A cracked tooth refers to a partial enamel rupture in a living tooth with dentinal and even pulpal involvement [1]. The causative factors of a tooth crack vary. A masticatory function is the most common factor related to vertical tooth fracture [2]. Depending on the crack’s extension depth and subsequent bacterial infections, cracked teeth can cause several clinical symptoms [3]. Typical symptoms include acute pain on chewing or unexplained pain as a result of exposure to a cold stimulus [4]. An incidence rate of 34–74% has been reported in adults between the ages of 30–50 [5], with a female predisposition ^[6][7][6,7]. The probability of a cracked tooth is higher in an aging population with a greater number of retained teeth [8]. There is an increase in the mineral density of enamel with aging. This is concurrent with a decrease in the volume of the protein matrix [9]. Cosmetic procedures such as bleaching can accelerate this deterioration in fracture toughness and mechanical properties of the enamel ^[10][11][10,11].

The mandibular molars are more susceptible to enamel fractures. Fractured enamel is more prevalent in the upper and lower bicuspids as well as the upper molar teeth [7]. The mesiopalatal cusp of the upper first grinder is quite protuberant. This creates wedging on the first mandibular grinders, which might also contribute to tooth cracks [12]. As a consequence, these teeth are more liable to fracture. Diagnosing a cracked tooth can be an arduous task for a clinician as its symptoms can be variable and bizarre [13]. The cracks will continue to spread if prompt treatment is not given. This can eventually lead to pulpitis or a complete tooth fracture [14]. The primary objective of its diagnosis is early detection. Visual inspection [15], periodontal probing, bite diagnosis, pulp vitality test [16], staining [17], transillumination [18], and computed tomography (CT) [19] were the most common clinical diagnosis methods. Surface crack detection methods such as optical coherence tomography (OCT) have recently attracted a lot of attention ^[20][21][20,21]. Cone-beam computed tomography (CBCT) and Micro-CT appeared to be more efficacious than traditional clinical dental imaging techniques in detecting longitudinal tooth cracks ^[22][23][22,23]. 

2. Clinical Features

The clinical symptoms differ according to the location and extension of the incomplete fracture ^[12][24][25][12,24,25]. It is possible to elicit a history of months of discomfort and acute pain while chewing or consuming cold beverages. “Rebound pain” or pain that occurs when pressure is released after eating fibrous foods is frequently observed. Patients may not be able to identify the tooth where the pain is felt. They may report a negative response to heat stimuli. Microleakage of bacterial byproducts and toxins can cause chronic pulpitis without causing clinical symptoms. Pulpal and periodontal symptoms may develop from cracks with pulpal involvement ^[26][27][26,27]. The physiological basis of chewing pain as proposed by Brannstrom and Astrom [28] states that the independent movement of the fractured tooth part with each other could cause a sudden transition in the fluids within the dentinal tubules. Subsequently, the A-type fibers get activated in the dental pulp leading to immediate pain. The toxic irritants may leak via the cracks producing hypersensitivity to cold. The release of neuropeptides as a result of the toxic irritant leakage within the dental pulp leads to a decreased pain threshold. These symptoms occur because of alternate compression and stretching of the odontoblast processes inside the crack. Asymptomatic enamel cracks can have multiple pathogenic consequences. Teeth with enamel cracks may be further damaged by ultrasonic instrumentation such as scaling and root planing [29].

3. Difference between a Fractured Cusp or a Split Tooth and a Cracked Tooth

A cracked tooth can be differentiated from a split tooth by checking the movements of the segment by wedging. A cracked tooth demonstrates no movement with wedging pressures. Under light pressure, a split tooth may break off, leaving patients with no further mobility. Under wedging stresses, a fractured cusp will display mobility and the mobile part will extend deep beyond the cementoenamel junction. Postoperative sensitivity as a result of broken restoration, areas of hyper occlusion due to dental restoration, microleakage from newly filled composite resin restorations, orofacial pain, and bruxism pain, can all be considered as a cracked tooth [30].

4. Causes of Enamel Cracks and Identification

The sources and causes of enamel cracks are multifactorial. They can develop glacially over a long time period. Enamel cracks can exist independent of dentin cracks. The pain from a cracked tooth can be difficult to distinguish from other conditions such as ear pain, atypical orofacial pain, migraine, sinusitis, or temporomandibular joint disorders. As a consequence, diagnosis may be clinically challenging and delayed. Early detection is critical because restorative treatment can prevent fracture propagation, microleakage, pulpal or periodontal tissue involvement, and catastrophic cusp failure [31]. The ease at which a fracture can be diagnosed varies depending on its location and extent. An extensive intracoronal repair is usually present on the tooth. Humans uniquely consume both hot and cold food, leading to repeated thermal changes that can cause expansion and contraction [32]. This is particularly relevant in the case of teeth restored with ceramics where thermal expansion and cyclic loading can lead to crack formation and propagation. Microscopic cracks at the edges of a restoration can indicate structural weakness [18]. Cracks at the margins of composite restorations may be due to polymerization contraction stress. It can present as a whitish margin [33]. Well-defined discoloration of a cusp can be a sign of a lack of structural integrity [18]. Pain can arise after dental procedures, such as the cementation of an inlay, and is often misdiagnosed to be due to the high spots on the recent restoration. The presence of underlying cracks could be indicated by regularly debonding the cemented intracoronal restorations like inlays.

5. Classification/Definitions of Tooth Fracture

The American Association of Endodontists has divided longitudinal tooth fractures into five classes: craze line, fractured cusp, cracked tooth, split tooth, and vertical root fracture [34]. An asymptomatic crack in the tooth enamel may progress along with dentin cracks. Clinically distinguishing between the various types of cracks can be difficult (Figure 1). Cracked tooth syndrome includes three phases:

• Craze Lines: Tiny and harmless cracks that occur in the outer enamel of the tooth.
• Cracks: It occurs when craze lines penetrate the body of the tooth (dentin).
• Fractures: A fracture occurs when cracks expand deep into the root of the tooth.

Craze lines are asymptomatic superficial lines that appear on enamel with age. They are frequently confused with cracks but can be distinguished with transillumination. Fractured cusps initiate from the coronal tooth portion, progress into the dentinal area, and end cervically. They are generally connected with large restorations that leave the cuspal enamel unsupported. A cracked tooth is defined as one that extends apically from the crown of the tooth without separating two segments, according to the AAE. A split tooth has a breakthrough both marginal ridges, totally dividing the tooth into two halves. Vertical root fractures begin from the root and are usually complete or partial. All classification methods have the difficulty of failing to relate the descriptions to clinical outcomes or treatment recommendations. A tooth fracture was divided into two types by Silvestri and Singh (1978): complete and incompletely fractured teeth [35]. Complete fractures were then separated into vertically directed and obliquely directed complete fractures. The authors believe that the full oblique fracture occurs most frequently as a result of a thorough restoration that undermines a cusp. The undermined cusp fractures off under the stresses of physiological functions. A vertically oriented complete fracture is seen as two individual mobile tooth and root segments in relation to one another. The incomplete tooth fractures are of two types: vertical and oblique. An oblique crack originates on the occlusal surface of the tooth, involves one or few cusps, extends through the dentin in an oblique direction beneath the cusps, and ends gingivally in the cementum or enamel. Tooth segments are not completely crushed. Vertical incomplete fractures start in the enamel and extend through the dentin and the root progress. With no complete separation of segments, the crack might involve one or both the marginal ridges extending in the mesiodistal direction. Figure 2 depicts a more detailed classification scheme, and was created by Talim and Gohil (1974): [36].