Background
Ingrown toenail or onychocryptosis is a common problem in routine clinical practice which leads to pain and disability. Several risk factors have been identified as associated with the pathogenesis. Extrinsic risk factors including improperly trimmed nails, tight or poorly shaped shoes, abnormal nail shape, and hyperhidrosis have been associated with the pathology [
1‐
3]. Intrinsic risk factors (congenital and heredity) have been also related to the pathology. Studies have noted anatomical abnormalities or certain forefoot shape as possible risk factors in developing ingrown toenail. Some authors have emphasized the internal pressure role that the distal phalanx on the lateral edge of the nail could play in developing onychocryptosis. Biomechanical imbalance between the great toe and the second toe in an ill-fitting shoe is the most commonly cited theory in the etiology. Some studies report a higher prevalence in people in which the first toe is shorter than the second toe (Greek foot) and also in people in whom the first and second toes are equal in length, but in whom the first metatarsal is shorter than the second (squared index minus foot type) [
4,
5]. However, other authors found a higher prevalence of onychocryptosis in subjects in which the hallux is longer than the second toe (Egyptian feet types) [
6].
Although many causes have been suggested based on clinical experience, current evidence suggests that the most probable cause is a combination of at least three of these factors [
7,
8]. Few studies have analyzed the association between abnormal hallux interphalangeal angle (HIA) and onychocryptosis with controversial results [
8,
9]. The main objective of the study was to analyze by a cross-sectional, analytical and observational case–control study the existence of a correlation between onychocryptosis and the presence of pathological hallux interphalangeal angle (HIA).
Results
The results of the measurements determined that the average value of the HIA from the study group was significantly higher than the control group (
p = .036) (Table
2). The percentage of subjects with HIA higher than 13.47° was 73.71 and 46.79 % for the study and control group, respectively. The percentage of individuals in the study group with HIA ≥ 15° was 69.72 %. No statistically significant differences were recorded in regard to the average HIA in the study group according to sex (Table
3). The maximum average value of HIA in the study group was recorded for the interval of subjects aged between 21–30 years old and the minimum average value aged 41–50 years old (Table
4) with statistically significant differences between age intervals (
P < .05). The Anova test found statistically significant differences in regard to the HIA according to the age range in both groups (
p < .001). The Scheffe multiple comparisons test determined the existence of statistically significant differences in regard to the HIA (
p = .002) recorded between the two age ranges 21–30 and 41–50 (Table
5). The ICC for intraobserver reliability was 0.96 and for interobserver reliability was 0.92. In regard to the frequency of nail edge affected, 50.64 % of those from the study group had both folds affected, 42.95 just the lateral edge and just 6.41 % for the medial edge. Aχ
2-Pearson correlation coefficient with continuity correction revealed there is no relationship between the existence of a pathological HIA (≥13.47°) and the affected nail edge (Table
6). In regard to the kind of digital formula recorded for those from the study group, the Egyptian foot digital formula was the most prevalent with 64.10 % of individuals followed by 25 % of individuals with a Greek foot formula and 10.90 % with square foot formula.
Table 2
Comparison of the mean HIA between groups (N = 312 toes, 165 patients)
Median (minimum, maximum) | (12.73, 14.20) | (16.44, 18.33) | |
Mean ± SD | 13.47 ± 4.66 | 17.39 ± 6.00 | .036a
|
Typical mean error | .374 | .481 | |
95 % CI (lower limit, upper limit) | (12.73-14.20) | (16.44-18.33) | |
Table 3
Distribution of the mean of HIA in the study group according to sex
Males (63 subjects) | 15.59 ± 6.31 | .42 | 14.49-16.16 |
Female (102 subjects) | 15.33 ± 5.30 | .50 | 14.59-16.58 |
Table 4
Means distribution of HIA values for both groups included according to age group analysed
Age intervals | | |
10-20 years | 15.64 ± 4.57 | 13.95 ± 3.58 |
21-30 years | 18.67 ± 6.72 | 14.71 ± 5.19 |
31-40 years | 18.17 ± 4.21 | 10.92 ± 4.06 |
41-50 years | 14.69 ± 6.82 | 11.74 ± 4.16 |
Table 5
Comparisons of the mean HIA performed within the different age groups in the study group
Age group of 21-30 | | |
VS 10-20 | 2.02 | .69 |
VS 31-40 | 2.36 | .100 |
VS 41-50 | 4.02 | .002 |
Table 6
Contingency table between abnormal HIA and nail edge affected
HIA | <13.47º | Recount | 4 | 37 | 41 |
| | Expected frequency | 2.6 | 38.4 | 41 |
| ≥13.47º | Recount | 6 | 109 | 115 |
| | Expected frequency | 7.4 | 107.6 | 115 |
Total | | Recount | 10 | 146 | 156 |
| | Expected frequency | 10 | 146 | 156 |
Discussion
Traditionally hallux abductus interphalangeus or valgus deformity has been defined as an increase in the HIA of greater than 10° [
10,
11]. Existing prior studies have revealed that the existence of an abnormal HIA may be related to the risk of suffering from ingrown hallux nail. In a case–control study on a sample of 80 patients, Darwish et al. analyzed the HIA and HAVA in subjects with onychocryptosis in the hallux (cases) and compared this with 80 healthy subjects (controls). As in our work, the percentage of abnormal HIA in the study group was also significantly higher (85 %) compared to just 25 % in the control group (
p < .005). In the same way, the authors also found statistically significant differences (
p < .005) when they analyzed the HIA of patients with prior episodes of ingrown hallux nail in just one of the toes and compared this with the healthy contralateral toe that had never suffered from onychocryptosis. The study concluded that the existence of abnormal HAVA and HIA represents a risk factor for the pathogenesis of onychocryptosis of the hallux [
9].
