➡️ Orthodontic tooth movement is a unique process where a solid object (teeth) is made to move through a solid medium (bone).

➡️ Orthodontic treatment is possible due to the fact that whenever a prolonged force is applied on a tooth, bone remodeling occurs around the tooth resulting in its movement.

➡️ Physiologic tooth movements are naturally occurring tooth movements that take place during and after tooth eruption. They include:

• a. Tooth eruption
• b. Migration or drift of teeth
• c. Changes in tooth position during mastication

a) Tooth Eruption:

➡️ Tooth eruption is the axial movement of the tooth from its developmental position in the jaw to its final position in the oral cavity. A number of theories have been put forward to explain how the eruption process take place.

a. Blood pressure theory: According to this theory, the tissue around the developing end of the root is highly vascular. This vascular pressure is believed to cause the axial movement of the teeth.

b. Root growth: According to the proponents of this theory, the apical growth of root results in an axially directed force that brings about the eruption of the teeth. This theory has not been accepted. Teeth move a greater distance during eruption than the increase in root length. In addition the onset of root growth and eruption do not seem to coincide. Teeth that are malformed and lack roots also erupt in a no. of cases.

c. Hammock ligament: According to Sicher, a band of fibrous tissue exists below the root apex spanning from one side of the alveolar wall to the other. This fibrous tissue appears to form a network below the developing root and is rich in fluid droplets. The developing root forces itself against this band of tissue, which in turn applies an occlusally directed force on the tooth.

d. Periodontal ligament traction: This theory states that the periodontal ligament is rich in fibroblasts that contain contractile tissue. The contraction of these periodontal fibres result in axial movement of the tooth.

b) Migration or drift of teeth:

• Migration refers to the minor changes in tooth position observed after eruption of teeth. Teeth show a lifelong ability to drift through the alveolar bone, a process called physiologic tooth migration.
• It is generally believed that the direction of tooth migration varies from species to species. The dentition in humans and primates shows a natural tendency to move in a mesial and occlusal direction. This is largely true in case of the maxillary dentition. However the lower jaw could show certain variations.

• Tooth migration is usually a result of proximal & occlusal wear of teeth. As the teeth undergo occlusal and proximal wear, they move in a mesial and occlusal direction to maintain inter-proximal and occlusal contact. Change in state of equilibrium such as by loss of neighbouring or opposing teeth may induce further movement.

• The remodeling process that accompanies physiologic migration is a normal function of the supporting structures. Osteoclasts are seen in lacunae along the alveolar bone wall towards which the tooth is moving. This is the resorptive surface. The alveolar bone wall on the opposite side from which the tooth is moving away is the depository surface and is characterized by osteoblasts depositing non mineralized osteoid which later mineralizes to form mature bone. In addition to this mesio-distal movement of teeth, they also exhibit a continued eruption throughout life accompanying the vertical development of their surrounding tissue.

C) TOOTH MOVEMENT DURING MASTICATION:

➡️ During mastication, the teeth and periodontal structures are subjected to intermittent heavy forces, which occur occur in cycles of one second or less and may range from 1-50 kg based on the type of food being masticated. A tooth subjected to these heavy forces, exhibits slight movement within its socket and subsequently returns to its original position as soon as the load is removed.

➡️ When a tooth is subjected to such heavy forces, the tissue fluid present in the periodontal space, being incompressible, prevents major displacement of the tooth within the socket. Therefore the forces are transmitted through the tissue fluids to the adjacent alveolar bone. Recent studies have shown that the alveolar bone can bend in response to heavy masticatory forces.

➡️ Whenever the forces of mastication are sustained and are more than the usual one second cycle, the periodontal fluid is squeezed out and pain is felt as the tooth is displaced into the PDL space.

References: Textbook of Orthodontics The Art and Science 6th Edition By Bhalajhi!…Image source: Google