Study of Pterygospinous and Pterygoalar Bars in Relation to Foramen Ovale in Dry Human Skulls

Published online: 04.11.2019

THIEME Original Article 97

Arvind Kumar Singh1 Richa Niranjan1

1Department of Anatomy, Government Medical College, Haldwani, Address for correspondence Richa Niranjan, MBBS, MD (Anat), Uttarakhand, India Department of Anatomy, Government Medical College, Haldwani, Uttarakhand 263139, India (e-mail: [email protected]).

Natl J Clin Anat 2019;8:97–100

Abstract Background Anatomical knowledge of bony bridges around the foramen ovale may be helpful for diagnostic and invasive neurosurgical procedures like electroencephalogram anal- ysis, trigeminal rhizotomy, biopsy of cavernous sinus tumors, and mandibular nerve block. Lateral pterygoid plate forms an important landmark for mandibular anesthesia; therefore, any variation related to lateral pterygoid plate is likely to create confusion during the maneuver of anesthesia. Aims and Objective The aim of the study was to explore any bony obstacle within and around Foramen ovale. Obstacles in form of ossified complete or incomplete ligaments. Additional foramina formed by ligaments or any bony enlargement might disturb the structures passing through the Foramen ovale. Methods Around 530 dried crania (from medical colleges in Uttarakhand and Uttar Pradesh) were observed to find ossified ligaments and foramen formed by them. Crania associated with bilateral enlarged lateral pterygoid plate other than the average width of 1.5 cm were included in this study. Length of ligaments and width of pterygoid plate were measured by digital Vernier calipers. Results Out of 530 crania, unilateral 52 ossified pterygospinous ligament (incomplete 31 and complete 21) were observed. Among them some rare variation was found along with ossified pterygospinous and pterygoalar ligament, one cranium along with unilateral pterygospinous bar was also having bar within foramen ovale, forming an accessory osseous compartment, found to be rare kind of variation. Another unilateral complete pterygospinous bar was enclosing two foramens instead of one. Only single cranium was found to have pterygoalar bar, related medial to foramen ovale. One cranium was associated with bilateral enlarged lateral pterygoid plate (i.e., 2.5 cm) was also found to be obliterating the space, that is, retropharyngeal and parapharyngeal space for surgeons. Conclusion This study would provide the anatomical knowledge of these bony Keywords bridges around the foramen ovale and may be helpful for diagnostic and neurosurgical ►►pterygospinous procedure like microvascular decompression by percutaneous trigeminal rhizotomy ligament for trigeminal neuralgia, percuatneous biopsy of cavernous sinus tumor and mandib- ►►pterygoalar ligament ular nerve block by lateral subzygomatic route. Thus, lateral pterygoid plate forms an ►►foramen ovale important landmark for mandibular anesthesia.

Introduction formation of the foramen ovale (FO) is observed at the sev- enth fetal month and the latest in 3 years after birth1 and the In the sphenoid bone, the first ossification center appears in ossification takes place around the large trunk of mandibular the greater wing of sphenoid; the earliest perfect ring-shaped nerve. The appearance of the various bony outgrowths of the

FO like spine, spur, tubercle, bony plate, and so forth, indicate bony overgrowth during developmental process. Ossification of ligaments of cranial base can also obstruct surgical corri- dors and interfere with operative access in this area.2 The presence of ossified pterygospinous (PS) and pterygoalar (PA) bars obliterates the space between the lateral pterygoid plate and spine of sphenoid which restricts access to retropharyngeal and parapharyngeal space to neurosur- geons. Presence of these ligaments should be kept in mind when one finds difficulty in approaching to the base of skull.

