Effect of superior laryngeal nerve stimulation on phonation in an in vivo canine model

Neurophysiology of the Superior Laryngeal Nerve in an In Vivo Rat Model

OBJECTIVE

The superior laryngeal nerve (SLN) is fundamental in laryngeal sensation, cough reflex, and pitch control. SLN injury has substantial consequences including altered sensation, aspiration, and dysphonia. To date, in vivo measurement of the SLN remains elusive. The purpose of this study was to assess the feasibility of recording motor and sensory evoked potentials in a rat SLN model.

METHODS

Twenty-two rat hemi-laryngeal preparations (n = 11) were obtained from 4-month-old Sprague-Dawley rats and included in this study. Compound motor action potentials (CMAPs) and motor unit number estimation (MUNE) were calculated by stimulating the SLN at the point of medial extension near the carotid artery and by placing a recording electrode on the cricothyroid muscle. Sensory response was determined through stimulation of the SLN and laryngoscopic visualization of a laryngeal adductor reflex (LAR). SLN and cricothyroid muscle cross-sections were stained and histologic morphometrics were quantified.

RESULTS

Laryngeal evoked potentials were successfully obtained in all trials. Mean CMAP latency and negative durations were 0.99 ± 0.57 ms and 1.49 ± 0.57 ms, respectively. The median MUNE was 2.06 (IQR 1.88, 3.51). LAR was induced with a mean intensity of 0.69 ± 0.20 mV. Mean axon count, myelin thickness, and g-ratio were 681 ± 192.2, 1.72 ± 0.26, and 0.45 ± 0.04, respectively.

CONCLUSIONS

This study demonstrates the feasibility of recording evoked response potentials following SLN stimulation. We hypothesize that this work will provide a tractable animal model to study changes in laryngeal sensation and cricothyroid motor function with aging, neurodegenerative disease, aspiration, or nerve injury.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:1778-1784, 2024.

Impact of Cricothyroid Muscle Contraction on Vocal Fold Vibration: Experimental Study with High-Speed Videoendoscopy

OBJECTIVES

The aim of this study was to evaluate the effects of cricothyroid muscle contraction on vocal fold vibration, as evaluated with high-speed videoendoscopy, and to identify one or more aspects of vocal fold vibration that could be used as an irrefutable indicator of unilateral cricothyroid muscle paralysis.

STUDY DESIGN

This was an experimental study employing excised human larynges.

METHODS

Twenty freshly excised human larynges were evaluated during artificially produced vibration. Each larynx was assessed in three situations: bilateral cricothyroid muscle contraction, unilateral cricothyroid muscle contraction, and no contraction of either cricothyroid muscle. The following parameters were evaluated by high-speed videoendoscopy: fundamental frequency, periodicity, amplitude of vocal fold vibration, and phase symmetry between the vocal folds.

RESULTS

Although neither unilateral nor bilateral cricothyroid muscle contraction altered the periodicity of vibration or the occurrence of phase asymmetry, there was a significant decrease in fundamental frequency in parallel with decreasing longitudinal tension. We also found an increase in vibration amplitude of right and left vocal folds, which were similar in terms of their behavior for this parameter in the various situations studied.

CONCLUSION

Our results suggest that differences in vibration amplitude and phase symmetry between vocal folds are not reliable indicators of unilateral cricothyroid muscle paralysis.

Intraoperative neural monitoring in thyroid surgery: lessons learned from animal studies

Recurrent laryngeal nerve (RLN) injury remains a significant morbidity associated with thyroid and parathyroid surgery. In the past decade, surgeons have increasingly used intraoperative neural monitoring (IONM) as an adjunct technique for localizing and identifying the RLN, detecting RLN injury, and predicting the outcome of vocal cord function. In recent years, many animal studies have investigated common pitfalls and new applications of IONM. For example, the use of IONM technology in animal models has proven valuable in studies of the electrophysiology of RLN injury. The advent of animal studies has substantially improved understanding of IONM technology. Lessons learned from animal studies have immediate clinical applications in establishing reliable strategies for preventing intraoperative RLN injury. This article gives an overview of the research progress on IONM-relevant animal models.

Long-term Results with Hydroxylapatite Middle Ear Implants

PURPOSE: This study evaluated long-term results in patients who had ossicular reconstruction with a Goldenberg hydroxylapatite implant.

