Tendinous insertions in the human thyroid cartilage plate: macroscopic and histologic studies

The zones of tendinuous insertion and origin of the sternothyroid muscle, the thyrohyoid muscle, the inferior constrictor muscle of the pharynx and the external part of the cricothyroid muscle were studied in adult larynges by macroscopy and light microscopy. The sternothyroid muscle is inserted in the superior and inferior thyroid tubercula and also in a tendinous arch, which extends between the two tubercula. In contrast to the descriptions found in most textbooks, the sternothyroid muscle has no attachment to the oblique line. The thyrohyoid muscle and the thyropharyngeal part of the inferior constrictor muscle of the pharynx originate in ventral and dorsal symmetry at the oblique line. X-rays permit a measurement of their original angles, resulting in a mean value of 36 degrees. According to these measurements, and electromyographic findings by other authors, both muscles can be regarded as a bipennate muscle from a functional point of view. Histologic studies indicate that the oblique line's zone of origin structurally resembles an areal periosteal-diaphysary tendinous insertion. In the non-ossified thyroid cartilage, the insertions in the region of the thyroid tubercula are similar to chondral-apophysary insertions of tendons in the limb skeleton. After ossification of the thyroid cartilage, they show the shape of circumscribed periosteal-diaphysary insertions. Osteogenesis of the laryngeal skeleton therefore affects the structure of the thyroid tubercula's tendinous insertions. From a mechanical point of view, the insertions in the thyroid cartilage function as extension-checking mechanisms, and also serve to balance the different elastic modules of the tendinous tissue and the cartilaginous or bony tissue.(ABSTRACT TRUNCATED AT 250 WORDS)