Surgical Anatomy of the Nose and Paranasal Sinuses

Abstract

This chapter delves into the intricate surgical anatomy of the nasal cavity and paranasal sinuses, highlighting their relevance in otolaryngological procedures. Understanding these anatomical details is crucial for performing safe and effective surgical procedures, minimizing complications, and enhancing patient outcomes. By integrating anatomical knowledge with clinical insights, this chapter aims to equip surgeons with a comprehensive understanding necessary for successful surgical intervention of nasal and sinus pathologies.

1. Introduction

The anatomical complexity of nasal cavity and paranasal sinuses is heightened by the proximity of significant surrounding structures, including the orbits and skull base which demand meticulous surgical navigation.

This chapter elucidates the basic anatomy of the nasal cavity and paranasal sinuses, emphasizing their clinical significance and the intricate relationships with adjacent critical structures. Mastery of this anatomical knowledge is imperative for the otolaryngologist to perform precise and safe surgical interventions, thereby optimizing patient outcomes and reducing intraoperative risks.

2. External nose anatomy

The external nose is composed of both bony and cartilaginous structures (osteocartiliginous framework), providing it with support and shape.

The bony part of the nose consists primarily of two nasal bones united in the midline by internasal suture. These nasal bones articulate with the frontal bone superiorly through the nasal process of the frontal bone and to the maxillary bones laterally through the frontonasal process of the maxilla (Figure 1).

Figure 1.

Showing the terminology for nasal surgery topography and the external nasal osteocartilaginous framework. 1: upper lateral cartilage; 2: lower lateral cartilage; 3: accessory cartilage; 4: cartilaginous part of nasal septum; 5: frontonasal process of maxilla; 6: nasal bone; 7: fibroconnective tissue. Adapted from Ref. [1].

The cartilaginous part of the nose consists of the nasal septum in the midline; in addition to the upper lateral cartilages and lower lateral cartilages (also known as allar cartilages).

The nasal septum divides the nasal cavity into two sides. It consists of the columellar septum which is the most anterior part of the septum, it is formed by the medial crura of alar cartilages which are connected by fibrous tissue and covered by skin. The membranous part of the septum, which lacks any bony or cartilaginous support, is situated between the columella and the lower edge of the septal cartilage. These two parts of the septum can be movable from side to side. The septum proper consists of an osteocartilaginous framework covered by nasal mucosa. The cartilaginous part is formed by the quadrangular septal cartilage anteriorly while the bony part is composed of the perpendicular plate of the ethmoid postero-superiorly and the vomer postero-inferiorly (Figure 2).

Figure 2.

Illustration of the nasal septum anatomy parts. Adapted from Ref. [2].

3. Blood supply and lymphatics

The nose is richly supplied with blood, with arterial contributions from both the internal carotid (including the anterior and posterior ethmoid arteries from the ophthalmic artery) and the external carotid (including the sphenopalatine, greater palatine, superior labial, and angular arteries).

The dorsal nasal artery, a branch of the ophthalmic artery, traverses the medial palpebral ligament to reach the dorsum of the nose, primarily supplying the bony portion of the external nose. Meanwhile, the external carotid artery gives rise to the facial artery, which provides branches to the cartilaginous part of the external nose; however, these branches are not specifically described in anatomical terminology [3].

The superior part of the nasal septum is nourished by the anterior and posterior ethmoidal arteries, as well as the sphenopalatine artery. The anterior septum and nasal floor receive blood from terminal branches of the superior labial artery. Kiesselbach’s plexus, also known as the Little area, is located in the lower anterior third of the nasal septum and is formed by the convergence of the superior labial, anterior ethmoidal, greater palatine, and sphenopalatine arteries.

The venous drainage of the nose, which mirrors the arterial pattern, is carried out by the anterior facial vein, the sphenopalatine vein, and the ethmoid veins. Notably, these veins directly communicate with the cavernous sinus and lack valves, facilitating the potential intracranial spread of infections. Lymphatic drainage from the nose flows anteriorly through the upper lip lymphatics and posteriorly through the deep cervical and retropharyngeal lymph nodes [4].

4. Nasal cavity

The nasal cavity is a complex structure comprising two cavities separated by the nasal septum, with each cavity having distinct anatomical features including the anterior nares, choana, roof, floor, medial, and lateral walls. The anterior nares are bounded by the columella medially and the ala laterally, while the choana is framed by the vomer medially and the medial pterygoid plate laterally. The cribriform plate of the ethmoid bone forms the roof of the nasal cavity, while the hard palate makes up the floor.

