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Home , Body of sphenoid bone, Foramen rotundum, Greater palatine canal, Inferior orbital fissure, Infraorbital foramen, Lesser palatine foramina

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Thomas von Arx1 Scott Lozanoff2 Extraoral anatomy in CBCT - Michael M. Bornstein3,4 a literature review 1 Department of Oral Surgery and Stomatology, School of Dental Medicine, University of Bern, Switzerland Part 3: Retromaxillary region 2 Department of Anatomy, Biochemistry and Physiology, John A. Burns School of Medi- cine, University of Hawaii, Honolulu, USA 3 Oral and Maxillofacial Radiol- ogy, Applied Oral Sciences KEYWORDS and Community Dental Care, Anatomy Faculty of Dentistry, The Uni- CBCT versity of Hong Kong, Prince Retromaxillary region Philip Dental Hospital, Hong Sphenoid Kong SAR, China 4 Department of Oral Health Pterygopalatine & Medicine, University Center for Dental Medicine Basel UZB, University of Basel, Basel, Switzerland

CORRESPONDENCE Prof. Dr. Thomas von Arx SUMMARY Klinik für Oralchirurgie und Stomatologie In this review about extraoral anatomy as depict- lesser wings, and pterygoid processes. Important Zahnmedizinische Kliniken ed by cone beam computed tomography, the ret- neurovascular structures pass through the sphe- der Universität Bern romaxillary region is discussed. A medium-sized noid bone: the optic and the ophthalmic Freiburgstrasse 7 (6 × 6 cm) or large (≥ 8 × 8 cm) field of view of the via the optic , the CH-3010 Bern will inevitably depict the retromaxillary via the , and the pterygoid Tel. +41 31 632 25 66 Fax +41 31 632 25 03 region that can be considered a “transition” zone nerve via the Vidian canal. The central body of E-mail: between the viscerocranium and the neurocrani- the also contains the highly vari- [email protected] um. Major structures of the region include the able sphenoid sinus that is the most posteriorly sphenoid bone and the pterygopalatine fossae. located paranasal sinus. The bilateral pterygopal- SWISS DENTAL JOURNAL SSO 130: 216–228 (2020) The sphenoid bone is a single but complex bone atine fossae behind the maxillary sinuses contain Accepted for publication: located between the maxilla and the . It is several important neurovascular structures that 16 September 2019 composed of a central body, bilateral greater and supply the maxilla and the midface.

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Introduction This third literature review about extraoral anatomy as depicted and seen in cone beam computed tomography (CBCT) scans ad- dresses the retromaxillary region. The latter can be defined as the zone immediately posterior to the maxillary on both sides. The region includes the sphenoid bone and the pterygo- palatine fossae. Both anatomical structures are usually depicted on CBCT scans of the posterior maxilla and/or the performed, for example, for treatment planning prior to dental implant placement including sinus floor elevation proce- dures or periapical surgery of molars. The single sphenoid bone presents a complex morphology and it can be regarded as a “transition” bone between the viscerocranium and the neuro- cranium (Fig. 1). The sphenoid bone contributes to the orbital apex, to the anterior and middle cranial fossae, and to the later- al wall of the (Budu et al. 2013). The sphenoid bone also contains the highly variable sphenoid sinus that may have clini- cally perilous anatomical relations with the optic and the internal carotid . The bilateral pterygopalatine fossae are located behind the posterior aspect of the maxillary bones. The fossae are consid- ered the “control center” of the maxilla since they contain the maxillary nerve, the , and the , all supplying the maxilla as well as the entire midface. Due to its inherent complex location, the Fig. 1 The sphenoid bone (highlighted in blue) is a viscerocranial component can potentially act as a natural conduit for the spread of inflam- of the craniofacial skeleton that forms a transition zone articulating with matory and neoplastic diseases across the various deep spaces contiguous neurocranial elements. of the head and neck (Tashi et al. 2016).

Sphenoid bone extends laterally as the posterior clinoid processes (Chong et al. The anatomy of the sphenoid bone (“sphen” in Greek means 1998). The sphenoid body also contributes to the that ex- wedge-shaped) is complicated since it is not a compact struc- pands posteriorly from the and slopes downwards ture but rather presents with multiple wings and processes to the (Fig. 3). The inferior of the clivus originating from a central body (Chong et al. 1998; Jaquesson represents the roof of the nasopharynx (Chong et al. 1998). et al. 2017) (Fig. 2). Furthermore, the sphenoid bone contains Three bony canals run through the lateral aspects of the sphe- several fissures, canals and foramina that convey neurovascular noid body: superiorly the , midway the foramen ro- structures from the middle to the maxillofacial re- tundum, and inferiorly the pterygoid (Vidian) canal. These ca- gion. The sphenoid bone contributes superiorly to the anterior nals transmit significant neurovascular structures to the orbits and middle cranial fossae, anterolaterally to the orbital walls, and pterygopalatine fossae (see below). laterally to the temporal fossae, and inferoanteriorly to the nasal cavities and to the pterygopalatine fossae. Sphenoid sinus The sphenoid bone has long been recognized as comprising The sphenoid sinus (SPS) is located within the central portion four parts (Fawcett 1910): of the sphenoid bone, i. e. the sphenoid body (Fig. 3–5). The SPS – central body shows great variability with regard to size, shape and compart- – greater wings mentalization. The SPS is the most posterior of the paranasal si- – lesser wings nuses. It drains into the sphenoethmoidal recess that is located – pterygoid processes superior or posterior to the superior nasal turbinate (Anusha et al. 2014; von Arx et al. 2019). Usually, the SPS is divided by an When viewed from anterior, the greater wings resemble the intervening septum into two compartments. Since the septum outstretched wings of a bird with the pterygoid processes ap- is rarely located in the midsagittal plane but rather deviated pearing as extended limbs (Chong et al. 1998). laterally, the SPS has asymmetric cavities. Often, there is a dominant cavity on one side (Tan & Ong 2007). Some SPS may Central body demonstrate subdivision into several recesses by accessory sep- The central body of the sphenoid bone has a cuboidal shape and ta. The average volume of the SPS ranges between 3 and 10 ml is actually hollowed by the sphenoid sinus. The body is the ori- (Anusha et al. 2014). gin for the paired greater and lesser wings as well as for the The extent of the SPS is commonly divided into three main pterygoid processes (Fig. 2). Superiorly, a bony depression (sella types (Budu et al. 2013; Anusha et al. 2014): (1) the conchal type turcica = Turkish saddle) serves as the hypophyseal or pituitary represents a very small sinus or pneumatization is completely fossa. Anterior (large) and posterior (small) clinoid processes absent and the “sinus” is filled with trabecular bone; (2) the represent the four corners of the . Lateral to the sella presellar (or juvenile) type has pneumatization extending to the lies the paired , a large venous cavity. The dor- sella turcica but not posterior to it; (3) the sellar (or adult) type sum sellae forms the posterior boundary of the sella turcica and demonstrates complete pneumatization of the sphenoid body.

