Barodontalgia: A review, and the influence of simulated diving on microleakage and on the retention of full cast crowns
Lyons, Karl M
This paper reviews the causes of barodontalgia and reports on a study that indicates a possible cause of barodontalgia in the diver. In the study, extracted teeth had full cast crowns cemented with either a zinc phosphate, a glass ionomer, or a resin cement, and simulated diving to 30 m (3.0 atmospheres) was performed. During simulated diving, the teeth were pressure cycled 15 times to 3 atmospheres and microleakage was monitored. The force required to dislodge the crown was then tested; a significant difference was found between the zinc phosphate and the glass ionomer cement groups (p
Environmental pressure variations have been shown to cause dental pain in divers and aviators; however, the effect of these variations on the retention of crowns to teeth and on the development of microleakage is largely unknown. Clinically, environmental pressure cycling has been found to be associated with barodontalgia, which is pain that is experienced in the teeth that is initiated by changes in barometric pressure. 2 This symptom was originally used to describe the dental pain developed by pilots in unpressurized cockpits during the early 1940s.2 Although relatively uncommon, it was often severe enough to incapacitate the pilot.3 The problem has been serious enough to stimulate continued research into barodontalgia,l24-14 which has been reported to occur in aviators from an altitude of 3,000 m (0.75 atmosphere) and in the diver at 10 m (1 atmosphere). However, Strohaver7 reported that the incidence of barodontalgia in pilots has decreased since World War II.
Strohaver7 has advocated the differentiation of barodontalgia into direct and indirect types. In the direct type, reduced atmospheric pressure contributes to a direct effect on a given tooth, whereas in the indirect type, dental pain is secondary to stimulation of the superior alveolar nerves by a maxillary barosinusitis. Direct barodontalgia is generally manifested by moderate to severe pain, which usually develops during ascent, is well localized, and the patient can frequently identify the involved tooth; indirect barodontalgia is a dull, poorly defined pain that generally involves the posterior maxillary teeth and develops during descent.7 If pain occurs during descent, indirect barodontalgia attributable to barosinusitis should be suspected. If indirect barodontalgia is diagnosed, the patient should be referred to a medical practitioner or an ear, nose, and throat specialist for treatment.
Certain generalities have been established to help with the diagnosis of direct barodontalgia. Posterior teeth are more frequently involved than anterior teeth, and maxillary teeth are affected more often than mandibular teeth. Teeth with amalgam restorations are more likely to be involved than unrestored teeth, and recently restored teeth are particularly susceptible. The character of the pain may be useful in determining whether the pulp involvement is acute or chronic; however, in an individual patient, this degree of diagnostic precision may not be possible.
Examination of a patient complaining of barodontalgia should include an estimation of the age of restorations in the suspected area, exploration for caries or defective restorations, percussion of any suspected tooth, the patient’s response to application of electrical stimulation and/or cold and heat, and radiographic examination. 12 Appropriate radiographs of the suspected teeth should be obtained, with the understanding that a negative radiograph does not rule out pulpitis. Reexposure to altitude in a chamber can also be used to confirm a doubtful diagnosis or to determine the effectiveness of therapy. Direct barodontalgia can be largely prevented by high-quality dental care. However, after dental treatment involving deep restorations, Strohaver7 has recommended that flying duties be restricted for 48 to 72 hours to allow time for the dental pulp to “quiet down” or stabilize. This would also apply to divers, particularly deep sea divers. Overall, regular dental examinations are essential for aircrew and divers; any dental problem that might predispose to barodontalgia should be corrected to prevent the development of symptoms.
Strohaver7 has also noted that the incidence of reported barodontalgia has decreased since World War II, possibly as a result of better dental care for fliers and improvements in cockpit and passenger compartment conditions, particularly cabin pressurization. To date, barodontalgia has not been a problem in space flight because astronauts operate in a pressurized cabin and are not exposed to the spectrum of pressure changes that are encountered in flights in modern aircraft.
