# Knee MRI Underestimates the Grade of Cartilage Lesions
## 4 Discussion  
The key finding of this study is the underestimation of cartilage lesion by MRI. Cartilage lesions are one of the most common findings during knee arthroscopy [1]. Moreover, it has a poor healing capacity and often requires surgical intervention. Therefore, an adequate preoperative assessment of the cartilage status is an important factor for surgical planning. Multiple factors can affect the cartilage, such as overuse, trauma, post-surgical gait changes, or rheumatoid diseases, joint inflammation, or malalignment [30; 31; 32; 33]. In our study, cartilage lesions were found in the vast majority of patients who underwent knee arthroscopy, and if apparent, then in the majority of cases, the lesions were multifocal. Femoral condyles and the medial knee compartment were found to be most commonly affected. Unfortunately, there are no specific clinical tests for the evaluation of articular cartilage, and distinguishing between chondral or meniscal lesions is impossible during physical examination [34]. For this purpose, MRI becomes the only available diagnostic modality for a routine cartilage examination [35]. In our study, we have chosen arthroscopic visualization as a gold standard in grading cartilage lesions. The overall good inter-observer reliability [36] can be lowered by the lack of surgeon experience [37]. Therefore, for this study, only experienced surgeons that specialized in sports were selected. Literature data show gross discrepancy regarding the sensitivity of MRI in the cartilage evaluation ranging from 45% to 94% [38]. However, most of the studies were performed on small patient groups. In our study, we have found 453 chondral lesions affecting different anatomical locations in 190 patients. The overall diagnostic accuracy in detecting chondral lesions was rather weak, with Kappa values lower than 0.6. Only healthy cartilages and grade IV lesions were evaluated with acceptable accuracy. Similar findings are reported by [PERSON] et al. [4], showing that diagnostic accuracy increases with the chondral lesion severity (in a prospective study on 190 knees). This phenomenon is also found in 3 T MRI examinations [16], where sensitivity for grade I lesions reached only 8.8%. The diagnostic accuracy was also dependable on the anatomic location, with the highest accuracy in the patellofemoral joint and the medial compartment of the knee. New MRI-based compositional and functional techniques are being proposed in the literature. Techniques such as DWI have a potential to distinguish between the healthy and diseased cartilage and, moreover, have the potential to distinguish the healthy cartilage from a repaired one [8]. Methods of functional cartilage evaluation are also proposed in the literature, including serial T1 and T2 mapping under loading [39]. Techniques such as T1 rho have the ability to assess the proteoglycan concentration in the cartilage [21; 40]. These techniques seem to be the most promising due to providing insight into the histologic composition of the cartilage. In addition, they require no administration of contrast agents and present an excellent range of interobserver reliability and capacity to detect even mild cartilage changes [41]. These new imaging techniques may be of wide use in the future and may perform better in grading and evaluating cartilage defects than the standard morphologic MRI. However, at the moment, the availability of such modalities is limited and papers concerning actual diagnostic performance in vivo are limited, as well. These techniques require further investigation. The medial compartment of the knee was most commonly affected with grade III and IV lesions. As shown in our study as well as previous studies, the diagnostic accuracy increases with the lesion grade, which can explain the higher overall accuracy in the medial compartment of the knee. The results found in PF can be explained by a great visualization of the cartilage on transverse views, where one can appreciate the lesions with little effort.