Clinical Relevance Preparation designs for bondable partial coverage restorations are varied. As little as 1 mm of thickness of a remaining buccal cusp can be kept when restoring maxillary premolars with bondable partial coverage restorations. SUMMARY Indirect partial coverage restorations have become increasingly popular in recent years as new and improved adhesive materials have been developed. These restorations can preserve substantial amounts of tooth structure. However, there are some aspects of indirect partial coverage restorations for which no clear protocol exists. This study investigated the minimal thickness of the nonfunctional cusp that must be left in a bondable ceramic partial coverage restoration in order to resist compressive force. Ninety sound human maxillary premolar teeth were obtained and used in one of the following three ways. Ten of the sound teeth were used as a control without further preparation. Forty other sound teeth had cavities designed and were tested as “unrestored teeth.” The remaining 40 sound teeth received not only cavities but also restoration and were tested as “restored teeth.” Both the restored group and the unrestored group were prepared either with an overlay or with varying buccal cusp thicknesses of 1, 2, or 3 mm. In total, there were nine experimental groups with 10 in each group (n=10). The prepared teeth were digitally scanned, and the restorations were designed and fabricated from IPS e.max computer-aided design (CAD) software using a CAD/CAM machine (CEREC MC XL, Dentsply Sirona, Bensheim, Germany). The restorations were cemented with resin cement (Panavia V5). All samples underwent thermocycling and dynamic fatigue simulating approximately one year of actual use. All the teeth were then subjected to compressive load until the point of fracture, and the mode of each fracture was analyzed. Results show that the fracture resistance of the restored groups was significantly higher than the nonrestored groups ( p <0.001) and the sound teeth ( p <0.05). Crucially, this study determined that 1 mm of remaining buccal cusp thickness in bondable partial coverage restorations for maxillary premolars is sufficient to withstand normal use of the tooth without breakage.