Why Does Supragingival Calculus Form Preferentially on the Lingual Surface of the 6 Lower Anterior Teeth?
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Clinical P RACTIC E Why Does Supragingival Calculus Form Preferentially on the Lingual Surface of the 6 Lower Anterior Teeth? Contact Author Colin Dawes, BSc, BDS, PhD Dr. Dawes Email: Colin_Dawes@ umanitoba.ca ABSTRACT Many authors have assumed that the reason supragingival calculus tends to form pref- erentially on the lingual surface of the 6 lower anterior teeth is because saliva from the adjacent submandibular ducts is a source of calcium and phosphate ions and because loss of CO2 as the saliva enters the mouth increases the local pH. However, the fluid phase of plaque in all locations is supersaturated with respect to the calcium phosphates in calculus and there is always a tendency for calculus to deposit, except after sugar con- sumption, when plaque pH may fall below the critical level and the plaque fluid becomes unsaturated. pH is least likely to fall below the critical level in plaque lingual to the lower anterior teeth, as this plaque is very thin, sugar concentration after sugar intake is lowest in that area and its clearance rate is fastest, and the high salivary film velocity there promotes loss of any acid formed in plaque. A high salivary film velocity also brings more salivary urea to the site, which facilitates plaque alkalinization. These factors all contribute to the development of shallow Stephan curves of short duration and together provide a more reasonable explanation for the fact that supragingival calculus depos- ition progresses most easily on the lingual surface of the lower anterior teeth. © J Can Dent Assoc 2006; 72(10):923–6 MeSH Key Words: dental calculus/etiology; dental calculus/prevention & control; dental plaque; This article has been peer reviewed. surface properties alculus is mineralized dental plaque and plaque bacteria to ammonia, which increases mineralization can only occur if the plaque pH.1,2 Both classes of patients are very Cfluid phase of plaque is supersaturated susceptible to calculus deposition.1 with the components of calculus. Saliva and All dentists and dental hygienists will have plaque fluid are normally supersaturated with noted that supragingival calculus almost al- respect to various calcium phosphates, except ways occurs predominantly on the lingual when fermentable carbohydrates are being surface of the 6 lower anterior teeth, with consumed, and thus most people are suscept- lesser amounts on the buccal surface of the ible to calculus deposition, albeit at different upper molars. In contrast, there is little or no rates. The degree of supersaturation of plaque site specificity in the deposition of subgingival fluid increases when its pH is high. This oc- calculus.3 The proportion of supragingival curs in patients who are tube fed, as their calculus on the lingual surface of the 6 lower plaque is not exposed to fermentable carbo- anterior teeth has been reported to range hydrates.1 It also occurs in patients on dialysis from 63%3 to 88%4; the amount of calculus for renal disease, as their salivary urea levels on the lateral incisors and canines is 70.2% are high and the urea can be converted by and 44.5%, respectively, of that on the central ���JCDA • www.cda-adc.ca/jcda • December 2006/January 2007, Vol. 72, No. 10 • 923 ––– Dawes ––– Hydroxyapatite Mineral deposition Plaque 3.0 pH solubility or dissolution LLM = Lingual lower molars FLM = Facial lower molars 9 2.5 Supersaturation Calculus deposition FUI = Facial upper incisors and enamel white-spot PUI = Palatal upper incisors remineralization e 2.0 8 FLI = Facial lower incisors os FUM = Facial upper molars cr 1.5 7 su LLI = Lingual lower incisors % 1.0 6 0.5 Critical Saturation pH 5 0.0 Caries and early 0 2 4 6 8 10 4 Unsaturation calculus dissolution Time (min) Figure 1: The effect of plaque pH, which may range from about Figure 2: Mean changes in the percentage of sucrose in saliva 4–9, on hydroxyapatite solubility and on the tendency for mineral at different sites in the mouth over 10 minutes beginning a deposition or dissolution. At the critical pH, which varies slightly few seconds after the mouth was rinsed for 1 minute with a among individuals, plaque fluid is just saturated with respect to 10% sucrose solution, and after consumption of a doughnut, hydroxyapatite and there is no tendency for mineral to deposit or mint candy or orange juice. dissolve. incisors.5 Wirthlin and Armitage6 and many previous pH, plaque fluid will be supersaturated with respect to authors have attempted to explain this site specificity hydroxyapatite, while below the critical level it will be on the basis that saliva leaving the submandibular– unsaturated. In