A saddle-nose deformity is the one in which there is loss of dorsal height of the external nose. A variety of conditions that lead to cartilaginous destruction can cause this condition. They may be infectious such as leprosy and congenital syphilis, immune mediated e.g. Wegener's granulomatosis or even iatrogenic when not enough cartilage is preserved along the dorsum while resection of the septum. Treatment is surgical. Augmentation rhinoplasty is performed to fill the defect. Image via: https://www.drhilinski.com/rhinoplasty-tutorial/saddle-nose-deformity/