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Fig. 2 Anterior view of the isolated sphenoid bone 1 = ; 2 = sphenoid crest; 3 = aperture of sphenoid sinus; 4 = lesser wing; 5 = orbital surface of greater wing; 6 = cranio-orbital foramen; 7 = ; 8 = anterior opening of optic canal; 9 = foramen rotundum; 10 = pterygoid (Vidian) canal; 11 = vaginal process; 12 = palatovaginal (pharyngeal) canal; 13 = vomerovaginal canal; 14 = lateral plate of pterygoid process; 15 = medial plate of pterygoid process; 16 = sphenoid spine; 17 = dorsum sellae

Furthermore, the SPS may expand into the greater and lesser Greater wings sphenoid wings, into the pterygoid processes, as well as into The paired greater wings originate from the lateral aspects of the the anterior clinoid processes (Unal et al. 2006; dal Secchi et al. central sphenoid body (Fig. 2). The greater wings have three sur- 2018). Pneumatization of the by the : (1) an anterior orbital surface contributing to the lateral sphenoid sinus has been reported in 6–29.3% (Anusha et al. orbital wall – the orbital surface is bounded by the superior and 2015). Generally, extensive pneumatization of the SPS can be inferior orbital fissures; (2) an external surface forming part of associated with irregularities in the sinus walls featuring pro- the lateral wall of the cranium; and (3) an internal posterior sur- trusions and recesses (dal Secchi et al. 2018). face that is oriented horizontally and contributes to the anterior Four vital structures run along the sphenoid sinus: the optic portion of the . The greater wings contain nerve in the superolateral wall, the in the three important foramina, i. e. rotundum, ovale and spinosum. midlateral wall, the maxillary division of the in The foramen rotundum is located anteromedially while the fora- the inferolateral wall, and the Vidian nerve in the floor of the si- men ovale and are positioned posterolaterally nus (Tan & Ong 2007). All of these neurovascular structures may (Chong et al. 1998). The sphenoid spine, a sharp bony projection indent the walls of the SPS or even show bulging into the sinus behind the foramen spinosum, also belongs to the greater wing. cavity (Unal et al. 2006). The internal carotid artery (ICA) coursing through the lateral Lesser wings wall of the SPS is essentially a reflection of its intracavernous The paired lesser wings protrude from the anterosuperior aspect portion (Anusha et al. 2015). In fact, the ICA is the most medial of the central sphenoid body (Fig. 2). They have a triangular element of the cavernous sinus, and it often has a direct relation shape, fuse anteriorly with the , and contribute to with the lateral wall of the SPS. Variations of the ICA position the . The lesser wings are oriented in the may include dehiscence of the overlying bone or a protrusion axial plane and show pointed bony projections extending poste- of the ICA into the SPS. Depending on the degree of pneumati- riorly over the sella turcica, i. e. the anterior clinoid processes. zation, bulging of the ICA into the sinus has been reported in The medial attachments of the lesser wings to the sphenoid 34–93% (Budu et al. 2013). Similarly, the optic canal running body have two roots that define the optic canal (Chong et al. through the roof of the SPS may show a dehiscence or may pro- 1998). The lower root, also known as the optic strut, separates trude from the sinus roof into the SPS (Anusha et al. 2015). the optic canal from the superior orbital fissure. The optic canal

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A

B

Fig. 3 Midsagittal CBCT image of a 55-year-old male 1 = body of sphenoid bone; 2 = sphenoid sinus; 3 = sella turcica; 4 = dorsum sellae; 5 = clivus; 6 = syndesmosis between sphenoid bone and ; 7 = vomer; 8 = ; 9 = soft palate; 10 = nasopharynx; 11 = atlas; 12 = C dens axis; 13 = body of axis; 14 = base of tongue; 15 = tip of epiglottis

