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Background context: Report of sacroiliac morphology changes during growth is limited in the literature and the interest of such morphology and its consequence for surgery is increasing.Purpose: Aims of this work are 1) to anatomically define the sacroiliac joint (SIJ), 2) to assess the influence of growth on the sacroiliac morphology and the pelvic parameters.Study design: Forty-nine young subjects from 6 months to 18 years old (y/o) and 20 adults aged from 18 to 50 y/o were selected from our institutional patient database.Methods: They underwent a CT examination on a 128-MDCT (GE Healthcare Optima CT660). Transverse CT image datasets were reconstructed, anonymized and segmented with ITK-SNAP. Landmarks and surfaces were selected and a SIJ orientation analysis was performed using costumed Python scripts.Results: The subjects were divided into four groups: infants (1.9±1 y/o), children (6.9±1.7 y/o), adolescents (13.7±1.8 y/o) and adults (27.3±5.6 y/o). Differences between SIJ orientation were found significant between young subject groups for synovial sacrum SIJ orientation (p<0.001) and iliac total SIJ orientation (p=0.036).Both orientations of younger subjects were found significantly different from the adult group (p<0.035). SIJ synovial sacrum and iliac total orientations correlated significantly with age (p<0.03). All orientations correlated with pelvic incidence (PI) (p<0.04) except for synovial sacrum SIJ orientation (p=0.2). No gender or symmetrical differences were found significant in any group. Conclusion:Morphological definition of the SIJ confirmed the independency of the gender during growth. Such results will be beneficial for the analysis and management of vertebral pathology.

The sacroiliac joint (SIJ) is a known pain generator that, in severe cases, may require surgical fixation to reduce intra-articular displacements and allow for arthrodesis.The objective of this computational study was to analyze how the number of implants affected SIJ stabilization with patient-specific characteristics such as the pelvic geometry and bone quality. Detailed finite element models were developed to account for three pelvises of differing anatomy. Each model was tested with a normal and low bone density (LD) under two types of loading: compression only and compression with flexion and extension moments. These models were instrumented with one to three cylindrical, threaded and fenestrated implants through a posterior oblique trajectory, requiring less muscle dissection than the more common lateral trajectory used with triangular implants. Compared with the noninstrumented pelvis, the change in range of motion (ROM) and stress distribution were used to characterize joint stabilization. Noninstrumented mobility ranged from 0.86 to 2.55 mm and from 1.37°to 6.11°. Across patient-specific characteristics, the ROM reduction with one implant varied from 3% to 21% for vertical and 15% to 47% for angular displacements. With two implants, the ROM reduction ranged from 12% to 41% for vertical and from 28% to 61% for angular displacements. Three implants, however, did not further improve the joint stability (14% to 42% for vertical and 32% to 63% for angular displacements). With respect to patient characteristics, an LD led to a decreased stabilization and a higher volume of stressed bone (>75% of yield stress). A better understanding of how patient characteristics affect the implant performance could help improve surgical planning of sacroiliac arthrodesis.