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Ossicular Chain Reconstruction: Maximizing Success and Minimizing Errors
Surgical reconstruction of the ossicular chain has evolved into a reliable procedure with multiple options available to restore sound transmission through the middle ear. Even in expert hands, however, hearing loss, prosthesis displacement, and tympanic membrane perforation may occur. The literature contains a plethora of publications on ossiculoplasty, but comparing results between studies is difficult because a uniform protocol for preoperative risk stratification and analysis of results has yet to be accepted. Consequently, many surgeons simply continue to use the same techniques and prostheses they learned as a resident rather than unravel the sometimes ambiguous claims in the literature.
This chapter reviews prosthetic materials, designs, and surgical techniques to minimize errors and maximize success of ossiculoplasty. Ossicular reconstruction is usually performed in the face of other disease processes such as eustachian tube dysfunction (ETD), cholestea-toma, and tympanic membrane perforation. These disease states must also be addressed to ensure successful ossiculoplasty.
■ Error Reduction
Microsurgery is an inherently difficult task often exacerbated by the severity of disease, scar, and congenital variations. Errors in otologic surgery, as in life, cannot all be avoided, even for the most fastidious, experienced surgeon. The only way to ensure the absence of human error is to omit humans. Because the latter is impossible, superior training, experience, and a consistent protocol are essential.
There may be many ways to perform a task correctly, but there are a staggering number of ways in which a task can go wrong. The reader is strongly encouraged to review the literature on error, particularly as it pertains to medicine, to better understand the cognitive causes of error.1 An error can be defined as an occasion when a planned sequence of mental or physical events fails to achieve its intended goal.2 Much has been learned from analysis of errors contributing to major industrial disasters, such as the space shuttle Challenger, Three Mile Island, and Bhopal.3 It is only recently that the medical field has begun to examine the cause and cost of human error.4 It is estimated that 1 million preventable medical errors result in 120,000 deaths each year.2 Once the reality of medical errors is acknowledged, efforts can be focused on error reduction.
Most errors in medicine are likely due to errors of attention. These errors are often due to (1) unrecognized changes in conditions, (2) fatigue, or (3) disruption of an automatic behavior array. Many of our daily tasks are performed in an almost automatic fashion with little conscious thought.5 They consist of a sequence of behaviors that are triggered by external or internal stimuli. These automatic behavior arrays (ABAs) account for a majority of our daily tasks. Without them, each task would require unreasonably high levels of mental effort that are typically reserved for only infrequent, short periods of problem solving throughout the day.
Errors can be categorized as either slips or mistakes. Although these terms are often used interchangeably in common speech, in human factors sciences they have specific meanings. A slip refers to formulating the correct plan but failing to execute it correctly. A mistake refers to choosing the wrong plan. The inexperienced and poorly trained are most susceptible to mistakes. However, poor habits that rarely cause problems can suddenly surface and reveal long-standing latent flaws for even the most experienced. For example, until recent years, surgeons rarely marked the ear they were about to operate on. In otologic surgery, a common example of a slip would be skipping of a drill bit—depending on anatomical conditions, an adjacent structure such as the facial nerve could be injured.6 In contrast, a mistake might refer to using a Bovie cautery to obtain hemostasis adjacent to the facial nerve.
■ Chronic Otitis Media
Ossiculoplasty is most reliable in dry, clean ears without underlying ETD such as occurs following trauma. Most reconstructions, however, are performed for ears with chronic otitis media wherein the severity of underlying pathology can significantly affect the outcome of even the most meticulous surgery. Some authors differentiate between two pathological forms of this disease: tubotympanic and atticoantral. In the former, milder variety, there is typically a perforation of the pars tensa with ossicles either intact or with lenticular incus erosion only. In the latter, more aggressive variety, there is typically an attic retraction cholesteatoma with bone erosion. In either case, granulomatous mucosal disease or ETD can negatively affect outcomes. Some draining ears are readily cured when middle ear or attic blocks are surgically debrided. Others remain recalcitrant when due to ETD, intrinsic mucosal disease, or systemic allergies.
Identifying these factors preoperatively can be helpful in counseling patients regarding reasonable expectations of success as well as the possible need for staged ossiculoplasty.
■ Implant Materials
The ideal prosthesis for ossicular reconstruction should be biocompatible, stable, and capable of yielding optimal sound transmission. Implant materials can be divided into autografts, homografts, or alloplastic prosthetics. Each of these materials possesses characteristic advantages and disadvantages when exposed to the environment of the middle ear. Problems include displacement or extrusion of the implant, graft failure, and persistent or recurrent conductive hearing loss.
Autografts
Autografts of bone or cartilage were one of the first materials used for ossicular chain reconstruction. Transposing a partially necrotic incus can yield low extrusion rates and good sound transmission, but care must be taken to avoid reimplanting residual disease in cases involved with cholesteatoma. In chronically diseased ears and revision cases, autologous ossicles may be unavailable. Unless there is a wide middle ear space, careful sculpting of the incus is essential to prevent subsequent ankylosis to surrounding bone. However, with the high cost of operative time (averaging $500.00 per 15 minutes), autografts sculpted intraoperatively can in fact become costly. Cartilage is an excellent biocompatible tissue for tympanoplasty, but it is less than ideal when used alone for ossicular chain reconstruction because it may undergo resorption and it lacks the rigidity required for optimal sound conduction.
