WHAT IS THE DISTRIBUTION OF MRI-ASSESSED CARTILAGE DAMAGE AND OSTEOPHYTES WITHIN RADIOGRAPHIC KL GRADE?

INTRODUCTION

Previous studies have shown that mild-to-moderate radiographic disease severity of knee osteoarthritis (OA), i.e. grades 2 and 3 on the Kellgren-Lawrence (KL) scale, reflects a wide spectrum of cartilage morphology including knees with KL2 having no cartilage damage at all, and KL 3 knees having far-advanced wide-spread full-thickness cartilage loss, raising questions on the validity of the KL grading system to classify knees. Radiographic KL grade 2 or 3 is often an inclusion criterion in disease- modifying drug (DMOAD) trials, with the assumption that these knees represent mild-to-moderate OA, i.e. definite OA but not end-stage. No data is available on whether KL0 and 1 knees, - considered pre-radiographic OA -, exhibit osteophytes (OPs) or cartilage damage to a relevant extent or whether more advanced disease, i.e. KL3 and 4, may also reflect knees without or only little relevant cartilage damage or OP presence.

OBJECTIVE

The current study aimed at investigating the distribution of MRI-based measures of OP and cartilage damage scores by radiographic KL grade.

METHODS

Centrally MOAKS MRI and radiographic KL readings were included from the following Osteoarthritis Initiative (OAI) substudies: FNIH Biomarker consortium, POMA and BEAK. In order to match the anteroposterior (a.p.) radiograph, four locations for OPs assessed in the coronal plane (central medial femur, central medial tibia, central lateral femur, central lateral tibia) were considered. Eight tibiofemoral subregions matching the a.p. radiograph were considered for cartilage damage: anterior medial tibia, central medial tibia, posterior medial tibia, central medial femur, anterior lateral tibia, central lateral tibia, posterior lateral tibia and central lateral femur (Figure 1). Cartilage was classified as focal damage only (MOAKS 0, 1.0, 1.1), damage with no advanced full thickness wide-spread damage (MOAKS 2.0, 2.1, 3.0, 3.1), and full thickness wide-spread damage (MOAKS 2.2, 3.2, 3.3). Meniscal damage was assessed with MOAKS, which considered both meniscal morphology and extrusion scores. Descriptive statistics were used to show the frequencies of maximum MOAKS osteophyte and cartilage grades by radiographic KL grade.

RESULTS

In total, the dataset includes 4924 visits from 1981 participants contributing 2276 knees for up to four timepoints. The radiographic KL distribution for the sample is KL0 n=1463 (29.7%), KL1 n=1457 (29.6%), KL2 n=1282 (26.0%), KL3 n=703 (14.3%) and KL4 n=19 (0.4%). There was a definite trend of increasing cartilage damage and osteophyte score with an increasing KL grade (p<0.001 for both). However, there was marked heterogeneity in both measures within KL grade. More than 20% of KL 0 knees showed wide-spread superficial cartilage damage and a minority wide-spread full-thickness damage (Figure 2, Panel A). Almost 20% had a definite MRI-defined osteophyte (Figure 2, Panel B). For KL1, considered no structural OA, almost 50% showed wide-spread superficial or full-thickness cartilage damage. Among KL2 knees, i.e. definite OA as defined by a definite osteophyte on the a.p. X-ray, more than 20% did not have osteophytes on MRI, while 35% did not have more than focal cartilage damage. 462 (23%) of 2004 knees with radiographic OA (KL grade ≥2) did not have more than focal cartilage damage in any of the 8 tibiofemoral subregions. Of these, 42% had no meniscal tear and a meniscal extrusion score of 0 (<2mm), 19% had no meniscal tear and meniscal extrusion >2mm, and 39% had a meniscal tear with or without extrusion >2mm.

CONCLUSION

Different radiographic KL grades represent a wide range of cartilage damage and OP presence and severity. Knees without OA have OPs to a large extent and knees with advanced OA may not exhibit full thickness cartilage damage. Meniscal damage may explain lack of cartilage damage for some, - but not all -, knees with radiographic OA and no cartilage damage on MOAKS. This study focused on only four TFJ locations for OP presence and eight subregions for cartilage damage and ignored the patellofemoral joint and the posterior femur. We conclude that the radiographic KL grade is not an ideal instrument to classify knees according to OP presence and cartilage damage in the matching TFJ locations and subregions as assessed on MRI.