Until recently, treatment of obstructive salivary gland disease involved a transoral surgical approach in the case of submandibular stones located in the mid-distal portion of Wharton's duct or the transcervical surgical gland removal in case of deeper stones or unsuccessful conservative treatment such as interventional sialendoscopy (1–6).

Today, the rate of gland removal has been significantly reduced <5% and the development of a variety of minimally invasive techniques has led to a fundamental change in therapeutic perspectives (4, 7): sialendoscopy, alone or in combined endoscopic and incisional approach for larger stones, plays an essential role in the management of salivary gland benign pathology (4, 8). The conservative transoral approach, preserving the Wharton’s duct in its entirety, is used and it is now considered the gold-standard treatment for the treatment of hilo-parenchymal salivary stones (9).

The recent diffusion of transoral robotic surgery has favoured its application not only in the oropharyngeal and hypopharyngeal-laryngeal fields, the anatomical sites of major use of the method, but also in the oral floor for the removal of neoplasms and other pathologies of the salivary glands (10). By this approach, the risk of the most common complications of traditionally performed transcervical surgery is lowered, maximizing the possibilities of gland preservation (11).

Up to date transoral robot-assisted management of submandibular gland stones and transoral robotic submandibular gland removal have been described in case reports and small, heterogeneous series of patients (12–16).

The aim of our study is to describe a different approach to hilo-parenchymal stones of the submandibular gland by means of transoral robotic surgery and to evaluate safety and effectiveness of sialendoscopy-assisted TORSS procedure performed by means of two different robotic system in the treatment of patients with submandibular stones, by presenting out 3-year experience.

The final analysis was based on the findings related to 54 patients including 33 (61.1%) males with a mean age of 53.3 ± 12.6 years (Table 1). All patients showed unilateral submandibular stones.

In most (68.5%) of cases, the stone was hilo-parenchymally located, while 24.1% of the stones were non-palpable pure parenchymal, and the remaining 7.4% were placed at the hilum (Table 1).

Sialendoscopy-assisted TORSS was successful in 44 out of 54 (81.5%) cases: in the remaining 10 cases, complete stone removal was achieved by contextual traditional transoral surgery (8 cases); in the remaining two cases also transoral surgery was ineffective, and then a trans-cervical submandibular sialadenectomy (1 patient) and a trans-oral submandibular sialadenectomy (1 patient) were performed. In one patient, sialendoscopy check at the end of TORSS achieved the visualization of a residual peri-hilar microlith which was successfully retrieved by means of an operative basket.

Considering the characteristics of the stone, sialendoscopy-assisted TORSS was unsuccessful in 7 out of 13 (53.8%) non-palpable pure parenchymal stones compared to 3 out of 41 (7.3%) palpable (hilar/hilo-parenchymal) stones. Among the seven failures occurring into the group of patients with non-palpable pure parenchymal stones, 5 out of 7 (71.4%) were conservatively surgically managed by means of traditional transoral stone removal; in the remaining two cases (2 out of 7; 28.6%) it was necessary to convert to transoral (1 patient) or transcervical (1 patient) submandibular sialadenectomy. All the three failures occurring into the group of patients with palpable hilar/hilo-parenchymal stones (due to partial stone removal during TORSS) were effectively managed by means of traditional transoral stone removal.

With regards the stone size, it is found to be smaller in cases unsuccessfully managed by means of sialendoscopy-assisted TORSS compared to the ones completely removed by means of primary robotic surgery (7.8 ± 1.8 mm vs. 9.8 ± 2.4 mm).

No major complications occurred, as only transient lingual nerve impairment (3 out of 54 patients, 5.5%), infectious sialadenitis (2 out of 54 patients, 3.7%), and mild oral floor oedema (1 out of 54 patients, 1.8%) were reported. Interestingly, all the patients with post-operative transitory lingual nerve impairment had undergone TORSS by means of DaVinci Xi system. However, in all but one case (experiencing mild lingual hypoesthesia till one year after surgery) a complete recovery occurred a few weeks after surgery.

