Biomechanical effects of combined transforaminal and interlaminar endoscopic decompression for lumbar spinal stenosis across different degrees of disc degeneration: a finite element analysis

Percutaneous endoscopic transforaminal decompression (PETD) and percutaneous endoscopic interlaminar decompression (PEID) are effective minimally invasive procedures for lumbar spinal stenosis (LSS). However, a single approach may be insufficient for multiregional stenosis at the same responsible segment. Combined PETD and PEID may extend the decompression range, but its biomechanical effects under different degrees of disc degeneration remain unclear. A three-dimensional finite element model (M0) of L1–S1 was established and validated based on lumbar CT data from a healthy male volunteer. PETD (M1), PEID (M2), and combined PETD and PEID (M3) models were created at the L4–L5 segment under normal, mild, moderate, and severe disc degeneration. Biomechanical outcomes included range of motion (ROM), intradiscal pressure (IDP), and facet joint stress (FJS). Percentage changes were calculated relative to the normal nonoperative M0 model. Under the normal disc condition, ROM, IDP, and FJS were higher in all surgical models than in M0, with M3 showing the highest values. As disc degeneration increased, ROM generally decreased whereas IDP continuously increased. At the same disc degeneration grade, M3 showed higher ROM, IDP, and FJS than M1 and M2 in all motion directions. Under severe disc degeneration, ROM in M3 decreased by 46.9%–61.2%, whereas IDP increased by 70.2% to 104.0% relative to M0. FJS showed a direction dependent pattern, decreasing during extension, increasing during axial rotation, and fluctuating during flexion and lateral bending as disc degeneration progressed. Under severe disc degeneration, FJS in M3 increased by 87.1% during left rotation and 91.8% during right rotation. Across all degeneration grades and motion directions, FJS in M3 was higher than that in M1 and M2. Combined PETD and PEID produced more pronounced segmental biomechanical changes than either approach alone under the same disc degeneration background. Disc degeneration was associated with reduced ROM, increased IDP, and motion dependent changes in FJS, particularly during axial rotation. These findings suggest that disc degeneration may influence segmental biomechanics after endoscopic decompression. Preoperative disc degeneration and the anticipated extent of structural resection should be considered when assessing combined PETD and PEID decompression.