Can sacroiliac joint surgery change motion, load transfer, and contribute to persistent pain if instability remains?

Sacroiliac (SI) fusion/fixation is intended to stabilize a joint with normally small motion. The claim is well supported for its mechanical premise; the proposed link from residual mechanical abnormalities to persistent pain is clinically credible but not proven as a direct causal relationship.

Biomechanical effects

• Cadaveric testing consistently shows reduced local SI motion after fixation/fusion. One threaded-rod construct reduced rotational and translational range of motion by 65–71% versus intact joints; the degree and plane of stabilization vary with implant configuration and trajectory.
• Finite-element studies likewise show reduced SI displacement, reduced SI-ligament tensile forces, and redistribution of stress into pelvic bone and across the implant–fusion interface. Thus, surgery changes load sharing rather than simply “removing” load from the region.
• Device behavior under fatigue matters: some constructs maintained the bone–implant interface, whereas others showed migration or subsidence, potentially undermining intended stabilization.

Persistent postoperative pain and mechanical failure

• Residual instability is a plausible pain pathway: bone–implant micromotion may impede osseointegration, favor fibrous tissue, and contribute to loosening or pseudarthrosis.
• In an 18-joint revision series, pseudarthrosis, loosening, and partial fusion with persistent pain were revision indications. Revision with decortication, grafting, compression, and rigid fixation achieved 88.9% CT-confirmed fusion at 1 year, but only 55.6% achieved clinically important pain improvement.
• Symptomatic pseudarthrosis and implant malposition are documented reasons for revision; malposition may produce radicular or SI-region pain independently of nonunion.

Clinical interpretation

• Bottom line: SI surgery reliably alters local motion and load transfer. Persistent pain can reflect inadequate stability, nonunion, loosening, or malposition, but mechanical findings do not establish a sole pain generator; lumbar, hip, neural, soft-tissue, and infectious causes may coexist and require targeted assessment.