---
title: "Posterior Superior Iliac Spine | Anatomy"
description: "Posterior Superior Iliac Spine is the rear bony landmark on the ilium used to locate the pelvis, check alignment, and study sacroiliac stability in Anatomy and Physiology I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/posterior-superior-iliac-spine"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 8"
---

# Posterior Superior Iliac Spine | Anatomy

## Definition

The posterior superior iliac spine is the bony bump at the back top edge of the ilium. In Anatomy and Physiology I, it is a palpated landmark for locating the pelvis and sacroiliac region.

## What It Is

The posterior superior iliac spine, or PSIS, is the bony prominence you can feel at the back of the pelvis on the iliac crest. In Anatomy and Physiology I, it shows up as a surface landmark for the pelvic girdle, especially when you are identifying the position of the pelvis on a model, image, or real body.

It sits on the posterior part of the ilium, near the top of the sacrum. Because it is close to the sacroiliac joint, the PSIS helps you orient where the pelvis sits and how the two halves of the pelvic girdle line up. If you can find both PSIS points, you can compare them to check whether the pelvis looks level or rotated.

The PSIS matters because bones are not just shapes on a diagram. They are also attachment sites for ligaments and muscles that keep joints stable. The posterior sacroiliac ligament and the iliolumbar ligament attach in this region, which helps brace the sacroiliac joint so it can transfer body weight without moving too much.

You can usually palpate the PSIS through the skin, especially in lean people, which is why it shows up in physical exams and some clinical procedures. It can be used as a reference point for placing electrodes, estimating pelvic position, or checking symmetry during a movement assessment. That makes it useful both on lab models and in real anatomy.

One easy way to think about it is that the PSIS is a landmark first and a structure second. You are not usually memorizing it because it does something dramatic by itself. You are learning it because it helps you find nearby structures, interpret pelvic orientation, and connect surface anatomy to the deeper pelvic girdle.

## Why It Matters

The posterior superior iliac spine is one of those landmarks that turns the pelvis from a memorized diagram into something you can actually locate and read. In Anatomy and Physiology I, that matters when you are identifying bones on a model, tracing where the pelvic girdle sits, or comparing the left and right sides of the pelvis.

It also connects surface anatomy to function. The PSIS sits near the sacroiliac joint, so if you understand where it is, you can better understand how the pelvis transfers weight from the trunk to the lower limbs. That connection shows up when you study stability, posture, and how ligaments keep joints from moving too far.

This term also helps you recognize why some exam questions and lab practical items use landmarks instead of full bones. A professor might point to the back of the pelvis and ask you to identify the PSIS, or ask which structures are nearby. If you know the landmark, you can reason out the rest of the region instead of guessing.

It is also a good example of how anatomy uses bony bumps for orientation. Once you can find the PSIS, it becomes easier to map the iliac crest, sacrum, and sacroiliac joint in the same area.

## Connections

### Iliac Crest

The PSIS sits on the posterior part of the iliac crest, so these two terms are closely linked. If you can find the iliac crest by tracing the top rim of the ilium, the PSIS is the rear bony point at that rim. In lab, this helps you move from a general outline of the pelvis to a specific landmark you can palpate.

### Sacroiliac Joint

The PSIS is near the sacroiliac joint, which is why it is often used to orient that area of the pelvis. The joint itself is where the sacrum meets the ilium, and the PSIS helps mark the posterior side of that connection. That makes the landmark useful when you are studying pelvic stability or checking alignment.

### Pelvic Girdle

The posterior superior iliac spine is part of the bony framework that helps you identify the pelvic girdle as a whole. Since the pelvic girdle transfers body weight and supports the lower limbs, landmarks like the PSIS help you see where the ring of the pelvis begins and how the left and right sides relate.

### [Auricular Surface](/anatomy-physiology/key-terms/auricular-surface)

The auricular surface is the ear-shaped area on the ilium that articulates with the sacrum, and it sits close to the PSIS region. These two structures are often discussed together because they help you locate the posterior pelvis and understand where bone contact and joint stability happen.

## On the AP Exam

A lab practical may point to the back of a pelvis model and ask you to identify the PSIS, so you need to know that it is the posterior bony bump at the top of the ilium. In image-based questions, you might use it to orient the pelvis and decide where the sacroiliac joint or iliac crest is located. If a question asks why it matters clinically, connect it to palpation, pelvic alignment, and ligament attachment. On quizzes, you may also see it in multiple-choice distractors with other pelvic landmarks, so the safest move is to tie the term to its position, not just its name.

## posterior superior iliac spine vs Iliac Crest

The iliac crest is the whole curved top edge of the ilium, while the posterior superior iliac spine is one specific bump at the back end of that crest. If you picture tracing the crest with your fingers, the PSIS is the rear point you would land on, not the entire rim.

## Key Takeaways

- The posterior superior iliac spine is a palpable bony landmark on the back of the ilium at the top of the pelvis.
- It helps you orient the pelvic girdle and locate the posterior side of the sacroiliac region.
- Ligaments in this area, including the posterior sacroiliac ligament and iliolumbar ligament, help stabilize the pelvis.
- You may use the PSIS in lab, physical exam practice, or imaging to judge pelvic position and symmetry.
- It is easiest to remember the PSIS as a landmark that points you to the rest of the posterior pelvis.

## FAQs

### What is the posterior superior iliac spine in Anatomy and Physiology I?

It is a bony projection on the back top part of the ilium, near the iliac crest. In Anatomy and Physiology I, you learn it as a surface landmark for the pelvis and sacroiliac region. It is useful because you can often feel it through the skin and use it to orient nearby structures.

### How do you find the posterior superior iliac spine?

You usually palpate the back of the pelvis and look for the two small dimples near the top of the buttocks, which often mark the PSIS area. On a model or diagram, it is the posterior end of the iliac crest. The exact prominence can vary depending on body composition and muscle mass.

### Is the posterior superior iliac spine the same as the iliac crest?

No. The iliac crest is the entire top border of the ilium, while the PSIS is one specific bump at the posterior end of that border. They are related, but not interchangeable. Confusing them can make it harder to orient the pelvis correctly on a lab practical.

### Why does the posterior superior iliac spine matter in the pelvis?

It matters because it helps locate the sacroiliac region and serves as an attachment area for ligaments that stabilize the joint. It is also a reliable landmark for checking pelvic alignment. In practice, that makes it useful in anatomy labs, exams, and movement assessments.

## Related Study Guides

- [8.3 The Pelvic Girdle and Pelvis ](/anatomy-physiology/unit-8/3-pelvic-girdle-pelvis/study-guide/EShWjJBXvDy4ipVL)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
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