X-rays are an essential part of dental diagnosis, and it is very important that any material that remains implanted in any part of the patient’s body, including his teeth, be radiographically distinguishable from natural structures or disease processes. In other words, any material or device implanted in teeth or in any other part of the body must be visible on an x-ray. Materials like amalgam, gold and titanium (for implants or posts) are made of metal and are naturally radiopaque (i.e. they block x-rays and cast a white shadow on x-ray film).
Materials like restorative composites, porcelain, or various dental cements are not inherently radiopaque and without modification of their composition, would not be visible on an x-ray film except as a dark spot if deposited in bone or tooth structure. Decay in teeth shows up as a dark area on an x-ray film, and in the early days of composite technology, before the addition of radiopacifiers, it was often difficult to distinguish between a composite filling or an area of decay in a tooth when looking at an x-ray. The addition of zirconium dioxide, barium oxide or Ytterbium oxide to any radiolucent (the opposite of radiopaque) material will impart the property of radiopacity. These three oxides are chosen for their compatibility with the chemistry of composites. Note that Barium Sulfate is used as a “milkshake” or enema when taking medical x-rays for the observation of the gastro-intestinal tract.
The addition of radiopacifiers is especially important in the production of dental cements used to lute crowns and bridges. Even though the cement will spend its lifetime under the crown, excess cement will be forced out from between the crown and the tooth during placement, and often end up between the teeth or under the gums where it cannot be seen by direct observation. When this happens, it can cause inflammation of the gums and even eventual loss of the tooth. As long as the cement is visible on the x-ray, it will reveal the presence of the cement so that it can be removed.
It is important to remember as a clinician that bonding agents are not generally radiopaque, and thick layers, especially at internal prep line angles, may yield dark lines around composites on an x-ray. These dark lines can mimic decay, and quite a few perfectly adequate composite restorations have been replaced needlessly for this reason.