Which restoration option has the highest risk of fracturing the tooth?

Inlays, being indirect restorations that are custom-made to fit within the confines of the tooth structure, can indeed pose a higher risk of fracturing the tooth, particularly in areas that experience significant occlusal stress. Because inlays cover only a portion of the tooth, they often do not provide enough structural support for teeth that are already compromised or extensively decayed. This limited coverage can lead to situations where the remaining tooth structure is subjected to increased forces that it is unable to withstand, resulting in cracks or fractures.

In contrast, complete crowns encapsulate the entire tooth, providing comprehensive coverage and effectively distributing occlusal forces. Partial coverage crowns offer more support than inlays as they cover a substantial portion of the tooth's surface, albeit not as much as a complete crown. Onlays, while also a type of partial coverage, extend over the cusps and often offer more reinforcement than inlays, thus also reducing the likelihood of tooth fracture.

This fundamental difference in the nature of coverage and support leads to the conclusion that inlays, while aesthetically pleasing and minimally invasive, have a higher risk of leading to tooth fractures compared to the other restoration options listed.