METHODS: A total of 233 patients underwent implantation; of these, 77 had 5-year or longer follow-up and are the subjects of this study of long-term hearing results. All 233 patients were included for analysis of extrusion rate and postoperative otorrhea.

RESULTS: The hearing success rate at long-term follow-up was 56.8%; the mean air-bone gap was 21.1 dB. Prosthesis extrusion occurred in 5.29% of the 233 patients, and visible slippage occurred in 7.7%. Overall, 50.6% of patients met the criteria for successful hearing, which included no extrusion and a dry ear. Better hearing before surgery and presence of the malleus long process were factors associated with a successful hearing result, as was tympanoplasty alone and canal wall up tympa-nomastoidectomy.

CONCLUSION: Hydroxylapatite hybrid prostheses provide stable hearing results over time with low extrusion and a dry ear overall.

Nonlinear analyses of elicited modal, raised, and pressed rabbit phonation

OBJECTIVES/HYPOTHESIS

The purpose of this study was to use nonlinear dynamic analysis methods such as phase space portraits and correlation dimension (D2) as well as descriptive spectrographic analyses to characterize acoustic signals produced during evoked rabbit phonation.

METHODS

Seventeen New Zealand white breeder rabbits were used to perform the study. A Grass S-88 stimulator (SA Instrumentation, Encinitas, CA) and constant current isolation unit (Grass Telefactor, model PSIU6; West Warwick, RI) were used to provide electrical stimulation to laryngeal musculature, and transglottal airflow rate and stimulation current (mA) were manipulated to elicit modal, raised intensity, and pressed phonations. Central 1 second portions of the most stable portion of the acoustic waveform for modal, raised intensity, and pressed phonations were edited and then analyzed via phase space portraits, Poincaré sections, and the estimation of the D2. In an attempt to limit the effects of the highly variable and nonstationary characteristics of some of the signals being analyzed, D2 analysis was also performed on the most stable central 200-millisecond portion of the acoustic waveform. Descriptive analysis of each phonation was also conducted using sound spectrograms.

RESULTS

Results showed that the complexity of phonation and the subsequent acoustic waveform is increased as transglottal airflow rate and degree of glottal adduction are manipulated in the evoked rabbit phonation model. In particular, phonatory complexity, as quantified via D2 analyses and demonstrated via spectrographic characteristics, increases from "modal" (ie, phonation elicited at just above the phonation threshold pressure) to raised intensity (phonation elicited by increasing transglottal airflow rate) to pressed (phonation elicited by increasing the stimulation current delivered to the larynx). Variations in a single dynamic dimension (airflow rate or adductory force) resulted in significantly increased productions of nonlinear phenomenon, including bifurcations from periodicity to regions of subharmonic content, fundamental frequency, and harmonic jumps, and evidence of periodicity within aperiodic regions ("chaos").

CONCLUSIONS

The evoked rabbit phonation model described in this study allows for the elicitation of various types of phonations under controlled conditions and, therefore, has the potential to provide insight regarding important variables that may elicit examples of nonlinear phenomena such as subharmonics and deterministic chaos.

Total ossiculoplasty: advantages of two-point stabilization technique

Objective. Evaluate a porous polyethylene prosthesis with two-point stabilization in total ossiculoplasty. This approach utilizes a lateral as well as a medial graft to stabilize a total ossicular prosthesis (TOP). Study Design. Retrospective cohort review of total ossiculoplasty. Methods. All patients who underwent total ossiculoplasty during the years 2004-2007 were included in the study group. Only five patients (10%) had primary surgery whereas 45 (90%) underwent revision surgery. Cartilage grafts covering the prosthesis (Sheehy, Xomed) laterally were used in all patients with areolar tissue being used for medial stabilization at the stapes footplate. Follow-up examination and audiometrics were performed a mean of 8.1 months following surgery. Results. The percentage of patients closing their ABG to within 10 dB was 44% with 66% closing their ABG to within 20 dB. The mean four-frequency hearing gain was 15.7 dB. The mean postoperative ABG was 15.7 dB. Conclusion. Audiometric results following total ossiculoplasty surgery using two-point stabilization exceeded results from the otologic literature. Proper two-point fixation with areolar tissue and stabilization utilizing cartilage were the keys to achieving a relatively high percentage of success in chronic ear disease in this sample.

