Maxillary Sinus Anatomy
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Direct answer
The largest air cavity in the skull sits directly above the roots of the upper posterior teeth, often separated from them by paper-thin bone — the maxillary sinus, a pyramidal chamber within the maxilla whose apex points toward the zygomatic bone and whose base is the lateral wall of the nose. It is present at birth as a small cavity and pneumatizes progressively through life, sometimes dipping between molar roots, even dehiscent over them, so upper molar extractions can breach the floor. Drainage is anatomically inefficient: the ostium opens high on the medial wall into the middle meatus of the nose (through the hiatus semilunaris), relying on mucociliary clearance that moves mucus upward and medialward against gravity. Sensory supply is maxillary (infraorbital plus the superior alveolar branches), which is why sinusitis masquerades as toothache across the upper quadrant.
What you must remember
- Shape and size: largest paranasal sinus, roughly pyramidal, average adult volume in the region of 15 millilitres (textbook figures vary); pea-sized at birth and pneumatizing well into adulthood, even into the alveolar process after extractions.
- Relations: floor — alveolar process over the molars and premolars (roots may project into or through it); roof — floor of the orbit carrying the infraorbital canal; medial wall — lateral nasal wall with the ostium; posterior wall — infratemporal and pterygopalatine fossae; apex — zygomatic process.
- Ostium: opens into the middle meatus through the hiatus semilunaris, positioned high on the medial wall near the roof — poorly placed for gravity drainage; an accessory ostium is present in a substantial minority.
- Closest roots: first and second molars most often related (classically the second molar's buccal roots closest, with the first molar's distobuccal root a close second), then third molars, premolars least; canines rarely.
- Nerve supply: maxillary division via infraorbital and the superior alveolar branches; the sinus shares its innervation with the posterior teeth, the anatomical basis of odontogenic sinusitis presenting as diffuse upper posterior toothache, worse on stooping.
- Lining: ciliated pseudostratified columnar (respiratory) epithelium over the Schneiderian membrane, with cilia beating toward the ostium.
- Clinical anchors: oroantral communication or fistula after upper molar extraction, odontogenic sinusitis (a commonly cited cause in roughly 10-40% of maxillary sinusitis, varying by series), sinus lift (lateral window) before implants, and Caldwell-Luc approach historically used for chronic disease.
An extraction that opens the sinus
An upper first molar with long divergent roots comes out with a crunch and slight nasal bleeding; when the patient is asked to blow gently, air bubbles escape through the socket — an oroantral communication, whose management is pure sinus anatomy. First confirm: the Valsalva manoeuvre through the socket and a fogging mirror test; suspect it whenever a root disappears during extraction (a displaced root is now a sinus foreign body requiring radiographic localisation and often retrieval). If the communication is small (under about 2 millimetres) and the membrane intact, it usually heals with a stabilising suture, socket dressing, antibiotics and strict instructions — no nose blowing or straw use, sneezing with the mouth open — because positive nasal pressure dislodges the clot. Larger communications and established fistulae need a flap: a buccal sliding advancement flap or, when tissue is deficient, a palatal rotation flap based on the greater palatine artery. Every step of that ladder is a relation of the sinus floor applied in order.
The dentist's sinus rules
Indian oral surgery vivas converge on five sentences: the second molar's buccal roots usually lie closest to the floor; roots displaced into the sinus demand retrieval; suspected communication is confirmed by the nose-blowing test; small communications heal, large ones need a flap; and sinusitis of dental origin is diagnosed when unilateral upper posterior pain accompanies an apical lesion communicating with the floor. The theory trap is the ostium: candidates write that the maxillary sinus drains "by gravity" into the inferior meatus — wrong on both counts; it drains into the middle meatus by mucociliary action against gravity, which also explains why the osteomeatal complex is the target of functional endoscopic surgery.
Frequently asked questions
Which tooth roots lie closest to the maxillary sinus floor?
The maxillary second molar's buccal roots, closely followed by the first molar's buccal roots; premolars and canines are progressively further away.
Where does the maxillary sinus drain, and why does it matter?
Through its ostium into the middle meatus via the hiatus semilunaris, positioned high on the medial wall, so clearance depends on ciliary action and is easily obstructed.
What is an oroantral communication and how is it recognised?
A breach of the sinus floor, usually after upper molar extraction, suggested by nasal bleeding and confirmed by air escape through the socket on gentle nose blowing.
Why does maxillary sinusitis present as toothache?
The sinus and the upper posterior teeth share the superior alveolar and infraorbital branches of the maxillary nerve, so inflammatory pressure is referred to the molar and premolar teeth, typically worsening on stooping.
What is a sinus lift and why is it done?
A lateral-window procedure that elevates the Schneiderian membrane and grafts bone beneath it to create vertical height for implants in the pneumatized posterior maxilla.