Introduction
Dentistry is concerned with preventing and treating oral disease, eliminating pain, restoring the oral apparatus to function and improving aesthetics. The oral cavity is a complex environment, in which hard and soft tissues (teeth, palate and gums) are continuously bathed in saliva and are subjected to vary- ing loads during mastication and deglutition (swallowing). Saliva varies in composition and pH from person to person, and from hour to hour for a given individual; it comprises a mixture of inorganic anions (predominantly chlorides and phosphates), organic acids, enzymes, bacteria and gastric secretions such as mucin’-3. The salinity of saliva approaches that of sea- water and tends to be highly corrosive to most non-noble metals. The total forces exerted during mastication vary with the age, musculature and other physiological factors for the patient and the existing dentition, typically being ca. 60 kg for the molar teeth in a healthy dentate patient, giving rise to stress values in the range 3-17 MN/m2, depending upon the food being c h e ~ e d ~ . ~ . Clearly, the mouth is a hostile environment and dental materials are required to be mechanically strong and resistant to degradation caused by stresses and the oral environment.
Decayed teeth are treated by removing the decay and then mechanically shaping the cavity to provide optimum retention of a restoration or ‘filling’.
Anterior (front of the mouth) teeth are restored with aesthetic materials such as composite resins which are modified epoxies or other glassy polymers con- taining inorganic fillers, such as quartz or glass. Badly decayed or broken anterior teeth are often given porcelain crowns. Posterior teeth, which per- form the masticatory functions of crushing and chewing food, require mechanically stronger materials. Teeth with one or more cusps in situ are restored with dental amalgam while those requiring more extensive restora- tion are provided with metallic crowns covering all or part of the tooth.
These crowns may be covered by porcelain for aesthetic reasons. Missing teeth may be replaced by pontics which are attached, usually by soldered joints, to crowns on the adjacent or abutment teeth to form a bridgework or fixed partial denture (FPD).
Larger numbers of lost teeth are replaced by a removable partial denture (RPD), a framework or base to which are attached the replacement teeth.
RPDs are fabricated from a variety of noble and base metal alloys as well
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as polymers such as acrylic resin. Complete dentures, provided when all the teeth are lost, are usually made of acrylic resin but sometimes metals. If there has been significant loss of the bony support, metallic or ceramic implants may be placed in the remaining bone with posts projecting through the gums into the oral cavity, and dentures are attached to the posts. Orthodontics, the specialty dealing with tooth realignment and positioning, commonly uses stainless steel brackets, bands and wires attached to the teeth to effect tooth movement.
Clearly a variety of metals and alloys are used in dentistry. Corrosion occurring in the oral cavity generally does not result in catastrophic failure but rather discolouration of restorations, staining of the teeth and, in some cases, allergic reactions due to the release of metallic ions into the biosystem.
Corrosion can also cause disruption of restorations, leading to leakage at the restoration-tooth margins and to secondary dental decay. There has also been a report of corrosion of implanted plates and screws following oral surgery that delayed healing of a fracture and resulted in osteomyelitis of the mandible 5.
Dental Amalgam
Traditionally, the most widely used restorative material was dental amalgam, some 80% of all restorations comprising this material wholly or in part, and even today, dental amalgams constitute a high percentage of posterior restorations. Dental amalgam is prepared by triturating or alloying mercury with filings or spheres of a silver-base alloy to form a plastic mass which is inserted into the prepared tooth cavity where it sets to a hard mass, For decades, the composition of the conventional amalgam alloy powder remained largely unchanged from 65-70% Ag, 25-29% Sn, 0-6% Cu and 0-2'70 Zn, the principal component being the silver-tin eutec- tic, Ag,Sn, known as the gamma phase. When this reacts with mercury, two new phases, gamma-1, Ag,Hg,, and gamma-2, Sn,,Hg, are formed so that the set mass contains residual gamma particles, gamma-1 and gan1ma-2~~~. The gamma-2 phase is mechanically weak and susceptible to corrosion.
