Physics of nail conditions: why do ingrown nails always happen in the big toes?

Cyril Rauch,Mohammed Cherkaoui-Rbati

doi:10.1088/1478-3975/11/6/066004

Abstract

Although surgical treatment of nail conditions can be traced back centuries to the writings of Paul Aegineta (625–690 AC), little is known about the physical laws governing nail growth. Such a poor understanding together with the increasing number of nail salons in the high street should raise legitimate concerns regarding the different procedures applied to nails. An understanding of the physics of nail growth is therefore essential to engage with human medicine and to understand the aetiology of nail conditions. In this context, a theory of nail plate adhesion, including a physical description of nail growth can be used to determine the transverse and longitudinal curvatures of the nail plate that are so important in the physical diagnosis of some nail conditions. As a result physics sheds light on: (a) why/how nails/hooves adhere strongly, yet grow smoothly; (b) why hoof/claw/nail growth rates are similar across species; (c) potential nail damage incurred by poor trimming; (d) the connection between three previously unrelated nail conditions, i.e. spoon-shaped, pincer and ingrown nails and; last but not least, (e) why ingrown nails occur preferentially in the big toes.

Highlights

The human nail is a keratinized structure and window to the nail bed, held in place by lateral nail folds
On the underside of the nail plate there is a complementary set of ridges as if the nail plates were held to the nail beds via a set of longitudinal rails
The aetiology is not fully understood a change in nail growth related to a weaker growth force with advanced age has been suggested

Summary

Introduction

The human nail is a keratinized structure and window to the nail bed, held in place by lateral nail folds (the cutaneous folded structures providing the lateral borders of the nail). Nail adhesion to the nail bed involves a number of well characterised microscopic adhesive units These units are apposed in a pattern along longitudinal epidermal ridges (or lamellae) stretching to the lunula, the half moon, pale convex portion of the matrix seen through the nail (figure 1(A)). Even though there exists an in depth and complex cross-species description of macroscopic/microscopic anatomical and cellular/sub-cellular structures, how nail and hoof growth inform their shape remains unclear. This apparently simple question is central to medicine as the first diagnosis of a nail/hoof condition by medics or vets is necessarily a physical and visual appraisal of the shape or form of the nail/hoof. In this context it is worth noting that nail cutting and hoof trimming have traditionally been advocated to alleviate pain and reshape the nail/hoof with time, there is little theory on which to ground these

C Rauch and M Cherkaoui-Rbati

Findings

Discussion