Lumbar/lumbosacral fusion is currently the mainstay of surgical procedures for treating degenerative lumbar disc disease, a condition marked by the gradual deterioration of intervertebral disc that predominantly impacts elderly people, often resulting in chronic lower back pain, limited mobility, and a diminished quality of life [1], [2]. Osteoporosis is also an age-related disease with the increasing prevalence resulting from the aging population. According to the clinical observations, the patients over 50 years old requiring spine surgery have osteoporosis rate as high as 31% [3], [4]. Consequently, osteoporosis should be a major consideration for the patients undergoing spine surgery. Osteoporosis is characterized by the deterioration of bone tissue with the loss of bone strength, potentially leading to biomechanical alterations in postoperative spine and thereby influencing surgical outcomes. The biomechanical effects of osteoporosis on the fusion surgery have been investigated and reported. For example, several researchers found that range of motion (ROM) at the surgical level increased when osteoporosis occurred, indicating that osteoporosis decreases the stability of postoperative spine [5], [6]. Also, osteoporosis was found to increase the risk of implant failure, such as screw loosening and cage subsidence [7], [8], [9].

Although these previous studies have provided significant insights into the link between osteoporosis and outcomes of the fusion surgery, the majority of them have concentrated primarily on lumbar spinal segments, while scant attention has been directed toward the adjacent sacroiliac joint (SIJ). SIJ, the articulation between the ilium and the sacrum on each side of the pelvis, plays a pivotal role in load transfer and stability maintenance, ensuring a smooth transition of forces from the spine to pelvis and lower extremities [10]. SIJ is a widely described source of lower back pain. Researchers have identified it as a pain generator in approximately 13% to 30% of patients after lumbar/lumbosacral fusion procedures [11], [12]. Recent evidence suggests that the fusion procedures may alter the SIJ biomechanics, for example, several biomechanical studies reported that lumbosacral fusion increased ROM and stress across the SIJ [13], [14], [15], which could be a probable reason for lower back pain in postoperative patients.

Given the significant impact of osteoporosis on the spinal fusion outcomes and the recognition of SIJ as a potential pain generator following fusion procedures, it is necessary to understand the biomechanical effects of osteoporosis on SIJ subsequent to the surgical interventions. However, to the best of our knowledge, there was no reported studies to date. Therefore, this study aimed to determine the effects of osteoporosis on SIJ biomechanics after lumbosacral fusion, and to identify the potential implications for postoperative outcomes.