19.2 Special Investigations and Reaching a Diagnosis
Key Takeaways
- An investigation is justified only if its result could change management, which is the general principle behind the IR(ME)R justification requirement.
- Radiographic intervals in the UK are risk-based rather than fixed, so bitewing frequency follows the individual caries risk assessment.
- Sensibility tests measure nerve response rather than blood supply and must be interpreted against a control tooth in the same arch.
- Pulp vitality is the single most discriminating test when assessing a periapical radiolucency, separating inflammatory lesions from fibro-osseous and developmental ones.
- An unexplained oral ulcer persisting more than three weeks is referred urgently rather than biopsied in primary care by an inexperienced clinician.
The Purpose of a Special Investigation
A special investigation is justified only when the result could change management. The IR(ME)R justification requirement for radiographs is the statutory expression of a principle that applies to every test: if you already know what you will do whatever the result shows, the test is not indicated.
Radiographic Selection Criteria
Radiographs must be selected for the individual patient, never taken as a routine package. UK selection criteria give risk-based intervals rather than fixed ones:
| Situation | Typical investigation |
|---|---|
| Caries risk assessment, primary and mixed dentition | Bitewings, interval determined by caries risk |
| Caries risk assessment, permanent dentition | Bitewings, interval determined by caries risk |
| Periodontal bone loss, mild to moderate (5 mm or less) | Horizontal bitewings |
| Periodontal bone loss, moderate to severe (more than 5 mm) | Vertical bitewings or periapicals |
| Suspected periapical pathology | Periapical radiograph |
| Unerupted tooth localisation | Two views with parallax, or a panoramic plus an anterior occlusal |
| Third molar assessment near the inferior alveolar canal | Panoramic; CBCT only where it will change surgical management |
| Suspected jaw fracture, cyst or extensive pathology | Panoramic plus further imaging as indicated |
Other Investigations
- Sensibility testing — cold, electric pulp testing and heat. These test nerve response, not blood supply, and must be interpreted with a control tooth on the same side and arch.
- Percussion, palpation and mobility — testing the periodontium rather than the pulp.
- Transillumination and dye staining — for cracks.
- Study models and photographs — for occlusal analysis, orthodontic assessment and medico-legal baselines.
- Biopsy — incisional for suspicious lesions, with the technique described in the oral medicine chapter. An unexplained ulcer of more than three weeks is referred urgently, not biopsied in primary care by a clinician without the relevant experience.
- Microbiology — a swab for candidiasis where the clinical picture is atypical or treatment has failed; pus for culture and sensitivity in spreading or non-resolving infection.
- Haematology — full blood count, ferritin, vitamin B12 and folate for recurrent ulceration; HbA1c where diabetes is suspected; INR before invasive treatment in a warfarinised patient.
- Salivary flow rates — for objective confirmation of hyposalivation.
Building a Differential Diagnosis
The expected method is to generate a differential, then use investigations to discriminate between its members rather than to confirm a favourite. A structured way to generate the list, often called the surgical sieve, runs: congenital, acquired — inflammatory, infective, traumatic, neoplastic, metabolic, degenerative, autoimmune, iatrogenic, idiopathic.
Applied to a solitary radiolucency at the apex of a tooth:
- If the tooth is non-vital: periapical granuloma, radicular cyst, periapical abscess, periapical scar.
- If the tooth is vital: cemento-osseous dysplasia, simple bone cyst, incisive canal cyst, Stafne defect, or a lesion unrelated to the tooth that happens to overlie the apex.
Pulp vitality is the single most discriminating test in jaw radiolucency diagnosis, and failing to perform it before treating an apparent periapical lesion is a classic examination trap.
Recording the Diagnosis
A complete diagnosis is rarely a single word. In periodontics the British Society of Periodontology requires extent, stage, grade, stability and risk factors. In endodontics both a pulpal and a periapical diagnosis are required. The habit of writing a full diagnostic statement is what allows a treatment plan to be justified, communicated and audited.
Exam link. A stem describing a well-defined radiolucency at the apex of a lower premolar in a 30-year-old woman with no symptoms is testing whether you will check vitality. If the tooth is vital, periapical cemento-osseous dysplasia is the likely answer and root canal treatment would be actively harmful.
Vitality Testing and Its Pitfalls
Special investigations in dentistry are examined largely through their limitations. Pulp sensibility tests — cold, heat and electric — test the nerve supply, not the blood supply, and so do not measure vitality directly. They produce false negatives in teeth with calcified canals, in heavily restored or crowned teeth, in recently traumatised teeth with transient loss of sensibility, and in immature teeth with incompletely developed nerve supply; they produce false positives from multi-rooted teeth with one vital canal, from stimulation of adjacent teeth, and from liquefaction necrosis conducting stimulus. For this reason a diagnosis is never made on a single test: the suspect tooth is compared with a contralateral and an adjacent control, and the findings are integrated with history, percussion, palpation, mobility, periodontal probing and radiographs.
Choosing the Right Investigation
Each investigation should be selected to answer a specific question. A bitewing answers questions about approximal caries and crestal bone. A periapical answers questions about the root, apex and periapical tissues. A panoramic answers questions about the whole dentition and jaws, third molars and gross pathology, but is poor for caries and for fine bony detail. Cone beam computed tomography answers three-dimensional questions — the relationship of a third molar to the inferior alveolar canal, complex endodontic anatomy, implant planning — and must be justified individually under IR(ME)R because of its higher dose. Vitality testing, transillumination, tooth sleuth testing, study casts, photographs, sensibility mapping, blood tests and biopsy each answer their own question. Ordering an investigation whose result cannot change the management is both poor practice and, for ionising radiation, unlawful.