Vitality assessment of teeth is a critical diagnostic procedure, but is complicated by the fact that pulp tissue is encased within calcified tissue and cannot directly be accessed without invasive procedures. Pulp condition cannot be determined from a single test; diagnosis involves a fusion of history, radiological examination, clinical examination and special tests. Pulp testing may be indicated prior to operative procedures. It should be remembered that pulpal deterioration can occur without supplementary symptoms (Kramer, 1954).
The origin of most oral pain is pulpal (Ehrmann, 1977). Pulp testing is valuable in the diagnosis of pain in the trigeminal area (Mumford and Bjorn, 1976). A normal response may exclude the diagnosis of pulpal pathology. Pulp testing aids in investigation of radiolucent areas. Periapical radiolucencies may represent normal structures, or indicate extension of pulpal pathology or other pathological processes such as periodontal lesions, cysts, fibrous lesions, congenital abnormalities or neoplastic processes (Harris, 1971).
Vitality testing aids in treatment of teeth affected by surgical trauma (Johnson and Hinds, 1969) and teeth involved in jaw fractures (Roed-Petersen and Andreasen, 1970). Pulp testers may be used to assess whether a tooth is anaesthetised prior to operative procedures (Grossman, 1978). I think this would be more useful in investigation of analgesics in the laboratory, rather than in the clinical setting. Vitality testing would help in the prognosis of pulp-capped teeth or teeth with deep restorations. The aim of this paper is to explore the reliability of current pulp testing methods.
Pulpal innervation Nerve bundles within the coronal pulp diverge towards the pulpo-dentine border and emerge from their myelin sheaths (Dahl and Mjor, 1973). Smaller fibre groups travel towards the dentine and form the nerve plexus of Raschkow, and terminal axons pass between odontoblasts as free nerve endings. The occlusal aspect of the pulp chamber, along with the lateral wall of coronal and cervical dentine, are rich in this nerve plexus. Two types of sensory fibres are in the pulp: myelinated (A) fibres and unmyelinated (C) fibres.
A fibres which principally innervate the dentine, are sub-grouped into AB (10%) and AS (90%) fibres, depending on their conduction velocities and diameter. AB fibres may be more sensitive to stimulation than AS fibres (Gopikrishna et al, 2009). AS fibres respond to stimuli which do not activate C fibres, as they have lower electrical thresholds (Olgart, 1974). They mediate acute, sharp pain and are excited by hydromechanical events in dentinal tubules (Byers, 1984). In contrast, C fibres mediate dull, burning pain and are activated by intense temperature changes of the tooth crown.
C fibre pain (associated with tissue injury) is modulated by increases in pressure, vascular changes and inflammatory mediators (Markowitz and Kim, 1990). Sensitivity testing Vitality testers assess AS fibre nerve integrity. Successful stimulation produces a short, sharp sensation. Enamel and dentine thickness, and density of nerve fibres in the underlying pulp affect the response to pulp testing. As fluid in dentinal tubules conducts electrical impulses from the tester electrode to the pulp, lower resistance to current flow is achieved by shortening the distance between the electrode and pulp (Bender et al, 1989).
Sometimes, for example after trauma, even though there may be blood supply to the pulp, the AS fibres may not function, responding negatively to the vitality test. Thermal testing Cold test Spraying ethyl chloride or dichlorodifluoromethane onto a cotton pledget results in ice crystal formation prior to tooth application. This contracts dentinal tubules, resulting in rapid outward flow of fluid within patent tubules. The sharp sensation produced in a vital tooth (by hydrodynamic forces acting on AS fibres) lasts for the duration of the thermal test (Trowbridge et al, 1980), which should be a maximum of 15 seconds.
If the pain lingers after the stimulus is removed, there may be irreversible pulpitis, whereas if pain subsides immediately, there may be reversible pulpitis. Frozen CO2 (dry ice / CO2 snow) is particularly effective when testing teeth heavily restored with metal. While its application for 5 minutes does not damage the pulp or enamel surface, its application for 5 seconds may cause pitting on porcelain restorations (Krell, 1985). It is important to note that multi-rooted teeth may respond positively, even when only one root contains vital tissue (Grossman, 1978).
A colder stimulus results in more effective testing (Fuss et al, 1986). Heat test Heated gutta-percha or hot water is applied to a Vaseline- coated tooth surface. It may be difficult to perform this test on posterior teeth due to limited access. Excessive heating may result in pulp damage (Mumford, 1964) and prolonged heating will result in initial bi-phasic AS stimulation, followed by C fibre activation. As C fibre activation may result in lingering pain, heat application should be limited to 5 seconds. Conversely, the stimulus may be too weak if the gutta-percha is poorly heated.
Electric pulp test This is a technique sensitive method which requires tooth isolation to prevent spread of electrical impulses to gingiva or adjacent teeth. The tester should be placed on the tooth surface adjacent to a pulp horn (highest density of nerve fibres). According to Lin and co-workers (2007), the optimum site of tester placement on molars is the mesiobuccal cusp tip. An ionic shift in dentinal tubule fluid causes local depolarisation and action potential generation from intact Ad nerves, giving a positive result where the patient feels warmth or a tingling sensation.
