Introduction

Unilateral vocal cord paralysis is the most common clinical glottic insufficiency disease, mainly showing hoarseness, aspiration, cough and other symptoms that can have a profound effect on the personal and professional life of the patient [1]. Treatment of unilateral vocal cord paralysis aims to eliminate aspiration and improve voice quality and ease of phonation. Before the operation, patients should accept voice modification treatment if voice modification treatment cannot recover effective voice and swallow functions. Surgical techniques usually include laryngoplasty, laryngeal framework surgery and laryngeal nerve transplantation. Since its introduction by Isshiki et al. [2] in the 1970s, thyroplasty has become an increasingly popular method for correcting unilateral vocal cord paralysis. Thyroplasty can improve the vocal quality by inserting an implant to adjust the degree of medialisation. However, despite its benefits, thyroplasty with implant has several disadvantages in both technique and implant material [3]. In response to the disadvantages and limitations, thyroplasty has been improved constantly [4–9]. Recently, Schneider-Stickler et al. [10] developed an adjustable laryngeal implant made of titanium for permanent and precise medialisation of paralysed vocal cords through long-term observation. Thyroplasty implant systems have attempted to standardise the shape and size of laryngeal implants to reduce trauma and surgical time.

This study aims to demonstrate a successful external vocal fold medialisation with a new titanium thyroplasty implant that can be adjusted by a screw, ensuring an uncomplicated application and sufficient fixation in the thyroid cartilage. This method offers the advantage of adjusting the depth of the titanium screw to allow arytenoid adduction. Through this approach, optimal medialisation of the vocal fold and maximum glottal closure can be achieved.

Materials and methods

Design of the adjustable titanium screw and the principle of adjusting phonation

The adjustable titanium screw was designed to allow arytenoid adduction and internal rotation simultaneously according to the principle of arytenoid movement during phonation. This design enables the optimal medialisation of the vocal fold and achieves maximum glottal closure. The screw is composed of a titanium plate and a titanium nail (Fig. 1a). The titanium screw was 15 mm in length (depending on the anatomy of the canine laryngeal) and 2 mm in diameter. The titanium plate was designed into a U-shaped bayonet, with the apical view of the square being 4 mm × 4 mm. Screw holes (1 mm thickness and 2 mm diameter) were placed at the centre of both layers of the titanium plate. The mouth of the U-shaped titanium plate was stuck upside on the 2/3 of the inferior edge of the lateral thyroid cartilage plate and fixed on the inferior edge of the thyroid cartilage plate. The titanium nail was screwed in the lateral thyroid cartilage through the outer U-shaped plate and then screwed through the inner hole of the U-shaped plate (Fig. 1b–e), with its top attached to the fovea oblonga of the arytenoid cartilage. The depth of the screwed titanium nail was used to control the degree of arytenoid adduction and internal rotation to control the degree of glottis closure (Note: The preparation of the titanium nail was assisted by the Heavy Metal Material Department of Tongji University in Shanghai).

Fig. 1
figure 1

The configuration of adjustable screw and appearance of the adjustable screw fixed on the canine laryngeal. a Adjustable screw, b before screw implanting, c after screw implanting, d the process of screw implanting, e titanium implant fixed on thyroid cartilage

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Animals and treatment

Six experimental Beagle dogs, irrespective of gender, exhibiting loud bark and with body weight of 10–13.5 kg, were randomly divided into experimental and control groups. The left recurrent laryngeal nerve of the dogs was cut off to prepare the left vocal cord paralysis animal model. The experimental group underwent adjustable titanium metal stent implantation to make the left vocal cord adduct; no other operation was carried out in the control group. This study was carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. The animal use protocol was reviewed and approved by the Institutional Animal Care and Use Committee of Second Military Medical University, Shanghai.

Details of the operation

The dogs underwent preoperative fasting for 6 h and were intraperitoneally injected with anaesthesia with 3 % sodium pentobarbital at 30 mg/kg body weight. Each experimental dog underwent anterior median incision, separating the cervical strap muscles to find the left recurrent laryngeal nerve in the tracheoesophageal groove. The left recurrent laryngeal nerve was cut off to make the unilateral vocal cord paralysis animal model. The experimental group underwent left vocal cord medialisation thyroplasty operation and was implanted with adjustable titanium metal stents; no other operation was conducted in the control group after cutting off the left recurrent laryngeal nerve.

Observation after the operation

The subjects immediately underwent postoperative video laryngoscopy (Olympus, type 240) after the operation. One dog in each group underwent video laryngoscopy, laryngeal CT scanning (SOMATOM Definition AS +) and 3D reconstruction at 1, 2 and 4 months postoperation. The subjects were killed painlessly after the examination, and laryngeal specimens were collected for pathological examination.

