Pulp (tooth)

The plexus of Raschkow monitors painful sensations. By virtue of their peptide content they also play important functions in inflammatory events and subsequent tissue repair. There are two types of nerve fiber that mediate the sensation of pain: A-fibers conduct rapid and sharp pain sensations and belong to the myelinated group, whereas C-fibers are involved in dull aching pain and are thinner and unmyelinated. The A-fibers, mainly of the A-delta type, are preferentially located in the periphery of the pulp, where they are in close association with the odontoblasts and extend fibers to many but not all dentinal tubules. The C-fibers typically terminate in the pulp tissue proper, either as free nerve endings or as branches around blood vessels. Sensory nerve fibers that originate from inferior and superior alveolar nerves innervate the odontoblastic layer of the pulp cavity. These nerves enter the tooth through the apical foramen as myelinated nerve bundles. They branch to form the subodontoblastic nerve plexus of Raschkow which is separated from the odontoblasts by a cell-free zone of Weil, therefore this plexus lies between the cell free and cell rich zones of the pulp.

Legend: 1 - Odontoblast layer; 2 - Cell-free zone of Weil; 3 - Cell-rich zone; 4 - Pulp core

As the dental pulp is a highly vascularised and innervated region of the tooth, it is the site of origin for most pain-related sensations.[13] The dental pulp nerve is innervated by one of the Trigeminal nerves, otherwise known as the fifth cranial nerve. The neurons enter the pulp cavity through the apical foramen, and branch off to form the nerve plexus of Raschkow (as mentioned earlier). Nerves from the plexus of Rasckow give branches to form a marginal plexus around the odontoblasts, with some nerves penetrating the dentinal tubules.

The dental pulp is also innervated by the sympathetic division of the autonomic nervous system.[12] These sympathetic axons projects into the radicular pulp, where they form a plexus along the blood vessels. Their function is mainly related to the blood vessel constriction within the dental pulp.[12] Hence, a sharp fall in pulpal blood flow maybe very possibly be caused by a stimulation of these nerves.There is no evidence for a parasympathetic pulpal innervation.

There are 2 main types of sensory nerve fibres in the pulp, each densely located at different locations. Moreover, the differing structural features of the 2 sensory nerve fibres also results in the different types of sensory stimulation.

• Myelinated A-Fibres:
  • The A-Fibres present in the pulp can be further classified into 2 different types, the A-delta Fibres which makes up 90% of the A-Fibres, while the rest are A-Beta Fibres.[14]
  • A relatively low-threshold sensory apparatus.
  • Mainly located at pulp-dentine border at the top of the pulp, and more specifically concentrated in the pulp horn.[12]
  • It has a relatively small diameter, hence has a relatively slow conduction velocity. However, it is still faster than C-Fibres.[12]
  • A-Fibres transmit signals to the brainstem are then to the contralateral thalamus.
  • Able to respond to stimuli through a shell of calcified tissue due to the stimulus-induced fluid flow in dentinal tubules.[15] This is known as the hydrodynamic theory. Stimuli that displaces the fluid within the dentinal tubules will trigger the intradental myelinated A-Fibres, leading to a sharp pain sensation,[15] commonly associated with dentine hypersensitivity

• Unmyelinated C-Fibres:
  • They are mainly located at the core of the pulp, and extends into underneath the odontoblastic layer.
  • On the contrary, C-Fibres have higher thresholds, responsible for detecting inflammatory threats.[16]
  • Since it is heavily influenced by modulating interneurons before it reaches the thalamus, hence C-Fibre stimulation often results in a “slow pain”, often characterised as a dull and aching pain.[12]

In a healthy tooth pulp, the pulp is well-protected from infection by the enamel and dentin layers.

