Ossiculoplasty Workup

• A thin-section CT scan of the temporal bones with axial and coronal views may provide the following information:

  • Extent of cholesteatoma or chronic ear disease (if present)
  • Size of the middle ear cavity
  • Presence, absence, and continuity of the ossicular chain
  • Presence of attic fixation of the malleus
  • Presence of otosclerosis
  • Bony structure of the inner ear
• The decision to perform a CT scan is based on the patient's history and physical examination findings. The final decision to perform a CT scan is at the discretion of the examining physician.

• Perform a thorough otoscopic examination, preferably with binocular microscopy.
• Inspect the preauricular skin and auricle. The presence of preauricular pits or tags or auricular deformities may suggest a congenital abnormality.
• Examine the external auditory canal for size, shape, and defects.
• Note the status of the tympanic membrane and, as much as possible, the middle ear.
• Perform pure tone audiometry with air/bone thresholds and masking, if necessary, prior to surgical planning. Include acoustic reflexes. The presence of acoustic reflexes with a conductive hearing loss may be associated with inner ear conductive hearing loss.

Middle ear implants are unique in that one end must be coupled to the tympanic membrane and the other to bone or soft tissue. Ideally, the implant should not contact other tissue between the implant's 2 ends. The material also must maintain its shape, rigidity, and acoustic transmission properties. The outcome of middle ear implantation is determined largely by the status of the middle ear. Eustachian tube dysfunction can lead to early extrusion, and infection can lead to breakdown and resorption of an implant.

Ossicular reconstruction materials are divided into autografts, homografts, or alloplastic prosthetics. Each of these materials possesses unique properties when exposed to the environment of the middle ear. Similarly, unique problems are associated with each of these materials. Problems include graft failure, implant extrusion, and persistent or recurrent conductive hearing loss.

Autologous cartilage was one of the first materials used for ossicular chain reconstruction. In 1971, Smyth reported that cartilage struts removed at revision surgery showed erosion, suggesting the graft would not be stable over time (< 3 y).^[3] Similar work by Merchant and Nadol in 1994 showed loss of rigidity on microscopic examination, chondromalacia on histologic examination, and resorption.^[4] The loss of stiffness is probably due to ingrowth of blood vessels with subsequent chondritis. These findings led the authors to conclude that cartilage struts are unsatisfactory as long-term implants.

Autologous incus grafts also have been used for some time. A strut or crutch in the short process of the incus is created for the malleus, with a cup made for the stapes capitulum. Autograft ossicular struts maintain their contour, shape, size, and physical integrity for at least 11 years.^[4] Ossicular grafts eventually may become nonvital because of loss of blood supply. Thermal injury during sculpturing may contribute to the loss of blood supply. Autologous ossicles subsequently undergo new bone formation and remodeling. The process is characterized by a slow creeping substitution of revascularized bone. The neo-osteogenesis is not vital for the transmission of sound.

Plasti-Pore, a HDPS, is an alloplastic material with a long clinical history. This material sets the standard by which the National Bureau of Standards tests other implant materials. HDPS has nonreactive properties and sufficient porosity to encourage tissue ingrowth.

Histologic examination of HDPS implanted for 1-4 years has shown extensive invasion of the porous spaces with fibrocytes, small round cells, and foreign-body giant cells. An envelope of fibrous tissue with a lining membrane of mucosal epithelium often forms around the implant. Some studies have demonstrated partial resorption of HDPS and replacement by fibrous tissue.

Clinical experience has shown the necessity of covering these HDPS alloplasts with cartilage to minimize the incidence of extrusion. Extrusion rates have averaged 3-5% in large series with 5-10 years of follow-up monitoring. Although most extrusions have occurred within the first year, some extrusions have occurred up to 5 years postoperatively.

Hydroxylapatite is currently one of the most common alloplastic materials used for ossicular reconstruction. Hydroxylapatite is a polycrystalline calcium phosphate ceramic that has the same chemical composition as bone. This material chemically attaches to bone and is osteoconductive. It forms a direct bond with bone at the hydroxylapatite/tissue interface. This bond is associated with an electron-dense layer at the interface composed, at least in part, of calcium phosphate in the form of hydroxylapatite. This epitaxy or continuity between the artificial and biologic hydroxylapatite crystals might explain the bonding osteogenesis at the interface. If placed next to the scutum, osseointegration can occur, with subsequent conductive hearing loss. Cartilage is not required when using hydroxylapatite.

Within the first 2 weeks of implantation, a large proportion of hydroxylapatite implants are covered with an epithelial layer. With time, the implant gradually becomes completely covered by this epithelial layer. The final epithelial layer contains all cell types characteristic for the middle ear. An epithelial covering resembling that in the normal middle ear indicates good biocompatibility of an implant material.

Hydroxylapatite is produced in both porous (pore size >100 µm) and dense (pore size < 30 µm) forms. The dense form is the type used in ossicular prosthetics. The size of the macropore has a direct influence on the kind of tissue that grows into these pores. Generally, a pore size larger than 100 µm favors the ingrowth of bone, whereas a pore size of 30 µm allows fibrous tissue ingrowth. With the porous implant materials, a firmer fixation and a smaller fibrous capsule occur compared to the dense form.

Titanium is another common alloplastic material. Studies in rabbits have shown that within 28 days after implantation, a thin, noninflamed, even layer of epithelium forms over the inserted implant. Similar results in human studies have shown the same type of reactivity. Titanium forms a biostable titanium oxide layer when combined with oxygen. Titanium has shown significant biostability in the middle ear for the past decade.

The properties of titanium make it possible to manufacture an extremely fine and light prosthesis with substantial rigidity in the shaft. Furthermore, differential processing of the material surfaces triggers various tissue reactions. For example, if titanium implants are rough milled, their contact points are increased. Rough-milled surfaces are most appropriate in areas that contact cartilage or the stapes head or footplate. Conversely, the smoother the surface, the less connective tissue reaction occurs, and the epithelial covering is minimized.