Anatomy of the distal incus in humans

Abstract

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 microm 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 microm 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.

FIG. 1

Drawing of the human ossicular chain taken from the Glasscock–Shambaugh Surgery of the Ear textbook (5th edition, 2005, B.C. Decker; reproduced with permission). Note that the lenticular process is depicted as a cylindrical disc, and the diameter of the incus long process is similar in size to the diameter of the lenticular process.

FIG. 2

A A representative histological section of the distal incus in the axial (horizontal) plane (from a 22-year-old male, H&E stain), showing the distal long process with its connection to the pedicle and the plate of the lenticular process. Note the narrow bony pedicle of the lenticular process and the abundant connective tissue surrounding it. B Structural parameters of the distal incus measured in our specimens. See Table 1 for measurement results.

FIG. 3

A histological section in the vertical plane (from a 33-year-old female) showing the narrow bony pedicle and the widened plate.

FIG. 4

Representative histological sections in the axial plane of the distal incus from four individuals at various ages. In each section, a narrow bony pedicle is seen connecting the long process to the plate of the lenticular process. The inset in (B) shows the area outlined in the box under higher magnification (×100). Note the slight scalloping of the bony pedicle as the capsular fibers insert. These fibers travel for a short distance into the bone, and are termed Sharpey’s fibers.

FIG. 5

Composition of fibers of I-S joint capsule using Movat’s Pentachrome Stain (from a 22-year-old male). Elastic fibers are stained black, and collagen fibers are stained yellow/orange. Note the prominent elastic tissue component of the I-S joint capsule around the joint space. This slide is representative of all three specimens.

FIG. 6

Comparison of histological and gross images of the distal incus. In the histological image (A), the narrow bony pedicle is readily appreciated. However, in the gross images (B and C), taken from another specimen, the diameter of the pedicle looks similar to the diameters of the distal long process and the plate. The outlines drawn in (C) represent an estimate of the shape of the bony distal incus based on our observations of other specimens.

FIG. 7

3-D reconstructions of the distal incus in four subjects. In all cases, the lenticular process consists of a narrow bony pedicle connecting to an expanded plate. Please note that the four panels are not on precisely the same scale. The scale bars represent the projection of a 0.25-mm length aligned in the medial–lateral direction in each panel.

FIG. 8

The relationship between the percentage of osteocyte-containing lacunae and age. The straight line is the regression line based on the data points. Also see Table 1.

FIG. 9

Schematic of the I-S joint at rest (solid line) and displaced (dashed line). The flexibility of the pedicle helps to isolate the stapes from the upward, downward, and sideward motion of the incus while faithfully translating piston-like lateral-to-medial motions from incus to stapes.