## 4 Discussion  
In the present study, the learning curve of robotic TT with concomitant CCND with regard to operation time was divided into three phases: phase 1 (the initial learning period, 1 st-19 th case), phase 2 (the challenging period, 20 th-121 st case), and phase 3 (the competent phase, 122 nd-194 th case). The learning curve with regard to perioperative complication showed that 119 cases were required to maintain a temporary hypoparathyroidismrian rate \(<30\%\). However, after adjusting for potential risk factors, this case number increased to 173. Several studies have evaluated the learning curves of different surgical procedures [13; 28; 29; 30; 31]. In these studies, learning curves were analyzed with respect to rates of surgical failure or perioperative complications as well as the operation time using quantitative statistical methods [13; 28; 29; 30; 31]. The learning curve for robotic thyroid surgery with regard to the operation time has been evaluated several times since robotic surgery was introduced to the field of thyroid surgery in South Korea in 2007 [5; 6; 7; 8; 9; 10]. The first assessment of the learning curve for robotic thyroidectomy using a gasless trans-axillary approach by a single surgeon showed that the operation times for robotic thyroidectomy reached a baseline after approximately 40 to 45 operations [5]. Regarding robotic thyroidectomy using bilateral axillo-breast approach (BABA), the surgeons completed the learning curve after performing 40 surgeries [10]. Also, in a multicenter study, the operation times were markedly decreased after completion of 50 robotic total thyroidectomies with central compartment node dissection [6]. However, these earlier studies had several limitations in that the learning curve was evaluated by mixing total thyroidectomy and lesser than total thyroidectomy [5; 10], with respect to the operation time [5; 6; 10]. Moreover, previous studies [5; 6; 10] (70-100 patients) did not enroll the sufficient number of patients to accurately determine the long-term learning curve in robotic total thyroidectomy. In the present study, the learning curve of robotic thyroid surgery was evaluated in terms of operation time and the incidence of temporary hypoparathyroidismochrom, which is the most common and challenging surgical complication following TT, by applying the CUSUM and RA-CUSUM methods to the data of \begin{table}
\begin{tabular}{l c c} \hline \hline \multirow{2}{*}{**Characteristics**} & \multicolumn{2}{c}{**Number of Patients (_n = 194_)**} \\ \cline{2-3}  & **Odds Ratio (95\% CI)** & _p-Value_ \\ \hline Age, years & 0.98 (0.94–1.02) & 0.285 \\ Gender \({}^{*}\) & & \\ Female & Ref & \\ Male & 3.83 (0.70–20.87) & 0.121 \\ Primary tumor size, cm & 0.55 (0.29–1.07) & 0.078 \\ Multifocality & & \\ No & Ref & \\ Yes & 0.57 (0.30–1.08) & 0.083 \\ Lymph node metastasis & & \\ No & Ref & \\ Yes & 1.86 (0.93–3.73) & 0.078 \\ Thyroid weight, gram & 1.04 (0.99–1.09) & 0.136 \\ \hline \hline \end{tabular} Ref = reference. \({}^{*}\) This parameter was corrected using FIRTH’s bias correction because of the small proportion of males.  
\end{table} 
Table 4: Potential risk factors associated with the development of temporary hypoparathyroidism as determined by backward selection (\(p<0.15\)). patients treated by TT and concomitant CCND. In addition, potential risk factors for development of temporary hypoparathyroidism were selected and adjusted. To the best of our knowledge, the present study is the first to evaluate the learning curve of robotic thyroid surgery with respect to a perioperative complication. In the present study, the learning curve for robotic thyroid surgery with regard to the operation time consisted of three phases. The initial learning phase (1 st-19 th cases), during which additional time is required for creating a working space and robotic docking. Unlike intra-abdominal surgery, the neck area has no preformed space, and thus, additional time is required for working space creation and robotic docking.