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Table of Contents
Year : 2011  |  Volume : 1  |  Issue : 1  |  Page : 14-21

Pulse oximetry and laser doppler flowmetry for diagnosis of pulpal vitality

1 Department of Conservative Dentistry and Endodontics, Kothiwal Dental College Research Centre and Hospital, Moradabad, India
2 Department of Pedodontics, Kothiwal Dental College Research Centre and Hospital, Moradabad, India

Date of Web Publication4-Mar-2011

Correspondence Address:
Dakshita Joy Vaghela
Department of Conservative Dentistry and Endodontics, Kothiwal Dental College Research Centre and Hospital, Moradabad
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2229-5194.77191

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The usual pulpal diagnostic instruments have been shown to be unreliable in diagnosing the pulpal status of the teeth following a traumatic injury, especially for teeth with immature root formation and open apex. Compounding the problem with these testing methods is that they all are very subjective, dependant on cooperation and understanding of the situation by the patient, which can lead to a further difficulty in cases involving young children. It is important to note that the usual pulp vitality tests provide information only about the presence or absence of nerve receptors in the pulp and not about the pulpal blood supply. Recent efforts for assessing pulpal circulation have involved the use of laser Doppler flowmetry and pulse oximetry. Though both methods are in their infancy and are not yet ready for general clinical applications, but hopefully before long, these technologies will become part of dentists' diagnostic armamentaria. The PubMed database search revealed that the reference list for Pulse Oximetry featured 2196 articles; in dentistry-121 articles and for Laser Doppler Flowmetry-932 articles and in dentistry-18 articles. A forward search was undertaken on selected articles, author names, and contemporary endodontic texts. A review is presented on the key developments in the arena of these pulp-tests to familiarize the modern dentists with the new advances in endodontic diagnosis.

Keywords: Endodontic diagnosis, laser Doppler flowmetry, pulp vitality, pulse oximetry, trauma

How to cite this article:
Vaghela DJ, Sinha AA. Pulse oximetry and laser doppler flowmetry for diagnosis of pulpal vitality. J Interdiscip Dentistry 2011;1:14-21

How to cite this URL:
Vaghela DJ, Sinha AA. Pulse oximetry and laser doppler flowmetry for diagnosis of pulpal vitality. J Interdiscip Dentistry [serial online] 2011 [cited 2023 Apr 1];1:14-21. Available from: https://www.jidonline.com/text.asp?2011/1/1/14/77191

The assessment of pulp vitality is a crucial diagnostic procedure in the practice of dentistry. [1] Pulp vitality test is crucial in monitoring the state of health of dental pulp, especially after traumatic injuries. The traditional pulp testing methods such as thermal and electrical depend on the innervation and often yield false positive and negative responses. In addition, each is a subjective test that depends on the patient's perceived response to a stimulus as well as the dentist's interpretation of that response. [2] The newer pulp testing devices, some of which are still in the developmental stage, detect the blood supply of the pulp and are considered to be more accurate and noninvasive. [3],[4]

   Pulse Oximetry Top

The pulse oximeter is a noninvasive oxygen saturation monitoring device widely used in medical practice for recording blood oxygen saturation levels during the administration of intravenous anesthesia. This technique has been used to detect vascular integrity in the tooth. [5],[6] Its wide acceptance in the medical field results from its ease of application and its capability of providing vital information about the patient's status. [7],[8]

Historical perspective

In 1935 Carl Matthes built the first device to continuously measure blood oxygen saturation in vivo by transilluminating tissue. He used two wavelengths of light, one of which was sensitive to changes in oxygen saturation and the other, which was in the infra-red range, was used to compensate for changes in tissue thickness, hemoglobin content and light intensity. However, the device had limitations as it was difficult to calibrate and absolute values could not be obtained.

J.R. Squire in 1940 devised a technique of calibration by compressing tissue to eliminate the blood. This was later incorporated in the first generation of pulse oximeters used in the operating theatres.

In the early 1940s, Glen Millikan coined the term "oximeter" to describe a lightweight earpiece to detect the oxygen saturation of hemoglobin, for use in aviation research to investigate high altitude hypoxic problems. Soon, similar devices were used during anesthesia to detect episodes of arterial desaturation in patients.

