Biomechanical risk factors and flexor tendon frictional work in the cadaveric carpal tunnel Aaron M. Kociolek, Jimmy Tat, Peter J. Keir n Department of Kinesiology, McMaster University, Hamilton, Ontario, Canada article info Article history: Accepted 8 December 2014 Keywords: Carpal tunnel Tendon Subsynovial connective tissue Gliding resistance Viscoelastic abstract Pathological changes in carpal tunnel syndrome patients include fibrosis and thickening of the subsynovial connective tissue (SSCT) adjacent to the flexor tendons in the carpal tunnel. These clinical findings suggest an etiology of excessive shear-strain force between the tendon and SSCT, underscoring the need to assess tendon gliding characteristics representative of repetitive and forceful work. A mechanical actuator moved the middle finger flexor digitorum superficialis tendon proximally and distally in eight fresh frozen cadaver arms. Eighteen experimental conditions tested the effects of three well-established biomechanical predictors of injury, including a combination of two wrist postures (01 and 301 flexion), three tendon velocities (50, 100, 150 mm/sec), and three forces (10, 20, 40 N). Tendon gliding resistance was determined with two light-weight load cells, and integrated over tendon displacement to represent tendon frictional work. During proximal tendon displacement, frictional work increased with tendon velocity (58.0% from 50–150 mm/sec). There was a significant interaction between wrist posture and tendon force. In wrist flexion, frictional work increased 93.0% between tendon forces of 10 and 40 N. In the neutral wrist posture, frictional work only increased 33.5% (from 10–40 N). During distal tendon displacement, there was a similar multiplicative interaction on tendon frictional work. Concurrent exposure to multiple biomechanical work factors markedly increased tendon frictional work, thus providing a plausible link to the pathogenesis of work-related carpal tunnel syndrome. Additionally, our study provides the conceptual basis to evaluate injury risk, including the multiplicative repercussions of combined physical exposures. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Carpal tunnel syndrome (CTS) is a peripheral entrapment neuropathy due to compression of the median nerve at the wrist. While the etiology of CTS is multi-factorial (Staal et al., 2007), common histopathologic changes include fibrosis and thickening of the subsynovial connective tissue (SSCT) surrounding the flexor tendons in the carpal tunnel (Barr et al., 2004; Ettema et al., 2006; Jinrok et al., 2004). The SSCT loosely attaches the flexor tendons to the visceral synovium, and is comprised of collagen bundles that form adjacent layers interconnected by small perpendicular fibrils. During flexor tendon displacement, the SSCT is strained, with deep layers close to tendon moving before superficial layers near the visceral synovium (Guimberteau et al., 2010). Repetitive tendon motion is thought to promote shear damage at the tendon- SSCT interface, which is supported by the finding that fibrosis is exacerbated in SSCT layers adjacent to flexor tendon (Ettema et al., 2006; Schuind et al., 1990). Moore et al. (1991) used 12 biomechanical measures to model tissue loads in the wrist/hand and found that flexor tendon frictional work was the best predictor of injury. Tanaka and McGlothlin (1993) also developed a heuristic model of the wrist using frictional energy to predict work-related CTS. Both of these methods utilized the well- established belt-pulley model that likens a tendon gliding over a curved surface, such as the transverse carpal ligament in wrist flexion, to a belt around a pulley (Armstrong and Chaffin, 1979). Keir and Wells (1999) determined the radius of tendon curvature in the carpal tunnel decreased with both wrist flexion angle and tendon force magnitude thereby increasing normal stress on the median nerve in addition to the effect of tendon tension on contact stress. More recently, flexor tendon gliding resistance was experimentally measured (in vitro) within the carpal tunnel (Filius et al., 2014; Zhao et al., 2007). Zhao et al. (2007) found that tendon gliding resistance increased exponentially with tendon displacement, primarily due to the viscoelastic SSCT. The researchers also quantified the effect of wrist flexion angle on tendon gliding resistance; however, the slow test velocity of 2 mm/sec likely diminished viscoelastic shear-strain. There Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics http://dx.doi.org/10.1016/j.jbiomech.2014.12.029 0021-9290/& 2014 Elsevier Ltd. All rights reserved. n Correspondence to: Department of Kinesiology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1. Tel.: þ905 525 9140 x23543; fax: þ905 523 6011. E-mail address: pjkeir@mcmaster.ca (P.J. Keir). Journal of Biomechanics 48 (2015) 449–455