The purpose of this study was to investigate the effect of four direct restorative materials that can be used in the proximal box elevation (PBE) technique. Materials and Methods: Seventy-five molar teeth were randomly assigned to one of five groups (n=15): Type II glass-ionomer (GI), Type II resin-modified glass-ionomer (RMGI), resin-based composite (RBC), bulk fill (BF) resin-based composite, and a control with no box elevation procedure. Specimens were prepared for a standard CAD-CAM ceramic onlay preparation with mesial cervical margins located 1 mm above CEJ (cemento-enamel junction) and distal cervical margins located 2 mm below the CEJ. PBE was used to elevate the distal margins to 1 mm above the CEJ in all groups except the control group. For the control group the onlay margin was placed directly on the prepared distal tooth without PBE. A Lava UltimateTM, CAD/CAM Resin, nano-Ceramic onlay Restorative (LAVU) was milled and bonded on all specimens with RelyX UltimateTMAdhesive Resin Cement. The margin quality of the tooth-PBE material and PBE material-onlay interface was evaluated with scanning electron microscopy (SEM) using epoxy replicas before and after mechanical loading (100,000 cycles, 1.2 Hz at 65N). In addition to margin quality, the fracture resistance of each group was measured using a universal testing machine. Fracture pattern was recorded by visual examination. One-way ANOVA was performed followed by Least Square Means. Results: For dentin margins, a statistically significant difference was detected between RMGI and control group at baseline (p=0.0442). All other groups GI, RBC, and BF showed no difference to control at baseline (p>0.05). No statistical significance was observed among groups for post-mechanical fatigue (p=0.8735). For onlay margins, no statistical significance was observed among groups for pre-mechanical fatigue, post-mechanical fatigue, or change (p=0.9713, p=0.528, p= 0.4385 respectively). No significant difference was observed for the fracture resistance among groups or for the type of break by material used (p=0.1593, p=0.77 respectively). Conclusion: Within the parameters of this study, following mechanical fatigue, the materials used for PBE: resin-modified glass-ionomer and glass-ionomer, resin-based composite and bulk-fill composite, did not influence results in terms of margin quality and fracture resistance. Therefore, collective findings suggest that these materials might be suitable for proximal box elevation procedures. Nevertheless, clinical caution is recommended with any PBE procedure and further testing of GI materials is needed.