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Dental Tribune Middle East & Africa Edition July-August 2015

Dental Tribune Middle East & Africa Edition | July-August 2015 7mCME < Page 6 an elastic modulus that more closely approaches that of den- tin (Fig. 8). The flexural strength of fiber and metal posts was re- spectively four and seven times higher than root dentin, and there is still debate on whether a post strengthens th tooth.16,17 The basic purpose of a post is to aid in retention of the core. The absence of a cervical ferrule has been found to be a determin- ing negative factor, giving rise to considerably higher stress levels within the root. When no ferrule was present, the prefabricated metal post/composite combina- tion generated greater cervical stress than cast post and cores. Yet, the ferrule seemed to can- cel the mechanical effect of the reconstruction material on the intensity of the stresses. With a ferrule, the choice of re- construction material had no impact on the level of cervical stress. The root canal post, the purpose of which is to protect the cervicalregion, was also shown to be beneficial even with suf- ficient residual coronal dentin. In the presence of a root canal post, cervical stress levels were lower than when no root canal post was present. Pierrisnard concluded that the higher the elasticity modulus, the lower the stress levels.18 The material the post is fabricat- ed from should have the same modulus of elasticity as the root dentin to distribute the applied forces along the length of the post and the root and not con- centrate them at the apical tip of the post. Studies have shown that when components of differ- ent rigidity are loaded, the more rigid component is capable of re- sisting forces without distortion. This stress is concentrated when the post is the stiffer material at the post’s apical tip. The less- rigid component fails invariably when a post is used that is stiffer than the root’s dentin.19 Posts with modulus of elastic- ity significantly greater than that of dentin create stresses at the tooth/cement/post interface, with the possibility of post sepa- ration and failure. As repetitive loading occurs on the endodon- tically restored tooth, the cement eventually fails at the interface between the metal post and root dentin, allowing microslippage of the post. This allows higher stressestobeexertedontheroot, leading to vertical root fracture andcatastrophiclossofthetooth. The higher modulus (rigidity) of the metallic posts makes it stiff and unable to absorb stresses. In addition, transmission of oc- clusal and lateral forces through a metallic core and post can con- centrate stresses, resulting in the possibility of unfavorable frac- ture of the root.20 Dentin’s modu- lus of elasticity is approximately 14 to 18 GPa. Fiber posts have modulus that is approximately 9 to 50 GPa, depending on the manufacturer of the post. This provides a similarity in elasticity between the fiber post and dentin of the root, allowing post flexion to mimic tooth flex- ion. The fiber post absorbs and distributes the stresses and thus shows reduced stress transmis- sion to the root.21 The longitu- dinal arrangement of fibers in the fiber post and the modulus of elasticity of a post that is less than or equal to that of the den- tin may redistribute the stress into the tooth and away from the chamfered shoulder to increase the likelihood of failure of the post core\root interface instead of root fractures. When failure does occur due to overloading, failure typically is in the coronal portion, fre- quently demonstrating fracture of the core at the tooth interface and leaving the possibility of re- restoring the tooth and not cata- strophic loss.22 The flexural properties of fiber posts were higher than the metal post and similar to dentin.23 Whereas prefabricated, stain- less-steel post exhibited a signifi- cantly higher fracture resistance at failure when compared with the fiber posts., the mode of fail- ure of the carbon fiber post was more favorable to the remaining tooth structure when compared with the prefabricated, stainless- steel post and the ceramic post.24 Ceramic posts were introduced prior to fiber posts as a more esthetic alternative to prefabri- cated metal posts, and, although not widely used today, they are still available. Modulus of elas- ticity of ceramic posts is 170-213 GPa, which is approximately 15 times that of dentin. Because these ceramic posts are too rigid and transmit more stress to the root canal than the fiber posts, which lead to irreversible root damage via vertical root fracture seen with metal posts, their use is not recommended in restor- ing endodontically treated teeth today.25 Decision making for restora- tion of teeth treated endodon- tically Restoration of endodontically treated teeth needs to take an engineering view of how best to reconstruct the remaining tooth for the best long-term survival. With this in mind, the practition- er needs to categorize the tooth based on how much native tooth structure is present following endodontic treatment and how much existing restorative mate- rial is currently present in the tooth. Minimal tooth missing or pre- viously restored Posterior teeth gain strength when the marginal ridge area and proximal surface is natural tooth structure and has not been restored. Teeth that have under- gone endodontic treatment — when either occlusal decay was present in the pits and fissures leading to pulpal involvement, or a small- to moderate-sized previously placed amalgam or composite restoration is present — require conservative restora- tion (Fig. 9). These teeth can be restored with removal of the existing restora- tive material and cleaning the pulp chamber of obturation ma- terial, including 2.0 to 3.0 mm of the canal. Placement of a conventional composite bonded within the tooth provides a good long-term restorative solution to these teeth, and a crown typically is not needed. The access or ex- isting restoration should leave most of the cuspal width present. When the preparation following removal of decay and existing restorative materials invades the width of the cusp leaving half of this tooth structure missing, more extensive restoration is in- dicated. Moderate tooth structure miss- ing or previously restored When the tooth to be restored is missing one or both marginal ridges and these areas have been previously restored or will be restored, placement of a bonded composite will not suf- fice as the final restoration (Fig. 10). The