# Knee MRI Underestimates the Grade of Cartilage Lesions
## 3 Results  
In our study, 74 females and 116 males were included with an average age of 41.3 (SD 15.3). The main indication for arthroscopic surgery were meniscal lesions (\(n=88\)), followed by ACL tears (\(n=75\)). Isolated chondral lesions without any concomitant intraarticular lesions were found in 27 patients. The mean time between the onset of symptoms to the MRI examination was 31 months (median 5.2). On average, patients waited nearly 4 months (3.9) for arthroscopic surgery after obtaining the MRI results. During arthroscopic surgery, chondral lesions were diagnosed in 139 knees, out of which 112 had concomitant injuries such as meniscal lesions or ACL tears. Grade II lesions were the most common in our study. Femoral comdyles were affected more commonly by chondral lesions rather than by tibial condydes. Moreover, chondral lesions were most frequently found in the medial compartment. Overall, 453 chondral defects were encountered during arthroscopy. The lesions showed predilection for MFC (\(n=118\)), followed by MTC (\(n=91\)) and PFJ (\(n=90\)). LFC and LTC were affected evenly and 77 lesions were found in each location. Chondral defects were multifocal and in over 90% (\(n=106\)) of patients, multiple anatomical locations were involved. Only 7.2% (\(n=33\)) of chondral lesions affected one location. The majority of isolated chondral lesions were located in MFC 54.5% (\(n=18\)). Moreover, grade VI and III lesions were most prevalent on MFC, where nearly a half of grade IV defects was found 46.7% (\(n=43\)), and one-third of grade III lesions 33% (\(n=36\)). Grade IV chondral defects in the lateral compartment accounted only for 11% (\(n=10\)) of all grade IV lesions. The results of the arthroscopic evaluation are shown in Table 1.  
Grade II lesions were encountered most frequently, followed by grade I lesions. Even though grade IV lesions were the rarest, they accounted for one-fifth of all the lesions. An adequate description of cartilage status on MRI was found only in 59 patients in all anatomical locations during arthroscopy. In 119 patients, at least one anatomical location was underestimated regarding the chondral lesion grade, and in 48 patients, at least one location was overestimated. In 36 patients, at least one anatomical location was overestimated and at least one was underestimated in regard to lesion severity. The MRI showed low sensitivity ranging from 24% to 42% depending on the anatomical location. The highest sensitivity was found in MFC and PFJ. The mean specificity of MRI in detecting chondral lesions ranged from 82% to 87.4%. Moreover, the specificity was highest in \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multicolumn{5}{c}{**Cartilage Status According to ICRS**} & \multirow{2}{*}{**Cartilage Lesions in Total**} \\ \cline{2-2} \cline{4-6}  & **0** & **1** & **2** & **3** & **4** \\ \hline MFC & 72 & 15 & 24 & 36 & 43 & 118 \\ LFC & 113 & 29 & 25 & 19 & 4 & 77 \\ MTC & 99 & 23 & 32 & 19 & 17 & 91 \\ LTC & 113 & 32 & 28 & 11 & 6 & 77 \\ PFJ & 100 & 20 & 24 & 24 & 22 & 90 \\ Total & 497 & 119 & 133 & 109 & 92 & 453 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Grading and location of chondral lesions in an arthroscopic view. MFC and PFJ. A healthy cartilage was properly evaluated in over 60% regardless of the anatomical location with sensitivity over 90% and specificity of 39%. The sensitivity of MRI was increasing proportionally with an increasing grade of the cartilage lesion. Grade I lesions showed sensitivity only of 4.8%, and grade IV of 39%. The specificity of MRI in detecting a healthy cartilage was estimated as only 39%. If the cartilage lesion was observed, then the specificity of the MRI ranged from 96% to 97%. A low sensitivity was found for grade I lesions not exceeding 10% in any anatomical location. The sensitivity of 0% was found for grade I MFC lesions and for grade III LTC lesions. Overall, the MRI capacity for detecting lesions was evaluated with ROC curves.