Abstract: Purpose: The cross sectional analysis of differences in femorotibial cartilage morphology (MRI) between knees with and without radiographic joint space narrowing (JSN) is limited by a high inter-subject variability and by differences in cohorts with and without JSN. We compared contra-lateral knees of identical subjects, where one knee has JSN and one has not, to determine which cartilage (sub) regions display the greatest differences between JSN and non-JSN knees Methods: The subsample studied was drawn from 2678 cases from the OA Initiative (OAI public-use datasets 1.2.1 Clinical Data set and 1.B.1 Imaging Data set). Participants were required to report bilateral frequent pain, BMI>25, medial JSN (OARSI grade 1-3) in one knee, and medial JSN 0 in the contra-lateral knee. 73 participants (46 women, 27 men, age= 61±9 yrs., BMI= 31±4) were identified, of which 45 had medial JSN 1, 21 JSN 2, and 7 JSN 3 (affected knee). The medial femorotibial cartilages were segmented by 7 experienced readers using sagittal DESSwe MR images. The mean cartilage thickness was computed in 5 subregions of the medial tibia (MT) and in 3 subregions of the weight bearing (cMF) and posterior femur (pMF), respectively. The difference between JSN 1, 2 and 3 knees and the contra-lateral (non-JSN) knee was computed in % cartilage thickness and in standard deviations (intersubject SD) of non-JSN knees. Results: Knees with JSN had significantly thinner cartilage in the medial tibia (p<0.001) and weight-bearing medial femur (p<0.001), and less thinning in the posterior medial femur (p<0.05). Differences between JSN and non-JSN knees became greater with increasing JSN (Table 1). Within the medial tibia, the greatest differences in cartilage thickness were observed in the external and central cartilage subregions, and within the weight-bearing medial femur in the central subregion (Table 1). Side differences (diff) in cartilage morphology (% and SD) between knees with JSN and contra-lateral JSN1 % diff JSN1 SDdiff JSN2 %diff JSN2 SDdiff JSN3 %diff JSN3 SDdiff MT -4.3 -0.27 -9.2 -0.58 -32 -2.05 cMT -10.4 -0.53 -16.1 -0.82 -41.5 -2.12 eMT -7.5 -0.51 -23.1 -1.57 -66.7 -4.55 iMT +1.9 +0.08 -1.6 -0.07 -12.9 -0.57 aMT -4.8 -0.30 -6.0 -0.37 -17.6 -1.08 pMT +1.1 +0.06 0.0 0.0 -30.5 -1.83 cMF -6.3 -0.37 -25.4 -1.48 -54.8 -3.18 ccMF -10.9 -0.56 -39.6 -2.05 -71.3 -3.68 icMF -4.1 -0.21 -17.3 -0.89 -41.4 -2.13 ecMF -2.6 -0.14 -18.9 -1.04 -56.4 -3.09 pMF -1.0 -0.07 -5.1 -0.35 -16.1 -1.12 cpMF -2.7 -0.17 -9.2 -0.59 -19.8 -1.27 ipMF -0.1 -0.01 -4.0 -0.25 -11.1 -0.69 epMF +1.6 +0.10 +1.9 +0.12 -15.5 -0.97 (c=central, e=external, I = internal, a=anterior, p= posterior) Conclusions: Changes in cartilage thickness (MRI) associated with radiographic JSN a) were greater in the medial tibia and weight-bearing medial femur than in the posterior femur, b) were greater in external and central subregions of the medial tibia and in the central subregion of the medial weight-bearing femur than in other subregions of these cartilage plates, and c) increased quantitatively with greater JSN.

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