Achilles Tendon Ruptures in Middle-Aged Rats Heal Poorly Compared With Those in Young and Old Rats

Abstract

Background: Achilles tendon ruptures are painful and debilitating injuries and are most common in middle-aged patients. There is a lack of understanding of the underlying causes for increased rupture rates in middle-aged patients and how healing outcomes after a rupture might be affected by patient age. Therefore, the objective of this study was to define age-specific Achilles tendon healing by assessing ankle functional outcomes and Achilles tendon mechanical and histological properties after a rupture using a rat model. Hypothesis: Rats representing the middle-aged patient population would demonstrate reduced healing capability after an Achilles tendon rupture, as demonstrated by a slower return to baseline ankle functional properties and inferior biomechanical and histological tendon properties. Study Design: Controlled laboratory study. Methods: Fischer 344 rats were categorized by age to represent young, middle-aged, and old patients, and Achilles tendon ruptures were induced in the right hindlimb. Animals were allowed to heal and were euthanized at 3 or 6 weeks after the injury. In vivo functional assays and ultrasound imaging were performed throughout the healing period, and ex vivo tendon mechanical and histological properties were assessed after euthanasia. Results: Rats representing middle-aged patients displayed reduced healing potential compared with the other age groups, as they demonstrated decreased recovery of in vivo functional and ultrasound assessment parameters and inferior mechanical and histological properties after an Achilles tendon rupture. Conclusion: These findings may help explain the increased rupture rate observed clinically in middle-aged patients by suggesting that there may be altered tendon responses to daily trauma. Clinical Relevance: The results provide novel data on age-specific healing outcomes after an Achilles tendon rupture, which underscores the importance of considering a patient’s age during treatment and expectations for outcomes.

Plantar flexor deficits following Achilles tendon rupture: a novel small animal dynamometer and detailed instructions

“…However, validating these models with patient conditions should consider clinically relevantand preferably, clinically implementedfunctional outcome measures. Using our small animal dynamometer, we confirmed that a preclinical rat model (Freedman et al, 2016;Hillin et al, 2019;Huegel et al, 2019;Leahy et al, 2022) is a valid and relevant model to study the effects of Achilles tendon ruptures on plantar flexor function that directly translate similar measurements made in patients (Baxter et al, 2019(Baxter et al, , 2018Hullfish et al, 2019;Silbernagel et al, 2012). The angle-torque curves we measured in our non-surgically repaired rats compare favorably to the same measurements of plantar flexor function made in a cohort of patients treated non-surgically for an acute Achilles rupture (Figure 3).…”

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

“…In contrast to humans, hindlimb tendons of rodents in advanced age regularly exhibit higher stiffness or elastic modulus (Wood et al 2011 ; Danos et al 2016 ; Wood and Brooks 2016 ; Leahy et al 2022 ). The magnitude of the increase is typically close to 50%.…”