The data available about the assessment of femoral and/or tibial torsions using US in children have been assessed in terms of validity and reliability, and the proximal and distal anatomical references have been described. Eight studies were considered eligible and were analysed.

Imaging techniques

All the studies assess the FT, but only one of them, Keppler [52], also analyses TT. Four different imaging techniques were used to assess the validity of US. CT is considered the gold standard for the assessment of torsional disorders [29, 36, 54, 55], and it is the technique used in 4 of the eight studies [47, 51,52,53]. MRI [51, 53], X-ray [38, 49, 50] and clinical assessment [51] were also used.

A fundamental difference between US and the other imaging tests is that only the surface of mature bone can be observed. This limitation results in differences in the reference planes utilised for US assessments compared to those for CT and MRI, which can capture cross-sectional images to identify epiphyseal planes. In addition, US cannot simultaneously capture both proximal and distal bone landmarks, necessitating separate acquisitions. This can be a limitation if the subject moves between assessments, potentially affecting the results. To address this, simple, secure and comfortable fixation systems can keep the child’s limb immobile on the examination table, preventing any movement between the proximal and distal acquisition [22, 56]. In contrast, CT, MRI and X-ray allow the simultaneous acquisition of both references.

Anatomical referencesProximal femur

Since the US only allows to observe the cortical surface to assess the proximal femur angle, a plane tangent to the bone surface, commonly the anterior plane, is often used. In contrast, the axis of the femoral neck can be used both by CT and MRI. Thus, most studies refer to “true anteversion” as determined by CT and MRI and “anterior anteversion” as determined by US.

Only Prasad et al. [48] assesses the true anteversion by US, since the studied population comprises children aged 0 to 6 months, in which the ossification of the femoral head is non-existent or very incipient, and thus the entire cartilaginous contour can be seen. However, he concludes that the reliability in the assessment of true anteversion with US is not acceptable at this age. Comparing true and anterior anteversion, Terjesen et al. [38] observes a consistent difference of 5° to 10° higher in the latter. Thus, he suggests a correction factor of 5° in children up to 12 years of age and between 5° and 10° in children over 12 years of age and adults.

Anatomical references in proximal femur

The anterior anteversion can be determined by different anatomical references, and they may influence the reliability, according to Elke et al. [50]. The references used in the selected studies are the anterior head to the trochanter tangent [38, 49, 53] and the anterior aspect of the femoral neck [48,49,50,51,52]. Elke et al. [50] recommends a degree of inclination of the probe over the anterior proximal femur that allows a correct visualization of the intertrochanteric plane and ensures maximum reliability during the measurement. Alternatively, Terjesen and Anda [49] uses 2 proximal references in his study and concludes that in children it is easier to determine the anterior tangent between the femoral head and the greater trochanter than the anterior tangent of the femoral neck, as the latter is too short. Berman et al. [47], the only author who does not recommend the general use of US for the assessment of FT, uses the posterior aspect of the femoral neck as a reference and finds differences greater than 10° compared to CT in 8 of the 19 femurs analysed. The soft tissues between the probe and the bone and the author’s recognition of some estimation of planes may have influenced the low accuracy of the assessments obtained.

Technique with horizontal probe vs inclined probe (tilted transducer technique)

Another feature that influences the reliability is the position of the ultrasound probe on the proximal femur. Berman et al. [47] and Prasad et al. [48] place the probe completely vertically by attaching a spirit level to it. Subsequently, the inclination of the femoral neck is calculated on the image obtained. With this methodology, the accuracy decreases as the torsional value increases, since the lateral area of the femoral neck will move further away from the probe, leading to distortions and measurement errors. To overcome this limitation, Elke et al. [50] proposes a variation of the methodology for large femoral true anteversions: he positions the subject with an internal hip rotation of 40°, thus increasing the parallelism between the femoral neck and the plane of the probe, which is placed completely horizontal, and then, these 40° are added to the value obtained. In his study, Terjesen and Anda [49] proposed a variation of the technique in which the probe is tilted over the proximal femur until it is displayed horizontally on the screen (tilted transducer technique). If an inclinometer is associated with the probe, it will directly provide the degree of inclination of the femoral neck. This is the most widely used technique in subsequent studies.

Distal femur

For the distal femur, also different anatomical references have been used: Keppler et al. [52] and Terjesen and Anda [49] used the posterior tangent of the femoral condyles. Gunther et al. [51], Terjesen et al. [38] and Tomczak et al. [53] used a simpler methodology: the horizontal intercondylar plane is inferred using the tibia as a perpendicular reference. This inference is made by flexing the leg vertically beyond the edge of the examination table so that the verticality of the leg assumes the horizontality of the femoral condyles. Prasad et al. [48] assesses infants (0 to 6 months old) by positioning them in lateral decubitus and with the knees flexed 90°, thus, the vertical represents the intercondylar plane. This inference of planes may detract from the validity of the methodology, as the assumed complete planar perpendicularity may not exist. On the other hand, it is much more repeatable, being useful in the monitoring of the same individual, where the initial error in the inference should not vary significantly in successive measurements.

