Tympanoplasty and Ossiculoplasty

Functional Considerations

Restoration of the middle ear transformer mechanism requires a secure connection between an intact tympanic membrane (TM) and the inner ear fluids. The TM should close an air-filled, mucosa-lined middle ear cavity. Traditional teaching attributes most of the middle ear gain to the hydraulic effect, the ratio of the effective vibrating area of the TM to the area of the mobile stapes footplate (see Chapter 128 ). Round window protection to avoid phase cancelation, which occurs when sound impacts the oval and round windows at the same time, was also thought to be an important contributor to effective sound transmission to the inner ear. More recent investigations of human middle ear mechanisms modified these classic teachings in the following ways :

• 1

  Sound can be transmitted from the ear canal to the cochlea by two mechanisms: ossicular and acoustic coupling. Ossicular coupling is the sound pressure gain that occurs through the TM and the ossicular chain. Acoustic coupling is the difference in the sound pressure that is acting directly on the oval and round windows. In normal ears, acoustic coupling is negligible, but it may play a significant role in diseased and reconstructed ears.

• 2

  When sound input to the cochlea depends solely on acoustic coupling (e.g., ossicular interruption behind an intact TM or complete absence of the TM and ossicles), the stimulus to the cochlea depends on the relative magnitude and the phase (timing difference) of the pressures on the oval and round windows. The magnitude difference is more important than the phase difference.

• 3

  The pressure gain resulting from ossicular coupling in normal ears is frequency dependent, and its magnitude is less than traditionally believed. The mean middle ear gain is about 20 dB between 250 and 500 Hz; it reaches a maximum of about 25 dB around 1 kHz and then decreases by about 6 dB per octave at frequencies greater than 1 kHz.

• 4

  The impedances at the stapes-cochlea interface and the round window membrane also contribute to middle ear function. Normally, motion of the stapes footplate is “opposed” by the annular ligament, the cochlear fluids and partition, and the round window membrane. Pathologic changes in the impedance of the annular ligament, cochlea, or round window can cause hearing loss, such as that resulting from otosclerotic stapes fixation. Also, in a nonaerated middle ear, the presence of fluid or fibrous tissue within the round window niche can increase the round window impedance and cause conductive hearing loss.

• 5

  Aeration of the middle ear plays an important role in sound transmission. In addition to contributing to normal stapes-cochlea impedance (see previously), aeration is crucial for normal ossicular coupling. The compressible air within the middle ear allows the TM and the ossicles to move. Impaired middle ear aeration can adversely affect ossicular coupling by altering a pressure difference that is important to middle ear function. In the normal human ear, sound pressure in the ear canal is higher than sound pressure within the middle ear; TM motion is driven by this pressure difference. Reductions in middle ear air volume that result from disease or surgery lead to elevated middle ear airspace impedance. This leads to a reduction in the pressure difference across the TM, with a subsequent reduction in TM and ossicular motion. The minimal amount of air required to maintain ossicular coupling within 10 dB of normal has been estimated to be 0.5 mL.

A perforation of the TM causes hearing loss by reducing the difference in sound pressure across the two sides of the TM, which causes decreased ossicular coupling. Hearing loss is proportional to the size of perforation and is frequency dependent, with the largest losses occurring at the lowest sound frequencies. It also varies inversely with the volume of the middle ear airspace, including the mastoid; this may explain why seemingly identical perforations in size and location produce different degrees of hearing loss and why hearing loss may fluctuate in a given perforation with the presence or absence of otorrhea, which would decrease the middle ear and mastoid airspace and increase hearing loss. Studies also suggest that hearing loss does not vary appreciably with the location of the perforation, in contrast to the long-held belief that posteroinferior perforations result in greater hearing loss than perforations at other locations, owing to increased phase cancelation at the round window.

Preoperative Evaluation

A detailed history and careful physical examination using an otomicroscope are essential to the planning of the surgical approach and the counseling of patients regarding the expected outcome. It is important to inquire about a history of infections, drainage, and prior surgery, including pressure-equalization tube placement as well as the course and severity of otologic symptoms. The extent of TM perforation and the condition of the ossicular chain and middle ear mucosa are evaluated. The perforation is observed to be either central, or circumferentially surrounded by residual TM, or marginal, without a TM remnant between at least part of the perforation and the bony ear canal wall. The health of the remaining membrane is assessed, paying particular attention to atrophic areas and the degree of myringosclerosis. The anticipated size of the perforation after the diseased segments are removed is estimated, as repairs of perforations comprising greater than 50% of the membrane have lower success rates. The size of the external canal is also evaluated, and canalplasty is planned if a prominent anterior canal wall prevents complete visualization of the perforation. Comprehensive audiometric evaluation is performed, and tuning fork tests are used to confirm the audiogram. Radiographic evaluation is usually not needed if clinical examination reveals a dry central perforation.