A favourable outcome was also attested in terms of patient's satisfaction and recovery time, as 37 out of 54 (68.6%) patients were discharged the first day after surgery, 42 patients did not report pain on the VAS pain after a period of 30 days from surgery. All the interviewed patients would recommend this surgical procedure to other patients suffering from the same condition.

During the follow-up period (mean 13.4 ± 9.7 months), three out of 54 (5.5%) patients experienced recurrent obstructive sialadenitis without ultrasonographic evidence of residual stones, all but one adequately managed by means of medical treatment alone. In the remaining case, the patient underwent traditional submandibular sialadenectomy. In one patient (1.8%) a residual hilo-parenchymal stone was identified at ultrasonographic follow-up examination and removed by means of traditional transoral surgery.

Recently, transoral robotic surgery (TORS) has become a valuable approach in head and neck cancer surgery, and it has been progressively adopted also for anterior oral floor diseases (11). Sialendoscopy check up to the hilum performed at the end of the procedure is advisable to enhance the therapeutic effectiveness, as it achieves the identification and removal of possible residual microliths.

Initial TORSS experiences with stones have been performed with the Da Vinci robotic system (10). Its limitations are the rigid and relatively bulky robotic arms, a limited number of cutting devices available, and high costs.

Our results confirm our previous experience (14, 20, 21) documenting the effectiveness of sialendoscopy-assisted TORRS for submandibular stone removal, given a global success rate of 81.5%. In 9 out of 10 cases, the surgeon turned to a traditional one because the stones were deeply located inside the submandibular gland and the patients had reduced buccal opening not allowing optimal robotic arm movements (8 converted to traditional transoral stone removal, and one to transoral surgical sialadenectomy). In the remaining case, a transoral robotic-assisted submandibular sialadenectomy was effectively performed. Traditional transoral stone removal as a rescue treatment after TORSS failure may be needed in case of impossibility to intraoperatively detect the stone for non-palpable pure parenchymal stone, or in case of fragmentary partial stone removal during TORSS in patients with palpable hilar/hilo-parenchymal stones. With regards to this, the routinely use of CB-TC in the pre-operative diagnostic planning achieves a three-dimensional stone visualization, with clear definition of its shape and size. This piece of information is very important to be compared to the intra-operative findings, in order to be sure that the stone is completely retrieved during TORSS. Under these conditions, it is mandatory an accurate pre-operative counselling (with double surgical consent administration), in order to make the patient aware of the possibility of surgical conversion to submandibular sialadenectomy, in case of impossibility to intra-operatively detect a non-palpable pure parenchymal stone.

The global success rate here detected is slightly reduced compared to that reported by literature, and ranging between 91.9 to 100% of cases (14, 20, 21); in particular, Wen et al. (20) reported a 91.9% success rate in 24 patients, all with palpable >5 mm stones undergoing TORSS-assisted combined procedure. This difference probably lines within different patient selection criteria: as fact, in our case-series, after data stratification, we found a better surgical outcome in case of palpable hilar/hilo-parenchymal stones than in case of non-palpable pure parenchymal ones, with a success rate in the former sub-group of patients of 92.7% that sharply drops to 46.2% in the latter one. Under this condition, therapeutic effectiveness of sialendoscopy-assisted TORSS for palpable hilar/hilo-parenchymal stone completely lines within data reported by literature.

Based on this, and considering the corresponding figures for traditional trans-oral stone removal (98.5%) (18), an accurate pre-surgical staging considering the above mentioned features should be desirable in order to identify which patients could be more effectively managed by means of sialendoscopy-assisted TORSS (also considering non-negligible direct and indirect TORSS costs). The better three-dimensional magnification of the surgical field during TORSS can achieve some advantage over traditional surgery in case of deep locations. In addition, we believe that this therapeutic gap could be reduced by the routinely use of CB-TC a mean to objectively assess the precise stone location.

