The sacrum has paired anterior (pelvic) and posterior sacral foramina at four levels (S1-S4). The anterior sacral foramina transmit the anterior primary rami of the sacral nerves and the lateral sacral arteries. The posterior sacral foramina are smaller and transmit the posterior primary rami. The S1 foramen is the largest and most clinically accessed. The foramina are identifiable on CT and radiograph and provide percutaneous access to the sacral nerve roots.
Sacral nerve root blocks and sacral nerve stimulation (neuromodulation) target the sacral foramina percutaneously under fluoroscopic guidance. S3 stimulation through the third sacral foramen is the standard approach for sacral neuromodulation (InterStim) treating overactive bladder, urgency incontinence, and faecal incontinence. Sacral fractures through the foramina produce a Denis zone II pattern associated with sacral nerve root injuries causing bowel, bladder, and sexual dysfunction. Pelvic ring disruption surgery uses iliosacral screws inserted through the posterior sacral mass at S1 and S2 levels.
Vertical shear or compression fractures through the sacral foramina (Denis zone II) cause partial sacral plexus injuries including unilateral S1-S3 deficits producing weakness of plantar flexion, bowel and bladder dysfunction, and saddle anaesthesia depending on which foramina are disrupted.
A permanent electrode lead placed through the third sacral posterior foramen under fluoroscopic guidance delivers low-voltage stimulation to the S3 sacral nerve root, modulating bladder, bowel, and pelvic floor function for refractory overactive bladder and faecal incontinence.