A healthy normal pulp: -is vital and normally responsive to pulp testing -is symptoms and inflammation free -has a mild or transient response clinically to thermal and cold testing which should last no more than 1–2 seconds after stimulus is removed

A mild to moderate inflammation of dental pulp caused by any momentary irritation or stimulant whereby no pain is felt upon removal of stimulants.[17] The pulp swells when the protective layers of enamel and dentine are compromised. Unlike irreversible pulpitis, the pulp still gives a regular response to sensibility tests and inflammation resolves with management of the cause. There is no significant radiographic changes in the periapical region hence further examination is mandatory to ensure that the dental pulp has returned to its normal healthy state.[18]

- Bacterial infection from caries

- Thermal shock

- Trauma

- Excessive dehydration of a cavity during restoration

- Irritation of exposed dentine

- Repetitive trauma caused by bruxism (tooth grinding) or jaw misalignment

- Fractured tooth exposing pulp

- Temporary post-restoration sensitivity

- Pain is non- spontaneous and is milder compared to irreversible pulpitis

- Short sharp pain due to a stimulant

- X-rays to determine extent of tooth decay and inflammation

- Sensitivity tests to see if pain or discomfort is experienced when tooth is in contact with hot, cold or sweet stimuli

- Tooth tap test (lightweight, blunt instrument gently tapped onto affected tooth to determine extent of inflammation)

- Electric pulp test

- Treatment aetiology should resolve reversible pulpitis; treating it early may help prevent irreversible pulpitis - Follow-up after treatment required to determine whether the reversible pulpitis has returned to a normal status

- Regular check-ups for carcinogenic or non-carcinogenic caries

- When preparing cavities, dehydrate with adequate amount of alcohol or chloroform and apply enough varnish to protect the pulp

Pulpitis is established when the pulp chamber is compromised by bacterial infection. Irreversible pulpitis is diagnosed when the pulp of the tooth is inflamed and infected beyond the point of no return, healing of the pulp is not possible. Removal of the aetiological agent will not permit healing and often root canal treatment is indicated. Irreversible pulpitis is commonly a sequel to reversible pulpitis when early intervention is not taken.[6][8] It is key to note that in this stage of the disease progression, the pulp is still vital and vascularised; it is not classified as ‘dead pulp’ until necrosis occurs.[3]

Irreversible and reversible pulpitis are differentiated from each other based on the various pain responses that they have to thermal stimulation. If the condition is reversible then the pulp's pain response will last momentarily upon exposure to cold or hot substances; a few seconds. However, if the pain lingers from minutes to hours, then the condition is irreversible. This is a common presenting complaint that facilitates diagnoses before further investigations e.g. sensibility tests and peri-apical radiographs are proceeded.[3][5]

Diagnosis of irreversible pulpitis is branched into two sub-divisions: symptomatic and asymptomatic. Asymptomatic irreversible pulpitis is a transition of symptomatic irreversible pulpitis into an inactive and quiescent state. This is due to the nature of its causation; the inflammatory exudate can be quickly removed e.g. through a large carious cavity or previous trauma that caused painless exposure of pulp. It is the build-up of pressure in a confined pulp space that stimulates nerve fibres and initiates pain reflexes. When this pressure is relieved, pain is not experienced.[4][7]

As the names imply, these diseases are largely characterised by the symptoms they present, duration and location of pain, causing and relieving factors. A clinician's role is to gather all this information systematically. This will be a compilation of clinical tests (cold ethyl chloride, EPT, hot gutta percha, palpation), radiographic analysis (peri-apical and/or cone beam computed tomography) and any further tests deemed necessary. Thermal tests are subjective by nature, so performed on not only the compromised tooth, but the adjacent and contra-lateral teeth as well, allowing the patient to compare information and give the clinician more accurate responses. Normal healthy teeth are used as a baseline for diagnoses.[19][8][6]

The following are key characteristics of each pathology:

Symptomatic irreversible pulpitis:

-      Very spontaneous and unpredictable pain. Can occur at any time of day and specific causing factors cannot be labelled.

-      Patient may complain of sharp lingering pains, that last longer than 30 seconds even after removal of stimulus.

-      There may be referred pain

-      Pain may be more pronounced after changes of posture e.g. from lying down to standing up

-      Analgesics tend to be ineffective

-      As the bacteria have not yet progressed to the peri-apical region, there is no pain on percussion.