Fig. 5 Coronal CBCT images of three patients showing three different config- urations of the sphenoid sinus and its neighboring structures: (A) Foramina rotunda and pterygoid canals are fully embedded in the body of the sphenoid bone (59-year-old female). (B) The sphenoid sinus reaches the level of the pterygoid canals (13-year-old male). (C) The sphenoid sinus extends deep into the base of the pterygoid process (53-year-old female). Fig. 4 Cadaveric dissection showing midsagittal view of sphenoid sinus and 1 = sphenoid sinus; 1* swelling of sphenoid sinus membrane; 2 = sphenoid contiguous structures sinus septum; 2* = accessory septum; 3 = foramen rotundum; 4 = pterygoid 1 = sphenoid sinus; 2 = sella turcica with hypophysis; 3 = clivus; 4 = lower (Vidian) canal; 5 = base of pterygoid process; 6 = lateral plate of pterygoid portion of central body of sphenoid bone; 5 = nasopharynx; 6 = perpendicu- process; 7 = medial plate of pterygoid process; 8 = hamulus; 9 = pterygoid lar plate of ; 7 = vomer; 8 = hard palate; 9 = soft palate fossa; 10 = vomer

is the only structure passing through the lesser wing (Chong et from the middle cranial fossa to the pterygopalatine fossa. al. 1998). The foramen is predominantly circular with an average size of 3.9 × 3.1 mm (Edwards et al. 2018). Berge & Bergman (2001) who Optic canal evaluated 100 randomly selected dry reported an average The paired optic canal (OPC) runs through the superior aspect size of 3.3 × 2.7 mm (range 2 × 2 to 4.5 × 3.5 mm). In a CT study of of the sphenoid body in an anterolateral direction towards the 100 patients, the mean distance from the posterior wall of the apex of the orbital cavity (Fig. 2). The OPC is the most superior maxillary sinus to the anterior opening of the foramen rotun- opening of the middle cranial fossa. It transmits the dum was 6.3 mm (range 4.4 to 8.5 mm) (Vuksanovic-Bozaric et (CN II) and the to the . The diameter and al. 2019). The distance from the to the the length of the canal range between 3.7 and 6.3 mm and 8 and foramen rotundum was evaluated in 50 Chilean adult human 12 mm, respectively (Edwards et al. 2018). dry skulls (Soto et al. 2015). The mean lengths were 31.95mm on the right side and 32.49 mm on the left side. Foramen rotundum Theforamen rotundum lies in the anteromedial portion of the greater sphenoid wing (Fig. 2 and 5). It represents the gate- The foramen ovale (FOv) lies in the posterolateral portion of the way for the maxillary division of the trigeminal nerve (CN V2) greater sphenoid wing (Fig.6–8). The predominant shape of the

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FOv is oval, with average dimensions of 7.11 × 3.60 mm (range nerve, i. e. the mandibular nerve (CN V3). It may further contain 5 × 2 mm to 8 × 7 mm) (Berge & Bergman 2001). The FOv connects an accessory branch of the middle meningeal artery, small em- the middle cranial fossa with the . Intracra- issary veins, or even a small venous plexus joining the pterygoid nially, the internal carotid artery runs directly posterior to the plexus to the cavernous sinus (Edwards et al. 2018). foramen. The FOv transmits the third division of the trigeminal The contents of the FOv are frequently interrupted by liga- ments that can ossify. The pterygospinous ligament (ligament of Civinini) that connects the sphenoid spine with the lateral plate of the pterygoid process can ossify and hinder the access to the FOv (Tubbs et al. 2009). Similarly, the pterygoalar liga- ment (Hyrtl’s ligament) can ossify resulting in the porus crota- phiticobuccinatorium (Hyrtl’s foramen or pterygoalar foramen) also potentially compressing branches of the mandibular divi- sion of the trigeminal nerve (Kamath & Vasantha 2014). Frequently, a small emissary sphenoidal foramen (also known as foramen of Vesalius) is present anteromedially to the FOv (Fig. 7). This foramen contains the emissary sphenoidal vein as an additional venous pathway connecting the cavernous sinus with the pterygoid venous plexus (Natsis et al. 2018). It can ap- pear as single or double openings that are uni- or bilateral (Kale et al. 2009) and vary between 0.5 and 2.86 mm (Boyd 1930; Rey- mond et al. 2005). The foramen of Vesalius is an anatomical vari- ant, but ipsilateral enlargement of this opening may relate to carotid-cavernous fistulas or may also relate to intracranial perineural or nasopharyngeal extension (Lanzieri et al. 1988).

Fig. 6 Illustration of the central portion of the skull base (inferior view) Foramen spinosum 1 = horizontal plate of ; 2 = greater palatine foramen; 3 = lesser palatine foramen; 4 = pyramidal process of palatine bone; 5 = hamulus; 6 = The foramen spinosum (FSp) got its name from the sphenoid medial plate of pterygoid process; 7 = lateral plate of pterygoid process; 8 = spine lying posteriorly to the foramen (Krayenbühl et al. 2008) ; 9 = vomer; 10 = inferior surface of body of sphenoid bone; (Fig. 6 and 7). The FSp is located posterolaterally to the FOv. 11 = pharyngeal opening of palatovaginal canal; 12 = pharyngeal opening of The average size of the FSp is 2.4× 2 mm (range 1 × 1 to 4 × 3 mm) vomerovaginal canal; 13 = posterior opening of pterygoid (Vidian) canal; 14 = inferior surface of greater wing of sphenoid bone; 15 = foramen ovale; 16 = (Berge & Bergman 2001). Occasionally, the FSp may be absent or foramen spinosum; 17 = foramen lacerum; 18 =

Fig. 7 Inferior perspective of the foramen ovale and contiguous foramina Fig. 8 Cadaveric dissection of the right retromaxillary region (lateral view). (photograph courtesy of Dr. Robert W. Mann, Dept. of Anatomy, Biochemistry The lateral wall of the maxillary sinus and the lateral plate of the pterygoid & Physiology, University of Hawaii School of Medicine) process have been removed. 1 = horizontal plate of palatine bone; 2 = greater palatine foramen; 3 = lesser 1 = foramen ovale; 2 = mandibular division of trigeminal nerve (CN V3); palatine foramen; 4 = ; 5 = hamulus of medial plate 3 = foramen spinosum; 4 = middle meningeal artery; 5 = maxillary artery; of pterygoid process; 6 = lateral plate of pterygoid process; 7 = foramen of 6 = external carotid artery; 7 = superficial temporal artery; 8 = inferior alveo- Vesalius; 8 = foramen ovale; 9 = foramen of Arnold; 10 = foramen spinosum; lar nerve; 9 = lingual nerve; 10 = tensor veli palatini muscle; 11 = pterygopala- 11 = foramen lacerum; 12 = pharyngeal opening of palatovaginal canal; 13 = tine fossa; 12 = ; 13 = maxillary sinus; 14 = medial plate of vomer; 14 = pharyngeal opening of vomerovaginal canal; 15 = inferior surface pterygoid process; 15 = of body of sphenoid bone; 16 = occipital bone; 17 = inferior surface of greater wing of sphenoid bone

