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Effects of Thermal and Mechanical Challenges burs).19 Collectively, these results indicate that pulpal 8 HS air reactions to various restorative procedures17-19 are not necessarily caused by excessive heat production. However, it is difficult to precisely position tempera­ ture sensors to detect heat generated during cut­ ting. In addition, the poor thermal conductivity of dentin can result in thermal burns to surface dentin without much change in pulpal temperature.20 HS air-water Pulpal reactions to restorative procedures may / - - - ...... in part be caused indirectly. It is possible that a high LS air-water surface temperature can thermally expand the den­ tinal fluid in the tubules immediately beneath poorly -5 0 5 10 15 20 25 30 35 irrigated burs. If the rate of expansion of dentinal Time(s) fluid is high, the fluid flow across odontoblast pro­ cesses, especially where the odontoblast cell body fills the tubules in predentin, may create shear forces Fig 15-2 Changes in pulpal temperature during low-speed (LS) and high­ sufficiently large to tear the cell membrane21 and speed (HS) cavity preparation with and without air-water cooling. (Modified from Zach and Cohen10 with permission.) induce calcium entry into the ce//,22 possibly leading to cell death.23 This hypothesis suggests that ther­ mally induced fluid shifts across tubules serve as the transduction mechanism for pulp cell injury without causing much change in pulpal temperature. An additional factor that can cause pulpal irrita­ that produced in pulp tissues. However, the out­ tion is evaporative fluid flow.24 Blowing air on dentin come may be influenced by the fact that the blood causes rapid outward fluid flow that can induce the flow per milligram of tissue is higher in the periodon­ same cell injury as the inward fluid flow caused by tal ligament (PDL) than in the pulp.9 heat. For this reason, dry cutting with air is not rec­ In their classic study of pulpal reactions to cavity ommended. Although air blasts lower pulpal tem­ preparation, Zach and Cohen10 demonstrated that perature, 10-13 they induce very rapid outward fluid pulpal temperature was actually lowered during flow in dentinal tubules24-26 that can create shear cavity preparation with high-speed handpieces and stress across odontoblasts and subodontoblastic air-water spray because the water spray was cooler cells and may tear their membranes. than the temperature of the pulp (Fig 15-2) and Recent studies have not been able to confirm because of the high heat capacity of water. They the earlier reports of pulpal damage from thermal recommended what they called the washed-field stress. Because many dental procedures can elevate technique of tooth reduction, in which the tooth sur­ pulpal temperatures by 9°C to 15°C, Baldissara et face is exposed to the air-water spray for 5 seconds al27 evaluated the pulpal response to these tempera­ before cutting. After initial cutting, the bur is lifted ture changes in normal young premolars scheduled off the surface for 1 second following every 4 sec­ for extraction for orthodontic purposes. They placed onds of cutting. When this technique was used, the custom-fabricated metal plates on the teeth. Ther­ pulpal temperature never rose above basal tempera­ moresistors were attached to the plates to produce tures (see Fig 15-2). controlled heat flows. The surface temperature of Cutting at high speeds with air alone used as a the test teeth was measured before and during coolant lowered pulpal temperature prior to cavity controlled heating in nonanesthetized patients who preparation; however, the pulpal temperature rap­ were able to record both prepain and pain sensa­ idly rose as much as 8°C higher than normal during tions during these procedures. the procedure.10 This observation has been con­ The rate of heat application in this study27 was firmed by others.8·9·11-14 The frictional heat produc­ much lower than that used by Zach and Cohen7 and tion will depend on rotational speed15 and torque,14 was selected based on the rate of heating reported the amount of force applied to the bur,1&-1s the cool­ in the literature from a variety of restorative proce­ ing efficiency of the irrigant, and the prior wear and dures (Fig 15-3). In monkeys, Zach and Cohen7 found design of the bur (eg, cutting blades such as carbide that a pulpal temperature of 40.5°C produced pulpal fissure burs or grinding surfaces such as diamond necrosis in 60% of the tested teeth. In the human 352 www.shayanNemodar.com Pu/pal Responses to Mechanical Stimuli During CavityPreparation study conducted by Baldissara et al,27 heating of --- Zach and Cohen' (adapted) 52 teeth to 39.5°C to 50.4°C (average 44.5°C) caused ---- Direct provisional crowns (Tjan et al28) pain. This was perceived first as a "swelling" of the 50 Baldissara et al27 tooth, but as the thermal stimulus continued, the 48 - Tooth preparation (air cooled) (Zach and Cohen 10) pain became more intense in magnitude, dull in per­ 46 ceptual character, and poorly localized. These symp­ � 44 toms are hallmarks of unmyelinated C-fiber nocicep­ � � /� � 42 tor pain (see chapters 7 and 8). The occurrence of ., c. postoperative symptoms was followed for 63 to 91 E ., 40 days, during which time none of the patients report­ I- 38 ed any spontaneous tooth pain. Histologic examina­ tion of the teeth failed to demonstrate any signs of 36 inflammation or reparative dentin. 