Patients and study design

We retrospectively reviewed 496 consecutive patients who underwent intraoral submandigular stone removal at a single medical center from January 2006 to December 2022. Patients who underwent a revision surgery for sialolithiasis and those diagnosed with SMG neoplasm were excluded. Patients who did not have preoperative CT scans were also excluded from this study. Among the 496 patients, 31 individuals were excluded, and 616 stones from 465 patients were retrospectively analyzed. Demographic factors (sex, age, alcohol consumption, and smoking status), anesthesia, and features of sialolithiasis (number and total volume of stones per patient) were analyzed. Postoperative complications, such as tongue hypoesthesia, hematoma, and ranula, were evaluated by patient’s symptom report and physical examination at the first outpatient clinic visit 7–14 days after surgery. Long-term follow-up was not routinely conducted, but in cases of symptom recurrence, a CT scan was performed to differentiate between residual stones. The location, number, and size of each stone were evaluated based on CT images. This study and the written consent waiver were approved by the Institutional Review Board (IRB no. 2021-08-031-006).

The categorization of stones by location and size in computed tomography images

The stone location was categorized into five parts: two superficial parts (distal and middle) and three deep parts (proximal, hilar, and intragland). An imaginary line from the orifice of Wharton’s duct to the SMG was drawn on a CT image at the level of the geniohyoid muscles and the upper half of the SMG. The line from the orifice of Wharton’s duct to the posterior border of the mylohyoid muscle (defined as the broadest part of the mandible) was equally divided and defined as “the distal” and “the middle.” The remaining part was defined as “the proximal.” The “intragland” was defined as the location of a stone in the middle of the gland. The stone was defined as “the hilum” if it was located across the duct and gland. This categorization is illustrated in Fig. 1. The length of the stone was measured as a line parallel to the duct, and the width was measured as a line perpendicular to the duct. Height was measured in the coronal view of the CT scanner. We calculated the area (the perpendicular plane area to pass the duct) using the width and height, and calculated the volume considering the stone as a sphere.

Fig. 1

The stone location. The location of the stone was categorized using the preoperative computed tomography scan axial image

Two types of surgical procedures (mediolingual and laterogingival approaches)

The type of approach was selected as follows: The stones located in the superficial part (distal and middle) were removed via the “mediolingual approach,” while the stones located in the deep part (proximal, hilar, and intraglandular) were removed via the “laterogingival approach.” (Fig. 2). These two approaches were designed considering the anatomy of the SMG duct to avoid injury to the sublingual gland (SLG), which may cause ranula formation, and the lingual nerve, which may cause postoperative hyposensation of the tongue.

Fig. 2

Two surgical approaches. The stones in the superficial part (distal and middle) were removed via the “mediolingual approach,” and the stones in the deep part (proximal, hilar, and intraglandular) were removed via the “laterogingival approach”

The distal part of Wharton’s duct runs under the SLG and is easily identified on the lingual side of the SLG. Therefore, when the stones were located in the distal and middle parts, a mucosal incision was made between the medial side of the SLG and the tongue. After the mucosal incision, the SLG was exposed and pushed laterally to identify Wharton’s duct (mediolingual side approach). A laterogingival approach is preferred to remove stones in the proximal portion. The lingual nerve runs across Wharton’s duct from the lateral to the medial side. Therefore, a mucosal incision should be made more laterally to prevent injury of the lingual nerve when approaching the proximal part of the duct and gland. After mucosal incision, the lingual nerve is easily identified under the connective tissues and can be pushed medially with a cotton swab (laterogingival approach). “The surgical triangle” is substantially useful to expose the gland and hilum. The boundary of “the surgical triangle” was the medial border of the mandible, the posterior border of the mylohyoid muscle, and the lingual nerve [7]. After the SLG was shifted to the side, the posterior border of the mylohyoid muscle and lingual nerve were exposed. The lingual nerve was pushed medially, and the surgical triangle was completely opened. A dilated duct was often observed at the center of the landmark.

After cleaning the oral cavity with iodine solution, the mouth was opened using a lip retractor. Denhardt mouth gag can be used together in the case of general anesthesia. The patient’s tongue was pushed in the opposite direction using a cotton swab, and a mucosal incision was made on the floor of the mouth, depending on each approach. After mucosal incision, the SLG and lingual nerve were identified and pushed away from Wharton’s duct with a cotton swab. An incision in the duct should be made just above the stone, which can be easily identified by palpation or visual suspicion. After stone removal, the floor of the mouth mucosa was sutured without marsupialization using 4 − 0 polyglactin (Vicryl; Ethicon, New Brunswick, NJ, United States).

The type of anesthesia was selected according to the stone location. General anesthesia was usually administered when the stones were located in the gland, hilum, or proximal duct. All distal and middle stones were removed under local anesthesia. For general anesthesia, the patient was anesthetized via nasotracheal intubation.

Statistical analysis of stone size in relation to stone location

To evaluate the potential influence of demographic factors on stone volume, we compared the total stone volume between patients with and without a history of alcohol consumption and smoking using independent t-tests.

We classified the stones across the five locations into two categories based on the criteria for applying the two surgical methods: distal, medial (mediolingual approach) and proximal, hilar, and intraglandular stones (laterolingual approach). For the purpose of comparing stone sizes between these two categories, we utilized independent t-tests for each of the five size parameters. We performed a receiver operating characteristic (ROC) curve analysis to establish a threshold for distinguishing between superficial and deep stones. For each potential cutoff value of stone width, sensitivity (the true positive rate) and specificity (the true negative rate) were calculated to assess the discriminatory ability. The ROC curve was constructed by plotting sensitivity against 1-specificity, with each point on the curve representing a specific threshold value. we evaluated all five size parameters—length, width, height, volume, and surface area—to identify which parameter was the most effective in distinguishing between superficial and deep stones. All statistical analyses were performed using SPSS version 25 (IBM Corp., Armonk, NY, USA) with statistical significance considered p < 0.05.