marginal ridges provide resistance to cuspal flexure of the tooth, improving its strength. When these are missing, func- tional loading of the tooth will allow greater cuspal flexure and consequentially a higher chance of fracture under masticatory function. Restoration of these teeth will require a core buildup with optional pins or other re- tentive elements for the core fol- lowed by a full coverage crown. Posts are often not needed, as the remaining tooth structure at the cusps after crown preparation is sufficient to retain the core, and a ferrule can be achieved. A post may be considered in those patients who are bruxers or clenchers or whose occlusion may place higher forces on the restored tooth due to the tooth’s position relative to the occlusal plane. When a ferrule cannot be achieved, the practitioner should consider osseous crown length- ening or forced eruption to im- prove the ferrule. Inlay restorations should be avoided in endodontically treat- ed teeth because the access cre- ated to perform the endodontic treatment weakens the tooth from a cuspal flexure standpoint and the inlay even when bonded may act as a wedge forcing the cusps apart and leading to frac- ture of the tooth. An onlay res- toration may be utilized, and its design should include shoeing of the cusps to limit cuspal flexure. Significant tooth structure missing or previously restored These teeth are a challenge to restore when removal of the old restorative material and decay leaves significant portions of the tooth needing replacement (Fig. 11). These teeth will require placement of posts to retain the core of the remaining tooth structure. Becausethepurposeofpostsisto retain the core, it is recommend- ed that in multi-canal teeth a post be placed into each canal to cross-pin the core to the remain- ing tooth structure (Fig. 12). Pro- jection of the posts in posterior teeth due to the angulation of the canals leads to convergence of the posts in the coronal portion of the tooth. This locks the core in place and assists in preventing fracture of the post or dislodge- ment under function that is ob- served when only a single post is placed. Use of pins may also be consid- ered to assist in retaining the core portion when cusps are missing and as an augment to posts being placed. These teeth require a full coverage crown to limit cuspal flexure under load. As with teeth with moder- ate missing tooth structure, use of inlays should be avoided be- cause they do not restrict cuspal flexure. An onlay may be used if desired in some cases but should include shoeing the cusps as part of the preparation design to limit cuspal flexure. Again, when fer- rule is not achievable, consider osseous crown lengthening or forced eruption to improve the ferrule. mCME SELF INSTRUCTION PROGRAM CAPPmea together with Dental Tribune provides the opportunity with its mCME - Self Instruction Program a quick and simple way to meet your continuing education needs. mCME offers you the flexibility to work at your own pace through the material from any location at any time. The content is international, drawn from the upper echelon of dental medicine, but also presents a regional outlook in terms of perspective and subject matter. Membership: Yearly membership subscription for mCME: 900 AED One Time article newspaper subscription: 250 AED per issue. After the payment, you will receive your membership number and Allowing you to start the program. Completion of mCME • mCME participants are required to read the continuing medical education (CME) articles published in each issue. • Each article offers 2 CME Credit and are followed by a quiz Questionnaire online, which is available on http://www. cappmea.com/mCME/questionnaires.html. • Each quiz has to be returned to events@cappmea.com or faxed to: +97143686883 in three months from the publication date. • A minimum passing score of 80% must be achieved in order to claim credit. • No more than two answered questions can be submitted at the same time • Validity of the article – 3 months • Validity of the subscription – 1 year • Collection of Credit hours: You will receive the summary report with Certificate, maximum one month after the expiry date of your membership. For single subscription certificates and summary reports will be sent one month after the publication of the article. The answers and critiques published herein have been checked carefully and represent authoritative opinions about the questions concerned. Articles are available on www.cappmea.com after the publication. For more information please contact events@cappmea.com or +971 4 3616174 FOR INTERACTION WITH THE AUTHORS FIND THE CONTACT DETAILS AT THE END OF EACH ARTICLE. Fig. 9. Minimal tooth missing or previously restored following en- dodontic treatment. Fig. 11. Significant tooth missing or previously restored following endodontic treatment Fig. 12. Multiple fiber posts placed into a molar to lock the core to the remaining tooth structure. Fig. 10. Moderate tooth missing or previ- ously restored following endodontic treat- ment Dr. Gregori M. Kurtzman He can be reached at dr_kurtz- man@maryland-implants.com. About the Author Conclusion For restoration of endodonti- cally treated teeth, an engineer- ing view is needed to ensure long-term survival. Ferrule is often overlooked in today’s age of adhesive dentistry, but it is as critical today as it was in the past. Lack of ferrule has been shown to affect survival of the tooth, and the literature supports use of 2.0 mm of ferrule, which is more critical in maxillary anterior teeth due to the direction of load- ing during mastication. Additionally, how we restore the remaining tooth plays a role in potential issues in the long term. Metal posts are being used less frequently due to vertical root fractures that can occur when the tooth is overloaded, and the direction has increasingly moved to the use of fiber posts, which mimic the roots modulus of elasticity. When teeth restored with a fiber post are overloaded, fracture typically occurs in the coronal (supragingival) portion, leaving sufficient tooth remain- ing to re-restore the tooth. Teeth rarely fail when they are over- engineered, but many fail due to under-engineering. References 1. Barkhodar RA, Radke R, Ab- basi J: Effect of metal collars on resistance of endodontically treated teeth to root fracture. J Prosthet Dent 61:676, 1989. Editorial note: The full list of references is avail- able from the publisher. +97143616174

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