The precise mechanism for pain production in barodontalgia has not been determined, although exposure to altered atmospheric pressure is obviously a significant factor. Exposure to reduced barometric pressure is evidently a precipitating factor, with disease of the pulp a probable cause. Adler15 classified symptoms of barodontalgia as being caused by either trapped gases or evolved gases. He placed barodontalgia in the trapped gas category primarily because symptoms frequently appeared at altitudes well below those considered necessary to produce evolved gases. Orban and Ritchey,4 on the other hand, concluded that the mechanism had to be the liberation and expansion of gases from the blood and tissue fluids; however, they were able to demonstrate empty, bubble-like spaces in only 6 of the 75 teeth that they studied. Air trapped under restorations has also been suspected of producing tooth pain, but experimentally produced air bubbles under dental restorations followed by exposure to reduced barometric pressure did not result in symptoms. The effects of pressurized oxygen and low temperature have also been considered, but disease of the pulp appears to be the most significant dental factor.
Whether pressure changes elicit pain in teeth with normal pulps, regardless of whether the tooth is intact, carious, or restored, is uncertain. Orban and Ritchey4 and Ferenstik and Aker6 stated that normal pulp tissue would not produce pressure-associated pain, regardless of whether restorations or caries were present. However, Sognnaes,5 Shiller,6 Hodges,8 and Hutchins and Reynolds”7 have reported that dental pain could be produced in apparently healthy teeth when the atmospheric pressure was increased to a level corresponding to a depth of 3 atmospheres.
Pain caused by rapid pressure variation seems to occur most commonly in heavily restored teeth without adequate lining cement, in teeth with extensive crown preparations, and in teeth with infected root canals. Experimental research indicates that barodontalgia may depend on an increased pulpal pressure induced by pressure variations in the permeability of the dentinal tubules.10 Other authors have concentrated on the possible variations in volume, caused by pressure changes, or microbubbles of air trapped within insufficiently filled restorations, cements, or root canals.1 Barodontalgia has been found to occur during diving, in teeth with carious lesions, or where there are periapical lesions, periodontal abscesses, maxillary sinus congestion, and recently crowned teeth. Dahl18 and Richardson et al.19 have shown that during a crown preparation, dentin permeability increases as dentin is prepared closer to the pulp. This should not cause a problem if the restoration placed is sealed and there is no microleakage.20-22 However, if microleakage occurs, it may indicate a deficient margin or disruption of the cement. At sea level, there may be no symptoms, but several meters below the surface barodontalgia may present clinically before crown debonding. The importance of microleakage in clinical dentistry is well recognized, but the exact amount that becomes clinically significant is at present undefined.
It is possible that repeated exposure to the environmental pressure variations experienced by divers could also affect the bond strength of crown luting cements. An in vitro study by Musajo et al.23 found that the bond strength of zinc phosphate cement was significantly reduced by pressure cycling. Thus, the mechanical failure of a luting cement may be another cause of barodontalgia. Additionally, the danger resulting from dislodgment of a fixed prosthesis during a dive is obvious.
Zinc phosphate cement has, until relatively recently, been the most commonly used luting agent (dental cement) for crowns and bridges. However, glass ionomer cements, and lately resin cements, have been used increasingly as luting agents, principally because of their adhesion to tooth structure and, in some cases, to the restorations as well. What effect the pressure variations that divers and aviators are exposed to have on the development of barodontalgia, on the retention of crowns to teeth, and on the development of microleakage is still largely unknown.
The aims of this study were to determine (1) whether during environmental pressure cycling, there would be any microleakage associated with extracted teeth that had full cast crowns cemented with one of three different cements-a zinc phosphate, a glass ionomer, or a resin cement-and the stage during pressure cycling that microleakage occurred, and (2) whether environmental pressure cycling would affect the retention of full cast crowns cemented to extracted teeth.
