the nerve maintains a symmetrical course in the hemimandibles of all the subjects examined. The anatomical course of the IAN did not vary with age. Anatomical results In all cases examined, the IAN penetrates the cancellous bone on the medial side of the ramus and runs immediately in the central portion of the mandible maintaining a ventral course in relation to the root apices of (a)
(c)
the mandibular molar teeth M3–1 (Triadan 311/411-309/ 409). In 95% (38 horses) of the subjects examined, the nerve changes direction between the mesial root of M1 and the distal root of PM4 (Triadan 308/408) and then runs close to the lingual cortex of the mandible. At the level of PM4, the nerve is thus localized both lingually and coronally to the apices of this tooth (Fig. 2b). In the remaining 5% of the subjects examined (2 horses), a variation was found in the course of the nerve at this site. Precisely, the IAN runs lingually and coronally as described above, but passes between the distal and the mesial roots of PM4 instead. The nerve continues rostrally on the lingual aspect of the root of PM3 returning to a position running at the same level of the apices (Fig. 2c). The nerve continues rostrally and then moves to the vestibular side of the mandible upon reaching the roots of PM2 (Triadan 306/406). Subsequently, part of the nerve continues its course towards canine and incisor teeth, while the external branches emerge from the mental foramen rostral to PM2. CT scans Analysis of CT scan data confirmed what was observed by direct visual dissection. The nerve runs in a specific canal on the lingual side of the mandible, close to the inner (b)
Fig. 3. CT images of the mandible including the canal for the IAN. (a) duplication of the branches of IAN in the mandibular canal (red stars) in relation to the root of PM3 and (b) location of mandibular canal (red star) in relation to the root of PM2.
© 2014 Blackwell Verlag GmbH Anat. Histol. Embryol. 44 (2015) 333–337
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limit of the cortical. Using this technique, we also recognized a number of variations in the course of the nerve that would be otherwise impossible to detect, such as duplication of the nerve branches (Fig. 3 a, b). The two canals of the duplicate nerve run one above the other, but not side by side. Discussion The direct visualization of the course of the equine IAN through the removal of the bone cortical and periosteal of the mandible bone in a rectangular box-like section was a satisfactory and economical mean to thoroughly follow both the course and exact location of the nerve and its relationship with the roots of the mandibular cheek teeth. The use of CT confirmed the data obtained with anatomical dissection and provided more accurate and detailed information with potential clinical value for equine practitioners. The path of the IAN in the horse and its relationship to the roots of the mandibular cheek teeth has occasional importance for surgery in the area. However, the anatomical images obtained from this study show a close proximity between the nerve and the roots of some specific mandibular cheek teeth. Our observations, summarized in Fig. 4, show that the course of the IAN is located in the central portion of the mandibular bone and runs ventrally to the root apices of the molars (M1–3). Consequently, the risk of damaging the nerve in surgical procedures involving the roots of the molar teeth is rather low, as the general consensus is to use a lateral or ventro-lateral surgical approach to these structures. During its rostral course, at the level of PM4, the IAN then runs lingually to the distal and mesial roots and is found coronally to the apices of both roots. Consequently, surgical procedures involving a ventro-lateral approach to the roots of PM4 should be relatively free of risks of accidental iatrogenic damage of the IAN. Available references show that the percentage of apical infections involving PM4 is 9.5% (Dixon et al., 2000). Both PM2 and PM3 are the most frequent mandibular cheek teeth that are affected by apical infections according to the literature (Dixon et al., 2000). Specifically, the percentage of apical infections involving PM3 and PM2 is 34.5 and 31%, respectively (Dixon et al., 2000). Our observations show that the IAN runs indeed very close to the roots apices of PM3 and PM2, with potential negative consequences in the case of various local surgical procedures such as endodontia, dental repulsion, fractures, apicectomy, etc. The variability of the pathway of the IAN, together with the narrowness of the mandibular body and the ves-
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Fig. 4. Schematic representation of variations in the course of the IAN in the mandible. Right side – blue line, commonest route of the nerve (95% of the subjects). Left side – green line, route of the nerve in 5% of the cases.
tibular course of the nerve, is among the potential factors that could lead to post-surgical complications of the equine mandible. To this effect, CT scans may be a valuable diagnostic and pre-operative tool to avoid possible damages to the IAN in the area of the premolar tooth roots, as is also shown in the dog (Martinez et al., 2009). Interestingly, the topography of the equine IAN is similar to the human (Gowgiel, 1992). This comparative anatomical similarity merits further study among mammalian species, especially considering the ample variation of both food type and mastication among the various species. © 2014 Blackwell Verlag GmbH Anat. Histol. Embryol. 44 (2015) 333–337
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To the best of our knowledge, injuries to the IAN have never been reported in the literature as a complication of equine dental or surgical procedures including extractions, apicectomy and endodontic treatments. However, the potential risks of damaging the IAN close to the apex of the roots of PM2-4 should be taken into account, especially considering the anatomical variations of its course in proximity to the roots of PM4. Acknowledgements The Authors wish to thank prof. Alessandro Zotti of the Department of Animal Medicine, Production and Health of the University of Padova for providing the CT scans, and Mr. Emanuele Zanetti of the Department of Comparative Biomedicine and Food Science of the University of Padova for his skillful technical assistance. References Butson, R. J., J. Robinson, M. C. Schramme, and S. A. May, 1997: Endodontic therapy for periapical infection in ten equine cheek teeth: preliminary results, Proceedings of 5th World Veterinary Dental Congress, 1997 Birmingham, England. Dacre, I. T., S. Kempson, and P. M. Dixon, 2008: Pathological studies of cheek teeth apical infections in the horse: 4 aetiopathological findings in 41 apically infected mandibular cheek teeth. Vet. J. 178, 341–351. Dixon, P. M., W. H. Tremaine, K. Pickls, L. Kuhns, C. Hawe, J. McCann, B. C. McGorum, D. I. Railton, and S. Brammer, 2000: Equine dental disease, Part 4: a long-term study of 400 cases: apical infections of cheek teeth. Equine Vet. J. 32, 182–194. Earley, E. T., 2010: How to identify potential complications associated with cheek teeth extraction in the horse. AAEP Proceedings 2010 vol. 10, 435–480 Baltimore, MA, USA. Facchini, C., S. Bono, M. Faughnan, and I. Iacopetti, 2014: Endodontic treatment of a mandibular premolar abscess in a horse: a case study. Equine Vet. Educ. 26, 194–199. Gowgiel, J. M., 1992: The position and course of the mandibular canal. J. Oral Implantol. 4, 383–385. Hillerup, S., 2007: Iatrogenic injury to oral branches of the trigeminal nerve: records of 449 cases. Clin. Oral Investig. 11, 133–142. Lischer, C. J., U. Walliser, P. Witzmann, E. M. Wehrli, and S. Ohlerth, 2005: Fractures of the paracondylar process in four horses: advantages of CT imaging. Equine Vet. J. 37, 483–487. Martinez, L. A. V., M. A. Gioso, C. M. Villegas Lobos, and A. C. B. Fonseca Pinto, 2009: Localization of the mandibular canal in Brachycephalic dogs using computed tomography. J. Vet. Dent. 26, 156–163.
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