The cervical approach involves three to four small ports including a 12-mm endoscopic port and two to three operating instrument ports in the anterior or lateral neck (Fig. 1A) [1,3]. The working space is maintained by CO₂ insufflation at low pressure. MIVAT is a direct access method using a 1.5–2 cm central neck incision without CO₂ insufflation (Fig. 1B) [2]. The main advantage of MIVAT is that it is a minimally invasive procedure rather than that it improves cosmesis because it ultimately leaves a scar in the neck although one that is smaller than in the conventional procedure. Many workers have reported that MIVAT causes less pain and results in better cosmesis than conventional thyroidectomy [4].

The anterior chest approach with CO₂ insufflation involves three small ports on the anterior chest wall, comprising a 12-mm endoscope port and 5-mm ports for endoscopic instruments (Fig. 1C) [5]. A gasless anterior chest approach has also been developed using three ports on the anterior chest wall and the cervical region-lifting device without CO₂ insufflation [6]. In another gasless anterior chest approach, the working space is created by CO₂ insufflation, and an external retractor system is used to maintain it [7]. Video-assisted neck surgery involves a type of anterior chest approach or infraclavicular approach (Fig. 1D) [8]. A 3–4 cm main oblique incision is made in the anterior chest wall below the clavicle, and a 5-mm incision is made in the lateral neck for inserting a 5-mm endoscope. After a skin flap has been dissected, two pieces of Kirschner wire are used to maintain the cervical working space without CO₂ insufflation.

The axillary approach with CO₂ insufflation was developed by Ikeda et al. [9] in 2000, and uses three axillary incisions (Fig. 1E). A gasless transaxillary approach was developed later by Chung et al. [10,11] and has gained in popularity. Initially the gasless transaxillary approach used a 6-cm axillary incision and one small anterior chest port (Fig. 1F), but it has progressed to using a single axillary incision without an anterior chest port (Fig. 1G) [12]. The gasless trans-axillary approach has also been modified in the form of a gasless unilateral axillary (GUA) approach (Fig. 1H) and a gasless unilateral axillo-breast (GUAB) approach (Fig. 1I) by Tae et al. [13-16]. The GUAB approach makes use of a small breast port around the areola in addition to the axillary incision. The breast port provides a wide angle between the robotic or endoscopic instruments, making it easier to manipulate the instruments and avoid collisions. However, the robotic GUAB approach has evolved into the GUA approach without a breast port, for better cosmesis.

The gasless transaxillary approach is generally used for remote access thyroidectomy because of the good exposure and excellent surgical view of the thyroid gland and lateral neck. Also, total thyroidectomy and bilateral central neck dissection is possible through a unilateral axillary approach if it is performed by experienced surgeons, although dissection of the contralateral thyroid lobe and recurrent laryngeal nerve (RLN) is a little more difficult.

The breast and axillo-breast approaches

The breast approach using CO₂ insufflation was developed by Ohgami et al. in 2000 (Fig. 1J) [18]. This approach uses two breast ports and one parasternal port [18,19] and the scar in the parasternal area tends to hypertrophy. Therefore, various modifications have been developed to avoid a parasternal port by adding one or two axillary ports. The ABBA was developed by Shimazu et al. [20], and uses two breast ports and an axillary port (Fig. 1K) [20]. The unilateral or bilateral axillo-breast approaches with CO₂ insufflation use one breast port and two axillary ports on one or both sides (Fig. 1L) [21]. The BABA was developed and popularized by Youn et al [22]. The BABA is a modification of the ABBA with an additional axillary port. It requires four incision sites, two in the areola, and an incision in each axillary area (Fig. 1M).

Facelift (retroauricular) thyroidectomy was first developed by Terris et al. [24] using a surgical robot (Fig. 1N). However, it was mainly popularized by the Korean group, including Koh, Jung and Tae [25-27]. The postauricular facelift approach employs postauricular and occipital hairline incisions, and is familiar to head and neck surgeons as it is widely used in parotidectomies and to excise submandibular gland and neck masses [31-34]. It allows a smaller dissection area and a shorter distance from the incision site to the thyroid gland than the transaxillary approach [35]. Postoperative cosmesis is good, and the scar is concealed beneath the auricle and hair, particularly in female patients. The disadvantages of this approach are the narrow working space and the difficulty in approaching the contralateral thyroid lobe via the unilateral incision [27]. A contralateral postauricular incision is sometimes needed for contralateral lobectomy when the exposure of the contralateral lobe is inadequate.

