Critical Appraisal of in Vitro Steady-State Frictional Resistance Studies

Mechanics is divided into two branches: (1) statics and (2) dynamics. Statics deals with bodies at rest (unaccelerated motion), and dynamics deals with accelerated motion. The orthodontic in vitro studies that investigate resistance to sliding generally fall into one of two categories: (1) pulling a bracket down a wire, or (2) pulling a wire through a series of brackets in a linear form or typodont form. The average movement per month in these in vitro studies is very high, which puts it in the dynamic branch of mechanics. Clinical tooth movement is in the static section of mechanics. Use of the dynamic branch of mechanics to explain clinical tooth movement is inappropriate and unscientific. Very little useful information, from a clinical perspective, can be interpolated from these in vitro investigations.