ObjectiveTo evaluate the correlation between the Mohawk (MKX) expression level and the collagen fiber diameter of autologous hamstring tendon graft during the stable graft remodeling phase after anterior cruciate ligament (ACL) reconstruction.MethodsBetween January 2018 and August 2018, patients who underwent arth-roscopic single-bundle anatomical ACL reconstruction with autologous hamstring tendons for at least 48 months and also underwent second-look arthroscopy were enrolled in study. During the second-look arthroscopic procedures, ACL graft biopsies were performed from the surface of central part of the ligament. MKX expressions of ACL grafts were analysed by real-time fluorescent quantitative PCR (qRT-PCR). The ultrastructure of collagen fibers of grafts were evaluated by transmission electron microscopy, which included average diameter of collagen fibers (Dc), average diameter of large-diameter collagen fibers (DL), average diameter of small-diameter collagen fibers (DS), and large-small collagen fibers ratio (RL/S). The correlation between MKX expression level and graft collagen fiber diameter was calculated.ResultsTwenty-six patients met the selection criteria and their ACL graft specimens were enrolled in the study. The interval between ACL reconstruction and second-look arthroscopy was 52-128 months, with an average of 78.6 months. Arthroscopic graft remodeling score was 3-6 (mean, 4.8). There were 17 cases of excellent remodeling and 9 cases of fair remodeling. All ACL grafts showed typical bimodal distributions of both large-diameter collagen fibers and small-diameter collagen fibers, but the ultrastructural characteristics of the graft collagen fibers were different according to different remodeling status under arthroscopy. The DC, DL, DS, and RL/S of the graft specimens were (65.2±9.3) nm, (91.6±10.5) nm, (45.7±8.6) nm, and 0.73±0.12, respectively. The relative expression level of MKX was 1.42±0.11, which was positively linearly correlated with DC, DL, and RL/S, and the correlation coefficient was statistically significant (r=0.809, P=0.000; r=0.861, P=0.000; r=0.942, P=0.000), while there was no significant correlation between DS and relative expression level of MKX (r=0.147, P=0.238). Regression analysis showed that the relative expression level of MKX could predict the DC, DL, and RL/S results of the ACL graft specimens (P<0.05).ConclusionAfter autologous hamstring tendon grafts stepped into stabilized remodeling phase, MKX expression level could predict the diameter measurement results of collagen fibers and be used as an important evaluation basis for graft collagen anabolic metabolism.