OCCLUSION OF TEETH

Occlusion means simply the contact between teeth. More technically, it is the relationship between the maxillary and mandibular teeth when they approach each other, as occurs during chewing or at rest.

The study of occlusion is an important aspect of dentistry. The study and practice of most branches of dentistry should be based on a strong foundation of the knowledge of occlusion. Angle defined occlusion as the normal relation of the occlusal inclined planes of the teeth when the jaws are closed. The definition is an oversimplification of what actually it constitutes. Occlusion is a complex phenomenon involving the teeth, periodontal ligament, the jaws, the temporomandibular joint, the muscles and the nervous system.

TERMINOLOGY:
A number of terms are used in the study of occlusion. The following are the commonly used terms.

• Ideal Occlusion : It is a preconceived theoretical concept of occlusal, structural and functional relationship that includes idealized principles and characteristics that an occlusion should have.
• Physiologic Occlusion : This refers to an occlusion that deviates in one or more ways from ideal yet it is well adapted to that particular environment, is esthetic and shows no pathologic manifestations or dysfunction.
• Balanced Occlusion : An occlusion in which balanced and equal contacts are maintained throughout the entire arch during all excursions of the mandible.
• Functional Occlusion : It is defined as arrangement of teeth, which will provide the highest efficiency during all the excursive movements of the mandible which are necessary during function.
• Therapeutic Occlusion : An occlusion that has been modified by appropriate therapeutic modalities in order to change a non-physiologic occlusion to one that is at least physiologic if not ideal.
• Traumatic Occlusion : Traumatic occlusion is an abnormal occlusal stress, which is capable of producing or has produced an injury to the periodontium.
• Trauma from Occlusion : It is defined as periodontal tissue injury caused by occlusal forces through abnormal occlusal contacts.

PERIODS OF OCCLUSAL DEVELOPMENT:

Occlusal development can be divided into the following developmental periods:

1. Pre - dental period.
2. Deciduous dentition period.
3. Mixed dentition period.
4. Permanent dentition period

(1) PRE - DENTAL PERIOD : This is the period after birth during which the neonate does not have any teeth. It usually lasts for 6 months afterbirth.
The alveolar processes at the time of birth are known as gum pads. The gum pads are pink, firm and are covered by a dense layer of fibrous periosteum. They are horse shoe shaped and develop in two parts. They are the labiobuccal portion and the lingual portion. The two portions of the gum pads are separated from each other by a groove called dental groove. The gum pads are divided into ten segments by certain grooves called transverse grooves. Each of these segments consists of one developing deciduous tooth sac.
The gingival groove separates the gum pad from the palate and floor of the mouth. The transverse groove between the canine and first deciduous molar segment is called the lateral sulcus. The lateral sulci are useful in judging the interarch relationship at a very early stage. The lateral sulcus of the mandibular arch is normally more distal to that of the maxillary arch.
The upper and lower gum pads are almost similar to each other. The upper gum pad is both wider as well as longer that the mandibular gum pad. Thus when the upper and lower gum pads are approximated there is a complete overjet all around. Contact occurs between upper and lower gum pads at the first molar region and a space exists between them in the anterior region. This infantile open bite is considered normal and it help in suckling.
The neonate is without teeth for about 6 months of life. At birth the gum pads are not sufficiently wide to accommodate the developing incisors which are crowded in their crypts. During the first year of life the gum pads grow rapidly permitting the incisors to erupt in good alignment.
Very rarely teeth are found to have erupted at the time of birth. Such teeth that are present at the time of birth are called natal teeth. Sometimes teeth erupt at an earlu age. Teeth that erupt during the first month of age are called neonatal teeth. The natal and neonatal teeth are mostly located in the mandibular incisor region and show a familial tendency.

(2) DECIDUOUS DENTITION PERIOD : The initiation of primary tooth buds occurs during the first six weeks of intra-uterine life. The primary teeth began to erupt at the age of about 6 months. The eruption of all primary teeth is completed by 2½ to 3½ years of age when the second deciduous molars come into occlusion.
The mandibular central incisors are the first teeth to erupt into the oral cavity. They erupt around 6-7 months of age. (Click here to read chronology of eruption). The average eruption of the deciduous dentition is given in Table 1. The timing of tooth eruption is highly variable. A variation of three months from the mean age has been accepted as normal. The sequence of eruption of the deciduous dentition is :

The primary dentition is usually established by 3 years of age on eruption of the 2nd deciduous molars. Between 3 - 6 years of age the dental arch is relatively stable and a very few changes occur.
Space usually exists between the deciduous teeth. These spaces are called physiological spaces or developmental spaces. The presence of spaces in the primary dentition is important for the normal development of the permanent dentition. Absence of spaces in the primary dentition is an indication that crowding of teeth can occur when the larger permanent teeth erupt.
Spacing invariably occurs mesial to the maxillary canines and distal to the mandibular canines. The physiological spaces are called Primate spaces or Simian spaces as they are seen commonly in primates. These spaces help in placement if the canines of the opposing arch.