Syphilis paralysis, abdominal hernia repair, and post-parotidectomy rehabilitation, to name a few.

By design, the structure of the acellular dermis is to provide a "scaffold" upon which ingrowth of tissue and neovascularity occurs. This has been demonstrated in the rabbit abdominal wall model [6], Little histological information about the fate of the micronized material is available, however. It has been concluded from a randomized prospective clinical trial studying lip augmentation that the cumulative volume enhancement in the micronized acellular dermis group was found to be greater than that in a group receiving bovine collagen [7]. While this is promising, the parallel study in laryngology has not been executed. The histological fate of injected micronized dermis has been evaluated as part of Courey's study [8] using a rabbit model. Neither ingrowth nor persistence of the injectate, however, was demonstrated in the killed animals. It is not known what the impact of paralysis on implant persistence would be.

Klemuk and Titze[9] recently published a study investigating the rheological properties of micronized acellular dermis in comparison with bovine collagen as well as to a hyaluronic acid derivative. The viscoelastic properties observed in the study revealed similarities between the bovine and human dermis preparations, though the hyaluronic acid derivative used had a better "match" to vocal fold mucosa. Further investigations will likely be forthcoming to determine the ideal use for each of the material available for soft tissue replacement in the vocal fold.

Advantages and Disadvantages of Micronized Acellular Dermis for Clinical Use

Advantages and Disadvantages of Micronized Acellular Dermis for Clinical UseThe principal advantage of micronized acellular human dermis reflects its origin as human tissue. It is not believed to be allergenic and therefore requires no pre-testing prior to its use. This allows it to be used in nearly all clinical scenarios on short notice. This is in contrast to bovine collagen preparations, such as Zyplast/Zyderm (Inamed, Santa Barbara, Calif.), which do require allergy testing. The testing process itself is not an obstacle, but it can delay the treatment process up to 4 weeks, which has been the recommended observation period following skin test prior to injection for bovine collagen. It is noteworthy that many practitioners, including the present author, choose to wait only lweek following skin testing for bovine collagen injection laryn-goplasty. The advantage remains with micronized acellular dermis, however, as the patient with aspiration or difficulties with travel cannot wait or is inconvenienced by the return trip to clinic necessitated by the skin test.

One modest disadvantage of micronized acellular dermis relates to the preparation process. It does require some time and features several steps which are prone to error. There have been several modifications in response to concerns from users, including providing a larger amount of substrate and the option to use saline instead of plain lidocaine (1%) as the mixing agent (see below). Its short half-life once reconstituted is a drawback compared with the available bovine collagen preparations, which may be held in a refrigerator for several years.

While some may describe the impermanent nature of injected micronized acellular dermis as a limitation, the clinical scenarios in which a practitioner treats glottic insufficiency are varied enough that there are many, if not the majority of, cases in which transience is an asset rather than a liability. Though cumulative, lasting effects have been described for lip augmentation using micronized acellular dermis [7], no similar study has been reported in humans.

Micronized Human Acellular Dermis in Clinical Laryngology

Early reports of success using micronized acellular dermis have been promising. Pearl et al. [1], published a series of injections performed under general anesthesia with excellent results over a 3-month period. Their patients experienced significant improvement in phonation time, reduction in airflow, and improvement in the voice handicap index (Fig. 8.3.1). This initial study was followed by a comprehensive evaluation of a series of 10 patients reported by Karpenko et al. [10]. While many of the same parameters were improved following augmentation with micronized acellular dermis at 1 month, they felt that it was limited as a material for vocal injection because of the lack of persistence in clinical benefit at the 3-month follow-up period. The authors also stated that these findings «have strengthened our beliefs that patients with irreversible vocal paralysis may receive the best long-term symptom resolution with from medialization thyroplasty.» While this is likely true, the role of micronized acellular dermis as a temporary augmentation material for vocal paralysis is becoming better established. It should not be compared to open thyroplasty for long-term results.

Recently, a promising study from the Cleveland Clinic [11] was published in which 20

patients having undergone injection laryngo-plasty using micronized human dermis were reviewed. 75% of patients had significant benefit past 6 months; 8 of the 20 patients had long term (greater than 12 months) improvement. One of the key strengths of the study, in addition to prospectively acquired subjective and objective voice quality data demonstrating benefit in many parameters, was that all of the treated patients had well-established paralysis, making the likelihood of spontaneous neural recovery unlikely as a confounding variable.

Beyond vocal paralysis, injection laryngo-plasty is often the treatment of choice for vocal paralysis and non-paralytic glottic insufficiency. Though no published study to date has focused on this, Woo has recently presented a series [12] of injections performed for reasons other than paralysis. Based upon the results of his study, he concluded that «micronized dermis injection laryngoplasty is a viable soft tissue replacement in membranous glottal defects due to bowing, scar, and sulcus vocalis. The success rates when treating sequelae of laryngeal fracture, partial laryngectomy defects, and intubation injury was limited, with only 8 of 21 (38%) of their patients experiencing improvement in his early report. This parallels the present author's experience. Not surprisingly, Woo [12] also found that smaller defects were more readily managed than large defects. Further

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