Molecular origin of a loading-induced black layer in the deep region of articular cartilage at the magic angle.
OBJECTIVE
To investigate the molecular origin of an unusual low-intensity layer in the deep region of articular cartilage as seen in magnetic resonance imaging (MRI) when the tissue is imaged under compression and oriented at the magic angle.


METHODS
Microscopic MRI (μMRI) T2 and T1 ρ experiments were carried out on 18 specimens, both native and degraded (treated with trypsin). The glycosaminoglycan (GAG) concentrations in the specimens were quantified by both sodium ICP-OES and μMRI Gd(DTPA)(2-) -contrast methods. The mechanical modulus of the specimens was also measured.


RESULTS
Native tissue shows no load-induced layer, while the trypsin-degraded tissue shows clearly the low-intensity line at the deep part of tissue. The GAG reductions were confirmed by the sodium ICP-OES (from 81.7 ± 5.4 mg/mL to 9.2 ± 3.4 mg/mL), MRI GAG quantification (from 72.4 ± 6.7 mg/mL to 11.2 ± 2.9 mg/mL). The modulus reduction was confirmed by biomechanics (from 4.3 ± 0.7 MPa to 0.3 ± 0.1 MPa).


CONCLUSIONS
Both T2 and T1 ρ profiles in native and degraded cartilage show strongly strain-, depth-, and angle-dependence using high-resolution MRI. The GAG reduction is responsible for the visualization of a low-intensity layer in deep cartilage when it is loaded and oriented at 55°.
Department of Physics and Center for Biomedical Research, Oakland University, Rochester, Michigan, USA.
Nian
N
Wang
David
D
Kahn
Farid
F
Badar
Yang
Y
Xia
D000818:Animals; D002358:Cartilage, Articular / Q000033:anatomy & histology* / Q000737:chemistry*; D004285:Dogs; D006025:Glycosaminoglycans / Q000032:analysis* / Q000737:chemistry*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D057054:Molecular Imaging / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014018:Tissue Distribution
D006025:Glycosaminoglycans
MRI; T1ρ; T2; anisotropy; cartilage; laminar appearance; loading; magic angle