Introduction

Vocal fold injection augmentation is a known treatment modality for glottal closure insufficiency. Causes of glottal closure insufficiency include vocal fold paralysis, paresis, atrophy, sulcus vocalis, scarring and vocal fold deficiency after laryngeal surgery.^{Reference Rosen1}

The original materials used in injection augmentation, such as paraffin, silicone and Teflon, caused foreign body reactions,^{Reference King and Simpson2–Reference Rubin4} and safer materials have since replaced them. Injectable materials are currently categorised as temporary or long-term. The temporary materials available are collagen-based, hyaluronic acid based and carboxymethyl cellulose based. The long-term materials available are calcium hydroxyapatite and autologous fat.^{Reference Misono and Merati5,Reference King and Simpson2,Reference Kwon and Buckmire6–Reference Lagier, Polle-Pelletier, Giovanni and Marie8} Injecting material into the vocal fold should improve glottal closure, and consequently improve patients’ voice. Other important features include easy application, low cost and appropriate duration of action (12–36 months).^{Reference King and Simpson2,Reference Simpson and Amin9–Reference Mallur, Morrison, Postma, Amin and Rosen11} The ideal injectable would also have similar biomechanical properties as the vocal fold component that is augmented.^{Reference Kwon and Buckmire6} None of the materials currently available for injection augmentation have all the characteristics desired of an ‘ideal’ injectable material.^{Reference Simpson and Amin9,Reference Lagier, Polle-Pelletier, Giovanni and Marie8}

Injectable materials differ in their physicochemical structure and their rheological properties.^{Reference Sundaram, Voigts, Beer and Meland12} The degree of viscosity and elasticity affect both the stiffness of the material during injection and its ability to maintain its shape under the tension of vocal fold movement.^{Reference Sundaram, Voigts, Beer and Meland12,Reference Kimura, Mau and Chan13} The rheometric measurement of viscosity and elasticity under conditions that simulated vocal fold movement was shown to differ by 3 per cent between bovine collagen, Cymetra®, calcium hydroxyapatite and hyaluronic acid, and to differ substantially from the vocal fold cover.^{Reference Kimura, Mau and Chan13} No data are currently available regarding whether the different rheological properties of an injectable material influence the improvement in voice following injection augmentation of the vocal folds.

Voice improvement after injection augmentation of the vocal folds (Table 1) has been reported for collagen,^{Reference Kimura, Nito, Sakakibara, Tayama and Niimi14–Reference Hoffman, McCabe, McCulloch, Jin and Karnell16} hyaluronic acid,^{Reference Szkiełkowska, Miaśkiewicz, Remacle, Krasnodębska and Skarżyński7,Reference Upton, Johnson, Zelazny and Dailey17,Reference Wang, Chang, Jiang, Lai, De Virgilio and Wang18} carboxymethyl cellulose^{Reference Mallur, Morrison, Postma, Amin and Rosen11} and ArteSense®.^{Reference Jang, Lee, Ahn and Son19} Vocal improvement was also reported with the long-standing injectables calcium hydroxyapatite^{Reference Rosen, Gartner-Schmidt, Casiano, Anderson, Johnson and Remacle20–Reference Amin25} and autologous fat.^{Reference Laccourreye, Papon, Kania, Crevier-Buchman, Brasnu and Hans26} Other reports that investigated a few injectables together showed improvement of the voice^{Reference Carroll and Rosen27–Reference Fritz, Peng, Born, Cerrati, Verma and Wang29} and glottal closure,^{Reference Andrade Filho, Carrau and Buckmire30} and an increased likelihood of undergoing permanent medialisation laryngoplasty.^{Reference Yung, Likhterov and Courey31} Comparable voice improvement was shown following the use of hyaluronic acid versus collagen,^{Reference Hertegård, Hallén, Laurent, Lindström, Olofsson and Testad32} and following autologous fat versus calcium hydroxyapatite^{Reference Zeleník, Walderová, Kučová, Jančatová and Komínek33} for vocal fold augmentation. A meta-analysis of voice outcome with calcium hydroxyapatite versus silicone thyroplasty also showed comparable results.^{Reference Shen, Damrose and Morzaria34} While both carboxymethyl cellulose and calcium hydroxyapatite are established vocal fold injectables, no published research has compared the two in terms of either voice improvement or action duration.

Table 1. Vocal improvement after vocal fold injection augmentation – literature review

*Exact distribution of direct laryngoscopy and in-office injection augmentation not specified. GRBAS = grade, roughness, breathiness, asthenia and strain scale; MPT = maximal phonation time; MFR = mean flow rate; S/Z ratio = standard test of vocal function (obtained by timing the longest duration that a patient can sustain the individual phonemes /s/ and /z/); VHI = Voice Handicap Index; GFI = Glottal Function Index; DSI = dysphonia severity index; CMC = carboxymethyl cellulose; HNR = harmonic-to-noise ratio; CaHA = calcium hydroxyapatite; CAPE-V = Consensus Auditory-Perceptual Evaluation of Voice; VPQ = voice performance questionnaire; VAS = visual analogue scale

When we first started treating patients with injection augmentations at our centre, we preferred to assess the success of vocal fold injection augmentations using a short-action injectable, namely carboxymethyl cellulose. Once the effect of the carboxymethyl cellulose injection wore off, the same patients were injected with calcium hydroxyapatite. Only after gaining experience did we eventually feel confident in using a long-action injection augmentation, namely calcium hydroxyapatite, for the first injection. This scenario of events created a research opportunity: specifically, the possibility of comparing voice improvement after injection augmentation using carboxymethyl cellulose or calcium hydroxyapatite in the same group of patients.