We assessed in vivo inter-rater, test-retest, and intra-rater reliability of tibiofemoral contact area and centroid location measurements for UO-MRI scans in both sagittal and coronal planes. We evaluated the accuracy of our contact area measurements by comparing measurements made using the UO-MRI to measurements made in a high resolution 7T MRI for a bovine knee model. All measures of contact area reliability, including inter-rater, test-retest, and intra-rater, ranged from good to excellent for coronal and sagittal scans. Qualitatively, there was close correspondence between contact regions identified by different readers (Fig. 2). The accuracy analysis found an overall mean error of 4.8% between areas found from 7T MRI and from the UO-MRI. Our results suggest that sagittal or coronal scans are similarly well-suited to evaluate cartilage contact and centroid location in the tibiofemoral joint, with slightly higher repeatability values resulting from sagittal plane acquisition and evaluation.
Our assessment of SDC95, the smallest amount of change that provides 95% confidence that a true change has occurred and is not due to inherent measurement error, may provide useful information for planning research studies that compliments the more widely-used ICC values. For example, our finding of SDC95 of 3–5% for changes in contact location (using sagittal plane images) suggests that changes in the anteroposterior direction larger than 2.5 mm can be detected (based on a 50 mm tibial plateau) using this method. This is smaller than the 4.2 mm difference reported between knees with ACL rupture and healthy knees estimated using a biplanar radiography/MRI image registration approach24, which suggests that our UO-MRI approach can effectively detect differences in centroid location due to ACL deficiency.
Our measures of contact area and centroid location reliability in weightbearing MR are comparable to those from 3T conventional closed-bore scans despite using a lower resolution scanner. For inter-rater reliability, our findings for contact area ICC in the medial compartment of 0.95 and in the lateral compartment of 0.83 are consistent with findings in 3T MRI of 0.90 medially and 0.92 laterally25. The inter-rater contact location ICCs (0.99 medially and 0.95 laterally) were also similar those found in 3T (0.99 medially and 0.91 laterally)25. For intra-rater reliability our findings for contact area ICC were 0.99 medially and 0.91 laterally, which was again consistent with 3T MRI findings of 0.97 both medially and laterally25. Our intra-rater contact location ICCs (0.99 medially and 0.98 laterally) were similar to those found in 3T (1.00 medially and 0.91 laterally)25. No previous study has evaluated the test-retest reliability of contact area and centroid location in vivo, although one cadaveric study examined the patellofemoral joint using a 1.5T magnet and found a test-retest ICC value of 0.98, which is comparable to our results26. The slightly higher variation in test-retest reliability in the current study is likely due to slight differences in participant posture and positioning between test dates, which may be easier to control in a cadaveric study. The test-retest reliability measures will be of value in experimental design, especially for studies requiring testing on more than one day. Our accuracy results, which found a mean error of 4.8%, suggest higher accuracy for our method than the results from a cadaver study using a silicone casting technique reference standard, which found a standard error of measurement of 14%12. This may be due to the substantial differences in the reference method for assessing contact area between the two studies.
The primary strength of this study is that it provides a comprehensive assessment of the role of the intra- and inter-individual differences in raters, and repeated scans, on the reliability of tibiofemoral contact measures. The good to excellent reliability results are supported by a large number of data sets and the inclusion of an accuracy assessment. Incorporation of both sagittal and coronal plane assessment and reporting of SDC95 may be useful in protocol development for future studies. Given the clear advantages for ecological validity with the UO-MRI approach for these assessments compared to traditional supine MRI, we feel that our findings have important implications for the study of knee joint mechanics and function.
The findings should be considered in light of some limitations. First, reliability was assessed in ACL-ruptured knees only. The cartilage of these participants may not be representative of cartilage in un-injured knee joints. The effect of this limitation may be that our study underestimated the reliability of our methods, because cartilage contact in healthy knee joints may be easier to identify and segment. Second, the number of samples used in the accuracy assessment was low and the reference method (7T MRI) did not represent a completely independent measure of contact area. We chose 7T MRI as it was the highest resolution possible with which we could ensure similar loads by using the same loading rig. The lengthy scan time and cost of the 7T scanner hindered our ability to process more samples for accuracy assessment; similarly, we were not able to establish the reliability of measuring contact area in the 7T MRI before we used it as the reference standard. Third, our study was limited to two readers, and further assessment might be required for an application where a large number of readers would be involved.