What is the function of the human toenails and why do we have them? I was trained by a functional anatomist who invested much time in teaching homologous (comparative) aspects of human anatomy and histology to that of mammals. When it comes to analogizing the human foot and toenails, the most common mammalian (quadruped) analogy or homologue to a human toenail is a mammalian claw or hoof, whose primary functions are for protection and adaptive tools for digging and grasping. However, in higher primates and humans, the function of the fingernails and toenails has adapted through diversification1 to accommodate finer dexterity and the “human machine interface,” such as tools and shoes.
It should come as no surprise that the claw of a mammal, bird, or reptile is susceptible to some of the diseases (dermatophytes) that afflict human nails, the keratin,2 or “horny plate,” which, in humans, covers part of the dorsal surface of the fingers or the toes. Weitzman and Summerbell3 use Ajello’s4 dermatophyte classification, which includes (1) zoophilic, which resides predominantly in animals and can result in inflammatory reactions in humans who have contact with infected cats, dogs, cattle, horses, birds, or other animals; (2) geophilic, which is usually recovered from the soil but occasionally infects humans and animals; (3) anthropophilic, which is specific to the human host and manifests as fungal infections in the web spaces of the toes and the nail beds. Dermatophyte (tinea) infections are named by the body site they infect.3 For example, mycotic invasion of the nail plate by a dermatophyte is termed tinea unguium. An infection of the nail by nondermatophytic fungi is called onychomycosis, which anatomically affects the subungual nail (distal and proximal) and often presents with superficial white mycotic infection (leukonychia trichophytica).5 The most common dermatophyte infections are T mentagrophytes and T rubrum.
Onychomycosis is a common condition that causes discoloration, thickening, and distortion of the nail; according to Paige and Nouvong,5 90% of these infections are caused by dermatophytic fungi; the remaining 10% are candidal infections in the diabetic and immunocompromised population. However, only approximately 50% of “nail dystrophy” can be ascribed to onychomycosis.4,5 Other common causes include psoriatic arthritis, lichen planus, eczema, and trauma. Risk factors of toenail onychomycosis among the population seen by the wound care specialist include advanced age, poor health, diabetes, tobacco use, critical limb ischemia, and poor tissue oxygenation. Systemic conditions involve immune compromise through illness and/or immunosuppressive drugs, frequent trauma, and a constantly moist environment.5,6 Although powerful systemic treatment options exist, liver toxicity is a serious issue with pharmacologic antifungal systemic agents. Therefore, a pharmacodynamics and pharmacokinetic consult should be obtained.5 Topical adjunct therapies are not as successful in treating onychomycosis.5,6 Infections of the toenail, trauma, and diabetes are associated with changes in the architecture of the toenail, which lead to ingrown toenails, pain, and infection.
This month’s continuing education article on “Influence of Smoking on Wound Healing in Patients Undergoing Nail Matrix Phenolization: A Prospective Randomized Clinical Study” by Álvarez-Jiménez et al (page 229) creates an opportunity to connect several important wound-healing concepts, using the ingrown toenail as a model for impaired wound healing. As mentioned previously, risk factors of nail disease and/or ingrown toenails include tobacco use and diabetes—an unfortunate combination that is ubiquitous in the wound care patients for whom we care. Álvarez-Jiménez et al make an eloquent case for smoking cessation in light of the negative influence nicotine has on tissue oxygenation and impaired wound healing. Smoking cessation programs are central to help prevent wounds, enhance the quality of wound healing, and improve the quality of life, and these programs need to be covered by healthcare insurance.7
Editor’s note: For an informative article to supplement this month’s continuing education, see the article by Richert 8 on surgical management of ingrown toenails.
Richard “Sal” Salcido, MD, EdD
1. Shubin N, Tabin C, Carroll S. Fossils, genes and the evolution of animal limbs. Nature 1997; 388 (6643): 639–48.
2. de Berker DAR, Baran R. Science of the nail apparatus. In: Baran R, de Berker DAR, Holzberg M, Thomas L, eds. Baran & Dawber’s Diseases of the Nails and Their Management. 4th ed. Oxford, England: Wiley-Blackwell, 2012: 1–50.
3. Weitzman I, Summerbell RC. The dermatophytes. Clin Microbiol Rev 1995; 8: 240–59.
4. Ajello L. Taxonomy of the dermatophytes a review of their imperfect and perfect states. In: Iwata K, ed. Recent Advances in Medical and Veterinary Mycology. Tokyo, Japan: University of Tokyo Press; 1977: 289–97.
5. Paige NM, Nouvong A. The top 10 things foot and ankle specialists wish every primary care physician knew. Mayo Clin Proc 2006; 81: 818–22.
6. Thomas J, Jacobson GA, Narkowicz CK, Peterson GM, Burnet H, Sharpe C. Toenail onychomycosis: an important global disease burden. J Clin Pharm Ther 2010; 35: 497–519.
7. Salcido R. Smoking cessation: an important factor in wound care. Adv Skin Wound Care 2007; 20: 576–8.
8. Richert B. Surgical management of ingrown toenails—an update overdue. Dermatol Ther 2012; 25: 498–509.