> ## Documentation Index
> Fetch the complete documentation index at: https://docs.spine.altician.com/llms.txt
> Use this file to discover all available pages before exploring further.

# S-M Distance

> Measure the surface path from a skin entry point to the midline

## What Is S-M Distance?

S-M Distance measures the **surface path length** from a skin entry point (S-point) to the midline plane. The measurement follows the surface of the volume rather than a straight line through space, giving a more realistic sense of approach depth.

The result is displayed in **millimeters (mm)**.

## Requirements

S-M Distance is calculated automatically when both of the following are present:

1. At least one **S-point** (skin entry marker created via [Skin Detection](/tools/skin-detection))
2. A defined **Midline** plane (see [Midline Tool](/tools/midline))

No manual calculation is needed — the distance updates as soon as both references exist.

## Where the Distance Appears

The S-M distance for each S-point is shown in the **right panel** next to the corresponding S-point entry. If multiple S-points exist, each displays its own distance to the midline.

## Path Visualization

The surface path used for the measurement is drawn on the 3D volume as a colored line connecting the S-point to the nearest midline crossing. This lets you visually verify the path and confirm it follows the anatomy as expected.

## Comparing Approach Distances

When multiple arrows and S-points are placed on different sides or levels, S-M distances can be compared side by side in the panel. This makes it straightforward to assess which approach is shorter or more symmetric.

| S-point       | S-M Distance |
| ------------- | ------------ |
| S1 (Left L4)  | 42 mm        |
| S2 (Right L4) | 38 mm        |

<Note>
  S-M Distance updates automatically when the midline or S-point position changes. No manual recalculation is required.
</Note>

<Warning>
  S-M Distance is a reference measurement based on user-defined inputs. Verify that both the S-point and midline are accurately placed before relying on the value.
</Warning>