Biomechanical compensatory chain and residual compensation potential in aging spinal deformity: a narrative review and multimodal assessment framework

Background/objectives As a prevalent condition among the elderly, adult spinal deformity (ASD) initiates progressive decompensation not only in the spine but also throughout the lower limbs. This systemic impairment reduces the elderly’s mobility, elevates the risk of falls, and consequently places a considerable burden on public health. Current assessment approaches frequently neglect the physiological ‘cost’ of such compensatory adaptations. This review aims to clarify the biomechanical relationship between sagittal spinal imbalance and secondary lower limb pathologies, and to propose “Residual Compensation Potential” (RCP) innovatively as a quantifiable framework for evaluation. Methods A narrative review was conducted by retrieving PubMed, Web of Science, and Embase databases (up to December 2025). We synthesized evidence regarding spinal-pelvic parameters, lower limb kinematics, and joint degeneration mechanisms to construct a ‘imbalance-compensation-degeneration’ closed-loop model. Results The compensatory chain follows a hierarchical sequence: posterior pelvic tilt, hip hyperextension, knee flexion, and ankle dorsiflexion. While initially protective, prolonged maintenance of these postures alters mechanical loading and leads to specific degenerative changes: anterior hip cartilage wear from hyperextension, patellofemoral overload from knee flexion, and Achilles tendinopathy from sustained dorsiflexion. We introduce the RCP as a proposed framework, which combines EOS imaging, 3D gait analysis, and surface electromyography (sEMG) to quantify the remaining compensatory capacity of these joints. The resulting RCP score stratifies patients into compensation-preserved or compensation-depleted categories, informing targeted surgical and rehabilitative planning. Conclusion The evaluation of adult spinal deformity should extend beyond static spinal alignment to include the functional status of the lower limbs. The proposed RCP framework offers a novel method to assess the risk of secondary joint degeneration and postoperative imbalance. Consequently, clinical management must evolve from a focus on radiographic correction alone toward the holistic recovery of the entire compensatory chain, ensuring that spinal realignment translates into stable and functional gait.