Ossiculoplasty: revisited

Conductive hearing loss from ossicular chain abnormalities may result from either discontinuity or fixation of the ossicular chain. The ideal prosthesis for ossicular reconstruction should be biocompatible, stable, safe, readily available, and capable of yielding optimal sound transmission. At present ossiculoplasty techniques using alloplast materials are becoming popular but the fate of these synthetic materials in human middle ear requires further study. Autologous ossicle or cortical bone grafts maintain their morphologic contour, size, shape, and physical integrity for long periods of time, over 25 years making them still the choice at present. The choice of technique will still depend on the causative pathology, availability of graft, surgical experience.

Experiments on Analysing Voice Production: Excised (Human, Animal) and In Vivo (Animal) Approaches

Experiments on human and on animal excised specimens as well as in vivo animal preparations are so far the most realistic approaches to simulate the in vivo process of human phonation. These experiments do not have the disadvantage of limited space within the neck and enable studies of the actual organ necessary for phonation, i.e., the larynx. The studies additionally allow the analysis of flow, vocal fold dynamics, and resulting acoustics in relation to well-defined laryngeal alterations.

PURPOSE OF REVIEW

This paper provides an overview of the applications and usefulness of excised (human/animal) specimen and in vivo animal experiments in voice research. These experiments have enabled visualization and analysis of dehydration effects, vocal fold scarring, bifurcation and chaotic vibrations, three-dimensional vibrations, aerodynamic effects, and mucosal wave propagation along the medial surface. Quantitative data will be shown to give an overview of measured laryngeal parameter values. As yet, a full understanding of all existing interactions in voice production has not been achieved, and thus, where possible, we try to indicate areas needing further study.

RECENT FINDINGS

A further motivation behind this review is to highlight recent findings and technologies related to the study of vocal fold dynamics and its applications. For example, studies of interactions between vocal tract airflow and generation of acoustics have recently shown that airflow superior to the glottis is governed by not only vocal fold dynamics but also by subglottal and supraglottal structures. In addition, promising new methods to investigate kinematics and dynamics have been reported recently, including dynamic optical coherence tomography, X-ray stroboscopy and three-dimensional reconstruction with laser projection systems. Finally, we touch on the relevance of vocal fold dynamics to clinical laryngology and to clinically-oriented research.

Phonatory characteristics of excised pig, sheep, and cow larynges

The purpose of this study was to examine the phonatory characteristics of pig, sheep, and cow excised larynges and to find out which of these animal species is the best model for human phonation. Excised pig, sheep, and cow larynges were prepared and mounted over a tapered tube on the excised bench that supplied pressurized, heated, and humidified air in a manner similar to that for excised canine models. Each excised larynx was subjected to a series of pressure-flow experiments with adduction as major control parameter. The subglottal pressure, electroglottograph (EGG), mean flow rate, audio signal, and sound pressure level were recorded during each experiment. EGG signal was used to extract the fundamental frequency. It was found that pressure-frequency relations were nonlinear for these species with large rate of frequency changes for the pig. The average oscillation frequencies for these species were 220+/-57 Hz for the pig, 102+/-33 Hz for the sheep, and 73+/-10 Hz for the cow. The average phonation threshold pressure for the pig was 7.4+/-2.0 cm H(2)O, 6.9+/-2.9 cm H(2)O for the sheep, and 4.4+/-2.3 cm H(2)O for the cow.

Comparison of the Mechanical Performance of Ossiculoplasty Using a Prosthetic Malleus-to-Stapes Head with a Tympanic Membrane-to-Stapes Head Assembly in a Human Cadaveric Middle Ear Model

HYPOTHESIS

Ossiculoplasty using prosthetic reconstruction with a malleus assembly to the stapes head will result in better transmission of vibrations from the eardrum to the stapes footplate than reconstruction with a tympanic membrane assembly to the stapes head. Both types of reconstruction will be affected by tension of the prosthesis.

BACKGROUND

Theories (and some clinical studies) that the shape of the normal tympanic membrane is important suggest that prosthetic reconstruction to the malleus performs better than reconstruction to the tympanic membrane. This has not been previously tested by directly measuring vibration responses in the human ear. Our previous work suggests that tympanic membrane assembly to the stapes head type prostheses performed best under low tension. This had not been previously tested for malleus assembly to the stapes head type prostheses.

