Diagnosing Cracked Teeth With Fibre-Optic Transillumination (FOTI)
A Clinical Selection Guide

FOTI is indicated when a patient presents with sharp, fleeting pain on biting or release, which cannot be attributed to caries, defective restorations, or periodontal issues. This classic cracked tooth symptom, especially when localized to a specific cusp but difficult to pinpoint visually, is a primary trigger for using a dental transilluminator.

The underlying mechanism is cuspal flexure during mastication, which irritates the pulp. The pain upon release of biting pressure is pathognomonic, as the crack segments spring back into position. A dental transilluminator works by passing a focused beam of light through the tooth; a fracture line, having a different refractive index, disrupts this light path and appears as a distinct dark line, confirming the source of the elusive pain.

• Sharp pain on biting: Especially when chewing granular foods like grains or nuts.
• Pain on release of pressure: A key indicator, as the fractured segments move.
• Localized thermal sensitivity: Particularly to cold, with no other identifiable cause.
• Inconclusive radiographs: Fractures, especially in the mesiodistal plane, are often invisible on 2D X-rays.
• No visible evidence: The tooth may appear clinically sound with no caries or restoration failure.

The primary difference between FOTI and methylene blue dye is that FOTI is a non-invasive, real-time diagnostic aid, while dye staining is an invasive technique that requires tooth preparation and can cause permanent discoloration. FOTI is the preferred initial method for vital teeth as it provides immediate chairside information without altering the tooth structure.

Methylene blue dye works via capillary action, seeping into the fracture line and making it visible upon removal of the excess. While effective, it can permanently stain dentin, complicating aesthetic outcomes. A high-intensity LED dental transilluminator, by contrast, relies purely on light physics and is completely reversible, making it a superior choice for initial assessment and patient communication.

• Reversibility: FOTI is completely non-invasive and reversible; dye staining is not.
• Speed: Transillumination provides an instant result, whereas dye requires application, waiting, and cleanup.
• Scope: FOTI can help visualize cracks through intact marginal ridges, while dye is most effective on exposed surfaces after restoration removal.
• Patient Acceptance: Patients readily accept a light-based examination over the application of a strong, potentially staining chemical.

FOTI diagnosis is most limited by the presence of certain dental materials and the location of the crack itself. Full coverage crowns, particularly PFM or full metal, completely block light transmission, rendering FOTI useless for detecting cracks underneath them. Similarly, large, multi-surface composite restorations can scatter the light, obscuring underlying fracture lines.

The technique's efficacy diminishes significantly for cracks that extend deep subgingivally or are located entirely within the root structure. The light from a dental transilluminator cannot adequately penetrate bone and soft tissue to illuminate a vertical root fracture. Therefore, while excellent for coronal cracks in minimally restored teeth, FOTI is not a definitive tool for assessing restoratively complex teeth or suspected root fractures.

• Restorative Materials: Opaque materials like metal, PFM, and some ceramics will block light.
• Crack Location: Does not work for subgingival or radicular fractures.
• Tooth Discoloration: Intrinsic staining or pulpal necrosis can create dark areas that mimic cracks.
• Operator Skill: Correct angulation of the light source is critical for accurate interpretation.

Using a dental transilluminator, a crack is identified as a distinct dark line that stops or significantly impedes the transmission of light across the tooth, whereas a craze line typically does not. This is the fundamental difference in their FOTI presentation and is key to avoiding overtreatment.

Craze lines are superficial, incomplete fractures confined to the enamel and are almost always asymptomatic. Because they are shallow and often filled with organic material, they do not significantly alter the path of light. A true fracture, however, propagates into dentin, creating a physical separation that acts as a barrier to light. When transilluminating, sweep the light from buccal to lingual; a crack will appear as a sharp, dark border, while a craze line may be faintly visible or not at all.

• Light Transmission: A crack blocks light; a craze line usually transmits light.
• Termination Point: A fracture line will often terminate at a marginal ridge or extend onto the occlusal surface.
• Symptomatology: Correlate FOTI findings with clinical symptoms. Craze lines are asymptomatic.
• Location: Craze lines are common on anterior teeth and buccal surfaces of posterior teeth. Symptomatic cracks are most frequent in posterior teeth.