The primary objective of this study was to develop an MRI-based technique for analyzing the spatial pattern of cartilage loss in the MF condyle based on anterior-posterior subregions, with and without medial-lateral trimming, and to determine changes in cartilage thickness over 1 year with 3-T MRI in participants of the OAI. Additionally we explored whether the spatial pattern of cartilage loss depended on radiographic OA status, namely, the grade of mJSN. We found that the rate of change varied significantly between anterior-posterior subregions of the MF, with greatest changes observed 30–60° posterior to the trochlear notch. The rate of change was greater in central aspects than over the entire medial-lateral extension of the MF after trimming the edges medially and laterally. Sensitivity to change was greatest in the 45–75° subregions but was only marginally affected by medial-lateral trimming.
The algorithm was designed to compute cartilage thickness (ThCtAB) in overlapping regions and to achieve sufficient spatial resolution to track regional patterns of cartilage loss in the MF while keeping the size of each subregion large enough to reduce noise in the data. Our results show that the algorithm provided here is capable of capturing significant differences in the rate of change between anterior-posterior subregions in OA knees with frequent knee pain and various degrees of radiographic involvement. Additionally, the algorithm was designed to allow for various degrees of medial-lateral trimming, to exclude peripheral areas of the MF, which are not orientated perpendicular to the image plane and in which therefore the bone interface and cartilage surface are not as clearly delineated due to partial volume effects. The results show that rates of change measured in the central areas of the MF were significantly higher than for the entire medial-lateral extension of the MF, although sensitivity to change (SRM) was minimally affected after excluding peripheral aspects of the MF. This must, however, be put into context of the acquisition protocol, which involved a 0.7mm sagittal DESS sequence with near-isotropic spatial resolution (18
). Many other longitudinal OA studies relied on 1.5mm sagittal images (1
) and, excluding the peripheral parts of the MF, may be more effective at lower spatial resolution because partial-volume effects increase with greater slice thickness. In contrast to a previously published algorithm (24
), which focused on the weight-bearing region of the femur only, the current algorithm covers the entire femoral condyle in anterior-posterior direction. A previous report has separated the femur into the trochlea (femoral trochlea), as well as into anterior, central, and posterior aspects of the femoral condyles, but has not used a finer grid of ROIs or applied medial-lateral trimming (26
A limitation of the current technique is that, in comparison with a landmark-based registration algorithms (i.e., Saha et al. (27
)), the precise spatial comparability of the subregions between subjects is likely less between different subjects than when also taking into account several specific aspects of the individual shape of each femoral condyle. Another limitation of the study is that data used were originally collected to compare the rate and sensitivity of change in participants with mJSN in one knee and no JSN in the contralateral knee (28
). Therefore, only the MF, but not the lateral femoral condyle, was examined. Application of the algorithm to the lateral femoral condyle, however, is straightforward, and its application to the MF in the current study allowed us to study the spatial pattern of cartilage loss across knees with well-defined grades of medial radiographic disease. As this study represents work in an exploratory cohort, however, the findings should be replicated in a larger validation cohort.
The results of this study support previous radiographic findings that JSN in OA is more severe when knees are flexed to 20–30° (6
) since the posterior aspects of the weight-bearing region of the femoral condyle (30–75°) displayed greater rates of change than those located somewhat more anterior. One reason the posterior aspect of the MF located at >90° displayed smaller rates of change than the total MF may be that these areas are not involved in load-bearing during frequent activities, such as walking, but only during very deep knee bends, an activity likely avoided in people with knee OA. Another reason why the weight-bearing regions of the femur that are in contact with the tibia in 20–30° flexion display greater rates of change than the more anterior region of the condyles is that greatest femorotibial contact stresses occur with the knee in 24–28° flexion (3
Previous papers using coronal imaging protocols have relied on a femoral ROI, defined as covering 60% of the slices between the trochlear notch and the most posterior aspect of the femoral condyle (18
). To relate the current findings to these coronal ROIs, we applied this 60% criterion to all 160 knees investigated and found the posterior end of the 60% ROI to be located at 56.9 ± 3.7°. The coronal 60% ROI thus includes most of the femoral ROI with the greatest rate of change (30–60°) but does not entirely include the ROI with the greatest SRM (45–75°). Coronal images are limited since partial-volume effects are too high posterior (>60%) to support segmentation (18
), but when segmenting the weight-bearing part of the MF condyle in sagittal images (18
), the posterior cutoff value for the weight-bearing region can be set more posterior. When applying a 75% cutoff, for example, the posterior end of the femoral ROI is located at 73.5 ± 4.8° in the knees investigated here. This femoral ROI would thus include the regions with the highest SRM and the regions with the greatest rates of change across all mJSN grades. Our data indicate that there may be subtle differences in the spatial pattern of femoral cartilage loss between knees with different mJSN grade, potentially due to alterations in femorotibial biomechanics. However, this finding should be confirmed in future work since the current study was not powered to conclusively answer this question.
In conclusion, this study presents an MRI-based technique for analyzing the spatial pattern of cartilage loss in the femoral condyle (MF) based on anterior-posterior subregions, with and without medial-lateral trimming. When applying the algorithm to an exploratory cohort of 160 knees examined longitudinally over 1 year with 3-T MRI, we found the rate of change to vary significantly between anterior-posterior subregions of MF. The greatest changes were observed 30–60° posterior to the trochlear notch, and the greatest sensitivity to change, 45–75° posterior to the notch. When trimming the MF medially and laterally to analyze its central aspect, the rate of change was greater than for the total medial-lateral extension of the MF, but sensitivity to change was minimally affected. These results indicate that the highest sensitivity to change in cartilage thickness of the MF in OA may be achieved when focusing on an ROI covering the posterior aspect of the weight-bearing (central) region of the MF condyle.