Neck Block
Not a medical condition
Neck Block
Most fretted instruments have necks that are attached to the body at the upper end where there is a big heavy chunk of wood to reinforce and stabilize the area.
Looking in through the sound hole of this guitar, we can see the neck block with the bolt that attaches the neck:
Here's a century-old Martin guitar with the back removed during repair:
It's easy to see how heavy and thick the neck block is.