The ethmoid bone is a small unpaired bone, located in the midline of the anterior cranium - the superior aspect of the skull that encloses and protects the brain. The term ‘ethmoid’ originates from the Greek ‘ethmos’, meaning sieve, which is reflected in its lightweight, spongy structure. In this article, we shall look at the anatomy of the ethmoid bone – its location, relations, and structure.
The ethmoid bone is one of the 8 bones of the cranium. It is situated at the roof of the nasal cavity, and between the two orbital cavities. It contributes to the medial wall of the orbit and forms part of the anterior cranial fossa, where it separates the nasal cavity (inferiorly) from the cranial cavity (superiorly). Additionally, it forms a significant portion of the nasal septum and lateral nasal wall. The olfactory nerve (CN I) has a close anatomical relationship with the ethmoid bone. Its numerous nerve fibres pass through the cribriform plate of the ethmoid bone to innervate the nasal cavity with the sense of smell.
The ethmoid bone is made up of three parts – the cribriform plate, the perpendicular plate, and the ethmoidal labyrinth. The cribriform plate forms the roof of the nasal cavity and is pierced by numerous olfactory nerve fibres, which gives it a sieve-like structure. Projecting superiorly from the cribriform plate is the crista galli, which provides an attachment point for the falx cerebri (sheet of dura mater that separates the two cerebral hemispheres). Another projection of bone descends from the cribriform plate – the perpendicular plate. It forms the superior two-thirds of the nasal septum while the ethmoidal labyrinth is made up of two sheets of bone - the orbital plate (lateral sheet of bone that forms the medial wall of the orbit), and the medial sheet (forms the upper lateral wall of the nasal cavity, from which the superior and middle conchae extend into the nasal cavity).
The ethmoid bone articulates with 13 other bones – paired bones which include the nasal bones, maxillae, lacrimal bones, palatine bones, and the inferior conchae, as well as unpaired bones such as the frontal, vomer, and sphenoid bones.
The ethmoid bone can be fractured in cases of facial trauma – most commonly hitting the dashboard in a collision, or a fall from height. Some signs and symptoms of fracture are related to the anatomy of the ethmoid bone:
A fracture to the cribriform plate may also allow communication between the nasal cavity and the central nervous system. Consequently, cerebrospinal fluid (CSF) can enter the nasal cavity and drain out from the nose. This manifests clinically as a clear watery discharge from one side of the nose – and is known as CSF rhinorrhoea.
In conclusion, the ethmoid bone is an essential component of the cranium. Its anatomy and articulations play a significant role in the functioning of the nasal cavity, orbit, and central nervous system. Furthermore, the fracture of the ethmoid bone may have important clinical implications. Therefore, it is important for medical professionals to have an understanding of the anatomy and physiology of the ethmoid bone, in order to diagnose and manage any related medical conditions.
The ethmoid bone is a single, midline bone found within the anterior cranial fossa, located between the frontal and sphenoid bones, immediately in front of the sphenoid. It is the smallest and most complex bone of the skull, and divides the nasal cavity from the brain. The ethmoid bone consists of three processes, which contribute to its attachment to various bones of the cranium and face, as well as its involvement in the formation of the orbits and the structures associated with it. These three processes are known as the orbital or lateral, the middle, and the posterior.
The orbital process of the ethmoid bone forms the medial walls of the orbits, and articulates with the frontal, lacrimal, sphenoid, maxilla, and palatine bones. It also gives origin to some of the muscles of the orbit and provides structural support for the olfactory nerve and the openings of the anterior ethmoidal foramina. The posterior ethmoidal process is covered laterally by the wings of the ethmoid bone and provides the openings of the posterior ethmoidal foramina. Anteriorly, it articulates with the frontal bone and superiorly with the sphenoid, and is attached to the planum sphenoidale, lamina papyracea, and orbital roof posteriorly.
Clinical Relevance
The ethmoid bone, due to its complex attachment to other bones of the cranium, is subject to fracture in cases of facial trauma such as colliding with the dashboard in a car accident or falling from a significant height. Some signs and symptoms of fracture are related to the anatomy of the ethmoid bone, and can include:
Additionally, fractures to the cribriform plate may cause communication between the nasal cavity and the central nervous system. This manifests clinically as a clear, watery discharge from one side of the nose – known as cerebrospinal fluid (CSF) rhinorrhoea. Most cases of CSF rhinorrhoea will stop spontaneously and can be managed conservatively; however, surgery may be necessary. CSF rhinorrhoea can also occur due to congenital or acquired defects in the ethmoid bone.
The ethmoidal labyrinth, located at either side of the perpendicular plate and containing the ethmoidal air cells or sinuses, also adds to the clinical relevance of the ethmoid bone. In general, its existence serves to, together with the other processes of the ethmoid, provide structural support and increase the surface area of the nasal cavity for the conduction of air.
In conclusion, the ethmoid bone is a vital structural bone of the cranium, is highly involved in the formation of the orbits, and is susceptible to fracture as a result of facial trauma. The ethmoid bone also contains the ethmoidal labyrinth and its air cells, which add to its complexity and enhance the surface area of the nasal cavity for conduction of air. Clinically, several signs and symptoms can arise from facial trauma to the ethmoid bone, including epistaxis, anosmia, and CSF rhinorrhoea.
