8.3 Radiographic Interpretation of Caries, Periapical & Bone Lesions
Key Takeaways
- Radiographs underestimate the true depth of a carious lesion, so a lesion appearing to reach the dentinoenamel junction has usually progressed into dentin histologically
- A dentigerous cyst attaches at the cementoenamel junction of an unerupted tooth, while an odontogenic keratocyst grows anteroposteriorly with minimal expansion and has a high recurrence rate
- Loss of the lamina dura with a periapical radiolucency on a non-vital tooth indicates periapical inflammatory disease, whereas an intact lamina dura with a widened ligament space on a vital tooth suggests occlusal trauma
- A well-defined corticated border suggests a slow-growing benign process, while a ragged, ill-defined, moth-eaten border suggests malignancy or osteomyelitis
- Bitewing radiographs are the standard for detecting interproximal caries and assessing crestal bone height, whereas periapical images are required to evaluate the apex and surrounding bone
Radiographic Interpretation of Caries, Periapical & Bone Lesions
Why this matters on the INBDE: Case items frequently pair a dental chart with a described radiographic finding and ask for the diagnosis or the next step. The candidates who do well describe the lesion systematically before they reach for a name.
A Systematic Description Framework
Before naming any lesion, describe six features. This framework converts a guess into a defensible differential.
- Location — which jaw, which region, relationship to the inferior alveolar canal or maxillary sinus, above or below the canal.
- Relationship to teeth — pericoronal (around a crown), periapical, interradicular, or unrelated.
- Border — well-defined and corticated (slow, benign); well-defined but non-corticated; ill-defined or moth-eaten (aggressive, infectious, or malignant).
- Internal structure — completely radiolucent, mixed, radiopaque, multilocular (soap-bubble, honeycomb), or containing tooth-like structures.
- Effect on surrounding structures — displacement of teeth, root resorption, cortical expansion or perforation, displacement versus destruction of the inferior alveolar canal.
- Number and symmetry — solitary or multiple; multiple keratocysts point to Gorlin syndrome.
Two highest-yield alarm signs: root resorption and destruction (rather than displacement) of a cortical boundary. Either finding elevates a lesion from watchful waiting to biopsy.
Caries Interpretation
Bitewings are the standard projection for interproximal caries and for crestal bone height. Caries appears as a radiolucency because demineralized tooth structure attenuates fewer photons.
| Classification | Radiographic extent |
|---|---|
| E1 | Outer half of enamel |
| E2 | Inner half of enamel, not reaching the DEJ |
| D1 | Outer third of dentin |
| D2 | Middle third of dentin |
| D3 | Inner third of dentin, approaching pulp |
Critical limitation: radiographs consistently underestimate lesion depth. A lesion that appears to just reach the dentinoenamel junction has usually penetrated into dentin histologically. Approximately 30% to 40% demineralization must occur before a lesion is radiographically detectable at all, which is why radiographs cannot detect early occlusal caries and why they are always interpreted alongside clinical examination and risk assessment.
Mimics of caries that must be excluded: cervical burnout (a triangular radiolucency at the cervical region between the enamel edge and alveolar crest, caused by differential penetration in a thin area of tooth), restoration overhangs, and abrasion or abfraction notches.
Periapical Radiolucencies
The first question is always vitality, not appearance.
| Entity | Pulp status | Distinguishing features |
|---|---|---|
| Periapical granuloma | Non-vital | Small, well-defined, continuous with a widened PDL space; loss of lamina dura |
| Radicular (periapical) cyst | Non-vital | Larger, corticated; may displace adjacent roots; arises from epithelial rests of Malassez |
| Acute apical abscess | Non-vital | May show no radiographic change early — bone loss lags symptoms by days; severe pain, swelling, percussion sensitivity |
| Condensing osteitis | Non-vital or inflamed | Radiopaque, not lucent, at the apex of a tooth with pulpal disease |
| Periapical cemento-osseous dysplasia | VITAL | Anterior mandible, often multiple, evolves radiolucent → mixed → radiopaque; no treatment |
| Surgical/anatomic mimic | Vital | Incisive foramen, mental foramen, maxillary sinus recess superimposed over an apex |
Radiographic changes do not distinguish granuloma from cyst reliably — histology does. On the examination, the correct answer is usually that both are managed initially with nonsurgical root canal therapy, with biopsy reserved for lesions that fail to resolve.
