Foot loading is different in people with and without pincer nails: a case control study

Abstract

Background: Recent studies suggest that pincer nails are caused by lack of upward mechanical forces on the toe pad. However, clinically significant pincer nails are also often observed among healthy walkers. It was hypothesized that in these cases, the affected toes do not receive adequate physical stimulation from walking and loading. To test this, the gait characteristics of pincer nail cases were assessed by measuring plantar pressure during walking.

Methods: In total, 12 bilateral pincer nail cases (24 affected feet) and 12 age- and sex-controlled healthy control subjects (24 ft) were enrolled in this prospective case-control study. Plantar pressure during free ambulation in both the barefoot and shod state was assessed using a digital pressure-plate system named S-Plate platform (Medicapteurs Co. France). First toe pressure and the frequencies of peak pressure in the first toe, metatarsal head, or other foot areas were calculated.

Results: In both the barefoot and shod state, the pincer nail group had significantly lower pressure on the first toe than the control group. In both the barefoot and shod state, the peak pressure area was mostly the metatarsal head area in the pincer nail group, whereas it was mostly the first toe area in the control group. Binomial logistic regression analysis revealed that peak pressure area was a significant risk factor for pincer nail development.

Conclusion: Walking behavior appears to contribute to pincer nail development. Pincer nails of walkers could be treated by correcting the walking behaviour so that more pressure is placed on the toe pad.

Keywords: Ingrown nail; Mechanical force; Nail deformity; Pincer nail.

Fig. 1

Measurement of nail parameters. The nail height, nail width and the thickness of the central part of the nail plates were measured by using a pair of calipers. The nail width at the distal ends and the nail height were used to quantify the curve index

Fig. 2

Pressure on the first toes of the pincer nail and control groups. The feet of the pincer nail group (n = 24) had significantly less pressure on the first toe than the feet of the control group (n = 24). Moreover, in the pincer nail group, the pressure on the first toe was significantly lower when the patients wore shoes than when the patients were barefoot

Fig. 3

Peak pressure areas of the feet of the pincer nail and control subjects. The metatarsal head area was generally the peak pressure area for the pincer nail group feet (n = 24), while the first toe was generally the peak pressure area for the control group feet (n = 24). * < 0.05. The standardized residuals are indicated on top of each bar

Fig. 4

Typical plantar pressure findings of pincer nail cases (a) and healthy walkers (b). Representative cases are shown. The first toe nail of the pincer nail case (4a, lefthand image) shows significantly greater inward curvature than the first toe nail of the control case (4b, lefthand image). The plantar pressures are indicated by a color gradient that ranges from red (very high pressure) to dark blue (very low pressure). The pressure on the first toe in the pincer nail case is much lower than the pressure on the first toe in the control case regardless of whether the feet were bare (lefthand foot in panels a and b) or shod (righthand foot in panels a and b)

Fig. 5

Hypothesis for the development of pincer and ingrown nails. Under normal conditions, the upward mechanical force and the constitutive nail shrinkage force are generally well-balanced and the nail remains normally curved. However, in some people, the constitutive nail shrinkage force may exceed the upward mechanical force, either because the patient lacks sufficient mechanical forces on the nail due to being bedridden or wearing ill-fitting shoes or because the constitutive nail shrinkage force is excessive due to genetic or systemic disease influences. In these cases, the nails curve inward. Inappropriate nail cutting promotes nail overcurvature by inducing inflammation and pain, which causes the patient to seek to reduce pressure on the nail further by reducing ambulation, changing the gait, or wearing ill-fitting shoes. In walkers, this vicious cycle may be abrogated by correcting the gait and shoes