This study was approved by the UBC Clinical Research Ethics Board (H18-01459). All participants provided informed, written consent (Appendix B).
A sample of 5 patients from a larger comparative cohort study volunteered for reliability analysis. The cohort study was a convenience sample of 18 patients with prior ACL rupture. Patients were recruited through posted notifications and targeted e-mails (Appendix A). The five patients selected for this study were the only patients from the larger cohort who consented to complete the scanning process, a three-hour procedure, on two separate dates, thus allowing for test-retest reliability analysis.
Inclusion criteria for the cohort study were: 1) adult participants between the ages of 18-50 years old with unilateral, isolated ACL ruptures; 2) intact cartilage and evidence of complete ACL rupture on MRI; 3) reported ACL rupture within the last 5 years and if reconstructed, done within 1 year from injury; and 4) have completed a full rehabilitation program and returned to regular sport or recreational activities.
Exclusion criteria were: 1) associated ligament rupture other than the ACL (though incomplete MCL ruptures were not excluded); 2) known knee osteoarthritis diagnosed by a physician; 3) presence of other joint disease; 4) incompletely rehabilitated injury, defined as a range of motion less than 0-130 degrees, quadriceps atrophy, or persistent mechanical symptoms; 5) individuals prohibited from undergoing MRI based on the MRI screening form (Appendix C); 6) history of fainting, or evidence of change in orthostatic blood pressure; 7) prior or subsequent knee surgery other than diagnostic arthroscopy; 8) history of corticosteroid injection to either knee; and 9) bilateral ACL rupture or ACL re-rupture.
Demographic data from participants were collected including age, height, body mass, date of injury, time from injury to surgery (if applicable), and time from injury to study participation.
Participants were scanned standing in a 0.5T upright, open MRI (MROpen, Paramed, Genoa, Italy). All scans were done in the morning, participants were instructed not to do any impact exercise prior to scanning, and participants were seated for 30 minutes prior to scanning, during which time questionnaires were administered. Participants wore compression socks to minimize venous pooling in the lower extremities during standing scans. Participants then stood for 15 minutes prior to acquiring standing scans to ensure a cartilage deformation equilibrium had been reached. Each participant wore a chest harness suspended from an aluminum ceiling track safety-rated to 450 lbs (Handicare, Concord, ON) as a precautionary measure in case the participant fainted during upright scanning. No weight was borne through the bars or the harness. Standing scans of the ACL-injured leg were acquired with the knees in full extension, with the participant instructed to stand comfortably and distribute their weight equally between legs. Three support bars (shins, buttocks, and hands) were placed to help the participant remain still during scanning. We obtained sagittal and coronal images with a double echo steady state T2 sequence (Table 1) using a commercial 2-channel knee coil (ParaMed) suspended around the knee. The sequence was optimized to provide excellent cartilage signal quickly enough to minimize the effects of patient movement and fatigue while standing. The data were denoised using an optimized blockwise nonlocal means denoising filter16, and the component DESS images were subsequently fit to a signal model with a global T1 estimate of 0.517.
Table 1. Imaging parameters used for UO-MRI scan and for the high-resolution 7T MRI scan.
||2D multi-slice RARE
|Repetition time (ms)
|Echo time (ms)
|Field of view (cm)
||22 x 22 x 16
||6 x 6
|Acquisition matrix size
||256 x 256 x 38(zero filled to 256 x 256 x 64)*
||256 x 256, 50 slices
|Slice thickness (μm)
|Slice gap (μm)
|Voxel dimensions (μm)
||859 x 859 x 2500
||23.4 x 23.4 x 35.0
|Flip angle (°)
|Total scan time (min)
*Note the voxel dimensions are interpolated in the slab direction
Two trained raters, A.M.S and D.J.S, with two years and three years’ experience respectively, performed segmentation for all data sets. Both raters were trained in knee joint segmentation by a post-doctoral fellow with 10 years of experience in segmenting MSK data. Prior to the study both raters established a set of general guidelines for segmentation. All data sets were anonymized, and a numerical code was assigned to each patient. Raters identified tibiofemoral contact regions by manually tracing regions with no visible separation between cartilage surfaces on each image slice using the Editor module in 3D Slicer18 (http://www.slicer.org) in both the coronal and sagittal planes (Figure 1.A). Raters selected voxels of cartilage that were in direct contact and did not contain any contribution from other structures (e.g. meniscus or synovial fluid). Volumes were created that represented medial and lateral contact areas, each with a known number of voxels (Figure 1.B). We multiplied the number of voxels in contact by their axial dimensions (length and width) to calculate contact areas for the medial and lateral compartments. To account for differences in size between subjects, the cartilage contact area measurement in the axial plane was normalized by taking the ratio (%) of the contact area over the maximum axial cross-sectional area of the tibial plateau.