An editorial in Anaesthesiology in 1951 [9] concluded prophetically "on many occasions this instrument has detected anoxemia when observations of pulse, blood pressure, color of the patient and peripheral vascular tone have shown no abnormalities." This confirmed Comroe's classic work, [10] which emphasized the unreliability of cyanosis in detecting hypoxemia.

Millikan's ear oximeter was not calibrated. In order to overcome the problem of calibration, using Squire's concept, Earl Wood added a pneumatic cuff to measure the light increase when the ear was blanched.

In 1964, a surgeon, Robert Shaw, built a self-calibrating ear oximeter, which was marketed by Hewlett Packard in 1970 for use in physiology and cardiac catheterization laboratories.

Takuo Aayogi (1972) at the Nihon Kohden Corp. working on a dye dilution cardiac output monitor using a ear densitometer, found artifacts due to pulsatile flow. He noted that the washout curves were modified by pulsatile variations. While attempting to eliminate these variations, he discovered that the absorbency ratios of these pulsations at different wavelengths varied with the oxygen saturation. Thus, he could minimize the pulsatile component by balancing the red light signal with an infrared light signal where the dye had no absorption. As this compensation was dependent on oxygen saturation, he incorporated the technique of reducing noise in his signal to measure oxygen saturation. [11],[12],[13]

The subsequent development of light emitting diodes (LEDs), photo detectors and microprocessors further refined the technique and pulse oximeters were widely introduced into clinical practice. Modern pulse oximetry was born with the realization that pulsatile changes in light transmission through living tissues are due to alteration of the arterial blood volume in the tissue. Measurement of the pulsatile component would eliminate the variable absorption of light by bone, tissue, skin, pigment, etc. from analysis.

The most important premise of pulse oximetry, is that the only pulsatile absorbance between the light source and the photo detector is that of arterial blood. [8],[14] Earlier studies by Schnettler and Wallace [5] reported a correlation between pulp and systemic oxygen saturation readings using a modified ear pulse oximeter probe on a tooth. They recommended its use as a definitive pulp vitality tester. Kahan et al. [15] designed, built, and tested a reflectance tooth probe by using a Biox 3740 oximeter (Ohmeda Louisville, CO). Pulse-wave readings from the teeth were found to be synchronous with the finger probe but not consistently. They concluded that the accuracy of the commercial instrument was disappointing and in its present form it was not considered to have predictable diagnostic value. [15] Gopikrishna et al. [14] developed a custom made pulse oximeter sensor holder for an existing Nellcor Oximax Dura-YD-YS multisite oxygen sensor (Tyco Healthcare group LP, Pleasanton, CA) and showed the utility of the pulse oximeter dental probe in assessment of human pulp vitality. They further evaluated the sensitivity, specificity, positive predictive value and negative predictive value of this device in comparison with thermal and electric pulp-testing methods and concluded that pulse oximeter is an objective and accurate method of assessing pulp vitality. [16],[17]

Because pulp vitality is purely the function of vasculature health, a vital pulp with an intact vasculature may test nonvital if only the nerve fibers are injured. This situation is commonly encountered in recently traumatized teeth.[18] On the other hand, pulp fibres are more resistant to necrosis than the vascular tissue. [19] Therefore, thermal and electric tests may give a false-positive response if only the pulp vasculature is damaged.

In cases of trauma, teeth often do not respond to conventional pulp testing methods immediately after injury. This temporary loss of response is caused by injury, inflammation, pressure or tension on the nerve fibres in the apical area because of trauma. [20] Usually 1-8 weeks can lapse before a normal pulpal response can be elicited. However, greater observation periods may be required. [21] According to Ozcelik et al., [22] early neuronal degeneration in cases of trauma is manifested as intramyelin edema, axonal swelling, and partial loss of myelin sheaths. Bhaskar and Rappaport [4] reported their clinical observation on 25 anterior teeth that had been traumatized and did not respond to conventional vitality tests. When the pulp chambers were opened, all revealed vital pulps. They concluded that conventional vitality tests are in reality sensitivity tests and have questionable predictive value of the vitality of the pulp tissue. For this reason, they recommended that endodontic therapy should be delayed in traumatized teeth and the affected pulp tissue should be considered vital unless apical radiolucencies or sinus tracts develop. A more accurate assessment of pulp vitality can be made by determining the presence of a functioning blood supply, thus allowing the healing potential to be evaluated at an earlier stage. Moreover, delay in diagnosis can lead to severe complications such as inflammatory root resorption. [23] Therefore, it is important to determine the status of pulp in such cases to evaluate the necessity for root canal treatment.