The corrosion mechanism of dental amalgam has been studied extensively and it is accepted that the gamma-2 phase is anodic to the other phases p r e ~ e n t ~ * ~ " . Corrosion of gamma-2 releases mercury which may react with residual gamma or the gamma-1 phase, the former reaction forming more gamma-2 and gamma-1 phases, while the latter forms mercury-rich gamma-1. The formation of additional gamma-1 and gamma-2 phases ensures continued corrosion of the Sn-rich phase throughout the body of the restoration. The mercury-rich gamma-1 phase is cathodic to gamma-1 '**
but the reaction is slow and polarises rapidly. Zero resistance ammetry studies" have shown that amalgam is susceptible to crevice corrosion and readily establishes galvanic cells, notably differential pH cells, cells between polished and corroded areas on the surface as well as those arising from com- positional differences within a restoration. Large galvanic currents are generated when amalgam is in contact with gold (or aluminium) and the magnitude of the current is affected by the type of alloy used to prepare the
CORROSION IN THE ORAL CAVITY 2: 157 amalgam, but saliva exerts an inhibitory effect on these galvanic currents ".
While there is good correlation between in vitro corrosion and fracture of restoration margins'f.13, in vitro studies correlates poorly with the in vivo corrosion of amalgam, which is lower than predicted. This low in vivo corro- sion rate has been ascribed to the formation of various salts such as the oxides, hydroxides, phosphates and oxychlorides of tin, copper and zinc present in the amalgam and the cathodic inhibiting action of the C@-- HCO; system in saliva2*'o* 14. Amalgam corrosion, however, does have the benefit that the corrosion products seal the margins between tooth and restoration.
The poor mechanical properties, corrosion and other problems associated with gamma-2 containing (conventional) dental amalgams led to the devel- opment of the high copper, reduced-tin content, amalgam alloys which are strengthened by the presence of Ag-Cu particles. When triturated with mercury, the gamma-2 phase reacts with Ag-Cu to form the eta phase, Cu,Sn, and more gamma-1 . The reduced gamma-2 content of these amal- gams results in greater strength, better clinical behaviour and lower corro- sion rates than the conventional amalgams4v6.
There have been numerous reports of possible allergic reactions to mer- cury and mercury salts and to the mercury, silver and copper in dental amalgam as well as to amalgam corrosion p r ~ d u c t s ' ~ - ~ ~ . Studies of the release of mercury by amalgams into distilled water, saline and artificial saliva tend to be conflicting and contradictory but, overall, the data indicate that mercury release drops with time due to film f o r m a t i ~ n ~ . ~ ' and is less than the acceptable daily intake for mercury in food". Further, while metallic mercury can sensitise, sensitisation of patients to mercury by dental amalgam appears to be a rare occurrence'*. Nevertheless, there is a growing trend to develop polymer-based posterior restorative materials in order to
eliminate the use of mercury in dentistry.
Noble Metal Alloys
Traditionally, full or partial coverage crowns, bridgework, RPDs, porcelain-fused-to-metal restorations (PFMs) and even complete denture bases were cast from high carat gold alloys. These alloys contain > 75% Au and are based on the ternary Au-Ag-Cu alloys with the mechanical proper- ties improved through additions of Pt, Pd and Ir. These high gold alloys possess near ideal casting characteristics as well as excellent corrosion resistance.
In recent years, however, the increasing price of gold has resulted in greater use of alloys containing less gold and greater amounts of silver and palladium as well as various base metal alloys (see later) for dental castings.
Lower gold content alloys have good mechanical properties and can be accurately cast but they do not possess the oral corrosion resistance of high gold alloys. Further, these alloys are often metallurgically heterogeneous or at least less homogeneous than the high noble-metal alloys, which will affect their corrosion susceptibility due to galvanic coupling effectsz3.
Cast restorations often exhibit little corrosion but rather an unaesthe- tic tarnishing or discolouration. There appears in fact to be an inverse
relationship between oral corrosion and tarnish for many low gold and copper-rich alloys in potential ranges where the corrosion rate is limited by the rate of the cathodic reaction’””. This effect is possibly due to the uniform distribution of anodic and cathodic sites in single phase alloys with no preferential deposition of corrosion products and consequently little tarnish. In contrast, the anodic and cathodic reactions in multiphase alloys are separated and the different nobilities of these phases may give rise to numerous bimetallic galvanic cells. Other systems, typically Ag-Pd alloys, do appear to show a correlation between tarnish and corrosionz5.
