Integrating qPLM and biomechanical test data with an anisotropic fiber distribution model and predictions of TGF-β1 and IGF-1 regulation of articular cartilage fiber modulus.

A continuum mixture model with distinct collagen (COL) and glycosaminoglycan elastic constituents was developed for the solid matrix of immature bovine articular cartilage. A continuous COL fiber volume fraction distribution function and a true COL fiber elastic modulus (\(E^\mathrm{f})\) were used. Quantitative polarized light microscopy(qPLM) methods were developed to account for the relatively high cell density of immature articular cartilage and used with a novel algorithm that constructs a 3D distribution function from 2D qPLM data. For specimens untreated and cultured in vitro, most model parameters were specified from qPLM analysis and biochemical assay results; consequently, \(E^\mathrm{f}\) was predicted using an optimization to measured mechanical properties in uniaxial tension and unconfined compression. Analysis of qPLM data revealed a highly anisotropic fiber distribution, with principal fiber orientation parallel to the surface layer. For untreated samples, predicted \(E^\mathrm{f}\) values were 175 and 422 MPa for superficial (S) and middle (M) zone layers, respectively. TGF-\(\upbeta \)1 treatment was predicted to increase and decrease \(E^\mathrm{f}\) values for the S and M layers to 281 and 309 MPa, respectively. IGF-1 treatment was predicted to decrease \(E^\mathrm{f}\) values for the S and M layers to 22 and 26 MPa, respectively. A novel finding was that distinct native depth-dependent fiber modulus properties were modulated to nearly homogeneous values by TGF-\(\upbeta \)1 and IGF-1 treatments, with modulated values strongly dependent on treatment.