Principle and working

The principle is based on a modification of Beer Lambert's law, which relates the absorption of light by a solute to its concentration and optical properties at a given light wavelength. It also depends on the absorbance characteristics of hemoglobin in the red and infra-red range. [24],[25] In the red region, oxyhemoglobin absorbs less light than deoxyhemoglobin and vice versa in the infrared region. [7],[8]

The system consists of a probe containing a diode that emits light in two wavelengths: red light of approximately 660 nm and infra-red light of approximately 940 nm. A silicon photo detector diode is placed on the opposing surfaces of the tooth, which is connected to a microprocessor. The probe is placed on the labial surface of the tooth crown and the sensor on the palatal surface. Ideal placement of the probe is in the middle third of the crown. If placed in the gingival third, disturbances from gingival circulation or any gingival trauma or bleeding will interfere with the readings, whereas incisally less pulp tissue is present for adequate detection of the pulse.

The critical requirement of using pulse oximeter in dentistry is as follows.

  1. Sensor should conform to the size, shape, and anatomical contours of teeth.
  2. Light-emitting diode sensor and the photoreceptor should be as parallel as possible to each other so that the photoreceptor sensor receives the light-emitted from LED.
  3. The sensor holder should allow firm placement of the sensor onto the tooth to obtain accurate measurements [Figure 1]. [14]
Figure 1: Working of a pulse oximeter. (a) LED-emitting red light at 660 nm. (b) LED emitting infrared light at 940 nm. (c) Photodetector. (d) Pulse oximeter monitor. (e) Pulse oximeter sensor. (f) Custom-made pulse oximeter sensor holder. HbO2, oxygenated hemoglobin; HbR, deoxygenated hemoglobin; SpO2, oxygen saturation of arterial blood

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  1. Effective and objective method of evaluating dental pulp vitality.
  2. Useful in cases of impact injury where the blood supply remains intact but the nerve supply is damaged.
  3. Pulpal circulation can be detected independent of gingival circulation.
  4. Pulp pulse readings are reproducible.
  5. Smaller and cheaper commercial oximeters are now available for routine clinical use in an average dental office.
  1. Background absorption associated with venous blood and tissue constituents is not differentiated.
  2. Probes should be specific for the anatomy of a tooth as the oxygen saturation values from the teeth routinely register lower than the readings from the patient's finger. [16],[17],[26]

   Laser Doppler Flowmetry Top

Pulp vitality implies that blood supply is present within the tissues. Hence, only a test that actually measures or assesses pulp blood flow can be called a vitality test.[27],[28] Laser Doppler flowmetry (LDF) is a noninvasive, painless, electro optical technique, which allows the semi-quantitative recording of pulpal blood flow. [29],[30],[31],[32],[33] It measures blood flow even in the very small blood vessels of the microvasculature. [7],[34]

Historical perspective

Laser Doppler method was used by Yeh and Cummins [35] to estimate the velocity of red blood cells in capillaries. LDF was developed to assess blood flow in microvascular systems, e.g., in the retina, gut mesentery, renal cortex, and skin. [36-38] It has since been widely adopted for the measurement of blood flow especially in soft tissues. [39],[40],[41],[42],[43],[44],[45],[46],[47] This original technique utilized a light beam from a helium-neon (He-Ne) laser emitting at 632.8 nm, which, when scattered by moving red cells underwent a frequency shift according to the Doppler principle. A fraction of the light back-scattered from the illuminated area, shifted frequency in this way. This light was detected and processed to produce a signal that was a function of the red cell flux. This information was used as a measure of blood flow, the value being expressed as a percentage of full-scale deflection at a given gain. This method was adopted to monitor blood flow in intact teeth in animals [48],[49] and in man. [50],[51],[52],[53],[54] Other wavelengths of semiconductor laser have also been used: 780 nm [55],[56] and 780-820 nm. [57],[58],[59],[60] Zang et al. [56] demonstrated greatly improved results using forward scattering detection as opposed to conventional backward scattering detection. [61] These results were confirmed by Sasano. [61] Odor et al. [62] reported that the 810 nm wavelength showed good sensitivity but poor specificity and that the 633 nm wavelength showed good specificity but poor sensitivity. Nonlaser light (peak output at 576 nm) has also been used for the detection of pulpal perfusion. [63] In general, infrared light (780-810 nm) has a greater ability to penetrate enamel and dentine than shorter wavelength red light (632.8 nm). [57] LDF techniques are united in their validity for pulp vitality testing as they reflect vascular rather than nervous responsiveness. [64] Due to some of the inherent problems associated with this technology, Sasano et al. [65] considered it to be limited in its usefulness for human pulp vitality testing. The lasers used for LDF are usually at a low-power level of 1 or 2 mW and no reports on pulp injury by this method have been made. The other use of laser for diagnostics related to endodontics was the application of an excimer laser system emitting at 308 nm for residual tissue detection within the canals. [38],[66],[67],[68]