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confluent with the FOv (Ginsberg et al. 1994). In a quantitative study of twelve cadaver heads, the median distance between the FSp and the FOv was 4.75 mm (range 2 to 7.5 mm) (Kray- enbühl et al. 2008). The FSp contains the middle meningeal artery and the middle meningeal veins that anastomose with the venous plexus of the FOv or the cavernous sinus. Further- more, the recurrent branch (nervus spinosus) of the mandibular nerve runs through the FSp (Krayenbühl et al. 2008; Edwards et al. 2018). The foramen petrosum (foramen of Arnold) is a variably ap- pearing foramen between the FSp and FOv (Fig. 7). It transmits the lesser petrosal nerve that runs as a parasympathetic branch of CN IX to innervate the parotid gland.

Pterygoid process The bilateral pterygoid process (PTP) is the most inferior portion Fig. 9 The four-sided pyramidal shape of the pterygopalatine fossa is taper- of the sphenoid bone (Fig. 2 and 6). The PTP originates inferolat- ing inferiorly. It has a large lateral opening (). The erally from the sphenoid body. The PTP is characterized by two three other walls are hidden inside the skull: anterior wall (red), medial wall bone plates (lamina medialis and lamina lateralis) that fuse superi- (blue), and posterior wall (green). 1 = foramen rotundum; 2 = pterygoid (Vidian) canal; 3 = palatovaginal (pha- orly and anteriorly (Chong et al. 1998). When viewed from pos- ryngeal) canal; 4 = vomerovaginal canal; 5 = ; 6 = teriorly, a bony depression (pterygoid fossa) can be seen be- ; 7 = pterygopalatine canal tween the medial and lateral plates. The medial plates of the PTP form the lateral margins of the choanae. The medial plates end inferiorly with a slender bony hook, i. e. the . This small bony projection is of great functional importance for several muscles, i. e. the tensor veli palatini curving around the hook, the palatopharyngeus originating inter alia from the hamulus, and the upper portion of the superior pharyngeal constrictor originating from the edge of the medial pterygoid plate and from the hamulus (Oz et al. 2016). Pterygopalatine fossa The bilateral pterygopalatine fossa (PPF) is one of the most complex anatomical regions to understand (Bannon et al. 2018) (Fig. 9–11). It is a small and clinically poorly accessible space deep in the face bordered by three bones: maxillary, pal- atine, and sphenoid bones (von Arx & Lozanoff 2017). The PPF is bounded posteriorly by the base of the sphenoid bone and the fusion zone of the plates of the pterygoid process, anteri- orly by the posterior surface of the maxillary bone, and medi- ally by the upper part of the perpendicular plate of the palatine bone (Daniels et al. 1998). The PPF has a four-sided pyramidal shape with its apex Fig. 10 Cadaveric dissection of the left retromaxillary region highlighting the pointing inferiorly towards the maxillary tuberosity. The mean pterygopalatine fossa (triangular dotted outline) volume of the PPF ranges from 0.7 to 1.2 ml (Coronado et al. 1 = sphenoid sinus; 2 = sella turcica; 3 = clivus; 4 = pterygoid nerve in ptery- 2008; Stojcev Stajcic et al. 2010; Hwang et al. 2011a). Median goid (Vidian) canal; 5 = pterygopalatine ganglion; 6 = infraorbital nerve; 7 = values of height, width and depth of 159 PPF from Caucasian anterior superior alveolar nerve; 8 = lateral wall of maxillary sinus; 9 = de- scending palatine nerve in pterygopalatine canal; 10 = skulls were reported as 17.5 mm, 5 mm and 14 mm, respectively exiting the greater palatine foramen; 11 = hard palate; 12 = soft palate; 13 = (Stojcev Stajcic et al. 2010). Rusu et al. (2013) viewed the PPF lower portion of central body of sphenoid bone as an “intersinus space” since it is surrounded by several para- nasal sinuses, i. e. the sphenoid sinus superoposteriorly, the posterior superomedially, and the maxillary vomerovaginal canal (Fig. 12). The lateral wall (pterygomaxil- sinus anteriorly. lary fissure), the anterior wall (inferior orbital fissure) and the The PPF serves as a major crossroad between the middle cra- medial wall (sphenopalatine foramen) each have one opening. nial fossa, oral and nasal cavities, nasopharynx and orbit (Erdo- Finally, the most inferior opening of the PPF is the pterygopal- gan et al. 2003; Hwang et al. 2011b; Tashi et al. 2016). The mul- atine canal. tiple pathways of communication from the PPF to contiguous areas are summarized in Table I. According to Meng et al. (2016), Pterygomaxillary fissure the majority of openings are located in the posterior wall of the This is the principal opening of the PPF on the lateral aspect to- PPF, i. e. from superolateral to inferomedial, the foramen ro- wards the infratemporal fossa. The main structure entering the tundum, the , the palatovaginal canal, and the fissure is the maxillary artery. Immediately after reaching the

