Background: Neck surgery is one of the newest fields of application of minimally invasive surgery. The technique of minimally invasive video-assisted thyroidectomy (MIVAT) developed by Miccoli  is the method that has so far become most widespread. Limiting factors of this method include the bothersome 20-mm cervical incision and consequently the specimen size to remove. Several papers describing an access outside the front neck region have been published. Such approaches are via the chest, axillary, a combined axillary bilateral breast, or a bilateral axillary breast approach [2-5]. The development of cervical scarless thyroid surgery is a great step toward better cosmetic outcomes. However, these techniques just moved the scars from the front neck region to the axilla or the chest where they are still visible. And the mentioned minimally invasive accesses as well as the conventional approaches to the thyroid gland do not respect the anatomically given surgical planes. This may result in complaints by the patients, e.g., scar development and swallowing disorders. Furthermore, the extracervical approaches do not comply with the use of the term "minimally invasive," because they are associated with an extensive dissection of the chest and neck region, thus being rather maximally invasive for the patients. The main goal of this project was the introduction of a technique of thyroid resection that fulfills the following criteria: (i. Respecting surgical planes and minimizing surgical trauma in thyroidectomy, ii. The access itself should be close to the thyroid gland to achieve a minimally invasive procedure, iii. Achieving an optimal cosmetic result may only be obtained by performing a scarless operation, iv. This optimal cosmetic result with scarless surgery should be achieved with minimal trauma, v. The minimally invasive character of this approach and the optimal cosmetic result may not be reached at the expense of patient's safety.). The technique that meets all of these criteria is the transoral access because the distance between the sublingual place and the thyroid gland is short, thus avoiding extensive dissection maneuvers. Furthermore, the mouth mucosa can be sutured without difficulties and repairs itself without leaving any visible scars. Feasibility of the transoral access has been recently demonstrated by a member of our group in a porcine model by using a modified axilloscope . However, the described technique is a hybrid one because an additional medial access (3.5-mm incision) 15-mm below the larynx was necessary for the insertion of a fixation forceps through a trocar. The main goal of our project was the investigation and introduction of a technique of totally endoscopic thyroid resection that is minimally invasive and safe for the patient and at the same time cosmetically optimal (scarless).
Methods: For this purpose, a total of five human cadavers were used. In three cadavers, safety and reproducibility to reach and resect the thyroid gland was assessed according to a defined road map. At the end of the procedure, the cadavers were dissected to evaluate all defined anatomical key structures regarding possible injuries and also allow an evaluation of the surgery performed. The TOVAT itself was performed on two more human cadavers with the help of one 5-mm and two 3-mm trocars that were introduced through the mouth floor and the vestibulum of the mouth subplatysmal. A working space was created by insufflating CO(2) at a pressure of 4-6 mmHg ("air dissection"). Surgical dissection of the further working space was realized with 3-mm bipolar scissors. The procedure consists of the following steps: (i. Patient in supine position and nasotracheal intubation, ii. 5-mm small incision between the carunculae sublinguales, iii. Penetration through the mouth floor along the superficial fascia colli with a blunt instrument, iv. Insertion of a 5-mm trocar, v. Blunt dissection subplatysmal by CO(2) insufflation ("air dissection"), vi. CO(2) insufflation (4-6 mmHg) and creation of a working space, vii. Insertion of two 3-mm trocars in the vestibulum oris on the right and left side, viii. Separation of the platysma from the strap muscles approximately at level of the larynx, extending up to the suprasternal notch. Laterally, this dissection can be continued up to the medial border of the sternocleidomastoid muscles, ix. Division of the linea alba coli and exposure of the strap muscles, x. Separation of the strap muscles from the thyroid gland, xi. Isthmus transection and blunt dissection of the thyroid gland from the trachea, xii. Dissection and division of the upper pole arteries and medial thyroid vein closely to the gland, xiii. Division of branches of the inferior thyroid artery closely to the gland, xiv. If necessary, preparation of the retro-thyroidal area, including visualization of the recurrent laryngeal nerve, xv. Thyroid resection from cranial to caudal and transoral removal of the specimen through the 5-mm midline incision. If the gland is too large, the midline incision can be extended longitudinally, xvi. All three incisions are closed with absorbable sutures.)
Results: Description of landmarks of surgical steps and dissection of defined anatomic structures could be achieved. The subplatysmal space could be reached without any major problems within a short time. Anatomical dissection showed intact muscles and vascular structures. One-side subtotal thyroid resection could be successfully performed without any additional skin incision in 60 minutes.
Conclusions: The minimally invasive aspect and the scarless character of TOVAT form the rationale for the preclinical investigation of this method in human cadavers. We could succeed in defining objective parameters, which describe the procedure in details and also allow an evaluation of the surgery performed. Access and feasibility of TOVAT could be demonstrated. The next step will be its application in living pigs before it may be applied in humans. To our knowledge of the literature, this is the first report on NOS application in thyroid surgery and also the first totally and scarless performed video-assisted thyroidectomy.