These results help describe the distribution of collagen fibers within the lamina propria of the human vocal fold and have implications to understand the cover-body theory of voice production both in the adult and newborn.
If length and latency asymmetry of the right and left RLNs in humans can be demonstrated, the synchronicity of the vocal folds requiring precise controlled variations within intrinsic laryngeal muscles needs further investigations.
The common trunk of the SHM and STM is the prime choice for laryngeal reinnervation using AC-RLN non-selective anastomosis. If the main trunk is missing, the branch to the STM can be an attractive candidate.
The aim of this study was to validate a procedure to identify and preserve the external branch of the superior laryngeal nerve (EBSLN) during thyroid surgery. The present study also aimed to analyze the EBSLN and the vagus nerve activities after stimulation and demonstrate an operative association between all the laryngeal muscles. Dissection of the cervical region was performed bilaterally in four adult cadavers. In a second step, 144 patients undergoing total thyroidectomy were included. Intraoperative stimulations of the cervical vagus nerve and the EBSLN in the sternothyroid-laryngeal triangle were performed bilaterally. Potentials in the thyroarytenoid muscle and the cricothyroid muscle were registered on each side using the NIM3 Medtronic System. EBSLN was identified according to Cernea's classification as type 1 in 62.5%, type 2a in 25%, and type 2b in 12.5% of cadaver's dissection. According to Friedman's classification, 50% of EBSLN were classified type 1, 25% type 2 and 25% type 3. The EBSLN was identified in 267 cases out of 288 peroperative dissections (92.7%). Direct stimulation (1 mA) of this branch led to a recordable contraction of the cricothyroid muscle with a mean latency of 1.42 ± 0.66 ms on the right side and 1.43 ± 0.61 ms on the left side. The stimulation of the EBSLN also led to a recordable contraction of the thyroarytenoid muscle in 211 cases (73.3%) with the same latencies. A contraction of the cricothyroid muscle was also recorded when the vagus nerve was stimulated in 219 cases (76.0%). The sole visual identification of the EBSLN during thyroid surgery is not a reliable method to prevent nerve injury. Direct stimulation of the EBSLN in the sternothyroid-laryngeal triangle is a simple and rapid procedure to detect and preserve the nerve during surgery. Functional associations between vagus nerve and EBSLN in laryngeal muscles' contractions were also identified.
The functional results of a partial laryngeal surgery or a laryngeal reinnervation depend on the precise knowledge of the intra laryngeal anatomy of the inferior laryngeal nerve (ILN). Ten human larynges without known laryngeal disorders were obtained from human cadavers for ILN microdissection. Intra laryngeal ILN branching patterns were determined bilaterally. The lengths of the vertical, genu and oblique segments of the anterior division of ILN and the distance between the nerve within the paraglottic space and the cricothyroid articulation (CTA) were measured with a digital microcaliper. The mean lengths of the vertical, genu and oblique segments were 10.82, 5.89 and 9.29 mm, respectively. The mean distance between the nerve in the paraglottic space and the CTA was 11.20 mm. Key anatomical landmarks of the abductor division (vertical and genu segments of ILN) were the lateral border of posterior cricoarytenoid (PCA) muscle and the superior ligament of the CTA. The two-branch pattern for the lateral border of the PCA muscle has been the most frequent (50%). A branch of interarytenoid muscle (IA) originated from the genu segment. One or two branches for the PCA muscle has been identified in 75% of cases from the IA neural plexus on the front side of PCA muscle. The adductor division for the thyroarytenoid muscle and the lateral cricoarytenoid muscle was the oblique segment of the nerve. We conclude that abductor and adductor divisions of intra laryngeal ILN can be readily identified and the knowledge of key landmarks allows preservation of the ILN during partial surgery of the larynx and possibly selective muscle reinnervation.
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