Outright comprehension of the surgical anatomy of RLN, including its variations, is crucial for a holistic approach to safe thyroid surgery. Full exposure to RLN is a prerequisite to avoiding nerve injury. The nerve lies more superficially in the last 2 cm of its course. Hence, this site is most vulnerable to iatrogenic lesions [16]. Anatomical landmarks may be of help in identifying the RLN intraoperatively. However, surgeons must be well versed in their consistency and reliability before using them in the operating theatre.
In our cadaveric dissections, the RLN was coursing superficial to the BL in 53 cases (88.3%), which coincides with the prevalence of superficial RLN/BL relationships noted by Asgharpour et al (88.1%) [3]. Henry et al [13] also described the superficial RLN/BL relationship as the most common (78.2%) pattern in their meta-analysis. A piercing pattern of the RLN/BL relationship was seen in 8.3% of cases, which aligned with the findings of Kaisha et al [17]. During surgical intervention, such a pattern needs scrupulous attention because of the higher risk of lesions associated with glandular traction [1, 16]. RLN lying deep to the BL was seen least commonly (3.3%). Although in such a location, it is wearisome to visualize the nerve undoubtedly during glandular manipulation, the chances of injuring it are less in most cases. The nerve escapes the area by piercing the larynx at this level. Henry et al [13] noted substantial ethnical and geographical variability in RLN/BL relationship. Most Asian studies [16, 24, 25] have reported a lower prevalence of superficial relationships (59.3%) than their North American [4], African [20], and European counterparts [6, 18]. In most cases (76.7%), we noted symmetrical RLN patterns concerning the BL. This information is crucial for surgeons performing total thyroidectomies, as when one nerve is identified in a particular location, the exact location should be searched first on the contralateral side. In a meta-analysis by Henry et al [13], the RLN/BL relationship was bilaterally symmetrical in 75.5% of cases.
The symmetrical behavior of the RLN to the BL (76.7%) was more pronounced than the symmetrical pattern (63.3%) with TEG, according to the assessment of the bilaterally symmetrical behavior of the RLN to the BL and TEG. As a result, head and neck surgeons consider the BL to be one of the most trusted landmarks in neck surgery to prevent iatrogenic RLN damage [3, 11, 28]. Our results corroborate the same notion. As a result, "the rule of thumb" is that no structure should be dissected or ligated until the RLN is found, and the BL, as a constant anatomical marker, may be utilized to find the nerve intraoperatively.
It is also crucial for the surgeons to consider the less common piercing pattern or RLN lying deep to the BL for a hassle-free procedure. The piercing pattern, where the nerve can be intricately intertwined with the BL, poses a unique peril as, in such cases, nerve fibres can easily be severed while incising the ligament during thyroidectomy [30]. For obvious reasons, such patterns are associated with the highest morbidity of nerve palsy [30]. Serpell [26] highlighted that the substantial variability in the relationship of the nerve to the BL could arise from a lack of appropriate anatomical assessment of the ligament itself [30]. They also noted that the BL comprises two fascial layers. The outer one is false, and the inner one is true. In a few instances, the RLN can be found in the deeper fibrous layer (the "true" BL) [13, 26] and can lead to fallacious reporting of the penetrating pattern of the BL. Unfortunately, most studies reporting variations in the RLN/BL relationship have not made any lucid statements about such variants. In our cadaveric study, we have considered such phenomena while reporting piercing patterns. Furthermore, Serpell suggested that a meticulous dissection of the RLN posterolaterally helps to reduce nerve traction, allowing for safe BL division and a reduction in nerve palsies [26].
Our study results revealed that the reliability of TEG as an identifiable landmark for locating RLN is less established. Only in 71.70% of cases was the nerve coursing within the TEG throughout its course. Various authors have studied the location of RLN with the TEG amongst different geographic populations. Armstrong and Hinton dissected 40 cadavers in USA and found RLN within the groove in 60.0% of cases [2]. Chen et al [8] studied 90 Chinese cadavers and located RLN within the groove in all the cases. Lang et al dissected 43 cadavers and found RLN within the grove in 37.2% of cases. Henry et al [13] dissected 40 cadaveric necks and found RLN within the groove in 68.1% of cases. As far as the Indian subcontinent is concerned, no such studies have been done to assess RLN/TEG relations. A Meta-analysis conducted by Henry et al [13] showed a pooled prevalence of 63.7% of nerves coursing within the TEG. After reviewing available literature, we believe that the RLN/TEG relationship shows ethnical and geographical differences, with Asian studies generally reporting more RLNs coursing in the TEG (75.9%) than European ones (50.9%) [13].
The location of the RLN outside the TEG ranges from anterior, anterolateral, and lateral to posterolateral. In our cadavers, it was most commonly (63.3%) located lateral to the TEG, similar to the findings of Henry et al[13]. At the same time, the meta-analysis differed concerning the location of the RLN when it was outside the TEG. Most RLNs outside the TEG (45.7%) were located anteriorly in the meta-analysis ranging from anterior to lateral [13].
