Contraction of the muscles that insert on the hyoid apparatus is important in upper airway stability and dilation. The purpose of this experiment was to determine the effect of traction on the tongue (T), geniohyoideus (GH), sternohyoideus (SH), sternothyroideus (ST) and combinations of these muscles on the position of the hyoid apparatus and the dimensions of the nasopharynx. Four cadaver horse heads were used. Nylon suture was secured to the paired ST, SH, GH and tongue. The head was positioned in ventral recumbency. An endoscope was passed through the left naris to the level of the left guttural pouch opening. The fluoroscope was used for imaging while the endoscope was used for examination of the larynx and nasopharynx. Still images were taken while traction was placed on each structure.
Traction on the tongue did not significantly alter the position of the hyoid apparatus, nor did it alter the dimensions of the nasopharynx. Rostral traction on GH caused cranioventral displacement of the basihyoid bone (P<0.05). Caudoventral displacement of the basihyoid resulted from caudal traction on ST or SH (P<0.05). Caudal traction on ST caused ventral displacement of the epiglottis (P<0.05). The angle between the stylohyoid bone and ceratohyoid bone was increased following traction on either GH, ST or SH (P<0.05). The angle between the stylohyoid bone and caudal ramus of the mandible was decreased by traction on SH (P<0.05). Traction on either GH, ST or SH increased the dimensions of the nasopharynx based on endoscopic examination.
Traction on the muscles that attach to the hyoid apparatus produces nasopharyngeal dilation by lengthening the hyoid apparatus. This is achieved by increasing the angle formed by the ceratohyoid and stylohyoid bones and displacing the basihyoid bone ventrally.