METHODS

Hydroxyapatite prostheses were used to reconstruct a missing incus defect in a fresh cadaveric human ear model. Two types of prostheses were used, one from the stapes head to the malleus (malleus assembly to the stapes head), the other from the stapes head to the tympanic membrane (tympanic membrane assembly to the stapes head). Stapes footplate center responses were measured using a laser Doppler vibrometer in response to calibrated acoustic frequency sweeps.

RESULTS

Tension had a very significant effect on both types of prostheses in the lower frequencies. Loose tension was best overall. The malleus assembly to the stapes head type prostheses consistently performed better than the tympanic membrane assembly to the stapes head type prostheses when stratified for tension.

CONCLUSION

Tension has a significant effect on prosthesis function. Malleus assembly to the stapes head type prostheses generally result in better transmission of vibrations to the stapes footplate than tympanic membrane assembly to the stapes head type prostheses.

Malleus Relocation in Ossicular Reconstruction: Managing the Anteriorly Positioned Malleus: Results in a Series of 268 Cases

OBJECTIVE

The objective of this study was to report an original method of malleus relocation allowing for better placement of both partial and total prostheses in ossicular reconstruction.

STUDY DESIGN

We conducted a retrospective review of clinical and audiometric findings.

SETTING

A tertiary referral center.

MATERIALS AND METHODS

This is a study of 268 patients who underwent ossiculoplasty surgery from October 1997 to October 2000 for chronic otitis media, noninflammatory disease, and otosclerosis revision. Malleus relocation with total and partial ossicular replacement prostheses was used in all cases. Audiometric assessment included pre- and postoperative audiometric evaluation using conventional audiometry. Air-bone gap, bone-conduction thresholds, and air-conduction thresholds were measured.

RESULTS

A postoperative air-bone gap closed to within 10 dB was achieved in 56% of cases. An air-bone gap smaller than 20 dB was obtained in 78% of cases. Postoperative improvement of air-conduction thresholds superior to 20 dB was found in 41.5% of cases. The postoperative bone-conduction thresholds were unchanged in 98% of cases. One case of total postoperative sensorineural hearing loss was seen in this series (0.4%). Extrusion of the protheses was not observed in this series. Follow up ranged from 6 to 36 months (mean, 12.4 months).

CONCLUSION

This study shows that malleus relocation is a safe and efficient technique for ossicular reconstruction. The ideal position of the relocated malleus allows easier and more stable placement of middle ear prostheses.

Current use of implants in middle ear surgery

OBJECTIVE

The authors report the results of a survey of members of the American Otological Society (AOS) and the American Neurotology Society (ANS) regarding their use of prostheses currently available for ossiculoplasty and stapedectomy. These findings are compared with a similar study presented by one of the authors in 1989.

METHODS

Questionnaires were sent to the entire membership of the AOS and ANS with questions regarding biomaterial and prosthesis usage for stapes and chronic ear surgery, as well as satisfaction with each type of prosthesis used. Of the 575 questionnaires mailed, 274 (47%) were returned. Only 248 of the respondents performed middle ear surgery (43%), and their responses constitute the database for this study.

RESULTS

For those respondents performing stapes surgery in both 1989 and 1999, the mean number of cases per year has increased from 32 to 37 (p < or = 0.004). The mean number of chronic ear cases has also increased from 95 in 1989 to 110 in 1999 (p < or = 0.001). As a biomaterial, hydroxyapatite prostheses are used by most surgeons (82%), followed by autograft and homograft bone (72%), autograft and homograft cartilage (62%), and Plastipore (59%). (Although 62% of respondents use cartilage, only 4.4% ranked it first in preference.) In 1989, bone was used most (93%), followed by cartilage (78%) and Plastipore (81%). Hydroxyapatite, which had just been introduced as a biomaterial, was used by only 9% of respondents. For stapes prostheses in 1999, the majority of respondents used stainless steel/platinum (71%), bucket handle (69%), or partial fluoroplastic (56%) prostheses. There was a high overall satisfaction rate in the use of most of these prostheses (> 85%), with several exceptions. The lowest satisfaction rate was 71% for Plastipore partial ossicular replacement prosthesis and total ossicular replacement prosthesis. Usage and satisfaction rates are presented for specific types of implants and compared with the earlier survey findings.

