Repositioning - of a tooth refers to its movement within the entire dentition which involves the remodelling of the tooth socket. Repositioning of teeth occurs naturally due to continued eruption and mesial (or distal) drift.

Rest Position - a position the jaw1 adopts when at rest with the lips lightly together.

Reticular fibres - are fine type III collagen fibres forming a net-like supporting framework or reticulum. They are found around small blood vessels, nerve cells, muscle fibres and in particular beneath epithelial membranes3 as part of thebasal lamina.

Reticular formation - in the central core of the medulla, it consists of several structures, including the periaquaductal grey. The reticular formation integrates information from many sources and influences sensory motor and autonomic activity. It is involved in aversive drive (behaviour which is an instinctive turning away from the unpleasant).

Retinoic acid - a product of retinol(Vitamin A) which binds onto cell membranes and controls cell division and differentiation through gene expression.

Ribosomes - - structures in the cy lasm of cells which attach onto messenger RNA. At the ribosome, the code of nucleotides on the mRNA is translated into a series of aminoacids.

RNA - Ribosenucleic acid - seenucleic acids.

Root resorption - resorption of cementum and underlying root dentine by osteoclasts. Temporary zones of root resorption may occur during orthodontic tooth repositioning. More extensive and irreversible root resorption may occur if the root becomes ankylosed.

Rugae - raised ridges of epithelium, each with its core of lamina propria, found on the anterior wall of the hard palate.