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Weakening teeth during root canal treatment
have fractured in clinical trials and monolithic or fullcontour zirconia crowns are essentially an unveneered core. The material offers very high flexural strength and has been tested in large multi-unit restorations. The material allows more conservative tooth preparations than the other available ceramics or PFM. Also, when wellpolished, they are gentle on opposing tooth structures and demonstrate less wear on enamel than occurs with other ceramics. The monolithic zirconia restorations tend to be opaque and have reduced esthetics compared to layered restorations. They are usually used for posterior teeth, where esthetic concerns are less important, especially for second molars. They can be fabricated with significantly less tooth preparation, so they are appropriate for mandibular anterior teeth. They are inexpensive, with an average cost of $171. Monolithic zirconia cannot be etched but can be subjected to airborne particle abrasion bonding with MDP primers and resin cements. However, the bonds formed are relatively weak and deteriorate with aging. Particle abrasion may also transform the crown or core. The internal surface of these crowns is usually contaminated with saliva and blood during try-in, and salivary proteins are not readily removed. Crowns can therefore be prematurely dislodged. Sodium hydroxide applied for 20 seconds, then a water rinse helps to cleanse these crowns and increase the bond strength.
Discussion.—The various ceramic systems now available have strengths and weaknesses that need to be recognized and tailored to the clinical situation. Layered leucite-reinforced crowns provide excellent esthetics when used on maxillary anterior teeth and etched and bonded appropriately. Monolithic lithium disilicate crowns work best on premolars and first molars. Layered lithium disilicate crowns are appropriate for anterior teeth and premolars. Monolithic zirconia crowns perform well on molars and mandibular anterior teeth.
Clinical Significance.—PFM remains the gold standard for esthetic crowns but the cost can be a serious drawback for patients. Ceramic materials have been developed to address the issue and achieve good to excellent results with adequate longevity. Clinicians should carefully evaluate the requirements of each situation and choose an appropriate material to meet the need.
Ahmed SN, Donovan TE: Evaluation of contemporary ceramic materials. J Esthet Restor Dent 27:59-62, 2015 Reprints available from TE Donovan, Dept of Operative Dentistry, UNC School of Dentistry at Chapel Hill, Chapel Hill, NC; fax: 919537-3990; e-mail: [email protected]
Endodontics Weakening teeth during root canal treatment Background.—The structural changes in dentin that make a devitalized tooth more prone to fracture remain to be determined. Removing tooth tissue during root canal treatment can affect devitalized teeth, altering their mechanical behavior even after they are restored. The relative contributions of cavity preparation steps to overall tooth weakening were evaluated. Methods.—The decrease in tooth strength was analyzed using the finite element method (FEM) to elicit the separate and combined influences of a two-surface Class II preparation and root canal treatment. Root canal treatment was broken into access cavity preparation and root canal enlargement to determine the influence of each of these steps and of the combination of the two. After each phase, the crown was restored with composite resin and FEM was done on the restored tooth. Four models
were developed: model 1 was the intact tooth, model 2 included MO Class II cavity preparation restored with composite resin, model 3 had a MO Class II restoration and access cavity to the pulp chamber but no root canal enlargement, and model 4 was based on scans of the treated tooth and included MO Class II cavity, an access cavity preparation, and root canal enlargement. A failure index based on maximum principal stress criterion (MPCS) was applied. Compressive and tensile stresses were also analyzed, with corresponding failure indices applied as appropriate. Results.—The compression test identified the critical breaking force for the intact tooth as 1025 N at a compressive strain of 0.9 mm. The critical breaking force for the treated tooth was 710 N at a compressive strain of 1.02
Volume 61
Issue 1
2016
45
mm. The site of fracture for both teeth was in the buccal cervical region. The distribution of principal stress during polymerization shrinkage was analyzed in the treated tooth (models 2, 3 and 4). There was a sharp transition between the tooth tissue and the composite filling. The compressive stress level was within the composite material. With bonding to enamel and dentin, tensile stress is generated where these tissues contact the filling. In model 4 the tensile stress was 2.35 MPa, with compressive stress of 9.4. The principal stress values of the other models were lower. All teeth were loaded axially with a force of 710 N and principal stress distribution was tracked. Tensile stress was generated in the approximal enamel of the untreated tooth but compressive stress was on the occlusal surface and in dentin at the buccal cervical area. In the enamel at the approximal surface, tensile stress exceeded the enamel’s tensile strength, creating a critical zone. A crack in the enamel may develop under these conditions. The maximal values of compressive stress in the buccal cervical area remained less than the compressive strength of the dentin, so dentin was likely to remain intact. Loading the intact tooth to 1025 N, compressive stress in the dentin surpassed the failure risk point. Loading the treated teeth with 710 N showed tensile stress generation in the buccal enamel that exceeded the enamel’s tensile strength. The zone of high risk for failure increased, with the greatest extent shown in model 4. Compressive stress was detected on the occlusal surface of each model and in the composite filling for models 3 and 4. All of the models had extreme levels of compressive stress in the cervical buccal region. The model 2 tooth was at increased risk for failure, but the levels in models 3 and 4 exceeded the compressive strength of dentin, indicating that failure would occur in the cervical buccal area. Tensile stress was the dominant force in the dentin-filling interface
of model 4. The stress generated in the dentin wall of the root canal was low and did not change appreciably after root canal enlargement. Von Mies stress distribution indicates when the highest risk for fracture occurs. The tooth was weakened most during access cavity preparation. Discussion.—This study provides a general framework for performing FEM analysis and experimental validation of the influence of tooth restoration and root canal preparation on the fracture resistance of devitalized teeth. Widening of the root canals did not affect stress distribution. The area at most risk for fracture is the buccal cervical area.
