Regional functional anatomy of the anterior sacroiliac ligament: Segmental resection reveals differential restraint of sacroiliac joint motion.

BACKGROUND

The anterior sacroiliac ligament (ASL) is often described as a thin ventral capsular reinforcement, potentially obscuring regional functional specialisation. We tested whether its superior and inferior regions provide distinct three-dimensional restraint during unilateral and bilateral anterior pelvic loading.

METHODS

Twenty-seven fresh-frozen pelves (19 male and 8 female; mean age, 57.1 ± 15.5 years) were tested under anteriorly directed loading applied at the anterior superior iliac spines from 10 to 100 N in intact, superior-resected, inferior-resected, and complete-resection states. Loading order and the superior-first versus inferior-first resection sequence were randomised. Optical tracking quantified three-dimensional translation and rotation. For each test condition, the mean of three repeated measurements was analysed. Data were analysed by two-factor repeated-measures ANOVA with Greenhouse-Geisser correction and Holm-adjusted contrasts.

RESULTS

Ligament state, loading mode, and their interaction affected every axis-specific and resultant outcome (all corrected p < 0.001; partial η² ranges, 0.981-0.992, 0.932-0.994, and 0.655-0.931). Superior resection increased all translational and rotational components. Inferior resection minimally affected unilateral X- and Y-axis translation but increased rotation and reduced local bilateral translation, indicating kinematic redistribution. Complete resection produced the greatest resultant motion. Bilateral loading increased resultant translation by 31.1%, 12.5%, 0.6%, and 17.1% in the intact, superior-resected, inferior-resected, and complete-resection states, respectively; the inferior-resection change was not significant.

CONCLUSIONS

The ASL is a regionally organised ligament rather than a mechanically uniform capsular reinforcement. Superior fibres provide broad multi-axial restraint, whereas inferior fibres are associated with a more rotation-dominant, loading-mode-dependent kinematic response. This regional functional anatomy provides a mechanistic basis for interpreting ventral SIJ disruption.