Validation of an Empirical Damage Model for Aging and in Vivo Injury of the Murine Patellar Tendon

Abstract

While useful models have been proposed to predict the mechanical impact of damage in tendon and other soft tissues, the applicability of these models for describing in vivo injury and age-related degeneration has not been investigated. Therefore, the objective of this study was to develop and validate a simple damage model to predict mechanical alterations in mouse patellar tendons after aging, injury, or healing. To characterize baseline properties, uninjured controls at age 150 days were cyclically loaded across three strain levels and five frequencies. For comparison, damage was induced in mature (120 day-old) mice through either injury or aging. Injured mice were sacrificed at three or six weeks after surgery, while aged mice were sacrificed at either 300 or 570 days old. Changes in mechanical properties (relative to baseline) in the three week post-injury group were assessed and used to develop an empirical damage model based on a simple damage parameter related to the equilibrium stress at a prescribed strain (6%). From the derived model, the viscoelastic properties of the 300 day-old, 570 day-old, and six week post-injury groups were accurately predicted. Across testing conditions, nearly all correlations between predicted and measured parameters were statistically significant and coefficients of determination ranged from R² = 0.25 to 0.97. Results suggest that the proposed damage model could exploit simple in vivo mechanical measurements to predict how an injured or aged tendon will respond to complex physiological loading regimens.

Biomechanical and structural response of healing Achilles tendon to fatigue loading following acute injury

“…Future studies will investigate regional changes to the injury site that may further enhance the ability of measured structural properties to predict mechanical properties that may be reduced by averaging throughout the entire tissue. Work is being completed to develop a healing model (Buckley MR, 2013; Duenwald-Kuehl et al, 2012) that can predict mechanical properties throughout healing from structural properties.…”

The Tendon Injury Response is Influenced by Decorin and Biglycan

“…13,14 This may suggest novel differences in the biology of damage resulting from either aging processes or the repair response to injury. 7 For example, it is possible that higher strains pose a greater risk to healing tendons while prolonged repetitive stress poses a greater risk to the aging tendon.…”

The injury response of aged tendons in the absence of biglycan and decorin

“…Pilot studies demonstrated that murine patellar tendon mechanical properties decline significantly between 120 and 270 days post-natal (data not shown). Therefore, 120-day old mice were considered “mature” while 270-day old mice were considered “aged.” At 270 days post-natal, 30-35 aged animals of each genotype (WT: n=30, Bgn −/− :n=31, Dcn −/− :n=35) underwent bilateral surgery on their patellar tendons as previously described (Beason et al, 2012; Buckley et al, 2013; Dunkman et al, 2013b; Lin et al, 2006). Briefly, after cutting the skin, incisions were made along the side of the tendon to allow for a rubber coating backing to slip under the tendon.…”

“…Throughout preparation, the tendon was kept hydrated with PBS. The dynamic testing protocol consisted of 1) preconditioning, 2) stress relaxation at strain levels of 4%, 6% and 8%, 3) a sinusoidal frequency sweep (10 cycles at 0.01, 0.1, 1, 5, and 10 Hz) at each strain level, 4) return to gauge length, and 5) ramp to failure(Buckley et al, 2013; Dourte et al, 2012; Dourte et al, 2013; Dunkman et al, 2013a, b; Lujan et al, 2009). …”