Tibial assessment

Out of the selected studies, only Keppler et al. [52] analyses the validity and reliability of the tibial torsion. He uses the posterior tangent of the tibial condyles and the tangent of the distal anterior tibial face as references. Similar to the proximal femur, some of the reference axes used by CT and MRI to determine tibial torsion, are not detectable by US, for example the intermalleolar axis.

Several studies evaluate US in the assessment of tibial torsion [14, 21, 22, 39], but to the best of the authors’ knowledge, no study has analysed its reliability and validity in a paediatric population without neurological alterations.

The anatomical references used by the authors for the assessment of femoral and tibial torsion are listed in Table 4.

Validity of US vs other imaging tests

Regarding the imaging tests used to analyse the validity of the US, Berman et al. [47] and Keppler et al. [52] compare it with TC, Gunther et al. [51] and Tomczak et al. [53] compare to CT and MRI, and Elke et al. [50] and Terjesen [38, 49] use biplanar radiography.

Gunther et al. [51], Terjesen [38, 49] and Tomczak et al. [53] conclude that US is a good alternative, endowed with sufficient precision for routine clinical examinations and monitoring of FT in children. However, for the preoperative planning of torsional disorders, the use of CT, MRI or biplanar radiography is preferable due to their higher accuracy. One of the major drawbacks of US assessment of femoral and tibial torsion is that both ends of the bony structure are not visualised simultaneously. Thus, those methodologies that use both proximal and distal reference to determine torsion require the subject to remain completely immobile during the assessment [47, 49, 52]. Keppler et al. [52] discusses a method of assessment with US that incorporates markers on the probe and 3D reference systems, which eliminates dependence during assessment on the subject’s position or movements. This allows him to also indicate US for pre-surgical assessment of both FT and TT. Only Berman et al. [47] does not recommend the general use of US for FT assessment due to the low validity results obtained. To be noted, his study was conducted in 1987; the evolution of US apparatus and the protocols may have increased the validity of the assessment.

Evaluation of inter- and intra-observer reliability

Inter-observer and intra-observer reliability have been analysed by 4 out of the 8 studies selected [48, 51,52,53]. The results were considered good for FT. Gunther et al. [51] and Tomczak et al. [53] obtain identical reliability data for both interobserver (r = 0.88) and intra-observer (r = 0.88). These results are good but lower than the reliability obtained with CT and MRI, with r values greater than 0.95. Furthermore, Gunther et al. [51] also compares it with the reliability of the clinical assessment, which obtains low inter- and intra-observer values; r = 0.47 and r = 0.77, respectively. In the Prasad infant study [48], the reliability of FT assessment with US using anterior anteversion as a proximal reference was clearly superior to the true anteversion. US observation of the entire cross-section of the head and trochanter in infants is unclear, given the lack of sharpness of the posterior face. This leads to an inaccurate determination of the real axis of the femoral neck and a low reproducibility.

Keppler et al. [52] incorporates a 3D reference system to the US that allows a non-motion-dependent assessment of the patient during acquisition. This method obtains better reliability results to CT for both femoral torsion and tibial torsion.

Most of the studies analysed conclude that US is a reliable method for the assessment and follow-up of femoral torsional alterations in children. It is useful due to its immediacy, convenience and safety, as it does not expose the children to ionising radiation. However, the validity and reliability data obtained by MRI, CT and biplanar X-ray are usually superior to US. Thus, MRI and biplanar X-ray would be the methods of choice in the surgical planning of femoral torsional alterations in children, due to their null or low exposure to ionising radiation, respectively. Despite being considered the gold standard, the use of CT is not recommended for follow-up in children due to the high exposure to ionising radiation it represents. Data for TT are only available from the Keppler study [52].

The method proposed by Keppler [52] obtains excellent results in terms of both validity and reliability, only slightly lower than those obtained with CT. Thus, he is the only author who validated US not only for the follow-up of torsional alterations of the lower extremity but also for surgical planning in adults and children. Noteworthy, this study is the only one in which a 3D reference system is used, clearly increasing them.

While some studies report good reliability for femoral torsion, particularly in paediatric populations, there is limited data available for tibial torsion assessment without neurological alterations. Future research should address this gap to provide a more comprehensive understanding of the reliability of US in assessing lower extremity torsional abnormalities in paediatric population.