Phase-specific changes in hip joint loading during gait following sacroiliac joint fusion: Findings from a finite element analysis.

Background: Low back pain affects over 80 % of adults, with sacroiliac joint dysfunction accounting for 15-30 % of these cases. Sacroiliac fusion is a surgical procedure for refractory joint pain. While the biomechanics of the joint and its fusion relative to the spinal column are well-known, the hip-spine relationship post-fusion remains unclear. Understanding the biomechanical state following fusion can enhance patient recovery and optimize surgical outcomes. This study uses finite element analysis to assess hip joint biomechanics following sacroiliac joint fusion.

Methods: CTs of a 55-year-old male were used to create a biomechanical model, validated against a cadaveric study. Three triangular titanium alloy implants were placed across the sacroiliac joint in a unilateral and bilateral configuration. The model, loaded with pelvis and hip joint kinematics during a gait cycle, calculated joint reaction forces, contact stress and area on the hip joint across various gait phases.

Findings: Hip joint contact stresses varied with fixation configurations and gait phases. Unilateral right fusion reduced joint reaction forces by 2 % but increased contact stress by 3.7 %. Bilateral fusion increased joint reaction forces by 6.7 % and contact stress by 3.25 %, with higher stress during foot flat and heel off phases compared to unilateral fixation.

Interpretation: Fusion alters hip loading patterns during specific gait phases, with bilateral fusion producing the highest stresses during foot flat and heel off. These findings may suggest the need for fusion-specific rehabilitation protocols and warrants further investigation of long-term joint health outcomes.