Background
Measurement of the relative difference between the lengths of the first and second toes as well as the first and second metatarsals is commonly used to examine the relationship between protrusion differences and foot problems such as metatarsal stress fractures [
1], myofascial trigger point (MTP) activation [
2], hallux rigidus [
3], hallux valgus [
4], hyperkeratosis [
5] and midfoot arthrosis [
6].
Several studies have utilised x-ray methodology [
4],[
7]-[
11] in order to estimate relative metatarsal length. However, this method has many disadvantages due to cost, accessibility and ionising radiation exposure [
12],[
13]. Osteological methods are based on direct bone measurements [
14]-[
16] on cadaveric models, which could be influenced by the presence of necrosis [
17]. Finally, clinical palpation is frequently used to identify metatarsal head position. However, many studies do not establish or describe the reliability and validity of this method. Spooner
et al.[
13] used clinical palpation in comparison to radiological measurements in order to establish metatarsal formula. Glasoe
et al.[
18] measured the relative length of the first and second metatarsals, using a caliper and reference bone marks such as the navicular tubercle and the dorsal crease of the first and second metatarsophalangeal joints, observing a poor inter-rater reliability. Based on this method, Davidson
et al.[
12] used a caliper in order to assess first and second toe and metatarsal length differences. They measured 36 feet of 18 participants, performing three different tests, one of which is described by Glasoe
et al.[
18]
. The aim of this research is to use a larger sample to study the intra-rater reliability of these three methods used by Davidson
et al.[
12]
.
Results
The sample consisted of 39 men and 62 women with a mean age of 23.7 (5.6) years, a mean body weight of 69.3 (15.5) kg, a mean height of 1.69 (0.96) m and a mean BMI of 24.0 (4.7) kg/m
2. The intra-rater reliability of the three different tests (ICC, SEM [
20] and MDC) is shown in Table
1. For the three tests, the ICCs are significant (p < 0.001). All the ICC values show a high degree of reliability [
22]. The small SEM and MDC values revealed a high level of accuracy. The value of the difference in millimeters as well as the percentage of accuracy, up to ±1 mm, obtained in the three tests is shown in Table
2.
Table 1
Intra-rater reliability of the three tests
TEST-RETEST 1 | 0.97 | 0.97 to 0.99 | <0.001 | 0.07 | 0.21 |
TEST-RETEST 2 | 0.86 | 0.80 to 0.90 | <0.001 | 0.10 | 0.30 |
TEST-RETEST 3 | 0.88 | 0.82 to 0.92 | <0.001 | 0.11 | 0.31 |
Table 2
Difference between and percentage accuracy of the three tests
TEST-RETEST 1 | ± 3 mm | 85% |
TEST-RETEST 2 | ± 5 mm | 62% |
TEST-RETEST 3 | ± 5 mm | 64% |
The different means of toe and metatarsal protrusion are shown in Table
3.
Table 3
Toe and metatarsal protrusion means of the three tests
TEST-RETEST 1 | 1.30 mm | 1.35 mm |
TEST-RETEST 2 | 1.75 mm | 1.45 mm |
TEST-RETEST 3 | 1.60 mm | 1.60 mm |
Discussion
The aim of this research was to determine the intra-rater reliability of these three tests used to measure toe and metatarsal protrusion of the feet.
The first test, measuring the difference between the ends of the first two toes, showed the highest reliability rate (ICC = 0.97), which is similar to the results obtained by Davidson
et al. (ICC = 0.98) [
12], although the sample used in this study is larger (202 feet versus 36 feet). Using this first palpation-based test, the mean protrusion of the first toe versus the second (Egyptian foot) and the second toe versus the first (Greek foot), was 1.30 and 1.35 mm, respectively. The ±3 mm difference in the results of this first test and the 85% accuracy up to 1 mm demonstrate the precision of this technique.
