Combined cartilage thickness and mechanical property mismatch drives local strain amplification at the patellar osteochondral allograft interface

Patellar osteochondral allograft (OCA) transplantation treats focal cartilage defects, but donor-recipient cartilage thickness and mechanical compatibility are not routinely quantified during graft selection. This idealized computational study evaluated associations of donor-to-recipient (D/R) thickness ratio, absolute stiffness, stiffness contrast, and depth-dependent stiffness with interface-local strain. Two-dimensional axisymmetric finite element models were subjected to a spatially uniform nominal pressure of 1.00 MPa. Nine D/R ratios (0.33-3.25) and eleven homogeneous donor/recipient (DC/RC) Young's modulus combinations (2.50-7.00 MPa) yielded 99 homogeneous cases; nine functionally graded material (FGM) cases were evaluated. Primary outcomes were 95th percentile (P95) values of the nodewise compressive strain magnitude and tensor maximum shear strain within a fixed 0.25-mm local interface region of interest (ROI). This robust upper-tail measure is less sensitive than a single nodal maximum. In the matched homogeneous control, P95 values were 0.031 and 0.015. The largest homogeneous values were 0.412 and 0.347 at D/R=3.25 and DC/RC=7.0/2.5 MPa. Same-side concentration factors normalized local P95 to median strain in the same model's 0.75-1.00 mm far-field band. Pooled factors were 3.71 ± 2.04 for compression and 6.03 ± 3.79 for maximum shear in homogeneous cases (n = 198 model-sides), and 3.51 ± 2.17 and 5.69 ± 4.03 in FGM cases (n = 18). FGM values were lower than both extreme homogeneous mismatch references at every D/R ratio. Across formulations, thickness disparity remained associated with interface-local upper-tail strain. These results support evaluating donor-recipient cartilage thickness and material compatibility alongside surface congruity and osseous integration in future experimental and patient-specific modeling studies.