Contemporary Restoration of Endodontically Treated Teeth. Nadim Z. Baba
not available to guide treatment planning. Therefore, tooth conditions and structural integrity remain key factors when clinicians assess anterior ETT and propose the most appropriate treatment regimen.
Physical Properties and Characteristics of Pulpless Teeth
As to the question of whether or not ETT are as brittle as many perceive, numerous studies have compared different physical properties and characteristics of both vital and nonvital teeth. While some definitive differences have been identified, there also are some conflicting findings. In fact, not all the data conclusively support the presence of substantial differences between vital and nonvital teeth. Additionally, some of the outcomes have not been replicated in multiple studies by different investigators. Nevertheless, a comprehensive review of available evidence provides insight into what happens or may happen to teeth following endodontic therapy. Therefore, this particular question is best addressed by assessing various physical properties and characteristics of pulpless teeth.
Moisture content
Conflicting information exists as to the moisture content in teeth before and after root canal therapy. One study of the dentin in dogs determined that pulpless teeth had 9% less moisture than comparable vital teeth.33 Yet in another investigation of 23 matched pairs of human ETT and their vital contralateral teeth, the moisture levels (12.3% in vital teeth and 12.1% in nonvital teeth) were not statistically significantly different.34 In some teeth that had undergone root canal therapy as many as 15 to 20 years earlier, the moisture content was not necessarily reduced, even after extended periods of time.34 It has even been stated that dehydration alone does not account for changes in physical properties of dentin.35
Flexibility
At least two studies have shown that ETT have less flexibility (ability to bend and then return to their original shape) than corresponding vital teeth.36,37 Another study found a measurable decrease in tooth stiffness and proportional limit as a result of root canal treatment.35 Stiffness also has been assessed as it relates to the type of restoration placed in vital teeth and the corresponding impact on root canal treatment. For example, a one-surface occlusal preparation was found to produce a 20% decrease in stiffness while an MO preparation caused a 46% reduction. Tooth stiffness was reduced even further (a 63% reduction) following placement of an MOD preparation. However, according to one investigation, endodontic treatment alone decreased stiffness by only an additional 5%.38
Cuspal deflection
Aside from moisture level and stiffness changes, cuspal movement or, better yet, resistance to deflection is another important characteristic. The cuspal deflection (separation of the cusps) that occurs on maxillary first premolars has been measured by applying a load to a steel ball positioned in the occlusal fossa. One study found the separation of the facial and palatal cusps to be 1.0 μm for an intact, vital tooth. The actual amount of deflection increased dramatically in premolars when these teeth were prepared for restorations. In fact, cuspal deflection increased from 1.0 μm (baseline) to 16.0 μm when there was a Class I occlusal cavity preparation, to 20.0 μm for a minimal width MO cavity preparation, and to 24.0 μm for a minimal width MOD cavity preparation in teeth that had not undergone endodontic therapy. Following a pulpotomy, the amount of deflection rose to the highest level, 28.0 μm.39 The authors concluded that breaking the continuity of the enamel layer reduces tooth rigidity, and teeth that have a wide isthmus, as in a Class II MOD cavity preparation, should have some form of cuspal protection.39
Another study of cuspal deflection determined that intact mandibular molars had cuspal deflections of up to 1.0 μm.40 While MO cavity preparations changed the deflection to less than 2.0 μm of movement, MOD cavity preparations produced 3.0 to 5.0 μm of movement. Endodontic access preparations produced 7.0 to 8.0 μm of movement in the MO group and 12.0 to 17.0 μm of movement in the MOD group (a twofold to threefold increase).40
A third study of cuspal movement in 10 maxillary premolars reported a mean deflection ranging from 3.0 to 12.0 μm in intact teeth. The amount of the mean deflection actually increased from 14.0 to 26.0 μm following root canal treatment, removal of the marginal ridges, and restoration with composite resin.41
Proprioception
The ability of teeth to respond to stimuli, such as possessing a sense of being contacted by opposing teeth or other hard objects, is known as proprioception. One clinical study used a spring device to apply force to 155 normal teeth (incisors, canines, premolars, and molars) until patients indicated that they first felt the sensation of pressure.42 The proprioception threshold, or point at which a pressoreceptive response is initiated, was significantly higher (57%) in nonvital teeth than vital teeth.42 This threshold level also increased significantly from anterior to posterior teeth.42
Another clinical investigation, with three patients, involved crowns with buccal bars placed on vital teeth and their adjacent or contralateral endodontically treated teeth.43 Weights and their corresponding loads were applied at different positions on the bars until the subjects experienced pain. Nonvital teeth had pain threshold levels that were more than twice as high as those of their contralateral or adjacent vital tooth.43
Conversely, a study of 29 patients compared the response of 59 vital teeth with that of 22 endodontically treated maxillary teeth when a pushing force was directed from the incisal edge parallel to the long axis of each tooth. The load was applied at an incremental speed of 1 N/s until the patient pushed a button to indicate touch was sensed. In this investigation, the authors did not find a significant difference between the tactile sensibilities of ETT and vital teeth.44
Classic physical properties
Considerable variation exists among the classic physical property tests, such as hardness, load to fracture, toughness, and strength (compressive, shear, and tensile), used to compare outcomes for vital and nonvital teeth. This makes it challenging to draw definitive conclusions and comparisons for specific properties. For example, it has been reported that pulpless teeth have decreased dentin strength.45, 46 It also has been reported that dentin strength is not decreased following endodontic therapy.47, 48 When vital and nonvital dentin hardness were measured, one group of researchers found comparable hardness values49 while another group noted a significant (3.5%) reduction in hardness.50
A study of shear strength and toughness determined that ETT exhibited significantly lower values than corresponding vital teeth for both these tests.46 A subsequent study by different investigators did not find differences in shear strength, toughness, or load to fracture between vital and nonvital dentin.50 Comparable compressive and tensile strengths also were recorded for vital and nonvital dentin.35
Guidelines for Restoration Selection
Recommendations for the type of restoration to be placed following endodontic treatment differ for posterior and anterior teeth.
Restoration of posterior teeth
Most endodontically treated posterior teeth should be restored with complete-coverage crowns to enhance their longevity, particularly teeth previously restored with large MOD, MO, or disto-occlusal intracoronal restorations. Such teeth benefit from crowns that encompass the cusps to prevent fracture from the occlusal forces responsible for cuspal separation. However, posterior teeth that are intact or minimally restored with a conservative endodontic access opening and are not subjected to heavier-than-normal occlusal forces can be restored with composite resin restorations. Dental amalgam restorations may be placed in situations where the restorative material covers the cusps at a thickness of at least 2.0 mm.
Restoration of anterior teeth
In many instances, the access opening in endodontically treated anterior teeth can be restored with a conservative composite resin restoration. According to Sorensen and Martinoff,19 complete-coverage crowns are not always necessary unless a tooth has been weakened or compromised by large or multiple restorations or its color or form cannot be effectively corrected with conservative treatment. As previously discussed, however, Salehrabi and Rotstein20 found that failure rates were