LAUSR

The subchondral lamella of the sacroiliac auricular surface is morphologically inconsistent. Its morpho‐mechanical relationship with dysfunction (SIJD) remains unstudied. Here, the iliac and sacral subchondral bone mineralization is compared between morphological subtypes and in large and small surfaces, in SIJD joints and controls. CT datasets from 29 patients with bilateral or unilateral SIJD were subjected to CT‐osteoabsorptiometry. Surface areas and posterior angles were calculated and surfaces were classified by size: small (<15 cm3) and large (≥15 cm3), and morphological types: 1 (>160°), 2 (130°–160°), and 3 (<130°). Mineralization patterns were identified: two marginal (M1 and M2) and two non‐marginal (N1 and N2). Each sacral and iliac surface was subsequently classified. Dysfunctional cohort area averaged 15.0 ± 2.4 cm2 (males 16.2 ± 2.5 cm2, females 13.7 ± 1.6 cm2). No age correlations with surface area were found nor mean Hounsfield Unit differences when comparing sizes, sexes or morphology‐type. Controls and dysfunctional cohort comparison revealed differences in female sacra (p = 0.02) and small sacra (p = 0.03). There was low‐conformity in marginal and non‐marginal patterns, 26% for contralateral non‐dysfunctional joints, and 46% for dysfunctional joints. The majority of painful joints was of type 2 morphology (59%), equally distributed between small (49%) and large joints (51%). Larger joints had the highest frequency of dysfunctional joints (72%). Auricular surface morphology seems to have little impact on pain‐related subchondral lamella adaptation in SIJD. Larger joints may be predisposed to the onset of pain due to the weakening of the extracapsular structures. Dysfunctional joints reflect common conformity patterns of sacral‐apex mineralization with corresponding superior corner iliac mineralization.