Background <p>The Forgotten Joint Score (FJS) has emerged as a vital metric for assessing the outcomes of total knee arthroplasty (TKA). However, its post-TKA trajectory patterns and predictive factors remain controversial. This study aimed to elucidate the evolution trajectory of the FJS within the first year following TKA and identify its predictive factors.</p> Methods <p>This was an longitudinal, prospective, observational, single-center study. A total of 157 patients who underwent primary TKA from May 2023 to July 2023 were included. Preoperatively, patient demographics and comorbidities were collected, and composite scores (Knee Society Score, FJS, Hospital Anxiety and Depression Scale, ASA) were assessed. FJS assessments were obtained monthly using standardized protocols during the 2–12 month postoperative follow-up period. Latent growth modeling techniques were employed to characterize FJS trajectory patterns. Multivariable logistic regression analysis was performed to identify predictors associated with trajectory membership.</p> Results <p>A total of 129 patients completed postoperative FJS assessments. The FJS trajectory following TKA exhibited a triphasic progression: rapid improvement (2–5 months), decelerated growth rate (5–10 months), and functional plateau from 10 to 12 months. The predictors were body mass index (BMI) (OR = 1.278, 95% CI:1.080–1.513), preoperative Knee Society Score (KSS) (OR = 0.951, 95% CI:0.907–0.997), depression (OR = 1.322, 95% CI:1.023–1.710), anxiety (OR = 1.717, 95% CI:1.266–2.330), and chronic postoperative pain (OR = 15.223, 95% CI 1.201-192.923).</p> Conclusion <p>The FJS appeared to demonstrate a phased improvement trajectory during the first postoperative year following TKA. Worse joint awareness was associated with chronic pain, high BMI, low preoperative KSS, and higher anxiety/depression. Surgeons must remain vigilant for these risk factors.</p> Trial registration <p>ChiCTR2500107625. Registration date 15/08/2025. Retrospectively registered.</p>