Clinical Significance.—Teeth with twosurface composite restorations that undergo root canal treatment are not as strong as untreated teeth and cannot withstand the occlusal loads handled by intact teeth. The specific action that reduces fracture resistance most is access cavity preparation, not canal enlargement, as some have believed. The process of analyzing fracture resistance and load characteristics may be helpful in determining the potential risk for tooth fracture.
Zelic K, Vukicevic A, Jovicic G, et al: Mechanical weakening of devitalized teeth: Three-dimensional Finite Element Analysis and prediction of tooth fracture. Int Endod J 48:850-863, 2015 Reprints available from M Djuric, Laboratory for Anthropology, Inst of Anatomy, School of Medicine, Univ of Belgrade, 4/2 Dr Subotica. 11000 Belgrade. Serbia; fax: þ381 11 2686 172; e-mail: [email protected]
Mineral trioxide aggregate for direct pulp capping Background.—Direct pulp capping (DPC) is done when a healthy pulp has been exposed during trauma or iatrogenic incident. A medication is placed directly over the exposed area to stimulate healing. No further treatment may be needed. Calcium hydroxide (CH) has been the most commonly used medicament for DPC but several disadvantages with this material have prompted the search for alternatives. Mineral trioxide
46
Dental Abstracts
aggregate (MTA) has demonstrated favorable outcomes, with good chemical and physical properties, antibacterial activity, biocompatibility, and sealing properties. The pulpal tissue reaction to MTA differs from that of CH. Dentin bridge formation with MTA is more homogenous and more localized than that formed with CH. Drawbacks to MTA include discoloration potential, difficult handling characteristics, long setting time, and difficulty removing
Discussion.—The various ceramic systems now available have strengths and weaknesses that need to be recognized and tailored to the clinical situation. Layered leucite-reinforced crowns provide excellent esthetics when used on maxillary anterior teeth and etched and bonded appropriately. Monolithic lithium disilicate crowns work best on premolars and first molars. Layered lithium disilicate crowns are appropriate for anterior teeth and premolars. Monolithic zirconia crowns perform well on molars and mandibular anterior teeth.
Clinical Significance.—PFM remains the gold standard for esthetic crowns but the cost can be a serious drawback for patients. Ceramic materials have been developed to address the issue and achieve good to excellent results with adequate longevity. Clinicians should carefully evaluate the requirements of each situation and choose an appropriate material to meet the need.
