To determine the necessity of an additional posterior lumbar interbody fusion (PLIF) after a posterolateral fusion (PLF) for the treatment of degenerative spondylolisthesis (DS).

A retrospective study, after a minimum follow-up of 2 years was conducted on forty patients who underwent a single level decompression and instrumented fusion for DS with spinal stenosis at the L4-5 level. A PLF was performed in 21 patients, and a circumferential fusion (CF) with an additional PLIF in 19 patients. According to the fusion methods and preoperative segmental mobility, the patients were divided into four groups; s-PLF group (PLF in the stable group, n=13), s-PLIF group (CF in the stable group, n=11), u-PLF group (PLF in the unstable group, n=8), and u-PLIF group (CF in the unstable group, n=8). Clinical and radiographic comparisions between the PLF and PLIF groups were performed.

The mean decrements of Oswestry Disability Index (Visual Analog Scale) scores were 29% (5.5), 29% (5.9), 22% (2.6) and 42% (5.9) respectively for the s-PLF, s-PLIF, u-PLF and u-PLIF groups, and a statistical difference was found only between the u-PLF and u-PLIF groups (ODI: p=0.032, VAS: p=0.004). Fusion rates were 92%, 100%, 88% and 100% respectively. The mean slip angle increments were serially 2.5 °, -3.1 °, -1.5 ° and -0.3 °, and the mean percent slip decrements were 6.7%, 8.7%, 5.1% and 3.7%, and the mean disc height increments were -0.4 mm, 1.8 mm, 0.5 mm and 3.0 mm, and the mean lumbar lordosis increments were 8.6 °, 4.7 °, -1.9 ° and 1.9 ° and the mean sacral tilt increments were 3.8 °, 3.4 °, -1.3 ° and 0.9 °. Statistical differences were found only between the s-PLF and s-PLIF groups in slip angle increments (p=0.029) and between the s-PLF and s-PLIF groups (p=0.043) and between the u-PLF and u-PLIF groups (p=0.042) in disc height increments.

PLF alone provided successful clinical outcome in stable group, but CF provided better clinical outcomes in the unstable groups. This study suggests that preoperative segmental mobility may be a criterion to determine whether or not an additional PLIF is necessary in the treatment of lumbar DS.