The Journal of

Laryngology and Otology{Founded in 1887 by MORELL MACKENZIE and NORRIS WOLFENDEN)

July 1964 THE DEVELOPMENT OF THE MIDDLE EAR SPACES AND THEIR SURGICAL SIGNIFICANCE*By BRUCE PROCTOR (Detroit)

THE recent advances in techniques for surgery of the middle-ear cleft has revived interest in the microanatomy of the middle ear. In the present study the previous works of anatomists and otologists working in this field was intensively reviewed. Fresh temporal bones were then procured at autopsy and preserved by deep freezing. Later micro-dissections were made, photographed and then sketched by a competent medical illustrator. Our attention will be directed first to the contents of the middle ear, then to the embryological origin and finally to the surgical significance of these structures. The middle ear contains the ossicular chain with its ligaments and the tendons of the tensor tympani and stapedius muscles. Along with the chorda tympani nerve these structures may be considered as the "viscera" of the tympanic cavity (Braus, i960). Attached to the ossicular chain are various mucosal folds (Ballance and Green, 1919; Hammar, 1902; Maisonnet and Coudane, 1950; Politzer, 1909; Sobotta, 1920; Lowndes Yates, 1936) which are of considerable clinical importance in that they carry blood vessels to the ossicles and also divide the middle ear into several definite compartments. They may* Read at the Section of Otology, Royal Society of Medicine, London, December 6th 1963-

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Bruce Proctorbe considered as the "mesenteries" of the tympanic cavity. These mucosal folds are easily seen in the living and in fresh temporal bones. They disappear or disintegrate rapidly if the temporal bones are permitted to dry or are immersed in fixative solutions. They may also disintegrate in disease processes involving the middle ear. Mucosal folds and their remnants are often considered as residues of inflammation or as adhesions. It must be emphasized, therefore, that they occur in fairly constant anatomical positions which can be explained by the embryological development of the middle ear.Compartments and Folds of the Middle Ear

The attic or epitympanum is almost completely separated from the mesotympanum by the ossicles and their folds except for two small but constant openings which it is proposed to call the isthmus tympani anticus and the isthmus tympani posticus (Plate I). The anterior opening lies posterior to the tensor tympani tendon and anterior to the stapes and long crus of the incus. The posterior opening is bounded posteriorly by the pyramidal process and posterior tympanic wall, laterally by the short process of the incus and posterior incudal ligament and anteriorly by the medial incudal fold which extends from the short to the long process of the incus. Its medial boundary is the stapes and the stapedial tendon. The attic usually extends forward through the incisura tensoris and anterior to the tensor tendon as the anterior malleolar space or anterior compartment of the attic. This space lies above the tensor fold which extends laterally from the semicanal for the tensor tympani muscle to the anterior malleolar ligament. The attic, however, may extend anteriorly only to the level of the tensor tendon where it is limited by a medial extension of the superior malleolar fold instead of by the tensor fold which in such an instance does not form. The space anteriorly would be in communication with the mesotympanum and eustachian tube. This space, when present, is called the supratubal space. Posterior to the transversely-placed superior malleolar fold lies the larger posterior compartment of the attic. That portion of this compartment lateral to the superior incudal fold may be considered as the superior incudal space and that portion medial to the superior incudal fold as the medial incudal space. Laterally the floor of the superior incudal space is formed by the lateral malleolar fold and by the lateral incudal fold which extends posteriorly to the posterior incudal ligament. The entrance into Prussak's space is usually located between the lateral malleolar fold and the lateral incudal fold. Medially the posterior compartment of the attic is separated from the mesotympanum by the dihedral-shaped medial incudal fold which632

The Development of the Middle Ear Spacesextends from both crura of the incus to the pyramidal eminence and stapes (Plates II and III). Beneath the floor of the attic and in the upper mesotympanum there are three compartments. They are the inferior incudal space and the anterior and posterior pouches of von Troeltsch. The inferior incudal space extends from the inferior surface of the incus laterally to the posterior malleolar fold (Plate IV). It is limited medially by the medial incudal fold and anteriorly by the interossicular fold which lies between the long crus of the incus and the upper two-thirds of the malleus handle. Between the posterior malleolar fold and the tympanic membrane lies the posterior pouch of von Troeltsch (Plate V). The chorda tympani nerve lies in the free margin of the posterior malleolar fold, although it may cross the posterior tympanum independent of this fold. The shallow anterior pouch of von Troeltsch lies between that portion of the drumhead anterior to the malleus handle and the anterior malleolar fold which is draped on the anterior malleolar ligament. Between the malleus handle and the drumhead and superior to the umbo lies the shallow manubrial fold. Five folds may be recognized as stapedial folds (Plate VI). They are the obturatoria stapedis between the crura, the anterior stapedial fold between promontory and anterior crus, the posterior stapedial between promontory and posterior crus, the plica stapedis between pyramidal eminence and the posterior crus and the superior stapedial folds which extend from the long crus of the incus to either crus of the stapes or from the facial canal to the crura.Prussak's Space

