Medical devices are classified by the US Food and Drug Administration (FDA) under three different

classes depending on the risks the medical device may impose on the user. Class I devices are

considered to pose the least amount of risk to the user and require the least amount of control. Class I

devices include simple devices such as arm slings and hand-held surgical instruments. Class II devices are

considered to need more regulation than Class I devices and are required to undergo specific

requirements before FDA approval. Class II devices include X-ray systems and physiological monitors.

Class III devices require the most regulatory controls since the device supports or sustains human life or

may not be well tested. Class III devices include replacement heart valves and implanted cerebellar

stimulators. Many implants typically fall under Class II and Class III devices.

Metals, because of their density, are opaque to x-rays. This gets in the way of a complete and reliable analysis of the image. High-performance plastics, on the other hand, are x-ray transparent and therefore invisible, allowing good monitoring of bone growth and the healing process,