PRINCIPLES OF OCCLUSION

Most restorative procedures affect the shape of the occlusal surfaces. Proper  dental care ensures that functional occlusal contact relationships are restored in harmony with both dynamic and static conditions. Maxillary and mandibular teeth should contact uniformly onclosing to allow optimal function, minimize trauma to the supporting structures, and allow for uniform load distribution throughout the dentition. Positional stability of well aligned teeth is crucial if arch integrity and proper fuunction are to be maintained over time.

Most dentitions deviate from optimal alignment and occlusion. Many patients adapt well to less than optimal occlusion, but malocclusion may be associated with undesirable changes to the teeth, the musculature, the temporomandibular joints (TMJs), or the periodontium. As an aid to the diagnosis of occlusal dysfunction, it is helpful to evaluate the condition of specific anatomic featurres and functional aspects of a patients occlusion with reference to a concept of (optimum) or (ideal) occlusion.

Deviation from this concept can then be measured objectively and may prove to be a useful guide during treatment planning and active treatment phases.

Over time, many concepts of (ideal) occlusion have been proposed. In the literature, the concepts of what is (ideal,) (acceptable,) and (harmful) continue to evolve. 

ANATOMY

Temporomandibular Joints

The major components of the TMJs are the cranial base, the mandible, and the muscles of mastication with their innervation and vascular supply. The TMJs are ginglymoarthrodial, meaning that they are capable of both a hinging and a gliding articulation. An articular disk separates the mandibular fossa and the articular tubercle of the temporal bone from the condylar process of the mandible.

The articulating surfaces of the condylar processes and fossae are covered with avascular fibrous tissue (in contrast to most other joints, which have hyaline cartilage). The articular disk consists of dense connective tissue; it also is avascular and devoid of nerves in the area where articulation normally occurs. Posteriorly , it is attached to loose highly vascularized and innervated connective tissue: the retrodiscal pad or bilaminar  zone (called bilaminar because it consists of two layers: an elastic superior layer and a collagenous inelastic inferior layer). The retrodiscal pad connects to the posterior wall of the articular capsule surrounding the joint. Medially and laterally, the articular disk is attached firmly to the poles of the condylar process. Anteriorly, it fuses with the capsule and with the superior lateral pterygoid muscle. Superior and inferior to the articular disk are two spaces: the superior and inferior synovial cavities. These are bordered peripherally by the capsule and the synovial membranes and are filled with synovial fluid. Because of its firm attachment to the poles of each condylar process, the articular disk follows condylar movement during both hinging and translation, which is made possible by the loose attachment of the posterior connective tissues.

Ligaments

The body of the mandible is attached to the base of the skull by muscles and three paired ligaments: the temporomandibular (also called the lateral), the sphenomandibular, and the stylomandibular ligaments.

Ligaments cannot be stretched significantly, and thus joint movement is limited. The temporomandinular ligaments restrict rotation of the mandible, limit border movements, and protect the structures of the joint. The sphenomandibular and stylomandibular ligaments limit separation between the condylar process and the articular disk; the stylomandibular ligaments also limit forward (protrusive) movement of the mandible.

Musculature

Several muscles are responsible for mandibular movements. These can be grouped as the muscles of mastication and the suprahyoid muscles. The former include the temporal, masseter, and medial and lateral pterygoid muscles; the latter are the geniohyoid, mylohyoid, and digastric muscles. Their respective origins, insertions, innervation, and vascular supply.

Muscular Function

The functions of the mandibular muscles are well coordinated and complex. Three paired muscles of mastication provide elevation and lateral movement of the mandible: the temporal, masseter, and medial pterygoid muscles. The lateral pterygoid muscles eaach have two bellies that function as two separate muscles, which contrct in the horizontal plane during opening and closing; the inferior belly (inferior lateral pterygoid muscle) is active during protrusion, depression, and lateral movement of the mandible; the superior belly (superior lateral pterygoid muscle) is active during closure. Because the superior belly has been shown to attach to the articular disk and the neck of the condyle, it is thought to assist in maintaining the integrity of the condyle-articula disk assembly by pulling the condylar process firmly against the articular disk.

