Effect of saline solution on creep fracture of Delrin

Delrin as an implant material has been widely used for more than 26 yr. To determine the combined effect of stress and body fluid environment on the stress limits of polyacetal, the creep rupture behaviour of Delrin in saline (0.9% NaCl) solution at 37 degrees C was studied. A creep rupture model was used to predict the upper (immediate fracture) and lower (no fracture) stress limits and the elastic ratio (Ee/Ea). The results showed that saline solution adversely affected the lower stress limit and Ee/Ea, but not the upper stress limit. Brittle fracture with a strain < 5% occurred for most specimens. This may explain why Delrin can fail prematurely in orthopaedic implants. Design stresses using Delrin as implant material have to be reduced by four times to 5 MPa and the strain limited to 5% as opposed to 20%, as previously believed. The increase in Ee/Ea also implied that the microdeformation mechanism is changed. The presence of surface cracks indicated that Delrin is susceptible to environmental stress cracking in saline solution.