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A

B C

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Fig. 11 Coronal (A) and axial CBCT images at four different levels (B, C, D, E) demonstrating the pterygopalatine fossa and various communications (60-year- old female) 1 = sphenoid sinus; 2 = superior orbital fissure; 3 = foramen rotundum; 4 = foramen lacerum (posterior end of pterygoid canal); 5 = vomer; 6 = opticcanal; 7 = ethmoidal air cells; 8 = cranio-orbital foramen; 9 = greater wing (orbital surface) of sphenoid bone; 10 = pterygopalatine fossa; 11 = maxillary sinus; 12 = middle turbinate; 13 = ; 14 = sphenopalatine foramen; 15 = palatovaginal (pharyngeal) canal; 16 = pterygoid (Vidian) canal; 17 = vomerovagi- nal canal

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Tab. I Pterygopalatine fossa (PPF): connections and contents (according to von Arx & Lozanoff 2017)

PPF connection to Location of connection Vascular content Neural content Comment within PPF

Middle cranial fossa

Foramen rotundum Posterior wall Venous plexus Maxillary nerve (CN V2)

Pterygoid canal Posterior wall Pterygoid artery and vein Pterygoid nerve Also known as Vidian canal

Orbit

Inferior orbital fissure Anterior wall and vein Infraorbital nerve; zygo- matic branch of maxillary nerve

Nasal cavity

Sphenopalatine foramen Medial wall and Nasal branches of maxil- vein lary nerve

Nasopharynx

Palatovaginal canal Posterior wall Pharyngeal branch of Pharyngeal branch of Also known as pharyngeal maxillary artery maxillary nerve canal

Vomerovaginal canal Posterior wall Unspecified small arteries Unspecified small nerves

Palate

Pterygopalatine canal Inferior Descending palatine artery Greater and lesser palatine Pterygopalatine canal di- and vein nerves vides inferiorly into greater and lesser palatine canals

Infratemporal fossa

Pterygomaxillary fissure Lateral Maxillary artery (entering), Posterior superior alveolar Main opening to the lateral posterior superior alveolar nerve(s) (PSAN) aspect of the skull artery (PSAA) (leaving) and vein

PPF, the maxillary artery gives rise to the posterior superior see von Arx et al. 2020). It transmits the infraorbital nerve and alveolar artery (PSAA) that, in turn, leaves the PPF in an an- vessels as well as the zygomatic branch of the maxillary nerve teroinferior direction through the pterygomaxillary fissure. (Erdogan et al. 2003). The fissure is bounded anteriorly by the posterior aspect of the os maxilla, superiorly by the body of the sphenoid bone, and Sphenopalatine foramen posteriorly by the lateral plate of the pterygoid process. Inferi- The sphenopalatine foramen (SPF) is located superomedially orly, the pterygomaxillary fissure is tapering and terminates within the PPF and it connects the fossa with the at the pterygomaxillary suture. (von Arx & Lozanoff 2017) (Fig. 14). The foramen is formed by two processes (orbital and sphenoid processes) of the vertical Pterygopalatine canal plate of the palatal bone joining the body of the sphenoid bone The pterygopalatine canal is the most inferior opening of the (Morton & Khan 1991). With regard to its nasal aspect, the SPS PPF (Fig. 13). It is the only connection from the PPF to the oral is positioned at the posterior end of the superior or at cavity. The pterygopalatine canal tapers and runs through the the posterior border of the middle concha (von Arx & Lozanoff posterolateral wall of the maxillary sinus. Shortly after entering 2017). The SPF often has an hour-glass shape with an average the os maxilla, the pterygopalatine canal divides into the greater horizontal diameter of 5.1 mm (range 4–7 mm) and an average and lesser palatine canals before terminating at the homony- vertical height of 6.2 mm (range 4.5–7.5 mm) (Nalavenkata et mous foramina. Rapado-Gonzalez et al. (2015) measured the al. 2015). In a radiographic analysis of computed tomography in width of the pterygopalatine canal at the transition level to the 50 adult patients, the SPS was located on average 26.6 ± 2.6 mm greater and lesser palatine canals in 150 CBCTs. The mean width above the nasal floor and 9.2± 1.4 mm anterior to the choanal in the sagittal plane was 2.94 mm (right side) and 2.90 mm (left arch (Maxwell et al. 2017). side), whereas the mean width in the coronal plane amounted The SPF transmits the artery and vein of the same name as to 2.16 mm (right side) and 2.18 mm (left side). well as the nasal branches of the maxillary nerve (Erdogan et al. 2003). Variability in the number of SPF with one to three Inferior orbital fissure foramina per side has been reported (Scanavine et al. 2009; Anterosuperiorly, the PPF communicates with the orbit via the El-Shaarawy & Hassan 2018), probably due to a branching of posteromedial portion of the inferior orbital fissure (for details, the sphenopalatine artery before leaving the PPF.

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C

Fig. 12 Depiction of the right pterygopalatine fossa (triangular dotted outline) in a dry skull after partial removal of the : lateral view (A), high magnification of lateral view (B). Inferior view of skull base (C). Colored cables have been inserted to highlight the pterygoid canal (blue cable), the palatovag- inal canal (green cable), and the vomerovaginal canal (red cable). 1 = medial wall of orbit; 2 = lateral wall of orbit; 3 = optic canal; 4 = superior orbital fissure; 5 = inferior orbital fissure; 6 = foramen rotundum; 7 = sphenopala- tine foramen; 8 = entrance to pterygopalatine canal; 9 = vomer; 10 = inferior surface of body of sphenoid bone; 11 = occipital bone; 12 = foramen lacerum; 13 = foramen ovale; 14 = foramen spinosum