Results

Video laryngoscopy

Immediately after the operation, the left vocal cords of three dogs in the experimental group shifted to the median position, and the glottis could properly close during phonation. The left cord of two dogs had slight oedema, whereas the left vocal cord of one dog had no oedema. The left vocal cords of three dogs in the control group were fixed in paramedian position, but with no oedema; the glottal closure had obvious gap during phonation. Similar manifestation was observed after 1 and 2 months. Specifically, the left vocal cords of the experimental group dogs had no oedema in the median position, with good glottal closure when phonating (Fig. 2a–d). The left vocal cord of the three dogs in the control group had no oedema and was fixed in median position, with right vocal cord compensatory, but with obvious gap (glottal closure Fig. 3a, b). Four months after the operation, the left vocal cords of the two groups were slightly atrophic. In addition, the position and closure effect exhibited similar trends 1 month after the operation.

Fig. 2
figure 2

1 and 2 months postoperation position of vocal cord in experimental group after implant of titanium adjustable screw during inspiration and phonation. a Inspiration 1 month postoperation, b vocalisation 1 month postoperation, c inspiration 2 months postoperation, d vocalisation 2 months postoperation

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Fig. 3
figure 3

Movement of vocal cord in control group during inspiration and phonation, 2 months after cut of left recurrent laryngeal nerve. a inspiration, b phonation

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Laryngeal CT scanning

After 1, 2 and 4 months, the CT scan showed roughly the same result, i.e. the laryngeal tissue of the experimental group dogs had no abnormal density changes. Moreover, the implant position was incumbent; no abnormal change of laryngeal tissues, such as thyroid cartilage plate and annular cartilage, was observed; the implant was also not expelled into the laryngeal cavity. After 4 months, the CT scan showed shadow formation around the implantation site of the adjustable titanium stent (Fig. 4a), which may affect the interpretation of the meticulous structure to some extent. Nevertheless, the 3D reconstruction image (Fig. 4b) can show clearly the position of the titanium metal stent within the laryngeal tissue, as well as the angle of the titanium nail. The imaging showed that the titanium screw was fixed properly, and the direction of the titanium screw tip pointed to the location of the arytenoid cartilage from the angle of the implant.

Fig. 4
figure 4

Expression of CT in 4 months after implantation of adjustable screw. a CT scan, b 3-D CT

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Laryngeal biology observation

General observation

At each time point, the laryngeal specimens of the two groups had normal colour and had no tissue necrosis, decay and deterioration. In the experimental group, the titanium stent was in the vocal plane, and the implant was exactly fixed at each time point, with no abnormal shifting and no implant titanium stent expelling out. After 1 month, the muscle at the titanium stent implantation site of the lateral thyroid cartilage plate had slight congestion and swelling. After 2 months, the congestion and swelling subsided. Scar tissue hyperplasia was observed after 4 months.

Observation under the microscope

After 1 and 2 months, no significant interface bone cortex thickening, no obvious hyperplasia of collagen in muscle fibre tissues, no muscle fibre atrophy, and no tissue necrosis, haemorrhage, and other performances were observed. After 4 months, bone cortical thickening was noted at the interface of the cartilage and the implant. Moreover, collagen hyperplasia and a few lymphocyte infiltrations were observed in the muscle fibre tissue around the implant. Four months after surgery, thyroarytenoid muscle exhibited slight atrophy at the operation side of the two groups (Fig. 5).

Fig. 5
figure 5

Histopathology expression, 4 months after implantation of adjustable screw (Scale bar 50 μm)

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Complications

During the 4-month follow-up after the operation, we observed that the wound had no infection, and the incision healed well. In addition, no difficulty in swallowing, breathing, eating aspiration and complications, such as implant rejection, were observed.

Discussion

Unilateral vocal cord paralysis, which is common in clinics, is mainly induced by trauma, surgery and tumour [11, 12] and can result in obvious hoarseness, aspiration, and other symptoms. Unilateral vocal cord paralysis can also cause aspiration pneumonia because of aspiration. The treatment for this disease includes intracordal injection [13], thyroplasty, arytenoid adduction and nerve grafting [14]; among which, thyroplasty is the most widely used method for treatment. Vocal cord movement is associated with the movement of the arytenoid cartilage, which is completed by the coordination of the thyroarytenoid muscle, lateral cricoarytenoid muscle, posterior cricoarytenoid muscle and interarytenoid muscle, whose basic forms are rotating, turning and moving. Type I thyroplasty allows the vocal cord plane to close only to the median but disallows arytenoid cartilage shift. Therefore, the procedure is not ideal to patients with large glottic posterior chink or vocal cord plane dislocation [15]. Clinical treatment for unilateral vocal cord paralysis often includes the arytenoid adduction surgery to narrow the glottis [16].

