*2.4. Statistical Analysis*

Kappa statistics and generalized McNemar tests were used to assess and test for agreement. The shape of the first metatarsal head was polycotomized into three groups; "round", "flat", and "crest". As suggested by Landis and Koch, we interpreted the kappa values as follows: <0.20 indicates poor agreement, 0.21–0.40 fair, 0.41–0.60 moderate, 0.61–0.80 good agreement, and >0.80 indicates excellent agreement [44].

The McNemar Bowker test describes whether the marginal distributions of two measures are similar, as one would expect if the measures agree.

Data were analyzed using IBM SPSS Statistics, version 22 statistical software (SPSS Inc, Chicago, IL, USA). Statistical significance was set at *p* < 0.05, and Confidence Interval (IC) to 95%.

#### **3. Results**

The interobserver agreement by Kappa analysis (Table 1) showed a moderate agreement at 15◦ , good agreement at 0◦ , 5◦ , 10◦ , 20◦ and 25◦ and was excellent at 30◦ .


**Table 1.** Interobserver agreement about shape first metatarsal head in beam angle 0–30◦ .

Abbreviations: F, frequency.

To calculate intraobserver agreement, results were compared against angle beams. Table 2 shows the intraobserver A agreement regarding when the first metatarsal head gets distorted and appears crested. Results indicate that this occurs when the angle of the X-ray beam is at 20◦ of inclination. These results are similar for intraobserver B (Table 3), where the distortion of the same head occurs at 20◦ relative to 15◦ (*p* < 0.001).


**Table 2.** Intraobserver A agreement about shape first metatarsal head in beam angle: 0◦ vs. 5◦ , 5◦ vs. 10◦ , 10◦ vs. 15, 15 vs. 20◦ , 20 vs. 25◦ , 25◦ vs. 30◦ .

Abbreviations: F: frequency. \* The McNemar Bowker test was used to compare the relative prevalence of the different grades and is given as *p*-values.

**Table 3.** Intraobserver B agreement about shape first metatarsal head in beam angle: 0◦ vs. 5◦ , 5◦ vs. 10◦ , 10◦ vs. 15, 15 vs. 20◦ , 20 vs. 25◦ , 25◦ vs. 30◦ .



**Table 3.** *Cont*.

Abbreviations: F, frequency. \* The McNemar Bowker test was used to compare the relative prevalence of the different grades and is given as *p*-values.

Finally, after dissecting the 103 anatomical specimens, we found that all the first metatarsal heads had a round shape and none with a square shape or a crested head, showing perfect intra and interobserver agreement.

#### **4. Discussion**

The purpose of this study was to determine the presence of a distortion effect in the first metatarsal shape, due to the angulation of the X-ray beam.

Most articles on the measurement of dorsoplantar radiographs report a 15◦ [33,42] or 20◦ craniocaudal tube angulation. The American Orthopaedic Foot and Ankle Society has recommended a tube angulation of 15◦ [40].

We used observations of radiographs in this study. This technique used a craniocaudal tube angulation in a sagittal plane 0◦ , 5◦ , 10◦ , 15◦ , 20◦ , 25◦ and 30◦ , to evaluate the shape of the first metatarsal and including dissection of 103 feet embalmed cadaver by both observers.

We focused on the distorting effects of the tube angulation in the shape of the first metatarsal. We found that the distortion of the shape of the first metatarsal was minimal when the radiograph was made without angulation, or the beam angle was less than 20◦ .

Both observers agree that the shape of the metatarsal head is distorted in projections in which the X-ray beam with angulations is equal to or greater than 20◦ (Figure 2).

In this study, an association between a flat- or crested-shaped head of the first metatarsal with pathologies, such as hallux rigidus or hallux limitus, cannot be supported, because these shapes are the result of distortion caused by tube angulation.

Another reason for the distortion of the first metatarsal head with the X-ray beam correctly positions at 15◦ is that the normal first metatarsal declination angle is 21◦ angle between the axis of the first metatarsal and a horizontal linear [45], and in the flat foot, the first metatarsal declination angle is lower.

So, we postulated that when tube angulation in a sagittal plane is 15◦ [33,42] in a normal foot with a first metatarsal angle declination of 21◦ , the possibility of deformation or distortion of the first metatarsal head is minimized.

Instead, if the first metatarsal bone is dorsiflexed as a flat foot, the first metatarsal angle declination is lower, and the angle between the X-ray beam and the axis of the first metatarsal bone is a higher, thus maximizing distortion of the first metatarsal head.

In light of these findings, it seems necessary to control the beam angulation to 5–10◦ in dorsoplantar X-rays of the flat loading foot, to avoid the presence of the crested or flat shape, which are artifacts produced by the angulation of the tube.

**Figure 2.** Views of a first metatarsal head showing distortion to appear crest shaped in a radiographic image performed to 30◦ (**A**) and after dissection revealing a round shape (**B**).

#### **5. Conclusions**

All of the articles that we identified state that there are three types of shapes of the first metatarsal head, and all authors relate each type of head to the diagnosis of a foot pathology, such as hallux valgus or hallux rigidus. This study demonstrates that there is only a round shape, and not three types of metatarsal head shape, and therefore, no diagnoses related to the shape of the first metatarsal head can be made.

A clinician should be aware that, in patients with flat feet, dorsoplantar with weight projection should be taken at an angle of the 5 to 10◦ beam.

**Author Contributions:** All authors: concept, design, analyses, interpretation of data, drafting of manuscript or revising it critically for important intellectual content. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Case Report*