Materials and Methods
Preparation of the Teeth
Sixty extracted single-rooted premolar teeth were used for this study. The teeth were cleaned of gross debris and blood under running water after extraction, then stored in 10% formalin. Storage in 10% formalin, has been shown in a number of studies, including that of Goodis et al.,24 not to have a significant effect on dentin permeability and bond strength of cements to the tooth. Two millimeters of the root apex was removed using a fast-running, diamond-impregnated, copper disc running in water.25.zs The root canals were prepared from the apical end of the teeth using standard K-files (United Dental Manufacturers, West Palm Beach, Florida), Hedstrom files (United Dental Manufacturers), and Gates Glidden drills (Komet, Gebr Brasseler, Lembgo, Germany). During instrumentation, the canals were irrigated with 5.25% NaOCl. The canals were prepared to a diameter that would allow subsequent placement of a cannula (21-gauge hypodermic needle, Becton Dickinson Medical Products Pte. Ltd., Singapore).
The crown preparations were cut by one person and were standardized by cutting uniform vertical and horizontal grooves, 0.5 mm deep, into the tooth using laminate veneer diamond depth guides (Brasseler Laminate Veneer System, Berlin, Germany), and a 0.5-mm shoulder margin preparation was prepared. Preparation for full-coverage cast crowns was completed using a water-cooled high-speed straight medium diamond bur (number 837KR, Komet, Berlin, Germany). The shoulder of the preparations followed the cement-enamel junction. The surface area of each preparation was assessed using occlusal indicator wax (Kerr, Emeryville, California), which was adapted to the crown preparation and then carefully removed and measured against 1-mm-square graph paper. This confirmed that the areas of the respective crown preparations were approximately the same (95 + 2 mm2). Preparations outside of these measurements were rejected.
Preparation of the Cast Crowns
An additional silicone (Exaflex, G.C. America, Inc., Chicago, Illinois) impression of each crown preparation was made, from which a type V die stone (Suprastone, Kerr, Romulus, Michigan) model of each tooth was made. Three layers of die spacer (TruFit, George Taub Products and Fusion Co., Inc., Jersey City, New Jersey) were placed over the crown preparation, after which a wax coping (Plastodent-Set, Degussa GB Dental, Frankfurt, Germany) was produced and invested in a phosphate-bonded investment material (Hi Temp, Whipmix Corp., Louisville, Kentucky). Metal castings (Spartan Plus, Williams, Amherst, New York) were subsequently made with a loop cast on the occlusal surface to allow testing of the retentive strength.
The 60 teeth with full cast crowns were assigned to three experimental groups of 20 crowns each according to the cement used for luting. The crowns were cemented with one of three cements: a zinc phosphate cement (S.S. White Manufacturing Ltd., Gloucester, England), a tri-cured glass ionomer cement (Vitremer, 3M Dental Products, St. Paul, Minnesota), or a resin cement (Panavia 21 dental adhesive, Kuraray Co., Ltd., Tokyo, Japan). The crowns were cemented to the teeth under a static load of 2.0 kg held for 15 minutes. After the initial setting period of 15 minutes, the excess cement was removed and the specimens were stored in 0.9% NaCI solution for 7 days at 37 deg C.
Preparation of the Experimental Groups for the Pressure Chamber
After 7 days, each group of 20 teeth was randomly divided into two groups of 10 so that the experimental and control groups both consisted of 10 teeth with crowns cemented with zinc phosphate cement, 10 cemented with glass ionomer cement, and 10 cemented with resin cement. The experimental and control specimens were secured with autopolymerizing methyl methacrylate (Ostron, GC Corp., Tokyo, Japan) within a brass cylinder that had been threaded internally and externally. Each tooth was placed within a brass cylinder so that the cemented crown margin was clear of methyl methacrylate. A cannula was then inserted into the prepared root canal of each of the teeth in the experimental groups and sealed in place with methyl methacrylate (Fig. 1). The methyl methacrylate was allowed to cure for 24 hours at 37C in 0.9% NaCl before pressure testing.
