Depth-dependent changes in cartilage T2 under compressive strain: a 7T MRI study on human knee cartilage

Design of a double acting pneumatic cartilage loading device for magnetic resonance imaging

Studies of osteoarthritis initiation and progression that measure strain in cartilage require physiological loading levels. Many studies use magnetic resonance (MR) imaging, which necessitates a MR-compatible loading device. In this study, the design and validation of a new device, the cartilage compressive actuator (CCA), is presented. The CCA is designed for high-field (e.g., 9.4 T) small-bore MR scanners, and meets a number of design criteria. These criteria include capability for testing bone-cartilage samples, MR compatibility, constant load and incremental strain application, a water-tight specimen chamber, remote control, and real time displacement feedback. The mechanical components in the final design include an actuating piston, a connecting chamber, and a sealed specimen chamber. An electro-pneumatic system applies compression, and an optical Fibre-Bragg grating (FBG) sensor provides live displacement feedback. A logarithmic relationship was observed between force exerted by the CCA and pressure (R² = 0.99), with a peak output force of 653 ± 2 N. The relationship between FBG sensor wavelength and displacement was linear when calibrated both outside (R² = 0.99) and inside (R² = 0.98) the MR scanner. Average slope was similar between the two validation tests, with a slope of -4.2 nm/mm observed inside the MR scanner and -4.3 to -4.5 nm/mm observed outside the MR scanner. This device meets all design criteria and represents an improvement over published designs. Future work should incorporate a closed feedback loop to allow for cyclical loading of specimens.

T2 Relaxation Time Changes in the Distal Femoral Condylar Cartilage of Children and Young Adults with Discoid Meniscus

OBJECTIVE

To investigate compositional changes in the distal femoral condylar cartilage (FCC) of children and young adults with and without discoid meniscus by T2 relaxation time mapping.

DESIGN

We retrospectively reviewed knee magnetic resonance images including sagittal T2 maps of distal FCC performed in patients with or without discoid meniscus. Combined meniscal pathology such as degeneration or tears was also reviewed. Regions of interest were selected, and T2 relaxation time profiles were generated according to medial and lateral and FCC and according to weight-bearing and non-weight-bearing FCC. Nonparametric comparison tests using median values were performed.

RESULTS

Seventy-nine knees from 73 patients (2-20 years) including 45 knees with lateral discoid meniscus (discoid group) were studied. T2 values of FCC showed negative correlation with age in both the discoid and nondiscoid groups (P < 0.01), except for medial weight-bearing FCC. In the discoid group, T2 relaxation times of lateral weight-bearing FCC (median, 46.5 ms) were lower than those of lateral non-weight-bearing (median, 53.2 ms; P < 0.001) and medial weight-bearing (median, 50.5 ms; P = 0.012) FCC. Lateral weight-bearing FCC also showed lower T2 values than other areas in patients with meniscal pathology in the discoid group. However, T2 relaxation times did not differ between the discoid and nondiscoid groups in patients without meniscal pathology.

CONCLUSIONS

Children and young adults with discoid meniscus have lower T2 relaxation times in lateral weight-bearing FCC compared with non-weight-bearing or medial FCC, suggesting compositional changes have occurred in these patients.

Characterizing the transient response of knee cartilage to running: Decreases in cartilage T2 of female recreational runners

Cartilage transmits and redistributes biomechanical loads in the knee joint during exercise. Exercise-induced loading alters cartilage hydration and is detectable using quantitative magnetic resonance imaging (MRI), where T₂ relaxation time (T₂ ) is influenced by cartilage collagen composition, fiber orientation, and changes in the extracellular matrix. This study characterized short-term transient responses of healthy knee cartilage to running-induced loading using bilateral scans and image registration. Eleven healthy female recreational runners (33.73 ± 4.22 years) and four healthy female controls (27.25 ± 1.38 years) were scanned on a 3T GE MRI scanner with quantitative 3D double-echo in steady-state before running over-ground (runner group) or resting (control group) for 40 min. Subjects were scanned immediately post-activity at 5-min intervals for 60 min. T₂ times were calculated for femoral, tibial, and patellar cartilage at each time point and analyzed using a mixed-effects model and Bonferroni post hoc. There were immediate decreases in T₂ (mean ± SEM) post-run in superficial femoral cartilage of at least 3.3% ± 0.3% (p = .002) between baseline and Time 0 that remained for 25 min, a decrease in superficial tibial cartilage T₂ of 2.9% ± 0.4% (p = .041) between baseline and Time 0, and a decrease in superficial patellar cartilage T₂ of 3.6% ± 0.3% (p = .020) 15 min post-run. There were decreases in the medial posterior region of superficial femoral cartilage T₂ of at least 5.3 ± 0.2% (p = .022) within 5 min post-run that remained at 60 min post-run. These results increase understanding of transient responses of healthy cartilage to repetitive, exercise-induced loading and establish preliminary recommendations for future definitive studies of cartilage response to running.

Reliability of tibiofemoral contact area and centroid location in upright, open MRI

BACKGROUND

Imaging cannot be performed during natural weightbearing in biomechanical studies using conventional closed-bore MRI, which has necessitated simulating weightbearing load on the joint. Upright, open MRI (UO-MRI) allows for joint imaging during natural weightbearing and may have the potential to better characterize the biomechanical effect of tibiofemoral pathology involving soft tissues. However open MRI scanners have lower field strengths than closed-bore scanners, which limits the image quality that can be obtained. Thus, there is a need to establish the reliability of measurements in upright weightbearing postures obtained using UO-MRI.

METHODS

Knees of five participants with prior anterior cruciate ligament (ACL) rupture were scanned standing in a 0.5 T upright open MRI scanner using a 3D DESS sequence. Manual segmentation of cartilage regions in contact was performed and centroids of these contact areas were automatically determined for the medial and lateral tibiofemoral compartments. Inter-rater, test-retest, and intra-rater reliability were determined and quantified using intra-class correlation (ICC_3,1), standard error of measurement (SEM), and smallest detectable change with 95% confidence (SDC₉₅). Accuracy was assessed by using a high-resolution 7 T MRI as a reference.

RESULTS

Contact area and centroid location reliability (inter-rater, test-retest, and intra-rater) for sagittal scans in the medial compartment had ICC_3,1 values from 0.95-0.99 and 0.98-0.99 respectively. In the lateral compartment, contact area and centroid location reliability ICC_3,1 values ranged from 0.83-0.91 and 0.95-1.00 respectively. The smallest detectable change in contact area was 1.28% in the medial compartment and 0.95% in the lateral compartment. Contact area and centroid location reliability for coronal scans in the medial compartment had ICC_3,1 values from 0.90-0.98 and 0.98-1.00 respectively, and in the lateral compartment ICC_3,1 ranged from 0.76-0.94 and 0.93-1.00 respectively. The smallest detectable change in contact area was 0.65% in the medial compartment and 1.41% in the lateral compartment. Contact area was accurate to within a mean absolute error of 11.0 mm².

CONCLUSIONS

Knee contact area and contact centroid location can be assessed in upright weightbearing MRI with good to excellent reliability. The lower field strength used in upright, weightbearing MRI does not compromise the reliability of tibiofemoral contact area and centroid location measures.