Conversely, in a retrospective study performed by Kose et al. the x-rays of 81 patients with onychocryptosis were studied and compared to those of 100 healthy subjects. The HAVA, HIA and intermetatarsal angles (IMA) were measured; no statistically significant differences were found between the groups in regard to any of the three angles assessed. The average HIA value obtained in the case group was slightly higher but without any statistically significant differences (11.9° ± 4.1 vs. 12.0° ± 5.1;
p = .123). The authors conclude that the study results do not reveal a correlation between forefoot misalignments and the existence of onychocryptosis, and they suggest that management of the pathology should not be based on correction of forefoot misalignments but rather managing the pathology itself [
8].
In both studies, the authors set out as normal HIA values those that do not exceed 10° and that has been considered a reference value for adults subjects in most studies performed [
10,
12,
13]. However, this value does not correspond to observational studies performed on the general population; in fact, other authors establish mean reference values above 10°. Barnett in a study performed on 346 x-ray from feet in 173 British healthy subjects (mean age: 21.4 year) with shoes set out as mean HIA values for men 13° ± 3.72 for the left foot; 13.1° ± 2.34 for the right foot; and in women 13.3° ± 3.54 for the left foot and 13.5° ± 3.53 for the right foot [
14]. Sorto et al. determined a mean HIA value in x-rays from 100 ft in subjects of different ages and for both sexes of 13.4° and they considered feet with HIA above this value as pathological [
15].
The main aim of our work was to determine whether there is a direct correlation between the existence of ingrown hallux nail and the presence of a pathological HIA. Therefore, we have used a more precise system of measuring HIA than that used in prior studies. The mean HIA value obtained in the study group was significantly higher than the control group. The results indicate to us that it is difficult for a subject with HIA values ≥ 15.5° not to suffer or be a future sufferer of onychocryptosis in the great toe. This highlights the impact of abnormal HIA on the onset of ingrown hallux nail. The authors recommend appropriate follow-up of these subjects to assess need for realignment surgery.
In the study performed by Darwish et al., the measurement technique used may have caused significant margins of error in measuring the angles studied, because these were performed with manual markers and rulers [
9]. In the same way, in the study by Kose et al. the angles were measured by a collaborator outside the study, but without specifying the method used for this measurement [
8]. The results obtained in this study are supported by the measurement technique used to measure the different angles. Barnett attributed the angular relationship between the longitudinal bisections of the distal and proximal phalanges of the great toe to two factors: an oblique orientation of the articular surface of the head of the proximal phalanx (obliquity), and an asymmetrical form of the distal phalanx itself (asymmetry) [
14]. For Sorto et al. and Duke et al. the HIA value is in turn the sum of the three angles; angle of obliquity, asymmetry and combined joint deviation (angular relationship between the articular surfaces of the base of the distal phalanx and head of the proximal phalanx) which have been appropriately taken into consideration to perform this work [
15,
16]. Farber et al. revealed that measuring certain angles on x-rays using a digital system is fully valid and leads to improving inter and intraobserver reliability in comparison with the use of the analogous technique by means of goniometer and pencil [
17]. The program used in this work has the property of increasing the image size by zoom without distorting it; this enables reducing the margin of error when we mark the points that serve as a reference to determine the mean axis of the phalanges.
The possible relationship of HIA with age was part of a secondary aim that we had not set in this study. The age range with a higher HIA value was 21–30 and the lower value 41–50. In regard to sex no statistically significant differences were found in regard to HIA values recorded in both groups. In regard to the digital formula, we found that 64 % of the patients in the study group had a Greek foot digital formula. These results coincide with those reported by Viladot and are substantially different to those obtained by Günal et al. [
4,
18]. We agree with the first author who found more pathological changes in the forefoot of patients with Egyptian feet. For the interphalangeal hallux deformity not only does this lead to deformity in the transverse plane but also in the frontal plane, which also causes more compromise of space with footwear in people with Egyptian foot digital formula. We consider therefore that individuals with Greek foot or square digital foot are less at risk of suffering from forefoot pathologies because of the footwear as a consequence of the lower vulnerability of these to the standard triangular form of the shoe tip which facilitates better accommodation of the foot to the footwear.
In regard to the frequency of nail edge affected, in the study of Darwish et al. the 85 % of patients in the study group had affected the lateral edge [
9]. Kose et al. excluded patients in their study group with medial or bilateral ingrowing nail edge [
8]. Günal et al. did not report any information regarding affected percentage of edges although reports that patients treated with a spacer between the first and second toes healed in about three weeks [
4]. We have found that more than half of the patients in our study group had affected both nail edges. Overall the lateral edge was affected by more than 93 % of cases.
The findings of this work are based on by a cross-sectional, analytical and observational case–control study, however, the limited population and the presence of others common risk factor related with the pathology may have introduced a selection bias. Future research should focus on validating the results of this study, a prospective study looking at the feet of an unaffected populations and then observing whether patients with increased HIA developed ingrown toenails would be definitive.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
All authors were fully involved in the preparation of the study procedures. The material within has not been and will not be submitted for publication elsewhere. All authors read and approved the final draft of the manuscript.