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6 Dental Tribune Middle East & Africa Edition | July-August 2015mCME > Page 7 Restoration of endodontic teeth: An engineering perspective By Gregori M. Kurtzman, USA I ntroduction Identifying the canals and negotiating them to be able to instrument and obturate the tooth is necessary to clinical success. But restoration of the endodontically treated tooth is critical to long-term success. It does not matter if we can com- plete the endodontic portion of treatment if the tooth cannot be restored. With this in mind, we need to look at the restora- tion phase from an engineering perspective. What is needed to reinforce the remaining tooth so that it can manage the repetitive loading that occurs during mas- tication? This article will discuss the importance of ferrule in ad- hesive dentistry as well as when to use posts and what materials are best. Ferrule: How important is it today? Ferrule has long been an im- portant concept in dentistry but has been de-emphasized with the bonding evolution. Yet this concept is as important today as it was prior to dental bonding. But what is a ferrule? A ferrule is a band that encircles the exter- nal dimension of residual tooth structure, not unlike the metal bands that exist around a barrel to hold the slats together. Sufficient vertical height of tooth structure that will be grasped by the future crown is necessary to allow for a ferrule effect of the future prosthetic crown; it has been shown to significantly re- duce the incidence of fracture in the endodontically treated tooth.1,2 Important to this concept is the margin design of the crown preparation, which may include a chamfer or a shoulder prepa- ration. Because a chamfer mar- gin has a beveled area that is not parallel to the vertical axis of the tooth, it does not properly con- tribute to ferrule height. There- fore, when a chamfer is utilized it would require an additional 1 mm of height between the edge of the margin and the top aspect of the coronal portion of remain- ing tooth structure. Thus, use of a chamfer may not be the best margin design when restoring endodontically treated teeth or those teeth with significant portions of missing tooth structure. With today’s movement toward scanning and milling for fixed prosthetics, whether done in the practition- er’s office or at the laboratory, it should be noted that it is difficult to scan the internal aspect of a shoulder preparation, and it has been uniformly recommended that a rounded shoulder be used. The rounded shoulder prepa- ration provides the maximum vertical wall at the margin, with the internal aspect being slightly rounded versus at a 90-degree angle. This ensures better rep- lication of the margins when scanned and milled. Some studies suggest that while ferrule is certainly desirable, it should not be provided at the expense of the remaining tooth/ root structure.3 Alternatively, it has also been shown that the difference between an effective, long-term restoration and re- storative failure can be as small as 1.0 mm of additional tooth structure that, when encased by a ferrule, provides greater pro- tection. When such a long-lasting, func- tional restoration cannot be pre- dictably created, osseous crown lengthening should be con- sidered to increase what tooth structure is available to achieve a ferrule, but this is also depend- ent on the periodontal status of the tooth, and when ferrule can- not be achieved then extraction should be considered.4 Ichim, et al, stated succinctly, “The study confirms that a ferrule increases the mechanical resistance of a post/core/ crown restoration.”5 How much ferrule is required? When rebuilding an endodon- tically treated tooth, it is best to maintain all dentin that is avail- able, even thin slivers. These thin slivers of dentin provide a strong connecting link between the core and tooth’s root and be- tween the crown and root.6 It is important to attempt to re- tain as much tooth structure as possible, and this aids in achiev- ing ferrule as well as maintain- ing cervical strength of the tooth where loading concentrates. Un- der masticatory loading, strain concentrates at the cervical por- tion of teeth; thus it is important to avoid over-preparation of this portion of the tooth during endo- dontic treatment and preserve this area during restoration of the tooth (Fig. 1). Multiple studies discussing how much ferrule is required have found that teeth with at least 2.0 mm of ferrule have significantly greater long-term prognosis from a restorative standpoint than those with less or no fer- rule. Libman, et al, reported, “Fatigue loading of cast post and cores with complete crowns of different ferrule designs provide evidence to support the need for at least a 1.5- to 2.0-mm ferrule length of a crown preparation. Crown preparation with a 0.5- mm and 1.0-mm ferrule failed at a significantly lower number of cycles than the 1.5-mm and 2.0- mm ferrules and control teeth.”7 Libman further demonstrated when loading at an off-axis di- rection, which occurs in the maxillary anterior, at the resto- ration’s margin, the side where the load is originating is under tension, whereas the opposing side is under compression (Fig. 2). This repetitive loading and micro strain due to tension at the lingual margin leads to the mar- gin opening, which may lead to recurrent decay and/or failure of the endodontic seal or restora- tion (Fig. 3). Additionally, if we look at strain studies by Libman and others comparing ferrule of different heights, we observe that in a fer- rule of 0.5 mm there is greater strain at the margin under ten- sion and concentrates at mid tooth where the core or post