Foramen rotundum floor of the sphenoid body in a downward and medial direction The foramen rotundum has been discussed above in the section and terminates in the PPF (Osawa et al. 2009). The upper margin “Sphenoid bone”. of the PTC may protrude from the sinus floor into the SPS de- pending on the extent of pneumatization of the sphenoid body Pterygoid canal (Chen & Xiao 2015). The PTC is roughly located in the same axial The pterygoid canal (PTC), also termed Vidian canal (Vidius, Ital- plane as the sphenopalatine foramen (Daniels et al. 1998). ian anatomist, 1509–1569), originates from the foramen lacerum In a quantitative study of 200 adults using computed to- in the middle cranial fossa (Fig. 11, 12 and 14). The PTC transmits mographic angiography, the mean length of the PTC was the homonymous artery and nerve that comprises complex sym- 14.6 ± 1.23 mm (Cheng et al. 2016). In another study with CT pathetic and parasympathetic connections (Domenech Mateu & scans of 137 patients, the mean length of the medial canal wall Pueyo Mur 1980). The canal is located within the fusion zone of amounted to 13.6 ± 2.2 mm (Fu et al. 2014). Markedly longer the pterygoid process and the sphenoid body. It runs through the PTC distances were measured in a MRI study of 91 Japanese

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A

B

Fig. 14 Axial CBCT image at the level of the sphenopalatine foramina (83-year-old female) 1 = pterygopalatine fossa; 2 = pterygomaxillary fissure; 3 = infratemporal fossa; 4 = sphenopalatine foramen; 5 = pterygoid (Vidian) canal; 6 = pala- C tovaginal (pharyngeal) canal; 7 = foramen ovale; 8 = foramen spinosum; 9 = sphenoid sinus; 10 = maxillary sinus; 11 = middle turbinate; 12 = maxillary ostium; 13 = nasolacrimal canal; 14 = ; 15 = vomer; 16 = inferolateral portion of body of sphenoid bone; 17 = glenoid fossa of (for mandibular condyle); 18 = external acoustic canal

The contents of the canal include the Vidian nerve (aka, nerve of the pterygoid canal) that comprises both sympathetic and parasympathetic components. The Vidian artery is particularly important since it exists as a potential collateral circulation connecting intra- and extracranial arterial circulatory systems particularly in the event of an occlusive lesion of the internal carotid artery (Takeuchi et al. 2005; Siddiqui & Chen 2009).

Palatovaginal canal The palatovaginal canal (PVC), also known as the pharyngeal canal, is a short tunnel running below the inferior wall of the sphenoid sinus (Fig. 6, 7 and 12). It originates from the PPF and Fig. 13 Sagittal (A), coronal (B) and axial (C) CBCT images depicting the pter- ygopalatine and greater/lesser palatine canals (66-year-old male) terminates posteriorly as a groove in the roof of the nasophar- 1 = pterygopalatine fossa; 2 = pterygopalatine canal; 3 = greater palatine ynx (Meng et al. 2016). It is commonly identified during routine canal; 4 = lesser palatine canal; 5 = greater palatine foramen; 6 = maxillary CT and MRI imaging (Rumboldt et al. 2002). Within the sphe- sinus; 7 = perpendicular plate of ethmoid bone; 8 = vomer; 9 = inferior turbi- noid body, the PVC is located between the Vidian canal (lateral- nate; 10 = ; 11 = lateral plate of pterygoid process; 12 = posterior nasal spine; 13 = median palatine suture; 14 = nasopalatine ly) and the vomerovaginal canal (medially, see below). The PVC canal transmits the pharyngeal artery and nerve. In a quantitative study in 200 adults using computed to- mographic angiography, the mean length of the PVC was patients with a mean length of 19.8 mm on the right side and 7.8 ± 0.41 mm (Cheng et al. 2016). In a study with CT imaging 19.3 mm on the left side (Tsutsumi et al. 2018). of the PVC in 10 patients (20 sides), the mean diameter of the Reported mean diameters of the PTC range from 1.4 to PVC was 1.7 mm (range 1.4–2.2 mm) (Herzallah et al. 2012). 3.5 mm (Hwang et al. 2011b; Sepahdari & Mong 2013; Fu et al. The same authors also reported a mean distance of 3.8mm from 2014; Chen & Xiao 2015). Commonly, the pterygoid canal is nar- the PVC to the Vidian canal (range 2.9–4.8 mm). rowest in its middle portion and widens anteriorly and posteri- orly, thus exhibiting a double funnel shape (Chen & Xiao 2015). Vomerovaginal canal The course of the PTC was found to be predominantly straight The vomerovaginal canal (VVC) is located between the lateral (41%) or arch-shaped (37%). An ascending or tortuous course wing of the vomer and the vaginal process of the sphenoid bone was less frequently observed (Tsutsumi et al. 2018). (Meng et al. 2016) (Fig. 6, 7 and 12). The VVC and the PVC are in In a CT study of 100 patients, the mean distance from the close proximity or sometimes even overlapping one another, posterior wall of the maxillary sinus to the anterior opening of thereby wrongly identifying the medially located VVC as the the PTC was 8.3 mm (range 6–9.2 mm) (Vuksanovic-Bozaric et PVC (Meng et al. 2016). The VVC conveys a branch from the al. 2019). sphenopalatine artery.

