We reported that excessive disc space distraction caused by insertion of large cages during posterior lumbar interbody fusion (PLIF) combined with pedicle screws (PS) induces adjacent segment disease (ASD). Spinous process plate (SPP) is known to allow cage subsidence when used in PLIF, since they cannot share vertical loads. We therefore hypothesize that the incidence of ASD after PLIF with SPP should be lower than that after PLIF with PS due to this loss of disc space distraction.

Methods: Radiographic ASD is defined as development of spondylolisthesis > 3 mm, decrease in disc height > 3 mm, or intervertebral angle at flexion lesser than -5°. Symptomatic ASD is defined by a decrease of ≥ 4 points in Japanese Orthopedic Association score. One hundred and thirty patients with L4 spondylolisthesis were treated with either PLIF with SPP (n = 45) or PLIF with PS (n = 85) and followed up for a minimum of 2 years (mean, 39.0 months).

Results: L4-5 disc space distraction in the SPP group was significantly smaller (0.4 mm) as compared to the PS group (1.8 mm). The incidence of clinical ASD in the SPP group (2%, 1/45) was also significantly less than that in the PS group (15%, 13/85). Multivariate analysis showed that disc space distraction was the most significant risk factor.

Conclusions: Among various risk factors for ASD after PLIF, the excessive distraction of disc space proved to be a potent risk factor for ASD.