What Are Frown Line Fillers? Frown lines are the vertical lines that can develop between the eyebrows over time. Some are mainly caused by repeated muscle movement, while others become more visible even when the face is at rest. Anti-wrinkle treatment can help reduce the muscle activity that contributes to these lines. However, in some people, a deeper static line may remain even when the muscle is relaxed. In carefully selected cases, a small amount of dermal filler may be u