PSis

PSIS stands for the posterior superior iliac spine, a prominent bony landmark on the pelvis. It is the posterior end of the iliac crest and can often be felt as a pair of small dimples on the lower back, roughly at the level of the S2 vertebra in many individuals.

Anatomy and relations: The PSIS is located on the posterior aspect of the ilium and marks the boundary of the greater sciatic notch. It serves as an attachment point for ligaments of the sacroiliac joint, including components of the posterior sacroiliac ligaments. The PSIS is used as a surface reference in clinical examination and imaging to assess alignment of the pelvis and sacroiliac region.

Clinical relevance: The PSIS functions as an important surface landmark in physical examination and clinical procedures. It helps clinicians palpate the sacroiliac joints, assess pelvic alignment, and guide certain injections or interventions in the gluteal region, though modern practice emphasizes safer injection sites and techniques. The prominence and symmetry of the PSIS can be evaluated to detect pelvic tilt or leg length discrepancies and to monitor posture or spinal–pelvic mechanics.

Variation and imaging: The prominence and exact position of the PSIS can vary between individuals. In some people, the dimples are less noticeable or not easily palpable. Imaging modalities such as X-ray, CT, or MRI can visualize the PSIS and surrounding structures for diagnostic or procedural planning.

Overall, the PSIS is a key anatomical landmark used in anatomy, physical examination, and clinical procedures involving the pelvis and sacroiliac region.