Comparison of posterior versus transforaminal lumbar interbody fusion using finite element analysis

Objectives: To compare the influence of posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) on adjacent segment degeneration. Methods: The study was carried out in the Traumatology and Orthopedics Laboratory, Department of Traditional Chinese Medicine, Medical School, Jinan University, Guangzhou, China, between December 2013 and November 2014. A normal, healthy finite element model of L3-5 was developed, a PLIF and a TLIF model were modified from the normal model, and interbody fusions were performed in the L4-5 segment. An 800 N compressive loading plus 10 Nm moments simulating flexion, extension, lateral bending, and axial rotation were imposed on the L3 superior endplate. Intradiscal pressure and intersegmental rotation in L3-4 were investigated. Results: The values of intradiscal pressure and intersegmental rotation in the PLIF or TLIF model were higher than those in the normal, healthy model, but the values in the TLIF model were relatively lower than those in the PLIF model in all directions. Conclusion: Posterior lumbar interbody fusion has more adverse influence on the superior adjacent segment than TLIF. Saudi Med J 2015; Vol. 36 (8): 993-996 doi: 10.15537/smj.2015.8.11759 B transforaminal lumbar interbody fusion (TLIF) and posterior lumbar interbody fusion (PLIF) are standard techniques of lumbar fusion to treat degenerative lumbar disorders.1,2 Many clinical studies have been performed to compare the surgical results of the 2 techniques, most of which suggested TLIF and PLIF had no significant difference in clinical outcomes.3,4 In addition, Zhang et al5 found in a metaanalysis that patients undergoing PLIF had a higher incidence of complications than those undergoing TLIF. In a retrospective study of 163 patients, Hey and Hee6 found a reduced risk of vessel and nerve injury, shortened operating time, and reduced intraoperative bleeding in TLIF. Subsequently, TLIF became an optimal selection when spine surgeons developed treatment strategies. However, in terms of adjacent segment degeneration (ASD), and the long-term complications after lumbar fusion, few studies have been performed to compare the influence of PLIF and TLIF on the adjacent segment. In a unique biomechanical study performed using human cadavers, Sim et al7 found PLIF and TLIF had similar biomechanical properties regarding range of motion, intradiscal pressure, and laminar strain at adjacent segments. In our opinion, these 2 techniques have differences in cage selection and excision of posterior elements, which may affect the conduction of stress, and produce different influences on adjacent segment. In addition, Sim et al’s7 study was an immediate test after cage placement and instrumentation fixation, but ASD usually occurs after solid fusion and a study using samples with solid fusion, may be better in clarifying the issues. The limitations in clinical study make discrete characterization of the effects of lumbar interbody fusion on the adjacent segment significantly difficult. In addition, specimens for cases and controls in clinical studies are difficult to obtain and standardize.8 By contrast, the finite element technique, which is highly reproducible and repeatable, can mitigate these problems. A finite element model can be adjusted in material properties, loading mode or structural shape, to simulate normal, degenerative, fusion or other different situations. Compared with other experimental methods, a finite element method presents many advantages, which facilitates a comparative study among models with different biomechanical situations.8 Therefore, we developed a 3-dimensional finite element model of L3-5 for the normal, healthy spine, along with a PLIF and a TLIF model, our aim was to compare the biomechanical influence of PLIF and TLIF on adjacent segments. Methods. The study was carried out in the Traumatology and Orthopedics Laboratory, Department of Traditional Chinese Medicine, Medical School, Jinan University, Guangzhou, China, between December 2013 and November 2014. A normal, healthy model of L3-5 was created (Figure 1) and validated in previous studies.8 A PLIF and a TLIF model were modified from the normal, healthy model. Both TLIF and PLIF were performed at L4-5. We assumed the elements of L3-4 segments including intervertebral disc, facet joints, endplates, and vertebral bodies were normal in all models. To mimic PLIF and TLIF, L4-5 Brief Communication