Nail Disorders

Antonella Tosti

• Publisher: Elsevier
• Year: 2018
• ISBN:
  9780323544344
• Topic: Medicine
• Subtopic: Dermatology

Get a quick, expert overview of nail diseases and procedures with this concise, practical resource. Dr. Antonella Tosti covers high-interest clinical topics including anatomy and physiology of the nail, benefits and side effects of nail cosmetics, nail diseases in children and the elderly, and much more.

• Covers key topics such as nail psoriasis, nail lichen planus, onychomycosis, traumatic toenail disorders, self-induced nail disorders, the nail in systemic disorders, nail disorders in patients of color, and more.
• Includes basic nail procedures useful to students, residents, fellows, and practitioners.
• Consolidates today's available information and experience in this important area into one convenient resource.

Chapter 1

Anatomy and Physiology of the Nail

Emma K.C. Ormerod, BM, BCh, MA, MRCP, and David de Berker, BM, BCh, MA, MRCP

Abstract

The nail has a unique structure and function and is an integral part of the distal digit tip. Understanding the anatomy, development, physiology, and function of the nail as a whole and its individual subunits is helpful to the clinician to aid assessment of the patient, diagnosis, and treatment. Abnormalities in the nail unit can be seen in both systemic and localized pathology and may result in functional and cosmetic problems for the patient.

Keywords

Anatomy; Components; Embryology; Embryology and development; Glomus bodies; Histology; Histopathology; Immune system; Nerve supply; Physical properties; Physiology; Production; Vascular supply

Key Features

• The nail forms an important part of the digit, with a wide range of sensory and motor functions

• There are at least six different subunits to the nail, all of which have a unique structure and function

• The earliest signs of nail development begin at 8–9 weeks’ gestation, and by week 32 nearly all the nail components can be identified in the developing fetus

• There is a rich vascular supply to the nail unit: paired digital arteries (a large palmar and a smaller dorsal) extending either side of each digit, which connect via two main arches (proximal and distal). Distal capillary plexuses ensure the flow of blood from more than one direction and origin

• Periungual soft tissues are innervated by dorsal branches of paired digital nerves, which divide into three branches just distal to the distal interphalangeal joint (DIPJ), to supply the different constituents of the nail unit

• Rates of growth are on average 3.5 mm/month for fingernails and 1.6 mm/month for toenails, but a wide range of factors can affect growth

Introduction

The nail unit is an integral part of the distal digit tip. It is highly versatile and has a role in a wide range of sensory and motor functions, including physical protection of the digit tip, fine manipulation of objects, enhancing sensory discrimination, scratching and grooming, and as an aesthetic accessory.^1,2 Abnormalities in the nail unit therefore result in functional and cosmetic problems.

The human nail is composed of at least six different specialized subunits. An understanding of the anatomy, development, and function of each of these subunits is crucial for the accurate assessment, diagnosis, and treatment of patients with nail disorders.

Anatomy

See Table 1.1 for descriptive terms, function, and common pathology of the nail subunits (Figs. 1.1–1.4).

Nail Matrix

The nail matrix generates the nail plate. Immunohistochemical staining with proliferating cell nuclear antigen or Ki-67 demonstrates minimal proliferative activity in the nail bed but substantial activity in the matrix, supporting the theory that the matrix is the source of the nail plate.⁷ Along with other studies, this has provided evidence that most, if not all, of the nail plate is generated from the “classic” (intermediate) matrix.⁸ Indeed, the majority of the nail plate cells are believed to come from just the proximal half of the nail matrix⁹; therefore, surgery to the distal matrix is less likely to cause scarring than surgery to the proximal matrix. In keeping with this theory, Zaias showed that a longitudinal biopsy >3 mm wide is likely to leave permanent dystrophy of the nail plate, and once occurred, damage is difficult to repair.⁸ In contrast such significant problems are not seen following damage to the nail bed.¹⁰

Nail Bed

The nail bed extends from the distal part of the matrix (clinically visible as a lunula) to the hyponychium. It supports the nail plate. It has a longitudinal epidermal ridge pattern along it, which corresponds to a complementary set of ridges on the underside of the nail plate and is thought to provide a strong binding between the nail bed and nail plate. The ridges are lost on the free edge of the nail, suggesting that this is softer than the main nail plate structure, and the nail bed loses ridges with loss of the overlying nail plate. It is probable that the ridges are generated at the lunula margin on the ventral nail surface. Small vessels of the nail bed are orientated in the same axis as these, which is demonstrated by the observation of splinter hemorrhages.

TABLE 1.1

Descriptive Terms, Function, and Common Pathology of the Nail Subunits

Nail Component	Description	Function	Common Pathology
Nail plate (nail)	Durable protective structure made of translucent keratin Tough, semitransparent, slightly convex sheet of tightly packed keratinized epithelial cells (corneocytes), which covers the nail bed and matrix. The upper surface of the nail plate is smooth, gaining thickness and density as it grows distally^3	Fine manipulation, scratching, physical protection of the digit Longitudinal and transverse axis curvatures allow it to embed into the nail folds, providing a strong attachment with a useable free edge	Ridging, pitting, dystrophy, complete/partial loss, color change, onycholysis
Lateral nail folds	Cutaneous folded structures (continuations of epidermis) into which the lateral margin of the nail plate is embedded either side of the digit, joining the nail bed medially	Guide and hold nail in place Help to attach nail to soft tissues and provide a protective seal at the lateral margin against penetration of foreign materials and organism...