CONCLUSION

The current use of implants in middle ear surgery demonstrates a specific pattern with a high degree of user satisfaction. The preference for implants by respondents has remained stable over the past 10 years; there has been a decrease in the percentage of use of bone, cartilage, and Plastipore with a corresponding increase in the use of hydroxyapatite.

Evaluation of hydroxyapatite ossicular chain prostheses

Hydroxyapatite (HA) middle ear prostheses have gained popularity as an alternative to human autografts and homografts. This study reports on 3 HA prostheses types: total ossicular chain prostheses, used for grafting the stapes footplate to the tympanic membrane; partial ossicular chain prostheses, used for grafting the stapes superstructure to the tympanic membrane; and Kartush incus struts (Smith & Nephew Richards Inc), used for grafting the stapes superstructure to the undersurface of the malleus. This single-surgeon study of 33 consecutive cases revealed a statistically significant difference in mean postoperative air-bone gap and airbone gap closure between incus struts (14/26 dB) or partial (22/11 dB) or total (25/10 dB) ossicular chain prostheses (t test: P<0.05). Prognostic risk factors graded by the Middle Ear Risk Index indicate a tendency for worse postoperative hearing with increasing Middle Ear Risk Index. This study supports the use of HA ossicular prostheses and, in particular, the use of the malleus for ossicular chain construction.

Long-term results with hydroxylapatite middle ear implants

PURPOSE

This study evaluated long-term results in patients who had ossicular reconstruction with a Goldenberg hydroxylapatite implant.

METHODS

A total of 233 patients underwent implantation; of these, 77 had 5-year or longer follow-up and are the subjects of this study of long-term hearing results. All 233 patients were included for analysis of extrusion rate and postoperative otorrhea.

RESULTS

The hearing success rate at long-term follow-up was 56.8%; the mean air-bone gap was 21.1 dB. Prosthesis extrusion occurred in 5.29% of the 233 patients, and visible slippage occurred in 7.7%. Overall, 50. 6% of patients met the criteria for successful hearing, which included no extrusion and a dry ear. Better hearing before surgery and presence of the malleus long process were factors associated with a successful hearing result, as was tympanoplasty alone and canal wall up tympanomastoidectomy.

CONCLUSION

Hydroxylapatite hybrid prostheses provide stable hearing results over time with low extrusion and a dry ear overall.

Combination thyroplasty and the "twisted larynx:" combined type IV and type I thyroplasty for superior laryngeal nerve weakness

Vocal cord medialization through Isshiki type I thyroplasty is part of the standard approach for patients with unilateral vocal cord immobility secondary to recurrent laryngeal nerve paralysis. However, several other modalities have been used to treat the symptomatic "twisted" larynx caused by unilateral superior laryngeal nerve weakness. The Isshiki type IV thyroplasty (cricothyroid approximation) specifically addresses cricothyroid muscle weakness, but, canine studies at the Mayo Clinic demonstrated a trend toward decreased acoustic power and sound intensity with simulated cricothyroid activity. Thus it is reasoned that addition of an ipsilateral type I thyroplasty should help compensate for this power loss. Using videostroboscopic and acoustic analysis, 9 patients with unilateral superor laryngeal nerve weakness were treated with combination type IV and type I thyroplasty. Subjective dysphonia and objective visual and acoustic measurements revealed postoperative improvement in most patients. The combination type IV and type I thyroplasty is recommended for surgical treatment of patients with superior laryngeal nerve weakness, because it addresses cricothyroid muscle weakness without compromising vocal power.

Long-term model of induced canine phonation

Experimental induced phonation in the dog has been used in short-term studies by several investigators and has proved quite useful in laryngeal research. In this study a long-term canine phonation model is described that uses permanently implanted electrodes on the superior and recurrent laryngeal nerves. A serial induced phonation model has not been previously reported and is needed for laryngeal research in which voice results are a primary end point. Inexpensive, reliable, nontoxic electrodes were designed and fabricated. The laryngeal nerves were found to be quite susceptible to injury, necessitating a series of changes in electrode design. Electrode durability and laryngeal nerve viability improved with each design modification; the final design gave a recurrent laryngeal nerve viability rate of 100% at 6 weeks, 83% at 9 weeks, and 73% at 12 weeks. Induced phonation was successfully produced on a repeated basis by stimulating the recurrent laryngeal nerves while passing air through the larynx, in 22 (95.6%) of 23 animals. Stimulation of the superior laryngeal nerves increased vocal fold length and tension but was not required for phonation. Technical aspects of chronic implantation and stimulation of the laryngeal nerves are discussed. The development and successful long-term implantation of electrodes on the laryngeal nerves and their use in repeated induced phonation have not been reported previously.

