Effect of elastin digestion on the quasi-static tensile response of medial collateral ligament

Abstract

Elastin is a structural protein that provides resilience to biological tissues. We examined the contributions of elastin to the quasi-static tensile response of porcine medial collateral ligament through targeted disruption of the elastin network with pancreatic elastase. Elastase concentration and treatment time were varied to determine a dose response. Whereas elastin content decreased with increasing elastase concentration and treatment time, the change in peak stress after cyclic loading reached a plateau above 1 U/ml elastase and 6 h treatment. For specimens treated with 2 U/ml elastase for 6 h, elastin content decreased approximately 35%. Mean peak tissue strain after cyclic loading (4.8%, p ≥ 0.300), modulus (275 MPa, p ≥ 0.114) and hysteresis (20%, p ≥ 0.553) were unaffected by elastase digestion, but stress decreased significantly after treatment (up to 2 MPa, p ≤ 0.049). Elastin degradation had no effect on failure properties, but tissue lengthened under the same pre-stress. Stiffness in the linear region was unaffected by elastase digestion, suggesting that enzyme treatment did not disrupt collagen. These results demonstrate that elastin primarily functions in the toe region of the stress-strain curve, yet contributes load support in the linear region. The increase in length after elastase digestion suggests that elastin may pre-stress and stabilize collagen crimp in ligaments.

Figure 1

A: The unloaded elastin network recoils through entropic and hydrophobic mechanisms. B: Upon loading (F), elastin can significantly elongate. C: Elastase digestion may leave fragments of various sizes, with crosslinks intact. D: Elastin is localized between, along and around collagen fibers and fascicles in ligament and tendon.

Figure 2

A: Elastin content decreased as elastase concentration increased. * - significant w.r.t. matched control; dashed line - linear curve fit. B: The change in peak tensile stress upon elastase degradation increased up to 1 U/ml elastase, above which the change remained constant. * - significant w.r.t. control; dashed line - inverse polynomial curve fit.

Figure 3

A: Elastin content decreased as treatment time increased. * - significant w.r.t. matched control; dashed line - linear curve fit. B: The change in peak tensile stress upon elastase degradation increased up to 6 hr of treatment, above which the change remained constant. Change in peak stress for control tissues remained constant regardless of treatment time. * - significant w.r.t. control treated; dashed line - inverse polynomial curve fit.

Figure 4

Tensile stress-strain curves as a function of control (6 h, buffer only) or elastase treatment (6 h, 1 U/ml). A: Control and control treated samples (N = 8). B: Control and elastase treated samples (N = 8). C: Sigmoid curves fit the stress differential between treatment cases and their respective controls. D: The instantaneous tangent modulus for elastase treated samples was significantly smaller than matched controls up to 7% clamp strain.