is situated. Teeth with 2.0 mm of ferrule demonstrated signifi- cantly less strain loading at the margins or center of the cervical aspectofthetooth.Thelowerthe strain at the cervical midpoint, the less chance of overload and failure restoratively (Fig. 4). Detecting failure at the coro- nal seal It is not unusual to have a patient present for a routine recall ap- pointment and the clinician or hygienist note recurrent decay at a crown margin with the pa- tient unaware of the issue. This becomes more complicated with teeth that have previously un- dergone endodontic treatment, as there is no pulp present that could warn the patient an issue is present until often extensive decay occurs or the crown dis- lodges from the remaining tooth. Freeman,etal,intheirpublished study, stated, “Fatigue loading of three different post and core de- signs with the presence of a full cast crown leads to preliminary failure of leakage between the restoration and tooth that is clini- cally undetectable.”8 The literature supports that coronal leakage may be a ma- jor factor in failure of endodon- tic treatment.9-11 As previously discussed, when loaded during mastication, margins with inad- equate ferrule may demonstrate micro opening on the tension side, leading to leakage over time. This initially may be observed as recurrentdecay,butasitdeepens and exposure of the obturation material results, failure of the endodontics may result due to apical migration of oral bacteria. This is minimized when a bond- ed core or post/core is present, but given sufficient time when a ferrule of sufficient height is not present the endodontics or the restoration will fail. Do all posts function the same? Teeth function differently, de- pending on the material that the post is fabricated from, with loads distributed within the root relative to the modulus of elas- ticity of the post compared to the dentin of the root (Fig. 5). When a tooth restored with a fib- er post does fail due to overload, the mode of failure is coronal, protecting remaining root and tooth structure.12 This mode of failure with fiber-post-restored teeth typically allows the tooth to be restored, as vertical root frac- ture is a rare occurrence. Bitter reported, “Compared to metal posts, FRC posts revealed reduced fracture resistance in vitro, along with a usually re- storable failure mode”13 (Fig. 6). Whereas, with metal posts either prefabricated or cast, fail- ure was at a higher value for cast post and core: 91 percent of the specimens had fractured roots (none of the specimens with a fiber post demonstrated root fracture); and the post and core usually fractured at the tooth composite core interface.14 As stress concentrates at the api- cal tip of the metal post due to its higher modulus of elasticity than the surrounding root, vertical root fracture is a frequent occur- rence (Fig. 7). This may result also from breakdown of the ce- ment luting the post to the root, allowing slippage microscopi- cally of the post in the tooth un- der load, leading to torque at the cervical area and the resulting vertical root fracture. Because metal posts are stiffer (higher modulus of elasticity) than the dentin of the root, stress concentrates at the post’s api- cal tip, leading to vertical root fracture and catastrophic loss of the tooth. Ansari reported, “The risk of failure was greater with metal-cast posts (nine out of 98 metal posts failed) than with carbon fiber posts (using which, none out of 97 failed) risk ra- tio.”15 But with fiber posts having a flexibility equal to or greater than the root (lower modulus of elasticity), stress concentrated at the cervical region, leading to horizontal fracture of the post and core; and typically the tooth can be salvaged. The elastic modulus refers to the relative rigidity of the ma- terial. The stiffer the material, the higher its relative modulus. Whentwodifferentmaterialsare placed together, such as when a post is placed into a tooth’s root, the elastic modulus is influenced by whichever of the materials is stiffest. Dentin averages a modu- lus of elasticity of 17.5 (+/- 3.8) GPa, with glass fiber posts at 24.4 (+/- 3.4) GPa, titanium pre- fabricated posts at 66.1 (+/- 9.6) GPa, prefabricated stainless steel at 108.6 (+/- 10.7) GPa and cast high noble gold posts at 53.4 (+/- 4.5) GPa. Cast posts fabricated from no- ble or base metals have higher modulus then high noble alloys and approach stainless-steel prefabricated posts in their rela- tive stiffness. Fiber posts have Fig. 1. Strain analysis of a posterior tooth demonstrating concentration of strain on loading at the cervical. (Image/Provided by Dr. Gene McCoy) Fig. 2. As a maxillary anterior tooth is loaded during mastication, tension and compression occur at the crown’s margins. (Images/Provided by Dr. Gregori M. Kurtzman) Fig. 3. Opening of the margin on the tension side may lead in time to recur- rent decay or restoration and endodon- tic failure. Fig. 4.Difference of intensity of strain and location related to ferrule height during occlusal loading (Libman). Fig. 5. Comparison of load distribution of fiber post, cast metal post and prefab- ricated metal post. Fig. 6. Tooth restored with a fiber post demonstrating coronal horizontal frac- ture supracrestally, typically seen with teeth restored with fiber posts when overloaded. Fig. 7. Vertical root fracture of a tooth restored with a metal post. Fig. 8. Comparative modulus of elastic- ity of different post materials mCME articles in Dental Tribune have been approved by: HAAD as having educational content for 2 CME Credit Hours DHA awarded this program for 2 CPD Credit Points CAPPmea designates this activity for2continuingeducationcredits.