The thumb carpometacarpal (CMC) joint affords the thumb prehensile power and precision. The biconcave saddle joint geometry and ligamentous structures of this articulating juncture between the trapezium (TPM) carpal bone and the first metacarpal (MC1) enable rotations and translations of the joint in 6 degrees-of-freedom (DOF) (Wang et al., 2018). Characterization of thumb range-of-motion (ROM) has been largely relegated to four primary directions: flexion, extension, abduction, and adduction (Holzbauer et al., 2021, White et al., 2018). However, in vivo studies of specific thumb tasks indicate activities of daily living require combinations of these primary directions, e.g., flexion and abduction for pinch (Cheze et al., 2011, Halilaj et al., 2014, Li and Tang, 2007). Pathological reduction in such function severely impacts patient quality of life (Tenti et al., 2020), yet existing surgical interventions insufficiently recoup healthy biomechanics, with many patients experiencing only moderate improvement in ROM and grip strength (Field and Buchanan, 2007, Giddins, 2020, Martou et al., 2004).

Inadequate treatments may reflect the fact that the multidirectional CMC biomechanics, defined as ROM and stiffness, have not been fully characterized. ROM has not been determined in paths other than primary directions, and the literature on CMC joint stiffness is sparse. While varied properties of CMC stabilizing ligaments (Bettinger et al., 2000, D’Agostino et al., 2014) imply joint stiffness variation by direction, to the authors’ knowledge, only a single study has measured joint stiffness as a function of direction, and it only examined volar, dorsal, radial, and ulnar subluxation and internal and external rotation (Shrivastava et al., 2003). Stiffness corresponding to primary or combined directions has not been reported.

An in vitro methodology for determining the multidirectional passive biomechanics of the thumb CMC joint would provide a more-complete understanding of CMC joint biomechanics. Additionally, across CMC joints with a variety of conditions, this method could be used to contextualize the effects of pathological or traumatic change in joint structure, informing the development of future therapeutic and surgical interventions, as well as the development and preclinical testing of thumb arthroplasty devices.

For this study, we developed a method of quantifying thumb CMC biomechanics in vitro that utilizes a 6 DOF robotic musculoskeletal simulation system (RMSS) to determine multidirectional ROM and stiffness (K) of the joint and present data from 10 healthy CMC joints. We hypothesized that multidirectional ROM would be greatest, and K least, in directions oblique to the primary directions of flexion, extension, abduction, and adduction, as activities of daily living requiring substantial ROM rely on these combined directions. We additionally hypothesized that Flexion + Extension CMC ROM would not be significantly different from the Abduction + Adduction CMC ROM, reflecting thumb ROM data acquired in vivo (, 2015, Goubier et al., 2009, Vocelle et al., 2020).