The RLN is an important anatomical structure in thyroid surgeries. The RLN palsy following its injury is one of the most common complications of these surgeries [8] which can cause severe vocal, breathing, and swallowing difficulties creating major psychological and social difficulties for the patient [9]. Although some surgeons avoid dissection near the RLN, others promote routine RLN identification, claiming that dissection throughout the nerve's full course can reduce nerve injury [10].
As the course of the RLN varies, many authors have researched for its relation to anatomic structures such as the inferior thyroid artery [11], the Berry’s ligament [12], the thyroid cartilage[13] and the Zuckerkandl's tubercle [12], to identify it. Until the mid-20th century, reports on the relationship between ZT and RLN evoked little interest [14]. Later in 1998, Pelizzo et al. [15]described “ When the ZT is encountered, it looks like an arrow pointing toward the nerve. If the ZT is mobilized medially, it allows easy identification of the nerve before it turns below the inferior cricothyroid articulation.”
One might argue that nodules from diseased thyroids can cast a shadow on ZT. Nonetheless, ZT, which is made up of thyroid tissue, is predicted to have a role in hyperplastic or neoplastic disease. Also, ZT is embryologically identified as the main location for medullary cancer in particular [16].
In the present study, ZT was present in 75% cases (n = 33/44), 73.68% (n = 14/19) on the right side and 76.0% (n = 19/25) on the left side. Thus, applying the chi square test, p value was calculated to be 0.86 (p > 0.05), which is not statistically significant. The finding was similar to the study conducted by Singh et al where ZT was identified in 87.86% cases, with no significant difference in laterality [5].
In this study, among 33 identified ZT, 92.9 (n = 13/14) ZTs on the right side and 100% (n = 19/19) ZTs on the left side were unilobed in structure, whereas only 1 ZT (7.1%) on the right side was found bilobed in structure. A prospective study done by Mehanna et al among 156 patients undergoing thyroidectomy showed presence of bilobed ZT in 4.7% cases which is similar to findings of our study [17]. This can be explained by the fact that ZT develops as a single lateral projection from the thyroid lobe, therefore favoring the unilobed nature of the ZT.
This study showed all the ZTs (n = 33/33) to be sessile in nature. In a study by Gil-Carcedo et al, the shape of the ZT was sessile in 70.96% cases and pedunculated in 29.03% cases [18]. This also may be contributed by the fact that ZT is a part of the thyroid lobe rather than a separate entity.
Findings from this study showed that ZT is located at the posterior part of the thyroid lobe in 71.4% cases (n = 10/14), lateral part in 21.4% cases (n = 3/14) and anterior part in 7.1% cases (n = 1/14) on the right side. On the left side, ZT was located in the posterior part of thyroid lobe in 78.9% cases (n = 15/19) and lateral part in 21.1% cases (n = 4/19). Singh et al conducted a similar study which showed that ZT is located at the posterior part of thyroid lobe in 97.20%, anterior part in 0.56% and lateral part in 2.77% of the cases [5]. This may be due to attachment of ZT, connecting the thyroid lobe with pharynx posteriorly during the embryological period which gradually separates to become the swelling in the posterior part of the thyroid lobe.
In this study, on the right side, 26.31% ZTs (n = 5/19) were grade 0, 31.57% ZTs (n = 6/19) were grade 1, 36.8% ZTs (n = 7/19) were grade 2 and 5.2% ZT (n = 1/19) was grade 3. On the left side, 24% ZTs (n = 6/25) were grade 0, 20% ZTs (n = 5/25) were grade 1, 44% ZTs ( n = 11/25) were grade 2 and 12% ZTs (n = 3/25) were grade 3. Moreover, results from previous studies support the finding of this study. Irawati et al conducted a similar study in which ZT was grade 0 in 9.5%, grade I in 28.9%, grade II in 50.5% and grade III in 11% [6]. Further studies with greater sample size are needed for more accurate results.
In the present study, on the right side, RLN was present medial to the ZT in all cases (n = 14/14), while on the left side, RLN was present medial to ZT in 84.2% cases (n = 16/19) and lateral to ZT in 15.8% cases (n = 3/19). Similar finding was present in a study conducted by Gurleyik et al which showed a common relationship between RLN and ZT. In 94% tubercles, the nerve runs medial to the tubercle [19].
In this study, among the 14 cases on right side, RLN was adhered to ZT in 57.1% cases (n = 8) RLN was at the distance of less than 5 mm from ZT in 35.7% cases (n = 5), at distance of 5–10 mm from ZT in 7.1% cases (n = 1). Among the 19 patients on left side, RLN was adhered to ZT in 15.8% cases (n = 3), at the distance of less than 5 mm from ZT in 36.8% cases (n = 7), at distance of 5–10 mm from ZT in 31.6% cases (n = 6) and more than 10 mm distance from ZT in 15.8% cases (n = 3). The relationship between distance between Recurrent laryngeal nerve and ZT and the size of ZT was analyzed using Pearson’s correlation test which showed that these two factors are negatively correlated with P-value of 0.017 on the right side and P-value of < 0.001 on the left side, which is statistically significant. This means that as the size of the ZT increases, the distance between ZT and Recurrent laryngeal nerve decreases and vice versa.
Identification of ZT after meticulous and deliberate dissection can be a useful method for the identification of RLN and prevention of its injury during thyroid surgery due to their consistent relationship. This can definitely add some additional prospect to the commonly used landmarks like Inferior thyroid artery, Berry’s ligament and inferior cornu of thyroid cartilage for the identification of RLN. Therefore, thorough anatomical understanding of the nerve, related vasculature and structure in close proximity with RLN like ZT in thyroid surgeries aids recognition and identification of RLN and helps in better functional outcome.