Which of the following is not true regarding posterior composite class II placement?

The assertion that composite materials have the same coefficient of thermal expansion as tooth structure is not accurate. This is significant because a discrepancy in thermal expansion rates between the dental material and natural tooth structure can lead to stress and potential failure over time due to changes in temperature. When a material expands or contracts differently than the surrounding tooth, it can create gaps, leading to leakage or loss of retention.

In contrast, the other statements are true regarding posterior composite class II placement. The polymerization shrinkage of composite materials is a well-documented phenomenon, which occurs as the resin material hardens and can lead to gaps at the margins if not properly managed. Additionally, the placement of composite in posterior restorations can be challenging, as maintaining a dry field for the bonding process is critical to prevent contamination. Furthermore, achieving ideal proximal contacts is often difficult due to the fluid nature of composite materials during placement, which can lead to overfilling or underfilling those areas if not carefully controlled.