This study analyzed the prevalence of SI joint variations in DISH+ and DISH- patients who had undergone lumbar spine surgery. We demonstrated that bony bridging and ankylosis of the SI joint were frequently observed in DISH+ patients compared with DISH- patients. Further, anterior paraarticular bridging of the SI joint was the most common type of SI joint change. The middle to lower thoracic spine and SI joint were highly affected by DISH and introduced bony ankylosis. In addition, the lower end of vertebral ossification of DISH terminated from the thoracolumbar junction to the upper lumbar spine.
Stability of the SI joint is maintained through a combination of only some bony structures and very strong intrinsic and extrinsic ligaments [10]. The proximal and ventral aspects of the SI joints are connected with the ventral sacroiliac ligament (VSIL) and proximal sacroiliac ligament (PSIL), representing synovial joints [11]. On the other hand, the superior and posterior aspects contained strong fibrous joint spaces with interosseous ligaments. These ligaments produce the multidirectional and structural stability of the SI joint. Both the VSIL and PSIL connect with the border of the iliac and sacral cartilage. The transition zone from ligament to cartilage comprises fibrocartilage representing entheses. Entheses contain fibroblasts, chondrocytes, collagen fibers, and calcified matrix. Entheses could thus represent a site of endochondral ossification, resulting in paraarticular bony bridging of SI joints [17].
Our study revealed that the spinal level from the middle to lower thoracic spine and SI joints were highly affected by DISH and introduced bony ankylosis. We also found that ossification of vertebrae due to DISH terminated from the thoracolumbar to upper lumbar segment. Such ossified segments could presumably act as long lever arms, increasing mechanical stress on the lower lumbar spine, following lumbar spinal degeneration and hypertrophy of the ligamentum flavum [18]. Non-fused lower lumbar segments could thus represent major sites of lumbar spinal stenosis and disc herniation associated with DISH. Kagotani et al. reported the presence of DISH as significantly associated with the presence of lumbar spondylosis [16]. Further, Yamada et al. demonstrated DISH as a risk factor for LSS requiring surgery [19]. Although the contribution of DISH to the severity of lumbar spinal disorders remains unclear, mechanical overloading below ankylosed sites may be a key contributor to lumbar spinal stenosis in patients with DISH.
In terms of surgical treatments for lumbar spinal disorders accompanying DISH, Otsuki et al. reported short-segment lumbar interbody fusion as a factor in delayed pseudarthrosis and adjacent segment disease (ASD) [20]. Further, numerous studies have reported that surgical treatment for traumatic spine fracture accompanying DISH often requires multi-level fusion to avoid postoperative ASD [21,22,23]. To maintain postoperative sagittal alignment, pelvic screw insertion, as a strong anchor of spinal fixation, became an indispensable technique not only in patients with DISH, but also in many clinical scenarios such as adult degenerative scoliosis, flat-back syndrome and kyphosis [24]. S2-Alar-Iliac (S2-AI) instrumentation has spread rapidly as a pelvic anchoring method for penetrating the SI joint. Compared to the iliac screw, the advantage of the S2-AI method includes a lower profiling setting of the screw, less extensive dissection of tissue, and higher pullout resistance [25, 26]. Elder et al. reported use of the S2-AI as an independent predictor of preventing reoperation and surgical site infection [27]. However, the long-term influence of SI joint fixation remains unclear. According to our recent data, DISH+ patients frequently exhibited SI joint ankylosis. S2-AI fixation, traversing and disrupting the SI joint, thus would not represent a disadvantage for DISH patients with SI joint ankylosis. Knowledge of the presence and variations of SI joint changes could be helpful for deciding on operative procedures.
AS, which belongs to a group of related diseases termed spondyloarthritides (SpA) [11, 17], is widely known to also affect the SI joint and introduces ankylosis. Although both DISH and AS share several clinical and radiographic features in the spine, the characteristics of bone proliferation differ [12, 14]. AS introduces ossification within the peripheral part of the annulus fibrosus in the intervertebral discs. On the other hand, ossification of the anterior longitudinal ligament and adjacent connective tissue is common in DISH, but not generally observed in AS. Typical findings of the SI joint in AS include sacroiliitis including joint erosions, joint space narrowing, sclerosis, and intraarticular ankylosis, but none of these are common in DISH [8, 12]. According to recent progress in the treatment of SpA using biological disease-modifying antirheumatic drugs, including tumor necrosis factor inhibitors (TNFi) and interleukin 17 inhibitors (IL-17i) [28, 29], early diagnosis facilitates timely treatment and may minimize structural damage. The present findings may thus contribute to an understanding of radiographic changes in the SI joint associated with DISH and sacroiliitis from SpA.
Some limitations of this study must be considered. First, the evaluation of cases was retrospective, and the populations of both groups were limited to patients who had undergone lumbar spine surgery. Analysis of a general population would also be worthwhile to confirm SI joint alterations due to DISH. Second, general health status and histories, such as obesity and diabetes mellitus, were not the focus of this study. Relationships between clinical symptoms and SI joint alterations therefore need to be elucidated in future studies. Third, criteria for diagnosing DISH from CT have not been established.
