Measuring fascicle lengths of extrinsic and intrinsic thumb muscles using extended field-of-view ultrasound

Abstract

Complex motion of the human thumb is enabled by the balanced architectural design of the extrinsic and intrinsic thumb muscles. Given that recent imaging advances have not yet been applied to enhance our understanding of the in vivo properties of thumb muscles, the objective of this study was to test the reliability and validity of measuring thumb muscle fascicle lengths using extended field of view ultrasound (EFOV-US). Three muscles (FPL: flexor pollicis longus, APB: abductor pollicis brevis, and ECU: extensor carpi ulnaris) were imaged in eight healthy adults (4 female; age, 21.6 ± 1.3 years; height, 175.9 ± 8.3 cm)[mean ± SD]. Measured fascicle lengths were compared to cadaveric data (all muscles) and ultrasound data (ECU only). Additionally, to evaluate how fascicle lengths scale with anthropometric measurements, height, forearm length, hand length, and hand width were recorded. The EFOV-US method obtained precise fascicle length measurements [mean ± SD] for the FPL (6.2 ± 0.5 cm), APB (5.1 ± 0.3 cm), and ECU (4.0 ± 0.4 cm). However, our EFOV-US measurements were consistently different (p < 0.05) than prior cadaveric data, highlighting the need to better understand differences between in vivo and ex vivo fascicle length measurements. Fascicle length was significantly related to only hand length (r² = 0.56, p = 0.03) for APB, highlighting that anthropometric scaling may not accurately estimate thumb muscle length. As the first study to apply EFOV-US to measure thumb muscle fascicle lengths, this study expands the utility of this imaging technology within the upper limb.

Keywords: Hand; Medical imaging; Muscle; Muscle architecture; Upper limb.

Figure 1:

Imaging set-up for (A) extensor carpi ulnaris (ECU), (B) flexor pollicis longus (FPL), and (C) abductor pollicis brevis (APB). The origin and insertion sites for each muscle is marked by a black marker dot. The arrows indicate the direction the probe traveled during data collection. Due to differences in muscle size, the APB was imaged using the LH20–6 probe and the FPL and ECU were imaged using the L18–5 probe. Note, the LH20–6 probe is smaller in size, has lower depth penetration, and higher resolution than the L18–5 probe. The starting posture for each subject and muscle was controlled using a goniometer. As illustrated, for ECU, the shoulder and elbow were at 90° flexion and the hand was supported, such that the wrist was extended 20–30°. For FPL and APB, the shoulder was at 90° flexion, the elbow was fully extended, and the forearm was in a supine position, resting on an elevated desk. The thumb was in a relaxed, neutral posture for all muscles.

Figure 2:

Examples of ultrasound images of (A) extensor carpi ulnaris (ECU), (B) flexor pollicis longus (FPL), and (C) abductor pollicis brevis (APB). Each image was deemed measurable because the muscle body is clearly defined by white fascia and there are minimal image distortions. The yellow lines represent fascicles measured from each image. Note, fascicle length measurements were consistently taken in the mid-belly portion of the muscles, as shown.

Figure 3:

Fascicle lengths for the flexor pollicis longus (FPL), abductor pollicis brevis (APB), and extensor carpi ulnaris (ECU). Each boxplot displays the median, maximum and minimum, and upper and lower quartiles for each muscle data set. The dots represent individual fascicle measurements. With 10 measurements coming from each muscle and 8 subjects total, there are 80 individual data points for each muscle.

Figure 4:

Comparison of the total average fascicle lengths measured (solid bars) to values reported in the literature (non-solid bars). Error bars indicate standard deviation. Note, Brand et al. (1981), Amis et al. (1979), and Ruggiero et al. (2016, single cadaver) did not report a standard deviation in their results; Adkins et al. (2017) imaged ECU in two different wrist postures by two different sonographers, short posture by expert sonographer (wrist ulnarly deviated 30 degrees and extended 40 degrees) vs. neutral wrist posture by novice sonographer.

Figure 5:

Relationship between average fascicle lengths for the extensor carpi ulnaris (ECU), flexor pollicis longus (FPL), and abductor pollicis brevis (APB) with various anthropometric measurements. Each point represents a subject average, regression lines examine the relationship across subjects, and the shaded region represents the 95% confidence interval of the regression. Significant regressions are highlighted by inclusion of the r²- and p-values.