Pressure Chamber Cycling
A pressure chamber was used that featured a threaded port into which the threaded brass mounting rings containing the teeth could be successively secured. Pressure within the chamber was monitored by an electronic pressure transducer (Sensotec Transducer, model A-205, Columbus, Ohio), which was connected to one channel of an analog/digital converter (MacLab, ADInstruments, New South Wales, Australia), which was in turn connected to a computer (Apple Macintosh, Apple Computers, Cupertino, California). The brass cylinder containing the tooth was screwed into the pressure chamber so that the crown was sealed inside the pressure chamber and the cannula that had been inserted into the apex of the tooth was outside the pressure chamber.
A physiological pressure transducer (Stratham Transducers, model P23AC, Hato Rey, Puerto Rico) was attached to the cannula sealed within the root canal, at the apical end of the tooth, with polyethylene tubing. This was also connected to the MacLab and allowed the continuous monitoring of pressure changes occurring through the tooth during the pressure cycling.
Each experimental specimen was then subjected to 15 compression cycles over a range of 0 to 3 atmospheres (304 kPa). In each cycle, nitrogen gas was introduced into the pressure chamber at a rate of 1 atmosphere per minute, allowing the maximum pressure to be reached in 3 minutes. The chamber was held at 3 atmospheres for 3 minutes, then decompressed over a 3-minute period. The experimental procedure is summarized in Figures 2 and 3.
Tensile Strength Study
Twenty-four hours after pressure cycling, each specimen in the experimental and control groups was tested to determine the force required to dislodge the crowns. The tensile bond strength of the cemented crowns was tested in a universal testing machine (Instron, model 1193, Bucks, England).27 The load-cell amplifier of the testing machine was connected to the MacLab, and the data were displayed by the Macintosh computer. The testing machine cross-head speed was calibrated to move at 0.5 mm/min, and a 100-kg tension load cell was used.
Microleakage Detected during Pressure Cycling A time delay was observed from when pressure cycling commenced to when microleakage was first detected. Microleakage was detected in one to three pressure cycles for the zinc phosphate cement group (Fig. 4), in which all experimental samples developed microleakage, and in three to five pressure cycles for the glass ionomer cement group (Fig. 5), in which 7 of the 10 experimental samples developed microleakage during pressure cycling. Microleakage was greater in the experimental zinc phosphate cement groups and occurred after fewer pressure cycles than in the experimental glass ionomer group. No microleakage was detected in any of the experimental samples cemented with the resin cement.
A comparison was made between the tensile bond strength of the cements and the amount of microleakage during pressure cycling. A significant correlation was found between leakage and bond strength of the cement for zinc phosphate (S = 0.72, p
Retentive Strength of Crowns on Extracted Teeth
A summary of the results of the retention of the full cast crowns cemented to the extracted premolars is shown in Table I. The results indicate that the bond strengths of the zinc phosphate and glass ionomer cement experimental groups were significantly less than those of the controls (p
An analysis of variance showed that there was no significant difference between the glass ionomer and zinc phosphate control groups (Table II), but there was a significant difference between the glass ionomer and resin control groups and between the zinc phosphate and resin control groups.
Comparison of the experimental groups showed that there were significant differences between the bond strength of the glass ionomer and zinc phosphate cement groups, between the glass ionomer and resin cement groups, and between the zinc phosphate and resin cement groups (Table II).
There are a number of possible reasons why microleakage developed during pressure cycling with the glass ionomer and zinc phosphate cements and not with the resin cement. Porosities introduced into the cements during mixing may have expanded and contracted during pressure cycling, weakening the cement. Other possibilities include microcracks appearing as a result of volumetric contraction,28,29 or internal stress30 within the brittle, thin film of the zinc phosphate and glass ionomer cements, which, when subjected to the effects of pressure cycling, may have produced stresses that exceeded the strength of the material, resulting in disruption of the cement layer and allowing microleakage to occur. Whether this may contribute to the development of barodontalgia was not investigated in this study.
