## **Core Concept**
Polymethyl methacrylate (PMMA) is a widely used material for denture base fabrication due to its biocompatibility, ease of handling, and cost-effectiveness. However, it has several limitations, including polymerization shrinkage and poor mechanical properties. One of its major disadvantages is its poor impact resistance.

## **Why the Correct Answer is Right**
The correct answer, **impact resistance**, is a significant concern with PMMA denture bases. Impact resistance refers to the material's ability to withstand forces that could cause it to fracture or shatter upon impact. PMMA has relatively low impact resistance compared to other dental materials, making it prone to fracture under sudden forces, such as those encountered during mastication or accidental drops.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While PMMA does exhibit some degree of water sorption, which can affect its dimensional stability and mechanical properties over time, **water sorption** is not highlighted as one of its major disadvantages in the context of the question.
- **Option B:** *Thermal conductivity* is not typically listed among the major disadvantages of PMMA as a denture base material. Although PMMA does have a relatively low thermal conductivity, this property is not usually emphasized as a significant drawback compared to its impact resistance or polymerization shrinkage.
- **Option D:** *Compressive strength* is actually one of the better mechanical properties of PMMA. While it does have limitations in terms of flexural strength and impact resistance, its compressive strength is relatively good, making it suitable for withstanding the forces of mastication in a denture base.

## **Clinical Pearl / High-Yield Fact**
A key clinical consideration when working with PMMA denture bases is to minimize the risk of fracture by optimizing denture design, ensuring adequate thickness of the denture base, and avoiding sharp angles or thin areas that could concentrate stress. Clinicians should also be aware of the potential for polymerization shrinkage and take steps to minimize its effects, such as using techniques that reduce residual monomer content.

## **Correct Answer:** C. impact resistance