Warreth A A Cracked Tooth Syndrome: A Review of the Literature. Dent Update. 2023; 50:555-562
Tatum RC Two new schemes for classifying propagating cracks in human tooth structure. Compend Contin Educ Dent. 1998; 19:211-218
Lynch CD, McConnell RJ The cracked tooth syndrome. J Can Dent Assoc. 2002; 68:470-475
Zimet PO, Endo C Preservation of the roots--management and prevention protocols for cracked tooth syndrome. Ann R Australas Coll Dent Surg. 2000; 15:319-324
Fong J, Tan A, Ha A, Krishnan U Diagnostic and treatment preferences for cracked posterior teeth. Aust Dent J. 2023; 68:135-143 https://doi.org/10.1111/adj.12959
Chen S, Arola D, Ricucci D Biomechanical perspectives on dentine cracks and fractures: Implications in their clinical management. J Dent. 2023; 130 https://doi.org/10.1016/j.jdent.2023.104424
Kahler W The cracked tooth conundrum: terminology, classification, diagnosis, and management. Am J Dent. 2008; 21:275-82
Homewood CI Cracked tooth syndrome – incidence, clinical findings and treatment. Aust Dent J. 1998; 43:217-222
Yang SE, Jo AR, Lee HJ, Kim SY Analysis of the characteristics of cracked teeth and evaluation of pulp status according to periodontal probing depth. BMC Oral Health. 2017; 17 https://doi.org/10.1186/s12903-017-0434-x
Abbott PV Assessing restored teeth with pulp and periapical diseases for the presence of cracks, caries and marginal breakdown. Aust Dent J. 2004; 49:33-39 https://doi.org/10.1111/j.1834-7819.2004.tb00047.x
Yeng T Late Failure in endodontic treatment: an exemplary case study. J Oral Health Dent Res. 2023; 3:1-5
Clark DJ, Sheets CG, Paquette JM Definitive diagnosis of early enamel and dentin cracks based on microscopic evaluation. J Esthet Restor Dent. 2003; 15:391-401 https://doi.org/10.1111/j.1708-8240.2003.tb00963.x
Brannstrom M The hydrodynamic theory of dentinal pain: sensation in preparations, caries, and the dentinal crack syndrome. J Endod. 1986; 12:453-457 https://doi.org/10.1016/S0099-2399(86)80198-4
Nguyen Thi W, Jansson L Survival rate after endodontic treatment in general dentistry for cracked teeth with different coronal restorations. Acta Odontol Scand. 2021; 79:256-261 https://doi.org/10.1080/00016357.2020.1834615
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In cases of cracks, dentists face difficulties when relying solely on clinical examination and peri-apical radiographs because these may not provide sufficient information for accurate tooth assessment. Often, cracks, fractures, etc can be correctly diagnosed only by removing the existing restoration to explore the extent of the crack lines. This report describes several cases of symptomatic patients keen to investigate fully the prognosis before deciding to either have their tooth treated with root canal treatment or plan for an extraction. Without early identification of the problems, these patients would have continued to experience symptoms after root canal treatment was completed.
CPD/Clinical Relevance:
The removal of restorations allows cracks and fracture lines to be discovered prior to the commencement of root canal treatment. This prevents unnecessary patient expense and disappointment.
Article
The location, direction, and extent of a crack in a tooth can affect the choice of dental treatments.1 It is important, therefore, that cracks be classified according to their anatomical location (e.g. in cusps, marginal ridges, development grooves, inclined planes, or incisal edges), the direction (e.g. single direction, vertical or oblique); and their extent (e.g. complete or incomplete). A diagnosis will be a combination of at least two of the above. Some examples would be complete vertical, complete oblique, incomplete vertical and incomplete oblique.2 Successful diagnosis and management not only require an awareness of the existence of a crack,3 but also an understanding of how the crack line relates to the pulp and periodontal ligament,4 and determination of the apical extent of an incomplete crack on the root.5 The lack of understanding regarding cracked tooth biomechanics among dentists6 may hinder their ability to manage cracked teeth properly. Comprehension of the characteristics and weakening factors of tooth cracks and available strengthening mechanisms is fundamental to designing better treatment.7
For consistency, the American Association of Endodontists8 classifies cracks into five types:
Craze lines: long vertical craze lines usually appear in anterior teeth, while in posterior teeth craze lines may cross marginal ridges, and extend along the buccal/lingual surface.
