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34 I I trends _ tooth and restoration survival roots2_2011 When posts are needed in molars, they should be placed in roots that have the greatest dentine thickness.Theserootsareknownastheprimaryroots andtheyarethepalatalrootsofmaxillarymolarsand the distal roots of mandibular molars. However, it is importanttorememberthatextensionofapostmore than 7mm apical to the root-canal orifice in primary canals increases the risk of perforation.55 The mesial roots of mandibular molars and the facial roots of maxillary molars should be avoided if at all possible. Attentionshouldalsobegiventoavoidinginstrument pressure on the root surface towards the furcation, as this surface is thinned more easily than the outer surface owing to root curvature. With all teeth, the apical 5mm of the roots should be avoided because most root curvatures occur within 5mm of the root apex69 and entrance into this area increases the risk of excessive root thinning or perforation. _Principle #VI Acervicalferruleshouldengage2mmoftoothstruc- turetopreventrootfracture. Ferrules can be established by the core engaging toothstructure(coreferrule)70–73 orbythecrownover- lying/encompassing sound tooth structure apical to the core (crown ferrule).74–79 The data indicates that crown ferrules are more effective than core fer- rules71–73, 80 andcrownferrulesincreasethetooth’sre- sistancetofracture.74,75,80 Inspiteofthedatasupport- ing the benefit of crown ferrules, not all practitioners recognise their value. A survey published by Morgano etal.80 evaluated the percentage of respondents who feltaferruleincreasedatooth’sresistancetofracture: 56% of general dentists, 67% of prosthodontists and 73% of board-certified prosthodontists felt that core ferrules increased a tooth’s fracture resistance. Different lengths and forms of the ferrule have beenstudied.74,76,77,81 Thelengthandformareessential to the success of the “ferrule effect”. When possible, encompassing 2mm of intact tooth structure around the entire circumference of a core creates an opti- mally effective crown ferrule. Ferrule effectiveness is enhanced by grasping larger amounts of tooth struc- ture. The amount of tooth structure engaged by the overlying crown appears to be more important than thelengthofthepostinincreasingatooth’sresistance to fracture (Fig. 6). If insufficient cervical tooth structure remains to develop a ferrule, surgical crown lengthening or orthodontic extrusion should be considered to ex- pose more tooth structure. In some situations, it may be prudent to extract a tooth and replace it with an implant and crown when one or more of the follow- ing conditions is present: a ferrule cannot be devel- oped; crown lengthening would create an unaccept- able aesthetic environment or produce a furcation defect; or a short root is present that would not permit appropriate post length to be developed. Manibular teeth Central incisor Lateral incisor Canine First premolar Second premolar First molar Second molar Number of canals* 75 %: 1 canal 25 %: 2 canals 6 %: >2 canals 72 %: 1 canal 28 %: 2 canals 2 %: >2 canals 89.4 %: 1 canal 10.6 %: canal 74 %: 1 canal 25 %: 2 canals 1 %: 3 canals 93 %: 1 canal 7 %: 2 canals <0.05 %: 3 canals 2 rooted teeth mesial root 95.7 %: 2 or more canals 4.3 %: 1 canal distal root 68.4 %: 1 canal 31.6 %: 2 or more canals 3 rooted teeth mesial root 100 %: 2 or more canals distobuccal root 97.6 %: 1 canal 2.4 %: 2 or more canals distolingual root 100 %: 1 canal mesial root 86 %: 2 or more canals 14 %: 1 canal distal root 85.1 %: 1 canal 14.9%:2ormorecanals Number of roots* 100 %: 1 root 94.8 %: 1 root 5.2 %: 2 roots 95 %: 1 root 5 %: 2 roots 98.5 %: 1 root 1 %: 2 roots 0.5 %: 3 roots 80.9 %: 2 roots 12.8 %: 3 roots 0.04 %: 4 roots 84.1 %: 2 roots 1.5 %: 3 roots 2.5 %: 1 root Suggested post diameter in mm** 0.5–0.8 0.5–1 1–1.5 0.8–1 1 in the distal root Table II Table II_Suggested maximum diameter based on root dimensions and pulp morphology for mandibular teeth. * Adapted from data present in Dental Anatomy & Interactive 3-D Atlas and provided by Dr Blaine Cleghorn, Dalhousie University, Canada, November 2007. **Adapted from data published by Shillingburg, 1982 and Tilk, 1979.