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Understanding Anatomy of the Petrous Pyramid-A New Original Article Understanding Anatomy of the Petrous PyramideA New Compartmental Approach Mamdouh Tawfik-Helika1, Patrick Mertens2, Guilherme Ribas3, Michael D. Cusimano4, Martin Catala5, Ramez Kirollos6, Timothe´ e Jacquesson2,7 - BACKGROUND: Learning surgical anatomy of the - CONCLUSIONS: This new compartmental approach al- petrous pyramid can be a challenge, especially in the lows a comprehensive understanding of the distribution of beginning of the training process. Providing an easier, petrous pyramid contents. Dividing it into anatomic com- holistic approach can be of help to everyone with interest partments, and then navigating this mental map along in learning and teaching skull base anatomy. We present specific reference points, lines, spaces, and segments, the complex organization of petrous pyramid anatomy using could create a useful tool to teach or learn its complex a new compartmental approach that is simple to under- tridimensional anatomy. stand and remember. - METHODS: The surfaces of the petrous pyramid of two temporal bones were examined; and the contents of the petrous pyramid of 8 temporal bones were exposed through INTRODUCTION progressive drilling of the superior surface. he temporal bone belongs to the skull base and is known to be a complex bone, because of the large number of - RESULTS: The petrous pyramid is made up of a bony T named contents, and its particular tridimensional shape. container, and its contents were grouped into 4 compart- The unique anatomic construction of the temporal bone reflects ments (mucosal, cutaneous, neural, and vascular). Two its particular development process and connections. The temporal reference lines were identified (mucosal and external- bone is composed of 5 parts: squamous, petrous, mastoid, styloid, 1 internal auditory canal lines) intersecting at the level of and tympanic. Some researchers have described it as being composed of 32 or 4 parts, by considering the mastoid as an the middle ear. The localization of contents relative to 3-5 these reference lines was then described, and 2 methods of extension from both the petrous and squamous parts. All 5 parts contribute to the middle ear walls, with the petrous part segmentation (the X method and the V method) were then medially, the squamous part laterally, the mastoid part posteriorly, proposed. This description was then used to describe and both styloid and tympanic parts inferiorly.2,4-6 The middle and middle ear relationships, facial nerve anatomy, and air cell posterior cranial fossae are built on a large pyramid-shaped bone, distribution. called the petrous pyramid by Pellet et al.,2 which houses the Key words SCC: Semicircular canal - Anatomy TMJ: Temporomandibular joint - Middle ear - Petrous From the 1Department of Neurosurgery, Beaujon University Hospital, Assistance publique - Pyramid hopitaux de Paris, Clichy, France; 2Department of Anatomy, University of Lyon 1, Lyon, - Segmentation France; 3Department of Surgery, University of São Paulo Medical School, São Paulo, Brazil; - Space 4Division of Neurosurgery, St. Michael’s Hospital, University of Toronto, Toronto, Ontario, - Temporal bone Canada; 5Sorbonne Université, CNRS UMR 7622, INSERM ERL 1156, IBPS, Paris, France; 6Senior Consultant, National Neuroscience Institute, Singapore; and 7Skull Base Multi- Abbreviations and Acronyms disciplinary Unit, Department of Neurosurgery B, Neurological Hospital Pierre Wertheimer, CSF: Cerebrospinal fluid Hospices Civils de Lyon, Lyon Cedex, France EAC: External auditory canal To whom correspondence should be addressed: Mamdouh Tawfik-Helika, M.D. ET: Eustachian tube [E-mail: [email protected]] FO: Foramen ovale Citation: World Neurosurg. (2019) 124:e65-e80. FS: Foramen spinosum https://doi.org/10.1016/j.wneu.2018.11.234 GSPN: Greater superficial petrosal nerve Journal homepage: www.journals.elsevier.com/world-neurosurgery IAC: Internal auditory canal ICA: Internal carotid artery Available online: www.sciencedirect.com MA: Mastoid antrum 1878-8750/$ - see front matter ª 2019 Elsevier Inc. All rights reserved. WORLD NEUROSURGERY 124: e65-e80, APRIL 2019 www.journals.elsevier.com/world-neurosurgery e65 ORIGINAL ARTICLE MAMDOUH TAWFIK-HELIKA ET AL. PETROUS COMPARTMENTAL APPROACH organs of hearing and equilibrium and is crossed by cranial nerves vestibule and semicircular canals [SCCs]). The ICA was freed at and the internal carotid artery (ICA). The petrous pyramid is the the petrous apex, medial to the ET, below the GSPN and tri- area of interest in this work. geminal ganglion, toward its exit through the upper part of the Segmentation of the petrous pyramid has rarely been described foramen lacerum into the cavernous sinus. The major space- for surgical purposes.2,7,8 However, better