Material and Method

A total of 530 dried human skulls of unknown age and sex, from department medical colleges (Uttarakhand and Uttar Fig. 1 Pterygospinous bar: length 14.85 mm and width 3.2 mm. Pradesh), were studied. The base of skull was studied for the Foramen of Civinni: length 5.34 mm and height 3.07 mm. presence of ossified ligament, that is, PS bar, PA bar, and its Accessory foramen (yellow arrow) lateral to foramen ovalis: length 1.2 mm and width 1.0 mm. association with lateral pterygoid plate and accessory fora- Foramen Ovale: length 6.37 mm and width 3.9 mm. men. Measurements were taken by using a sliding digital Vernier calipers. Photographs of ossified ligament were taken with relation to FP.

Observation and Results

Out of 530 human skulls, 52 skulls showed unilateral PS bar, complete ossification were found in 21 skulls, and in rest of the 31 skulls 25 unilateral and 6 bilateral incomplete ossification were observed, incidence of these given in 2014.3 Among these some rare variation associated with bony bars were observed; there was rare association of bony bar within FO beside PS bar. Another skull PS bar was found to enclose two foramen in vicinity of FO. One crania had incomplete PS bar with larger lateral pterygoid plates than the average ones; free borders of it related lateral to FO. One cranium had complete unilateral PA ligament. Case 1 presented with complete pterygospinous bar on left side, of length 14.85 mm and width of 3.2 mm; a foramen Fig. 2 Pterygoalar ligament: length 7.6 mm and width 1.8 mm. formed underneath to bar is considered foramen of Civinni, Pterygoalar foramen (crotaphitico-buccinatorius) (yellow arrow): of size length 5.34 mm and height 3.07 mm. An accessory length 3.46 mm and width 1.73 mm. foramen, length 1.2 mm and width 1.0 mm was also observed within the FO, of size length 6.37 mm and width 3.9 mm, it might disturb the normal contents of the FO (►Fig. 1). In case 2 complete PA bar was observed on left side of skull of length 7.6 mm and width 1.8 mm; thus, the foramen formed from it was PA foramen (crotaphitico-buccinatorius) of length 3.46 mm and width 1.73 mm. This bony bar was related to medial of FO (►Fig. 2). Case 3 shows a right side complete PS bar and left side incomplete PS bar; it was associated with larger lateral pterygoid plate, that is, >10 mm.4 On the left side lateral pterygoid plate is a triangular shape with maximum width of 2.4 cm, whereas on the right side it was quadrangular in shape with maximum width of 2.5 cm. Both side bars related lateral to FO (►Fig. 3). Case 4 shows a right side complete pterygospinous bar lat- Fig. 3 Left side incomplete pterygospinous bar with quadrangular eral to FO and thus enclosing two foramina underneath to it. shape LPP of 2.5 cm (maximum width). The higher foramen of length 1.1 mm and width 0.8 mm and Right side complete pterygospinous bar (length 2 mm and width the lower foramen of length 2.2 mm width 3.0 mm (►Fig. 4). 1 mm) with triangular LPP of 2.4 cm (maximum width).

