The anatomic, acoustic and aerodynamic parameters measured in this study all showed significant changes when the medialization probe was advanced from anterior to posterior. Some of these changes were likely a consequence of changing geometry while others probably indicate improved phonatory conditions. Glottal area decreased in a predictable manner matching the shortening of the hypotenuse of the triangular glottic opening (). Medialization force doubled likely due to increased force dissipation from higher amounts of tissue between the medialization probe and the posterior vocal fold as well as losses from moving the arytenoid cartilage (). Phonation threshold flow decreased likely not only from increased friction due to airway narrowing, but also from improvement in phonatory conditions that lower phonation threshold, such as symmetry, tension or glottic closure.
Phonation threshold pressure showed a more complicated pattern. It decreased with anterior and middle medialization, but greater pressures were required for posterior medialization. This translated to anterior and middle medialization requiring less aerodynamic power than posterior medialization, although the resulting intensity and efficiency at phonation threshold were also much lower than seen with posterior medialization. This may be explained by considering that anterior and middle medialization require less displacement to achieve effective approximation to the contralateral vocal fold. Vocal fold entrainment and focal oscillation can thus be rendered even though there is a residual glottic gap posterior. The ensuing vibration will tend to be asymmetric and of lower amplitude, producing a less efficient signal of lower intensity. Posterior medialization requires greater aerodynamic power due to substantially smaller glottic area and entrainment of a longer vocal fold length. Once phonation is achieved with posterior medialization, because of better efficiency a greater signal intensity is possible than with more anterior shim locations.
The perturbation measures and SNR showed that posterior displacement also offers superior acoustic results to anterior and middle displacement. This is likely a reflection of the more even distribution of tension in posterior medialization combined with improved symmetry. These factors decrease the irregular and, at times, chaotic behavior that creates jitter, shimmer, noise, and hoarseness.
The results can be summarized in terms of the goals of medialization thyroplasty: to improve sound quality, intensity, ease of phonation, efficiency, and airway protection. Sound quality is subjective, and in this study we used objective acoustic measures of percent jitter, percent shimmer, and SNR to best describe this perceptual entity. Sound intensity was measured at the start of phonation, providing a measure of the intensity of sound with minimal effort. Aerodynamic power was calculated from phonation threshold pressure and flow and is a measure of the ease of phonation to produce minimal sound. Efficiency combines intensity and aerodynamic power. Airway protection is a function of the ability to reduce glottal area with maximal vocal fold adduction.
The results show that posterior placement was superior in terms of acoustic measures, efficiency, and airway protection () at a moderate cost in airway reduction. Middle placement had minimal improvement in sound quality but a larger reduction in the work of phonation. Anterior placement had similar sound quality and work of phonation results as middle placement but with poorer efficiency and airway protection, although consequently, the least reduction in airway caliber.
These findings are in agreement with the design preference of most preformed prostheses, such as the Montgomery® Implant System (Boston Medical Products, Boston., MA) and the Kurz Titanium Implant (Kurz Medical, Norcross, GA), which favor posterior displacement.8
They are also similar to findings in human postoperative thyroplasty studies. Jitter, shimmer, and SNR universally improve with thyroplasty.4,14,15
In a study by Omori et al. these improvements correlated with reduction in glottal area.4
This also occurred in this study but showed a more substantial improvement when factors such as symmetry and equalization of tension (expected in posterior medialization) were also present. The decrease in airflow seen in this study also agrees well with human studies and is a consequence of airway narrowing and improved phonotary conditions.12,16
Pressure estimates in humans show mixed results with most studies reporting a decrease in intraoral pressure,12
an indirect estimate for subglottal pressure. In this study, subglottal pressure showed a complicated pattern that decreased with anterior and middle medialization but increased with posterior medialization. As explained previously, this was likely due to differences in mechanics between anterior and posterior medialization.
The results in this study show some differences with previous excised larynx studies of medialization thyroplasty. In a study by Green et al., jitter, shimmer, and SNR worsened with medialization thyroplasty alone but improved the results of arytenoid adduction when combined with that procedure.13
The authors concluded that the poor results they experienced with medialization thyroplasty alone were likely due to the large posterior chink present between the vocal processes in dogs. In this study, we were able to achieve improvement in jitter, shimmer, and SNR without complete glottic closure by using a small probe that medialized a portion of the vocal fold until it touched the opposite fold instead of the triangular shaped shim used in the afore-mentioned study. This highlights the importance of displacing the vocal fold adequately enough to entrain the opposite vocal fold, while avoiding too much displacement, especially in anterior and middle locations, that could potentially impede vibration. As in the anatomic study by Noordzij et al., we observed that posterior medialization required a greater force to adequately medialize the vocal fold. Unlike that study, we could not achieve the same glottic closure with displacement of the middle vocal fold as we could with displacement of the posterior vocal fold.5
This was again likely due to the smaller probe used in this study compared to the large shims used in previous studies.
There are limitations in applying an excised canine larynx model to human glottic insufficiency, such as the aforementioned differences in anatomy and lack of compensatory behavior; however, these differences should not significantly affect the trends observed in the various medialization positions. Another potential limitation in our model is that all of the measurements were made at phonation threshold or slightly above in the case of jitter and shimmer. This was designed to detect the changes in aerodynamic and acoustic parameters with different medialization positions at the point of minimal effort in phonation. For higher pressures and flows, the parameters will likely maintain the relationships observed in this experiment; however, this is not certain. For example, the poor efficiency seen with anterior and middle medialization may be present only at low aerodynamic power and may improve with higher pressures, leading to higher intensities.
In applying the findings in this study, it appears that prosthesis design may be tailored to patient goals. Prostheses that emphasize posterior glottis medialization could be used in patients with good respiratory reserve. The surgeon can feel confident that additional prosthesis size and force of medialization can be translated into effective medialization of the membranous vocal fold. When intraoperative monitoring fails to show the desired result, additional medialization might be achieved through judicious anterior and midmembranous displacement. Arytenoid adduction remains an option when all of these measures prove ineffective and especially if the plane of correction is not matched to the opposite vocal fold. Prostheses emphasizing middle and anterior medialization could be chosen for patients who cannot tolerate any airway compromise. Effective displacement should be possible using only gentle force and small displacement prostheses. With care to avoid irregularities such an approach should prove particularly useful for patients with significant bilateral pathology or poor respiratory reserve.
A final important point is that many of the larynges did not display the final pattern seen in the averaged results as evident by the large standard deviations in the data. This reaffirms the importance of intraoperative voice assessment performed by many surgeons.17
Under ideal circumstance this should guide how the particular patient will respond to different probe placements. When intraoperative testing is not possible due to bleeding or edema18
or in the occasional patient done under general anesthesia, the data presented in this study give surgeons a general idea of what they can expect with different medialization positions.