The protympanum, or bony portion of the eustachian tube (ET), is a middle ear space that lies anterior to the mesotympanum. The space is also confluent with the epitympanum superiorly, hypotympanum inferiorly, and the cartilaginous ET anteriorly. The protympanum has been infrequently examined in the past due to its difficulty to view using the operating microscope. However, the area is now gaining relevance with endoscopic ear surgery because it can easily be seen with angled scopes. With surgery moving toward evaluation and restoration of ventilation pathways, the protympanum serves as the final common pathway between the tympanic cavity and external environment, drawing comparisons to the ventilatory function of the larynx in the airway.
True ossification of the protympanum only commences at the 18th fetal week because of its dependence on the bone growth of the otic capsule. Earlier studies reported a contribution to protympanum development from the tympanic part of the temporal bone, while more recent studies point to development solely from the petrous part of the temporal bone. From the 21st fetal week, 2 bony laminae develop around the carotid artery, eventually forming the carotid canal. The superior lamina is better developed, forming 2 prolongations: a superior, which is longer and connects to the tympanic annulus posteriorly, forming the lateral wall of the protympanum; and an inferior prolongation, which is shorter and forms the lateral wall of the carotid canal. Similarly, from the 23rd fetal week, the canal for the tensor tympani muscle forms from superior and inferior laminae. In addition, the tegmen tympani and promontory help to form the superior and medial walls of the protympanum, respectively.
The boundaries of the protympanum had been previously defined as the most anterior extent of horizontal and vertical tangents through the margins of the osseous tympanic ring. The boundaries can now be defined more clearly with angled endoscopes.
- Superior: tegmen tympani and entire tensor tympani canal, merging posteriorly with and including the supratubal recess if present, with the boundary defined here by the tensor fold;
- Inferior: from the protiniculum (an oblique bony ridge demarcating the transition from hypotympanum) posteriorly, extending anteriorly with the possible presence of protympanic air cells, an anterior extension of the hypotympanic cell complex;
- Anterior: confluent with the junctional and then cartilaginous portion of the ET; Posterior: confluent with the mesotympanum;
- Medial: lateral wall of the carotid canal, extending from the caroticocochlear recess anteriorly, with caroticotympanic vessels and nerves including anterior branches from Jacobson’s (tympanic branch of glossopharyngeal) nerve. More anteriorly, variations of false passages occur depending on pneumatization patterns.
- Lateral: bony wall separating the space from the mandibular fossa and extending to the anterior annulus, from the level of the protiniculum inferiorly to the anterior limit of the notch of Rivinus at the anterior tympanic spine.