Abstract Focal chondral defects of the knee present a heterogeneous clinical problem with no universally applicable solution. Lesion size, containment, subchondral bone involvement, joint alignment, meniscal integrity, patient activity demands, and system-level constraints variably influence durability, yet outcomes are often interpreted without adequate contextualization. As a result, treatment comparisons frequently reflect differences in indication rather than superiority of technique. This review proposes a constraint-based framework to guide treatment selection in focal knee chondral defects. Cartilage restoration is conceptualized as a continuum of joint-preserving strategies, each occupying a defined role determined by mechanical demands, biologic plausibility, and practical feasibility. Available interventions are organized into five categories: structural resurfacing and transfer techniques, cell-based repair, marrow stimulation-based repair, scaffold-augmented repair, and osteochondral allograft transplantation. A structured decision algorithm is presented emphasizing sequential evaluation of the joint environment, defect characteristics, and patient-specific modifiers. The framework emphasizes that malalignment, instability, meniscal deficiency, and subchondral bone pathology must be addressed to optimize durability, and that lesion geometry and depth narrow the range of biologically and mechanically plausible options. Patient expectations and system constraints refine, but do not override, lesion-driven decision-making. Rather than positioning techniques hierarchically, this model highlights appropriate application boundaries and expected trade-offs. Durable symptom relief and delay of arthroplasty remain shared endpoints across modalities. By shifting emphasis from modality comparison to constraint matching, this framework provides a unified structure for interpreting the cartilage restoration literature and establishes the foundation for the modality-specific analyses presented in subsequent parts of this series.
Smitterberg et al. (Thu,) studied this question.