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Discussion OPC optic canal This literature review presents an update of the clinical and ra- PC pterygoid canal diological anatomy of the retromaxillary region with regard to PPF pterygopalatine fossa CBCT. The retromaxillary region comprises the bilateral ptery- PSAA posterior superior alveolar artery gopalatine fossae, the single sphenoid bone, and the nasophar- PTP pterygoid process ynx. The latter structure will be addressed in detail in part 4 PVC palatovaginal canal about extraoral anatomy in CBCT, i. e. the pharyngo-cervical SPF sphenopalatine foramen region. SPS sphenoid sinus The retromaxillary region can be described as the transition VVC vomerovaginal canal zone between the viscerocranium (face) and the (brain). The anatomy is complex and many important neuro- Zusammenfassung vascular structures pass through the retromaxillary region. In dieser dritten Arbeit über die extraorale Anatomie im DVT As the pterygopalatine fossae communicate with multiple ana- wird die retromaxilläre Region diskutiert. Diese Region gilt tomical regions, the fossae represent major pathways for the als Übergangszone zwischen dem Gesichts- (Viszerokranium) spread of inflammatory or neoplastic diseases between these und dem Gehirnschädel (Neurokranium). Wesentliche skelet- (Rusu et al. 2013). tale Bestandteile der retromaxillären Region sind das Os sphe- Many of the discussed anatomical structures become visible noidale (Keilbein) sowie die Fossa pterygopalatina. Letztere in CBCT images of the posterior maxilla and/or the maxillary si- wird oft auch als «Kontrollzentrum» des Oberkiefers bezeich- nuses. However, the general dentist is usually not familiar with net. the anatomy outside the oral cavity, and consequently, the in- Das Os sphenoidale liegt als unpaarer Knochen zentral im terpretation of radiographs depicting the retromaxillary region Schädelskelett und ist Teil der Orbita, der vorderen und mittle- can be a daunting task. ren Schädelgrube sowie der Schädelaussenwand. Der massige While all the above-mentioned bones (and their processes, Körper des Os sphenoidale enthält die Keilbeinhöhle (Sinus sphe- canals or foramina) as well as spaces between bones (fossae, noidalis), die eine grosse Variabilität bezüglich Grösse, Form und fissures) can be found on the corresponding CBCT images, the Unterteilung mittels Septen zeigt. Der paarige Canalis opticus most striking anatomical structures of the discussed retromax- (für den Sehnerven und die A. ophthalmica) zieht durch den obe- illary region include the pterygoid processes of the sphenoid ren Teil des Keilbeinkörpers von der mittleren Schädelgrube zur bone and the sphenoid sinus. Once these structures are identi- Augenhöhle. Oberhalb des Keilbeinkörpers findet sich eine ty- fied, the observer can easily locate the sphenoid bone with its pische sattelförmige Einziehung, die sogenannte Sella turcica, many canals and foramina. in der sich die Hypophyse befindet. Die Unterfläche des Keil- Since the bilateral pterygoid processes are the most inferior beinkörpers bildet das Dach des Nasopharynx. parts of the sphenoid bone, they become readily visible on Typisch für das Os sphenoidale sind auch seine diversen knö- CBCT images of the retromaxillary region. The medial and later- chernen Fortsätze, wie die Alae minores und majores (lateral) al plates of the pterygoid processes appear as “extending limbs” sowie die Processi pterygoidei (inferior). Letztere bestehen aus from the body of the sphenoid bone. The latter also contains the einem lateralen und medialen Blatt. Am unteren Ende des me- sphenoid sinus that corresponds with the most posterior para- dialen Blattes befindet sich ein kleiner bogenförmiger Fortsatz, nasal sinus. As such, it can be easily differentiated from all other der Hamulus. Die beidseitigen medialen Blätter bilden zusam- paranasal sinuses that are bilateral in contrast to the single and men mit dem Vomer die Begrenzung der Choanen, also der hin- centrally located sphenoid sinus. teren Öffnungen der Nasenhöhlen. Die beiden grossen Flügel (Alae majores) des Keilbeins enthal- Acknowledgement ten drei wichtige Öffnungen: Foramen rotundum (für N. maxilla- The authors thank Bernadette Rawyler, medical illustrator, and ris), Foramen ovale (für N. mandibularis), und Foramen spinosum Ines Badertscher, media designer, School of Dental Medicine, (für A. meningea media). Während die erstere eine Verbindung University of Bern, Bern, Switzerland, for the illustrations and von der mittleren Schädelgrube zur Fossa pterygopalatina dar- preparation of figures. The authors acknowledge the generous stellt, ziehen die beiden letzteren zur Fossa infratemporalis. Die donation of the anatomical materials by anonymous individuals beiden kleinen Flügel (Alae minores) des Keilbeins bilden einen in the Willed Body Program, the University of Hawai’i John Teil der vorderen Schädelgrube. Zwischen den kleinen und A. Burns School of Medicine, Honolulu, HI, USA. grossen Flügeln befindet sich die spaltförmige Fissura orbitalis We also thank Dr. Odette Engel Brügger, oral surgeon in superior, durch die diverse Hirnnerven zur Augenhöhle gelan- Nidau, Switzerland, for the French translation of the summary. gen. Die Fossa pterygopalatina (Flügel-Gaumen-Grube) liegt ver- Conflict of interest steckt zwischen Oberkiefer und Keilbein. Zugang und Anato- The authors declare that there are no conflicts of interest related mie sind äusserst komplex. Die Flügel-Gaumen-Grube ist über to this review. mehrere Knochenkanäle bzw. Öffnungen mit Nachbarregionen verbunden. Über diese Verbindungen ziehen wichtige neuro- Abbreviations vaskuläre Strukturen zum Mittelgesicht bzw. von dort zum CBCT cone beam computed tomography Gehirn. Die grösste Öffnung der Flügel-Gaumen-Grube - die CN cranial nerve Fissura pterygomaxillaris - befindet sich lateral als Verbindung CT computer tomography zur Fossa infratemporalis. Die wesentliche Leitstruktur hier ist FOv foramen ovale die A. maxillaris. Nach unten verjüngt sich die Flügel-Gaumen- FSp foramen spinosum Grube trichterförmig zum Canalis pterygopalatinus, der sich in ICA internal carotid artery der Hinterwand der Kieferhöhle in den Canalis palatinus major