Pericoronal Radiolucencies
| Lesion | Attachment / features | Behavior |
|---|---|---|
| Dentigerous cyst | Attaches at the cementoenamel junction of an unerupted tooth; unilocular, corticated | Most common developmental odontogenic cyst; may displace the tooth substantially (mandibular third molar to the inferior border) |
| Odontogenic keratocyst | May be pericoronal; grows anteroposteriorly through the marrow with little cortical expansion; may be multilocular | High recurrence; multiple lesions signal Gorlin syndrome |
| Ameloblastoma | Often multilocular soap-bubble; posterior mandible; root resorption and marked cortical expansion | Locally aggressive, requires resection with margins |
| Adenomatoid odontogenic tumor | Anterior maxilla, young female, around an unerupted canine; radiolucency with fine snowflake calcifications; extends apical to the CEJ | Benign, enucleation is curative — the "two-thirds tumor" |
| Calcifying epithelial odontogenic tumor (Pindborg) | Mixed radiolucent-radiopaque around an impacted tooth; "driven snow" calcifications | Locally invasive |
Discriminator: attachment at the CEJ equals dentigerous cyst; extension beyond the CEJ down the root of an unerupted anterior maxillary canine in a teenage girl equals adenomatoid odontogenic tumor.
Multilocular Radiolucencies
The classic differential for a multilocular lesion of the posterior mandible: ameloblastoma, odontogenic keratocyst, odontogenic myxoma (fine "tennis-racket" or "step-ladder" septa), central giant cell granuloma (anterior mandible crossing the midline, younger patient), and aneurysmal bone cyst. Any patient with a giant cell lesion should have serum calcium, phosphorus, alkaline phosphatase, and parathyroid hormone checked to exclude the brown tumor of hyperparathyroidism, which is histologically identical.
Radiopaque Lesions
| Lesion | Key feature |
|---|---|
| Odontoma (compound) | Multiple small tooth-like denticles in a corticated sac; anterior maxilla; the most common odontogenic tumor |
| Odontoma (complex) | Amorphous radiopaque mass; posterior mandible |
| Idiopathic osteosclerosis | Homogeneous, no rim, vital tooth, no expansion |
| Cemento-osseous dysplasia (florid) | Multiquadrant, mixed to radiopaque, vital teeth, in middle-aged patients |
| Fibrous dysplasia | Ground-glass internal pattern with ill-defined blending into normal bone; expansion; superior displacement of the inferior alveolar canal |
| Osteoma | Well-circumscribed dense bone; multiples suggest Gardner syndrome |
| Sialolith | Radiopacity along the course of a duct, most often submandibular |
Malignancy and Osteomyelitis: The Aggressive Pattern
Features that must trigger urgent referral:
- Ill-defined, moth-eaten, or permeative borders.
- Widening of the periodontal ligament space around multiple adjacent teeth — a classic early sign of osteosarcoma.
- Teeth appearing to "float" in space with destroyed supporting bone — malignancy or Langerhans cell histiocytosis.
- Sunburst or onion-skin periosteal reaction — osteosarcoma and Ewing sarcoma respectively.
- Sequestrum formation and involucrum — osteomyelitis.
- Paresthesia of the inferior alveolar nerve with any of the above.
Choosing the Right Projection
| Clinical question | Best projection |
|---|---|
| Interproximal caries, crestal bone height | Bitewing |
| Periapical pathology, root morphology, endodontic working length | Periapical |
| Impacted third molars, jaw fractures, large lesions, developmental assessment | Panoramic |
| Localizing an impacted canine buccolingually | Two periapicals using the tube-shift (SLOB) rule, or CBCT |
| Sialolith or foreign body in the floor of mouth | Occlusal |
| Implant site bone volume, complex root anatomy, resorption assessment | CBCT, when the benefit justifies the higher dose |
The SLOB rule — Same Lingual, Opposite Buccal — states that when the tube head shifts, an object that moves in the same direction as the tube is lingual, and one that moves in the opposite direction is buccal.
A bitewing shows an interproximal radiolucency that appears to just reach the dentinoenamel junction on the distal of the mandibular first molar. What is the most accurate statement about the lesion's true extent?
A 16-year-old female has an unerupted maxillary canine. A radiolucency surrounds the crown and extends apically past the cementoenamel junction along the root, and it contains fine snowflake-like calcifications. What is the most likely diagnosis?
A panoramic radiograph shows uniform widening of the periodontal ligament space around several adjacent posterior mandibular teeth, with an ill-defined moth-eaten radiolucency and a sunburst periosteal reaction. The patient reports lower lip numbness. What is the appropriate action?
Two periapical radiographs of an impacted maxillary canine are taken, the second with the tube head shifted mesially. The canine image moves mesially relative to the adjacent lateral incisor. Where is the canine positioned?