Fractured cusps: just one cusp is affected, and usually results from a lack of cusp support because of a weakened marginal ridge.
Cracked tooth: a crack extends from the occlusal surface of the tooth apically without separation of the two segments and may cross one or both marginal ridges.
Split tooth: these cracks are usually mesiodistal in direction, crossing both marginal ridges, and the tooth segments separate completely. A split often results from long-term propagation of a cracked tooth.
Vertical root fracture: these begin in the root and may extend the length of the root or occur as a shorter crack at any level along the root in an axial direction.1
The human dentition is subject to many destructive forces that affect the longevity of individual teeth.9 These stresses may result in the development of microcracks in the dentine and enamel that can propagate and cause symptoms in vital teeth, possibly leading to crack line propagation and eventual tooth fracture.3 Cracked teeth tend to cause pain on mastication, especially when biting pressure is released10 and, if the crack extends into the pulp, pain occurs from direct stimulation of pulpal tissues.11 The existence of crack lines raises two issues: the first relates to the diagnosis in relation to pain and the second relates to the structural integrity of the tooth and periodontal/restorative management in the overall treatment plan.4 The challenge dentists face is that clinical examination and peri-apical radiographs may not provide sufficient information for accurate clinical assessment; thus, for cracks to be correctly diagnosed, the existing restoration should be removed.12 Removing the restorations may lead to a significant increase in the number of visible crack lines. Following a definitive diagnosis of the crack and the need for root canal treatment, the treatment planning stage should be straightforward.13
This article reports on the importance of visual assessment of cracks and how this information affects the treatment plan and the final management of cracked teeth.
Patient cases
Each patient was presented with a provisional diagnosis of a cracked tooth (see Figures 1–5) and was keen to investigate the prognosis further before choosing between treating the tooth with root canal treatment or planning for an extraction.
Figure 1. The lower right first mandibular molar (LR6) with an orthodontic band cemented by the dentistFigure 2. The lower left first mandibular molar (LL6) with an old amalgam restoration. Clinical evidence of multiple crack lines is evident.Figure 3. The lower right first mandibular molar (LR6) continued to be symptomatic after initial endodontic instrumentation.Figure 4. The lower left first mandibular molar (LL6) that presented with symptoms of a cracked tooth.Figure 5. The lower left second mandibular molar (LL7) presented with symptoms that support the diagnosis of cracked tooth syndrome
Patient 1
Figure 1 shows the mandibular right first molar (LR6) with an orthodontic band cemented by the dentist prompted by symptoms consistent with cracked tooth. As the symptoms persisted, a referral to the endodontist was made to investigate the cracked molar and potential root canal therapy. On further investigation, it was found that the crack line started from the distal marginal ridge and extended deep down into the distal canal orifice. Following a lengthy discussion with the patient, a decision was made to extract LR6, as the long-term outcome with endodontic treatment and crowning was unpredictable.
Patient 2
Figure 2 shows the mandibular left first molar (LL6) with an old amalgam restoration. The presence of multiple crack lines was evident clinically, which prompted a referral to the endodontist to determine the prognosis before formulating a treatment plan. Following removal of the amalgam restoration, the crack line was found to extend from the mesial marginal ridge and across the pulp chamber floor towards the distal marginal ridge. Extraction of the tooth was advised because of the poor prognosis.
