Reliability, Reproducibility, and Validation of Five Major Histological Scoring Systems for Experimental Articular Cartilage Repair in the Rabbit Model

Histological evaluation of the repair tissue is a main pillar in the advancing field of experimental articular cartilage repair. Despite their widespread use, the major histological scoring systems for cartilage repair have seldom been validated. We tested the hypotheses (1) that elementary scores have a better reproducibility compared with more complex systems and (2) that the data from these different histological scores correlate with the DNA and proteoglycan contents of the repair tissue. A total of 1,165 observations of cartilage repair based on histological sections (n=233) from an experimental investigation on the repair of standardized osteochondral defects in vivo were made by three investigators with different levels of experience in cartilage research to determine the inter- and intra-observer reproducibility of elementary (Pineda and Wakitani score) and complex (O'Driscoll, Sellers, Fortier score) histological grading systems. DNA and proteoglycan contents of the repair tissues from simultaneously created defects were determined and correlated with histological (a) overall score values, (b) matrix staining, and (c) cellular characteristics of the five scores. Finally, applying the proteoglycan content as validating test, sensitivity, and specificity of the grading systems were assessed. All histological scores provided high intra- (Pearson r=0.92–0.99) and inter-observer reliability (intra-class correlation=0.94–0.99), low numerical intra- and inter-observer differences, and high internal correlations (Spearman's ρ=0.63–0.91). No disparity in reliability and reproducibility was detected between elementary and complex scores or between investigators with different levels of experience (all p>0.05). Individual histological overall score values did not correlate with proteoglycan contents but with DNA contents of the repair tissue (O'Driscoll, Wakitani, Sellers score). In all systems, proteoglycan contents did not correlate with matrix staining (all p>0.05), but histological cellular characteristics correlated with total cell numbers (p<0.001). These data indicate that both elementary and comprehensive histological scores are suited to quantify cartilage repair. Histological and biochemical evaluations may serve as complementary tools to assess articular cartilage repair in vivo.