Anatomy of the Distal Incus in Humans

The anatomy of the distal incus, including the lenticular process, was examined in histological sections from 270 normal cadaveric human temporal bones aged between less than 1 month and 100 years. All but nine of these sectioned specimens showed signs of a bony connection between the long process of the incus and the flattened plate of the lenticular process, and in 108 specimens a complete bony attachment was observed in a single 20 mu m section. In these 108 ears, the bony lenticular process consisted of a proximal narrow "pedicle" connected to a distal flattened "plate" that forms the incudal component of the incudo-stapedial joint. A fibrous joint capsule extended from the stapes head to the pedicle of the lenticular process on all sides, where it was considerably thickened. Three-dimensional reconstructions made from serial 20 mu m sections of four bones provided views from all directions that easily convey the anatomical features of this region. Morphometric measurements of the bony architecture of the distal incus in 103 temporal bones were made, including lengths and cross-sectional areas, estimates of the percentage of lacunae containing osteocytes, and the degree of bone resorption. These measurements, analyzed as a function of age, provided an anatomic description over a large age range that can serve as a normal baseline against which structural pathology can be compared. Although none of the bony dimensions showed significant age dependence, the estimated percentage of bony lacunae that contain osteocytes decreased significantly with age. The results have implications for the roles of specific components on the coupling of ossicular motion across the incudo-stapedial joint, and provide insights regarding bone resorption at the level of the distal incus, which occurs clinically in some patients with chronic otitis media or after stapedectomy.