Finite element analysis of anterior lumbar interbody fusion: threaded cylindrical cage and pedicle screw fixation

Abstract

Study design: Pedicle screw fixation (PSF) has been used for the setting of a lumbar interbody fusion.

Objective: To investigate analytically the effects of additional PSF to the anterior lumbar interbody fusion (ALIF) with the cylindrical cages (Ray TFC) in the immediate postoperative state.

Summary of background data: An addition of PSF to the stand-alone ALIF increases segmental stiffness.

Methods: A finite element model of ALIF (L4,-L5) was constructed. Finite element analyses were performed to investigate relative motion and bone stress at the bone-cage interface. The results were then compared with those of the stand-alone ALIF. Bone stress surrounding pedicle screws was also predicted.

Results: An addition of PSF to the stand-alone ALIF led to higher segmental stiffness, smaller relative motion, smaller bone deformation, and lower bone stress level at the cage-bone interface. The bone stress level at the interface reduced substantially during extension. The bone stress level at the bone-screw tip interface was much lower than that of the bone-cage interface. The articulating facet had no significant load-bearing capacity because of stiff PSF. Little gap opening at the bone-cage interface was found during a compressive preload over 400 N; an excessive extension without preload caused considerable gap opening. Slip less than 0.15 mm occurred during all the loading modes except axial rotation; an excessive axial rotation caused to slip over 0.15 mm.

Conclusion: Geometric constraints caused by PSF will reduce substantially the bone stress level and the relative motion, and therefore be more likely to allow bone ingrowth at the bone-cage interface, compared with the stand-alone ALIF. The use of pedicle screw stabilization would have significant beneficial effects on the rate of interbody fusion, regardless of whether ALIF or PLIF was used.