<qa>  What is the key anatomical structure of the posterior sacral network?A recent cadaveric study by our research group has clarified the sensory innervation of the SIJ and the findings have implications for currently practiced SIJ sensory block and RFA procedures. The study confirmed that the posterior sacral network innervates the SIJ and runs along the lateral crest of the sacrum between the S1 and the S3 lateral sacral tubercles. Inasmuch as the lateral sacral crest is predictably and clearly visible on ultrasound, posterior sacral network PSN SIJ sensory blocks and RFA under ultrasound guidance may be feasible using the lateral sacral crest as the key anatomical structure. 