Intoeing gait is a symptom that commonly appears in childhood and can also be seen in normal young children. Its causes can be metatarsus adductus, internal tibial torsion, increased femoral anteversion angle (FAA) or combinations of these [1,2]. Since the FAA can be increased or reduced in various diseases including not only intoeing gait, but also congenital hip dislocation, polio, and cerebral palsy [3,4], its measurement is clinically important and requires an accurate assessment. The FAA was measured with different methods in various research studies. One of the methods using the position of external rotation in greater trochanter of the femur as a physical examination is not meaningful due to the very low accuracy [5]. The plane radiography has several errors resulting in low accuracy and reliability [6,7]. Other approaches such as ultrasound [7], magnetic resonance imaging [8], fluoroscopy were attempted as well, but their examination procedures are too complicated, and the reliability and usefulness cannot yet be verified. The two-dimensional computed tomography (2D-CT) shows a meaningful correlation between tibia and FAA in the physical examination of the child with intoeing gait. Several studies [9,10,11] have reported a significant correlation between physical examinations and femoral with tibial torsion angles by using 2D-CT in children with intoeing gait. However, measurements with 2D-CT have some limitations. Firstly, the measurement error more frequently occurs with larger FAA [12]. Secondly, positional error occurs when the patient's position and lower extremities are not rotated, abducted and adducted for exact vertical alignment during CT image acquisition. Since we examine children, who have more possibility of poor cooperation for positioning, 2D-CT measurements may be clinically challenging. Recently, the advancement of image technology has led to the development of 3D-CT reconstruction, which is widely used currently for the diagnosis of various diseases [13,14,15,16,17]. In 2010, Gose et al. [13] morphometrically analyzed femur of children with cerebral palsy by using 3D-CT. However, they did not analyze the accuracy of femur neck angle nor femoral anteversion measurements; rather, they compared correlation and other parameters with different examination methods. Kim et al. [14], reported that the measurement method of FAA is too complicated for clinical application. Therefore, we developed clinically applicable 3D-CT measurement method, which could be used in outpatient clinic with less effects of positioning than 2D-CT. In this study, the FAA of the reconstructed femur was obtained from the young children who visited the foot clinic with intoeing gait using the 3D-CT reconstruction method and further investigated the measurement reliability of the intra-raters and the inter-raters, in order to test the usefulness and availability of the method.

The multi-channel CT has a benefit of obtaining images in a short period of time, as the image acquisition can be processed at a rapid rate if the number of sections of the detector increases. Thus, since it acquires images using multi-channels in a brief period of time, the corresponding radiographic exposure is smaller in comparison with the sequential scanning technology. In addition, it includes all the anatomical details of the scanned area allowing the attainment of the volume data, which can be used to reconstruct the 3D image with the aid of 3D image processing technique [18].

The FAA can be increased or decreased in various diseases such as intoeing gait, congenital hip dislocation, polio, and cerebral palsy. Thus, its measurement is clinically important and requires accurate assessment. Several research studies on FAA measures used various methods with different merits and demerits, accuracy, and reliability; however, most methods had low reliability. However, 3D-CT reconstruction reportedly has high accuracy and reliability. Furthermore, FAA measured directly from the cadaver femur and from the reconstructed 3D-CT, showed high accuracy and reliability [15]. The results of a recent study on patients with cerebral palsy indicated high inter-rater and intra-rater reliabilities of the FAA measurement regardless of the femur neck angle, confirming the usefulness of the 3D-CT reconstruction in the measurement of FAA [19].

Likewise, in this study, the FAA was measured through the 3D-CT reconstruction method, as reported by Kim et al. [17]; the medial and lateral condylar axis was determined as the line tangent to the most posterior aspects of medial and lateral condyles of distal femur from the caudocranial of 3D image. In addition, the reference axis for proximal was determined as the line connecting the 2 center points of the neck of femur and the head of femur, craniocaudally. This standard allows simple and clear identification of the anatomical reference point for the determination of axis for distal than other standards, and is considered to have fewer intra-rater and inter-raters errors. The reliabilities of the intra-raters and inter-raters measured at the femoral neck angle were lower than the condylar angle (Tables 2, 3). This is due to the varying appearances of the neck and head of femur that makes it relatively difficult to determine each of the center points accurately, causing potential errors between the intraraters and the inter-raters. However, the correlation coefficients of the reliabilities of intra-raters and inter-raters were high, i.e., 0.89 for the rater A, 0.96 for the rater B, and 0.89 between the raters, when this standard was applied as well. The correlation coefficients of reliabilities of the intra-raters for the measurement of FAA, the difference between the femur neck angle and condylar angle, was high with 0.97 for the rater A, 0.95 for the rater B, and 0.89 between the raters. The average of FAA values measured twice by each rater were 27.3 and 27.2 in rater A, and 29.7 and 30.1 in rater B, respectively (Table 4). The differences between 2 raters' measurements were from 2.4 to 2.9. The differences might be caused by measurement error, since the femoral head and femoral neck used for the femur neck angle measurement, are irregular circle and irregular cylinder, respectively. Also, the difference between first and second values of average FAA in rater A and rater B might be due to the similar pattern used by each rater at each time, which resulted in consistent differences. The measurement method itself may cause inevitable differences between raters due to the irregular surface characteristics. This might lead to different measurement methods used by each researcher of different centers. However, the differences between the 2 raters' measurements were acceptable, since angle differences were <3°, and angle differences of same raters were almost the same at each measurement. Therefore, this measurement method would be clinically acceptable. The 3D-CT method to measure the femur neck angle and condylar angle, as well as the difference (FAA) can be useful for accurate clinical diagnosis and monitoring the progress. The radiological FAA measurement using 3D-CT in this study is relatively convenient, fast and accurate, as compared with conventional methods.

However, the 3D-CT has some disadvantages since it can only be conducted in large hospitals and not in primary care facilities, and is more expensive than other diagnostic tools. Furthermore, even though the CT scan for the legs and arms has lower risk for the incidence of diseases like leukemia and brain tumor among children under the age of 10 years, due to radiographic exposures, as compared to the CT scans for brain, chest, and abdomen [20], frequent scanning needs to be avoided, as it may instantly increase the possibility of radiographic exposure.

In conclusion, the 3D-CT method for measuring the FAA showed high intra-rater and inter-rater reliabilities and can be considered for use in children with intoeing gait to compare with normal young children, as well as for deciding treatment criteria for intoeing gait in the future. It may also be used for the diagnosis of patients under the risk of intoeing gait with an increased FAA and for determining treatment effect and follow-up exams.