Safety of medial dissection of the thyroid gland along the trachea based on anatomic constancy of the laryngeal entry point of the recurrent laryngeal nerve

PURPOSE: The purpose of this study was to determine the extent of safety of medial dissection of the thyroid gland along the trachea. Medial to lateral dissection of the thyroid gland along the trachea after early division of the isthmus has been known to be a useful technique in thyroid surgery, especially for difficult cases, but the risk of injury of the recurrent laryngeal nerve (RLN) has constrained thyroid surgeons from utilizing this technique to its full extent. METHODS: Distances of the laryngeal entry point (LEP) of 134 RLNs of 71 patients from the midline of the trachea, and some other anatomical distances, were measured intraoperatively. The relationships of the intraoperatively measured data with circumferences of the cartilaginous portion of the trachea (CCT) around LEP measured preoperatively by CT scan were evaluated. RESULTS: LEP was always located within 2 mm vertically from the horizontally extended line of the inferior border of the cricoid cartilage and was the closest point from the midline in the whole course of the RLN. The distance between LEP and the midline was very closely correlated with CCT measured on preoperative CT scan, and it can be accurately calculated with a regression equation; Distance between LEP and the midline = (0.42 × CCT) + (1.2 × sex) + 3.2 (mm) (sex: female=0, male=1; R(2) = 0.85). CONCLUSION: Early division of the isthmus and dissecting the thyroid off the trachea to the calculated extent is a safe and effective procedure.