• Full Name: MINGDE CAO
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: China

• Full Name: TIANYI ZANG
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: Hong Kong S.A.R.

• Full Name: TIANSHUN XIE
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: Hong Kong S.A.R.

• Full Name: JUNLIN YE
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: Hong Kong S.A.R.

• Full Name: ELVIS CHUN-SING CHUI
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: Hong Kong S.A.R.

• Full Name: PATRICK SHU-HANG YUNG
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: Hong Kong S.A.R.

• Full Name: MICHAEL TIM-YUN ONG
• Affiliation/Institution: The Chinese University of Hong Kong
• Country: Hong Kong S.A.R.

Background

Quadriceps muscle function is critical for anterior cruciate ligament reconstruction (ACLR). While muscle atrophy is well-documented after ACLR, the role of intramuscular fat infiltration—a marker of muscle quality—remains controversial. Recent literature presents conflicting evidence regarding the persistence of fatty infiltration in the chronic setting.

Objectives

To quantify fatty infiltration in individual quadriceps muscles following ACLR using artificial intelligence-based MRI analysis and evaluate its relationship with functional outcomes and patient-reported measures during early recovery.

Study Design

Prospective cohort study analyzing 80 patients undergoing primary ACLR with hamstring autograft between October 2022 and October 2024. An artificial intelligence-based segmentation tool (MuscleMap) quantified fatty infiltration in individual quadriceps muscles (vastus medialis [VM], vastus lateralis [VL], vastus intermedius [VI], rectus femoris [RF]) using Dixon MRI sequences at 4 months (n=62) and 6 months (n=50) postoperatively. Assessments included isokinetic strength testing (60°/s and 180°/s) and patient-reported outcomes (IKDC, IPAQ, Lysholm). SHAP analysis evaluated predictive relationships between fat infiltration and functional outcomes.

Results

At 4 months, the operated limb showed significantly increased fatty infiltration compared to the contralateral limb in VI (13.64±4.67% vs. 11.75±3.60%, P<0.001), VL (13.25±4.14% vs. 11.89±3.38%, P<0.001), and VM (11.80±3.97% vs. 10.53±3.29%, P<0.001). RF showed no significant difference (11.11±3.74% vs. 11.15±3.59%, P=0.947). Fat infiltration correlated negatively with knee extension strength at 60°/s (r=-0.26 to -0.40, P<0.05), with VM showing the strongest association (r=-0.40, P<0.01). At 6 months, fatty infiltration persisted with minimal improvement. The IPAQ score correlated significantly with fat infiltration of VI (r=-0.338, P=0.006), VL (r=-0.340, P=0.005), and VM (r=-0.311, P=0.01) at 4 months. SHAP analysis identified RF and VL fat infiltration as predictive factors for IPAQ scores.

Conclusions

Unlike reports from chronic follow-up studies, significant fatty infiltration is evident in VM, VL, and VI as early as 4 months post-ACLR, with RF uniquely spared. This infiltration correlates with functional deficits, suggesting muscle quality is a distinct and clinically relevant rehabilitation target beyond muscle volume restoration.