Longitudinal changes in infrapatellar fat pad fibrosis following anterior cruciate ligament reconstruction: a retrospective cohort study

Background Infrapatellar fat pad (IFP) fibrosis is implicated in anterior knee pain, limited range of motion (ROM), and posttraumatic osteoarthritis (PTOA) after anterior cruciate ligament reconstruction (ACLR). However, its precise longitudinal progression remains unclear. This study aimed to elucidate the dynamic changes in IFP fibrosis from preoperative to 12 months post-ACLR using magnetic resonance imaging (MRI). Methods We retrospectively analyzed consecutive patients who underwent anatomic double-bundle ACLR using autologous hamstring tendon grafts. IFP fibrosis was semiquantitatively graded (grades 0–5) using proton density-weighted (PDW) sagittal MR images obtained preoperatively and at 6 and 12 months postoperatively. Patients were categorized into mild (grades 0–3) and severe (grades 4–5) IFP fibrosis groups. Temporal changes in the proportion of the severe group across the three time points were evaluated using Cochran’s Q test, followed by the Bonferroni-corrected McNemar test for pairwise comparisons. Results Of 558 consecutive cases screened, 169 patients (mean age, 23.9 years) were included in the final analysis. The proportion of patients classified into the severe fibrosis group significantly changed over time (Cochran’s Q test, χ 2 (2) = 24.0, p < 0.001). Compared with the preoperative proportion (1.2%), the proportion of patients in the severe group increased significantly at 6 months postoperatively (8.3%, p = 0.003) and persisted at 12 months (8.3%, p = 0.003). Crucially, 91.1% of the patients showed no change in fibrosis grade between 6 and 12 months. This demonstrates that IFP fibrosis significantly progressed within the first 6 months post-ACLR, with minimal subsequent changes up to 12 months. Conclusions IFP fibrosis is an early and persistent pathological event following ACLR and is established primarily within the first 6 postoperative months. This early onset suggests a critical time window for targeted interventions to mitigate inflammation and prevent the irreversible establishment of IFP fibrosis, providing a basis to investigate the long-term PTOA risk.