I read this article with great interest and would like to add some important information concerning the current international literature, which should be additionally provided to readers.

I would like to comment on the statement “For the diagnosis of FAI, sonographic assessment compared to MRI has not been evaluated to our knowledge” in the introduction section.

There are already studies that have studied the sonographic diagnosis of FAI compared to the MRI or CT diagnosis of FAI (F.M. Buck et al. Eur Radiol 2011; 21: 167–175, Y.C. Hsu et al. Ultraschall in Med 2012; 33: E158-E165, S. Lerch et al. Int Orthop 2013; 37: 783–788, D.J. Robinson et al. Ultrasound Med Biol 2018; 44: 495–501). Furthermore, previous studies made this comparison with conventional radiography (S. Lerch et al. Arch Orthop Trauma Surg 2016; 136: 1437–1443, L. Mandema et al. Phys Ther Sport 2018; 32: 167–172). Postoperative ultrasound follow-up after arthroscopic osteochondroplasty has also been investigated (S. Lerch et al. Int Orthop 2015; 39: 853–858). Within these studies, qualitative criteria (F.M. Buck, J. Hodler et al. Eur Radiol 2011; 21: 167–175) and quantitative criteria such as the alpha angle have been developed (S. Lerch, A. Kasperczyk et al. Int Orthop 2013; 37: 783–788) and analyzed (D.J. Robinson, S. Lee et al. Ultrasound Med Biol 2018; 44: 495–501, S. Lerch, A. Kasperczyk et al. Arch Orthop Trauma Surg 2016; 136: 1437–1443, L. Mandema, I. Tak et al. Phys Ther Sport 2018; 32: 167–172, S. Lerch, A. Kasperczyk et al. Int Orthop 2015; 39: 853–858). Furthermore, additional quantitative parameters such as anterior offset, offset ratio, and/or anterior femoral distance have also been the subject of sonographic investigations (Y.C. Hsu, Y.C. Wu et al. Ultraschall in Med 2012; 33: E158-E165, S. Lerch, A. Kasperczyk et al. Int Orthop 2013; 37: 783–788).

Further distinguishing criteria in the aforementioned studies were the ultrasound probe used and the rotational position of the hip joint. In the chronologically first publications, a convex transducer was used (F.M. Buck, J. Hodler et al. Eur Radiol 2011; 21: 167–175, Y.C. Hsu, Y.C. Wu et al. Ultraschall in Med 2012; 33: E158-E165). The subsequent publication used a linear transducer in ventral longitudinal section according to DEGUM specifications (S. Lerch, A. Kasperczyk et al. Int Orthop 2013; 37: 783–788). A linear transducer was also used in subsequent studies (S. Lerch, A. Kasperczyk et al. Arch Orthop Trauma Surg 2016; 136: 1437–1443, L. Mandema, I. Tak et al. Phys Ther Sport 2018; 32: 167–172, S. Lerch, A. Kasperczyk et al. Int Orthop 2015; 39: 853–858). A more recent study again used a linear transducer, but with application of the “virtual convex” technique (D.J. Robinson, S. Lee et al. Ultrasound Med Biol 2018; 44: 495–501).

Regarding the rotational position, it should be mentioned that in the first two studies chronologically, measurements were performed only in neutral position (F.M. Buck, J. Hodler et al. Eur Radiol 2011; 21: 167–175, Y.C. Hsu, Y.C. Wu et al. Ultraschall in Med 2012; 33: E158-E165). In 2013 (S. Lerch, A. Kasperczyk et al. Int Orthop 2013; 37: 783–788), ultrasound examinations were performed and compared in 20° internal rotation, neutral position, and 20° external rotation. Here, the strongest correlation with MRI was seen for the ultrasound measurements in internal rotation, but also for the measurements in neutral position. However, no correlation for the measurements in external rotation was seen. Based on these results, subsequent studies performed their ultrasound measurements in internal rotation or neutral position (D.J. Robinson, S. Lee et al. Ultrasound Med Biol 2018; 44: 495–501, S. Lerch, A. Kasperczyk et al. Arch Orthop Trauma Surg 2016; 136: 1437–1443, L. Mandema, I. Tak et al. Phys Ther Sport 2018; 32: 167–172, S. Lerch, A. Kasperczyk et al. Int Orthop 2015; 39: 853–858).

Thus, the authorsʼ statement in the discussion section that they recommend 15° of external rotation of the leg is not consistent with the current literature. Unfortunately, they do not provide their own ultrasound images contrasting Cam imaging in external rotation and internal rotation. Comparative measurements in internal rotation supporting the authorsʼ recommendation would also be an interesting addition and would help to advance the international scientific discussion on this still novel and exciting topic.

The authors state that “this study shows for the first time that the hip alpha angle FAI can be reliably determined using sonography”. However, it should be noted that studies on the reliability as well as on the sensitivity and specificity of sonographic FAI diagnostics have already been published. In these studies, the reliability of ultrasound diagnostics has been established (S. Lerch, A. Kasperczyk et al. Int Orthop 2013; 37: 783–788, D.J. Robinson, S. Lee et al. Ultrasound Med Biol 2018; 44: 495–501, S. Lerch, A. Kasperczyk et al. Arch Orthop Trauma Surg 2016; 136: 1437–1443, L. Mandema, I. Tak et al. Phys Ther Sport 2018; 32: 167–172). Now that the study of Schamberger et al. has also confirmed the reliability of ultrasound diagnostics of the hip joint, I am looking forward to further scientific studies on this topic.

Received: 31 January 2022

Accepted: 05 March 2022

Article published online:
14 June 2022

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