INTRODUCTION
The lips are central to many critical human functions, for example, by facilitating breathing, speech, eating, and intimacy.1-4 The lips are also a common characteristic defining facial aesthetics. Full lips are associated with youth and beauty, while the loss of exposed vermilion and vermilion bulk during aging is considered less attractive.5 The lips may be secondary only to the eyes6,7 in terms of facial visual importance.
Anatomically, the lips act as the transitional junction between the keratinized skin of the face and mucosa of the oral cavity.3 The skin of the lips generally has the same basic features as the skin in other areas of the body 8: it serves as a protective barrier between the human body and the external environment, and it plays a role in metabolic processes, resorptive and thermoregulatory functions, and immune defense.9 But just as specific characteristics differentiate the facial skin from skin elsewhere on the body, the lip vermilion differs from the skin that surrounds it.8 While both facial and lip skin are stratified squamous epithelium,10 lip skin is non-keratinized.3 Keratins are strong fibrous proteins that provide strength and resilience to skin cells.11 Additionally, the lips have far fewer layers than facial skin, so lip skin has comparatively less barrier function.3,12 Other characteristics of the lips that make them particularly vulnerable to environmental damage from wind, sun, temperature, and smoking include low-hydration state and low moisture-retaining capacity.8,12 For example, hydration is significantly higher on the cheek compared with lip skin.8 Furthermore, the lips have fewer natural moisturizing factors (eg, lipids)13 and lower levels of natural ultraviolet protectants (eg, melanin, urocanic acid).14 These characteristics make the lips prone to conditions such as cheilitis simplex, perleche, actinic cheilitis, which can carry malignant potential, and eczematous dermatoses (eg, allergic and irritant contact dermatitis).15 Other lip conditions can be caused by nutritional deficiencies, systemic diseases, psychiatric disorders, certain medications, and irritating products.15
Despite the importance of the lips and their vulnerability to environmental damage, there are no established clinical measures of lip health. Photonumeric scales can counterbalance
Anatomically, the lips act as the transitional junction between the keratinized skin of the face and mucosa of the oral cavity.3 The skin of the lips generally has the same basic features as the skin in other areas of the body 8: it serves as a protective barrier between the human body and the external environment, and it plays a role in metabolic processes, resorptive and thermoregulatory functions, and immune defense.9 But just as specific characteristics differentiate the facial skin from skin elsewhere on the body, the lip vermilion differs from the skin that surrounds it.8 While both facial and lip skin are stratified squamous epithelium,10 lip skin is non-keratinized.3 Keratins are strong fibrous proteins that provide strength and resilience to skin cells.11 Additionally, the lips have far fewer layers than facial skin, so lip skin has comparatively less barrier function.3,12 Other characteristics of the lips that make them particularly vulnerable to environmental damage from wind, sun, temperature, and smoking include low-hydration state and low moisture-retaining capacity.8,12 For example, hydration is significantly higher on the cheek compared with lip skin.8 Furthermore, the lips have fewer natural moisturizing factors (eg, lipids)13 and lower levels of natural ultraviolet protectants (eg, melanin, urocanic acid).14 These characteristics make the lips prone to conditions such as cheilitis simplex, perleche, actinic cheilitis, which can carry malignant potential, and eczematous dermatoses (eg, allergic and irritant contact dermatitis).15 Other lip conditions can be caused by nutritional deficiencies, systemic diseases, psychiatric disorders, certain medications, and irritating products.15
Despite the importance of the lips and their vulnerability to environmental damage, there are no established clinical measures of lip health. Photonumeric scales can counterbalance
