Comparisons of the analyte concentrations in cartilage extract. Mean (standard error of the mean) concentrations of (a) isomerized aspartate (IsoAsp), (b) protein, and (c) glycosaminoglycan (GAG) in different regions within osteoarthritis (OA) cartilage and non-arthritic cartilage. Analytes were normalized to gram of wet weight cartilage. Statistical significance was determined using the Wilcoxon signed rank test, and results represent six individual patients. Using all 14 available cartilage extracts, we found no significant differences between the non-lesional and lesional cartilage extracts but did observe significantly higher (P < 0.05) levels of protein in the extracts of non-OA cartilage compared with any OA cartilage samples.