The suprahyoid muscles have a dual function: They can elevate the hyoid bone or depress the mandible. The movement that results when they contract depends on the state of contraction of the other muscles of the neck and mandibular region. When the muscles of mastication are in a state of contraction, the suprahyoid muscles elevate the hyoid bone. However, if the infrahyoid muscles (which anchor the hyoid bone to the sternum and clavicle) are contracted, the suprahyoid muscles depress and retract the mandible. The geniohyoid and mylohyoid muscles initiate the opening movements, and the anterior belly of the digastric muscle completes mandibular depression. Although the stylohyoid muscle (which also belongs to the suprahyoid group) may contribute indirectly to mandibular movement through fixation of the hyoid bone, it does not play a significant role in mandibular movement.

Dentition 

The relative positions of the maxillary and mandibular teeth influence mandibular movement. Many (ideal) occlusions have been described. In most of these, the maxillary and mandibular teeth contact simultaneously when the condylar processes are fully seated in the mandibular fossae, and the teeth do not interfere with harmonious movement of the mandible during function. Ideally, in the fully bilateral seated position of the condyle-articular disk assemblies, the maxillary and mandibular teeth exhibit maximum intercuspation. This means that the maxillary lingual and mandibular buccal cusps of the posterior teeth are evenly distruted and in stable contact with the opposing occlusal fossae. These functional cusps can then act as stops for vertical closure without excessively loading any one tooth, while left and right TMJs concurrently are in an unstrained position.

Howevver, in many patients, maximal intercuspal contact occurs with the condyles in a slightly translated position. This position is referred to as maximum intercuspation, which is defined as the complete intercuspation of the opposing teeth, independent of condylar position; this is sometimes considered the best fit of the teeth regardless of condylar position.

If the mesiobuccal cusp of the maxillary first molar is aligned with the buccal groove of the mandibular first molar, the orthodontic relationship is considered Angle classI; this is considered normal occlusion. In such a relationship, the anterior teeth overlap both horizontally and vertically. This position is defined as the dental relationship in which the anteroposterior relationship of the jaws is normal, as indicated by correct intercuspation of maxillary and mandibular molars, Orthodontic textbooks have traditionally described an arbitrary 2-mm horizontal overlap and 2-mm vertical overlap as being ideal. For most patients, however, greater vertical overlap of the anterior teeth is desirable  for preventing undesirable posterior tooth contact. Mandibular flexing during mastication also may contribute to such undesirable contact. Empirically, dentitions with greater vertical overlap of the anterior teeth appear to have a better long-term prognosis than do dentition with minimal vertical overlap.

CENTRIC  RELATION

Centric relation is defined as the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective articular disks with the complex in the anterosuperior position against the shapes of the articular eminences. This position is independent of tooth contact. It is also clinically discernible when the mandible is directed superior and anterior and is restricted to a purely rotary movement about the transverse horizontal axis.

Centric relation is considered a reliable and reproducible reference (and treatment) position. If maximum intercuspation coincides with the centric relation position, restorative treatment is often straightforward. When maximum intercuspation dose not coincide with centric relation, it is necessary to determine whether corrective occlusal therapy is needed before restorative treatment is initiated.

MANDIBULAR  MOVEMENT 

As any other movement in space, complex three-dimensional mandibular movement can be divided into two basic components: translation, in which all points within a body have identical motion, and rotation, in which the body is turning about an axis. Every possible three-dimensional movement can be described in terms of these two components. It is easier to understand mandibular movement when the components are described as projections in three perpendicular planes: sagittal, horizontal, and frontal.

Reference Planes

Sagittal plane

In the sagittal plane, the mandible is capable of a purely rotational movement, as well as translation. Rotation occurs around the terminal hinge axis, an imaginary horizontal line through the rotational centers of the left and right condylar processes. The rotational movement is limited to abbout 12 mm of incisor separation before the temporomandibular ligaments and structures anterior to the mastoid process force the mandible to translate. The initial rotation or hinging motion occurs between the condylar process and the articular disk. During translation, the inferior lateral pterygoid muscle contracts and moves the condyle-articular disk assembly forward along the posterior incline of the tubercle. Condylar movement is similar during protrusive mandibular movement.

