RO0211

I 33 trends _ tooth and restoration survival I roots2_2011 _Principle #V Large diameter posts increase the possibility of root thinning, root perforation and root fracture. It is recommendedthatpostsnotexceedone-thirdofthe rootdiameter. Increasing the diameter of a post weakens the remaining root. It has been determined that stresses increase in a root as the post diameter increases;56 larger post diameters decrease the resistance to toothfracture.53 Withlargediameterposts(1.5mmor more), it was determined57 that there was a six-fold increase in the potential for root fracture for every mm of increased post diameter. Studies have shown that root fracture is the sec- ondmostcommoncauseofpostandcorefailure.58–62 Multiple factors have been associated with the potential for root fracture, including large diameter posts,23, 50, 56 short posts,23, 49–52 and threaded posts (Fig. 4).44, 49, 50, 57, 63–65 It is recommended that the post diameter not exceed one-third of the root diameter55 and that the post diameter be proportionally related to average root dimensions. Toensurethatpostsdonotexceedone-thirdofthe root diameter, the post diameter should be between 0.6 and 1.2mm, depending on the tooth.66–68 Only post preparation instruments that match the desired diameter of the post space should be used. When using a particular brand of post, make sure that the matching drill belongs to the same type of post (Tables I & II). A good understanding of dental anatomy, the configuration of the roots and their variations, and use of an appropriate instrument angulation help in avoiding root thinning and perforation. Instruments shouldbeangledsothattheyfollowthecanal(Fig.5). When posts are needed in premolars, they are best placedinthepalatalrootofthemaxillarypremolarand the straightest root of any mandibular premolar with multiple roots. Root taper, curvature and depressions should be reviewed prior to post preparation. Figs. 6a & b_Post and crown loosened from maxillary canine a few months after placement. Both the core/prefabricated post and the crown came off (a). Clinical photograph shows the absence of cervical tooth structure (ferrule) for retention of the crown (b). Fig. 6a Fig. 6b Maxillary teeth Central incisor Lateral incisor Canine First premolar Second premolar First molar Second molar mesiobuccal root 59.2 %: 2 or more canals 40.8 %: 1 canal mesiobuccal root 51.5 %: 1 canal 47 %: 2 or more canals Number of canals* 100 %: 1 canal 83 %: 2 canals 12 %: 1 canal 2 %: 3 canals 48 %: 1 canal 43 %: 2 canals 0.6 %: 3 canals distobuccal root 98.3 %: 1 canal 1.7 %: 2 or more canals distobuccal root 99.7 %: 1 canal 0.3 %: 2 or more canals palatal root 99 %: 1 canal 1 %: 2 or more canals palatal root 99.9 %: 1 canal 0.1 %: 2 or more canals Number of roots* 1 root 75 %: 2 roots 23 %: 1 root 2 %: 3 roots 90.7 %: 1 root 9 %: 2 roots 0.3 %: 3 roots 95.9 %: 3 roots 3.9 %: 2 roots 88.6 %: 3 roots 7.8 %: 2 roots 2.8 %: 1 root 0.4 %: 4 roots Suggested post diameter in mm** 1–1.7 0.8 1–1.5 0.8–1 in the palatal root 1 in the palatal root Table I_Suggested maximum diameter based on root dimensions and pulp morphology for maxillary teeth. Table I