Imaging tests for knee osteoarthritis (OA) commonly use X-rays to classify disease progression using the Kellgren-Lawrence (KL) grading system [1]. However, radiographic evaluations cannot effectively quantify cartilage degeneration or assess the degree of degenerative changes in the cartilage in situ in early knee OA.

Magnetic resonance imaging (MRI) is an accurate, objective, and minimally invasive procedure for joint evaluation. 3-T high-field MRI equipment and the development of imaging sequences have enabled qualitative evaluation of signal changes within the cartilage. This has made quantitative evaluation of cartilage morphology, such as cartilage thickness and volume, possible. Recently, compositional MRI quantitative evaluation methods have emerged, which can non-invasively evaluate cartilage disorders by sensitively capturing biochemical and microstructural changes in cartilage well before the changes are visible to the eye/arthroscopist. Compositional MRI focuses on early and potentially reversible injury stages, and with the development of preventive and regenerative medicines for cartilage disorders, its importance in the diagnosis of cartilage-related diseases is expected to increase in the future.

T1ρ and T2 mapping techniques are part of the quantitative assessment of cartilage components by compositional MRI. T1ρ and T2 mapping reflect changes in glycosaminoglycan (GAG) and collagen content, respectively [[2], [3], [4]]. T1ρ mapping can non-invasively detect and quantitatively evaluate arthropathic changes at an early stage of OA, which is related to decrease of glycosaminoglycan content, and is expected to be applied for understanding the pathogenesis of cartilage degeneration and determining the efficacy of treatment [2,5,6]. Compared to conventional two-dimensional (2D) T1ρ mapping, three-dimensional (3D) T1ρ mapping allows quantitative evaluation of the whole articular cartilage, not just one section, and this technique has been found to correlate with the gross evaluation of cartilage lesions [7]. However, the validity of this technique from a histological standpoint, such as its difference from T2 mapping and whether it reflects molecular structural changes in the cartilage matrix, has not yet been verified. This study aimed to histologically validate the 3D T1ρ mapping technique for evaluating cartilage degeneration using resection specimens from patients who underwent total knee arthroplasty (TKA). We hypothesized that there would be a correlation between quantitative assessments of cartilage degeneration and T1ρ values obtained from 3D image analysis. Histological assessment of cartilage degeneration was performed as the primary outcome measure and gross assessment was performed as a secondary outcome measure.