The transoral approach via a single sublingual route was initiated by Witzel et al. [36] in cadavers and the pig model. It includes a sublingual approach and an oral vestibular approach with or without CO₂ insufflation [37-43]. Wilhelm et al. [40] and Wilhelm and Metzig [41] were the first to perform transoral thyroidectomy in humans, using one sublingual port and two oral vestibular ports (Fig. 1O). The oral vestibular approach was described first by Richmon et al. [42] in a cadaveric model, and uses three oral ports in the oral vestibule. Nakajo et al. [43] also developed gasless transoral video-assisted neck surgery using a single 2.5-cm incision in the oral vestibule.

Initially the transoral approach was considered difficult, due mainly to the limited working space, damage to the structure of the floor of mouth, hypoglossal nerve injury, and mental nerve injury. However, the transoral approach using three vestibular incisions has recently been revisited after a report of the use of the transoral endoscopic thyroidectomy vestibular approach in 60 human cases (Fig. 1P), and it is considered feasible and safe [44,45]. The transoral approach is viewed as a form of true natural orifice transluminal endoscopic surgery, and is less invasive in terms of working space than other types of remote access thyroidectomy. It makes it easier to perform total thyroidectomy than the transaxillary and facelift approaches because it provides midline access to both thyroid lobes. It also provides a top down operative view. Hence, central neck dissection can be performed easily up to level VII.

The patient is placed in the supine position, the neck slightly extended, and the lesion-side arm is raised to expose the axillary fossa. A 5- to 6-cm skin incision is made in the axillary fossa [15,16], and a skin flap is elevated under direct vision in the plane of the subplatysmal layer over the pectoralis major muscle from the axilla to the anterior neck area. The dissection is continued through the space between the two heads of the sternocleidomastoid (SCM) muscle and progressed below the sternothyroid muscle to expose the thyroid gland. Thereafter, an external retractor that is able to maintain adequate working space without CO₂ insufflation is placed. A second 0.5- or 0.8-cm skin incision is made just inferior to the axillary incision to insert a trocar. The purpose of this second axillary incision is to minimize the length of the main axillary incision. Three robotic arms are then inserted through the main axillary incision port. A 30-degree face-down robotic endoscope is placed in the center of the main axillary port, and Prograsp forceps is placed at the top of the working space immediately below the external retractor blade and close to the endoscope. The Harmonic curved shears (dominant hand side) is placed at the right end of the main axillary incision, and a Maryland dissector (non-dominant hand side) is placed at the second axillary incision port in a leftside approach (Fig. 2).

The superior thyroid vessels are cut individually close to the thyroid gland, using the Harmonic curved shears, to preserve the external branch of the superior laryngeal nerve. The superior parathyroid gland is identified and is carefully preserved with an intact blood supply. The thyroid gland is then retracted medially, and the paratracheal lymph nodes and perithyroidal soft tissue are dissected while preserving the whole course of the RLN. The isthmus is divided, and the ipsilateral total lobectomy with central neck dissection is completed. A suction drain is inserted, and the wound closed.

The patient is placed in supine position, with neck extended using a shoulder pillow. Both arms are abducted slightly to insert the trocars [22,46]. Diluted epinephrine solution is injected into the subcutaneous space in both chest and neck to reduce bleeding during the dissection. After making incisions on both upper circumareolar areas, a blunt dissection is performed using a vascular tunneler. The working space is elevated to the level of the thyroid cartilage superiorly and to the medial border of the SCM muscle laterally. After creating an adequate working space, an endoscope is placed through the 12-mm right breast port, and the left 12-mm breast port is used for the operating instruments. Two 5- or 8-mm cannulas are inserted through the axillae (Fig. 3). The working space is maintained using CO₂ insufflation at a pressure of 5–6 mmHg. The midline fascia between the strap muscles is divided, and the isthmus dissected. The thyroid gland is dissected, with preservation of the parathyroid glands and RLNs. The resected specimen is removed in a plastic bag through the 12-mm breast port. The midline of the strap muscles is reapproximated, a suction drain is placed, and the skin is closed.