Ahmed SN, Donovan TE: Evaluation of contemporary ceramic materials. J Esthet Restor Dent 27:59-62, 2015 Reprints available from TE Donovan, Dept of Operative Dentistry, UNC School of Dentistry at Chapel Hill, Chapel Hill, NC; fax: 919537-3990; e-mail: [email protected]
Endodontics Weakening teeth during root canal treatment Background.—The structural changes in dentin that make a devitalized tooth more prone to fracture remain to be determined. Removing tooth tissue during root canal treatment can affect devitalized teeth, altering their mechanical behavior even after they are restored. The relative contributions of cavity preparation steps to overall tooth weakening were evaluated. Methods.—The decrease in tooth strength was analyzed using the finite element method (FEM) to elicit the separate and combined influences of a two-surface Class II preparation and root canal treatment. Root canal treatment was broken into access cavity preparation and root canal enlargement to determine the influence of each of these steps and of the combination of the two. After each phase, the crown was restored with composite resin and FEM was done on the restored tooth. Four models
were developed: model 1 was the intact tooth, model 2 included MO Class II cavity preparation restored with composite resin, model 3 had a MO Class II restoration and access cavity to the pulp chamber but no root canal enlargement, and model 4 was based on scans of the treated tooth and included MO Class II cavity, an access cavity preparation, and root canal enlargement. A failure index based on maximum principal stress criterion (MPCS) was applied. Compressive and tensile stresses were also analyzed, with corresponding failure indices applied as appropriate. Results.—The compression test identified the critical breaking force for the intact tooth as 1025 N at a compressive strain of 0.9 mm. The critical breaking force for the treated tooth was 710 N at a compressive strain of 1.02
Volume 61
Issue 1
2016
45
mm. The site of fracture for both teeth was in the buccal cervical region. The distribution of principal stress during polymerization shrinkage was analyzed in the treated tooth (models 2, 3 and 4). There was a sharp transition between the tooth tissue and the composite filling. The compressive stress level was within the composite material. With bonding to enamel and dentin, tensile stress is generated where these tissues contact the filling. In model 4 the tensile stress was 2.35 MPa, with compressive stress of 9.4. The principal stress values of the other models were lower. All teeth were loaded axially with a force of 710 N and principal stress distribution was tracked. Tensile stress was generated in the approximal enamel of the untreated tooth but compressive stress was on the occlusal surface and in dentin at the buccal cervical area. In the enamel at the approximal surface, tensile stress exceeded the enamel’s tensile strength, creating a critical zone. A crack in the enamel may develop under these conditions. The maximal values of compressive stress in the buccal cervical area remained less than the compressive strength of the dentin, so dentin was likely to remain intact. Loading the intact tooth to 1025 N, compressive stress in the dentin surpassed the failure risk point. Loading the treated teeth with 710 N showed tensile stress generation in the buccal enamel that exceeded the enamel’s tensile strength. The zone of high risk for failure increased, with the greatest extent shown in model 4. Compressive stress was detected on the occlusal surface of each model and in the composite filling for models 3 and 4. All of the models had extreme levels of compressive stress in the cervical buccal region. The model 2 tooth was at increased risk for failure, but the levels in models 3 and 4 exceeded the compressive strength of dentin, indicating that failure would occur in the cervical buccal area. Tensile stress was the dominant force in the dentin-filling interface
of model 4. The stress generated in the dentin wall of the root canal was low and did not change appreciably after root canal enlargement. Von Mies stress distribution indicates when the highest risk for fracture occurs. The tooth was weakened most during access cavity preparation. Discussion.—This study provides a general framework for performing FEM analysis and experimental validation of the influence of tooth restoration and root canal preparation on the fracture resistance of devitalized teeth. Widening of the root canals did not affect stress distribution. The area at most risk for fracture is the buccal cervical area.
Clinical Significance.—Teeth with twosurface composite restorations that undergo root canal treatment are not as strong as untreated teeth and cannot withstand the occlusal loads handled by intact teeth. The specific action that reduces fracture resistance most is access cavity preparation, not canal enlargement, as some have believed. The process of analyzing fracture resistance and load characteristics may be helpful in determining the potential risk for tooth fracture.
Zelic K, Vukicevic A, Jovicic G, et al: Mechanical weakening of devitalized teeth: Three-dimensional Finite Element Analysis and prediction of tooth fracture. Int Endod J 48:850-863, 2015 Reprints available from M Djuric, Laboratory for Anthropology, Inst of Anatomy, School of Medicine, Univ of Belgrade, 4/2 Dr Subotica. 11000 Belgrade. Serbia; fax: þ381 11 2686 172; e-mail: [email protected]
Mineral trioxide aggregate for direct pulp capping Background.—Direct pulp capping (DPC) is done when a healthy pulp has been exposed during trauma or iatrogenic incident. A medication is placed directly over the exposed area to stimulate healing. No further treatment may be needed. Calcium hydroxide (CH) has been the most commonly used medicament for DPC but several disadvantages with this material have prompted the search for alternatives. Mineral trioxide
46
Dental Abstracts
aggregate (MTA) has demonstrated favorable outcomes, with good chemical and physical properties, antibacterial activity, biocompatibility, and sealing properties. The pulpal tissue reaction to MTA differs from that of CH. Dentin bridge formation with MTA is more homogenous and more localized than that formed with CH. Drawbacks to MTA include discoloration potential, difficult handling characteristics, long setting time, and difficulty removing