Effects of age on the collagen structure and mechanical properties of the human sclera

Abstract

 

Purpose

 

To determine the collagen structure and mechanical properties of the human sclera comparing for the effects of age.

 

Methods

 

The inflation response was measured for 12 posterior scleral cups from 10 human donors, age 46 to 91 (Coudrillier et al., IOVS, 2012). Wide-angle x-ray scattering (WAXS) was used to measure the collagen fiber structure of prior-inflated specimens (Pijanka et al., IOVS, 2012). Specimen-specific inverse finite element models were developed to determine the material properties (Coudrillier et al., Biomech. Model. Mechanobiol., 2013) by minimizing the difference between the measured and computed displacements. The stress response of the sclera was described using a distributed fiber model that used the WAXS measurements for the distribution of collagen fiber orientations. This allowed the mechanical properties of the sclera to be described by 3 parameters for the properties of the collagen fibers and background matrix. The outcome measures were the degree of anisotropy (ratio of preferentially aligned to total collagen fibers), the collagen fiber axial modulus, and the matrix shear modulus. A general linear statistical model was used to evaluate the effects of age accounting for the effects of diabetes diagnosis. The analysis for the degree of fiber anisotropy also considered the spatial auto-correlation of the different fiber alignment measurements.

 

Results

 

The degree of fiber anisotropy was significantly lower in older peripapillary scleras (Fig. a, p = 0.004), i.e. the fiber structure of older specimens was less oriented. Older age was associated with a significant increase in matrix shear modulus (Fig. c, p = 0.004). The fiber modulus increased with age, though the effect was not statistically significant (Fig. c). Diabetes was associated with an increase in the matrix modulus (p = 0.01) . The stress response of younger specimens was markedly more nonlinear than that of older specimens (Fig. b).

 

Conclusions

 

The mechanical properties and collagen structure of the peripapillary sclera change with age. These changes may significantly alter the biomechanical environment of the ONH.

 

 

Figure: (a) Degree of fiber alignment in the Nasal/inferior quadrant as a function of donor age. (b) Stress/strain response in the fiber direction. (c) Matrix shear modulus as a function of donor age. (d) Fiber axial modulus as a function of donor age.

 

Keywords: 413 aging • 473 computational modeling • 708 sclera