A Study of the Physical Properties of Latex as Related to Orthodontic Tooth Movement

Abstract

Latex elastics have long been used in orthodontics, and force decay has always been a disadvantage. Reports differ on the load-deflection properties of rubber. Decay in elastic bands has ranged from 8% to 74.9%, depending on test conditions. A study was designed to ascertain (1) whether better quality controls would minimize decay and give better reproducibility in force from one elastic band to the next; and (2) whether the amount of stretch to which the elastic body is subjected affects the rate of decay. The quality control testing consisted of selecting elastic bands of identical size, shape, and color. This was followed by testing each one with a 100 gram weight and separating out all elastics which tested out to a given length. This test-controlled sample was then used to study the physical properties of latex. A load-deflection study was performed from this test-controlled sample, and revealed a curvilinear relationship. From the load-deflection curve, four force values were chosen which would represent 1) minimal stretching in the elastic deformation range, 2) maximal stretching in the elastic deformation range, 3) minimal stretching in the permanent deformation range, and 4) maximal stretching in the permanent deformation range. In the decay phase of the study, four more groups of elastics from the test-controlled sample were loaded with the four force values previously mentioned. The force was then measured after one and 12 hours and after one, seven, 14 and 21 days. Thus the decay of force could be calculated for each group over a three-week period. The results showed that more rigid quality control does produce a more predictable decay rate and force. The load-deflection properties of latex elastic were shown to be curvilinear, and the decay rate was affected by the amount of stretch to which the elastic body was subjected. However, if the elastic was stretched within the elastic deformation range, the decay rate remained the same regardless of the length of stretch. Therefore, it is recommended that the clinician use latex elastic in the elastic deformation range and thereby reduce the decay rate.