Mylène Jansen

Cartilage collagen structure after KJD and HTO using T2-mapping 303 15 Introduction Cartilage degeneration and substance loss are hallmark features of knee osteoarthritis (OA). Cartilage thinning is an important parameter in the diagnosis of knee OA, in staging its severity and as outcome measure for monitoring disease progression and treatment effect. 1,2 Traditionally, cartilage thickness changes have been evaluated indirectly from radiographic joint space narrowing. Nowadays, MRI is frequently used for semi-quantitative scoring of OA-related parameters, but also to quantitatively measure cartilage thickness. 3,4 Quantitative analyses typically rely on 3D spoiled gradient recalled imaging sequences with fat suppression, which have been validated for measuring cartilage thickness and volume, but do not provide much information about cartilage quality. 5 In order to measure quality, sequences that can visualize cartilage composition are required, such as delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) and T2-mapping. 6,7 dGEMRIC MRI allows to depict the distribution of glycosaminoglycans, whereas T2-mapping is sensitive to changes in water content and the collagen fiber network, reflecting collagen content and orientation. 8,9 Compared to healthy cartilage, OA cartilage shows higher T2 relaxation times, as a result of loss of collagen content and matrix anisotropy (structure) and subsequent increase in permeability and water content. 8,10–12 T2-mapping is frequently used in observational studies 5 , but has also been applied to investigate cartilage quality after cartilage defect treatment, where quality of the repair tissue can be compared to that of the surrounding native cartilage. 7,10,13 Cartilage T2- mapping is, however, not typically applied to evaluate the effect of joint-preserving surgical treatments for severe OA in whole (tibiofemoral) cartilage plates. Two such treatments are high tibial osteotomy (HTO) and knee joint distraction (KJD), both used in younger knee OA patients to postpone a knee arthroplasty (KA). In KJD, the tibia and femur are temporarily placed at a distance with an external fixation frame, unloading the tibiofemoral compartments. In HTO, the mechanical leg axis is (over)corrected by wedging the bone, unloading the most affected compartment (MAC) permanently. 14,15 Both treatments have shown not only good and comparable clinical results, but also cartilage restoration activity, demonstrated by radiographs, MRI-based cartilage thickness, and second- look arthroscopy as well as biochemical marker analyses. 16–23 Cartilage quality was previously evaluated with dGEMRIC, which showed that values after KJD and HTO treatment were on average not different from pre-treatment. 24 T2-mapping, however, has not yet been assessed and compared. The objective of this exploratory study was to evaluate cartilage T2 relaxation times as a measure of collagen structure before and after treatment with KJD and with HTO, and compare results between the 2 treatments. To compare these results to natural progression that might be expected in comparable, untreated OA patients, retrospective data from the Osteoarthritis Initiative (OAI) was used.