Background: The blind sacroiliac joint (SIJ) block cannot always be performed accurately; it is commonly
performed in the office based setting because intraarticular and periarticular injections are effective for SIJ
pain. However, knowledge on the surface anatomy of the SIJ is lacking. The purpose of this study was to
analyze the surface anatomical location of the posterior-inferior margin of the SIJ.
Methods: After informed consent was obtained, fifty patients undergoing SIJ block in the prone position were
examined. The oblique angles where the anterior-inferior margin and the posterior-inferior margin of the SIJ
overlap on X-ray were evaluated. In addition, the surface anatomical relationships between the posterior-inferior
margin of the SIJ on X-ray and the posterior superior iliac spine (PSIS) and sacral hiatus by palpation were
assessed.
Results: The oblique angle was 5.4 ± 2.9o. The vertical and transverse distance between the posteriorinferior
margin of the SIJ and PSIS were 3.8 ± 0.8 cm and 0.9 ± 0.6 cm, respectively. The vertical and
transverse distance between the posterior-inferior margin of the SIJ and the midpoint of the sacral hiatus were
3.4 ± 0.7 cm and 3.9 ± 0.6 cm, respectively. Only the vertical distance between the posterior-inferior margin
of the SIJ and PSIS showed significant difference between the male and the female groups (P = 0.0016).
Conclusions: The measurements in this study can be used as a reference standards for the blind SIJ
block.