The medial wall of the nasal cavity is formed by the nasal septum which is comprised of cartilaginous and bony parts; the quadrangular cartilage being antero-inferiorly, the perpendicular plate of the ethmoid bone being postero-superiorly and the vomer is postero-inferiorly.

The lateral wall features three turbinates (conchae) and four corresponding spaces (meatuses), with the inferior turbinate being the largest and highly vascular, located above the inferior meatus where the nasolacrimal duct opens. The middle turbinate is of moderate size and is located centrally within the nasal cavity. Below it lies the middle meatus, which contains several notable structures: the bulla ethmoidalis, the largest cell of the anterior ethmoid sinus; the hiatus semilunaris, a semilunar groove below the bulla; and the uncinate process, a bone shelf medial to the bulla. The osteomeatal complex (OMC), crucial for drainage of the anterior group of sinuses, is bounded laterally by the lamina papyracea and medially by the middle turbinate. The superior turbinate is the smallest and highest, with the superior meatus below it, where the posterior ethmoid sinus opens. The sphenoethmoidal recess, located above the superior turbinate, drains the sphenoid sinus.

The nasal valve, the narrowest part of the nasal cavity, is located at the junction of the lower and upper lateral cartilages near the anterior end of the inferior turbinates.

The lateral nasal cavity, with its intricate anatomy, will be explored further in this chapter.

5. Ethmoid bulla

The ethmoid bulla is a prominent anterior ethmoid air cell originating from the lamina papyracea along the medial wall of the orbit. Despite the intricate nature of ethmoid bone anatomy, the ethmoid bulla remains the most recognizable and dependable ethmoid cell. It is located medial to the lamina papyracea, anterior to the vertical basal lamella of the middle turbinate, posterior to the uncinate process and postero-inferior to the frontal recess. Roughly 92% of patients exhibit well-pneumatized ethmoid bullae, making it a reliable landmark for anterior ethmoid sinuses during endoscopic procedures. In the remaining 8% of patients, the ethmoid bulla is either minimally pneumatized or completely absent, a condition referred to as torus lateralis [5].

6. Uncinate process, ethmoid infundibulum, and hiatus semilunaris

Uncinate process, a hook-shaped projection, is located anterior and inferior to the ethmoid bulla. Starting superiorly, near the agger nasi region at ethmoidal crest of the maxilla, the uncinate process extends downward and backward ending in a free edge with no bony attachment. This curving will result in both vertical and horizontal portions to the uncinate process (Figure 3).

Figure 3.

Parasagittal cadaveric dissection of the lateral nasal wall. The arrows are demonstrating the hook-shaped uncinate process with its vertical and horizontal parts. (*) Ethmoid bulla [6].

The way the vertical portion of the uncinate process attaches superiorly is significant, since it can influence the layout of the frontal recess and the outflow pathway of the frontal sinus. Not recognizing this alignment can result in insufficient drainage of the frontal sinus after surgery.

Most often, the uncinate process attaches laterally to the lamina papyracea. In this alignment, the frontal sinus drains medially to the uncinate process insertion and directly into the middle meatus. In uncommon instances, the uncinate process attaches to the upper section of the middle turbinate or directly to the skull base. In these scenarios, the frontal sinus drainage route will be lateral to the uncinate process, flowing into the ethmoid infundibulum [6].

Hiatus semilunaris, a two-dimensional space between the posterior free margin of the uncinate process and the ethmoid bulla that serves as an “entrance” to the three-dimensional ethmoid infundibulum.

The ethmoid infundibulum is a channel that is bordered anteromedially by the uncinate process, posteriorly by the ethmoid bulla, and laterally by the lamina papyracea. This structure is important because it acts as an innate drainage channel for the maxillary sinuses and often the frontal sinuses.

7. Agger nasi region

Agger nasi is an area of ridge bone on the anterior region of lateral nasal wall, when pneumatized, it is identified as an agger nasi cells which are the most anterior ethmoid sinus cells. The agger nasi region is bordered superiorly by the frontal sinus, superiorly and posteriorly by the frontal recess, by nasal bones anteriorly and by anciante inferomedially. Recognizing and excising the agger nasi cell is essential in the surgical management of frontal sinus disease, as it often plays a major role in narrowing the frontal recess and sinus outflow tract [5].

8. Middle turbinate

The middle turbinate consists primarily of three sections, oriented in various planes and firmly attached to the lateral nasal wall, orbit, and skull base.

