CBCT images of the TB are typically acquired with very thin slices and a small field of view to ensure the smallest possible voxel size (usually 0.15mm) and without intravenous contrast material.

Multiplanar reconstructions are essential for improved assessment of TB anatomy and of postoperative changes. Sagittal oblique planes parallel (Pöschl) or perpendicular (Stenvers) to the superior semicircular canals (SSCC) are useful for illustrating the long axes of the malleus and incus. Moreover, the Pöschl projection provides one of the best views of the incudostapedial joint [Fig. 1].

The most frequent pathologies responsible for conductive/mixed hearing loss typically affecting the tympanic membrane (TM), ossicular chain, middle ear cavity, oval and round windows include:

• malformations or destructive lesions (including traumatic injuries) of the external auditory canal (EAC), ossicular chain and/or oval window,
• chronic otitis media (COM),
• cholesteatoma 
• otosclerosis

Tympanoplasty

It is the surgical procedure used to repair the tympanic membrane (TM) and it is most frequently associated with the reconstruction of the ossicular chain. The reconstruction will depend on the degree of damage caused by the underlying disease and the possibilities of functional recovery.

The Wullstein classification (Type I – V) is the most often used tympanoplasty classification. CT and CBCT can easily distinguish between the different tympanoplasty types [Fig. 2]:

• Type I / myringoplasty: it consists in repairing only the TM using a graft (in cases with TM perforations and an intact ossicular chain) [Fig. 3]. These grafts are made from the temporalis fascia or auricular cartilage.
• Type II: it consists in placing the TM graft on the remaining malleus or incus (in cases with malleus erosions).
• Type III / myringostapediopexy: it consists in placing the TM graft on the stapes head (in cases with malleus and incus erosions) [Fig. 4]. Type III tympanoplasty is often combined with OCR (see below).
• Type IV: it consists in placing the TM graft on the stapes footplate and to shield the round window (in cases when the malleus, the incus and the stapes superstructure are eroded) [Fig. 5]. This type of tympanoplasty is usually performed in conjunction with canal wall down (CWD) mastoidectomy (i.e., removal of the superior and posterior bony EAC creating a common cavity, which includes the EAC, mastoid and epitympanum).
• Type V: it consists in total stapedectomy and footplate replacement by an adipose graft (Type Vb). Alternatively, the TM graft is placed directly on a fenestration made in the lateral semicircular canal (LSCC) [Fig. 6].

Tympanoplasty types I and III are the most performed procedures, while type V is very rarely done.

The transcanal approach is a surgical technique that provides access to the TM and middle ear solely through the EAC. Postoperative imaging may reveal subtle findings, e.g., soft-tissue thickening or bone changes, defects and flattening of the walls of the EAC. As the clinical history is often incomplete, it is crucial for the radiologist to be familiar with these findings to avoid misinterpreting them as erosion caused by disease, e.g., cholesteatoma or carcinoma.

Ossicular chain reconstruction (OCR)

OCR is used for re-establishing the mechanism of sound transmission, by graft or prosthesis, and restoring the continuity between the TM and the stapes footplate.

There are many kinds of prostheses of different designs and materials [Fig. 7]. The choice – when to use what prosthesis - depends on prosthesis properties, patient requirements, disease extent and surgeon experience.

OCR are classified as follows [Fig. 8]:

• Prosthesis for partial reconstruction: the prosthesis replaces the malleus and incus and is placed between the TM/graft and the stapes head. The stapes must be complete and mobile. The procedure is often used in cholesteatoma and COM, which are often associated with malleus or incus destruction.
  • When it is combined with tympanoplasty type III a subclass is distinguished: Tympanoplasty type III - minor columella reconstruction - a graft or strut is placed between the stapes head and the TM.
  • The graft or struts used include:
    • autograft ossicle strut, which comprises a malleus and incus columella; the affected long processes of the malleus/incus are resected, and the malleus head/incus body are then connected to the incus and stapes, respectively [Figs. 9 and 10].
    • autograft cortical bone strut,
    • or partial ossicular replacement prosthesis – PORP [Fig. 11]
  • Prosthesis for full reconstruction: the prosthesis is placed between the TM/graft and the footplate or oval window depending on whether the footplate is ankylosed or not.
    • The procedure is frequently combined with tympanoplasty type III. A subtype called Tympanoplasty type III - major columella reconstruction is characterized by a graft or strut (total ossicular replacement prosthesis - TORP) placed between the footplate and the TM [Fig. 12].
  • Stapes surgery: there are three possible procedures (otosclerosis being the main indication) [Fig. 13]:
    • The stapes superstructure is removed, and the prosthesis is connected to the incus.
    • After footplate resection, the piston is positioned on a graft over the oval window (stapedectomy – the preferred procedure at present).
    • By perforating the footplate, the piston is placed through the hole (stapedotomy).

Postsurgical Complications and Pitfalls

CBCT or high-resolution CT may be necessary to detect disease recurrence or to evaluate unexpected outcomes and complications after tympanoplasty and OCR. These include necrosis [Fig. 14], dislocation of the ossicular chain/graft or prosthesis displacement [Fig. 15], extrusion [Fig. 16], subluxation [Fig. 17 and 18] or fracture. If any of these complications occur, revision surgery may become necessary.

Postoperative conductive hearing loss can also occur after surgery due to the development of granulation tissue [Fig. 19], which can reduce the mobility of the reconstructed ossicular chain or prosthesis. At CBCT, granulation tissue appears as a nonspecific soft-tissue lesion around the prosthesis 4–6 weeks after intervention and should be carefully correlated with the clinical information and the audiogram, if available (particularly in cases of stapedial prosthesis).

Prosthesis fixation, malposition or dislocation can occur as a consequence of granulation tissue or of obliterative otosclerosis of the oval and round window [Fig. 20]. Obliterative otosclerosis tends to affect younger patients.

Other causes of a poor OCR outcome include dehiscence of the labyrinthine structures (the “third window” mechanism), labyrinthitis, intra-vestibular footplate dislocation and labyrinthine fistula. In the appropriate clinical context, a labyrinthine fistula should be suspected if imaging shows an unexplained middle ear effusion and/or pneumolabyrinth developing during follow-up [Fig. 21]; nevertheless, during the first week after stapes prosthesis surgery, a small quantity of pneumolabyrinth can be seen.

MRI can be helpful when serous or infectious labyrinthitis is suspected. Labyrinthitis manifests as high signal intensity on fluid-attenuated inversion-recovery sequences and contrast enhancement of the labyrinth.