A horizontal line above the upper lip when smiling is often called a “second smile line.” This crease results from the skin folding as facial muscles contract during expression. While initially visible only during movement, changes in skin structure can eventually etch the line into the skin, making it visible even at rest. Understanding the underlying anatomy and factors that degrade skin quality helps manage its appearance.
The Anatomy of Dynamic Lines
The horizontal line above the lip results from strong, hyperactive lip elevator muscles working against the skin. Smiling involves a complex orchestration of muscles that pull the upper lip upward and outward, causing the overlying skin to bunch or fold horizontally.
The key muscles forming this crease are the Levator Labii Superioris and the Levator Labii Superioris Alaeque Nasi. These muscles originate higher on the face and insert into the upper lip and the area around the nose. Powerful contraction during a smile creates a strong upward pull that compresses the skin tissue beneath the nose.
This compression results in a temporary horizontal crease perpendicular to the muscle fibers’ pull. Since the skin is anchored to these underlying muscle fibers, the folding is a normal anatomical response. The line’s appearance during movement indicates an expressive smile pattern.
Factors That Worsen Line Visibility
While the crease is initially dynamic, intrinsic and extrinsic factors cause it to transition into a static line visible even at rest. The most significant factor is the age-related breakdown of the skin’s structural proteins. After age 20, the body’s production of collagen, the protein providing firmness, begins to decrease by about one percent each year.
The decline of elastin, which allows skin to snap back after compression, is noticeable around the perioral area. As skin loses elasticity and resilience, it cannot immediately smooth out after the powerful elevator muscles relax. This structural degradation allows the temporary dynamic fold to become permanently etched into the skin’s surface.
Environmental exposure significantly accelerates this process, with ultraviolet (UV) radiation being a major contributor to visible aging. Photoaging causes the breakdown and disorganization of collagen and elastin fibers faster than natural aging alone. The delicate skin around the mouth is vulnerable to this damage, making the horizontal crease more pronounced and permanent.
Repetitive motions reinforce the crease, similar to repeatedly folding a piece of paper. Although the line is caused by the upward pull of smiling, habits involving pursing the lips, such as smoking or drinking through a straw, contribute to overall perioral skin damage. These actions, combined with genetic predisposition, dictate how quickly the crease becomes permanently visible.
Addressing the Appearance of Lip Lines
Management focuses on preventative measures and cosmetic procedures targeting muscle activity and skin structure. Daily use of broad-spectrum sunscreen with an appropriate Sun Protection Factor (SPF) is the simplest preventative action. Avoiding habits like smoking and excessive sun exposure minimizes extrinsic factors that accelerate collagen breakdown.
For a dynamic line caused by muscle movement, neuromodulators, such as botulinum toxin, can achieve temporary muscle relaxation. Tiny, precise injections are delivered to the hyperactive lip elevator muscles, slightly weakening their pull during a smile. This subtly reduces the skin-bunching effect that causes the horizontal crease.
When the line has become static due to structural loss, dermal fillers containing hyaluronic acid can smooth the area. Rather than directly injecting the crease, which can lead to an unnatural appearance, the filler is often placed strategically beneath the line. This technique restores lost volume, acting as a mechanical block that forces the muscle to work against the filler, thereby softening the overlying fold.
Topical treatments like retinoids improve skin texture by promoting new collagen formation and accelerating cell turnover in the upper layers. For more stubborn static lines, light resurfacing treatments can further stimulate collagen production and improve the surface quality of the perioral skin.