Object: Spinal fusion at the L4-5 disc space alters the normal biomechanics of the spine, and the loss of motion at the fused level is compensated by increased motion and load at the other unfused segments. This may lead to deterioration of the adjacent segments of the lumbar spine, called adjacent-segment disease (ASD). In this study, the authors investigate the distracted disc height of the fused segment, caused by cage or bone insertion during surgery, as a novel risk factor for ASD after posterior lumbar interbody fusion (PLIF).
Methods: Radiographic L3-4 ASD is defined by development of spondylolisthesis greater than 3 mm, a decrease in disc height of more than 3 mm, or intervertebral angle at flexion smaller than -5 degrees . Symptomatic ASD is defined by a decrease of 4 points or more on the Japanese Orthopaedic Association scale. Eighty-five patients with L-4 spondylolisthesis treated by L4-5 PLIF underwent follow-up for more than 2 years (mean 38.8 +/- 17.1 months). The patients were divided into 3 groups according to the final outcome. Group A comprised those patients without ASD (58), Group B patients had radiographic ASD (14), and Group C patients had symptomatic ASD (13).
Results: The L4-5 disc space distraction by cage insertion was 3.1 mm in the group without ASD, 4.4 mm in the group with radiographic ASD, and 6.2 mm in the group with symptomatic ASD, as measured using lateral spinal radiographs just after surgery. Multivariate analysis showed that distraction was the most significant risk factor.
Conclusions: The excessive distraction of the L4-5 disc space during PLIF surgery is a significant and potentially avoidable risk factor for the development of radiographic, symptomatic ASD.