Horizontal Plane

In the horizontal plane, the mandible is capable of rotation around several vertical axes. For example, lateral movement consists of rotation around an axis situatef in the working (laterotrusive) condylar process with relatively little concurrent translation. A slight lateral translation of the condyle on the working side in the horizontal plane-known as laterotrusion, Bennett movement, or mandibular side shift  is frequently present. This may be in a slightly forward  direction (lateroprotrusion) or slightly backward direction (lateroretrusion). The orbiting (nonworking) condyle travels forward and medially as limited by the medial aspect of the mandibular fossa and the temporomandibular ligament. In addition, the mandible can make a straight protrusive (anterior) movement. 

Frontal plane

In a lateral movement in the frontal plane, the nonworking (mediotrusive) condyle moves down and medially, whereas the working (laterotrusive) condyle rotates around the sagittal axis perpendicular to this plane. Again, as determined by the anatomy of the medial wall of the mandibular fossa on the mediotrusive side, transtrusion may be observed; as determined by the anatomy of the medial wall of the mandibular fossa on the laterotrusive side, this movement may be lateral and upward (laterosurtrusion) or lateral and downward (laterodetrusion). A straight protrusive movement observed in the frontal plane, with both condylar processes moving downward as they slide along the tubercular eminences. 

MUSCLES OF MASTICATION

 Muscles of Mastication

• Mastication is the process of chewing food in preparation for deglutition (swallowing) and digestion.
• All muscles of mastication originate on the skull and insert on the mandible. 
• All muscles of mastication are innervated by the mandibular division of the trigeminal nerve. 
•  All muscle of mastication are derivatives of the 1st pharyngeal arch.
• Movement of the mandible are classified as:  

1. Elevation 
2. Depression
3. Protrusion
4. Retrusion
5. Side-to-side (lateral) excursion 

• Mastication prepares food by chwing for deglutition and digestion.
• It is the 1st step in the breakdown of food by:

1.  Making smaller pieces from larger pieces( thus increasing the surface area for digestive breakdown).
2. Helping soften and lubricate the food with saliva.

BONES INVOLVED

Base of the skull and the mandible.

They articulate at the temporomandibular joint (between the squamous portion of the temporal bone {skull} and the condyle of the mandible)

MUSCLES INVOLVED

• 4 muscles of mastication:

1. Masseter
2. Temporalis
3. Medial pterygoid 
4. Lateral pterygoid

• All muscles of mastication are innervated by the mandibular division of the trigeminal nerve (nerve of the 1st pharyngeal arch).
• Mastication involves using the 4 muscles in different combinations to move the mandible in 1 of 3 planes in an antagonistic fashion:

1. Elevation - depression
2. Protrusion - retrusion
3. Side - to -side excursion

• Although the buccinator is not a muscle of mastication, it aids in keeping the bolus of food against the teeth to help in mastication.

 Access Cavity Preparation 

 

:Definition

Before going for access cavity preparation, a study of preparative periapical radiograph is necessary with a paralleling technique.

An ideal access preparation should have following features:

• An unobstructed view into the canal.
• A file should pass into the canal without touching any part of the access cavity.
• no remaining caries should be present in access cavity.
• obturating instruments should pass into the canal without touching any portion of the access cavity.

:Objectives of access cavity preparation

:Direct straight line access to the apical foramen helps in

Improved instrument control because of minimal instrumemt deflection and ease of introducing instrument in the canal.

Improved obturation.

Decreased incidence of iatrogenic errors.

complete deroofing of pulp chamber helps in:

Complete debridement of pulp chamber.

Improving visibility.

Locating canal orifices.

Permitting straight line access.

preventing discoloration of teeth because of remaining pulpal tissue.

Conserve sound tooth structure as much as possible so as to avoid weakening of remaining tooth structure.

Clinical tips:

• Recommended access opening bur is round bur . It prevents the overpreparation. Once (drop in) into the pulp chamber is obtained, round bur is replaced by tapered fissured bur.
• Avoid using flat ended burs as these result in highly irregular access walls, causing multiple ledges.

