I read the article by Freitas et al and appreciated the accomplishment of this clinical trial. The authors evaluated the salivary concentration of metal ions of silver, cadmium, copper, and zinc from silver solder in 30 patients with hyrax appliances and 30 controls with saliva samples taken at 6 times (before cementation; 10 minutes after cementation; and 1, 7, 30, and 60 days after cementation). The small salivary concentrations of metal ions varied with ion type and decreased with time, with the maximum at 10 minutes (eg, cadmium, 71 μg/L) and decreased closer or equal to the controls with time.
I strongly disagree with the term “toxic” or “toxicity” used indiscriminately in the title and multiple times throughout the text. Toxicity is defined as a quality of being poisonous. The authors evaluated the small concentrations of metal ions released into the saliva and did not evaluate or even comment on the amount of metal ion ingestion needed to elicit toxicity demonstrated in signs, symptoms, or blood levels.
Zinc and copper are essential elements needed for bodily functions. The recommended daily intake for zinc is 15 mg per day (15,000 μg). Zinc toxicity is not well established, since zinc ingestion in excess of 200 mg (200,000 μg) has caused nausea and abdominal pain. Many zinc-containing compounds are commonly found in health food stores and over-the-counter cold aids. The recommended daily intake for copper is 1 mg (1000 μg) with possible toxicity greater than 10 mg daily (10,000 μg).
Silver benefits are not established and toxicities are not well defined in humans, although large doses are toxic in animals. A human report of ingestion of 6 g (6,000,000 μg) caused a blue-gray discoloration of the skin and mucosa. An oral dose of silver, after absorption, undergoes a first pass effect through the liver, resulting in excretion into the bile, thereby reducing systemic distribution to body tissues. Silver amalgam fillings have been used for decades without noted toxicities.
Cadmium exposure should be minimized, since it has no known benefits and has an established metal toxicity in small amounts. A normal daily diet can contain 50 μg of cadmium. Cadmium exposure can increase with cigarette smoking or eating healthy foodstuffs, especially grains, cereals, and leafy vegetables grown in water or soil contaminated from nickel-cadmium batteries, plastic disposal, or mining effluents. Cadmium accumulates in the body and is excreted slowly with a half-life of 10 to 30 years. A cadmium blood level greater than 5 μg per deciliter is considered toxic.
Bottom line: which metal ions, released from silver solder into saliva, should concern orthodontists? Copper and zinc? No, they appear to be less than the recommended daily intake. Silver? No, the small amounts are not a concern and toxicity is not well defined with large amounts. Cadmium? It might be a concern, so efforts should be taken to minimize exposure. Since cadmium-free silver solder has been in use for many years, orthodontists should contact their orthodontic laboratories to ensure that cadmium-free silver solder is used in the construction of their appliances (including the laboratory making the appliances for this study).