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several accessory foramen and ligament; similar observation were observed by Sinha.14 Additional anterior spine of sphenoid and inferior foramen spinosum in human skull. Pterygoalar bar is the name given to an ossified pterygoalar ligament by Chouki and Hodes.15 It extends from the root of lateral pterygoid lamina of sphenoid bone to the under surface of its greater wing existing as a bony ridge or forming a foramen which lies along the lateral margin of FO. This foramen was termed as porus crotaphitico buccinatorius by Hyrtl.16 In the present study complete PA bar and PA foramen was observed on left side of the skull and was related medial to the FO. Kamath and Vasantha17 also observed a single complete PA bar and 29 incomplete ones. Natsis et al (2014)5 observed 4 complete and 26 incomplete PA bars 18 Fig. 4 Foramen 1: length–1 mm and width 0.8 mm. in Greek human skull. Goyal and Jain found only incom- Foramen 2: Length 2.2 mm and width 3.0 mm. plete ones that is, 6 bilateral and 11 unilateral PA bars. The PA bar may pass medial to FO (as in lemurs) or lateral to FO (pithecoid), or even across the lumen of the foramen. This can Discussion potentially press on the deep temporal, the lateral pterygoid, Osseus bridging in various parts of the body is a frequent buccal nerves (branch of ant division of mandibular nerve) age-dependent process poorly understood and represents and branches of auriculotemporal nerve. the outcome of secondary ossification of fibrous structure.5 Various inconstant patterns of grooves and foramina in The thickened cranial part of fascia, between the lateral and the vicinity of the FO can be interpreted as arising from the medial pterygoid muscles, stretches between the spine of interplay of various parts of the membranous bone and the sphenoid to upper posterior part of lateral pterygoid plate, emissary venous plexus from the middle meningeal veins resulting in fibrous bands which are named as PS ligaments. to the pterygoid venous plexus.19 Sphenoid accommodates Italian anatomist F Civinini (1805–1844)6 first described and numerous vessels and nerves, which can easily be entrapped coined the term “pterygospinous ligament” or “ligament of in the case of development of accessory osseous structures or Civinini.” Ossified ligament projects as a bar, medial or lateral calcification of the intrinsic ligaments. or sometimes across the FO; it may interfere with percutane- Entrapment and compression of lingual nerve may occur ous injection of the mandibular nerve. due to one of the following causes: partial or complete ossi- In the present study, the complete PS bar on left side, was fication of PS ligaments. Pterygospinous bar ridge separates lateral to the FO, along with presence of accessory foramen the trunk of lingual nerve into anterior and posterior branch- within FO, toward the lateral rim. In another case complete es. Anterior fibers may get compressed because of their PS bar on right side, with two foramen formed underneath course between the tensor veli palatini muscle and the bony to it, was related lateral to FO. In another case complete right ridge20 or because of a large lateral pterygoid plate.11,21,22 side complete PS bar with large triangular lateral pterygoid plate, left side incomplete PS bar with large quadrangular lat- Conclusion eral pterygoid plate is observed. A foramen which is created between the base of skull and Bony bar as one of the causes of failure of trigeminal nerve the complete ossified bar transmits neurovascular structures injection for relief of trigeminal neuralgia. Penman (1949)23 of the medial pterygoid muscles.7,8 The size of the foramen and Sweet24 for the same reasons emphasized the importance varies from 2 to 12 mm.8 Formation of accessory osseous of reontegenographic control during trigeminal injection. components within the FO or in its vicinity may influence The complete ossification of the PA ligament determines the the neurovascular structures passing through its lumen.9,10 formation of the PA foramen (crotaphitico-buccinatorius), Mandibular nerve occupies most of the space of FO and as which is much smaller in diameter compared with the PS such can be subject to compression by abnormal protrusion foramen (Civinini’s). The PA ligament completely prevents emerging from rim of FO. Compression of motor branches access to the mandibular nerve or trigeminal ganglion from of mandibular nerve can lead to paresis or weakness of the foramen ovale by the supra or infrazygomatic approach. Most innervated muscles.11 of the earlier investigations mentioned the incidence of the Elongation of the lateral lamina of the pterygoid process ossified ligaments in different parts of world. The clinical could result in weakening of the medial pterygoid muscle implications of these are very few. Since combination of oth- and paresthesia of the inner aspect of the cheek. Probable er features make them rarest of rare, so other obstacles along compression of the lingual nerve could lead to the weakening with the PS and PA bar have been studied. of general sensation from anterior two-thirds of the tongue In present study, we observed several rare types of bony unilaterally or may cause neuralgia.12,13 Situation of lateral obstacles around the FO and along with ossified ligaments pterygoid plate and adjoining foramina are always a prob- and analyzed morphometrically the homonyms of foramen lem due to morphological changes and may continue to give in collection of human skulls that could lead to a difficult and

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National Journal of Clinical Anatomy Vol. 8 No. 3/2019