Shape of pulp chamber is determined by:

• size of pulp chamber: In young patients, access preparation is wider than the older ones.
• shape of pulp chamber: Final outline form should reflect the shape of pulp chamber. It is triangular in anteriors, ovoid buccolingually in premolars and trapezoidal or triangular in molars.
• number, position and curvature of the canal: It can lead to modified access preparation , like Shamrock preparation in maxillary molar.

Laws of access cavity preparation for locating canal orifices:

• Law of centrality: Floor of pulp chamber is always located in the center of tooth at the level of cementoenamel junction.
• Law of cementoenamel junction: Distance from external surface of clinical crown to the wall of pulp chamber is same throughout the tooth circumference at the level of CEJ.
• Law of concentricity: Walls of pulp chamber are always concentric to external surface of the tooth at level of CEJ.
• This indicates anatomy of external tooth surface reflects the anatomy of pulp chamber.
• Law of color change: Color of pulp chamber floor is darker than the cavity walls.
• Law of symmetry: Usually canal orifices are equidistant from a line drawn in mesial and distal direction through the floor of pulp chamber.
• Law of orifice location: Canal orifices are located at the junction of floor and walls, and at the terminus of root development fusion lines.

TEMPORAL AND INFRATEMPORAL FOSSAE

 The entire area consists of 2 fossae divided by the zygomatic arch.

TEMPORAL FOSSA 

• Related to the temple of the head.
• Communicates with the infratemporal fossa beneath the zygomatic arch.

INFRATEMPORAL FOSSA

• An irregularly shaped fossa inferior and medial to the zygomatic arch.
• Communicates with the pterygopalatine fossa at the pterygomaxillary fissure.

MUSCLES

• Temporalis
• Lateral pterygoid 
• Medial pterygoid

ARTERIES

• Maxillary and its branches

VEINS

• Pterygoid plexus of veins and tributaries

NERVES

• Mandibular division of the trigeminal n. and branches
• Posterior superior alveolar (branch of maxillary division of trigeminal)
• Chorda tympani (branch of the facial n.)
• Otic ganglion 
• Lesser petrosal

Pulp and Periradicular Tissue  

 INTRODUCTION 

 The dental pulp is soft tissue of mesenchmal origin located in center of a tooth. It consists of specialized cells, odontoblasts arranged peripherally in direct contact with dentin matrix. This close relationship between odontoblasts and dentin is known as (pulp-dentin complex). The pulp is connective tissue system composed of cells, ground substance, fibers, interstitial fluid, odontoblasts, fibroblasts and other cellular components. Pulp is actually a microcirculatory system consists of arterioles and venules as the largest vascular component. Due to lack of true collateral circulation, pulp is dependent upon few arterioles entering through the foramen. Due to presence of the specialized cells, odontoblasts as well as other cells which can differentiate into hard tissue secreting cells; the pulp retains its ability to form dentin throughout the life. This enables the vital pulp to partially compensate for loss of enamel or dentin occurring with age. The injury to pulp may cause discomfort and the disease. Consequently, the health of pulp is important for successful completion of the restorative procedures. 

DEVELOPMENT OF DENTAL PULP

The pulp originates from ectomesenchymal cells of dental papilla. Dental pulp is identified when these cells mature and dentin is formed.

Before knowing the development of pulp, we should understand the development of the tooth. Basically the development of tooth is divided into bud, cap and bell stage.

The bud stage is initial stage where epithelial cells of dental lamina proliferate and produce a bud like projection into adjacent ectomesenchyme.

The cap stage is formed when cells of dental lamina proliferate to form a concavity which produces cap like appearance. It shows outer and and inner enamel epithelia and stellate reticulum. The rim of the enamel organ, where inner and outer enamel epithelia are joined is called cervical loop. As the cells of loop proliferate, enamel organ assumes bell stage.

The differentiation of epithelial and mesenchymal cells into ameloblasts and odontoblasts occur during bell stage. The pulp is initially called as dental papilla; it is designated as pulp only when dentin forms around it. The differentiation of odontoblasts from undifferentiated ectomesenchymal cells is accomplished by interaction of cell and signaling molecules mediated through basal lamina and extracellular matrix. The dental papilla has high cell density and the rich vascular supply as a result of proliferation of cells with in it.

