Intermaxillary elastics: the stated force applies at one specific extension, not always

Intermaxillary elastics are the cheapest orthodontic device and the one most dependent on the patient. A well-planned treatment fails if the elastics are not worn.

The commonest classification divides them by direction of pull. Class II elastics run from the upper canine to the lower molar and correct a distal relationship; Class III elastics take the opposite route.

Vertical elastics close an anterior open bite, triangular elastics stabilise a segment, cross elastics correct a posterior crossbite. Direction determines the effect, and every direction brings side effects with it.

Class II elastics, for instance, do not merely retract the upper segment: they extrude the lower molars and rotate the occlusal plane. In a hyperdivergent patient that effect worsens the picture rather than improving it.

The force stated on the packet — 2 ounces, 3.5 ounces, 4.5 ounces — is measured at a specific extension, typically three times the resting diameter. It is a figure seldom read and one that changes everything.

A 4-ounce elastic stretched to twice its diameter delivers far less than the nominal force; stretched to four times, far more. Diameter must be chosen relative to the actual distance between anchorage points.

Force decay is the phenomenon that makes frequent replacement necessary. Latex loses a significant part of its initial force within the first hours, and continues to decay slowly.

Hence the correct instruction is to replace them at least once a day, not when they break. An elastic worn for three days applies a force well below that prescribed, and the patient believes they are complying.

Synthetic latex-free elastics exist for allergic patients. Their force decay differs slightly and they cost more, but latex allergy is common enough to justify keeping them in the practice.

Compliance is achieved by explaining the mechanism, not by repeating the instruction. A patient who understands that removing elastics to eat and then forgetting them produces stop-start movement complies better than one merely told to wear them.

Hours matter more than force. The evidence on tooth movement indicates that a light continuous force is more effective than a heavy intermittent one, and elastics worn only at night largely produce an oscillating effect.

The patient should be told that twelve hours a day is not half the result of twenty-four: it is far less, because every interruption undoes part of the remodelling under way.

In summary: direction determines side effects as well as intended ones, diameter must be matched to the real distance, replacement is daily and not on breakage, and hours of wear count more than the force chosen.