Reversibility of medialization laryngoplasty. An experimental study

Medialization laryngoplasty has become a routine procedure for cases of unilateral vocal fold paralysis. In certain clinical situations, it may become desirable to reverse the procedure and remove the implant. This process was studied experimentally in eight dogs in a chronic model of induced canine phonation. A silicone polymer implant was inserted to medialize one normal vocal fold for a period of 1 month, after which it was removed. Motion of the cricoarytenoid (CA) joint and induced phonation were studied weekly while the implant was in place, and for another month following implant removal. Significant abnormalities were found even with this relatively short period of implantation. With the implant in place, impairment of CA joint mobility was found in seven of the eight dogs, precluding phonation. A dense fibrous capsule rapidly developed around the implant, making its removal technically difficult. Following implant removal, a gradual return to normal function was found in only three of the eight dogs. One of the animals had evidence of neural injury, while four had intact neural function but impaired mobility or fixation of the CA joint. Medialization laryngoplasty should not be considered a reversible procedure. The clinical implications of these findings are discussed.

Effects of driving pressure and recurrent laryngeal nerve stimulation on glottic vibration in a constant pressure model

Glottic phonatory parameters have been studied in constant flow models; however, the lung-thorax system is better viewed as a constant pressure source. Adjusting the driving pressure and recurrent laryngeal nerve stimulation as independent variables, rather than as dependent variables, may provide a more physiologic understanding of laryngeal function and glottic parameters, including subglottic pressure, airflow, fundamental frequency, and glottic area. In three dogs subglottic pressure and airflow were measured in two separate conditions: with constant recurrent laryngeal nerve stimulation and varying driving pressure, and with constant driving pressure and varying recurrent laryngeal nerve stimulation. Videostroboscopic measures on four dogs assessed glottic areas with constant recurrent laryngeal nerve stimulation at different driving pressures. With constant recurrent laryngeal nerve stimulation, increasing driving pressure had no effect on glottic areas, whereas subglottic pressure, fundamental frequency, and airflow increased significantly. However, changes in subglottic pressure were minimal in comparison with changes in driving pressure. At constant driving pressure, increasing recurrent laryngeal nerve stimulation increased subglottic pressure and fundamental frequency and decreased airflow. These findings suggest that during phonation subglottic pressure is primarily dependent on recurrent laryngeal nerve stimulation and laryngeal muscular contraction, but not on lung driving pressure.

Superior laryngeal nerve paresis and paralysis

Superior laryngeal nerve paresis and paralysis are relatively common but often difficult to diagnose with certainty. They are most commonly caused by viral infections, though other etiologies must be considered. A thorough history and physical examination, including strobovideolaryngoscopy and laryngeal electromyography, are needed for definitive diagnosis. It is essential to establish the diagnosis accurately to differentiate an apparent superior laryngeal nerve paresis from other conditions, such as myasthenia gravis. Laryngeal electromyography is used to confirm clinical impressions, as a guide for therapy, and as one measure of recovery. In our experience, accurate and early diagnosis assure the best phonatory outcome by directing therapy that will prevent or eliminate compensatory vocal abuses, which may themselves lead to even more serious vocal injury.

Characteristics of an in vivo canine model of phonation with a constant air pressure source

Many previous studies of laryngeal biomechanics using in vivo models have employed a constant air How source. Several authors have recently suggested that the lung-thorax system functions as a constant pressure source during phonation. This study describes an in vivo canine system designed to maintain a constant peak subglottic pressure (Psub) using a pressure-controlling mechanism. Increasing levels of recurrent laryngeal nerve (RLN) stimulation resulted in a significant rise in resistance followed by a plateau. For a given Psub, flow decreased significantly and precipitously with increasing stimulation and then quickly plateaued. Vocal intensity increased with increasing RLN stimulation until a peak was reached. After this peak, intensity dropped until a plateau was reached, corresponding to the flow minimum. At a given Psub, increasing levels of RLN stimulation resulted in a normal distribution of vocal efficiencies.