The cells of dental papilla appear as undifferentiated mesenchymal cells, gradually these cells differentiate into fibroblasts.The formation of dentin by odontoblasts heralds the conversion of dental papilla into pulp. The boundary between inner enamel epithelium and odontoblast from the future dentinoenamel junction. The junction of inner and outer enamel epithelium at the basal margin of enamel organ represent the future cementoenamel junction. As the crown formation with enamel and dentin deposition continues, growth and organization of pulp vasculature occurs.

At the same time as tooth develops unmyelinated sensory nerves and autonomic nerves grow into pulpal tissue. Myelinated fibers develop and mature at a slower rate, plexus of Raschkow does not develop until after tooth has erupted.

HISTOLOGY OF DENTAL PULP

When pulp is examined histologically, it can be distinguished into four distinct zones from periphery to center of the pulp .

Zones of pulp are:

1. Odontoblastic layer at the pulp periphery.
2. Cell free zone of Weil.
3. Cell rich zone.
4. Pulp core. 

Odontoblastic layer: Odontoblasts consists of cell bodies and cytoplasmic processes. The odontoblastic cell bodies form the odontoblastic zone whereas the odontoblastic processes are located within predentin matrix. Capillaries, nerve fibers (unmyelinated) and dendritic cells may be found around the odontoblasts in this zone.

Cell free zone of Weil: Central to odontoblasts is subodontoblastic layer, termed cell free zone of Weil. It contains plexuses of capillaries and small nerve fiber ramifications.

Cell rich zone: This zone lies next to subodontoblastic layer. It contains fibroblasts, undifferentiated cells which maintain number of odontoblasts by proliferation and differentiation.

Pulp core: It is circumscribed by cell rich zone. It contains large vessels and nerves from which branches extend to peripheral layers. principal cells are fibroblasts with collagen as ground substance.

SUPPORTIVE ELEMENTS

Pulpal Blood Supply

Teeth are supplied by branches of maxillary artery. Mature pulp has an extensive and unique vascular pattern that reflects its unique environment. Blood vessels which are branches of dental  arteries enter the dental pulp by way of apical and accessory foramina

Lymphatic vessels

Lymphatic vessels arise as small, blind, thin-walled vessels in the coronal region of the pulp and pass apically through middle and radicular regions of the pulp. They exit via one or two large vessels through the apical foramen

:Lymphatic can be differentiated from small venules in following ways

• Presence of discontinuities in vessel walls
• Absencce of RBC in their lumina

Regulation of pulpal blood flow

Walls of arterioles and venules are associated with smooth muscles which are innervated by unmyelinated sympathetic fibers. When stimulated by electrical stimulus (e.g. epinephrine containing local anesthetics), muscle fibers contract, decreasing the blood supply

Pulpal Response to Inflammation

Whenever there is inflammatory reaction, there is release of lysosomal enzymes which cause  

hydrolysis of collagen and the release of kinins 

These changes further lead to increased vascular permeability. The escaping fluid accumulates in the pulp interstitial space. Since space in the pulp is conined so, pressure within the pulp chamber rises. In severe inflammation, lymphatics are closed resulting in continued increase in fluid and pulp pressure which may result in pulp necrosis

 Effect of Posture on Pulpal Flow

In normal upright posture, there is less pressure effect in the structures of head. On lying down, the gravitational effect disappears; there is sudden increase in pulpal blood pressure and thus corresponding rise in tissue pressure which leads to pain in lying down position

Another factor contributing to elevated pulp pressure on reclining position is effect of posture on the activity of sympathetic nervous system. When a person is upright, baroreceptors maintain high degree of sympathetic stimulation which leads to slight vasoconstriction. Lying down will reverse the effect leading to increase in blood flow to pulp. In other words, lying down increases blood flow to the pulp by removal of both gravitational and baroreceptor effect

 

ANATOMY OF DENTAL PULP

Pulp lies in the center of tooth and shapes itself to miniature form of tooth. This space is called pulp cavity which is divided into pulp chamber and root canal

In the anterior teeth, the pulp chamber gradually merges into the root canal and this division becomes indistinct. But in case of multirooted teeth, there is a single pulp chamber and usually two to four root canals. As the external morphology of the tooth varies from person to person, so does the internal morphology of crown and the root. The change in pulp cavity anatomy results from age, disease, trauma or any other irritation

PULP CHAMBER

It reflects the external form of enamel at the time of eruption, but anatomy is less sharply defined. The roof of pulp chamber consists of dentin covering the pulp chamber occlusally. 