Arytenoid adduction surgery allows thyroarytenoid muscle tension to a certain extent because of vocal process adduction. This method also allows two vocal cords in a plane during phonation, but disallows interarytenoid muscle internal shift. The phonation quality depends on the compensation function of the contralateral vocal cord. However, the incision of arytenoid adduction surgery is large, and the range of its soft tissue injury is wide. To position the cricoarytenoid joint, this method needs to expose the posterior thyroid cartilage and the cricothyroid joint. Partial resection of the thyroid cartilage also needs to be conducted when necessary. However, this procedure may damage the laryngeal mucosa and lead to pyriform sinus perforation because the postoperative structural stability of the laryngeal cartilage framework may be affected. Local soft tissue injury may also cause local haematoma, oedema and other postoperative complications, which can cause difficulty in breathing. Moreover, discomfort on the part of patients may increase after the arytenoid cartilage is fixed [17]. Arytenoid adduction is a difficult surgical procedure [18, 19], and possible serious complications limit its wide application in clinics.

Since Friedrich first reported the use of titanium implants for thyroplasty [20], the titanium vocal fold medialising implant has been widely used in recent years. The major advantage of using this implant is the significant reduction of operative time in performing the implant.

This study used an adjustable titanium screw to fix the metal plate on the thyroid cartilage plate. Internal shift can then be carried out through the pushing force of the metal screws on the vocal process of the arytenoid cartilage to achieve vocal cord adduction. This method can be carried out with the use of local anaesthesia. According to phonation, intraoperative adjustment can be conducted on the position of the arytenoid cartilage through the screw to achieve the best phonating effect. Although such an adjustable titanium implant has already been reported [9], the shape of the implant and implant site are different with the one reported in the current study. Devos’s study aimed to describe a new porous titanium thyroplasty implant that can be adjusted with a screw, whereas the current study investigated the feasibility of arytenoid adduction by fixation of a surgical screw to the arytenoid cartilage by transthyroidal method. The new material for laryngeal stent implantation operation must shorten the surgical time and reduce the occurrence of complications [21]. On one hand, the application of the metal stent is similar to type I thyroplasty, which aims to avoid too much damage on the cervical tissue. In addition, repeated intraoperative removal of material size can be avoided when the metal stent is applied because the material is adjustable, thereby simplifying the surgery and shortening the surgical time. On the other hand, this method can narrow the glottis gap in the same way as that with arytenoid cartilage adduction surgery. The proposed method also can improve the quality of phonation because the titanium metal is compatible with the human body and avoids rejection, infection and other complications.

Video laryngoscope results showed that the adjustable titanium screw can effectively make the experimental paralysed vocal cord adduct and shift to the median immediately and 4 months after the implantation. These effects are beneficial to glottal closure. The titanium stent was also adjustable, thereby allowing the paralysed vocal cord to shift to an optimal position and enabling the vocal cords to achieve maximum tension and glottal closure. This study observed the postoperative titanium stent position of the experimental dogs through laryngeal CT scan and observed the position of the titanium stent in the laryngeal tissue and the position of the front of the screw nail. CT scanning results showed that the self-made adjustable titanium stent can be reliably fixed. No implant rejection, shedding, shifting and other complications were observed after 4 months of follow-up.

Titanium screw is widely used in orthopaedics, neurosurgery and maxillofacial surgery departments [22], as well as in laryngeal stent for the treatment of vocal cord paralysis [8–10]. The safety and tissue compatibility of the titanium screw is good, and no infection and rejection caused by the implant titanium screw or plate have been reported. Therefore, this study utilised titanium alloy to create an adjustable titanium screw. The material design and operation mode were improved, and the method used was simple, rapid, reliable, effective and convenient. In this study, the results of the laryngeal gross observation and microscope observation of the experimental group 4 months after surgery showed that the titanium stents were located at the vocal cord plane. No abnormal shift and expelling of the implant titanium stent were observed. In addition to scar tissue hyperplasia around the implant site, bone cortex thickness at the contact surface, hyperplasia of collagen muscle fibre tissue and a few lymphocyte infiltration around the implant, no infiltration of neutrophils, tissue necrosis, haemorrhage and other complications have been reported. The results confirmed that the designed and prepared titanium screw has good histocompatibility and showed that the adjustable titanium stent can meet the basic requirements of laryngeal stent material selection for clinical applications.

Although the adjustable titanium screw can be effectively used in the treatment of unilateral vocal cord paralysis, the advantages of this implant over the traditional laryngeal stent operation (e.g. thyroplasty and arytenoid adduction) remain unclear. Moreover, problems in clinical application still need to be solved, including the location of the thyroid cartilage plate fenestration and the selection of titanium screw implant angle and direction.