The absence of microleakage detected with the resin cements may be the result of obstruction of the dentinal tubules by resin tags, or it may be that the resin cement was sufficiently flexible to resist microfracture during pressure cycling. Therefore, the microleakage detected in this study may be attributable to a mechanical failure of the cement, and the degree of microleakage may be related to the bond strength of the cement. Fortin et al.31 found that the lower the bond strength of a dentine adhesive, the greater the amount of microleakage.
Pressure Cycling and the Timing of Microleakage
The time taken for the effect of pressure cycling to disrupt the zinc phosphate and glass ionomer cements may account for the time delay from the commencement of pressure cycling to when microleakage was detected. On average, a greater number of pressure cycles were required before the development of microleakage with glass ionomer cement than with zinc phosphate cement. This may be because glass ionomer cement is stronger and less brittle than zinc phosphate cement, and glass ionomer cement develops a strong chemical bond to dentin and zinc phosphate develops a weaker mechanical bond.
This study also showed an inverse relationship between microleakage and the bond strength of the crowns to the extracted teeth. Zinc phosphate cement showed greater microleakage and lower bond strength than glass ionomer cement. These findings indicate that low-strength, brittle cements suffer greater disruption during pressure cycling, leading to earlier and greater microleakage.
Crown Retention to Teeth
This study found that the bond strength of the zinc phosphate cement experimental group was reduced to about one-tenth the strength of the untreated controls and that the bond strength of a glass ionomer cement was reduced by about half after pressure cycling compared with the untreated controls. What effect this may have on the development of barodontalgia was not investigated in this study. The bond strength of the resin cement was not significantly affected by pressure cycling.
Microcracks have been found to appear as a result of volumetric contraction,2829 which, when subjected to the effects of pressure cycling, may have produced tensile stresses that exceeded the strength of the material, resulting in the significant reduction in tensile bond strength of the zinc phosphate and glass ionomer cements found in this study, It has also found that the bond strength of a resin cement (Panavia EX) was not significantly affected by volumetric contraction.29 Finally, Kamposiora et al.,30 in a study using finite element analysis, found that under normal functional loads zinc phosphate cement exhibited greater average internal stresses than glass ionomer or resin cements. These three studies indicate that brittle cements, such as zinc phosphate and glass ionomer, may be subject to greater internal stresses than resin cements. It is likely, therefore, that the brittle cements will be more affected by environmental pressure cycling than resin cements, as was found in this study.
Within the limitations of this study, the following conclusions can be made based on the results.
(1) Microleakage during environmental pressure cycling has been shown to occur in crowns cemented with zinc phosphate and glass ionomer cements; the microleakage was greater and more consistent with crowns cemented with zinc phosphate cement. Microleakage was detected with zinc phosphate cement after fewer pressure cycles than with glass ionomer cement. No microleakage occurred in crowns cemented with resin cement.
(2) Environmental pressure cycling affects the retention of crowns cemented with zinc phosphate and glass ionomer cements. The resin cement (Panavia 21) used in this study appeared to be unaffected by environmental pressure cycling.
The occurrence of microleakage and the reduction in the retention of crowns cemented with zinc phosphate and glass ionomer cements after pressure cycling may present clinically as barodontalgia before crown debonding. On the basis of the results of this study, dentists should consider cementing fixed prostheses with resin cements for patients who are exposed to marked variations in environmental pressure, such as recreational and professional divers, shortly after placement of the prosthesis.
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Karl M. Lyons, MDS FRACDS*
John C. Rodda, MDS FRACDS*
James A.A. Hood, BSc MDS FRACDS^
*Department of Oral Rehabilitation, and ^ Department of Oral Sciences and Orthodontics, School of Dentistry, University of Otago, Dunedin. New Zealand.
This manuscript was received for review in November 1997. The revised manuscript was accepted for publication in July 1998.
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