Achilles tendon enthesis behavior under cyclic compressive loading: Consequences of unloading and early...

Area of Science:

• Biomedical Engineering
• Musculoskeletal Biology
• Tissue Mechanics

Background:

• The Achilles tendon enthesis (ATE) is crucial for load transmission, composed of fibrocartilage rich in type II collagen and proteoglycans (PGs).
• Mechanical loading influences ATE remodeling, suggesting chronic load changes can alter its mechanical properties under compressive stress.

Purpose of the Study:

• To investigate the effects of hindlimb unloading and subsequent reloading on the fatigue behavior of the mouse Achilles tendon enthesis under cyclic compressive loading.
• To qualitatively assess changes in proteoglycans within the ATE fibrocartilage.

Main Methods:

• Cyclic compressive loading was applied to mouse ATE after 14 days of hindlimb suspension and 6 days of reloading.
• Mechanical parameters including displacement, hysteresis area, and stiffness were measured.
• Histological analysis of proteoglycans in the ATE fibrocartilage was performed.

Main Results:

• Unloading significantly impaired ATE mechanical behavior, with a 27% loss in displacement, a 27% decrease in hysteresis area, and a 45% increase in stiffness.
• Reloading resulted in a 28% decrease in hysteresis area and a 26% increase in stiffness compared to controls.
• These mechanical changes correlated with alterations in proteoglycans within the uncalcified ATE.

Conclusions:

• Chronic unloading and subsequent reloading significantly alter the mechanical properties of the Achilles tendon enthesis.
• Proteoglycan content in the ATE appears to play a role in load-dependent mechanical responses.
• Managing compressive loading is critical for prophylactic and rehabilitation exercises involving the ATE.