Intraocular lens replacement, which has long benefited adults suffering from cataracts, can also help children with certain vision problems. The sizes of the replacement lenses developed for adults are not, however, appropriate for young children. (The mean axial length of a newborn’s eye is 17 mm, whereas that of an adult is 23 to 24 mm.) In addition, the surgical procedure must accommodate developmental considerations such as lower scleral rigidity, greater elasticity of the anterior capsule, and higher vitreous pressure (see, e.g., Dahan, 2000; Ahmadieh and Javadi, 2001; Good, 2001; Pandey et al., 2001).

For certain implanted devices, reductions in size have brought benefits to both children and adults. The cardiac pacemaker is a notable example. Figure 2.1 and Figure 2.2 show the difference in size between an external pacemaker (circa 1957) that was designed to provide short-term life support and a recent implantable pacemaker that offers long-term support.

In some cases, sizing of a device is done at the time of use. For example, cardiac shunts or tubes that are used to connect two blood vessels come in

FIGURE 2.1 Child with early external pacemaker, c. 1957 (Used with permission of The Saturday Evening Post).