X is related posteriorly to Y

Solution 1:

Anterior and posterior are anatomical terms used across the sciences, particularly in anatomy. Anterior roughly means towards the front of the body; posterior, towards the back. (The actual definition is a little more complex but this is sufficient.)

These are the planes of the body with directional terminology:

The problem is, adding "related...to" in these examples is converting these into the OPPOSITE meaning.

Looking at anatomical position, clearly the bladder is anterior to the rectum:

The way your text uses it would mean that these two sentences are equivalent:

• The rectum is related anteriorly to the bladder.
• The bladder is located anteriorly to the rectum.

This is confusing and the usage simply seems wrong. But if you have found this in an actual medical text, it appears that this may be either (1) a poor translation into English, or (2) archaic terminology no longer used, or (3) a regional phrasing.

HOWEVER, when looking for a diagram, I came across this interesting slide from an unknown contributor, which uses "related to" to mean "attached to" or "sharing an anatomical boundary" without using the directional "anteriorly" terminology:

To me, this suggests that in some regions or contexts, it may be a way of phrasing that two things share an anatomical boundary, or lie against one another. Again, it is rather odd sounding or perhaps archaic, and the added directional terminology makes it seem to say the opposite. However, apparently it does exist in some contexts. I will update this if I run into any more clear explanations.