34 Similar in vitro studies27 were done on extracted 0 5 10 15 20 25 30 35 40 human teeth with thermocouples placed at the pulp­ Time (s) odentin border, immersed to the cementoenamel Fig 15-3 Increases in pulpal temperature following a variety of experimental junction in 37°C. When the same electric currents procedures. (Modified from Baldissara et al27 with permission.) were applied to the teeth in vitro, the authors could follow the rate and duration of changes in pulpal temperature (see Fig 15-3). This study concluded that young premolars could withstand increases in pulpal temperature between 8.9°C and 14.7°C with­ 3.4°C. This resulted in 36.4% and 33.4% reductions, out any histologic evidence of pulpal damage. Their respectively, in cell metabolism as measured by the rate of heat application was less than that used in 3-(4,5-d i methylthi azol-2-yl)-2,5-d i p he nyltetrazo Ii um the studies by Zach and Cohen.7·10 Thus, the rate of bromide (MTT) assay. The authors concluded that a delivery of heat is probably more important than the quartz tungsten halogen curing unit activated for 40 absolute rise in pulpal temperature. seconds could cause adverse cellular responses in This temperature range is similar to that mea­ 0.5-mm-thick dentin.36-37 sured in pulp chambers during finishing or polishing The reader is referred to a recent review on heat of restorations.28 Even higher increases in pulpal transfer in human teeth38 for further details. temperature have been measured during self-curing of provisional crowns28-30 and from visible light­ cured crowns.31 These studies were repeated using the turbo tips that concentrate light on smaller sur­ Pulpal Responses to Mechanical faces32 and new high-intensity light sources,32·33 with Stimuli During Cavity Preparation similar results. Some authors believe that in vitro studies of pulpal temperatures in response to various thermal Several studies have reported on the release of stimuli should be done while the pulp chamber is enzymes and other immunoreactive substances in perfused by fluid to simulate pulpal microcircula­ the dental pulp during mechanical tooth prepara­ tion. A recent study confirmed that perfusion of the tion procedures. The release of these substances pulp chamber significantly lowered the pulpal tem­ may be due to temperature increases, the mechani­ perature increases in response to external thermal cal stimuli of tooth preparation, or both. Because stimuli34 as well as to light-curing units.35 temperature effects have already been discussed, An in vitro experiment involved the use of 0.5- this section reviews only the mechanical, nonthermal mm dentin disks covered on their pulp surfaces causes of release of these substances. by odontoblast-like cells. When the investigators Many enzymes and other immunoreactive sub­ irradiated the upper dentin surface with a quartz stances are normally present in the pulp under tungsten halogen or a light-emitting diode (LED) unstimulated conditions. When the pulp is stimulated light-curing unit at 553 and 240 mW/cm2, respective­ mechanically, these substances are released by physi­ ly, the "pulp" surface temperature rose 6.4°C and ologic mechanisms (eg, exocytosis) or by disruption 353 www.shayanNemodar.com Effects of Thermal and Mechanical Challenges of cell membranes.39 An early study in monkey teeth was found in all pulp specimens but in only 25% examined the effect of cavity preparation on enzyme of dentin specimens. The results indicated varying release (alkaline and acid phosphatases and others) responses of plasma protein release in reaction to in the pulp.40 Tooth preparation by air turbine and mechanical injuries. adequate water cooling did not affect enzyme activ­ Another study examined changes in the distribu­ ity, nor did the application of corticosteroids. When tion of fibrinogen/fibrin and fibronectin in the pulpo­ calcium hydroxide was applied to the cavity floor, dentin complex after Class V cavity preparation in however, enzyme activity was increased after 24 hours maxillary rat molars.44 Fibrinogen was detected in in the odontoblastic and subodontoblastic cell layers the exudate and dentinal tubules at various times adjacent to the calcium hydroxide-covered dentin.

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