<p>This finite element study aimed to compare the biomechanical effects of endoscopic unilateral laminotomy for bilateral decompression (Endo-ULBD) on lumbar spinal stability and load distribution in both healthy and osteoporotic spine models, with a specific focus on identifying biomechanical changes in a reduced‑stiffness bone model that may serve as hypothesis‑generating risk indicators, recognizing that the model does not simulate full osteoporotic pathology. A validated L4–L5 finite element model was developed. Virtual Endo-ULBD was performed on both healthy and reduced‑stiffness (osteoporosis‑simulating) models. Segmental range of motion (ROM) and von Mises stress in the intervertebral disc and endplates were evaluated under flexion, extension, lateral bending, and axial rotation, with a 500 N compressive preload and 10 N·m pure moments. Nonlinear mechanical characteristics of spinal tissues and the potential modulating effect of paraspinal musculature were discussed as key contextual factors for interpreting in vitro results. The model was successfully validated against established biomechanical data. After Endo-ULBD, ROM percentage increases ranged from 2.80 to 16.90% in the healthy model and from 3.00 to 17.10% in the reduced‑stiffness (osteoporosis‑simulating) model (depending on loading direction), with all absolute ROM values remaining below 10°. The reduced‑stiffness (osteoporosis‑simulating) model showed slightly higher absolute ROM values than the healthy model both preoperatively and postoperatively, consistent with reduced bone stiffness. The percentage increases after Endo‑ULBD were similar between the two models. Disc stress showed increases ranging from 3.88 to 25.78% in the healthy model and from 2.47 to 10.48% in the reduced‑stiffness model. There is no established threshold to determine whether these increases are biomechanically significant. Endplate stress was markedly elevated in the reduced‑stiffness model (average 13.5% increase), suggesting a hypothesis‑generating concern for fracture risk. Endo-ULBD suggests preserved segmental stability under the tested loading conditions, with only modest changes in ROM in both healthy and reduced‑stiffness (osteoporosis‑simulating) models. These findings are computational predictions and require clinical validation. The ROM increase observed in our reduced‑stiffness (osteoporosis‑simulating) model (similar to that of the healthy model) should not be directly extrapolated to clinical osteoporosis, as our model does not include the degenerative changes (disc degeneration, facet hypertrophy, ligament thickening) that typically coexist with osteoporosis in elderly patients. Future models that incorporate both bone quality deterioration and age‑related soft‑tissue degeneration are needed to determine whether osteoporotic patients truly have a lower risk of iatrogenic instability after Endo‑ULBD. For now, the present finding is hypothesis‑generating only. However, the marked rise in endplate stress in osteoporotic bone suggests a potential biomechanical mechanism for increased fracture risk. It is important to note that our reduced‑stiffness (osteoporosis‑simulating) model represents reduced bone stiffness (elastic modulus) only, without simulating microarchitectural deterioration such as trabecular loss or cortical thinning. Therefore, the absolute magnitude of clinical risk cannot be directly inferred from these computational results. Instead, the findings should be considered hypothesis‑generating and require validation using more sophisticated models and clinical studies. It is important to note that our reduced‑stiffness (osteoporosis‑simulating) model represents only reduced bone stiffness and does not simulate microarchitectural heterogeneity (trabecular thinning, cortical porosity). These findings advance current knowledge by defining a bone quality‑specific biomechanical risk profile for Endo‑ULBD, suggesting that while segmental stability is preserved, osteoporotic spines are at higher biomechanical risk for endplate-related complications. This suggests that bone quality may be a relevant factor for further investigation in future clinical studies, but no surgical recommendations can be derived from this computational work.</p>