MMP9 and obstructive sleep apnea syndrome: When grouped into T90 quartiles, the MMP-9 levels were higher in the OSA patients in the highest quartile compared to those in the lowest quartile or the controls (90.6 ± 56.3 ng/mL vs. 56.9 ± 31.9 ng/mL, p = 0.022; 90.6 ± 56.3 ng/mL vs. 49.0 ± 18.6 ng/mL, p = 0.002, respectively).