Ligatures and springs: friction consumes part of the force before it reaches the tooth

The force applied to a wire does not reach the tooth intact. Part is consumed by friction between wire, bracket and ligature, and that part is larger than one imagines.

In sliding mechanics, where the tooth must travel along the wire, friction can absorb a substantial fraction of the applied force. It is why spaces close more slowly than expected.

Elastic ligatures are convenient, quick and available in every colour, which has real value with young patients. Their limitation is decay: they lose a significant part of their force within the first hours.

This has two opposite consequences. As a ligation system they hold the wire in the slot less well as the weeks pass; as a traction system, elastic chain loses effectiveness well before the next visit.

Steel ligatures do not decay and allow the tightening force to be adjusted. Tight, they give maximum wire control; loose, they leave sliding freer.

In space-closure mechanics, a loose steel ligature on the teeth that must slide reduces friction appreciably, and it is a technique that costs nothing.

Nickel-titanium springs share the advantage of archwires of the same alloy: they deliver an almost constant force across a wide range of deformation.

The open coil spring is used to create or maintain space, typically before bringing an impacted tooth into the arch or rebuilding the space of a lost element. It should be compressed only a little: excessive compression does not increase speed, only force.

The closed coil spring replaces elastic chain where a force constant over time is needed. It costs more, but the difference in performance between the third and sixth week is marked.

The choice between chain and spring depends on the interval between visits. With closely spaced appointments chain is adequate; with long intervals, the spring maintains force while the chain has already lost it.

The correct force for tooth movement is lower than one tends to apply. The ranges cited in the literature for bodily movement fall broadly between 100 and 150 grams, and exceeding them accelerates nothing.

A force gauge in the practice is an inexpensive instrument that allows one to verify the force actually being applied rather than estimating it. It is the simplest way to discover that one had been working with twice the force needed.

In summary: friction consumes part of the applied force, loose steel ligatures reduce it, NiTi springs hold force constant while chains decay, and the correct force is lower than the one applied out of habit.