Graduate Research Assistant University of Florida Gainesville, Florida, United States
Introduction:: The thumb is a complex structure with nine muscles that are crucial for its functionality. Yet, how changes within each muscle contributes to the progression of musculoskeletal diseases, such as osteoarthritis, remains poorly understood. For example, muscle fascicle lengths, which help determine the force-generating capabilities and operating range of muscles, may change due to disuse atrophy or altered neuromuscular control patterns. However, before fascicle lengths in the presence of disease can be understood, we must have robust baseline data from healthy adults. While extended field of view ultrasound (EFOV-US) imaging has been widely used to investigate the in vivo properties of lower limb muscles [1], data on thumb muscle fascicle lengths is particularly limited. Only one in vivo ultrasound study [2] and several cadaver studies [3-5] have previously examined thumb muscles. However, no previous studies have investigated all thumb muscles in a single study or by the same research team. In this context, the objective of this study is to establish and test an EFOV-US imaging protocol for assessing the anatomical features of thumb muscles in a young, healthy population.
Materials and Methods:: Four healthy adults (2 females, 2 males, age: 22.5 ± 0.58 years, height: 167.73 ± 6.95 cm, weight: 82.23 ± 22.96 kg) participated in this IRB-approved study. From each participant, ultrasound images of nine muscles [(abductor pollicis brevis (APB) and longus (APL), adductor pollicis (ADD), extensor carpi ulnaris (ECU), extensor pollicis brevis (EPB) and longus (EPL), flexor pollicis brevis (FPB) and longus (FPL), and opponens pollicis (OPP)] were captured using EFOV-US (Supersonic Imagine Mach 30). This preliminary analysis focuses on 6 muscles: APB, APL, ECU, EPL, FPB, and FPL. This selection enabled comparisons to the limited literature on in vivo fascicle lengths (APB, ECU, FPL) [2] and evaluated how the imaging methods generalized to both intrinsic and extrinsic muscles. For each intrinsic muscle, fifteen panoramic images were taken (LH20-6 probe), and one fascicle length per image was measured from the best ten images from each participant. For each extrinsic muscle, eight panoramic images were taken (L18-5 probe), and two fascicle lengths per image were measured from the best five images from each participant. Analyzed images were selected based on anatomical feature visibility and the absence of image distortion. Fascicle lengths were traced in ImageJ. The mean and standard deviations were calculated across subjects and compared to those reported in cadaveric [3-5] and in vivo ultrasound [2] studies.
Results, Conclusions, and Discussions:: This study demonstrates the reliability of the EFOV-US technique for all examined muscles, as indicated by small standard deviations of ±0.36 to ±0.60 cm across the four subjects. Fascicle lengths measured in this study are consistent with prior ultrasound literature, falling within one standard deviation of published data (Fig. 1A). However, among the examined muscles, three (APB, EPL, FPB) were within cadaver data, while three (ECU, FPL, APL) were not Fig. 1A&B). The difference in measurement results between cadaver and ultrasound studies may be due to the inactive state of cadaver muscles and dissection procedures, which may affect muscle architecture and fascicle length measurements.
This study successfully demonstrates the feasibility of obtaining in vivo fascicle length measurements for thumb actuators using EFOV-US imaging. The obtained data provides valuable information on healthy muscle properties and can serve as a reference standard for future studies. Immediate next steps will consist of increasing the sample size, expanding these analyses to all nine imaged muscles, and analyzing how subject size influences fascicle lengths.
Acknowledgements (Optional): : Funding from NIH NIAMS (R01 AR078817) is gratefully acknowledged.
References (Optional): : [1] Martin et al. (2001) J. Anat. (199):429-434 [2] Rakauskas et al. (2023) J. Biomech. 149(111512) [3] Jacobson et al. (1992) J Hand Surg Am. 17(5), 804–809 [4] Lieber et al. (1992) J Hand Surg Am, 17(5), 787–798 [5] Lieber et al. (1990) J Hand Surg Am. 15(2), 244–250