Comprehensive Foundations of Dental and Oral Anatomy

Tooth Development and Fundamental Anatomy

The study of dental and oral anatomy is essential for dental nurses to recognise healthy mouth development and detect abnormalities or initial symptoms of disease. The face and neck of an embryo become visible as early as five weeks after conception. The first signs of tooth development appear when the embryo is approximately six weeks old, specifically around $42$ days after conception. At this six-week mark, the deciduous teeth begin to form, while the permanent teeth start their development at $20$ weeks. The process of tooth development is categorized into three distinct stages: the bud stage, the cap stage, and the bell stage. Teeth form within the dental lamina, which is the dental arch. Support for the teeth is provided by a specialized bone within the jaws known as the alveolar bone. This bone surrounds the roots and performs a functional role; consequently, if a tooth is lost, the alveolar bone will resorb because it no longer serves a purpose.

Enamel: Structure and Mineralization

Enamel is the outer protective covering of the tooth crown. It is translucent and varies in shade from yellow-white to grey-white. As the most highly mineralized tissue in the body, it is also the strongest, capable of withstanding crushing stresses of approximately $100,000\,lb/in^2$. Despite its strength, enamel is brittle and prone to fractures. It is a calcified tissue composed primarily of calcium and phosphorus. Notably, enamel is highly resistant to destruction, often remaining as the only recognizable feature of a body following fire or decomposition. Because enamel contains no blood vessels or nerve tissues, it lacks sensitivity to pain and cannot repair itself. Any damage or loss, such as that caused by dental caries, is permanent.

Microscopically, enamel consists of solid, calcified rods called enamel prisms. These prisms are formed from hydroxyapatite crystals oriented at angles to the outer surface. The rods intertwine and run parallel to one another to provide maximum strength. Between these rods lies the interprismatic cement, which is composed of organic materials including keratinized cells. This cement is weaker than the prisms and is frequently the site of fractures. Thickness varies across the tooth, being greatest on the occlusal or biting surface and tapering toward the neck of the tooth.

Dentine: Composition and Types

Dentine constitutes the bulk of the tooth's tissues. Unlike enamel, it is sensitive to pain and reacts to injury or stimulation. It is yellowish, tough, slightly elastic, and softer than enamel. Dentine extends from the crown down through the root section. The junction where the enamel meets the dentine is known as the amelo-dentinal junction (ADJ). Microscopically, dentine is comprised of hollow tubes called dentinal tubules which run from the pulp to the enamel. Within these tubules are dentine fibrils, which are processes of living cells called odontoblasts, along with nerves and blood vessels.

Dentine is classified into three major types based on when and why it forms. Primary dentine is formed before tooth eruption and makes up the majority of the tooth structure. Secondary dentine begins forming after eruption and continues at a slow pace throughout a person's life, gradually reducing the size of the pulp chamber. Tertiary dentine is a localized deposit on the pulp chamber wall formed in response to irritation, such as attrition, erosion, dental caries, or various dental treatments.

Dental Pulp and Root Anatomy

Located at the center of the tooth, the dental pulp extends from the crown to the root. The portion within the crown is the coronal pulp, which includes pulp horns extending toward the cusp tips. The portion within the root is the radicular pulp or root pulp. The pulp consists of nerve tissue, blood supply, and connective tissues, providing nourishment and sensitivity. A layer of odontoblasts resides where the dentine meets the pulp. These cells, which originally formed the dentine during fetal development, retain the ability to produce new dentine throughout life. When they produce new dentine for repair or as a natural part of aging, it is referred to as secondary dentine. This slow, continuous process causes the pulp chamber and root canals to narrow over time. Blood vessels and nerves enter the tooth through a small opening at the tip of the root called the apical foramen.

Cementum and Tissular Proportions

Cementum is a thin, calcified layer covering the dentine of the root surface, functioning similarly to enamel. It is light yellow, darkens with age, and is softer than dentine. Its thickness varies and can change throughout life in response to physical stresses. The juncture between the enamel and cementum at the tooth's neck is called the cementoenamel junction (CEJ). Cementum serves to anchor the periodontal ligament to the tooth root.

The tissues of the tooth are composed of organic and inorganic matter. Organic matter refers to the living matrix of water, cells, fibres, and proteins. Inorganic matter consists of non-living mineral content, primarily complex calcium salts like calcium hydroxyapatite, which provide strength. The compositions are as follows:

Enamel: $96\%$ Inorganic, $4\%$ Organic Dentine: $70\%$ Inorganic, $30\%$ Organic Cementum: $45\%$ Inorganic, $55\%$ Organic

Types of Teeth and Dentition

Human dentition consists of four types of teeth. Incisors have a single, thin, blade-like cutting edge and act with a scissor-like motion to cut food. Canine teeth, or "eye teeth," are located at the corners of the mouth and are pointed for gripping and tearing. Premolars, also known as bicuspids because they typically have two cusps, are absent in the deciduous dentition and work with molars to crush food. Molars have broad, flat biting surfaces designed to grind food before swallowing.

