Guided bone regeneration is a core technique for reconstructing alveolar bone defects in implant dentistry，and its success critically depends on the space maintenance component of the PASS principle. Space-maintaining capability directly determines the outcome of bone augmentation and varies significantly across different defect morphologies：favorable bone defects，in which native bony walls constitute a rigid space-maintenance apparatus，generally yielding favorable prognoses，whereas unfavorable bone defects，lacking such natural barriers，must rely on artificial scaffolds for compensation，leading to a markedly elevated risk of augmentation failure. In this context，the present article focuses on unfavorable bone defects，systematically reviewing the advantages and disadvantages of commonly used bone substitute materials and barrier membranes in addressing high-tension space-maintenance demands，highlighting the key operational points and maintenance strength of current mainstream space-maintenance techniques，and，based on the morphological classification of bone defects，presenting a hierarchical surgical recommendation system，with the aim of providing clinicians with a reference for rationally selecting space-maintenance strategies for unfavorable bone defects of varying severity.