FRICTION AND PARTIAL DENTURE RETENTION

¹ Department of Prosthetics, The London Hospital Medical College, Turner Street, London

Some experiments were carried out to measure the coefficient of static friction, µ_s, between tooth enamel and various clasp materials in the presence of saliva. Five different materials were examined, chrome cobalt, white gold, stainless steel, acrylic resin, and an epoxy resin, Araldite.

For metals, µ_s was found to lie between 0.15 and 0.24 and this wide variation was largely due to surface contamination. The upper value was obtained when the surfaces were very carefully cleaned. Variations due to temperature were negligible and the nature of the saliva did not appear to be important. However, surface roughness had a marked effect under certain conditions. With chrome cobalt, which is harder than enamel, roughening the surface can double the friction. With the other materials tested, no such effect was observed.

Both resins have a higher coefficient of friction than the metals, but both are rather more sensitive to contamination, Araldite particularly so. When their surfaces are clean, their coefficients of static friction are:

acrylic resin - µ_s 0.4,

Araldite - µ_s = 0.5,

but under oral conditions a value between 0.3 and 0.35 is more probable.