Reproducibility may be compromised due to lack of intrinsic accuracy of electric pulp testers. Test cavity preparation This is usually a technique used as a last resort as it causes iatrogenic damage. A test cavity is made by drilling through the amelodentinal junction of an unanaesthetised tooth. If pain is felt upon contacting dentine, drilling is stopped, the cavity is restored and the tooth deemed vital. Local anaesthetic test When conventional pulp testing is inconclusive, and pain symptoms are poorly localised or referred, the most posterior tooth in the suspected area is anaesthetised.
If pain persists, the tooth mesial to it is anaesthetised, and so on, until pain wanes. Limitations of sensitivity testing A non-vital tooth may respond positively (false positive) to testing in anxious or young patients anticipating an unpleasant sensation (Bender et al, 1989). Reiss and Furedi (1993) have stated that reproducibility of sensitivity tests is an area of concern which may relate to the variable state of mind of the patient. A false positive may also result from electric currents conducted by necrotic pulpal breakdown products to viable nerve tissue in adjacent areas (Dummer et al, 1980).
A false negative (vital tooth responds negatively to testing) may be seen in teeth with incomplete root development as they have a higher threshold to testing. I think this is probably because neural development is incomplete when teeth are already functional. Vital traumatised teeth may not respond positively to sensitivity testing due to nerve rupture. Orthodontic tooth movement reduces blood flow to AS nerves, increasing response threshold to electric pulp testing; this can last for up to 9 months after treatment (Cave et al, 2002).
Cooley and Robison (1980) have stated that patients with psychotic disorders may not respond to pulp testing. Read and co-workers (2014) demonstrated that individuals under the influence of analgesics have an increased threshold to nerve excitation. Intriguingly, placebos produce the same effect (Blair, 1965). Several studies show a lack of correlation between pulp testing methods and the histological condition of the pulp. However, a statistically significant relationship exists between negative response to pulp testing and presence of a completely necrotic pulp (Marshall, 1979).
The stage of dental development should be considered in young patients, as erupting teeth have an increased threshold value for electric pulp testing or respond negatively, even though they are vital. However, thermal testing with CO2 snow has been shown to give consistent positive responses regardless of the stage of dental development (Fulling and Andreasen, 1976). Age seems to have no effect on thresholds to pulp testing (Mumford, 1963). I would think that threshold would increase with age due to decreased neural density and pulpal nerve calcification.
Assessment of pulpal vascularity Thermographic imaging It is a very accurate, non-invasive technique to measure crown surface temperature. Pogrel and co-workers (1989) demonstrated that following cooling, non-vital teeth were slower to re-warm than vital teeth. A major disadvantage of this complex technique is that subjects need to be at rest for 1 hour prior to testing, after isolation with a rubber dam. Laser Doppler Flowmetry It is used to assess blood-flow in microvascular systems by utilising a beam of infrared light. When this light enters the tissue, mobile red blood cells and still tissue elements scatter and adsorb it.
Photons interacting with moving elements are Doppler shifted and interpreted as an index of pulpal blood flow (Gazelius et al, 1986). It is essential to stabilise the measuring probe on the tooth surface. False positive results may be obtained from supporting tissues, so caution is advised. The clinician must keep in mind that blood pigments within a discoloured tooth crown can interfere with laser light transmission (Gopikrishna et al, 2009). Transmitted light photoplethysmography This non-invasive technique monitors pulpal blood flow and incurs less signal contamination from periodontal blood flow than for LDF.
Pulse oximetry It utilises the pulsatile changes in blood volume to determine the saturation of arterial blood. It is crucial that the sensor conforms to the size, shape and anatomical contours of the teeth. The test’s accuracy is dependent on a normal arterial blood flow. Measurements may be unobtainable during hypovolemia, hypothermia or intense peripheral vasoconstriction (Wilson and Shapiro, 2001). Specificity may be lowered when the pulp is undergoing calcific changes; a radicular vital pulp with coronal calcification may cause a false negative response (Gopikrishna et al, 2009).
Fibre-based laser speckle imaging It can detect the presence of pulsatile flow. This recently developed technique is based on the observation that dentine tubules act as light-guiding conduits (Regan et al, 2015). Conclusion Several methods are used to test pulp for vitality, including sensitivity tests (thermal test, electric pulp test, test cavity and local anaesthesia) and those that assess for vascularity (thermographic imaging, laser Doppler flowmetry, transmitted light photoplethysmography, pulse oximetry and the most recently developed fibre-based laser speckle imaging).
I believe that patient responses to sensitivity testing are subjective (due to the subjective nature of pain) and so assessment of a functioning blood supply is a more reliable method of testing tooth vitality. An ideal pulp test should be reproducible, simple, inexpensive, non-painful, non-injurious, objective, accurate and standardised. I think that the most reliable test available is the pulse oximeter test which has a sensitivity of 0. 93 and specificity of 1. A single sensitivity test on its own may not be sufficient for diagnosis, but the pulse oximeter test is quite accurate.