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Biomechanics

Biomechanics - Poster Session E

Poster E1 - The Effect of Age on Activity and Tendon Mechanics Following Overload

Saturday, October 26, 2024
10:00 AM – 11:00 AM EST
Location: Exhibit Hall E, F & G

Introduction:  Mechanical loading of tendons is necessary for tendon homeostasis. Moreover, moderate load increases lead to adaptation, and overload leads to degeneration. Clinical disorders with tendon degeneration include tendon rupture (without pre-existing pain) or tendinopathy (chronic pain). Incidence of these disorders increases with age.1 Aging is a complex process involving cellular, structural, and functional changes that decreases the ability of tendon tissue to maintain homeostasis. It is unknown how aging affects the tendon response to overload, which is due to an incomplete understanding of the mechanisms at which aging affects the tendon. Animal models can control age and loading to investigate the mechanisms that lead to tendon adaptation and degeneration. We previously introduced a tendon overload model using synergistic ablation (SynAb) in 3-month and 7-month old rats, where the Achilles tendons were resected to overload the adjacent plantaris tendon. We observed different responses, where younger animals appeared to have adaptive response, while older animals had a degenerative response.2,3 Therefore, the first objective of this study was to assess changes in activity and tendon mechanics due to normal aging from 3-months to 14 months. The second objective was to assess the effect of increased loading on rat activity and tendon mechanics 8-weeks post SynAb in 5-month old rats and 14-month old rats. We hypothesize that rodent activity will not change with age, however, mechanical properties will decrease with age. We further hypothesize that we will see a more degenerative change in tendon mechanics due to overload with increased age.

Materials and

Methods:  To assess the effect of age on activity and tendon mechanics, we used 3-month (n=10), 5-month (n=10), 7-month (n=6), 12-month (n=9), and 14-month old (n=4) female Long-Evans rats. To assess the effect of age on the tendon response to overload, a Young cohort of rats (3-months at the time of surgery, n=10/group) and an Older cohort of rats (12-months at the time of surgery, n=4/group) were assigned to two groups: the SynAb group where the Achilles was resected to overload the plantaris tendon and an Intact control group that did not undergo any surgery or incisions. Rats were euthanized after 8-weeks for mechanical testing.

Activity was evaluated using Infrared (IR) actimeters to obtain the total distance traveled (m) in the cage and the total number of times the rat stood on their hind legs (rearings) over a 12-hour dark cycle.

To assess tendon mechanics, each tendon underwent 5 cycles of preconditioning between 0 and 5% strain and then was ramped with a strain rate of 1%/s until failure. Tendon cross-sectional area (CSA), stiffness, modulus, and ultimate load, stress, and strain were calculated. To evaluate the effect of age, the activity and mechanics of the Intact group was assessed using a one-way ANOVA. To evaluate the effect of age on the response to overload a two-way ANOVA with post-hoc t-tests was used.

Results, Conclusions, and Discussions: There was no change in activity with increasing age (Fig 1A,B). The tendon CSA was not different between the 3, 7, and 14 month old groups, but was and significantly higher in the 5-month old rats compared with the other ages (Fig 1C ). The increase in CSA in the 5-month group was unexpected and is being further investigated. Tendon stiffness increased in the 7-month old group and then decreased in the 14 month-old group (Fig 1D). The increased stiffness in the 7-month group may reflect maturation of the tissue, although the cause for stiffness to return to lower levels at 14 months is unclear. The modulus and ultimate stress were not affected by animal age.

For the effect of age on the response to overload (SynAb group, Figure 2), in the young cohort, only the UT load was affected by overload and this load increased 21.7%. Thus, some positive adaptation occurred in the young animals due to overload, but it was relatively small. In contrast, in the older cohort, the overload (SynAb) group stiffness increased 17.8%, modulus decreased 53.0%, ultimate load increased 26.6%, and ultimate stress decreased 50.6% compared to the Intact group (Fig 2). This represents a degenerative response to overload in the older animals. However, the UT strain was significantly smaller in the Young cohort to the Older cohort, indicating that aging affects elongation of the tendon. These results support our initial hypothesis that aging would have a greater effect on the mechanical response to load than on younger rats due to an increased ability of the Young cohort to adapt to the SynAb treatment. In conclusion, rat activity stays consistent throughout our age groups, and tendon overload leads to a greater degradation in mechanical properties with aging.

Acknowledgements (Optional):  References
1) Svensson+ JAP 2016 2) Bloom+ ASME 2023 3) Lin+ ORS Tendon Conference 2024