Photoelastic analysis of stress induced from insertion of self-threading retentive pins

Abstract

W hen teeth have lost significant amounts of their original structure, it is common to use pins as auxiliary support for the restorative material. Three basic systems are used for the retention of auxiliary pins: (1) cement (threaded or nonthreaded pins are luted into a prepared channel that is slightly larger in diameter than the pin); (2) friction (nonthreaded, tapered pins are forced into a channel that is slightly smaller than the pin); and (3) threads (threaded pins are twisted into a channel that is prepared slightly smaller in diameter than the pin).’ Of the three systems, self-threading pins are used most frequently because of their *excellent retentive properties.2 However, teeth may be damaged as a result of their use; cracking and crazing of dentin has been observed in association with insertion of self-threading retentive pins in extracted teeth.3 Two-dimensional photoelastic models have been used to observe stresses associated with the placement of various kinds of loads on the three types of retentive pin systems and to investigate stress concentrations associated with pin-retained amalgam restorations.4a 5 A photoelastic study that specifically evaluated the self-threading self-shearing retentive pin has shown the presence of high stress zones in the lateral and most apical portions of the pin channel; the apical zone exhibits the most stress.6 It is important to minimize the stresses induced during placement of pins because of the relatively small amount of tooth structure between the pulp and the external tooth surface. It is particularly important that the high stress zone be minimized to reduce the potential risk of fracture and resultant pulpal involvement or tooth failure, The goal of effective pin placement should be adequate retention with elimination of the high stress zone.