The biomechanical effects of treating double-segment lumbar degenerative diseases with unilateral fixation through interlaminar fenestration interbody fusion surgery: a three-dimensional finite element study

Background: Transforaminal lumbar interbody fusion (TLIF) surgery has become increasingly popular in the surgical treatment of lumbar degenerative diseases. The optimal structure for stable double-segment fixation remains unclear.

Objective: To compare the biomechanical changes of unilateral fixation versus bilateral fixation in patients with lumbar degeneration undergoing double-segment TLIF surgery, and to explore the stability and feasibility of unilateral double-segment fixation.

Methods: A three-dimensional finite element model of L3-5 was established based on CT data from a recruited young male volunteer, and the model was validated to have reasonable predictive capability. Surgical procedures were simulated by adjusting bony structures to create models of unilateral and bilateral fixation for double-segment TLIF. Under a pure moment of 10 Nm, range of motion (ROM), extension, lateral bending, axial rotation movements, as well as stresses on interbody fusion devices, internal fixation, and endplates were recorded and compared.

Results: Unilateral fixation was fixed on the left side, with both groups performing flexion, extension, left lateral flexion, right lateral flexion, left rotation, and right rotation movements. All reconstructed conditions showed decreased motion from L3 to L5. Unilateral fixation had greater lumbar spine range of motion (ROM) in all directions compared to bilateral fixation. The greatest difference between the two occurred during right lateral flexion at the L3-4 segment, measuring 1.78°. During right lateral flexion at the L4-5 segment, the largest difference was 2.29°. Regarding stress on the fusion devices, unilateral fixation models exhibited higher stresses than bilateral fixation models, but no significant differences in stability were found. Terminal plate stress in unilateral posterior fixation was higher during flexion than in the bilateral model, showing a similar trend in stress changes. No significant difference was seen in internal fixation stress between the two groups during two-segment fusion, with the posterior internal fixation stress in unilateral fixation being 1.7 times higher during flexion and 1.9 times higher during left bending compared to bilateral fixation.

Conclusion: Unilateral fixation in two-segment transforaminal lumbar interbody fusion (TLIF) surgery can increase stability compared to bilateral fixation, with no significant differences observed between the two models. Unilateral two-segment fixation allows for greater lumbar spine mobility than bilateral fixation, albeit with a slight increase in stress on the posterior fixation and fusion devices under the unilateral fixation mode. This provides some biomechanical evidence for selecting surgical approaches for elderly patients who cannot tolerate long surgeries, suggesting that two-segment unilateral fixation may be advisable.

Clinical trial number: Not applicable.