3D full-field strain distribution across the osteochondral unit during osteoarthritis progression

Abstract

Strain patterns across the osteochondral unit play a key role in the early pathological changes that lead to osteoarthritis (OA). X-ray computed tomography and digital volume correlation compared residual strain distribution across the osteochondral unit during OA progression. Osteochondral plugs (Ø = 4 mm) from Dunkin–Hartley guinea pig tibias aged 2, 4 and 24 months (n = 6; representing pre-OA, early-stage OA and late-stage OA) were stained with a 1:2 hafnium-substituted Wells–Dawson polyoxometalate. Residual strains (εp1, εp3 and γ) were computed following unconfined compression. Applied loads corresponded to twice body weight (BW; L1; n = 3) or four times BW (L2; n = 3). OA severity was confirmed histologically. Under L1, peak strains were low in pre-OA (∼200,000 με, εp3; ∼160,000 με γ) and distributed evenly throughout the cartilage, calcified cartilage (CC) and subchondral bone (SB). In early OA, strain was localised to the cartilage surface (where degradative changes were also observed) and intensified with disease progression (∼300,000 με εp3; ∼200,000 με γ). Under L2, increased compressive and shear strain (peak εp3 = 258,395 ± 36,076 με; peak γ = 174,075 ± 32,522 με) were detected at the cartilage surface and in CC during pre-OA. Strain in the middle and deep zones increased as CC thickness increased. Microcracks in the SB in severe OA specimens were associated with high γ and high εp1. Changes in strain distribution are depth dependent and may be linked to the progressive thickening of CC during early OA development, which may even promote the disease.