TITLE: Risk Factors for Early versus Late Adjacent Segment Disease requiring revision surgery after Lumbar Fusion – are they comparable?

Background : Adjacent segment disease (ASDis) is a common but poorly understood complication following lumbar fusion surgery. It remains uncertain whether this association is a result of the altered biomechanics following the fusion or simply a continuation of the natural progression of degenerative disc disease. To date, no published data exists that identifies the risk factors that predict the need for surgical intervention in Early (<5 years) versus Late (≥5 years) ASD. Methods : We conducted a retrospective study analysing all lumbar fusion surgeries (≤2 levels) performed at a single center over a period of 25 years. 3973 patients were screened, of which 1395 were excluded and 134 were diagnosed with ASD. Demographic, clinical (initial diagnosis, BMD T-score, history of smoking), radiographic (pre-existing disc degeneration or facetal fluid, PI, LL, PT, PI-LL mismatch), and surgical data (type of fusion surgery, fusion levels, floating vs non-floating fusion, and interventions) were compared between the groups. Results : 62(2.40%) and 72(2.79%) patients were classified as Early- and Late-onset groups respectively. The mean post-operative duration for the development of Early and Late ASD was 2.82±1.11 and 7.96±2.41 years respectively. Incidence of ASD was more in the proximal segment (n=121, 90.29%) as compared to the distal segment (n=13, 9.70%). Pre-operative pathology at the operated level (p=0.769) and the type of adjacent segment degenerative changes seen Later were not significantly different between the groups (p=0.168). Similarly, preoperative disc and facet degeneration at adjacent segments showed no significant intergroup difference (p=0.998). The number of fusion levels, type of fusion, and floating vs non-floating fusion showed no correlation to the development of Early or Late ASD, however smoking/tobacco use ( p=0.022); LL (p=0.0001 ), SS ( p=0.0001) and PI-LL mismatch ( p=0.0009) measured after the index fusion surgery were associated with the development of ASD. Conclusion : Presence of asymptomatic degenerative changes at the adjacent segment level has no bearing on the timing of development of adjacent segment disease. Smoking/tobacco use is a major contributor to the development of Early ASD. Additionally, inadequate restoration of lumbar lordosis and pelvic parameters leads to the development of both Early and Late-onset ASD.