For the second test, Spooner et
al.[
13] marked the metatarsal heads when standing, in order to compare the results with radiographs. No statistical differences were found between the two methods of measuring length differences, but they did find errors in participants with the two first metatarsals of equal length. The ICC of 0.87 for this second test in our study showed a higher degree of reliability than that obtained by Davidson
et al. (ICC = 0.76) [
12]. The difference of ±5 mm in the results of this second test and the 62% accuracy up to 1 mm show that this technique is less specific than the first test. However, the differences of ±5 mm are smaller than those found by Davidson
et al. [
12] of up to 12 mm. Despite the accuracy of the technique, the palpation of the metatarsal heads and the caliper position may alter the results [
12],[
19]. Investigators should perform the test in an identical position in order to avoid measurement errors. For this reason, this is the most difficult technique for determining intra-rater reliability. In this second palpation-based test, the mean protrusion of the first metatarsal versus the second (index plus foot) and the second metatarsal versus the first (index minus foot), was 1.75 and 1.45 mm, respectively. In their study of 7167 feet using the radiographic method in 1949, Harris and Beath [
10] found a first-second and second-first metatarsal protrusion of 3 mm, with differences of between 1 and 12 mm. Hardy and Clapham [
9], with a control group of 504 feet in 1951, and Mancuso
et al.[
23], with a group of 100 healthy feet in 2003, found a first-second metatarsal mean of 2 mm
. In 2006, Domínguez
et al.[
7] found a second-first metatarsal mean of 1.88 mm. However, despite producing very similar results, the methods are different and not comparable.
For the third test, the ICC of 0.88 is higher than that obtained by Davidson
et al.[
12]. The difference of ±5 mm in the results of this second test and the 64% accuracy up to 1 mm shows that this technique is less specific than the first test. However, the differences of ±5 mm are less than the differences found by Davidson
et al.[
12] of up to 10 mm. The radiological technique used by Hardy and Clapham [
9], Mancuso
et al.[
13] and Domínguez
et al.[
7], is the most similar to that used in our study. This test may also be affected by the palpatory identification and the marking of the bony landmarks (metatarsal heads and navicular tubercle) [
12],[
19]. This test was first performed by Glasoe
et al.[
18], who found a poor inter-rater reliability (ICC = 0.36) in a sample of 15 subjects.
One of the strengths of this study was the confirmation of the greater intra-rater reliability of these three manual tests in determining toe and metatarsal protrusion first demonstrated by Davidson
et al.[
12] using only 36 feet. These tests could be useful in clinical practice, especially when clinicians are dealing with patients involved in physical activities. For example, a first longer toe may cause hallux valgus [
4] and is the best foot for ballet dancers [
3],[
5] with less pain and corns, while a second toe equal or longer than the first toe, is related to pain and swelling over the first metatarsophalangeal joint [
3],[
5] and hallux rigidus [
3]. In contrast, a correctly structured foot is the one in which both metatarsal have the same length or the first is longer than the second [
24]. An index minus foot could be related to Morton’s foot structure [
2],[
10], hallux valgus [
24], Freiberg’s disease [
25] and second metatarsal stress fractures in ballet dancers [
26],[
27]. In marathon runners, an index minus foot could be related to activation of peroneus longus myofascial trigger points (MTrPs) due to abnormal weight distribution and excessive pronation of the foot [
12],[
28],[
29]. Other muscles of the lower limb, whose muscles’ MTrPs could be activated are the vastus lateralis, tensor fasciae latae, gemellus superior and inferior, soleus, tibialis anterior and the peroneus brevis [
2],[
28]. However, an excessively index plus foot has been associated with hallux rigidus [
26], hallux limitus [
30] and hallux valgus [
4],[
7],[
9],[
20],[
23],[
31].
This study needs to be considered in light of three limitations. One limitation in this study was the inability to compare these tests with osteological or radiological methods in search of the gold standard. Our goal in the near future is to carry out further research in order to determine its validity and to study the inter-rater reliability of the three tests, previously studied by Glasoe
et al.[
18] for the third test. A second limitation is related to the sample of the study consisting of young, healthy adults aged 20–40 years instead of a younger population with growth physes still open, or older or diseased populations with deformities of the forefoot that could affect the results of the study. The final limitation to mention is the approach of analysing 202 feet from 101 participants as separate data items. This assumes that the characteristics of one foot are independent from the characteristics of the other foot, but while it is more common to have the same digital (76.2% versus 23.8%) and metatarsal formula (75.3% versus 24.7%) in both feet, sometimes there are differences. For example, a right Egyptian index-minus foot and a left square index-minus foot in the same subject [
19]. To overcome this issue, often only one foot per participant should be selected for inclusion in the analyses [
32].
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
CBM conceived of the study, was responsible for the design and data acquisition, contributed to the analyses and drafted the original manuscript. JZ contributed to the design, analysis and interpretation of data and the revision of the manuscript. JJ carried out the statistical analyses, contributed to the interpretation of data and the revision of the manuscript. RC and IP made substantial contributions to the drafting and revision of the manuscript. All authors read and approved the final version of the manuscript.