2The Tympanic Cavity: Walls, Recesses, and Surgical Landmarks

FA six-walled box

The tympanic cavity is an air-filled, mucosa-lined cleft hollowed out of the petrous temporal bone, no larger than a small pea yet crowded with the most delicate apparatus in the body. The classic teaching is to picture it as a rectangular box with six walls — a roof, a floor, and four sides — each of which abuts a structure of consequence. Naming the walls is not an anatomical formality: every wall is a surgical neighbour, and a misjudged millimetre can mean a dead nerve, a leaking dura, or a deafened ear[2016].

The lateral (membranous) wall is dominated by the tympanic membrane, completed above the membrane by a wedge of bone called the scutum, the lateral wall of the attic. The medial (labyrinthine) wall separates the middle ear from the inner ear and carries the promontory, the two windows, and the facial canal. The roof (tegmen tympani) is a wafer of bone over the middle cranial fossa; the floor (jugular wall) overlies the dome of the jugular bulb. Anteriorly, the carotid wall contains the eustachian tube orifice and the canal for tensor tympani, with the internal carotid artery lying just beyond; posteriorly, the mastoid wallopens through the aditus into the antrum and houses the pyramidal eminence and the recesses of the retrotympanum [2016].

Hold this box in your mind as you read on: almost every operation on the middle ear is an exercise in working through one wall to reach a target on another, while respecting the vital structures embedded in each.

FThree floors: attic, mid-cavity, basement

Cutting the box horizontally, anatomists divide the cavity into three storeys defined by the level of the tympanic membrane. The epitympanum, or attic, lies above the upper rim of the membrane and contains the head of the malleus and the body and short process of the incus — the bulk of the ossicular mass sits here, not at membrane level. The mesotympanum is the chamber directly medial to the pars tensa and holds the long process of the incus, the stapes, and the two windows. The hypotympanum is the shallow basement below the membrane, often a honeycomb of air cells over the jugular wall [2011].

Two further subdivisions are named in the anteroposterior plane. The protympanum is the funnel that narrows anteriorly toward the bony eustachian tube; the retrotympanum is the posterior region behind the membrane, riddled with the recesses that give surgeons most trouble. These compartments are not isolated rooms but communicating spaces; ventilation passes from the eustachian tube through the mesotympanum and up into the attic and mastoid, and a blockage anywhere along this route is the engine of chronic ear disease [1964].

TThe medial wall: the surgeon’s compass

Of the six walls, the medial wall is the one a surgeon reads most often, because every landmark on it locates something that must not be injured. Its central bulge is the promontory, raised by the basal turn of the cochlea. Posterosuperior to the promontory sits the oval window niche, sealed by the stapes footplate; posteroinferior lies the round window niche, hidden under an overhanging lip of bone. Between and around them run the bony ridges — the ponticulusabove the round window and the subiculumbelow it — that bound the retrotympanic recesses described next [2010].

Running across the medial wall, just above the oval window, is the horizontal (tympanic) segment of the facial nerve in its bony canal. In a substantial minority of ears this canal is naturally dehiscent over the oval window, leaving the nerve covered only by mucosa and exquisitely vulnerable to the curette and the drill [2016]. Above the facial canal, the bulge of the lateral semicircular canal marks the labyrinth; an eroded prominence here in cholesteatoma signals a labyrinthine fistula. For the ossiculoplasty surgeon, the medial wall is the bedrock against which a prosthesis is set: the footplate is the destination of a total prosthesis, the stapes head the destination of a partial one, and both rely on an intact, correctly identified oval window.

TThe retrotympanum and its blind spots

The posterior wall is where anatomy turns adversarial. Two recesses flank the descending facial nerve, and the difference between them is one of the most clinically loaded facts in otology. The facial recess lies lateralto the nerve, a triangle bounded medially by the facial canal, laterally by the chorda tympani, and superiorly by the fossa incudis. Because it opens toward the surgeon, it can be drilled from the mastoid — the posterior tympanotomy— to deliver a view of the mesotympanum without sacrificing the canal wall [2013].

The sinus tympani lies medial to the facial nerve, scooped into the labyrinthine wall between the ponticulus above and the subiculumbelow, with the pyramidal eminence as its lateral lip. This is the great blind spot of the middle ear: it points away from every line of sight afforded by the operating microscope, and a deep sinus tympani — Marchioni’s type C, which extends behind the facial nerve — can shelter cholesteatoma matrix that no straight instrument can see or reach[2010]. Endoscopic anatomy studies confirm that the retrotympanum is involved in roughly half of acquired cholesteatomas, and that residual disease, when it occurs, is found here more often than anywhere else[2010, 2010].

This is why the angled endoscope has reshaped chronic ear surgery: passed transcanal, it looks around the pyramidal eminence into the sinus tympani and forward into the anterior epitympanic recess and protympanum, recesses the microscope simply cannot illuminate. The chart below shows the characteristic rank order of recesses harbouring residual matrix at second-look surgery, with the sinus tympani at the top of the list.

TThe epitympanum and Prussak’s space

The attic is not one chamber but a partitioned loft. Mucosal folds — the malleal and incudal folds, the tensor fold, and a bony ridge called the cog— divide it into an anterior and a posterior epitympanic compartment and regulate how air reaches it. When these folds are complete, the anterior attic ventilates poorly, and a selectively ventilated pocket becomes a candidate for retraction and disease [1964].

The most surgically important of these compartments is Prussak’s space, a tiny recess immediately deep to the pars flaccida. It is bounded laterally by the flaccid part of the membrane and the scutum, medially by the neck of the malleus, inferiorly by the lateral process of the malleus, and superiorly by the lateral malleal fold; its only ventilation creeps in from behind, above the posterior malleal fold [1996]. This cramped aeration is precisely why Prussak’s space is the cradle of pars flaccida (attic) cholesteatoma: a retraction of Shrapnell’s membrane invaginates into the space, traps shed keratin, and then expands superiorly into the epitympanum and posteriorly toward the aditus and antrum. Knowing these escape routes tells the surgeon where to look beyond the visible pocket [2016].

CWhy the boundaries matter at operation

For the clinician planning ossiculoplasty or chronic ear surgery, the walls and recesses translate directly into operative decisions. The scutumis often eroded by attic disease and must be drilled down to expose Prussak’s space, then reconstructed with cartilage to prevent a recurrent retraction pocket. The tegmen must be inspected for dehiscence before any attic dissection, lest the temporal lobe or its dura be drawn into the field. The facial recess is the gateway of the canal-wall-up mastoidectomy and the access route for posterior ossicular and cochlear-implant work, but it is opened with the facial nerve and chorda tympani as its immediate walls [2013].

Above all, the sinus tympanidictates the completeness of cholesteatoma removal and therefore the durability of any reconstruction built afterward. A prosthesis placed on a footplate beneath retained disease is a prosthesis destined to fail. The modern answer is to map the retrotympanum deliberately — with angled endoscopes, and for a deep type C sinus tympani a retrofacial approach — before declaring the cavity clean[2010, 2010]. The medial wall, finally, is the foundation of hearing reconstruction itself: an intact mobile footplate, a correctly identified oval window, and a respected facial canal are the prerequisites for every ossiculoplasty technique discussed later in this atlas.

Mastery of the tympanic cavity is therefore less about memorising six walls than about reading them as a surgeon does: as a set of neighbours and hiding places, each with a structure to protect and a route through which disease may have already escaped [2016].