This study is the first to assess in depth the 3D conformation of the deposited bolus from vocal fold injection augmentation. The analysis used 3D image reconstruction techniques to highlight how the injected bolus relates to laryngeal structures, including mucosal surfaces. The geometric distribution of the bolus brings insights to both favorable and unfavorable consequences of injection augmentation that are not apparent from inspection of 2D images alone.
Boluses deposited by injections performed in a lateral or deep fashion can adopt irregular shapes, with nonuniform surfaces. This contrasts with block implant thyroplasty, in which the implants have well-defined contours, but may be similar to the result of Gore-Tex thyroplasty, in which the aggregate shape of the folded strips of Gore-Tex can be irregular.6
All 3 methods have been shown to produce good vocal outcomes, presumably because the intrinsic longitudinal structure of the TA muscle and the fibroelastic nature of the vocal ligament and the conus elasticus mask irregularities of the implants. While the irregular shape of the lateral boluses has no apparent effect on vocal outcome, it may have implications for revision or further surgery. Overcorrection can in principle be remedied by partial removal of the injected bolus,7,8
but complete removal of an irregularly shaped bolus would likely require extensive dissection of the paraglottic space. In addition, in the authors’ experience, silastic implant thyroplasty following injection of a relatively durable material such as calcium hydroxylapatite can be more challenging in some cases. Intraoperative trial medialization at different positions within the thyroplasty window may not produce direct one-toone displacement of the corresponding part of the vocal fold due to an intervening irregularly shaped residual bolus. An implant that deviates from the typical dimensions9
may be required to achieve the best vocal outcome.
The vertical thickness of the vocal fold is a determining factor in the ease of phonation in theoretical models. The phonation threshold pressure, the minimum subglottal pressure required to initiate vocal fold oscillation, is inversely proportional to the vertical thickness of the vocal fold.10
It has been suggested that injection augmentation can increase vocal fold thickness,11,12
but the vertical dimension of injected boluses has not been previously quantified to our knowledge. The bolus vertical depth averaged 7.6 to 9.5 mm in this study (see the Results section), which is slightly greater than the average vocal fold height in the noninjected larynges reported previously.2
This reflects the slight vertical expansion of the paraglottic space and raising of the floor of the ventricle with injection in some specimens. While resorption of carrier fluid within the first few days after injection in vivo will decrease the size of the bolus, it is clear that the bolus not only medializes the vocal fold but also has the potential to augment the vertical thickness of the vocal fold. This effect of injection augmentation may be particularly important in the setting of significant vocal fold atrophy, where the reduction in the cross-sectional area of the TA muscle affects both its width and height.
The vertical thickness of medial boluses was found to be more strongly correlated with injection volume than that of lateral boluses, a quantitative finding consistent with the visual observation that medial boluses were more compact. Medial bolus thickness was also less correlated with vocal fold thickness compared with lateral boluses, supporting the notion that the effect of medial boluses may be less subject to individual anatomical variations.
Findings from this study may explain additional clinical observations. We have noted anecdotally and others have reported that overinjection can result in vibratory stiffness.1
This is generally assumed to be an effect on the vocal fold mucosa. However, demonstration of TA muscle compression by lateral boluses in this study suggests at least another possible mechanism. Analytical modeling of vocal fold vibration based on the body-cover model entails participation of variable depth of the TA muscle in vibration.13
As the amplitude of vibration increases as in loud phonation, a greater depth of the TA muscle becomes involved. Increased stiffness of the TA muscle from bolus compression can be expected to impede vibration by reducing the amount of tissue deep to the cover that can move with the cover. This effect is maximal at the time of the injection. As the resorption of carrier fluid takes place within the first few days and of some injected material over weeks to months, the magnitude of TA muscle compression will presumably decrease, and the apparent vibratory stiffness should resolve. However, it is possible that the TA muscle may undergo some degree of remodeling due to mechanical compression as well as integration of the injected material. Demonstration of an effect on vocal fold vibration due to muscle-injectate interaction will require investigation in vivo.
Another observation relates to repeat injections. In the authors’ experience, patients who undergo a second injection for unilateral vocal fold paralysis after the benefit from the first injection had diminished sometimes derive less-than-expected vocal quality improvement from the second injection. The 3D conformation of the bolus offers a possible partial explanation for this anecdotal experience. While the initial bolus is constrained by the boundaries of the paraglottic space, the second bolus is additionally constrained by whatever material or tissue integration remains from the first injection. This residual effect may be significant when a more durable injectable was used initially. The second bolus therefore may distribute in a less predictable way.
Clinically, injection lateral to the vocal process is sometimes thought to provide additional medialization of the membranous vocal fold by rotating the vocal process medially.11
The 3D analysis calls the basis for this practice into question. The lateral boluses tend to abut the anterior face of the arytenoid body (–). The predominant contact involves an anterior-posterior interface. The direction of contact is unlikely to produce a medial rocking movement of the arytenoid, since the 2 directions are orthogonal. Rather, further medialization of the posterior membranous vocal fold is likely produced by passive movement of the vocal process as the vocal ligament is further medialized by the additional bolus.5
Several aspects of this study deserve clarification. First, although calcium hydroxylapatite was used, the general principles of bolus distribution may be applicable to many other commonly used injectables, such as collagen and hyaluronic acid products, because they are comparably deformable based on their injectability through similar-gauge needles. The 3D bolus conformation may be different for fat, which may not dissect through the paraglottic space as easily, and for Teflon paste, which is highly viscous and may form a more compact initial bolus. Second, the imaging data represent the bolus in its initial state after injection. In a live subject, the bolus may flatten somewhat from glottal closure tasks and will also undergo resorption over time.14
Some of our findings and conclusions therefore have the most implications for injections at the time they are performed and may be less relevant as resorption takes place. Third, even though medial boluses are geometrically better defined than lateral boluses, medial injections have indications specific to lamina propria defects, and only injectables with suitable tissue replacement properties should be used.15