Patient 3
Following initial root canal instrumentation, the referring dentist suspected that crack lines were the cause of the persistent symptoms experienced in the mandibular right first molar (LR6). An endodontic referral was made and further investigation found that remnants of pulpal tissues remained in the pulp, as shown in Figure 3. Following complete debridement, none of the crack lines extended into the pulpal floor or canal orifices. The prognosis was good, extraction of the tooth was not required, and the patient's symptoms resolved.
Patient 4
Figure 4 shows the mandibular left first molar (LL6) presenting with symptoms of a cracked tooth. With the removal of the existing composite resin restoration, a crack line was identified that began at the mesial marginal ridge and propagated towards the middle of the pulp chamber. With the roof of the pulp chamber removed and all the canal orifices located, the crack line extended down the mesial wall, but did not cross the pulpal floor or propagate into the mesio-buccal or mesio-lingual canals. In addition, there was no deep periodontal pocketing, no furcal or angular bone loss seen on the radiograph, and no peri-apical radiolucency. The treatment planned for this tooth was non-surgical root canal treatment followed by a crown restoration.
Patient 5
The mandibular left second molar (LL7), shown in Figure 5, presented in 2017 with symptoms consistent with the diagnosis of cracked tooth syndrome. Two crack lines were identified by the general dentist, one extending from the distal marginal ridge and the other crossing the mid-buccal wall. Both crack lines were propagating towards the pulp chamber. To assess whether this tooth should continue with root canal treatment or be extracted, a referral was made to the endodontist. Clinically, there was no periodontal pocketing detected, no tenderness to percussion, no pain on palpation around the peri-apical area, and the mobility of the tooth was within normal limits. The radiograph showed no furcal or angular bone loss, and no peri-apical radiolucency. After a discussion with the patient, it was decided to attempt root canal treatment with subsequent crowning of the tooth. A review of the tooth in 2022, 5 years after the treatment, showed the tooth surviving well, although some horizontal bone loss was detected radiographically because of periodontal disease.
Discussion
The existence of an undetected crack can affect the long-term outcome of treatment to a tooth and may result in late failure.14 Use of an operating dental microscope makes it easier to detect cracks and assess whether they involve the pulp.4 At a magnification level of x14 or greater, a dental microscope allows the detection of significant cracks long before incomplete coronal fractures and cracked teeth become symptomatic.15 By isolating the tooth with a contrast dye to detect the crack more easily and keeping the area free of saliva, the use of a rubber dam improves the probability of visualizing these cracks.9 The removal of existing restorations allows closer inspection of the remaining cavity and evaluation of the residual, mesial and distal marginal ridges, which tend to be weak areas.12
When planning treatment, it is crucial to inspect and confirm the presence of a crack, taking into account crack line propagation, dental pain, and the prognosis of the cracked tooth.
Crack line propagation
A recent Australian study reported that over 70% of respondent dentists demonstrated a poor understanding of cracked tooth biomechanics.6 As centrally located cracks tend to follow the lines of the dentinal tubules and head toward the pulp,7 it is important to know that such cracks will act as initiation points for progressive failure.1 With each load imposed on the cusp, a crack may propagate incrementally and, over time, the structural integrity of this part of the tooth will weaken.16 In the event of a vertically oriented crack, continued use may lead to the propagation of the crack and if this occurs, the tooth may develop the symptoms of an irreversible pulpitis, or the pulp may become necrotic.3 If a crack line increases in length and progresses apically, a vertical root fracture may result.17 In cases of peripheral cracks, however, these are likely to lead to cuspal fracture, with or without pulpal exposure.7 Functional cusps tend to fracture near the centre of the tooth to absorb occlusal forces and direct them axially, whereas non-functional cusps do not usually demonstrate this axial alignment.18 The non-functional cusps are more liable to crack generation as they guide the mandibular lateral excursion movements. Hence, they are non-axially loaded. According to Goel et al,19 cracks in the remaining tooth structure can be caused by the combination of stress gradient changes in the dentine and enamel, leading to cuspal fracture adjacent to the deepest portion of the cavity.