A pressure-regulated model of normal and pathologic phonation

Recent evidence suggests that the lung-thorax system functions as a constant pressure source during phonation. However, previous animal models used a constant flow source. This article describes an in vivo canine model that maintains a constant subglottic pressure during phonation to more closely simulate the pulmonary system. At any given subglottic pressure, increasing levels of recurrent laryngeal nerve stimulation resulted in a significant rise in resistance followed by a plateau. Increasing levels of superior laryngeal nerve stimulation, however, produced no significant change in glottal resistance. Three experimental conditions were studied: normal, unilateral recurrent laryngeal nerve paralysis, and paralysis followed by arytenoid adduction. In normal canines, maximal vocal efficiency values were the highest, indicating the best match between pressure and resistance. The vocal efficiency values were significantly lower in recurrent laryngeal nerve paralysis, indicating pressure-resistance mis-match. Arytenoid adduction increased the maximal vocal efficiency values and decreased the mismatch observed in the paralyzed state. These findings may provide insight into an understanding of normal and pathologic laryngeal behavior.

Function of the interarytenoid muscle in a canine laryngeal model

The interarytenoid (IA) muscle has rarely been studied in the living larynx. In this work, the role of the IA muscle in phonation was studied in three dogs by means of an in vivo phonation model. The isolated action of the IA muscle was studied by sectioning and stimulating its nerve branch. As IA activity increased, subglottic pressure increased significantly until a plateau was reached. In the absence of superior laryngeal nerve stimulation, the fundamental frequency rose with increasing IA activity. In the presence of superior laryngeal nerve stimulation, however, no significant change in fundamental frequency was observed with increasing IA activity. Measurement of adductory force demonstrated that the IA muscle adducts primarily the posterior vocal fold. In this canine model, phonation was not possible without IA stimulation, owing to a large posterior glottic chink.

Hydroxylapatite ossicular replacement prostheses: a four-year experience

Between 1987 and 1991, I have used 215 hydroxylapatite middle ear implants, in various styles, for hearing reconstruction. The first such implants were composed entirely of hydroxylapatite. Because of intraoperative difficulties in shaping and trimming these prostheses, hybrid prostheses using Plasti-Pore were developed. For each of four implant designs (incus, incus-stapes, PORP, and TORP), the head is constructed from hydroxylapatite and the shaft from Plasti-Pore. Extrusion rate for the hybrid prostheses is low (4.3%). Hearing results from 47 patients with the hybrid hydroxylapatite prostheses, 140 patients with total hydroxylapatite prostheses, and 75 control group patients with homograft bone or Plasti-Pore prostheses were compared. A "successful" hearing result was achieved in 51.1%, 51.4%, and 60.0% of the three groups, respectively. Surgical technique for use of the new hybrid hydroxylapatite prostheses is described.

Effect of superior laryngeal nerve on vocal fold function: an in vivo canine model

Assessment of laryngeal framework surgery requires an awareness of the effect of vocal fold mass, stiffness, and position on voice production. The vibratory pattern of vocal folds during phonation depends on the subglottic pressure as well as the mass and stiffness of the folds. To assess the effect of variations in vocal fold tension with contraction of the cricothyroid muscle on phonation, eight mongrel dog larynges were studied in vivo. Photoglottography, electroglottography, and subglottic pressure were simultaneously recorded as airflow rate and superior laryngeal nerve (SLN) stimulation were varied. Stimulation of the SLN was modified by varying the frequency and voltage of the stimulating electrical signal. Multiple regression analysis of the data revealed a direct relationship between the voltage of SLN stimulation and frequency of vibration (p less than 0.001) at constant subglottic pressure. Increases in the stimulating voltage to the SLN also led to an increase in open quotient (p less than 0.001), but no statistically significant change in speed quotient, subglottic pressure, or sound intensity. Changing the frequency of SLN stimulation had only a modest effect on the frequency of vibration. These results are consistent with the reported findings of an increase in frequency and open quotient with increased tension in an in vitro canine model. The glottographic measurement open quotient appears to be an estimator of cricothyroid contraction and longitudinal vocal fold tension, and may be clinically applicable to the assessment of superior laryngeal nerve injuries and laryngeal framework procedures.