The Use of Shimstock to Assess Occlusal Contacts: A Laboratory Study

Purpose: This laboratory study investigated the influence of varying occlusal forces, occlusal contacts, or small gaps (0 to 8 µm) on the force required to pull 8-µm shimstock from between a tooth and a restorative material specimen. Materials and Methods: An apparatus was developed and calibrated to (1) produce and measure very small gaps (0 to 20 µm) and apply variable occlusal forces between specimens, and (2) pull shimstock consistently from between the specimens and measure the removal force. An unworn maxillary molar cusp and polished amalgam were used as the opposing specimens. Combinations of occlusal forces from 0 to 400 g and gaps of 0 (contact), 2, 4, 6, and 8 µm were tested. Results: Shimstock removal forces increased linearly with increasing occlusal force with 0-, 2-, 4-, and 6-µm gap settings. With 0-, 2-, and 4-µm gaps, there was no significant difference in removal force. With a 6-µm gap, the forces rose but at a lower gradient. With the 8-µm gap, the mean force was low (less than 12 g). Conclusion: The shimstock removal force increased with occlusal force and was highest with the smaller gaps. Using 8-µm shimstock, 0-, 2-, and 4-µm gaps cannot be differentiated, a gap of up to 6 µm may still be assessed as a holding contact because the shimstock is still gripped, and even an 8-µm gap may be recorded as a light contact since there is still friction on removal.