I would like to see a picture too so that we would have a better idea of what it looks like.

From your description though, it looks like it is a stress rupture on the cane's external skin. This usually happens when the cane takes up too much water after the cane has finished its growth phase and the skin has already hardened.

Typically, the skin and underlying tissue of dendrobiums are soft and relatively pliable during their active growth phase. Towards the end of this growth phase, the cane's skin hardens to give the plant strength.

But sometimes the plant continues to take in too much water even after the skin has hardened and this results in the skin breaking from excess internal pressure. These look like vertical cuts along the plant's length.

The wound by itself is not a threat to the plant. However, this is an opening for disease and infection to bypass the plant's natural protection.
If the plant is not infected, the area around the wound would dry up and harden. This is the plant's natural way of sealing up wounds and protect the rest of the plant.

Another possibility is physical trauma. Something must have struck the plant and caused the skin to break. Bending a mature cane beyond its limit can also result in wounds on the skin as it folds into itself.