Most cast dental restorations are subjected to some form of subsequent heat treatment such as annealing, hardening or soldering. This often induces changes in the structural state or in the phases present and may establish local galvanic cells. Potentio-dynamic polarisation studiesx have shown that high gold alloys are unaffected by their thermal history but the corrosion susceptibility of low golds (containing <60% Au) and Ag-Pd alloys are markedly effected by heat treatment. As-cast and age-hardened structures were found to be more susceptible to corrosion than those annealed at 100’
below the solidus temperature. Other workersX*’’ also have reported that as-cast low golds are more prone to in vivo and in vitro tarnish than alloys solution heat-treated at 700°C. This does not appear to be the case for high Pd-Cu alloys used for porcelain bonding which show polarisation resistance values independent of the metal pre-treatmentz9.
Multiphase gold or palladium-based alloys never show dissolution of Au or Pd but often exhibit progressive surface ennoblement due to selective dissolution of copper or silver from the outer 2-3 atomic layers’”’’. Heat treatment often decomposes multicomponent alloys into a Pd-Cu rich com- pound and an Ag-rich matrix with corrosion of the latter phase in deaerated artificial saliva and S’--containing media”. Au-Cu-rich lamellae have simi- larly been observed, again with preferential attack on Ag-rich phases or matrix. These effects presumably arise from the ability of the noble alloy phases to catalyse the cathodic reduction of ~xygen’’-’~.
Generally, a greater Pd level in Pd-based single phase alloys results in increased corrosion resistance in the potential range and C1- level typical of the m ~ u t has might be expected. Corrosion of low gold alloys ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ , occurs in C1- media, typically with attack occurring on the Ag-rich matrix, but phosphate and bicarbonate ions have an inhibiting effect while the isothiocyanate ion can induce Various amino acids, typical of body fluids, can induce passivity but it appears that solution pH is the con- trolling factor in corrosion for high Ag-content alloys, passivation being enhanced by basic
Base Metal Alloys
Approximately 90% of all RPDs are now cast from base metal alloys con- taining principally chromium, cobalt and nickel, with chromium being the element present in all such alloys. Commonly, these cast chromium alloys contain various alloying elements, typically I 5% Mo, I 1% Fe, 25-30% Cr and the balance Co although there are some widely used alloys containing
CORROSION IN THE ORAL CAVITY 2: 159 15-20% Cr and ~ 5 % each of Al, Mn and Mo with the balance Ni. The physical properties of these base metal alloys are controlled by the minor alloying elements present, ie. Cy Mo, Be, W and Al. The upper Cr level is limited to 30% to avoid casting difficulties and the formation of the brittle sigma phase while the lower level is 15-20% to maintain corrosion resis- tance. There has also been a limited application of stainless steel for wrought RPDs. Oral implants are usually fabricated from Ti, Ta and Cr-Co alloys and most orthodontic appliances utilise stainless steel brackets and wires although there is increasing application of Co-Cr, beta-titanium and Ni-Ti alloys for orthodontic spring wires.
There has been comparatively little published on the corrosion behaviour of cast chromium alloys or on their in vivo corrosion products. The avail- able data are somewhat contradictory but it appears that these alloys tend to passivate in synthetic saliva solutions and in the potential range and chloride level corresponding to that of the oral cavity3842. It has been reported that in vitro polarisation results for Co-Cr alloys in isotonic (0.9%) saline closely correlate with in vivo polarisation in rabbits" and also that there are no significant effects on corrosion behaviour arising from mucin or the amino acids present in the proteins found in the mouth"*41. The corro- sion rates of stainless steel in isotonic saline were also found to be com- parable to or less than those measured in vitro". Other work indicates, however, that the corrosion rate of Cr-Co is several times greater in the mouth than that found in and proteins (from calf serum) were found to reduce corrosion rates and metal ion release from cast chromium alloys and a Ni-Cr-Be There is, however, no evidence of corro- sion of Be when present in Ni-base alloys@. There are no indications of pit- ting attack or tarnishing of cast chromium alloys in 5 % sulphide solution or in the mouth. Crevice corrosion of Co-Cr alloys does not occur in ferric chloride solution3" but it can occur when Co-Cr is in contact with gold in phosphate-buffered saline". Ni-base alloys, however, are susceptible to crevice corrosion 38 and galvanic coupling with gold increases the metal dis- solution rate4'.
