Anatomy of Paranasal Sinuses: Maxillary, Ethmoid Sinuses & More

• Paired sinuses are present in the facial bones. There are two groups of sinuses:
  • Anterior group, consisting of maxillary sinuses, frontal sinuses, and anterior ethmoid sinuses. 
  • Posterior group consisting of posterior ethmoid sinuses and sphenoid sinuses.

Maxilla and Maxillary Sinuses Development of Maxilla 

• It is the second-largest facial bone. It develops during the 6-7 weeks of intrauterine life from five ossification centers that fuse during the 4th fetal month.
• Maxilla has a body and four processes
  • Zygomatic process 
  • Frontal process 
  • Palatine process 
  • Alveolar process 
• It forms the roof of the mouth, floor of the orbit, lateral wall, and floor of the nasal cavity

Development of Maxillary Sinus

• This is the first sinus to appear radiologically (7–10 weeks). 
• It continues to grow during childhood at the rate of 2 mm vertically and 3 mm anteroposteriorly. 
• At 12 years, pneumatization reaches laterally towards the zygoma and inferiorly to the level of nasal bone. 
• After dentition, it goes below the nasal cavity floor. 
• It enlarges gradually and reaches its final size in the 17th to 18th year of life. 

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Maxillary Sinus (Antrum of Highmore)

• On the anterior side, subcutaneous tissue/skin is present. 
• The roof of the maxillary sinus is anatomically formed by the floor of the orbit. Infections can spread through this communication and lead to orbital complications.
  • The floor of the maxillary sinus is connected to the second premolar and the first molar teeth. 
  • The medial wall of the maxillary sinus separates the sinus from the lateral wall of the nose. 
  • The zygomatic process is present on the lateral side. 
  • Pterygo-palatine fossa is present on the posterior side. 
  • It is the largest paranasal sinus. 
  • It is the most common source of infection.
  • The capacity of the maxillary sinus is 15 ml. 
  • It is a pyramidal shape. 
  • It is lined by pseudo-stratified ciliated columnar epithelium.

Maxillary Sinus Ostium

• The maxillary sinus typically opens into the middle meatus at the level of the ethmoidal infundibulum. Occasionally, a small accessory ostium may be present further posteriorly, commonly known as posterior fontanalle having a posterior accessory ostium (30% of cases). 
• It is located in the superior aspect of the medial wall of the maxillary sinus. 
• The natural ostium of the maxillary sinus is typically elliptical and can vary in length, ranging from 1 to 20 millimeters. 

Relation of Maxillary Sinus

• The anterior wall forms the facial surface of the maxilla and the soft tissue of the cheek.
• The posterior wall forms the infratemporal and pterygopalatine fossa. 
• The medial wall forms the lateral wall of the nasal cavity. 
• The roof forms the floor of the orbit, infraorbital nerves, and vessels. 
• The floor forms the alveolar and palatine processes of the maxilla.

Blood Supply, Nerve Supply and Lymphatic Drainage

• The blood supply to the maxillary sinus is facilitated by ECA through branches of the facial artery, maxillary artery, infraorbital artery, and greater palatine artery. Venous drainage occurs through the anterior facial vein and the pterygoid plexus. 
• The nerve supply is provided by branches of the maxillary nerve (branch of the trigeminal nerve) via infraorbital, superior alveolar (anterior, middle, and posterior), and greater palatine nerves. 
• The lymphatic drainage is in the submandibular nodes and pterygopalatine fossa.

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Ethmoid Bone and Ethmoid Sinuses Development of Ethmoid Sinuses  

• It is formed at 9–10 weeks of gestation, where 6-7 folds appear in the lateral wall of the nasal capsule. 
• These folds are separated from each other by corresponding grooves. 
• The folds fuse to form crests with
  • Anterior ascending crests 
  • Posterior descending crests 
• There are three turbinals, namely, nasoturbinal, ethmoturbinal, and maxilloturbinal. Each of these turbinals contributes to some components of air cells.
  • Nasoturbinal: It forms the ascending portion of the agger nasi and the descending portion of the uncinate process. 
  • Ethmoturbinal: It forms the ethmoidal bulla, middle, superior, and supreme turbinates.
  • Maxilloturbinal: It forms the most interior fold and contributes to inferior turbinate. 