addition, when plaque pH is above the sublingual and parotid ducts provides a source of cal- critical level, there is a tendency for calculus to deposit cium and phosphate to dental plaque and that loss of CO2 and for remineralization of white-spot enamel lesions to as the saliva enters the mouth increases the local pH. occur, although a considerable degree of supersaturation They state, correctly, that this will make the saliva more is necessary before these processes take place.9 Conversely, supersaturated with calcium phosphates, which might when plaque pH is below the critical level, caries will tend promote calculus deposition in dental plaque on the teeth to occur and any small calcium phosphate crystals that in locations close to the salivary ducts. have not been integrated into the 3-dimensional network However, as calcium and phosphate concentrations present in calculus will tend to dissolve. Thus calculus in plaque fluid are higher than those in saliva,7,8 there is formation is most likely to occur when plaque pH re- normally no concentration gradient to allow these ions to mains well above the critical level for long periods. move from saliva into dental plaque. In addition, the loss of CO2 into air present in the mouth is far from instantan- Factors Influencing the Depth and Duration of eous, and one might expect that as saliva travels further the Stephan Curve away from the duct openings, more CO2 will be lost and Exposure to Fermentable Carbohydrate salivary pH will become even higher. If the explanation The main reason for a fall in plaque pH to below the 6 provided by Wirthlin and Armitage were correct, more critical level is exposure to fermentable carbohydrate, calculus would be expected at sites further from the sali- which allows the formation of acid by plaque bacteria. vary ducts. Thus their explanation for the site specificity of The fall in pH and its subsequent rise as acid is removed supragingival calculus deposition seems unlikely. is termed the Stephan curve. The higher the carbohydrate concentration in the saliva over the plaque, the greater The Mineral Phase of Calculus and Its Solubility the amount that will diffuse into plaque and be avail- The mineral phase of calculus is composed of cal- able as a substrate for acid formation. Ingested sugar is cium phosphate.9 Although 4 different forms (dicalcium not uniformly distributed in the saliva,12,13 and Fig. 2 phosphate dihydrate, octacalcium phosphate, tricalcium shows that, after the intake of various sources of suc- phosphate and hydroxyapatite) have been detected,10 rose (doughnut, mint candy, orange juice and a sucrose hydroxyapatite is the predominant one10 and the least rinse), there are marked differences in salivary sucrose soluble.9 All are more soluble at an acid pH and less sol- concentrations at different sites.12,13 Particularly striking uble at an alkaline pH. The concept of a critical pH for is the fact that sugar concentrations in saliva lingual hydroxyapatite solubility has been discussed recently in to the lower incisors and, to a lesser extent, buccal to this journal11 and Fig. 1 illustrates that above the critical the upper molars are much lower than at the other sites 924 JCDA • www.cda-adc.ca/jcda • December 2006/January 2007, Vol. 72, No. 10 • ––– Supragingival Calculus ––– 7.0 L H 86.2 mm/min 6.5 H = High L = Low 6.0 8.2 mm/min H H H VL = Very low 5.5 L H L pH 0.78 mm/min 5.0 L H H L 4.5 H HH H H, L, VL Sugar conc. H H , L Film velocity 4.0 VL 0 25 50 75 100 125 150 175 200 Calculus Time (min) L L Figure 3: The effect of the velocity of a salivary film, 0.1 mm Figure 4: The site specificity of supragingival calculus deposition, deep, on the Stephan curve developed in artificial plaque, the film velocity when salivary flow is unstimulated and the sali- 0.5 mm deep, after simulation of a rinse with 10% sucrose vary sucrose concentration after consumption of sucrose from any for 1 minute. The velocities of 8.2 and 0.78 mm/min are close source. to the maximum and minimum unstimulated rates, while that of 86.2 mm/min is about 10 times higher than the maximum when salivary flow is unstimulated. pH is the mean at the enamel surface of a plaque 6 mm in length. tested; this occurred with all 4 products13 and when gum the plaque, they must diffuse out into the overlying film containing sucrose was chewed.12 Thus, plaque on the of saliva, which averages just under 0.1 mm in thickness,15 lingual surfaces of the lower anterior teeth will have the and the salivary film must move across the plaque to keep least amount of substrate available for acid formation acid from accumulating there and reducing the diffusion compared with other locations in the mouth. Tube-fed gradient for further acid removal from the plaque.