The anterior part of the middle turbinate, which is the first portion of the middle turbinate to be seen during anterior rhinoscopy or nasal endoscopy, is oriented in the sagittal plane. It is attached anterosuperiorly to the crista ethmoidalis of the maxilla, and base of the skull near the lateral lamella of cribriform plate of the ethmoid bone with a free edge anteroinferiorly.

The middle turbinate basal lamella has a vertical part which is aligned in the coronal plane and a horizontal part aligned in an axial plane (Figure 4).

Figure 4.

Gross cadaveric dissections of the lateral nasal wall. (A) The three turbines are unbroken and in position. (IT) Inferior turbinate; (MT) middle turbinate; (ST) superior turbinate. (B) The vertical and horizontal portions of the MT basal lamella can be seen (arrows) after the parasagittal sections of the MT and ST have been excised. (C) The anterior ethmoid cavity is separated from the posterior ethmoid cavity by the vertical and horizontal parts of the MT basal lamella (demarcated in a black line) [6].

The vertical part divides the anterior and posterior ethmoid complexes and attaches to the skull base superiorly and to the lamina papyracea laterally. The horizontal part, which forms posteriorly, can be used to locate the sphenopalatine foramen.

Pneumatization of the middle turbinate is possible and the nomenclature of these structures is determined by the location of the pneumatized portion. It is referred to as a concha bullosa if the anterior parasagittal-oriented portion is pneumatized and it is referred to as an interlamellar cell if the vertical section of the basal lamella is pneumatized [5].

9. Posterior ethmoid complex

The posterior ethmoid sinus is bounded anteriorly by the vertical portion of the middle turbinate basal lamella, posteriorly by an anterior wall of the sphenoid sinus, laterally by the laminia papyacea and superiorly by the skull base.

An Onodi cell, which is the term anatomical variant, is a highly pneumatized posterior ethmoid cell that has persisted in growing superolaterally in relation to the true sphenoid. Studies have shown that the incidence of sphenoethmoidal air cells in individuals ranges from 3.4 to 60%, a variation that is probably attributed to differences in definitions rather than actual rates of occurrence [7].

Since the term “sphenoethmoid cell” better describes the anatomy in this region, it is now preferred over “Onodi cell.” sphenoethmoidal cell when present can be mistaken for a sphenoid sinus. Before sinus dissection, radiographic identification of sphenoethmoid cells is crucial to avoid incomplete dissection that may result from not identifying them beforehand.

The other primary reason for the significance of these air cells is that they are situated in close proximity to the internal carotid artery and optic nerve, with a mere 0.03 mm (median 0.08 mm) of bone dividing them [7].

10. Ethmoid roof and skull base

The anterior skull base is divided by the middle turbinate vertical lamella into the cribriform plate medially and the roof of the ethmoid laterally. The ethmoid roof is made up of two parts: a thinner vertical portion, named the lateral cribriform plate lamella (LCPL), and the thicker horizontal portion named fovea ethmoidalis which an extension of the orbital plate of the frontal bone.

The skull base is thicker laterally and as it gets medially it becomes thinner reaching only (0.2 mm thickness) along the cribriform plate which represents the thinnest point in the anterior skull base and the most frequent location for iatrogenic cerebrospinal fluid leaks during sinus surgery [6].

Furthermore, there is a positive correlation between the depth of the olfactory cleft and the chance of iatrogenic injury during surgery. The ethmoid roof is classified by Keros classification system into—shallow type I (1–3 mm), medium type II (4–7 mm), and deep type III (8–16 mm). As the depth increases the risk for complications increases during endoscopic endonasal surgery which can include CSF leak or intracranial hemorrhage [8].

10.1 Maxillary sinus

Out of all the paranasal sinuses, the maxillary sinus is the biggest and the first to develop. The maxillary sinus is positioned within the maxillary bone, it has a pyramid shape, with its base near the nasal cavity and its apex directed toward the zygomatic bone. The boundaries of the maxillary sinus include the floor, which is formed by the alveolar process of the maxilla housing the roots of the maxillary teeth; the roof is formed by the floor of the orbit; the medial wall is formed by the lateral nasal wall; and the lateral wall is facing the zygomatic process.

The usual opening of the maxillary sinus is through a single natural ostium positioned in the posterior part of the ethmoid infundibulum. Nonetheless, up to 23% of patients may develop accessory ostia through the anterior or posterior fontanelles. These fontanelles are situated in the lateral nasal wall and lack bone; in some instances, the mucosa and connective tissue covering them are insufficient, creating an alternative drainage route for the sinus [6].