Homografts
Homografts offer many of the advantages of autologous grafts, and some are presculpted to reduce operative time. In an end-stage ear with no drum or ossicles, a transplant tympanic membrane with attached malleus can be advantageous in reconstruction. However, the burden and costs to procure, store, and document that a graft is free of infected tissue led to the need for dedicated tissue banks. Concern for the possibility of disease transmission despite the absence of documented cases from U.S. otologic tissue banks has nevertheless markedly diminished the use of homografts in otology.7,8,9
Allografts
Due to the disadvantages of autografts and homografts, synthetic replacements, or alloplasts, were sought for ossicular reconstruction. The initial alloplast developed was a high-density plastic polyethylene sponge (HDPS) that has sufficient porosity to encourage tissue ingrowth, called Plasti-Pore (Gyrus, Memphis, Tennessee). The major disadvantage of Plasti-Pore is its reduced biocompatibility, which can lead to a high incidence of extrusion when in contact with the tympanic membrane, especially in the presence of infection or ETD.10 Extrusion rates have averaged 8 to 12% in many series.11 Extrusion can be reduced when cartilage is placed as a cap between an alloplastic prosthesis and the tympanic membrane.
Alternatives were sought to decrease the extrusion rate. Hydroxyapatite (HA), a polycrystalline calcium phosphate ceramic, is currently one of the most common alloplastic materials used for ossicular reconstruction. It has the same chemical composition as bone and can be used in the same fashion as an incus interposition. Similar to bone, it can be readily identifiable on computed tomographic (CT) scan, which can be helpful in evaluating what prosthesis was used if prior records are unavailable.12
This material has demonstrated the greatest biocompatibility of any alloplast used in the middle ear and serves as a measure of comparison for new synthetics. Infection and rejection are very uncommon in healthy middle ears, so that many surgeons place the prosthesis in direct contact with the drum. Nonetheless, cartilage is strongly considered in cases with an atrophic tympanic membrane and persistent ETD. A limitation of HA is that it may be difficult to trim because it can shatter easily when drilled. Using a diamond bur at a low speed will minimize this. Alternatively, multiple sizes for the prosthesis must be stocked.
For this reason, composite prostheses have been developed with plastic trimmable shafts beneath an HA head, which comes in contact with the tympanic membrane. In a review of 233 cases with a composite prosthetic reconstruction, Goldenberg and Driver demonstrated a 5.2% extrusion rate at 5 years.13 Using a composite prosthesis may decrease the likelihood of extrusion compared with pure plastic prostheses, but further investigation is required to determine if the plastic shaft may compromise long-term biocompatibility compared with pure HA prostheses.
Titanium has recently been adapted as a material for ossiculoplasty, with early studies performed in Germany (Heinz Kurz GmbH, Dusslingen). Titanium implants have a history of excellent biocompatibility elsewhere in the body, and titanium has been promoted as a potentially superior material because of its light weight, which, however, has yet to be demonstrated. Titanium implants are extremely stiff, which creates challenges for intraoperative trimming and may decrease these implants’ ability to resonate sound optimally. Bio-mechanical studies of middle ear dynamics have shown that normal ossicular function requires flexibility for impedance matching as well as stiffness to prevent abnormal resonances within the hearing range.14 The hypothesis is that structural stiffness is the critical design variable for middle ear replacement prostheses. Again, further investigation is required for the long-term results of newer prosthetic materials such as titanium.
Adjuvant Materials
Other materials used in tympanoplasty, mastoidectomy, and ossicular chain reconstructions include plastic sheeting, cements, and adhesives. Plastic sheeting placed in the middle ear can be helpful to reduce adhesions, but some patients may generate a foreign body response to this material that can result in infection, perforation, and loss of the prosthesis. There are a variety of cements such as Mimix (W. Lorenz, Jacksonville, Florida) that can be used for reconstructing canal wall and attic defects. Cartilage has often been used in reconstructing these areas, but it is difficult to stabilize. Cements have recently been used to reconstruct a necrotic lenticular process of the incus. The previous mobile incudostapedial joint becomes an immobile core of cement, however. Furthermore, because there is little contact area, the cement can fall off the residual incus. Adhesives, discussed in detail later, can provide a more malleable link and eventually are absorbed after tissue healing has occurred.
■ Implant Design
A wide variety of implant designs have been developed. Most prostheses are similar in design, with a wide, flat head to contact the tympanic membrane and a narrow shaft for placement onto the stapes superstructure or footplate. This design is so common, in fact, that two of the early trademarked names, TORP (total ossicular replacement prosthesis) and PORP (partial ossicular replacement prosthesis) (Gyrus-Smith and Nephew Richards, Memphis, Tennessee), became synonymous with this type of design. Many variations of this basic mushroom-shaped prosthesis have evolved over the years.
Several limitations occur with this geometry, however. Although the broad, flat top provides a large surface to appose the tympanic membrane, it also limits visibility to the stapes or footplate (Fig. 9–1A,B), which can compromise accuracy of prosthesis placement. To overcome this limitation, off set caps have been designed, but this off set can exacerbate the poor balance inherent in the top-heavy PORP/TORP design. Also, these prostheses are completely dependent on absorbable supporting material during the healing process and are prone to displacement. As a result, investigations into fixation of the prosthesis include altering the prosthetic design to take advantage of the malleus handle (when present) and adhesives. Fibrin glue, albumin, bone cement, and butylcyanoacrylates have been tried, each with its own advantages and disadvantages.15, 16
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