Technical NoteMeasuring Femoral Torsion In Vivo Using Freehand 3-D Ultrasound Imaging
Introduction
Femoral torsion exhibits considerable natural variation within healthy adults (19.0 ± 11.3°) (Kulig et al. 2010) and more so in children for whom it changes during growth (Cibulka, 2004, Mudge et al., 2014). Increased or decreased femoral torsion may alter the mechanical action of muscles important for locomotion (Davids et al. 2002). Musculoskeletal modelling is an important tool for quantifying lower-limb muscle function and joint loading during gait (Pandy and Andriacchi 2010). Analyses based on this approach often use generic models derived from healthy adult cadavers (Arnold et al., 2010, Delp et al., 1990), which may lead to inaccuracies in muscle moment arms for individuals with abnormal femoral torsion (Arnold et al., 2001, Correa et al., 2011). Techniques to incorporate patient-specific anatomy are required for musculoskeletal modelling to reach its full potential in the clinical domain.
Currently, medical imaging methods such as computed tomography (CT) and magnetic resonance imaging (MRI) may be used to obtain accurate representations of musculoskeletal geometry (i.e., bony torsion) (Sangeux et al. 2015). However, CT involves exposure to ionising radiation, and MRI is both costly and difficult to implement in patients who cannot remain motionless for the duration of a scan (≈30 min). Physical examination is an alternative to medical imaging, but the reported accuracy of physical exam measures has been mixed (Davids et al., 2002, Milner and Soames, 1998, Ruwe et al., 1992), and a recent retrospective study revealed a poor correlation between physical examination and CT measurements of femoral torsion (Sangeux et al. 2014).
Ultrasound imaging offers a potentially cost-effective and time-efficient method for measuring femoral torsion and incorporating patient-specific bone geometry into a computer model of the human musculoskeletal system. Previous studies have measured femoral torsion by coupling an ultrasound transducer with an inclinometer (Bråten et al., 1992, Hudson et al., 2006, Miller et al., 1993). Implementation of this method can be difficult as the top edge of the ultrasound image and the bony landmark of interest must be aligned parallel to each other, and the subject must remain motionless between imaging of the proximal and distal ends of the femur.
In recent years, the coupling of 3-D motion capture with conventional brightness mode (B-mode) ultrasound has given rise to freehand 3-D ultrasound imaging (Gee et al., 2004, Treece et al., 2003). In this technique, reflective markers are rigidly attached to the ultrasound transducer, allowing 3-D tracking of the image plane. This technique has previously been used to investigate muscle and tendon morphology (Barber et al., 2011, Obst et al., 2013) and to identify the location of the hip joint centre (Peters et al., 2010, Sangeux et al., 2011), but to our knowledge no study has used this method to measure bony torsion in vivo.
The aim of the present study was to quantify the accuracy and repeatability of using freehand 3-D ultrasound imaging to measure femoral torsion in vivo. Accuracy was determined by comparing freehand 3-D ultrasound measurements of femoral torsion obtained in healthy patients against corresponding measures obtained from MRI. A repeated-measures analysis was also performed to evaluate the repeatability of the methods.
Section snippets
Participants
Ten healthy adults (five male, five female) with no history of gait pathology, joint disease, injury or neurologic problems were recruited and gave written informed consent to participate in this study (Table 1). Ethics approval was obtained from The Royal Children's Hospital and The University of Melbourne research ethics committees. A priori power analysis was performed to determine the required sample size. This was performed on the basis that a difference of 5° (standard deviation [SD]: 5°)
Results
Femoral torsion measurements from freehand 3-D ultrasound were, on average, smaller than the corresponding measurements obtained from MRI, with a mean difference of 1.8° (SD: 2.7°) (Table 1, Fig. 2). No relationship was observed between the amount of femoral torsion and the difference obtained by applying the two methods (Fig. 2). The Pearson correlation coefficient was 0.94. Repeatability coefficients for the MRI and ultrasound methods were 3.1° and 3.7°, respectively.
Discussion
The aim of this study was to quantify the accuracy and repeatability of using freehand 3-D ultrasound imaging to measure femoral torsion in vivo. Accurate in vivo measurements of femoral torsion were obtained from freehand 3-D ultrasound with an average difference from MRI of 1.8° (SD: 2.7°). The 3-D ultrasound method used anatomic landmarks different from those used during MRI, but this did not appear to affect the accuracy of the estimates obtained from ultrasound. The ultrasound and MRI
Acknowledgments
This research was supported by a grant from the Australian Orthopaedic Association. Partial funding from an Innovation Fellowship from the Victorian Endowment for Science, Knowledge and Innovation to M.G.P. is also gratefully acknowledged. The sponsors had no involvement in the study design, the collection, analysis or interpretation of data, writing or the decision to submit the article.
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Conflict of interest disclosure: No financial or personal relationships were conducted with individuals or organizations that could inappropriately influence or bias this work.