Spinal fusion surgery is widely used to treat various lumbar spinal disorders, including spinal degenerative disease. Although fusion procedures provide stability and have good functional outcomes [1], [2], adjacent segment disease (ASD) is a potential postoperative complication that affects clinical outcomes. Previous studies have identified several risk factors associated with ASD, such as age, sex, body mass index, preexisting degeneration, violation of the adjacent facet during surgery, sagittal imbalance, and the length of fusion [3], [4].

A lumbosacral transitional vertebra (LSTV) is a common congenital anomaly. In 1984, Castellvi et al. [5] described a radiographic classification of LSTVs into four types based on morphological characteristics. Typically, degenerative changes occur more frequently at the level above an LSTV, possibly due to hypermobility and increased torque at the superior levels [6], [7]. A cadaveric biomechanical study showed that L4–5 fusion in patients with LSTV significantly increased torques and adjacent segment motion at all levels other than L5–S1 compared to LSTV alone [8]. Despite the prevalence of ASD, there is a lack of evidence elucidating the relationship between ASD and LSTV after lumbar fusion surgery.

To the best of our knowledge, the relationship between LSTV and ASD after L4–5 lumbar fusion has not yet been evaluated in detail. This study aimed to explore the association between ASD following L4–5 lumbar interbody fusion surgery and LSTV.