Deciduous dentition consists of $20$ teeth. Compared to permanent teeth, deciduous teeth are whiter, have rounder crowns, splayed roots (to accommodate erupting permanent teeth), thinner enamel, and larger pulp chambers. They are more prone to decay and wear. Deciduous roots are usually resorbed as permanent teeth erupt, though occasionally they may require extraction if resorption is incomplete. The average eruption ranges and characteristics are:

• Incisors: $6\text{--}12$ months, $1$ root, biting function.
• First Molars: $12\text{--}24$ months, $3$ upper/$2$ lower roots, $4\text{--}5$ cusps, chewing function.
• Canines: $14\text{--}20$ months, $1$ root, tearing function.
• Second Molars: $18\text{--}30$ months, $3$ upper/$2$ lower roots, $4$ cusps, chewing function.

Permanent dentition consists of $32$ teeth. Upper molars typically have three roots (mesial buccal, distal buccal, and palatal), while lower molars have two. The first permanent upper premolar is unique for having two roots, while most other non-molar teeth have one. The first molars erupt between $5\text{--}8$ years and may feature the cusp of Carabelli. Third molars, or wisdom teeth, erupt between $17\text{--}24$ years, though their presence and eruption are varied.

The Periodontium and Supporting Structures

The periodontium refers to the collective tissues holding teeth in their sockets, including the gingiva, cementum, alveolar bone, and periodontal membrane. The gingiva (gums) consists of pink mucous membranes and fibrous tissue. Attached gingiva is firm, stippled by collagen bundles, and tightly bound to bone. Free gingiva is contoured around the tooth neck, forming interdental papilla. The gingival crest is the unattached portion forming a natural pocket that is less than $2\,mm$ deep in a healthy mouth. The alveolar bone is specialized bone that supports the teeth and resorbs upon tooth loss. The lamina dura is a layer of compact bone lining the tooth socket, often appearing as a white line on radiographs. The periodontal ligament is made of non-elastic collagen fibers and a fluid "shock absorber" that connects the cementum to the lamina dura, providing flexibility and strength; this ligament is destroyed by periodontal disease, leading to tooth mobility.

Salivary Glands and Saliva Function

Saliva is produced by minor glands scattered throughout the oral mucosa and three pairs of major glands. The Parotid gland is the largest, located near the ear; its saliva enters via Stensen’s duct opposite the upper second molar. It produces $25\%$ of daily saliva and is the site of swelling during mumps. The Sublingual gland is the smallest and flattest, located near the lingual frenulum; its saliva (which is sticky) enters through the ducts of Ravinus or Bartholine (when joined with Wharton’s duct), accounting for $5\%$ of daily saliva. The Submandibular gland is irregular in shape, located below the mylohyoid line; its saliva enters via Wharton’s duct at the side of the lingual frenulum, producing $70\%$ of daily saliva.

Saliva is $99.5\%$ water and $0.5\%$ dissolved substances, including mucin (proteins), ptyalin (salivary amylase for starch digestion), lysozyme (antibacterial), serum proteins (albumin and globulin), waste (urea and uric acid), and inorganic ions (sodium, potassium, calcium, phosphate, and chloride). Humans produce approximately $1\,litre$ of saliva daily. Saliva serves as a lubricant for speech and swallowing, a cleanser for food debris, a digestive aid (bolus formation and taste dissolution), an antacid (buffering pH back to $6.8$), and an antibacterial agent. During an "acid attack" after eating, pH drops, causing demineralization. Saliva facilitates remineralization over a period of $20\,minutes$ to $2\,hours$.

Muscles of Mastication and Mandibular Anatomy

Chewing involves four main muscles. The Temporalis is a fan-shaped muscle originating in the temporal fossa and inserting into the coronoid process; it elevates and retracts the mandible. The Masseter is a quadrilateral muscle originating from the zygomatic arch and inserting into the ramus; it elevates the mandible. The Medial Pterygoid is a four-sided muscle originating from the sphenoid and inserting into the internal ramus; it elevates the mandible and protrudes the chin. The Lateral Pterygoid is cone-shaped, originating from the sphenoid and inserting into the condyloid process; it depresses the mandible to open the mouth and aids in chewing. The Buccinator, a muscle of facial expression, helps press the cheek against teeth to keep food out of the buccal vestibule.

The mandible is the largest and strongest facial bone. It consists of the body (horizontal), the ramus (vertical), and the angle. Key features include the anterior coronoid process, the posterior condyloid process (condyle), the sigmoid notch, the mental foramen (near the $2nd$ premolar), and the mandibular foramen (inner ramus, protected by the lingula). The temporomandibular joint (TMJ) connects the condyloid process to the glenoid fossa and articular eminence of the temporal bone, separated by a fibrous cartilage disc called the meniscus.

Nerve Supply and Anaesthesia

The maxillary nerve supplies the upper jaw and mid-face. Anaesthesia is typically delivered via infiltration. The three branches include the Anterior Superior Nerve (incisors, canines, labial gingiva), the Middle Superior Nerve (premolars, mesial part of the $1st$ molar, buccal gingiva), and the Posterior Dental Nerve (distal part of the $1st$ molar, $2nd$ and $3rd$ molars, buccal gingiva).

The Inferior Dental Nerve (IDN) supplies all lower teeth, entering the mandible through the mandibular foramen and giving off the mental nerve branch near the premolars. For posterior teeth, an IDN block at the mandibular foramen is required, often necessitating a long needle and resulting in whole-side numbness. For anterior teeth, infiltration may suffice. The Lingual Nerve supplies the lingual aspect of the lower teeth, the floor of the mouth, and the anterior two-thirds of the tongue; it is generally anaesthetised for extractions or lingual tissue procedures.