A tooth can hurt reliably while examination and imaging show nothing obviously wrong. Cracks are the common explanation, and the difficulty of finding them is a physical consequence of how they are shaped.
Radiographs miss most cracks
A radiograph records differences in how much radiation passes through tissue. A structure shows up when it changes that transmission enough to register.
A crack is often a very narrow separation with the two surfaces still in contact. It removes almost no material and therefore alters transmission very little.
Detection also depends on orientation. A crack running along the beam direction may show, while the same crack across the beam usually will not.
The pain pattern comes from movement of the segments
When a cracked tooth is loaded, the segments flex apart slightly. Releasing the load allows them to snap back together.
That movement disturbs fluid in the dentin tubules, stimulating nerve endings in the pulp. The result is often sharp pain on releasing a bite rather than on applying it.
Because the pain requires the right direction of force, it can be inconsistent, appearing with some foods and not others.
Localizing the tooth is its own problem
Pulp pain is poorly localized, since the nerve supply does not distinguish position well. Patients frequently identify the wrong tooth or only the general area.
Pain can also be referred between arches, so a lower tooth may be blamed for an upper one.
Clinicians therefore test teeth individually, using devices that load one cusp at a time to reproduce the symptom on a specific tooth.
Additional methods are used to reveal the line
Dyes applied to the tooth can seep into a crack and outline it. Transillumination, shining light through the tooth, can show a segment that fails to transmit light.
Magnification improves the odds of seeing a fine line on the surface, and removing an existing filling sometimes exposes a crack running beneath it.
Three-dimensional imaging is used in some cases, though very fine cracks remain difficult to capture with any imaging technique.
Why timing matters clinically
Cracks tend to extend under repeated loading. A crack confined to the crown has different implications from one reaching the root.
If it reaches the pulp, bacteria gain access and inflammation or infection can follow, changing what treatment is possible.
Persistent pain on biting, even with normal-looking teeth, is a reason to see a dentist rather than wait, because the assessment depends on tests that cannot be done at home.