The growing focus on cosmetic results and progress in medical technology has accelerated the development of minimally invasive procedures. Compared with traditional open thyroid surgery, endoscopic thyroidectomies via various approaches have gained universal favor since they were employed to bypass the anterior cervical incision, including subclavian (1), axillary (2–4), breast (5), oral (6), and hybrid of oral and submental approaches (7). Nonetheless, for these conventional endoscopic procedures, carbon dioxide (CO₂) insufflation for maintaining the working space remains an inherent limitation, since it may cause CO₂ embolism, pneumothorax, and subcutaneous emphysema (8, 9). CO₂ embolism can be fatal although it is a rare event (8, 9). To overcome this critical problem, an increasing number of surgeons have attempted endoscopic thyroidectomies without any gas insufflation via various remote access (10–14). However, these remote-access techniques do not comply with truly minimal invasiveness because an extensive dissection of subcutaneous tissue is required to reach the target site, except for gasless transoral endoscopic thyroidectomy via the vestibular approach (TOETVA). TOETVA emerging as a type of natural orifice transluminal endoscopic surgery (NOTES), has gained popularity over a relatively short time due to a more direct surgical route, shorter dissection distance, less invasion, and bilateral thyroid gland exposure (15, 16). However, some unavoidable weaknesses of TOETVA, including discomfort at the chin and lower lip caused by mental nerve injury and flap dissection, the difficult establishment of operative space, and the limitation of large tumor intact extractions via the chin due to anatomic obstacles, cannot be ignored. To overcome the above problems of TOETVA, a novel technique called a hybrid transoral and submental thyroidectomy (TOaST) seemed to have been preliminarily confirmed to simplify the TOETVA procedure and achieve aesthetic results equal to those of TOETVA, but it did not use the gasless insufflation technique (7). Use the gasless insufflation technique.

In this study, we designed an innovative retractor-suspension method to create a surgical space to accomplish gasless submental-transoral combined approach endoscopic thyroidectomy (STET), with the main advantage of preventing mental nerve injury and avoiding CO₂-related complications. Here, we attempted to present our preliminary experience and discuss the feasibility, and safety of gasless STETs.

To meet the demand for cosmetic incisions, the safety, efficacy and feasibility of minimally invasive thyroidectomy via various accesses have been evaluated and verified since the first endoscopic thyroid surgery was performed by Hüscher et al. in 1997 (21). Among these remote-access approaches for endoscopic thyroidectomy, the TOETVA and TOaST passing through the natural space are preferred because of the additional trauma to the patient associated with other approaches. However, an endoscopic thyroidectomy regardless of the approach requires air insufflation to retain the operating space. In the process of establishing a workspace with blunt instruments, some small vessels would inevitably be damaged. As such, there is a higher likelihood of CO₂ gas entering the circulatory system with life-threatening consequences, including subcutaneous emphysema, pneumothorax, pneumomediastinum, and CO₂ embolism (8, 9). Video-assisted neck surgery (VANS) using a subclavian approach was proposed as early as 1998, which did not need air insufflation and did not result in CO₂-related complications (22, 23). In recent years, the gasless TOETVA has been reported including ours (14), while TOaST without CO₂ insufflation has not been studied. Nevertheless, gasless TOETVA still has unique drawbacks that are inherent to the approach because of the special anatomical structure of the oral vestibule. First, the mental nerves have a branch-like distribution on the right and left sides of the oral vestibule. The position of the midline intraoral incision and two lateral incisions described previously in TOETVA has a significant risk to lead to a high incidence of mental nerve injury with long-term sensory loss to the anterior chin (16). Second, in our experience, prolonged compression of the fixed chin by a 10-mm trocar in the observation port and the instrument leverage on the mandibular prominence possibly cause chin pain and discomfort in many patients. Furthermore, some patients report residual pain and tightness, which may be associated with the extent and range of the flap dissection (24). Third, creating a surgical space in TOETVA requires premandibular dissection and separation of the soft tissues from the bone, which is difficult and prone to the complication of skin perforation, especially in patients with prominent chin projection and submental depressions. Fourth, the circuitous route from the access point to the target area and the rather small space in the central region of the mandible may lead to an increased risk of capsular disruption during tumor extraction.

In this study, we develop the gasless STET with a novel suspension system intending to thoroughly address the risks associated with CO₂ insufflation during the procedure and the series of drawbacks caused by inherent to the TOETVA while retaining the original benefits of TOETVA. We use the suspension retractor by attaching it to the anesthetic frame via a sterile bandage to eliminate complicated installation steps and reduce the cost of purchasing special equipment. In addition, it helps us prevent CO₂-related complications, such as hypercapnia, tumor implantation, and even gas embolism. Smoke and water mist from energy instruments during operation could be continuously removed by a suction device on the suspension retractor to provide a clear surgical field of view for gasless STET. Less wiping of the endoscope lens also helps us reduce the procedure time. In summary, compared with the previous retraction technique involving percutaneously placed stitches in both TOETVA and TOaST (7, 25, 26), this novel suspension system allows for a more stable, sufficient and clear space for endoscopic thyroidectomy.

