Bilaminar pattern of tibial condyle cartilage layer on the fat-suppressed 3D gradient echo images: artifact or structural and biochemical difference in composition of cartilage?

The purpose of this study was to examine if an unusual bilaminar pattern of lateral tibial condyle cartilage layer on the fat-suppressed three-dimensional (3D) spoiled gradient echo sequence is artifactual or correlates with structural and/or biochemical composition of cartilage. The laminar appearance of the lateral tibial condyle cartilage layer was studied on fat-suppressed 3D spoiled gradient echo MR images of the knee joint in 67 patients (mean age: 28y) performed at 1.0 Tesla. After i.v. administration of gadopentetate dimeglumine, diffusion of the contrast media into cartilage layer was qualitatively analysed over time on inversion recovery spin echo images of knee joints of five asymptomatic volunteers (mean age: 25y). In a patient with osteosarcoma and total knee replacement, MR examination of cartilage layer of lateral tibial plateau was compared with histologic specimens stained with Safranin-O, demonstrating proteoglycan distribution in cartilage. The retrospective analysis of 67 knee joints revealed a bilaminar appearance of lateral tibial condyle cartilage layer in the gradient echo images in the majority of cases (81%) with a statistically significant tendency to a trilaminar pattern in patients older than 20 years. With i.v. contrast administration, the contrast enhancement was only observed in the superficial zone of tibial cartilage layer. Histologic specimens in one patient demonstrated a good correlation between thickness of proteoglycan-free and proteoglycan-rich laminae of lateral tibial condyle on Safranin-O staining with hyperintense and hypointense zones, respectively, on corresponding fat-suppressed 3D spoiled gradient echo images (correlation coefficient of 0.87). Bilaminar pattern of tibial condyle cartilage layer on fat-suppressed 3D spoiled gradient echo images in younger subjects is not an artifact or an intrachondral lesion, but it may represent a regional difference in composition of extracellular cartilage matrix possibly produced by a highly-oriented collagen fiber structure associated with a high concentration of proteoglycans in the middle and deep portion of the cartilage layer.