A skin incision is made in the postauricular sulcus, curved posteriorly at the upper third of the auricle, and continued along the occipital hairline [24,27]. The skin flap is elevated in the plane of the subplatysma over the SCM muscle under direct vision, posteriorly to the posterior border of the SCM muscle, superior to the lower border of the mandible, and inferiorly to the sternal notch. The great auricular nerve and the external jugular vein are identified and preserved. The omohyoid, sternohyoid and sternothyroid muscles are dissected and retracted upwards to expose the thyroid gland, after which an external retractor is placed to maintain the working space. A 30-degree face-down endoscope and three robotic instruments including Maryland dissectors, Prograsp forceps and Harmonic curved shears are inserted through the postauricular incision (Fig. 4). It is usually necessary to retract the SCM muscle and skin flap laterally, using a self-retaining retractor or an assistant to obtain an adequate working space. The parathyroid glands are identified and preserved, and the RLN is identified in the tracheoesophageal groove and preserved. The Berry’s ligament and thyroid isthmus are dissected, and the lobectomy with isthmusectomy is completed. A suction drain is placed, and the wound is closed layer by layer.

The patient is placed in a supine position, and the neck is extended with a shoulder pillow under general anesthesia with orotracheal intubation [47]. A 1.5–2 cm horizontal incision is made at the end of the lower lip frenulum followed by lateral incisions on either side of the central incision, close to the oral commissure so as not to injure the mental nerve. Epinephrine diluted in normal saline is injected into the submental area for hydrodissection. Blunt dissection of the submental area is performed using a tunneler. A 30-degree rigid endoscope is placed in the center, and 5 or 8 mm trocars are inserted on either side of the endoscope for two endoscopic dissectors or monopolar electrocautery. The CO₂ insufflation pressure is set at 5–6 mmHg. A working space is usually created in the plane of the subplatysmal layer by endoscopy without a surgical robot. The skin flap is widened to the level of the sternal notch inferiorly and the SCM muscle laterally. After completing the working space, a 30-degree robotic endoscope and robotic instruments such as bipolar Maryland’s forceps or monopolar scissors are placed on either side of the endoscope (Fig. 5A). If necessary, a third robotic instrument such as Prograsp forceps or Cardinal forceps is inserted through the right axillary port (Fig. 5B). The midline fascia between the strap muscles is divided and the sternohyoid and sternothyroid muscles are dissected to expose the thyroid gland. An external stitch is made to retract the strap muscles laterally for better surgical view. First the isthmus is dissected and divided, and then the thyroidectomy is performed while preserving the RLN and parathyroid glands. Special care must be taken to preserve the RLN when dissecting the Berry’s ligament area. The specimen is removed using a plastic bag via the central oral incision. The divided strap muscles are re-approximated and the surgical wound in the oral vestibule is closed with absorbable sutures; usually no drain is required.

Although there are many reports of the feasibility and safety of remote access thyroidectomy, complication rates are potentially higher, especially during the learning curve and in the case of low-volume surgeons because of the challenging surgical technique. There can also be serious complications such as injury to the esophagus and trachea, compromised airways due to hematomas, and, in rare instances, serious CO₂ embolisms. Strict patient selection criteria are needed for successful surgical outcomes. The safety of patients must be the first priority, especially, during the steep learning curve. An appropriate training program for surgeons is also definitely required [28]. Also, the possibility of conversion to an open procedure should always be discussed with patients before surgery.

In meta-analyses, rates of complications such as RLN paralysis and hypoparathyroidism were not significantly different in robotic and conventional thyroidectomy [54-58]. However, in subgroup analyses, transient RLN paralysis was higher in the robotic procedure than the conventional procedure [55]; RLN injury, in particular, was more frequent early in the learning curve and with low-volume surgeons. Seroma is more often encountered in remote access surgery than in the conventional approach although it is easily controlled in the outpatient clinic by repeated aspiration [15,16].

