Background <p>Sacroiliac joint dysfunction is a common cause of lower back pain, especially in patients undergoing lumbar fusion surgery. While spontaneous bony fusion, i.e. auto-fusion, of the sacroiliac joint may provide mechanical stability, the underlying pathophysiology of this disease remains incompletely understood. Here, we present two patients with pre-operative auto-fused joints presenting with sacroiliac joint dysfunction who underwent successful minimally invasive sacroiliac joint fusion.</p> Case presentations <p>Two patients presented with pre-existing radiologic auto-fusion of their sacroiliac joints with intra-articular injection-proven sacroiliac joint dysfunction refractory to conservative management. Both patients underwent minimally invasive sacroiliac joint fusion and reported complete and sustained pain relief persisting at one year post-operatively. Radiologic findings confirmed the appropriate placement of fixation hardware without complications.</p> Discussion <p>These cases challenge the conventional understanding that bony fusion resolves sacroiliac joint dysfunction by highlighting the potential role of non-mechanical factors, including neural contributions, in driving persistent pain. Neural innervation of the sacroiliac joint, including lateral branches of the L5-S3 dorsal rami, may be implicated, as supported by recent evidence of effective neural-based interventions.</p> Conclusions <p>This report highlights that sacroiliac joint dysfunction may be present in patients with pre-existing auto-fusion. The presence of bony fusion of the sacroiliac joint(s) may not preclude a diagnosis of sacroiliac joint dysfunction, and patients may benefit from minimally invasive fusion surgery. The underlying mechanism of sacroiliac joint dysfunction is complex and incompletely understood.</p>