Biomechanical Effects of Posterior Lumbar Reconstruction on the Adjacent Segment

INTRODUCTION: To overcome the limitations of posterolateral fusion (PLF) in maintaining lumbar lordosis, posterior lumbar interbody fusion (PLIF) has been developed. This study compares the biomechanical effects of three different posterior reconstructions on the adjacent motion segment0and discusses indication for PLF and PLIF.

METHODS: Ten calf spines (L3-S1) underwent non-destructive flexion-extension testing (±6 Nm). After testing intact specimens, destabilization was performed at L5-L6-S1 and followed by pedicle screw fixation (PS) with and without interbody cages as follows: 1) PS with straight rods [ in situ PLF], 2) PS with kyphotically pre-bent rods [kyphotic PLF], and 3) PS and interbody cages with straight rods [ in situ PLIF/PLF]. Construct stiffness of the operative segments, intradiscal pressure and longitudinal lamina strain at the upper adjacent segment (L4-L5) were analyzed.

RESULTS : Construct stiffness of the in situ PLIF/PLF was significantly higher compared to both in situ and kyphotic PLF, however, statistical significance was not detected between two PLFs. The in situ PLIF/PLF presented greater intradiscal pressure and lamina strain compared to in situ PLF under flexion loading. Higher intradiscal pressure and lamina strain exhibited by the kyphotic PLF compared to in situ PLF under extension, were further increased in the in situ PLIF/PLF group.

CONCLUSIONS: This study investigated the intradiscal pressure and lamina strain as parameters of biomechanical effects on the unfused adjacent segment and indirectly evaluated the load transmission through the anterior and posterior columns. The kyphotic PLF indicated higher intradiscal pressure and lamina strain compared to the in situ PLF, suggesting that local kyphosis of the operative segments causes excessive load on the cranial adjacent segment. Importantly, the in situ PLIF/PLF demonstrated greater intradiscal pressure and lamina strain at the adjacent segment compared to the kyphotic PLF. Hence, additional PLIF may lead to higher adjacent level load transmission through both anterior and posterior columns due to increased construct stiffness, even if local kyphosis is corrected by performing PLIF.