Carpal tunnel syndrome (CTS) is a peripheral median neuropathy that is commonly characterized by thickening and fibrosis of the subsynovial connective tissue (SSCT) surrounding finger flexor tendons. The degenerative process affecting SSCT can be initiated with excessive relative motion between the tendon and SSCT that ruptures interconnecting collagen. We used colour Doppler ultrasound to evaluate flexor digitorum superficialis tendon motion at two movement speeds with palmar compression, forearm compression, and partial ischemia (via brachial blood pressure cuff). Partial ischemia decreased SSCT displacement (22.9 ± 3.3 mm vs. 22.0 ± 3.3 mm; p = 0.015) while tendon displacement did not change. There was also a trend for increased relative tendon-SSCT displacement and shear strain index (SSI – relative displacement normalized to tendon displacement), which suggested partial ischemia might increase the strain in collagen that connects tendon and SSCT. Forearm compression decreased tendon displacement (28.5 ± 4.1 mm vs. 27.0 ± 4.6 mm; p = 0.043) while SSCT displacement also trended to decrease (24.0 ± mm vs. 22.5 mm; p = 0.059). With a lack of change in relative tendon-SSCT displacement and SSI, maintaining flexion-extension range of motion may have meant that forearm compression strained the musculotendinous unit at a location where SSCT was uncompromised. Palmar compression did not significantly affect any dependent motion variables, which suggested palmar compressive forces likely do not affect tendon-SSCT shear injury risk. The fast movement speed increased relative tendon-SSCT displacement and SSI while decreasing mean velocity ratio (MVR), which suggested greater tendon-SSCT shear strain in all baseline and compression conditions. Previously, increased relative tendon-SSCT displacement with fast movement speed was only shown in cadaveric investigations, but we confirmed this effect is transferable in an in vivo model. We induced ischemia proximally and found a reduction in SSCT displacement at the distal carpal tunnel. This finding suggests that the vascular network integrated within SSCT may play a role in altering tendon-SSCT excursion, independent of other external mechanical factors previously shown to increase relative motion and potential shear injury risk. Overall, this thesis showed that external mechanical compression at the palm or forearm likely do not negatively affect relative tendon-SSCT motion and that local ischemia and carpal tunnel blood flow should be considered when evaluating tendon and SSCT motion in relation to CTS development and progression. / Thesis / Master of Science (MSc)

Data from the McMaster Occupational Biomechanics Laboratory were consolidated to evaluate overall trends relating to tissue motion and blood flow in the carpal tunnel. Regarding tissue motion, displacements of the flexor digitorum superficialis (FDS) tendon and its subsynovial connective tissue (SSCT) were found to decrease with greater movement speed and a flexed wrist posture. Notably, changes to shear outcomes including relative tendon-SSCT displacement, the shear strain index (SSI), and maximum velocity ratio (MVR) demonstrate that greater movement speed contributes to SSCT damage according to the shear strain mechanism of injury theorised to promote carpal tunnel syndrome (CTS). Median nerve blood flow was also found to be implicated by wrist flexion, and appeared to decrease with greater CTS severity status. Finally, induced blood flow alteration of the carpal tunnel was found to elicit a median nerve blood flow response similar to the level found in CTS subjects, confirming its effectiveness as an intervention to study tissue motion in a CTS-like state. The influence of altered blood flow on tissue motion was differential, where the higher supradiastolic condition altered FDS displacement, and the lower subdiastolic condition affected SSCT displacement and SSI. These findings provide valuable evidence for changes in median nerve blood flow—and by extension, the local fluid environment within the carpal tunnel—not only being a consequence of SSCT fibrosis characteristic of CTS, but potentially also acting as a cause for said changes in carpal tunnel tissue motion. / Thesis / Master of Science in Kinesiology / This thesis aimed to evaluate and summarize key findings from the McMaster Occupational Biomechanics Laboratory relating to tissue motion and blood flow in the carpal tunnel. Performing repetitive finger movements faster and with a flexed wrist posture were found to decrease the distance travelled of the underlying finger tendon. Blood flow of the median nerve, which is implicated in carpal tunnel syndrome (CTS), is higher with forceful exertion and flexed wrist posture, and lower with greater severity of CTS. Finally, altering blood flow to the carpal tunnel was found to create a CTS-like environment, affected tissue motion in the carpal tunnel, and promoted movement disparity between these tissues that is associated with injury. This suggests that fluid/blood flow changes affecting the carpal tunnel is a plausible mechanism for increasing the likelihood of developing CTS.

Carpal tunnel syndrome is characterized by non-inflammatory fibrosis of the subsynovial connective tissue next to the tendons in the carpal tunnel, suggesting a shear injury owing to repetitive wrist and finger motion at work. I tested the effects of several well-established biomechanical predictors of injury on tendon and subsynovial connective tissue motion and shear in the carpal tunnel. These included non-neutral finger and wrist posture, speed of work, and forceful exertion. A cadaveric paradigm was used to directly measure tendon gliding characteristics, which showed that concurrent exposure to multiple biomechanical risk factors disproportionately increased tendon frictional work (Chapter 2). Given that tendon shear cannot be directly measured in vivo, colour flow ultrasound was used to assess relative motion between tendon and subsynovial connective tissue as a metric of shear potential (Chapter 3 − 5). Healthy participants completed middle finger movements while colour flow ultrasound imaged carpal tunnel structures and optical motion capture recorded finger joint kinematics. From the data, I developed regression equations to predict both tendon and subsynovial connective tissue displacements as a function of finger joint angles, which can be used as an ergonomic method to calculate the relative displacement (Chapter 3). Furthermore, relative motion between tendon and subsynovial connective tissue increased with wrist flexion angle, suggesting a greater susceptibility to shear injury during repetitive work when the wrist is flexed (Chapter 4). Using colour flow imaging, electrogoniometry, and fine-wire EMG, relative displacement was found to increase with tendon velocity and force (Chapter 5). Relative displacements in Chapters 3 to 5 were combined into a prediction model, and further compared to a tendon friction model derived from Chapter 2. The relative displacement model showed an additive relationship with combined physical exposures, including finger and wrist position, tendon velocity, and force (Chapter 6). The relative displacement model was more responsive to lower physical exposures whereas the friction model produced greater overall changes (with higher exposures). While both models infer a greater risk of shear injury due to repetitive and forceful wrist/finger movement, future studies will aim to set protective guidelines based on tendon motion and shear during hand-intensive work. Overall, this thesis showed that tendon friction and relative motion between tendon and subsynovial connective tissue both increased in response to well-established biomechanical risk factors. We propose the current models for use in ergonomics, representing a move towards mechanistic-based injury risk assessment of the wrist and hand. / Thesis / Doctor of Philosophy (PhD)