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et minor aufteilt. Letztere enden im harten Gaumen palatinal vers le bas. Ces derniers présentent une lame latérale et une der zweiten und dritten Molaren als Foramina palatinum majus lame médiale. À l’extrémité de la lame médiale se trouve un et minus. crochet, l’Hamulus. Les deux lames médiales et le vomer for- Als anteriorer Kommunikationsweg von der Flügel-Gaumen- ment les choanes, orifices postérieurs internes des fosses na- Grube zur Augenhöhle dient die spaltförmige Fissura orbitalis sales. inferior, die auch die hintere Grenze des Orbitabodens darstellt. Les deux grandes ailes du sphénoïde présentent chacune trois Durch diese Fissur ziehen N. infraorbitalis und N. zygomaticus, bei- ouvertures importantes : le foramen rond pour le nerf maxillaire des Äste des N. maxillaris. Nach medial dient das Foramen spheno- reliant la fosse crânienne moyenne à la fosse ptérygopalatine, palatinum als Verbindung von der Flügel-Gaumen-Grube zur le foramen ovale pour le nerf mandibulaire et le foramen épineux Nasenhöhle. Dieses Foramen befindet sich etwa auf Höhe der pour l’artère méningée moyenne, conduisant les deux vers la hinteren Begrenzung der mittleren Nasenmuschel. Durch das Fossa infratemporalis. Les deux petites ailes du sphénoïde forment Foramen sphenopalatinum ziehen die gleichnamigen Arterien und une partie de la fosse crânienne antérieure. La fente sphénoïdale Venen zur Nase sowie die nasalen Äste des N. maxillaris. est un espace entre les petites et les grandes ailes du sphénoïde Am komplexesten sind die Verbindungen von der Flügel- qui conduit plusieurs nerfs crâniens vers l’orbite. Gaumen-Grube nach posterior, da sich hier gleich mehrere Öff- La fosse ptérygopalatine est cachée entre le maxillaire supé- nungen präsentieren: Foramen rotundum, Canalis pterygoideus, rieur et le sphénoïde. Son accès et son anatomie sont particuliè- Canalis palatovaginalis sowie Canalis vomerovaginalis. Die beiden rement complexes. Elle présente plusieurs orifices conduisant letzteren ziehen als dünne Kanäle zum Nasopharynx. Der Cana- des structures vasculo-nerveuses vers le cerveau et le massif lis pterygoideus ist deutlich grösser und länger, und verbindet facial. L’ouverture la plus grande – la fissure ptérygomaxillaire – (wie auch das Foramen rotundum) die mittlere Schädelgrube mit communique avec la fosse infratemporale et conduit l’Arteria der Flügel-Gaumen-Grube. Durch den Canalis pterygoideus ver- maxillaris comme structure la plus importante. La fosse ptérygo- laufen die gleichnamigen Arterien und Nerven. Etwas weiter palatine va s’affiner vers le bas en entonnoir et former le Canalis lateral und oberhalb des Canalis pterygoideus befindet sich das pterygopalatinus qui longe la paroi postérieure des sinus maxil- Foramen rotundum, dessen Leitstruktur der N. maxillaris ist. laires. Il se poursuit dans les Canalis palatinus major et minor qui se terminent au palais osseux à hauteur des deuxièmes ou troi- Résumé sièmes molaires et forment les Foramina palatinum majus et Ce troisième travail au sujet de l’anatomie extraorale au CBCT minus. présente la zone retromaxillaire. Cette zone délimite le splanch- La fosse ptérygopalatine communique vers l’avant, donc vers nocrâne du neurocrâne. Ses structures osseuses principales sont l’orbite, par la Fissura orbitalis inferior qui forme aussi la limite l’os sphénoïde et la fosse ptérygopalatine, connue comme « zone postérieure du plancher de l’orbite. Cette fissure conduit les de contrôle » du maxillaire supérieur. N. infraorbitalis et N. zygomaticus, des branches du N. maxillaris. L’os sphénoïde est un os impair et médian. Il constitue une La fosse ptérygopalatine communique vers médial avec la cavité part de l’orbite, des fosses crâniennes antérieures et moyennes nasale par le Foramen sphenopalatinum qui aboutit à l’arrière du ainsi que de la base du crâne. Le corps massif du sphénoïde con- cornet nasal moyen. Le Foramen sphenopalatinum conduit l’ar- tient les sinus sphénoïdaux. Ces derniers ont une dimension et tère et la veine du même nom ainsi que les branches nasales du une forme très variables. Ils sont divisés de façon irrégulière par N. maxillaris. des septums. La structure paire du canal optique, conduisant le Les orifices postérieurs de la fosse ptérygopalatine sont les nerf optique et l’Arteria ophtalmica, traverse la partie supérieure plus complexes : Foramen rotundum, Canalis pterygoideus, Canalis du sphénoïde reliant la fosse crânienne moyenne à l’orbite. La palatovaginalis et Canalis vomerovaginalis. Les deux derniers for- fosse hypophysaire – une excavation à la face supérieure du ment de fins canaux vers le rhinopharynx. Le Canalis pterygoi- corps de l’os sphénoïde – contient l’hypophyse. Elle présente deus – plus long et plus large – relie comme le Foramen rotundum la forme typique d’une selle d’où le nom de selle turcique. La la fosse crânienne moyenne à la fosse ptérygopalatine. Le canal face inférieure de l’os sphénoïde forme le plafond du rhinopha- ptérygoideus conduit l’artère et le nerf du même nom. Le Fora- rynx. men rotundum se situe en dessus, légèrement latéralement, du L’os sphénoïde présente différents prolongements osseux: les Canalis pterygoideus. La structure principale qu’il conduit est le petites et les grandes ailes en latéral et les processus ptérygoïdes N. maxillaris.

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