# Concavity Loss in Shoulder Instability

How the shoulder's shallow socket wears at the front rim, and three ways doctors measure that bone loss on a CT scan\.

By Ausberto Velasquez

Topic: Human Body & Medicine

[Open the interactive Jig](https://ask.jig.com/j/WfjiZ3o1NQeq)

## Transcript

### 1. A ball on a shallow dish

Your shoulder is a ball and socket\. The ball is the top of your upper arm bone\. The socket, called the glenoid, is a shallow dish on your shoulder blade\.

The dish is about 35 mm tall but only about 4 mm deep, more like a saucer than a cup\.

### 2. A squeeze keeps it centred

Muscles press the ball into the dish\. To slip out, the ball would have to climb the dish's rim, so it rolls back to the middle\. Doctors call this concavity\-compression\.

Concavity means the dish shape\. A deeper dish holds the ball more strongly\.

### 3. When the front rim wears away

In anterior instability the shoulder slips out towards the front\. Each slip can chip or wear the front lower rim\. The dish loses its concavity, so the ball slides out more easily\.

Drag the slider to wear away more or less of the rim\.

### 4. Way one: the best\-fit circle

Look straight at the dish, as on a CT scan, a 3D X\-ray picture of the bone\. Draw a circle that fits the healthy lower rim\. The missing width divided by the circle's width gives the bone loss\.

It is quick, but the answer depends on exactly where the circle is drawn\.

### 5. Way two: height and width

Measure how tall the dish is\. A healthy dish of that height should be a certain width\. Compare that with the width that is left\.

In one study this way gave the largest answers of the three\.

### 6. Way three: the curved surface

A scan of the healthy shoulder is flipped like a mirror and laid over the injured one\. Now we measure the whole curved surface inside the circle, not just one line across it\.

Different readers got almost the same answer: an agreement score of 0\.95 out of 1\.

### 7. Same shoulder, three answers

In one study of 36 patients, the three ways gave different average losses for the same shoulders\. So the numbers cannot be swapped between methods\. Pick a way to light it up\.

Surface 9\.4%, circle 14\.3%, height and width 17\.6%\. The thin bar marks 25%\.

### 8. Why a better measure matters

A loss of about a quarter of the dish width is often the point where surgeons add bone, not just repair the soft tissue \(the stretchy rim and ligaments that hold the joint\)\. Measuring the real curved surface helps choose the right fix\.

A dent on the ball itself matters too, so doctors look at both sides of the joint\.