Spatial and temporal variations of viscoelastic material properties in developing and aging goat articular cartilage

Articular cartilage possesses limited regenerative capacity, making it vulnerable to degeneration. Understanding its mechanical properties and how they change with age is vital for advancing regenerative medicine. This study investigated the time-dependent viscoelastic properties of goat articular cartilage across different developmental stages using indentation testing and a two-term Prony series viscoelastic model.

Articular cartilage samples were collected from Saanen goats (n = 5-6 per group) across seven age-groups (neonates to aged). Stress relaxation indentation experiments were performed on femoral condyles using a 1 mm indenter to map viscoelastic properties. Cartilage thickness was also measured. A two-term Prony series expansion of the generalized Maxwell model was employed to analyze the stress relaxation data, yielding instantaneous modulus (E₀), equilibrium modulus (E_eq), and two relaxation times (τ₁,τ₂). Statistical analyses of changes with age were performed using one-way ANOVA tests with Tukey correction.

Cartilage thickness was highest in neonates, decreasing significantly by 6 weeks and plateauing thereafter. E₀ increased significantly from neonates to 6 weeks, plateaued until 6 months, and then declined by 12 months of age with a further decrease observed in the aged group. E_eq followed a similar pattern, peaking at 6 weeks and declining by older age. Both τ₁ and τ₂ progressively increased with age, with aged goats exhibiting the highest values.

This study comprehensively characterized the age-dependent viscoelastic properties of goat articular cartilage. The findings reveal a rapid stiffness maturation in early postnatal life, and a progressive increase in viscoelastic relaxation times with age. These findings highlight dynamic remodeling of time-dependent properties and the importance of viscoelasticity in cartilage maturation, adaptation, and therapeutic design.