Autologous Chondrocyte Implantation (ACI) for the Knee: Historical Development, Age Related Outcomes, and Future Directions

Abstract Articular cartilage defects of the knee remain a major clinical challenge because hyaline cartilage possesses very limited intrinsic healing capacity. Untreated lesions often lead to pain, functional impairment, and progressive joint degeneration. Autologous chondrocyte implantation (ACI) was introduced in the 1990s as a transformative joint-preserving surgery designed to promote hyaline-like cartilage regeneration rather than simple repair. Since its inception, ACI has undergone significant technical evolution across three generations. First-generation ACI utilized a periosteal flap to cover injected chondrocytes, which demonstrated the feasibility of biological repair but was hindered by complications such as periosteal hypertrophy. To address these issues, second-generation techniques introduced collagen membranes, improving procedural safety. The third generation, matrix-assisted autologous chondrocyte implantation (MACI), utilizes biodegradable scaffolds to ensure uniform cell distribution and simplified surgical workflows. In Japan, the development of atelocollagen-based scaffolds has further standardized these procedures, allowing for reliable clinical application. Historically, advanced age and early-stage osteoarthritis were considered relative or absolute contraindications for ACI due to concerns regarding diminished chondrocyte biological activity. However, contemporary clinical evidence indicates that chronological age alone is not a definitive predictor of treatment failure. While younger patients often achieve higher absolute clinical scores, middle-aged and older individuals frequently realize clinically meaningful improvements in pain relief and daily function. The success of ACI in these populations is increasingly linked to a comprehensive "whole-joint" management strategy, where concomitant pathologies such as limb malalignment, meniscal deficiency, and ligamentous instability are addressed simultaneously. By optimizing the overall joint environment, ACI serves as a potential intervention to delay the necessity of total knee arthroplasty in patients over the age of 40 or 50. Future perspectives in cartilage repair focus on the potential for one-stage cell-based therapies to reduce donor site morbidity and treatment complexity. One area of ongoing investigation involves the possible utilization of osteophyte-derived chondrocytes, which are typically discarded during surgery. Preliminary preclinical studies suggest that these cells may retain sufficient proliferative capacity and chondrogenic potential for therapeutic use. Integrating minced osteophyte cartilage with next-generation biomaterials could eventually offer a strategy that reduces the need for healthy cartilage biopsy and cell culture, though further clinical validation remains essential. In conclusion, ACI has transitioned from an experimental treatment into a versatile joint-preserving procedure. As research continues to refine surgical indications and explore alternative cell sources, ACI remains an important biological option for managing a broader spectrum of patients with degenerative cartilage pathology.