8.8\ Ossicular Interposition

8.8.1\ Discussion

Ossicular interposition grafting is a form of ossicular reconstruction (Type 3 tympanoplasty mechanism) that consists of resecting the malleus head or incus and then reinserting it between the stapes and either the malleus manubrium or tympanic membrane after it has been sculpted with a drill bur. The most common form of this technique is incus interposition grafting, in which the long process is amputated and a notch is cut into the short process such that it can be wedged between the manubrium of the malleus and the stapes superstructure (Fig. 8.21). If the stapes superstructure is absent, the long process of the

a

incus is positioned in contact with the stapes footplate, and the notched short process is positioned below the manubrium (notched incus with long process); however, results of incus interposition are much poorer if the stapes superstructure is absent and the technique is only rarely utilized. The malleus head can also be used as an interposition graft by drilling a small groove at the point where the head was amputated from the malleus neck, thereby allowing the graft to be set securely between the stapes superstructure and the undersurface of the tympanic membrane. Complications related to ossicular interposition include encasement by granuloma, iatrogenic cholesteatoma, graft necrosis (Fig. 8.22), and graft dislocation (Fig. 8.23).

b

Fig. 8.21  Incus interposition. Illustration of incus interposition (a). Axial (b) CT image shows that only the malleus is present in the epitympanic space due to

disarticulation of the malleoincudal joint. Axial (c) and coronal (d) CT images show that the sculpted incus (arrows) articulates with the head of the stapes

Fig. 8.22  Osteonecrosis of incus interposition graft. Axial CT image shows a demineralized incus interposition graft (arrow)

Fig. 8.23  Incus interposition dislocation. Coronal CT image shows an air gap between the head of the stapes and the remodeled incus (arrow)

8.9\ Ossicular Reconstruction

with a Synthetic Prosthesis:

Partial Ossicular

Reconstruction Prosthesis

(PORP), Total Ossicular

Reconstruction Prosthesis

(TORP), Incudostapedial

Joint Reconstruction

Prosthesis, and Vibrating

Ossicular Reconstruction

Prosthesis

8.9.1\ Discussion

PORPs and TORPs are synthetic implants used for ossicular chain reconstruction typically composed of a head to engage the tympanic membrane and a shaft to engage the stapes. Most modern prostheses are composed of dense hydroxyapatite, titanium, or some combination of the two. Hydroxyapatite has the advantage of being compatible with direct contact to the tympanic membrane, whereas titanium has a tendency to erode through the drumhead if directly in contact; therefore, an overlying protective cartilage cap is mandatory if a titanium head is utilized and optional with hydroxylapatite. On CT, cartilage appears as a thickened segment of tympanic membrane overlying the prosthesis. Plastipore, Teflon, polyethylene, stainless steel, gold, platinum, nitinol, and cortical bone have also been used. Some PORPs and TORPs feature a notched head that is intended to stabilize the implant by engaging the malleus manubrium, while others are placed in direct contact with the posterior/superior quadrant of the tympanic membrane. PORPs extend to the head of the intact stapes and are set upon the superstructure with an open cradle located at the end of the

shaft. TORPs extend from the tympanic membrane to the stapes footplate where a cylindrical distal end of the shaft is set (Fig. 8.24). Occasionally a separate “footplate shoe” prosthesis is used in combination with a TORP to prevent it from slipping off of the footplate, since TORPs are often considered less secure than PORPs. A final class of incudostapedial joint reconstruction prosthesis exists to deal with the common scenario of isolated incus erosion involving the long process—including its articulation with the stapes superstructure. These prostheses can be observed spanning from the residual incus long process to the stapes capitulum. However, synthetic hydroxylapatite bone cement products are also commonly utilized for this purpose. Selected examples of various prostheses are shown in Figs. 8.25, 8.26, 8.27, 8.28, and 8.29 and listed in Table 8.2.

Vibrating ossicular reconstruction prostheses (VORPs) are part of an electronic implantable hearing device (Vibrant Soundbridge, Med-El, Austria) that may be used to treat conductive hearing loss in cases where the prognosis for a favorable hearing outcome with a PORP or a TORP is extremely poor, such as severe congenital middle ear anomalies or end-stage middle ear disease. A VORP can also be used in cases of mixed hearing loss where the amplification needs are beyond the capability of a conventional hearing aid. VORPs consist of an external sound processor that is held magnetically over an implanted receiver-stimulator located under the postauricular scalp. The receiver-stimulator is connected by a wire to a magnetic vibrating floating mass transducer that is either connected to the ossicular chain or placed directly onto the round window membrane (Fig. 8.30).

Fig. 8.24  Schematics of PORP (a) and TORP (b). The PORP inserts between the tympanic membrane or cartilage graft complex and the head of the stapes. In contrast, the TORP inserts between the tympanic membrane or car-

tilage graft complex and the stapes footplate at the oval window. Photographs of various ossicular prostheses (c) (Courtesy of Grace Medical)

Fig. 8.25  Cortical bone sculpted TORP. Coronal CT image shows a linear bone fragment that extends between the tympanic membrane cartilage graft and stapes footplate

Fig. 8.26  Goldenberg TORP. Coronal CT image shows the flathead prosthesis attached to the tympanic membrane with cartilage graft reconstruction and shaft well seated upon the stapes footplate

a

b

Fig. 8.27  Applebaum PORP. Axial (a) and coronal (b) CT images show the characteristic hollow L-shaped hydroxyapatite prosthesis, which articulates with the stapes medially and incus laterally

Fig. 8.28  Black oval-top PORP. Axial CT image shows the tympanic membrane and tragal cartilage graft draped over the C-shaped hydroxyapatite head of the prosthesis. The piston of the prosthesis, which articulates with the head of the stapes, is not conspicuous

Fig. 8.29  Dornhoffer PORP. Stenver reformatted CT image shows the hydroxyapatite head in contact with the reconstructed tympanic membrane and the titanium cradle in contact with the stapes superstructure

Table 8.2  Examples of ossicular prostheses

Fig. 8.30  VORP. The illustration (a) shows the components of the VORP including the floating mass transducer (arrow) in the round window niche attached to the incus.

Axial CT image (b) shows the floating mass transducer in the round window niche (arrow) (Courtesy of Christine Toh, MD)