immunity n. The ability of the body to resist infection due to the presence of circulating *antibodies and white blood cells. Natural (innate or non-specific) immunity is a defence mechanism which includes natural, mechanical, chemical, and biological barriers to infection and which has no degree of memory or specificity; it is present at birth prior to exposure to infective agents. Acquired immunity is achieved in response to foreign *antigens in the body and involves the production of *antibodies which react specifically with the foreign antigen. Active immunity is achieved when the body’s own cells produce and continue to produce appropriate antibodies in response to an infection or to artificial stimulation by the use of treated antigens to stimulate the body to produce its own antibodies (*immunization, *vaccination). Passive immunity is of short duration and occurs naturally at birth from maternal antibodies in the mother’s milk (colostrum) and placental blood or by the injection of antibodies contained in antiserum taken from another immune person or animal.

immunization n. The production of *immunity by artificial means. It may be achieved actively or passively.

immunodeficiency n. A condition resulting from a reduced or deficient *immune response. This may be a primary condition due to a defect in the immune system or a secondary (or acquired) condition as a result of a disease process such as *AIDS.

immunoglobulins n. A group of structurally related serum proteins synthesized by *plasma cells which act as *antibodies.

immunosuppressant n. A drug or agent that reduces the *immune response. They are useful in maintaining the survival of transplanted organs or tissues which would otherwise be rejected and destroyed by the immune response, and for treating certain *autoimmune diseases such as rheumatoid *arthritis.

impaction n. The condition of being tightly wedged together. Food impaction can occur between the teeth due to an open contact, a poorly contoured restoration, or the plunger effect of opposing teeth. Tooth impaction occurs when a tooth is prevented from erupting into a fully functional position. The preventing agent can be another tooth, alveolar bone, or soft tissues. The most commonly impacted teeth are third molars, second premolars, and canines; the impacted tooth may lie in a variety of different positions.

Types of impacted lower third molars

impairment n. Any loss or abnormality of psychological, physiological, or anatomical structure or function. It represents a deviation from a person’s usual biomedical state. An impairment is thus any loss of function directly resulting from injury or disease. A social model defines impairment as the functional limitation within an individual caused by restricted physical, mental, or sensory activity. Compare DISABILITY; HANDICAP.

impetigo n. A superficial bacterial infection of the skin. It is characterized by small pus-filled blisters that form a yellow crust, usually around the nose and mouth. Non-bullous impetigo is caused by Staphylococcus aureus or Streptococcus species or both, is highly contagious, and occurs mainly in young children. Bullous impetigo is caused by Staphylococcus aureus, is less contagious, and can occur at any age. Treatment is usually by topical antibiotics.

impingement n. The unusually forceful application of food, teeth, foreign objects, etc. onto tissues, teeth, or prostheses.

Further Reading: Esposito M., Worthington H. V., Coulthard P., Thomsen P. Maintaining and reestablishing health around osseointegrated oral implants: a Cochrane systematic review comparing the efficacy of various treatments. Periodontol 2000 2003;33:204–12.

Bishop K, Addy L, Knox J. Modern restorative management of patients with congenitally missing teeth: the role of implants. Dent Update 2007;34:79–84.

Types of implants

implantology (oral) n. A specialist branch of dentistry concerned with the surgical insertion of materials or devices into the hard or soft tissues of the oral cavity for the purpose of providing support or retention for tooth restorations or a prosthetic appliance.

implant restoration A restoration placed on an implant designed to restore normal function, aesthetics, speech, and comfort. It may be secured on the underlying implant by screw or cement retention, or a combination of the two. This allows the restoration to be removed for repair in the event of porcelain fracture, abutment fracture, or if the prosthesis needs to be modified such as when an implant has failed. The restorative prosthesis may consist of several units secured to a number of implants.

Impregum^® n. The trade name for a *polyether impression material.

impression n. An imprinted mould of an object such as a tooth, arch of teeth, or edentulous ridge, obtained by pressing an *impression tray containing an *impression material over the target object, from which a positive reproduction of that object can be made. A mucodisplacive (mucocompressive, functional) impression utilizes a technique in which the denture-bearing area of the mucosa is loaded to produce a uniform reduction in volume which will be subsequently compressed during denture wearing; the impression material used is of high viscosity such as *impression compound. A mucostatic impression creates an impression of the denture-bearing tissues in their natural stateand requires a low-viscosity impression material such as *plaster of Paris or *alginate. A primary (preliminary) impression is made for diagnostic purposes or for the construction of an impression in which a more accurate secondary impression can be taken; preliminary impressions may also be taken in order to construct a *model on which a custom-made impression tray can be fabricated for the purposes of taking a more accurate secondary impression. A dual impression technique is a method of taking an impression in layers using impression materials of different viscosities, chosen appropriate to the character and mobility of the soft tissues so as to provide differential compression. A working (secondary or final) impression is an accurate impression usually taken in a custom-made *impression tray used for making a master cast. A sectional impression is made in two or more parts. A pick-up impression is an accurate impression taken with an implant superstructure or one or more crown *copings in place so that they are included in the impression material; it provides an accurate impression of the oral mucosal tissue and the location of the metallic superstructure relative to the crown preparations or implants.