As mentioned previously, postoperative aeration of the middle ear is key to the success of TM repair and restoration of hearing. Aeration largely depends on eustachian tube dilatory function. However, no current test (e.g., pneumatic otoscopy, tympanometry) can accurately predict postoperative eustachian tube function in the presence of a perforation. Some indicators—the aeration of the opposite ear, increased age in children, fewer episodes of otorrhea, and normal middle ear mucosa—may suggest reasonable eustachian tube function. In many cases, however, surgery is the ultimate test.

The mere presence of a perforation is not an absolute indication for surgery. A small, dry, central perforation without significant hearing loss or drainage may be left alone. Similarly, in the setting of chronic eustachian tube dysfunction, it may be best to leave a perforation in place to act as a pressure-equalizing vent. On the other hand, even in the absence of noticeable conductive loss, patients may wish to undergo tympanoplasty to prevent otorrhea and infection with water exposure or hearing aid usage.

This chapter deals mainly with techniques for repair of dry TM perforations. Techniques for handling active chronic otitis media with or without cholesteatoma are described in Chapter 143 . In brief, it is recommended that the ear be dry for 3 to 4 weeks before proceeding with tympanoplasty. There are situations, however, in which the ear cannot be successfully treated medically, and tympanoplasty with or without mastoidectomy may be the best therapeutic option.

Graft Materials

Temporalis fascia is the most commonly used material for the repair of TM perforations; it was introduced for this use in the early 1960s. Temporalis fascia can be harvested at the time of tympanoplasty through a small incision posterosuperior to the helix in the hair-bearing scalp when a transcanal approach is being used or via superior dissection from a postauricular or endaural incision.

Other grafting materials have also been used, including loose areolar tissue overlying the temporalis fascia, vein, and fat. Perichondrium is frequently used in tympanoplasty, particularly with endoscopic and other transcanal approaches, if no temporalis fascia is available (e.g., in cases of multiple revisions), or if a composite cartilage/perichondrium graft is needed to prevent retraction of the reconstructed membrane when there is suspected persistent eustachian tube dysfunction. Cartilage grafting is discussed in more detail later in this chapter, under “Special Considerations.” When temporalis fascia is unavailable and the surgeon wishes to avoid the additional incision needed to harvest tragal perichondrium, the periosteum on the medial surface of the temporalis muscle can also be used as a graft material.

Nonautologous materials, including human cadaveric acellular dermal graft (AlloDerm; LifeCell Corp., Branchburg, NJ) and porcine submucosal collagen graft (Biodesign; Cook Medical Inc., Bloomington, IN) have also been studied and used in tympanoplasty. Advocates cite their usefulness when there is insufficient autologous material; also cited is the avoidance of donor-site morbidity, potential reduction of surgical time, and comparable success rates.

Minimalist Techniques

In some clinical situations, a minimalist approach to tympanoplasty, sometimes referred to as myringoplasty, is possible and desirable. Small, uninfected, established perforations 1 to 2 mm in diameter, as may arise following intratympanic steroid injections, can often be managed in an office setting in this way. The epithelium at the margins of the perforation is cauterized or removed and a fat plug slightly larger than the diameter of the perforation is removed from the lobule to be used as a graft; it is placed through the opening in a dumbbell fashion and covered with a dressing such as Gelfoam or Gelfilm (Pharmacia & Upjohn Company, Kalamazoo, MI). Other surgeons prefer simply to cauterize the perforation's edges with trichloroacetic acid and then apply a patch of Gelfoam, Gelfilm, cigarette paper, or a hyaluronic acid film (Epidisc; Medtronic Xomed, Jacksonville, FL). Traumatic perforations are also often managed by patching after the edges of the perforation edges have been realigned.

Small to medium-sized central perforations may be addressed without elevating a tympanomeatal flap and using a butterfly cartilage graft inlay technique. The perimeter of a precisely shaped composite cartilage-perichondrial graft is scored circumferentially to a depth of 2 mm and placed so that the freshened edges of the TM perforation are nestled between the medial and lateral butterfly wings.

Formal Tympanoplasty

Approaches and Incisions

Three main approaches are used in tympanoplasty: transcanal, endaural, and postauricular. The approach used depends on the size of the perforation, the anatomy of the external auditory canal, whether the surgeon is using an operative microscope or endoscope for visualization, and the surgeon's preference. Most importantly, the approach used should provide complete visualization of the perforation. When using a microscope, the transcanal approach is usually reserved for small posterior perforations or for medium-sized perforations, when the ear canal anatomy is favorable and the entire perforation and an anterior TM rim can be seen; it should be avoided when the anterior margin of the perforation is not well visualized, particularly in the hands of surgeons with less experience. Endoscopes may allow the experienced surgeon transcanal access to larger and more anterior perforations, as detailed further in Chapter 143 . The endaural approach can be used with all perforations and is more commonly used in Europe; it is most useful if a limited atticotomy is anticipated in conjunction with tympanoplasty. A self-retaining retractor can be used with this approach. The postauricular approach is the most commonly used approach for tympanoplasty in the United States. It can be used with all perforation sizes and offers a better angle of visualization of the anterior TM even without canalplasty. The use of self-retaining retractors allows for easier use of both hands for instrumentation and suctioning.

