Joint effusion after anterior cruciate ligament reconstruction: Associations with higher postoperative physical activity, patella alta and increased quadriceps tension

Purpose (1) To explore postoperative factors associated with effusion after anterior cruciate ligament reconstruction (ACLR), and (2) to investigate the associations between effusion and quadriceps tension and activation across rehabilitation and return‐to‐sport (RTS) phases. Methods In this cross‐sectional study, participants were assessed at approximately 3, 6, 12 and ~24 months post‐ACLR and further analysed by phase (3–6 vs. 12–24 months). Effusion (Anterior Cruciate Ligament Osteoarthritis Score [ACLOAS]) and patellar alignment (Insall‐Salvati ratio [ISR], bisect offset [BO], patellar tilt angle [PTA]) were quantified by 3.0‐T magnetic resonance imaging. Quadriceps tension and activation were quantified by shear wave elastography (SWE) and surface electromyography (EMG) during resting and isometric knee extension conditions. Physical activity was assessed using The International Physical Activity Questionnaire‐Short Form (IPAQ‐SF). Ordinal logistic regression tested associations with effusion; generalised linear models evaluated relationships between effusion and muscle outcomes ( α = 0.05). Results A total of 119 participants (75% male, age 29 ± 6 years) who underwent single‐bundle ACLR with hamstring tendon autograft were included in this study. Higher postoperative physical activity (MET‐min/week) was associated with higher effusion grade at 3‐month (odds ratio [OR] = 1.07, p < 0.01) and 24‐month (OR = 1.05, p = 0.05) groups; greater ISR was associated with higher effusion grade at ≥12 months (OR = 1.08, p < 0.01). Higher effusion grade was associated with higher vastus medialis and lateralis shear modulus at ≥12 months ( B = 7–11 kPa, all p < 0.04) during the isometric knee extension condition; No association was observed for normalised EMG activation (all p > 0.09). Conclusions Postoperative activity and patella alta were associated with joint effusion after ACLR. Additionally, residual effusion was associated with higher quadriceps tension under submaximal contraction beyond 1 year. These findings suggest that phase‐specific monitoring of physical activity and patellofemoral joint loading may benefit joint health and knee function after ACLR. Level of Evidence Level IV.