Unusual complications can also occur. Thus, transient brachial plexus injury has been reported in the robotic transaxillary approach [59], but the risk can be reduced by proper arm and shoulder positioning. Also the marginal branch of the facial nerve can be injured in the postauricular facelift approach, possibly as a result of compression of the nerve by a robotic instrument at the narrow postauricular port [27]. Mental nerve injury can also occur in the transoral approach [39], and CO₂-related complications such as serious CO₂ embolism can arise in procedures using CO₂ insufflation [47].

The operative time of robotic thyroidectomy is significantly longer than that of conventional thyroidectomy due to the longer flap dissection time and the time needed for robot docking [54-58]. There was no significant difference of operative time between the transaxillary and the facelift approach in robotic thyroidectomy [27]. The average increase in operative time of robotic thyroidectomy compared to the conventional transcervical approach was 43.5 minutes [56]. However, operative time did not differ between robotic and endoscopic procedures [56]. The total operative time of robotic thyroidectomy may decrease with experience. Due to the complexities of flap dissection and manipulation of the robotic instruments, learning curve for robotic thyroidectomy via the transaxillary approach comprises 35–50 cases [60,61], and the learning curve of the BABA is about 40 cases [62].

Surgical completeness and number of lymph nodes removed

Complete total thyroidectomy via the unilateral gasless transaxillary and postauricular approaches is difficult because of the inadequate view of the contralateral lobe, and poor access to it. Therefore, the surgical completeness of total thyroidectomy can be an issue in robotic and endoscopic thyroidectomy. Thyrotropin-stimulated thyroglobulin (Tg) levels and radioactive iodine uptake levels are frequently-employed surrogate markers for the surgical completeness of total thyroidectomy. The surgical completeness of total thyroidectomy via the gasless transaxillary approach was similar to that of conventional thyroidectomy if it was performed by experienced surgeons [63]. However, in a meta-analysis, the pre-ablation-stimulated Tg level was significantly higher in robotic thyroidectomy than in conventional controls [64]. Also, the surgical completeness of the BABA was comparable to that of conventional thyroidectomy [65]. In a meta-analysis the number of lymph nodes retrieved in the robotic approach was significantly lower than in conventional thyroidectomy [64].

Oncologic outcome is an important issue in the treatment of thyroid cancer and should not be neglected or overlooked in favor of cosmesis or functional outcomes. However, the literature on oncologic outcomes such as locoregional recurrence and disease survival after robotic or endoscopic thyroidectomy is very limited. In three studies, oncologic outcomes, including diseasespecific survival and recurrence rates, were not significantly different in robotic trans-axillary thyroidectomy and conventional thyroidectomy although the follow-up period was rather short [66-68]. Further studies with long-term follow-up and large patient samples are needed to assess the ultimate long-term oncologic outcomes of robotic/endoscopic thyroidectomy.

Cosmetic and functional outcomes and quality of life

Cosmetic excellence is the most important reason for patients and surgeons to choose robotic/endoscopic thyroidectomy. Cosmetic outcome is indeed superior in robotic/endoscopic thyroidectomy than in conventional surgery [15,57]. Long-term cosmetic satisfaction after scar maturation is also significantly greater in the transaxillary approach than in conventional thyroidectomy [69].

In terms of voice outcomes, robotic thyroidectomy via the gasless transaxillary approach yielded better subjective voice recovery after surgery, and also better results for acoustic parameters of voice pitch than conventional surgery [70,71]. However, one study reported similar postoperative voice outcomes in transaxillary and conventional thyroidectomy [72]. Further prospective studies are needed to assess voice function after robotic and endoscopic thyroidectomy. Postoperative swallowing function after robotic/endoscopic thyroidectomy was also evaluated in some studies [71,73]. However, the evidence is not yet definitive.

Pain and sensory disturbance in the anterior chest area is more severe and lasts longer after gasless transaxillary thyroidectomy than after conventional thyroidectomy [17]. However, this disadvantage can be reduced by minimizing dissection of the anterior chest during flap elevation. Other studies reported no difference of subjective early postoperative pain in the robotic transaxillary approach and the conventional method [74,75].

Health-related quality of life after transaxillary robotic thyroidectomy, including physical, psychological, social, and spiritual well-being, was similar to that of patients undergoing conventional thyroidectomy [76].