Physiologic Adaptation

Modeling is a surface specific activity (apposition or resorption) that produces a net change in the size and/or shape of the bone. It is an uncoupled process meaning that cell activation (A) proceeds independently to formation (F) or resorption (R). Modeling is the fundamental mechanism of growth, atrophy and re-orientation. Remodeling is defined as the turnover or the internal reconstructing of previously existing bone. It is a coupled tissue phenomenon. Activation of the precursor cells results in the following:

Activation (A)

Resorption (R)

Quiescence or reversal (Q)

Formation (F)

The duration of A - R(Q) - F remodeling cycle  is also referred to as the SIGMA CYCLE. It is about 6 weeks in rabbits, 12 weeks in dogs and 17 weeks in humans.