The centroid location was calculated as the geometric center generated from the contact area segmentations in the medial and lateral compartments (Figure 1.B). A validated joint coordinate system was employed to locate contact area centroids within a consistent coordinate frame19,20. Centroid location was reported in mm and also quantified as a percentage on the tibial plateau in the medial (0%) to lateral (100%) and posterior (0%) to anterior (100%) directions to account for differences in size between participants. The coordinate system was based on the bony landmarks and axis definitions of an established joint coordinate system convention19. Reference bony landmarks were established from supine scout scans of the hip, knee, and ankle, with the scan position relative to each other noted from the difference in UO-MRI scan table position. Positions and orientations of the coordinate systems in the upright posture were determined by registering supine images of the tibia and femur to corresponding upright images using Analyze 12.0 (AnalyzeDirect, Inc., Overland Park, KS). The test-retest reliability of using this joint coordinate system in the UO-MRI was assessed in a previous study20, and ICC reliability values ranged from 0.95-0.99 for joint rotations and translations.
We assessed the accuracy of contact area measurement by comparing our method in the UO-MRI to reference measurements of contact area made in a 7T MR scanner (Bruker Biospin, Ettlingen, Germany) for two cartilage preparations at two load levels. We created two cartilage contact preparations by dissecting a bovine knee and extracting medial and lateral tibial and femoral blocks using a handsaw. The block dimensions were approximately 30 mm by 30 mm in the anteroposterior direction and mediolateral direction and were approximately 20 mm in the axial (compressive) direction. The blocks were oriented on polycarbonate tissue mounts in a manner that maximized contact of the flattest part of the mating joint surfaces. The bony side of each osteochondral block was affixed to the tissue mounts with cyanoacrylate glue. The preparations were immersed in phosphate-buffered saline and positioned in an MR-compatible compression chamber such that axial compression could be applied by rotating a Delrin plunger (2 mm thread) within the capsule of the compression chamber. The samples were positioned such that opposing cartilage surfaces were touching but not compressed, and images were acquired. An axial load was then applied until cartilage compression could be visualized, and the specimen was re-scanned. Five minutes were allowed to pass in between cartilage compression and re-scanning in order to permit the cartilage to equilibrate. The displacement of the plunger was marked on the outside of the chamber so that the process could be repeated. On completion, the load was removed, and the cartilage was given time to re-equilibrate. The process was performed first in the UO-MRI and then in the 7T MRI, using imaging parameters listed in Table 1. In a previous study, intra-observer repeatability of segmentation of loaded tibial and femoral cartilage images in this 7T scanner was within 2.3 and 3.3 voxels for cartilage depth, 95% of the time21.
Inter-rater, test-retest, and intra-rater reliability statistics were calculated for tibiofemoral contact area and centroid location. Inter-rater reliability was obtained for two raters who individually segmented and calculated contact areas for each scan. Test-retest reliability was established by scanning each participant twice, with approximately one month between scans, with one rater (D. J. S.) segmenting both scans. Intra-rater reliability was obtained for one rater (A. M. S.) segmenting the contact areas for each sample 3 times, each 2 weeks apart. We calculated the intra-class correlation coefficient for fixed raters (ICC3,1) using the methods described by Shrout and Fleiss22, as well as the standard error of measurement (SEM), and the smallest detectable change with 95% confidence (SDC95). ICCs less than 0.5 indicated poor reliability; 0.5 to 0.75 moderate reliability; 0.75 to 0.9 good reliability; and greater than 0.9 excellent reliability. All metrics were obtained for both coronal and sagittal scans.
We assessed contact area accuracy by finding the mean absolute error (MAE) for contact areas measured using UO-MRI and those measured for the same region and load using 7T MRI from images obtained in the sagittal plane.