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

Abstract

Pedicle screw fixation (PSF) has been used for the setting of a lumbar interbody fusion. 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. An addition of PSF to the stand-alone ALIF increases segmental stiffness. 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. 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. 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.