Canal orifices are openings in the floor of pulp chamber leading into the root canals

A specific stimulus such as caries leads to the formation of irritation dentin. With time, pulp chamber shows reduction in size as secondary or tertiary dentin is formed

ROOT CANAL

Root canal is that portion of pulp cavity which extends from canal orifice to the apical foramen. The shape of root canal varies with size, shape, number of the roots in different teeth. A straight root canal throughout the entire length of root is uncommon. Commonly curvature is found along its length which can be gradual or sharp in nature. In most cases, numbers of root canals correspond to number of roots but a root may have more than one canal

FUNCTIONS OF PULP

1. Formation of dentin
2. Nutrition of dentin
3. Innervation of tooth
4. Defense of tooth

  Thyroid gland

General Information:

• Begins in the floor of the pharynx as an invagination at the foramen cecum.
• Descends inferiorly to its final position alongside the larynx.
• May be connected to the foramen cecum by the thyroglossal duct (which normally atrophies and disappears, remnants may persist and form cysts).
• Divided into 2 lateral lobes connected by an isthmus, from which a pyramidal lobe sometimes develops.
• Follicular cells are derived from the endoderm, parafollicular cells are derived from the ultimobranchial body.

Pharyngeal Pouch Abnormalities

Ectopic thyroid

• Thyroid tissue in an aberrant location.
• Often the only thyroid tissue in the affected person.
• Susceptiblscated at the base of the tongue (lingual thyroid).
• Common locations include:

1. Lingual thyroid.
2. Sublingual thyroid.
3. Thyroglossal duct remnant.
4. Anterior mediastinum.
5. Prelaryngeal.
6. Intralingual.
7. Intratracheal.

Pharyngeal Arch Abnormalities

PIERRE ROBIN

• First reported as a condition characterized by micrognathia, cleft palate, and glossoptosis.
• Now includes any condition with a series of anomalies caused by events initiated by a single malformation.
• In this micrognathia, the inferior dental arch is posterior to the superior arch.
• The clefting may affect the hard and the soft palate.
• Glossoptosis (posterior displacement of the tongue) may cause airway obstruction or apnea.
• The mandible usually grows fairly quickly during childhood.
• Multiple surgeries typically needed to correct the cleft palate and to aid speech development in children.

TREACHER COLLINS 

• A hereditary condition affecting the head and neck.
• caused by haploinsufficiency of the gene TCOF1 (Treacher Collins-Franceschetti syndrome 1) which is officially known as Treacle Ribosome Biogenesis Factor 1.
• The gene product is the treacle protein, which contributes to development of cartilage and bone of the face.
• Children of an affected parent have a 50% risk of having the syndrome.
• Clinical manifestations include:

1. Downslanting eyes.
2. Incomplete orbits.
3. Notching of the lower eyelids.
4. Hypoplastic mandible.
5. Hypoplastic zygomatic bones (malar hypoplasia).
6. Underdeveloped or malformed ears or "sideburns," or both, are prominent.

• Common associated problems include:

1. Hearing loss.
2. Eating/breathing difficulties.
3. Cleft palate.

DIGEORGE SYNDROME

• A rare condition caused by a deletion on chromosome22, characterized by a wide array of clinical manifestations.
• possible explanation: proper development is dependent on migration of neural crest cells to the area of the pharyngeal pouches.
• A lthough researchers decribed the syndrome as abnormal development of the 3rd and 4th pharyngeal pouches, defects involving the 1st to the 6th pouches have been observed.
• Thus, the affected individual is born without a thymus and parathyroid glands.
• Possible associated problems include:

1. Congenital heart defects (such as tetralogy of Fallot, right infundibular stenosis, truncus arteriosus, aberrant left subclavian artery, and ventricular septal defect).
2. Facial defects (such as cleft palate, microstomia, downslanting eyes, low-set ears, or hypertelorism).
3. Increased vulnerability to infections (due to impaired immune system from the loss of T cells associated with absence or hypoplasia of the thymus).