The difference in success rate by considering the site and characteristics of the stones (i.e.,: pure parenchymal vs. hilar/hilo-parenchymal; palpable vs. non-palpable; stone size) probably lines within the fact that during TORSS the advantages deriving from the tactile manoeuvre typical guiding the surgical dissection through stone identification during traditional transoral surgery are abolished. This hypothesis is supported also by the intra-operative detection of stones with smaller diameter (presumably more difficult to be intra-operatively located) in cases unsuccessfully managed by means of TORSS compared to the ones completely removed by means of TORRS. In our cohort, the mean stone size of 10.0 ± 3.7 mm was lower than that reported in other case-series: 12.4 and 12.3 mmm respectively in Wen et al. (20) and Razavi et al. ’s (21) studies.

Our results also confirmed the safety of sialendoscopy-assisted TORSS procedure, as no major complications occurred. As a fact, the most frequently reported untoward effects were a transient mild lingual nerve impairment (5.5%), completely resolved within a few weeks in all but one patient (who experienced a complete recovery by one year later), and sialadenitis (3.7%). The fact that all the patients reporting transitory lingual nerve dysfunction had been operated by means of Da Vinci Xi System, make us argue that the larger width of the surgical spatula in the Xi System compared to Si System (i.e., 8 vs. 5 mm) can results in a more traumatic effect on the delicate nerve structures placed in the anfractuous and restricted oral floor. In addition, the procedure resulted to be adequately tolerated by the patients, with an overall low morbidity attested by the high rate (68.6%) of hospital discharge on the first day after surgery, and the reported VAS pain on the first (6.0 ± 3.6) and the seventh (3.4 ± 2.2) days after surgery.

Beside costs and peculiar stone characteristics (i.e.,: location and palpability), the duration of the procedure (docking and pure surgical time) should be counted among possible disadvantages of TORSS: in our case-series the mean docking and surgical times were respectively 10.1 ± 2.6 and 75.6 ± 42.8 min, with a large variability (range 35–160 min) in surgical times based on surgical outcomes (64.2 ± 23.5 vs. 85.0 ± 32.5 min respectively in case of TORSS successful and failure) (data not showed). Compared to other case-series, our mean operation time (75.6 ± 42.8 min) is quite lower than that reported by Wen et al. (20) (103 min on average), being however within the range (62–120 min) (11, 14, 20, 21) of values described by literature. This probably is related to the improving surgical skills acquired during a period of three years from our two senior surgeons (i.e.,: PC and CV) working in two large University Hospital Centers used to perform major surgery, and taking advantage of robotic surgery skills acquired in other areas of the head and neck surgery.

The duration of the procedure (along with the probability of therapeutic success) considerably impacts on the cost-effectiveness of the approach, and it is primarily affected by three main factors, that are the experience of the surgeon, the location of the stone and specific anatomic insights. Regarding this last condition, an accurate pre-operative ENT assessment should be carried out to detect some possible red flags for TORSS: identification of hilo-parenchymal stones or proximally-located stones in a patient with unfavourable anatomy (i.e., reduced mouth opening) can result into a hard robotic procedure, usually not allowing the right help from the assistant surgeon in exposing the surgical field. Eventually, thanks to the 3D-HD view of the robotic camera and to the ability of the robotic arms to perform micro-movement with a much higher precision than the human hands, the transoral robotic surgical procedure to remove the salivary stones can be ruled out, whichever the utilized system is.

Despite sialendoscopy-assisted TORSS seems a safe and effective approach for submandibular stone removal, better surgical outcomes occur in patients with large, palpable and hilar/hilo-parenchymal stones. Accurate pre-operative patients’ stratification is mandatory to enhance the surgical effectiveness of sialendoscopy-assisted TORSS procedure.