The excision of the perforation edge to disrupt the epithelial union between the lateral squamous layer and the medial mucosal layer is an integral part of any tympanoplasty procedure, whatever the approach, incision, or technique used. Preferably before the elevation of the TM, the edge of the perforation is excised. A sharp, straight pick may be used to create small holes around the periphery of the perforation, similar to those of a postage stamp ( Fig. 142.1A ); this outlined edge can be removed with cupped forceps.

Transcanal incisions outline a medially based tympanomeatal flap. Superior and inferior incisions start at 12 and 6 o'clock. Either the incisions converge to meet on the posterior canal wall, forming a triangular or U -shaped skin flap (see Fig. 142.1A ), or each incision may be extended laterally for 6 to 7 mm, at which point the lateral ends of the incisions are connected by a horizontal incision that forms a rectangular flap (see Fig. 142.1B ). The outlined skin flap is elevated medially using a round knife. Care is taken to minimize suctioning on the flap; suctioning should be done between the instrument used to elevate the flap and the bony canal. To avoid tears, elevation should be performed along the entire breadth of the flap rather than by the creation of a tunnel in the middle. After the annulus is reached, it is elevated from the tympanic sulcus to expose the middle ear mucosa, which is divided to enter the middle ear. Elevation of the annulus from the sulcus is continued superiorly and inferiorly under direct vision; this minimizes the risk of injury to a dehiscent, high-riding jugular bulb. Care should be taken to avoid injuring the chorda tympani with superior elevation of the flap. When elevation has reached the limits of the incision, it is usually possible to tuck the tympanomeatal flap anteriorly, where it will not interfere with the surgical field.

The endaural incision has a vertical limb that usually starts at 12 o'clock at the bony cartilaginous junction and extends laterally and superiorly into the cartilage-free incisura terminalis between the superior aspect of the tragus and the root of the helix ( Fig. 142.2A ). The incision is deepened to expose the inferior edge of the temporalis muscle, where a fascial graft can be taken. Depending on the planned surgery, the endaural approach limb can be combined with one of the medially based tympanomeatal flaps described previously, or the vertical limb can be extended medially, forming an inferiorly based skin flap in the bony canal (see Fig. 142.2B ) or a laterally based Koerner flap that extends onto the concha (see Fig. 142.2C ), which allows cartilage excision and meatoplasty.

The postauricular incision usually extends from the mastoid tip to just above the attachment of the helix and is placed 5 to 10 mm behind the postauricular crease. The incision is deepened in layers, with care taken not to injure the periosteum. Care is also taken not to elevate the auricle as a separate layer from the periosteum, because this can lead to a lop-ear deformity postoperatively. Next, the temporalis muscle and fascia are exposed. Starting at the zygomatic root, the periosteum is incised along the linea temporalis, and a vertical limb is dropped in a “T” or “7” shape and is curved down to the mastoid tip. The presence of this sharp angle allows for the precise repositioning of the periosteum during closure to avoid any misalignment of the auricle or any change in its vertical position postoperatively. The periosteum is elevated anteriorly with a Lempert elevator to expose the spine of Henle and the ear canal.

Several options are available for handling the skin of the posterior canal wall, which contains the so-called vascular strip. Lying in the posterosuperior ear canal between the tympanosquamous and tympanomastoid sutures, the vascular strip is the area of skin through which the deep auricular branch of the maxillary artery sends vessels to supply the TM. The canal skin can be elevated from behind with care taken not to tear the skin, particularly at its attachment at the tympanomastoid and tympanosquamous suture lines. If a suture line is prominent, it is often helpful to incise the fibrous tissue within the suture line, using a sickle knife or a No. 5910 Beaver blade. The skin is elevated down to the annulus, which should be left intact at this time; vascular strip incisions can then be made from behind to access the ear canal. Vertical incisions placed at the 6 and 12 o'clock positions are connected by a horizontal incision just lateral to the annulus to create a long vascular strip (see Fig. 142.2C ). The other option is to perform the vascular strip incisions through the ear canal before starting the postauricular incision. When no perforation is evident and surgery is performed for a second look or for ossiculoplasty (especially if an incus interposition is contemplated), it is possible not to make any incisions into the canal and instead to elevate the canal skin and annulus in continuity, entering the middle ear for inspection and ossicular reconstruction.