The maxillary sinus can show different levels of pneumatization and occasionally, hypoplasticity. The orbital contents tend to fill a greater volume of the midface as the volume of the maxillary sinus decreases. Due to the increased orbital volume-to-maxillary sinus volume ratio in cases of maxillary sinus hypoplasia, paranasal sinus surgeons need to proceed with caution during surgery. The uncinate process is often shifted inferolaterally and positioned near the orbital wall in cases of the hypoplastic maxillary sinus. Furthermore, marked degrees of maxillary sinus hypoplasia may be associated with an underdeveloped uncinate process because of their shared developmental origins [5].

10.2 Sphenoid sinus

Positioned within the sphenoid bone at the central skull base, the sphenoid sinuses are a pair of large paranasal spaces found posterior to the ethmoid sinuses. The natural ostium of the sphenoid sinus connects the sphenoid sinus with the roof of the nasal cavity via the sphenoethmoidal recess.

The sphenoid ostium opens in the anterior superior wall of the sinus and it is situated approximately 1.0–1.5 cm above the superior edge of the posterior choana and it is typically situated approximately midway between the posterior insertion of the superior turbinate and the nasal septum [5].

Due to its variability, the sphenoid sinus is one of the most complex sinuses. Its proximity to critical neurovascular structures makes it challenging for endoscopic surgeons to navigate [9]. These relationships include the pituitary gland which is located posterior and superior to the sphenoid cavity. Above it lies the optic chiasm, with the optic nerves extending laterally toward the orbital apices. The internal carotid arteries are positioned lateral to the pituitary gland and clivus, often creating impressions in the sphenoid sinus walls and forming the opticocarotid recesses. The cavernous sinus, which houses oculomotor; trochlear; ophthalmic and maxillary divisions of trigeminal; and abducent cranial nerves, is situated lateral to the sphenoid sinus.

Additionally, the vidian nerve can be seen as an impression along the inferolateral aspect of the sinus floor. Understanding these anatomical relationships is vital to avoid damaging these structures during surgery.

The right and left sphenoid sinuses are separated by a sphenoid intersinus septum. It is common for these sinuses to develop unevenly, and demonstrate differences in size and pneumatization. When planning sphenoid sinus surgery, it is essential to thoroughly assess the sphenoid intersinus septum. This septum can deviate to one side and may attach near critical structures like the internal carotid artery or optic nerve [10].

Awareness of the anatomical layout and positioning of these neighboring structures can make interventions considerably safer, given the potentially devastating outcomes of damaging any of them.

10.3 Frontal sinus

The frontal sinus has a trapezoidal or triangular shape and it develops last among the paranasal sinuses. In newborns, the frontal sinus (FS) is barely noticeable, but it becomes detectable on radiographs around age 4. Its growth parallels that of the craniofacial area, attaining its maximum size approximately at age 18 [11].

The frontal sinus is bounded inferiorly by the orbital roof, anteriorly by the terminal frontal table of the frontal bone, and posteriorly by the posterior table dividing the sinus from the brain.

Despite the notable anatomical differences in the frontal sinus area, the right and left frontal sinuses are divided by a thin bone known as the frontal intersinus septum. The right and left frontal sinuses are often unequal in size, with the septum leaning toward one side. The frontal sinus is commonly bilateral; however, it might be unilateral in up to 15% of cases and absent in 8% of cases [12].

In some cases, the septum becomes pneumatized, resulting in an intersinus septal cell that can drain into the left or right frontal sinus. Surgeons may mistakenly think they have drained the opposite sinus by entering this cell.

Other pneumatized cells that should be taken into consideration in the frontal sinus region are the supraorbital ethmoid cells, which extend over the orbit, as they can also confuse without proper preoperative imaging.

Connecting the frontal sinus to the anterior ethmoid cells, the frontal recess is an inverted cone featuring a narrow upper end at the internal frontal ostium and a broader lower end that merges into the anterior ethmoid cells. Endoscopic surgeons need to have a comprehensive understanding of the frontal recess.

Despite that the drainage pathway can vary based on the superior insertion of the uncinate process, the frontal recess bony structures remain fairly the same. The frontal recess is bound medially by the anterior superior portion of the middle turbinate and laterally by the lamina papyracea. The frontal sinus forms its upper limit while the nasal cavity forms its lower limit. The agger nasi cells constitute the anterior border and the ethmoid bulla forms the posterior border [11].