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Foot loading is different in people with and without pincer nails: a case control study
© Sano et al. 2015
Received: 31 December 2014
Accepted: 6 August 2015
Published: 19 August 2015
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.
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.
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.
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.
Fingernails play a key role in protecting the distal phalanges, enhancing tactile discrimination and performing fine manipulation [1–4]. They are also used for scratching and grooming. Toenails contribute to pedal biomechanics. During daily life, nails are constantly exposed to physical stimuli such as counter pressure and shear force. These mechanical forces may therefore play a marked role in nail configuration and participate in the development of nail deformities.
The term pincer nail was introduced by Cornelius and Shellery in 1968  and is defined as a transverse overcurvature of the nail plate. Another condition is ingrown nail, which is a painful inflammatory condition caused by a nail edge that has usually been rendered sharp by improper trimming . Both conditions accompany each other at high rates. Notably, although pincer and ingrown nails are common diseases, their precise etiologies remain unclear. Although poorly fitting shoes have been considered to be one of the most common causes of pincer nails, our previous study showed that bedridden patients who do not wear shoes or bear their own weight have a high incidence of pincer nails . Furthermore, we showed that the nails of fingers that are subject to little physical stimulation tend to curve inward while the nails of fingers that are subject to a lot of physical stimulation tend to have a flat shape [7–9]. While it is generally believed that nail shrinkage is an abnormal phenomenon , cumulative research at our hospital has led us to hypothesize that human nails are constitutively equipped with an automatic shrinkage function that allows them to adapt to daily upward mechanical forces. In line with this, we proposed that a pincer nail may be caused by the lack of appropriate mechanical forces on the nail . At face value, the fact that pincer nails are also often observed among healthy walkers could be seen to contradict the latter hypothesis. However, we speculated that, in these cases, insufficient physical stimulation of the toe pad during walking or loading may be the causative factor. To test this hypothesis, the present study on the gait characteristics of pincer nail cases was performed. For this, the plantar pressure during walking was measured.
Demographic characteristics of the study participants
Pincer nail (n = 12 cases)
Control (n = 12 subjects)
6 females, 6 males
6 females, 6 males
48.5 ± 17.8
46.8 ± 11.9
163.8 ± 9.2
165.2 ± 7.9
61.3 ± 15.3
61.9 ± 10.3
22.6 ± 3.8
22.6 ± 2.7
Informed consent was obtained from all individual participants included in the study. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
Measurement of plantar pressure
A digital pressure-plate system named the S-Plate platform (Medicapteurs Co. France) was chosen to measure plantar pressure, because of its portability, accuracy and cost-effectiveness. It was implemented as a device to determine plantar pressure profile after pilot studies confirmed its repeatability. The S-Plate platform can collect data from the right and left foot at the same time during natural walking and is used to measure plantar pressure on both bare and shod feet during free ambulation. The foot was divided into eight anatomical regions: first toe, small toes, medial metatarsal, middle metatarsals, lateral metatarsals, medial arch and lateral arch, as reported by Boyd LA et al. . Three trials were sequentially performed under each condition on the same day. The average pressure of each first toe area was calculated. Moreover, the peak pressure area, which is defined as the site that receives the most intense pressure in the plantar, in each foot and in both in the barefoot and shod state, was determined. Peak pressure is defined as the maximum pressure of the peak pressure area. The frequency with which the first toe, the metatarsal head, or the remaining areas of the foot was the peak pressure area in the barefoot or shod state was calculated for both groups.
Analysis of first toe nail configuration
All data were expressed as means ± SD. Statistical analysis was performed using Microsoft Excel 97–2003 (Microsoft, USA) and SPSS statistical software (SPSS, Chicago, IL). Continuous data were compared by using the Mann-Whitney U test. Categorical data were compared using the chi-square test or Yates chi-square test followed by Haberman residual analysis. Bonferroni’s correction was used to correct for multiple comparisons. The relationship between pincer nail development (the dependent variable) and clinical covariates, including weight, height, BMI, age, sex, peak pressure area (first toe vs. other areas of the foot), and the pressure on the first toe were assessed by binomial logistic regression analysis. p < 0.05 was considered to indicate statistical significance.
The Mann-Whitney U test revealed that the pincer nail group had a significantly higher mean first toe nail curve index than the control group (57.9 ± 24.9 % vs. 13.9 ± 5.4 %, p < 0.001).
Binomial logistic regression analysis revealed that peak pressure area (p = 0.015, odds ratios = 20.8, 95 % confidence intervals 1.82–237.3) was a significant risk factor for pincer nail development.
The nail consists of the nail plate, the nail fold, the nail matrix, the sterile matrix and the hyponychium [13, 14]. The nail configuration is influenced by genetic factors, the shape of the distal phalangeal bone , mechanical force [6–8], malnutrition , neurogenic factors [17, 18], blood flow [19, 20], and factors that cause the thinning and softening of nails . The causative factors of pincer nail include hereditary, inappropriate physical forces due to ill-fitting shoes, and inappropriate nail cutting [22–26]. Recent researchers have revealed that mechanical forces may have a particularly pronounced effect on nail configuration and may be involved in nail deformity development [7–10]. Namely, pincer nails may be caused by the lack of upward mechanical forces . However, pincer nails are also often observed among healthy walkers. In these cases, the physical stimulation on the toe pad that is delivered by walking and loading may be insufficient. To test this notion, the gait characteristics of patients with a pincer nail were assessed in the present study.
Indeed, plantar pressure analysis of these patients and healthy control participants revealed that the first toes of the pincer nail group experienced significantly less pressure during free ambulation than the first toes of the control group, both in the barefoot and shod conditions. Moreover, compared to the barefoot condition, the pressure on the first toes of the pincer nail group was significantly reduced by wearing shoes. Given that poorly fitting shoes are considered to be a major cause of pincer nails, this observation suggests either that patients with pincer nail tend to wear bigger shoes to avoid pain or that pincer nail develops because the first toe nail does not receive sufficient pressure from the ill-fitting shoes.
The gait characteristics of cases of pincer nail were assessed in this study. The results suggested that walking behavior may contribute to the development of pincer nail.
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