Principle and working

The technique depends on the Doppler principle whereby light from a laser diode incident on the tissue is scattered by moving RBCs and as a consequence, the frequency is broadened. The frequency broadened light, together with laser light scattered is photo detected and the resulting photocurrent processed to provide a blood flow measurement. LDF is an optical measuring method that enables the number and velocity of particles conveyed by a fluid flow to be measured. The particles (1-20 μm) must be big enough to scatter sufficient light for signal detection but small enough to follow the flow faithfully. [69],[70],[71],[72],[73],[74] The original technique used a light beam from a helium-neon (He-Ne) laser emitting at 632.8 nm. Other wavelengths of semi-conductor laser have also been used: 780 nm and 780-820 nm. [38] Laser light is transmitted to the dental pulp by means of a fibre optic probe placed against the tooth surface. Two equal-intensity beams (split from a single beam) intersect across the target area. The scattered light beams from moving red blood cells are frequency-shifted whilst those from the static tissue remain unshifted in frequency. The unshifted light is returned by an afferent fibre within the same probe to photodetectors in the flowmeter and the signal is produced. [74],[75],[76],[77] The LDF output signal or Flux can be simplified as a function of the product of red blood cells' concentration as well as their mean velocity. [78] It should be emphasized that the optical properties of a tooth change when the pulp becomes necrotic and this can produce changes in the LDF signal that are not due to differences in blood flow. [79] In fact, as red blood cells represent the vast majority of moving objects within the tooth measurement of the Doppler-shifted backscattered light serves as an index of PBF. LDF evaluates dynamic changes in blood flow by detecting blood cell movement in a small volume of tissue (about 1 mm 3 ).[80],[81],[82] Most current laser Doppler devices give readout, in addition to the flux, in perfusion units (PUs).

If a wave with frequency ω is scattered from a moving particle with velocity v; the Doppler shift can be written as

where kI is the incident wave vector, ks is the wave vector of the scattered wave, and β is the angle between the velocity vector and the scattering vector, which is defined as (kI - ks) [Figure 2] and [Figure 3]. [68]
Figure 2: Principle of laser Doppler flowmetry: red light is emitted from a light source; if the light beam is scattered-off of stationary tissue or cells, there is no shift in the light spectrum. If, however, the light hits a moving cell in a blood vessel there is a shift in the light spectrum of the scattered light according to the Doppler flowmetry

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Figure 3: LDF monitor and probes