In addition, damage to the mental nerve is a TOETVA-unique complication, resulting in varying degrees of numbness around the chin. It has been demonstrated that the incidence of nerve injury can be effectively reduced by moving bilateral 5-mm incisions more lateral to the canines and closer to the free edge of the lower lip (17). Daqi et al. determined the location and distribution pattern of the mental nerve structures in relation to the oral vestibular incisions by cadaver dissection and showed that standard bilateral 5-mm incisions were safe for determining mental nerve integrity, while the transverse incision in the central oral vestibule was prone to injury of the branch of the mental nerve at a rate of 12–25% (18). Hence, the transverse incision of the oral vestibule is recommended to be moved to the submental area, away from the mental and other facial nerve branches, which theoretically reduces the probability of nerve injury. Additionally, by changing the position of the median transverse incision, long-term compression of the lower lip and chin nerve branches by the trocar can be avoided and the extent of flap dissection can be shortened, which is likely to further reduce postoperative discomfort. In this study, mild abnormal sensation of the lower lip occurred only in three patients (3/75, 4%) postoperatively, and all returned to normal one month later. This may have resulted from compression of the lip by trocars rather than injury to the mental nerve.

The process of surgical space establishment via the submental approach is not prone to the complication of skin perforation, even in patients with prominent chins and submental depressions. There were no cases of skin perforation in this study. Thanks to the change in the position of the main incision making it less difficult to establish the surgical space, our surgery time was even shorter at 141.1 ± 34.8 minutes for lobectomy + CND measured from the time of incision to the completion of skin suture. As the operators become more proficient at this procedure, the surgery time is expected to be further reduced.

Follicular lesions require evaluation of the intact tumor capsule to determine if there is evidence of invasion to diagnose malignancy, in which case tumor complete extraction is critical. When necessary, it is simpler to extend the submental incision laterally than using the transoral vestibular incision without concerns about damaging the mental nerves. Furthermore, the submental approach permits the extraction of larger specimens since it avoids the circuitous but required traversal path from the extraction site to the thyroid. As a result, the eligibility criteria for tumor size could be suitably adjusted. In this study, subject to ensuring complete excision and removal of the surgical specimen, the average size of benign tumors was 36.3 ± 11.2 mm, of which the largest tumor diameter was approximately 52.2 mm, and the average size of malignant tumors was 8.2 ± 4.9 mm, of which the largest tumor diameter was 3 cm.

In terms of cosmetic results, the submental incision was more discreet and less likely to cause scarring as the skin at the submental area was looser and located in the blind spot of the front view. No patient complained of not achieving the desired cosmetic results.

We found gasless STET is acceptably safe and feasible, meeting the criteria for open surgery and it provides bilateral access to the thyroid lobe as does TOETVA. Since we started gasless STET, there were 59 lobectomies with or without central neck dissection, and 16 total thyroidectomies with unilateral or bilateral central neck dissection, none of which were converted to conventional open surgery. All patients in our group had complete tumor resection and thorough lymph node dissection since the gasless STET permitted total-field exposure of the central region and posterior sternal lymph nodes. Our data presented a higher average number of totally resected lymph nodes, up to 7.2 ± 4.5 (27–29).

For surgical complications, the probability of occurrence of any complication was relatively low. As would be expected, there were no instances of permanent recurrent laryngeal nerve paralysis or hypoparathyroidism. Temporary hypoparathyroidism occurred in two patients, both of whom underwent total thyroidectomy and bilateral lymph node dissection in the central region, and both of whom had bilateral inferior parathyroid gland transplants intraoperatively. We recommend preserving the parathyroid glands in situ whenever possible, although these annoying symptoms tend to improve over time. One patient had a mild postoperative chylous fistula that improved after adjustment to a fat-free diet. Notably, gasless STET is no longer a traditional sterile incision; thus, it requires preoperative prophylactic use of antimicrobial medication and intraoperative disinfection, similar to the transoral vestibular approach. There was one case of subcutaneous redness and infection in this study, which was resolved with antimicrobial therapy. Additionally, during our average follow-up period of 13.6 ± 0.3 months, one patient was found to have a suspicious recurrence of lateral neck lymph nodes by ultrasound at the 6-month postoperative follow-up, and the postoperative pathology of this patient indicated a tumor size of 3 cm and 10 metastatic lymph nodes in the central compartment. Further studies on the efficacy of tumor treatment are needed because of the short follow-up period.

Our gasless STET still has several minor drawbacks. Specifically, gasless STET is somewhat restrictive in terms of who it is suitable for. In the premise of not having a scar constitution, patients with a prominent chin or those without a double chin are usually our preferred option for inclusion. If male patients have beards, then this procedure is more friendly to them. However, in others, especially patients with scarring, the incision in the submental area may not be completely hidden as in the TOETVA, thus affecting the aesthetics. Additionally, given the limited number of patients included in this study and the short follow-up period, a larger sample size and longer follow-up results are needed to further validate the importance of this surgical technique.

In conclusion, gasless STET using the innovative suspension system was shown to be a safe, feasible, and sensible alternative for selected patients. It has the potential to compensate for the shortcomings of mental nerve injury and gas embolism without sacrificing good cosmetic results to a certain extent, making it a new surgical approach for clinical application.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Zhejiang University of Medicine School Sir Run Run Shaw Hospital. The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

DZ contributed to the conception and design of the study. LG and LX organized the database. GH, JC, JL, XL, and XJ performed the statistical analysis. JJ wrote the first draft of the manuscript. DZ and JJ contributed to the manuscript revision. All authors contributed to the article and approved the submitted version.