Spontaneous Lumbar Interbody Fusion Following Posterolateral Fusion Surgery: A Retrospective Analysis of 5-Year Data.

BACKGROUND: Posterolateral fusion (PLF) surgery is frequently performed for a range of spinal disorders. However, spontaneous lumbar interbody fusion (SLIF) following PLF surgery is yet to be reported. Thus, we evaluated the incidence and characteristics of SLIF among patients that underwent PLF surgery.

METHODS: This retrospective study involved review of electronic medical records of 121 adult patients who underwent primary lumbar decompression with instrumented PLF between 2006 and 2011. The available radiographs of L2-S1 region were assessed for SLIF and PLF. At 1 year, modified Lee's and Lenke's criteria were used to assess SLIF and PLF, respectively. Differences between the patients in the fusion and non-fusion groups were evaluated.

RESULTS: At 1-year follow-up, 28.93 and 87.61% patients had SLIF and PLF, respectively. Moreover, 27.27% patients had both SLIF and PLF. L4-L5 ( n  = 13) was the most common segment involved in SLIF. SLIF rate was significantly greater among young adults ( p value = 0.001), and those with no pre-operative instability ( p value = 0.003) as well as who underwent pedicular fixation instrumented PLF surgery ( p value < 0.0001). While, PLF was significantly greater in patients who did not undergo discectomy ( p value = 0.049). SLIF was not significantly associated with sex, age groups, discectomy status, and level of PLF surgery (all p values > 0.05). PLF was not significantly associated with sex, age groups, pre-operative instability, type of instrumentation, and level of PLF surgery (all p values > 0.05). There was no significant association between patients with SLIF and PLF ( p value = 0.155).

CONCLUSIONS: More than a quarter of patients developed SLIF and majority of them had PLF. SLIF was significantly associated with younger age at surgery and use of pedicular fixation instruments. Level of Evidence III; retrospective cohort study.