HYPERSENSITIVITY
which cause inflammation and then produce allergic reactions, with symptoms such as rhinitis and asthma (Valliant, et. al., 2021; Abbas, 2021). The cell membranes of the mast cells and nearby tissue cells release arachidonic acid, which then interacts with enzymes to produce prostaglandins and leukotrienes (Abbas, 2021; Koike, 2018). In a type II reaction, or Ab-dependent cytotoxic hypersensitivity reaction, Ab (usually immunoglobulin G) is directed against self-antigen (Ag) or foreign antigen on cells, leading to phagocytosis, killer cell activity, or complement- mediated lysis Valliant et al., 2021; Taylor & Lindorfer, 2016). In a type III hypersensitivity, or serum sickness, reaction, immune complexes (Ag-Ab) are formed and deposited in the tissues (Valliant 2021; Li et al., 2016). Complement is activated, and polymorphonucleocytes are attracted to the site, causing local tissue damage and inflammation. In a type IV, or delayed type hypersensitivity reaction, antigen-sensitized T cells release lymphokines following a secondary contact with the same antigen (Hasegawa, Lino, & Sudo, 2016). These lymphokines induce inflammatory reactions and activate and attract macrophages, which release inflammatory mediators (Hasegawa et al., 2016).
An allergy or hypersensitivity is an increased susceptibility to a specific substance (Valliant, et al., 2021).Antigens are substances that stimulate immune reactions to defend against foreign materials (The Centers for Disease Control and Prevention, 2021b; Marieb and Hoehn, 2018; Bottaro, 2021). Immunoglobulin E (IgE) antibody (Ab) in an individual with allergies is responsible for the majority of allergic reactions (Marieb and Hoehn 2018; Abbas, et al., 2021). Hypersensitivity occurs when an adaptive immune response occurs in an exaggerated or inappropriate form (Li et al., 2016;; Valliant, et al., 2021). Many different antigens can provoke such responses, and they vary from individual to individual. There are four types of hypersensitivity reactions: types I, II, III, and IV (Abbas et al., 2021) Types I through III are Ab mediated, and type IV is mediated mainly by T cells and macrophages (Marieb and Hoehn, 2018; Li et al., 2016; Tomasiak-Lozowska, et al., 2018). In a type I hypersensitivity reaction, IgE is bound to mast cells or basophils via Fc receptors (Son, et al., 2018; Koike, et. al., 2018; Abbas, et. al, 2021). In the presence of an allergen, the IgE becomes cross-linked, inducing degranulation and the release of pharmacological mediators, such as histamine granules ,
METALS AND HYPERSENSITIVITY
allergenic sensitivity to base metals.” Chromium and cobalt are known to cause hypersensitivity reactions in orthopedic device use (Roberts, et al. 2017; Wawrzynski, et al., 2017). Many metallic elements that are necessary for life functions can cause allergic reactions when used in devices. Thus, although many metals have been shown to cause allergy, in bulk and in trace amounts they are also important for many biological functions of life (Marieb and Hoehn, et al., 2018; Al-Fartusie and Mohssan, 2017; Witkowska, et al., 2020). The metallic elements calcium, magnesium, sodium, and potassium are essential to life functions; other important bioactive trace metallic elements include zinc, iron, copper, manganese, chromium, germanium, molybdenum, and vanadium (Mehri 2020; Marieb and Hoehn, 2018). Many of these elements are necessary systemically but have been associated with contact allergies when used in dental and orthopedic devices (Pangi et al., 2016).
Precision casting of metal alloys, particularly gold alloys, has been used in dentistry since the early 1900s, with few problems in biocompatibility (Pocket Dentistry, 2016). Their high cost, though, prompted the development of base metal alloys, which lowered the cost of cast restorations to dentists and patients. A majority of prosthetic restorations used today are fabricated from chromium-cobalt or nickel-chromium alloys (American Dental Association, 2021; Alnazzawi, 2018). Beryllium was added to the nickel-chromium alloys to improve metal castability and facilitate porcelain bonding (Moslehifard, et al., 2019). Nickel- based alloys can cause dermatitis in patients with a known allergy and may also corrode over time in the mouth, causing small amounts of nickel and chromium to release into the body (Pangi, Shetty, Prasad, & Kanathila, 2016). In a chart on its website, the American Dental Association compares indirect restorative materials and classifies the biocompatibility of base metal alloys as “well tolerated, but some patients may show
METALLIC CORROSION
Another important factor in corrosion is galvanic cells, which are made up of mixed metals (Narayanan et al., 2018). Mixed metal galvanic cells form when two electrochemically dissimilar metals touch and come in close contact with an electrolyte or conducting solution (Mystkowksa, et al., 2018; James, 2020). An example of this in dentistry is the electric “shock” or pain a patient may experience when he or she bites down on a piece of aluminum foil with a tooth that contains an amalgam or gold restoration, with the patient’s saliva acting as an electrolyte (Calcaterra, 2016). In dentistry, this is often referred to as galvanic shock (James 2020). To minimize galvanic corrosion and galvanic shock of a certain metal, a metal with lower resistance to corrosion may be applied as a coating which is known as cathodic protection as in the application of zinc or aluminum- silicone coatings on steel (Narayanan, et al, 2018; Cathwell, 2019). In dentistry, the application of placing a coating of unfilled light- cured resin over the metal restoration will break the metal contact and the galvanic circuit (James, 2020). Multiple amalgam restorations of the same composition in the same oral environment may corrode at different rates based on several factors. Amalgam that comes in contact with a gold crown on an adjacent tooth will show more corrosion (“blackening”) when compared to other amalgam restorations in the mouth as oral galvanism accelerates the corrosion of dental restorations (James, 2020). Furthermore, exposure to different levels of dissolved oxygen will also influence corrosion
The oral cavity provides an ideal environment for metallic corrosion given the acidic environment, the cumulative effect of masticatory forces and the presence of chloride and fluoride ions in the saliva (Sukumaran, et al., 2020). Corrosion is the loss of the essential metallic characteristic or mass of a metal through reaction with its environment Narayanan, et al., 2018). The corrosion process involves an anodic, or oxidation, reaction and a cathodic, or reduction, reaction proceeding at equal but opposite rates (Fraunhofer, et al., 2017; Prikrylova, et al., 2019). An example of a corrosion reaction is that of iron in a dilute acid solution, where the iron dissolves and hydrogen gas is released (Haugli, et al., 2020). In neutral or basic (alkaline) solution, the predominating cathodic reaction is the reduction of dissolved oxygen (Mohamed, et al., 2021; Khotseng, 2018).The cathodic product, hydroxyl ions, may often react with the anodic reaction product Mystkowska, et al., 2018; Khotseng, 2018).An example of this is the reaction between ferrous ions and hydroxyl ions to form ferrous hydroxide, which undergoes further oxidation to form hydrated ferric oxide or “rust” (Rehman, Amin, & Abbasm, 2014). When two or more metallic elements are present in an alloy, as is common in dental alloys, then some or all of these elements can corrode to release metal ions (American Dental Association, 2021; Nierlich et al., 2016). Because the individual rates of metal dissolution may differ, the amount of particular metallic ions present in a solution will vary (Arregui, et al., 2021; Nierlich et al., 2016).
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