The TEG is a valuable landmark for identifying the RLN. It represents an asylum to safeguard the nerve inside the groove. Operating surgeons may quickly fail to spot the nerve while lying inside the groove, but its presence can be confirmed by simple palpation [7].This landmark has been proven reliable in identifying RLN in laparoscopic procedures of the neck [7]. In a series by Chang, the RLN was identified in 100% of laparoscopic thyroid procedures using the TEG [7].
We advocate that every time an attempt is made to identify and locate the RLN in the TEG, the course of the nerve should be traced upwards up to the level of BL. Once the BL confirms its position, surgical manipulation is to be done. We believe surgeons must stick to the dictum of "no manipulation until and unless the RLN/BL relationship is well established". However, such an identification method can get complicated due to local pathologies (e.g., large thyroid goitre) or anatomical nerve variations, such as extra laryngeal branching of the RLN. Henry et al reported that 76.6% of RLNs branch before entering the larynx [13]. In recent years, intraoperative nerve monitoring (IONM) has appeared as an alternative to identifying RLN intraoperatively. However, it has yet to be proven superior to concrete anatomical landmarks [7, 13]. That being said, IONM can be beneficial in cases where pathology does not permit these at-risk structures to be adequately visualized and identified. The use of the BL and TEG in nerve identification is valuable regardless of whether procedures are of primary or secondary nature and can provide valuable surgical information, allowing for the reduction of iatrogenic nerve injuries.
Although the anatomical relationship between RLN and ITA is primarily volatile, it still holds a position amongst head and neck surgeons as an essential landmark in identifying the nerve intraoperatively. The relationship between the artery and the nerve has been studied over the past century and several classification systems have been developed [21]. We concluded that the RLN usually ran deep to the ITA, with a prevalence of 65%. In addition, there were differences between the left and right sides of the thyroid gland, with the deep type being more common on the left, representing an occurrence of 67%. Uen et al studied 120 cadavers and reported posterior position in 65.8%, 70% being left posterior [28].
Fowler and later, Skandalakis et al [27] concluded that injury to the RLN is more common when the nerve lies anterior to or between the ITA. Surgeons should also take care when performing thyroid operations on the right side because of the predominance of the anterior configuration [10, 22]. Some studies suggest that intraoperative nerve monitoring (IONM) has not proven more effective than direct nerve visualization [9, 12]. However, when there is an anatomical variant, reoperation of the thyroid gland, or obscured pathology, IONM should be used to prevent iatrogenic injury of the RLN [23].
Henry et al (2016) reviewed multiple literatures and documented a symmetrical relationship with ITA only in 36.6% (95% CI 31.5–41.9). The current study found symmetrical bilateral relationships in 63% of cadavers, and the difference may be attributed to population differences or smaller sample sizes [15]. Two earlier systematic reviews by Henry et al (2016) and Noussios et al (2020) presented relatively different pictures [15, 21]. Henry et al (2016) observed an asymmetrical pattern predominantly (posterior configuration) with ITA in which posterior configurations were common on the left and anterior configurations were on the right sides [13, 14]. Noussios et al (2020) found that posterior configuration was more common on both sides.
Due to the substantially higher incidence of the anterior and between patterns of the RLN/ITA connection on the right side, more caution is required during operations. In bilateral operations, symmetry must not be presumed, and the RLN must be positively recognized on both sides. The significant incidence of RLN/ITA variation must be considered while planning, exposing, and performing neck procedures [21].
The midpoint of the posterior border of the thyroid gland could be another possible landmark. Moreover, few authors have defined this landmark as TZ, which is present only in 63–78% of subjects [10, 13, 22]. Henry et al conducted a cadaveric dissection and meta-analysis to determine the relationship between TZ and RLN, the pooled prevalence of TZ was 70.2%, the majority of which were considered Grade 0 tubercles (< 1.0cm). RLN ran posteromedially to the ZT in 82.7% of cases [14]. According to Gauger et al, in 93% of the cases, the RLN was located near this landmark on the tracheal surface or buried in the glandular capsule, while in the other cases, the nerve was placed lateral to the posterior border of the gland [14].
The limited frequency and its dubious grading system makes TZ a doubtful landmark, hence to avoid such uncertainty, we have taken the midpoint of posterior border as a valid anatomical landmark and studied the topography of the RLN. Almost in all the cases (95%), RLN was travelling in the area posterior to this landmark, with an average distance of 4.95 ± 2.23 mm ranging between 2.21mm to 12.1mm. However, in a few instances (5%), it was coursing anteriorly along the gland's lateral surface, making itself vulnerable to injury during partial thyroidectomy. RLN may have a very intimate relationship with the thyroid capsule along the posterior border. This distance is thus a possible indicator in calculating the safe zone in deciding the line of incision in subtotal thyroidectomy or near-total thyroidectomy. Although the distance between the RLN and the posterior border of the thyroid is crucial for hassle-free thyroid surgery, specifically deciding the plane of incision in partial thyroidectomy, very little is mentioned in the literature. Hence, we could not compare the measured distance with previous literature.