Asymptomatic irreversible pulpitis

-      There are no clinical symptoms

- Pulp will respond to sensibility tests as a healthy pulp would   [6][8]

Treatments: Require endodontic (root canal) treatment or tooth extraction. In endodontic therapy, removal of the inflamed pulp relieves the pain. The empty root canal system is then obturated with Gutta-Percha which is a rubber material that acts as a pressure and pain reliever.[20]

Pulp acts as a security and alarm system for a tooth. Slight decay in tooth structure not extending to the dentin may not alarm the pulp but as the dentin gets exposed, either due to dental caries or trauma, sensitivity starts. The dentinal tubules pass the stimulus to odontoblastic layer of the pulp which in turns triggers the response. This mainly responds to cold. At this stage simple restorations can be performed for treatment. As the decay progresses near the pulp the response also magnifies and sensation to a hot diet as well as cold gets louder. At this stage indirect pulp capping might work for treatment but at times it is impossible to clinically diagnose the extent of decay, pulpitis may elicit at this stage. Carious dentin by dental decay progressing to pulp may get fractured during mastication (chewing food) causing direct trauma to the pulp hence eliciting pulpitis.

The inflammation of the pulp is known as pulpitis. Pulpitis can be extremely painful and in serious cases calls for root canal therapy or endodontic therapy.[21] Traumatized pulp starts an inflammatory response but due to the hard and closed surroundings of the pulp pressure builds inside the pulp chamber compressing the nerve fibres and eliciting extreme pain (acute pulpitis). At this stage the death of the pulp starts which eventually progresses to periapical abscess formation (chronic pulpitis).

The pulp horns recede with age. Also with increased age, the pulp undergoes a decrease in intercellular substance, water, and cells as it fills with an increased amount of collagen fibers. This decrease in cells is especially evident in the reduced number of undifferentiated mesenchymal cells. Thus, the pulp becomes more fibrotic with increased age, leading to a reduction in the regenerative capacity of the pulp due its loss of these cells. Also, the overall pulp cavity may be smaller by the addition of secondary or tertiary dentin, thus causing pulp recession. The lack of sensitivity associated with older teeth is due to receded pulp horns, pulp fibrosis, addition of dentin, or possibly all these age-related changes; many times restorative treatment can be performed without local anesthesia on older dentitions.[2]

Pulp necrosis describes when the pulp of a tooth has become necrotic. The pulp tissue is either dead or dying, this may be for a number of reasons including: untreated caries, trauma or bacterial infection. It is often subsequent to chronic pulpitis. Teeth with pulp necrosis will need to undergo root canal treatment or extraction to prevent further spread of infection which may lead to an abscess.

Pulp necrosis may be symptomatic or asymptomatic for the patient. If the necrosis is symptomatic it could result in lingering pain to hot and cold stimuli, spontaneous pain that may cause a patient to wake up during sleep, difficulty with eating and being tender to percussion.[22][23] If the necrosis is asymptomatic it will be non-responsive to thermal stimuli or electric pulp tests, the patient may even be unaware of the pathology.[23]

If the pulpal necrosis is asymptomatic it may go unnoticed by the patient and so a diagnosis may be missed without the use of special tests. To determine if pulp necrosis is present a dentist may take radiographic images (X-rays) and sensitivity testing e.g. hot or cold stimuli (using warm gutta percha or ethyl chloride); or they may use an electric pulp tester. Vitality of a tooth, which refers to blood supply to a tooth, may be assessed using doppler flowmetry.[24] Sequelae of a necrotic pulp include acute apical periodontitis, dental abscess, or radicular cyst and discolouration of the tooth.[25]

If a necrotic pulp is left untreated it may result in further complications such as infection, fever, swelling, abscesses and bone loss.[26] Currently there are only two treatment options for teeth which have undergone pulpal necrosis.[27][28] Future treatments aim to assess revascularisation of pulp tissue using techniques such as cell homing.[27]