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  1. Estimation of the pulpal vitality: the diagnosis of a tooth with a necrotic pulp may be difficult particularly when referred pain is present. In these situations, a suitable test and its precise interpretation are of paramount importance. [83]
  2. Pulp-testing in children: sensibility tests are not reliable in children, because they are subjective and rely upon patient's response. LDF is a suitable method for the measurement of PBF in deciduous incisors. [84]
  3. Periapical radiolucencies may have nonendodontic origins, so application of vitality tests, such as LDF can help in differential diagnosis of these radiographic views. [85]
  4. It monitors age related changes in PBF. Using this system, it has been shown that the hemodynamics in the human pulp is reduced with age. [86]
  5. Monitoring the effect of exercise on PBF. It has been indicated that PBF varies during exercise, with a mean percentage change of 38% from the level at rest.
  6. Monitoring of reactions to local and systemic pharmacological agents (including local anesthetic solutions). [87],[88],[89]
  7. Monitoring of reactions to electrical or thermal pulp stimulation. [90],[91]
  8. Monitoring reactions to orthodontic procedures.
  9. Measuring PBF after orthognathic surgery. Among patients who undergo a segmental maxillary osteotomy or Le fort I osteotomy, significant reduction in pulpal sensibility has been noted in teeth in the osteotomized segment or maxilla.
  10. Measuring of PBF after traumatic injuries: Traumatized teeth may have their innervations damaged and give a negative response to pulp tests although their blood circulation and thus their true vitality is functional. LDF is an accurate and objective technique for assessment of pulpal vitality in these teeth.
  11. Monitoring of revascularization of replanted teeth: LDF readings correctly predict the pulp status in vital vs nonvital teeth. [34]
  • Accurate
  • Reliable
  • Reproducible
  • Nonpainful
  • Luxation injuries
Useful in young children whose responses are unreliable and its noninvasive nature helps to promote patient cooperation and acceptance. [7],[26],[74]


  • Too expensive a device for use in a dental office.
  • The sensor should be maintained motionless and in constant contact with the tooth for accurate readings.
  • The laser beam must interact with the moving cells within the pulpal vasculature. [74]
  • It is generally agreed that LDF assessment for human teeth should be performed at 4 weeks following the initial trauma and repeated at regular intervals until 3 months.
  • Blood pigments within a discolored tooth crown can also interfere with laser light transmission. Care must be taken to ensure that the false positive results are not obtained from the stimulation of supporting tissues. [75]

Comparative studies on pulp oximetry and laser Doppler flowmetry

Gopi Krishna et al. evaluated the efficacy of a custom-made pulse oximeter dental probe in comparison with the electrical and thermal tests for assessing pulp vitality. Sensitivity, specificity, negative predictive value, and positive predictive value for each test were calculated by comparing the test results with the actual pulpal status [Table 1] and [Table 2].
Table 1: Comparison of sensitivity (in %)

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Table 2: Comparison of specificity (in %)

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The results of the study showed that custom-made pulse oximeter dental probe is an effective, accurate, and objective method of evaluating pulp vitality. [16]

Gopi Krishna et al. compared the efficacy of a custom-made pulse oximeter dental probe with the electric pulp testing and thermal testing for measuring pulp vitality status of recently traumatized permanent teeth. Readings for pulp vitality for 17 recently traumatized maxillary incisors were taken with custom-made pulse oximeter dental probe (group 1), electrical pulp tester (group 2), and thermal testing (group 3) over a 6-month period. The proportion of recently traumatized teeth showing a positive responsiveness in thermal/electric pulp tests increased from no teeth showing responsiveness on day 0 to 29.4% teeth on the 28 th day, 82.35% of teeth at 2 months and 94.11% teeth at 3 months. However, pulse oximeter gave positive vitality readings that remained constant over the study period from day 0 to 6 months in all patients [Table 3]. [17]
Table 3: The proportion of recently traumatized teeth showing positive response (in %)

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Odor et al. investigated the pattern of light transmission through teeth of different species and examined laser light propagation within enamel from various animal sources. They concluded that light from a laser Doppler probe appeared to reach the dental pulp in all the species; however, in the mammals with smaller teeth, light may also have been able to reach the periodontium and thus the reflected signal may not be entirely of pulpal origin. [67]

   Conclusion Top

The unreliability of testing tooth pulp nerve response is well-documented. When nervous sensations are inhibited or abolished in the tooth, for example, following trauma, tooth transplantation procedures or during a general anaesthetic, conventional tests are of little value. However, a method based on the vascular response of the pulp need not be restricted under such conditions. Recording the pulpal blood flow would be an objective assessment of the status of the pulpal blood circulation, a true indicator of pulp vitality. Optical devices that exploit the various absorbance properties of different substances within the dental pulp are being studied to determine pulsation and blood volume. They offer the advantages of being objective, noninvasive, and atraumatic testing modalities, which result in greater patient acceptance and cooperation. Currently, the significance and reliability of these methods are being studied. It is hoped that newer technology will enable a more thorough study of the pulpal vasculature and define its role in pulp vitality testing.

   References Top

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  [Table 1], [Table 2], [Table 3]

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