Predicting ACI Outcomes with GMP In-Process Control Kinetic Metrics: Initial Chondrocyte Yield and Population Doubling Time as Prognostic Clinical Biomarkers

Purpose: To determine whether specific in vitro biologic characteristics and manufacturing kinetics of cultured human articular chondrocytes (HACs) can serve as predictive biomarkers for patient-reported and structural outcomes following second-generation autologous chondrocyte implantation (ACI) in the knee. Materials and Methods: This prospective cohort study evaluated 67 patients (mean age 25.1 ± 8.3 years) treated with second-generation ACI for large focal cartilage defects (mean size 5.4 ± 2.5 cm2) between 2017 and 2024. HACs were expanded under Good Manufacturing Practice (GMP) conditions using human platelet lysate-supplemented media. Four biological determinants were analyzed: (1) initial chondrocyte yield (ICY) isolated from the cartilage biopsy; (2) chondrogenic activity (relative ACAN and COL2A1 expression); (3) final culture confluence level; and (4) HAC population doubling time (PDT) pre- and post-cryopreservation. Clinical outcomes (KOOS and IKDC) and MRI outcomes (MOCART) were assessed at 2 years (T1) and at a mean final follow-up of 4.2 ± 1.7 years (T2) via univariate and multiple regression analyses. Results: Static biosynthetic markers (ACAN/COL2A1 expression) and final culture confluence did not significantly correlate with longitudinal clinical (KOOS/IKDC) or structural (MOCART) outcomes. Conversely, the retained kinetic parameters served as strong prognostic indicators. Multiple regression revealed that higher ICY values significantly predicted improvements across most KOOS subscales from baseline to T2, including KOOS Symptoms (β = 63.56; p < 0.01), KOOS Pain (β = 88.83; p < 0.001), KOOS ADL (β = 107.84; p < 0.001), and KOOS Sport and Recreation Function (β = 92.3; p < 0.05). Furthermore, a prolonged PDT, indicative of diminished in vitro proliferative vigor, inversely correlated with functional recovery in IKDC (β = −26.12; p < 0.01), KOOS Pain (β = −23.09; p < 0.05), and KOOS ADL (β = −21.71; p < 0.05) scores. Conclusions: The intrinsic proliferative vigor of the HAC cellular payload (shorter PDT) and higher initial cell yields are robust kinetic biomarkers associated with markedly superior mid-term clinical outcomes following second-generation ACI for large focal chondral defects in the knee. In contrast, standard morphological and gene expression metrics failed to predict in vivo functional success. These findings advocate for a risk-based paradigm shift in GMP quality control methodologies, emphasizing dynamic HAC growth kinetics over static cellular features to optimize patient-specific regenerative potential.