The annulus fibrosus, the dense fibrocartilaginous ring to which the radial fibres of the drum attach, leaves the sulcus tympanicus posteriorly (Plate VII). Its outer fibres insert on the posterior tympanic spine or extend in the stria membrana tympani posticus to the short process of the malleus. Its inner fibres insert on the medially-placed pretympanic spine (Dworacek, 1960; Pernkopf, i960) or radiate out, forming the supporting structure for the posterior malleolar fold and attaching on the posteromedial aspect of the upper third of the malleus handle. Between the posterior malleolar fold and the drum lies the posterior pouch of von Troeltsch. Anteriorly the annulus fibrosus leaves the sulcus tympanicus to attach in part to the anterior tympanic spine then continues on: (1) as the stria membrana tympani anticus to the short process, (2) to radiate out to help form the floor of Prussak's space, (3) to interdigitate with fibres of the lateral malleolar fold, and (4) to attach to the bony rim of the notch of Rivinus.633

Bruce ProctorThe lateral malleolar fold arises from the junction of the malleus head and neck and radiates out to insert on the entire bony rim of the notch of Rivinus, thus forming a firm roof for Prussak's space (Plate VIII).The Embryology

Between the third and seventh foetal month the gelatinous tissue of the middle-ear cleft is gradually absorbed. At the same time the primitive tympanic cavity develops by a growth, into the cleft, of an endotheliallined fluid pouch extending from the eustachian tube (Plate IX). Four primary sacs or pouches then bud out. They are saccus anticus, saccus medius, saccus superior and saccus posticus (Hammar, 1902). Where these pouches contact each other mucosal folds are formed. Between the mucosal layers of the folds are remnants of the mesoderm, including blood vessels supplying the "viscera" of the tympanic cavity. Saccus anticus is the smallest of the pouches. It extends upward anterior to the tensor tendon to form the anterior pouch of von Troeltsch. Its upward extent may be limited at the level of the semicanal for the tensor tympani by contact with the anterior-most saccule derived from the faster developing saccus medius (Plate X). The fold formed is the tensor fold and above it is the anterior compartment of the attic. Saccus anticus may, however, extend upward to the tegmen and as far posterior as the superior malleolar fold. In this instance there is developed a supratubal space instead of an anterior attic compartment (Plate XI). Saccus medius forms the attic. It extends upward through the isthmus tympani anticus and usually breaks up into three saccules. The anterior saccule may form the anterior compartment of the attic. The medial saccule forms the superior incudal space by growth over the malleoincudal body to the lateral incudal fold and posterior incudal ligament. The medial saccule usually sends an offshoot forward between the lateral malleolar and lateral incudal folds to form Prussak's space. Occasionally the medial saccule extends only to the level of a superior incudal fold in which case the lateral incudal fold is absent. The superior incudal space would in such an instance be developed from the saccus superior. The posterior saccule of saccus medius extends posteriorly to the anterior crus of the stapes, passes medial to the long crus of the incus and eventually pneumatizes that portion of the mastoid air cell system which is derived from the pars petrosa of the temporal bone. Saccus superior extends posteriorly and laterally in the interval between the malleus handle and the tip of the long crus of the incus. It forms the posterior pouch of von Troeltsch and the inferior incudal space. Its upper limit is the lateral incudal fold, although it may extend to the level of a complete superior incudal fold and also extend into Prussak's space. Posteriorly saccus superior extends medially to pass over the pyramidal eminence into the antrum. Eventually it pneumatizes that634

The Development of the Middle Ear SpacesFor Anatomical Key see page 647

PLATE I.

Floor of attic viewed from above. The mesotympanum is almost completely separated from the attic by the ossicular chain and mucosal folds. The only constant communication are the isthmus tympani anticus and the isthmus tympani posticus.

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PLATE II.

The ossicles viewed from posterior, superior and medial. The medial incudal fold extends medially and postero-inferiorly from the incus crura to the stapedial tendon.

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PLATE

III.

Lateral view of tympanum. Note the anterior compartment of the attic with the tensor fold as the floor, the interossicular fold between malleus and incus and the medial incudal fold with the isthmus tympani posticus behind.

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PLATE IV.

Postero-superior and lateral view of middle-ear contents showing embryological origin. Saccus medius forms the superior incudal space, Prussak's space and extending posteriorly into the mastoid (pars petrosa). Saccus superior forms the inferior incudal space, posterior pouch of von Troeltsch and extending into that portion of the mastoid developed from pars squamosa. Saccus posticus forms the round window niche, sinus tympani and lower half of oval window niche.

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PLATE V.

Section of malleus. Anterior aspect (right) and posterior aspect (left). Note anterior and posterior pouches of von Troeltsch.

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PLATE VI.

Five combinations of the various stapes folds showing their embryological origin. 640

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PLATE

VII.

Detail of Prussak's space. Terminal fibres of annulus fibrosus spread out to help form the boundaries of Prussak's space. The roof is formed by the lateral malleolar fold which fans out to attach along the rim of the notch of Rivinus.

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PLATE VIIT.

Prussak's space. Frontal section showing the posterior two-thirds of this space. Note the anterior malleolar fold extending from the anterior process of the malleus to the stria membrana tympani anticus. 642

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PLATE

IX.

Illustrates growth of pouches into the middle ear of 3- to 7-month-old embryos.

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G.S. L.ASHBROOK

PLATE X.

Development of anterior compartment of the attic by saccus medius with the saccus anticus stopping at the level of the semicanal and tensor tympani tendon resulting in formation of the tensor fold.

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PLATE

XI.

Development of the anterior compartment of the attic by extension of saccus anticus to superior malleolar fold.

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