Metal ion release from cast chromium alloys and stainless steel occurs in vivo as a result of corrosion and/or dissolution of passive films. In vitro
~ t u d i e s ' ~ indicates that the release of metals into artificial saliva is less than 0.2ppm over a two-month period but a recent in vivo shows that release of Cr and Co is readily detected in a short time ( 5 min) in the saliva of patients wearing Co-Cr dentures. Although metal release decreased with the age of dentures, presumably due to passivation effects, it still occurred over a long period, paralleling the results of other workers with Co-Cr and particularly Ni-Cr alloys over 35 week test periods". This long-term metal ion release would account for the many reports of mucosal and systemic allergic reactions to cast chromium alloy^^^*^^. The allergens in such cases are thought to be Ni and Cr but patients allergic to Ni are sometimes also allergic to Co. Although the literature on oral reactions to Cr and Co is sparse, allergic responses to Ni and Co were found in five out of ten Ni-sensitive patients patch-tested with Ni-containing Co-Cr alloyss3. In vivo release of Ni and Co through corrosion has been shown to occur with implanted alloys. The released metal localises in tissue near the implant and
to a lesser extent in various organs and is thought to result in allergic reaction^'^.^^. Additionally, Ni-Cr casting alloy powders have been shown to have cytotoxic potential’*.
There is, however, considerable debate over whether metal release from dental prostheses can cause sensitisation and/or allergic reactions and some consider it to be Overall there are relatively few reported cases of local or systemic hypersensitivity resulting from base metal dentures and it has been reported that the incidence of nickel allergy with intra-oral expo- sure to Ni alloys is only 4% compared to 6% in those without exposuresg.
In vivo studies with guinea pigsa indicate that exposure to Ni and Cr from dietary intake or intra-oral alloys can induce a state of partial tolerance to both metals. The authors suggested that individuals not hypersensitive to Ni or Cr may become partially tolerant as a result of intra-oral exposure to the metals. Further, they suggested that this could have application as a preven- tive protocol for those at high risk of sensitisationa.
There has been growing clinical interest in magnetic retention of over- dentures. Typically a permanent samarium-cobalt magnet, encapsulated in stainless steel or a high palladium alloy for strength and corrosion resistance, is fitted into the denture and a ferromagnetic alloy is cemented into the residual tooth root. The ferromagnetic alloys are principally Pd-Co alloys with small ( 5 2.5%) additions of Ga and ( 5 2Vo)P. These alloys appear to have good corrosion resistance in the oral potential range ( - 100 to 300 mV, vs S.C.E.) based on electrochemical studies6’ but there are, as yet, few reports on these materials which will probably be of increasing clinical importance in the future.
Galvanic Effects in the Mouth
Corrosion resulting from intra-oral mixed metal and other galvanic cells has been mentioned previously and dentists may often observe blackening of restorations and other effects but not always recognise their source. Many serious oral conditions such as lichen planus, leukoplakia and oral cancer have been ascribed to galvanic cells in the These couples may arise from different metals being in contact, electrical circuits being estab- lished between a gold crown and an amalgam core separated by a film of dental cement or even a soldered cast prosthesis with separations in the solder joints. Patients with such galvanic couples commonly have either a subjective complaint of a metallic taste or sensation or an objective com- plaint such as chronic inflammation of the mucosa and possibly neurologic complaints. Intra-oral measurements of electrode potentials and the proper- ties of patients’ saliva did not establish a relationship between the measured potentials of individual metallic restorations and orofacial complaints 65*66.
Intra-oral potential measurements made on patients with mixed metals in contact also indicated that large potential differences could exist between similar metals67. These authors indicated that a potential difference greater than 50 mV between metals was harmful. While the pathological effects arising from oral galvanic cells varied with individual patients and was not always proportional to the observed electrode potentials, regression and
CORROSION IN THE ORAL CAVITY 2: 161 often disappearance of oral lesions occurred when harmfully high potential differences were eliminated63*67.
Conclusions
Oral corrosion of metallic restorations does not, per se, generally result in serious damage to the structure. Corrosion can result, however, in various local and systemic effects, notably the hypersensitivity and allergic reactions reported by many workers. Galvanic cells created by mixed metal couples can delay fracture healing and induce oral lesions and cancer.
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