When cracks occur under cusps or in unrestored teeth, they may develop in one of two ways: either at the fulcrum of a cusp and possibly progress outwards to the outer enamel or cementum surface or, in unrestored or minimally restored teeth, from the enamel inwards to the dentine.4 If the crack extends in an oblique direction and is sufficiently small or hidden beneath an existing restoration, although it may continue to produce symptoms of a hypersensitive pulp, it may also escape detection for years.18 Over time, it may proceed toward the pulp and produce the same effect as the vertically orientated crack, or the cusp may fracture and fall away, relieving the patient's symptoms.20 Lagouvardos et al21 reported that vital teeth tend to fracture supragingivally, in contrast to non-vital teeth, where the fractures tend to end subgingivally.
Dental pain from cracks
The pulp is richly innervated and contains both A (myelinated) and C (unmyelinated) nerve fibres.22 Based on the concept of hydrodynamic theory23 rapid movement of dentinal fluid in dentinal tubules can either activate A-delta nerve fibres, which results in short, sharp pain, or it may affect the C-fibres, which are slow-conducting and produce a dull, poorly localized sensation.9 The pain from cracks may arise because of pulpal involvement related to:
The hydrodynamic theory of pain, as the fracture line opens and closes.
Direct pulpal involvement of the fracture line, which results in direct initiation of the pulp, e.g. irreversible pulpitis or pulpal necrosis.
Therefore, pain may be the result of a minute separation of the tooth during use with the consequent stimulation of the dentinal tubules, bacterial invasion, or both.24 If the crack splits further and deeper, inflammation will progress directly to the crestal epithelial attachment and periodontal ligament.25 When the fracture line in a root reaches the cementum and the periodontal ligament, the area becomes a source of chronic inflammation.17 Periodontal breakdown will occur, and the development of a deep periodontal pocket will be seen.26 Once the crack line propagates into a complete tooth fracture, there is a path available for bacterial invasion to the pulp, and hence pulpitis is expected.27 The resulting dental pain is often an expression of the inflammatory process around the root because of a fracture.14
Prognosis of a cracked tooth
Despite the presence of a crack being potentially detrimental to tooth survival, high survival rates have been reported.28 Survival rates vary from 54% in a 10-year study, to 68% in a 5-year study,29 to 82% in a 1-year study.30 In general, the long-term prognosis for cracked teeth is better when no crack is visible or the crack does not extend to the pulp chamber floor,12 and when the tooth is asymptomatic after banding or placement of a temporary crown.8 Compared to a propagating crack line terminating in dentine, and extending into the pulp, the prognosis is poor.10 It is important to inform the patient that, although treatment with full coronal coverage from crowning will probably be successful for many years,31 it is possible that some cracked teeth may continue to deteriorate and separate, and eventually require extraction.14 In cases where, after having unroofed the pulp chamber, the crack is found to extend through the floor of the pulp chamber, or through both marginal ridges, the predictability of any treatment is believed to be severely compromised and extraction is required.4 Generally, when there is a visible crack and severe periodontal defect, the prognosis is considered poor.10 Propagating crack line can cause progressive periodontal destructions, despite an overall stable periodontal state and proper endodontic treatment.17 When the tooth is completely fractured in a vertical direction, the preferred treatment is extraction.26 However, if the vertical crack is incomplete, and the tooth is restorable, temporarily banding the segments together is advised, and complete root canal treatment and a crown restoration are indicated.8 Thus, cracked teeth that have been restored with crown restorations report significantly higher survival rates.32 Although it is beyond the scope of this paper to discuss root canal preparation and crack formation in root dentine, there is a strong correlation between the amount of dentine removed and the development of cracks.33 The results of root canal treatment may be compromised if these cracks propagate.
Conclusion
A visual inspection of the position of the crack line provides a clearer insight into treatment planning, tooth management, and treatment prognosis. To reduce the risk of treatment failures, prompt restoration with a crown or an occlusal coverage restoration is crucial to the long-term survival of cracked teeth.
