I've touched on the subject of treeless vs. treed saddles before, and stated that there are pros and cons to both - BUT that each needs to be fitted correctly or problems will result. Here's a thermograph (see here for more on thermography) that shows what can happen when an incorrectly-fitted treeless saddle creates pressure on the spinous processes:
Remember, blue is coolest and white is hottest (see the scale on the bottom). That, in the words of the horse's owner, is a serious case of "wither sadness". To the owner's immense credit, she knew something was wrong with her horse and spent a lot of time and money trying to pin-point his mystery lameness ... and finally the thermographic image found the culprit.