Ethmoid Bone

• Ethmoid bone gives rise to three critical components of the nasal cavity and paranasal sinuses.
  • The perpendicular plate of the ethmoid bone forms the upper part of the septum, the cribriform plate of the ethmoid, and the roof of the ethmoid and ethmoidal air cells. These together form an ethmoidal complex. 
• These ethmoidal bones or air cells are sandwiched between the frontal sinus anteriorly and the sphenoid sinus posteriorly. These ethmoidal cells are divided into anterior and posterior air cells by the basal lamella of the middle turbinate. The vertical part of the middle turbinate superiorly attaches to the skull base, and the horizontal part is attached to the LP of the orbit. The junction of the horizontal and vertical portions is called the basal lamella/ground lamella. 
• The rood of the ethmoid is made by a thin plate of bone called foveolae ethmoidalis. It separates the ethmoidal air cells from the anterior cranial fossa containing the frontal lobe of the brain.
• The anterior ethmoidal cells will drain in the middle meatus, whereas the posterior ethmoidal air cells drain in the superior meatus. The sphenoid sinus drains in the spheno-ethmoidal recess.

Ethmoidal Sinuses

• They are the collection of cells and clefts in the ethmoidal labyrinth. 
• The anterior ethmoidal cells are smaller and more numerous (2–8), compared to the posterior ethmoidal cells (1-5).
• The roof of the ethmoidal labyrinth
  • Leads to the separation of the anterior cranial fossa from the nasal cavity. 
  • The pits/indentations overlying ethmoid spaces are called foveolae ethmoidalis.
  • The roof articulates with a lateral lamella of the cribriform plate (thinnest bone in the skull base). 
• The lateral wall of the ethmoidal sinus is formed by the LP of the orbit.
• During ethmoidal air cell surgical intervention, the lateral lamella can be accidentally traumatized. This can lead to cerebrospinal fluid (CSF) rhinorrhea.

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Keros Classification

• The ethmoidal air cells determine the depth of the olfactory fossa because the roof of the ethmoidal air cells attaches to the lateral lamella of the cribriform plate. Based on the depth of the olfactory fossa, there can be:
  • Low skull base, indicating higher chances of CSF leak. Also, the anterior ethmoidal artery (AEA) runs in mesentery, which can lead to traumatic events during the surgery. In normal conditions, the AEA has a bony covering; however, this covering can be absent when there is a low skull base. 
• The depth of the olfactory fossa can be divided based on CT measurements
  • Type 1 Kero: 1-3mm 
  • Type 2 Kero: 4-7mm 
  • Type 3 Kero: 8-17mm

Ethmoidal Air Cells 

• The ethmoid bone features several variations in its pneumatization patterns, leading to the formation of distinct structures:
  • Agger nasi: A lateral bone structure located on the anterior side. 
  • Haller's Cells: These develop when pneumatization occurs inferior to the orbit as the cells migrate toward the floor of the orbit. 
  • Frontal Cells: If these cells migrate superiorly towards the frontal sinus, frontal cells can be formed. 
  • Supraorbital Cells: When pneumatization extends above the orbit, supraorbital cells may be created. 
  • Concha Bullosa: If pneumatization extends to the middle turbinate, it can result in the formation of concha bullosa. 
  • Onodi Cells: Superolateral migration above the sphenoid bone can lead to the development of Onodi cells. 
  • Crista Galli: In some cases, pneumatization may extend to the crista galli, leading to the formation of air-filled cells within the ethmoid bone.

Blood Supply, Nerve Supply and Lymphatic Drainage to Ethmoidal Sinus

• The blood supply to the ethmoid bone and its associated structures is provided by the sphenopalatine artery, as well as the anterior and posterior ethmoidal arteries, along with their corresponding veins. 
• Nerve supply is delivered through the anterior and posterior ethmoidal nerves, as well as the orbital branches of the pterygopalatine ganglion. 
• Lymphatic drainage involves the submandibular nodes for anterior regions and retropharyngeal nodes for posterior areas. 

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