impression compound n. A material composed of fatty acids, shellac, glycerine, and filler used as a primary *impression material. When heated in a water bath at about 65°C (149°F) it becomes plastic and can be moulded in an impression tray and inserted in the mouth. The material becomes fairly rigid on cooling to mouth temperature and has low material flow at room temperature. It has a low thermal-expansion coefficient so that there is minimal dimensional change as the impression is cooled from mouth temperature and it does not adhere to moist oral tissue. It may be modified by the addition of an *alginate wash. Impression compound is usually used for taking primary impressions of the edentulous ridge prior to the construction of a custom-made *impression tray for complete *dentures. Impression compound is also available in sticks of various colours indicating different softening temperature ranges e.g. greenstick compound (composition); these are used for impression correction and moulding the borders of custom impression trays, particularly around muscle attachments.

impression material n. Any substance used for making a negative mould of a structure. Rigid impression materials are inflexible and are usually used to take impressions of structures which have no undercut surfaces such that when removed the material is not required to be deformed; such materials include *plaster of Paris, *impression compound, *zinc oxide, and *wax. Elastic impression materials are capable of deformation with minimal distortion; materials commonly used are reversible and irreversible *hydrocolloid, *silicone, *polyether, and mercaptan (*polysulphide). Syringable impression material may be used for obtaining an *occlusal record.

Further Reading: Stewardson D. A. Trends in indirect dentistry: 5. Impression materials and techniques. Dent Update 2005;32(7):374–6, 379–80, 382–4.

impression tray n. A rigid receptacle in which impression material is placed. Materials commonly used for impression trays are metal, *shellac, or resin. They are usually coated with tray adhesive immediately prior to use and may be perforated with 2–4mm diameter holes or made from metallic mesh to improve the adhesion of the impression material to the tray. Stock trays are rigid trays commercially manufactured in different sizes and shapes and usually made of resin or metal. Edentulous stock trays have a shape which conforms to the shape of the edentulous ridge of either the mandible or maxilla. Dentate stock trays have a box shape to accommodate the teeth in the arch. Custom trays are made from a primary cast and, because their shape conforms more closely to the shape of the teeth and tissues to give a more uniform thickness of impression material, it provides greater dimensional accuracy. Custom trays are usually made from shellac or resin. A sectional tray is used to take an impression of a specific area of the dental arch such as a quadrant or sextant.

IMZ implant system n. See IMPLANT.

incidence n. The number of occurrences of a disease, condition, or event within a defined time interval. The incidence rate is the number of cases of a specific disease, condition, or event diagnosed or reported during a defined period of time, divided by the number of people at risk for the disease.

incipient adj. In an initial stage; beginning to become apparent.

incisal adj. Relating to the cutting edge of an anterior tooth (incisor or canine). The incisal edge is the biting edge of an incisor or canine. The incisal angle is the angle between the incisal edge and the mesial or distal surface of an anterior tooth. The incisal guide table forms the base for the incisal guide *pin on an *articulator. Incisal guidance is the influence on mandibular movements caused by the contacting surfaces of the mandibular and maxillary anterior teeth during masticatory movements (eccentric excursions from centric *occlusion).

incision n. 1. The act of making a cut in body tissue. 2. The cutting action of incisor teeth.

incisor, permanent mandibular central n. One of two teeth which are the smallest in the permanent *dentition located between the mandibular lateral incisors either side of the midline. They replace the mandibular primary central incisor teeth. It has a convex labial surface with a flattened area in the incisal half. The lingual surface is concave, except above the cervical margin where it is convex. Unlike the upper central incisors, there is no well-defined *cingulum and no definite marginal ridges on the lingual surface. The mesial and distal surfaces of the crown are triangular in shape, to give a wedge-shaped appearance, and are almost at right angles to the incisal edge. On *eruption, the incisal edge has three small tubercles (*mamelons), which wear away to give a flat surface. It has a single root, oval in cross-section and flattened on the mesial and distal surfaces. The apical region may show some degree of bifurcation. The *pulp chamber has two mesial and distal pulp horns directed towards the mesial and distal angles of the incisal edge. The root canal is subject to variation and may divide to give buccal and lingual canals. *Calcification of the tooth begins at about 3–4 months after birth and the crown is normally complete by 4–5 years of age. The tooth erupts at about 6–7 years and the calcification of the root is complete at about 9–10 years.

incisor, permanent mandibular lateral n. One of two teeth located between the mandibular central incisors and the canines in the permanent dentition. They replace the mandibular primary lateral incisor teeth. It is similar to the lower central incisor but is wider mesio-distally and therefore more fan-shaped. The lingual surface usually shows more definite mesial and distal marginal ridges. On *eruption, the incisal edge has three small tubercles (*mamelons), which wear away to give a flat surface. *Calcification of the tooth begins at about 3–4 months after birth and the crown is normally complete by 4–5 years of age. The tooth erupts at about 7–8 years and the calcification of the root is complete at about 10 years.

Permanent incisors

incisor, permanent maxillary central n

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