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Orthopedic and Rehabilitation Engineering

Orthopedic and Rehabilitation Engineering - Poster Session D

Poster K6 - New Apparatus for Testing Tendon Wear Over Pulley Surfaces

Friday, October 25, 2024

3:30 PM - 4:30 PM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Shane S. Miller (she/her/hers)

Undergraduate Research Assistant
University of Tennessee, Knoxville
Clinton, Tennessee, United States

Primary Investigator(s)

DC

Dustin L. Crouch, PhD

Associate Professor
University of Tennessee, Knoxville
Knoxville, Tennessee, United States

Introduction: Throughout the body, tendons glide over various connective tissues to facilitate proper movement and support of the tendon [1]. A commonly studied example of this is the flexor pulley system of the hand, consisting of the annular and cruciate ligaments. These ligaments, composed of fibrous connective tissue and fibrocartilage, wrap around the flexor tendons of the digits [2]. They function to prevent bowstringing of the flexor tendon and transfer muscle force direction for proper control of the digit [2]. Naturally, these tendon pulleys experience smooth gliding and minimal wear, enabled by the lubrication of the synovium-lined tendon sheath [1]. When developing a synthetic tendon graft, it is therefore necessary to search for materials that are low friction and can withstand the incessant rubbing and wear against a pulley surface. Previous work highlights methods developed to measure the friction of cadaveric and in-vivo tendon-pulley systems [3,4], which allows for a basis to compare and refine synthetic tendon friction. No methods have been developed to test synthetic tendon wear affected by repetitive pulley interactions. Single point friction data is insufficient to model true tendon behavior, and long duration tendon-pulley wear is a vital factor to consider since friction behaviors change over time as wear occurs. To improve material selection, we designed a custom rig that tests the wear resistance of synthetic tendons as they undergo high-repetition pulley interactions.

Materials and

Methods: The wear testing rig was designed to meet several design requirements. First, it needed to closely mimic in-vivo conditions of the flexor pulley system of the thumb, since the system is well-studied and a potential clinical target for synthetic tendons. Second, to simulate the in vivo environment, the rig needed to fit in a heated water bath (FSGPD10, Fisher Scientific). Third, the rig had to be capable of applying and monitoring a tendon tensile load of at least 27 lbs, which is the reported maximum force generated by the flexor pollicis longus and extensor pollicis longus muscles, whose tendons cross the thumb [5]. Fourth, the rig needed to enable adjustment of the tendon wrapping angle around the pulley to account for different thumb joint angles. Fifth, the rig needed to generate relative motion between the tendon and pulley surface of at least 5 cm/s, based on thumb bone morphology and peak joint velocity of 375 degrees/second [6,7]. An initial model of the rig’s main body was created using CAD software (Autodesk Fusion) (Figure 1A).

Results, Conclusions, and Discussions: The current version of our custom wear-testing rig can test up to 8 tendons at a time and features a pulley surface that rotates against stationary tendons (Fig. 1B). The pulley surface is modeled using a polished stainless-steel shaft (⌀ = ¼”) that rotates via belt-and-pulley, controlled by a DC gearmotor (6Z916, Dayton). Speed control (4Z527, Dayton) allows for shaft speed up to 348 RPM, which equates to a linear velocity of 11.55 cm/s at the pulley-tendon interface for a ¼-inch diameter pulley. Tendons are loaded onto the rig on the right and left attachment plates, which feature adjustable angle of rotation and height. On the left attachment plate, samples are tied to hooks that have a threaded length of 2 ¼” to allow for tension adjustment (Fig. 1C). Compression springs are used between the plate and nut to maintain tension in the tendons even if they elongate due to creep or other phenomena. On the right attachment plate, samples run through freely rotating pulleys that are in series with compression load cells (LTH300, Futek) (Fig. 1D); the pulleys equalize the tension in each pair of tendons and reduce the total number of required load cells. The through-hole load cells sit against the outer surface of the attachment plate and have a maximum capacity of 250 lbs (or 125 lbs per tendon).
Instead of pulling tendons across a stationary pulley surface, the rig rotates the pulley surface against stationary tendons to simplify the design. Although the rotating pulley design does not match in-vivo tendon-pulley sliding mechanics, the same relative motion between the pulley and tendon surfaces can be achieved. A limitation of the design simplification is that tendon wear is isolated to the point of contact with the pulley; conversely, when tendons move across stationary surfaces in-vivo, the contact is distributed along the tendon. Additionally, biological tendons move back and forth, whereas the current design allows a single rotation direction. Future improvements to the device will include the addition of linkages that alternate the direction of pulley surface rotation to simulate the back-and-forth movement.

Acknowledgements (Optional): This study was funded by a seed grant from the University of Tennessee.

[1] Standring, Gray’s Anatomy E-Book, 41st ed., 2015. [2] Hirt, et al., Hand and Wrist Anatomy and Biomechanics, 1st ed., 2016. [3] Uchiyama, et al., J Orthop Res 13(1): 83-89, 1995 [4] Schweizer, et al., J Biomech 36(1): 63-71, 2003 [5] An et al., J Appl Physiol Respir Environ Exerc Physiol 54(3): 714-719, 1983 [6] Li, et al., Biomimetrics 8(2): 244, 2023 [7] Hoang, et al., J Hand Surg 46(2):149, 2021