Pulp (tooth)


The pulp is the connective tissue, nerves, blood vessels, and odontoblasts that comprise the innermost layer of a tooth. The pulp's activity and signalling processes regulate its behaviour.

Anatomy

The pulp is the neurovascular bundle central to each tooth, permanent or primary. It is composed of a central pulp chamber, pulp horns, and radicular canals. The large mass of the pulp is contained within the pulp chamber, which is contained in and mimics the overall shape of the crown of the tooth. Because of the continuous deposition of the dentine, the pulp chamber becomes smaller with the age. This is not uniform throughout the coronal pulp but progresses faster on the floor than on the roof or sidewalls.
Radicular pulp canals extend down from the cervical region of the crown to the root apex. They are not always straight but vary in shape, size, and number. They are continuous with the periapical tissues through the apical foramen or foramina.
The total volume of all the permanent teeth organs is 0.38cc, and the mean volume of a single adult human pulp is 0.02cc.
Accessory canals are pathways from the radicular pulp. These canals, which extend laterally through the dentin to the periodontal tissue, are seen especially in the apical third of the root. Accessory canals are also called lateral canals because they are usually located on the lateral surface of the roots of the teeth.

Development

The pulp has a background similar to that of dentin because both are derived from the dental papilla of the tooth germ. During odontogenesis, when the dentin forms around the dental papilla, the innermost tissue is considered pulp.
There are 4 main stages of tooth development:
  1. Bud stage
  2. Cap stage
  3. Bell stage
  4. Crown stage
The first sign of tooth development is known to be as early as the 6th week of intrauterine life. The oral epithelium begins to multiply and invaginates into ectomesenchyme cells, which gives rise to dental lamina. The dental lamina is the origin of the tooth bud. The bud stage progresses to the cap stage when the epithelium forms the enamel organ. The ectomesenchyme cells condense further and become dental papilla. Together the epithelial enamel organ and ectomesenchymal dental papilla and follicle form the tooth germ. The dental papilla is the origin of dental pulp. Cells at the periphery of the dental papilla undergo cell division and differentiation to become odontoblasts. Pulpoblasts form in the middle of the pulp. This completes the formation of the pulp. The dental pulp is essentially a mature dental papilla.
The development of dental pulp can also be split into two stages: coronal pulp development and root pulp development.
The pulp develops in four regions from the periphery to the central pulp:
  1. Odontoblast layer
  2. Cell-free zone – likely to be an artefact
  3. Cell-rich zone
  4. Pulp core

    Internal structure

The central region of the coronal and radicular pulp contains large nerve trunks and blood vessels.
This area is lined peripherally by a specialized odontogenic area which has four layers :
  1. Pulpal core, which is in the center of the pulp chamber, with many cells and an extensive vascular supply; except for its location, it is very similar to the cell-rich zone.
  2. Cell-rich zone, which contains fibroblasts and undifferentiated mesenchymal cells.
  3. Cell-free zone, odontoblasts, defence cells like histiocytes, macrophages, granulocytes, mast cells, and plasma cells. The nerve plexus of Raschkow is located central to the cell-rich zone.

    The plexus of Raschkow

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 fibers that mediate the sensation of pain: A-Fibres conduct rapid and sharp pain sensations and belong to the myelinated group, whereas C-Fibres are involved in dull aching pain and are thinner and unmyelinated. The A-Fibres, 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-Fibres 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. This plexus lies between the cell-free and cell-rich zones of the pulp.

Pulp innervation

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. 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. Nerves from the plexus of Raschkow provide 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. These sympathetic axons project into the radicular pulp, where they form a plexus along the blood vessels. Their function is mainly related to blood vessel constriction within the dental pulp. A sharp fall in pulpal blood flow may be caused by stimulation of these nerves. There is no evidence for a parasympathetic pulpal innervation.
There are two main types of sensory nerve fibres in the pulp, each densely placed at different locations. The differing structural features of the two sensory nerve fibres also result in different types of sensory stimulation.
  • Myelinated A-Fibres:
  • * The A-Fibres present in the pulp can be further classified into 2 different types. A-Delta Fibres make up 90% of the A-Fibres, while the rest are A-Beta Fibres.
  • * Have a relatively low-threshold sensory apparatus.
  • * Mainly located at the pulp-dentine border at the top of the pulp, and more specifically concentrated in the pulp horn.
  • * Have a relatively small diameter with a relatively slow conduction velocity. They are is still faster than C-Fibres.
  • * A-Fibres transmit signals to the brainstem and 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. 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 the sharp pain sensation commonly associated with dentine hypersensitivity
  • Unmyelinated C-Fibres:
  • * They are mainly located at the core of the pulp and extend underneath the odontoblastic layer.
  • * C-Fibres have higher pain thresholds, responsible for detecting inflammatory threats.
  • * They are heavily influenced by modulating interneurons before they reach the thalamus. C-Fibre stimulation often results in a "slow pain", normally characterised as a dull and aching pain.

    Functions

The primary function of the dental pulp is to form dentin.
Other functions include:
  • Nutritive: the pulp keeps the organic components of the surrounding mineralized tissue supplied with moisture and nutrients;
  • Protective/sensory: extremes in temperature, pressure, or trauma to the dentin or pulp are perceived as pain;
  • Defensive/reparative: the formation of reparative or tertiary dentin ;
  • Formative: cells of the pulp produce dentin which surrounds and protects the pulpal tissue.

    Pulp testing

The health of the dental pulp can be established by a variety of diagnostic aids which test either the blood supply to a tooth or the sensory response of the nerves within the root canal to specific stimuli. Although less accurate, sensitivity tests, such as Electric Pulp Tests or Thermal Tests, are more routinely used in clinical practice than vitality testing, which requires specialised equipment.
A healthy tooth is expected to respond to sensitivity testing with a short, sharp burst of pain which subsides when the stimulus is removed. An exaggerated or prolonged response to sensitivity testing indicates that the tooth has some degree of symptomatic pulpitis. A tooth that does not respond at all to sensitivity testing may have become necrotic.

Pulp diagnoses

Normal pulp

In a healthy tooth, enamel and dentin layers protect the pulp from infection.

Reversible pulpitis

Reversible pulpitis is a mild to moderate inflammation caused by any momentary irritation or stimulant whereby no pain is felt upon the stimulants' removal. The pulp swells when the protective layers of enamel and dentine are compromised. Unlike irreversible pulpitis, the pulp gives a regular response to sensibility tests and inflammation resolves with management of the cause. No significant radiographic changes are present in the periapical region. Further examination is required to ensure that the dental pulp has returned to its normal state.

Common causes

  • Bacterial infection from caries
  • Thermal shock
  • Trauma
  • Excessive dehydration of a cavity during restoration
  • Irritation of exposed dentine
  • Repetitive trauma caused by bruxism or jaw misalignment
  • Fractured tooth exposing pulp

    Symptoms

  • Temporary post-restoration sensitivity
  • Non-spontaneous and milder pain compared to irreversible pulpitis
  • Short sharp pain caused by a stimulant

    Differential diagnosis

  • X-rays to determine extent of decay and inflammation
  • Sensitivity tests to see if pain/discomfort is experienced when tooth is in contact with hot, cold or sweet stimuli
  • Tooth tap test
  • Electric pulp test