Animal studies show an excellent biocompatibility of PDMS particles [45]. Hypersensitivity is not an issue for subcutaneously located PDMS elastomers, neither in animals nor in humans [46], so skin testing is not required prior to injection. Particle size prevents the material from being phagocytosed. Hence, it does not serve as an antigen.

Several studies show favourable results in terms of voice restoration (Videos 10.4, 10.5, 10.6, and 10.7) similar to thyroplasty [1, 47–50]. PDMS particles provide a long-term augmentation with documented follow-up of 6 years in average [51]. Substance-associated complications seem to be a rare exception and can be difficult to separate from adverse findings as a consequence of malpositioning, i.e. by injecting to superficial, which may happen more frequently. As a last resort, surgical removal of displaced PDMS particles is technically feasible to a certain degree, though a marked stiffness of the erroneously injected subepithelial space cannot be restored completely [52, 53].

In the group of permanent, non-resorbable substances, longevity, safety and efficacy in terms of voice improvement are best documented for PDMS particles. Therefore, Vox Implants™ are most suitable for deep injection laryngoplasty addressing mild to moderate glottic insufficiency in unilateral vocal fold paralysis. Thus, PDMS particles are offering a true alternative to open thyroplasty for patients preferring an endoscopic approach.

Calcium hydroxylapatite (CaHA) is a calcium phosphorus compound found as basic component in the bone and teeth. There is a commercially available material consisting of CaHA microspheres and a carrier gel marketed under the brand name Radiesse that can be injected through a fine needle (25–27 gauge). Once injected, the carrier gel is replaced by fibrosis with the CaHA microspheres remaining in place.

The substance has been studied in both animal and human studies. In an in vivo canine vocal fold model, it has been shown that CaHA injection provided adequate medialization of the canine vocal fold up to 12 months follow-up without migration or resorption and a giant cell reaction without appreciable chronic inflammation [54]. A recent multi-institutional clinical trial revealed excellent results of 80% improvement at 12-month follow-up Rosen 2009 [55]. It is intended as a permanent implant, but resorption occurs to some degree. There are data [56] on longevity showing a loss of benefit in 64% of patients, with an average length of benefit of 18, 6 months (ranging from 8 to 36 months). Therefore, Radiesse is more appropriately labelled as slowly resorbable than non-resorbable. In many aspects similar to collagen, CaHA can be used for the same indications; also the technical process of injection is basically identical. Versatility and ease of use have made it a very popular substance for injection laryngoplasty in the office as well as in the operating room.

There is FDA approval for potentially long-term vocal fold injection. Nevertheless, some substance-associated complications have been described [57, 58]. Voice results seem to be well comparable to those accomplished using thyroplasty type I [59].