Objective:

To investigate the biomechanics of transforaminal lumbar interbody fusion (TLIF) with interspinous process device (IPD) or pedicle screw fixation under both static and vibration conditions by the finite element (FE) method.

Method:

A validated FE model of the L1-5 lumbar spine was used in this study. This FE model derived from computed tomography images of a healthy female adult volunteer of appropriate age. Then the model was modified to simulate L3-4 TLIF. Four conditions were compared: (i) intact; (ii) TLIF combined with bilateral pedicle screw fixation (BPSF); (iii) TLIF combined with U-shaped IPD Coflex-F (CF); and (iv) TLIF combined with unilateral pedicle screw fixation (UPSF). The intact and surgical FE models were analyzed under static and vibration loading conditions respectively. For static loading conditions, four motion modes (flexion, extension, lateral bending, and axial rotation) were simulated. For vibration loading conditions, the dynamic responses of lumbar spine under sinusoidal vertical load were simulated.

Result:

Under static loading conditions, compared with intact case, BPSF decreased range of motion (ROM) by 92%, 95%, 89% and 92% in flexion, extension, lateral bending and axial rotation, respectively. While CF decreased ROM by 87%, 90%, 69% and 80%, and UPSF decreased ROM by 84%, 89%, 66% and 82%, respectively. Compared with CF, UPSF increased the endplate stress by 5%-8% in flexion, 7%-10% in extension, 2%-4% in lateral bending, and decreased the endplate stress by 16%-19% in axial rotation. Compared with CF, UPSF increased the cage stress by 9% in flexion, 10% in extension, and decreased the cage stress by 3% in lateral bending, and 13% in axial rotation. BPSF decreased the stress responses of endplates and cage compared with CF and UPSF. Compared BPSF, CF decreased the facet joint force (FJF) by 6%-13%, and UPSF decreased the FJF by 4%-12%. During vibration loading conditions, compared with BPSF, CF reduced maximum values of the FJF by 16%-32%, and vibration amplitudes by 22%-35%, while UPSF reduced maximum values by 20%-40%, and vibration amplitudes by 31%-45%.

Conclusion:

Compared with other surgical models, BPSF increased the stability of lumbar spine, and also showed advantages in cage stress and endplate stress. CF showed advantages in IDP and FJF especially during vertical vibration, which may lead to lower risk of adjacent segment degeneration. CF may be an effective alternative to pedicle screw fixation in TLIF procedures.