understanding of its occupying contents of the petrous pyramid were then grouped into tridimensional architecture could further help teaching, imaging, four groups, or compartments, having different composition, and surgical planning. Despite seeming complexity, the petrous lining, and connections. These are the mucosal, cutaneous, neural pyramid structure shows certain patterns that can be better and vascular compartments; respectively. The other parts of the appreciated from a superior view. These patterns can simplify petrous pyramid, not occupied by any of the four compartments, understanding the anatomic architecture of intrapetrous contents. have also been adressed. Traditional description of temporal bone anatomy starts with its parts and goes directly into details; such as middle ear contents RESULTS and the facial nerve. Yet, having a clear understanding of the general plan, while facing a large number of details, may be The petrous pyramid can be described as being composed of 2 difficult in the beginning of the training process, which is already parts: the bony container and the contents, which are described in time limited. This may even make it harder to step up into use for terms of compartments. Four compartments are thus described clinical applications, especially for young surgical trainees. Having based on their nature and connections: mucosal, cutaneous, a holistic plan may provide an easier way to start learning. neural, and vascular. The mucosal compartment or line consists of Breaking down a complex map into smaller parts and under- the ET, middle ear, and MA and is used as the axis of reference. It standing their relative positions, before analyzing each of them, is drawn obliquely anteromedially, pointing to, and opening into might provide a useful way to conceptualize, learn, and navigate. the pharynx. The mucosal line is considered as the central piece of Like any map, it needs to have reference points and directions for this approach because the terms anterior, posterior, medial, and orientation purposes; as well as landmarks; for proper use. lateral are used relative to this axis, also called the mucosal axis; We propose a new anatomic compartmental approach to the not to the midsagittal plane. All major anatomic structures were petrous pyramid that could ease understanding and navigation thus located relative to the mucosal axis: the mucosal compart- along simple landmarks, lines, and spaces. As a result of different ment lies along the axis, the cutaneous compartment (EAC) lateral þ methods of segmentation, we aim to provide an educational tool to the middle ear, the neural compartment (IAC anterior laby- þ for surgeons; as one advances progressively from creating the rinth or cochlea posterior labyrinth made up of vestibule and map, to its use and applications. SCCs) medial to the middle ear, and the vascular compartment (ICA) medial to the ET. Then, 2 methods of segmentation were developed. These blueprints were used to describe the anatomy of METHODS the middle ear, facial nerve, and air cells. One dry head (2 temporal bones) was used to study the walls of the petrous pyramid. In addition, 4 fresh cadaver heads (8 temporal The Container: General Form and Location of the Petrous Pyramid bones) were harvested in the anatomy laboratory of University The petrous pyramid is the partition separating the middle and Lyon 1 (Lyon, France), prepared with 3% formaldehyde, and then posterior cranial fossae, as shown in Figure 1. It is pyramid shaped injected with colored latex (Aérographe Colorex Technics, with 3 lateral surfaces and a base, all triangular; and is surrounded Magenta and Cyan [Phocéenne de chimie, Marseille, France]). The by cortical bone. The base of the pyramid faces laterally whereas vertex and the whole brain were removed, including the brain- the apex faces the midline. The superior/anterior surface forms stem. Cranial nerves IIeXII were carefully left in place, as well as the posterior part of the floor of the middle cranial fossa and the ICA. The dura was gently removed at each side and the skull faces the temporal lobe, whereas the apical part faces Meckel’s base was then exposed with cisternal segments of all cranial cave. We adopted the description of its anterior limit as was nerves of the middle and posterior cranial fossa. proposed by Ribas et al.9 It is composed of a horizontal line Drilling (Midas Rex [Medtronic, Fort Worth, Texas, USA]) starting laterally at a preauricular burrhole, located just anterior started at the center of the superior aspect of the petrous pyramid, to the tragus, facing the dorsum sella medially, and passing to unroof the middle ear, removing the tegmen tympani, and through the FS. The posterior limit is the petrous ridge, exposing the malleus and incus.
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