Effect of cyclic loading on the vertical microgap of long-span zirconia frameworks supported by 4 or 6 implants

Abstract

Few studies have investigated the microgap of long-span complete-arch fixed dental prosthesis zirconia frameworks. The purpose of this study was to evaluate the effects of cyclic loading on the vertical microgap of maxillary 12-unit prostheses supported by 4 implants and on 14-unit prostheses supported by 6 implants. One-piece zirconia frameworks were fabricated with a computer-aided design/computer-aided manufacturing technique and divided into 2 groups (n=5): a group of 12-unit prostheses and a group of 14-unit prostheses. The vertical microgap between the frameworks and prosthetic abutments was measured with an optical microscope (80×) under 2 reading conditions. Condition 1 (1-screw test): 1A, the screw on implant 1 was tightened and readings were made on all implants; 1B, the screw was changed to implant 4 (implant 6 for the 14-unit group) and readings were made on all implants. Condition 2: the microgap was measured with all screws tightened before cyclic loading (2A). Specimens were submitted to 200 N underwater (37°C) cyclic loading at a 2-Hz frequency for 1×10(6) cycles. Microgap reading condition 2 was repeated after cyclic loading (2B). The data were submitted to a linear mixed-effects model for statistical comparison (α=.05). A lower (P<.05) vertical microgap (μm) was found for the 12-unit group (reading conditions: 1A, 47.93; 1B, 43.83; 2A, 11.77; and 2B, 11.25) compared to the 14-unit group (1A, 94.87; 1B, 112.56; 2A, 21.28; and 2B, 16.90). No differences were found when each group was compared before and after cyclic loading (P>.05). The vertical microgap was significantly reduced after tightening all the screws in the framework, possibly leading to a nonpassive situation. Longer-span frameworks showed an increased microgap. Cyclic loading had no influence on the vertical microgap within each group.