For more than four decades titanium implants have been and continue to be mainstream in implant dentistry. Most dentists today are trained to use and offer titanium and titanium alloy dental implants which are all metal. However there are increasing clinical reports and scientific research on instances of allergic reaction to titanium implants with spontaneous immediate or delayed implant failures. Other studies have investigated the stability of titanium dental implants and the crowns and bridges placed over them in the oral environment.
Thanks to the stability of the TiO2 layer (oxide layer) on their surface, titanium alloys are exceptionally resistant to corrosion but they are not inert to corrosive attack. When the oxide layer is broken down and then fails to reconstitute itself, titanium can be as corrosive as many other base metals. There is increasing evidence that titanium implants when exposed to the oral environment can corrode and result in compromised structural integrity of the implant but also lead to implant loss and potentially life threatening health conditions.
WHAT IS CORROSION?
Corrosion can be defined as the graded degradation of materials by chemical or electrochemical attack. This phenomenon is of concern particularly when metallic implants, metallic/silver fillings, or orthodontic appliances are placed in the hostile electrolytic environment provided by the human mouth. Corrosion can severely limit the fatigue life and ultimate strength of dental materials leading to mechanical failure.
WHAT TYPE OF CORROSION OCCURS IN THE MOUTH?
The type of corrosive reactions that occur in the oral cavity are electrochemical and are also called wet corrosion. Electrochemical corrosion requires the presence of water or some other fluid electrolytes and in the oral cavity saliva plays that role. This general mode of corrosion is important for dental restorations, implant-to-abutment joints and abutment-to-restoration (crown, bridge, retentive bars etc) connections. The complexity of the electrochemical process involved in the implant-to-implant superstructure joint and/or connection is linked to the phenomenon of galvanic coupling and stress and pit corrosion.
Galvanic corrosion is an electrochemical corrosion, it is the most common form of corrosion that occurs with dental implants. The use and connection of dissimilar metallic restorative materials is called galvanic coupling and may also generate corrosion. Therefore there is a great amount of concern regarding the types of materials used for suprastructures and crowns over titanium dental implants. When two or more dental prosthetic devices/restorations made of dissimilar alloys come into contact while exposed to oral fluids, the difference between their corrosion potential results in a flow of electric current between them. A galvanic cell is formed in the mouth and the galvanic current causes acceleration of corrosion of the less noble metal. High noble gold alloys are generally chosen as the material of choice for superstructures because of their excellent biocompatibility, corrosion resistance, and mechanical properties. However, these materials have become very expensive and as a result new more affordable less noble alloys such as Ni-Cr, Ag-Pd, and Co-Cr alloys are used instead. These alloys have good mechanical properties, they are less noble than titanium and their biocompatibility and corrosion resistance are of concern.
The galvanic current passes through the metal/metal junction and also through tissues, which causes inflammation and pain in the soft tissue (gums) and bone. In such cases saliva and other fluids in bone and soft tissue become electrolytes and allow the corrosive galvanic currents to take hold. These events trigger immune responses and ultimately possible implant loss.
STRESS AND PIT CORROSION
This is the second type of corrosion that occurs at the joint of the implant and the implant superstructure. Implant restorations and abutments can have small microscopic pits and crevices on their surface. With chewing cycles, implant and implant teeth (abutments and crowns) endure high forces stress of various types such as torsional compression and elongation and as a result stress and pit corrosion occurs.
Although not fully proven, microbial corrosion is another type of corrosion that can occur in the oral cavity. Titanium and the various alloys that are used to make restorations on implants are prone to retain a great amount of plaque compared to ceramic/zirconia implants. Wherever there is plaque there is bacteria and microbes living in it, and these bacteria release by-products that destroy bone and make natural teeth loose over time if not removed. In the same manner with titanium implants, those microbes and bacteria by-products are acidic in nature and can potentially corrode the titanium and the metal alloys used for restoration over the implants.
CLINICAL OBSERVATIONS WHEN CORROSION OCCURS IN THE MOUTH
As long as metallic dental restorative materials are employed, there will be galvanic currents associated with electrogalvanism in the oral cavity. For some patients, especially after the placement of a base metal restoration, pain caused by galvanic currents can occur and be a source of discomfort and ultimate implant failure. Corrosion leads to roughening metal surfaces, release of ions from the metal or alloy, and toxic reactions. The liberation of elements can produce discoloration of the soft tissues around the implant and allergic reactions such as oral edema, perioral stomatitis, gingivitis. Extraoral manifestation such as eczematous rashes in susceptible patients can occur. In a study by Kirpatrick, et al, it was found that the pathomechanism of poor wound healing is modulated by specific metal ions released by corrosion.
The mouth is the portal entry of the human body. It is also the habitat of microbial species that are kept wet by saliva. Oral tissues are exposed to a veritable bombardment of both chemical and physical stimuli as well as metabolism of about 30 species of bacteria. Teeth and dental implants function in one of the most inhospitable environments in the body, they are subject to the most extreme temperature variations, enduring temperatures as low as 0°C to hot foods and beverages. Multiple factors such as temperature, saliva, plaque, pH, and the physical and chemical properties of food and liquids as well as oral health conditions may influence corrosion. Yet, for the most part, oral tissues remain healthy. The combination of stress, ongoing corrosion, and bacteria contribute to implant structural failure and loss of bone integration.
As it has been the case in orthopedics for almost two decades, we now have alternatives in implant dentistry. Metal-free and metal alloy-free solutions are available for teeth replacement, from the implant embedded in bone to the visible crown in the oral cavity. Zirconia (ceramic) dental implants and all types of all-ceramic restoration (crown, bridge, retentive bars etc) are available. Futhermore, bioceramics accumulate very little plaque if at all thus reducing bacteria habitat, multiplication and by-products. Zirconia dental implants and restorations do not conduct electrochemical currents nor release ions to the oral cavity, surrounding bone and the rest of the body.
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