Thyroplasty is a phonosurgical technique designed to improve the voice by altering the thyroid cartilage of the larynx (the voice box), which houses the vocal cords in order to change the position or the length of the vocal cords. Thyroplasty is a phonosurgical technique designed to improve the voice by altering the thyroid cartilage of the larynx (the voice box), which houses the vocal cords in order to change the position or the length of the vocal cords.