Question: I just read your Q&A column for a horse with a swayback. We have a similar problem and have found treeless saddles work quite well in this situation, and some of the treeless seem to be very "fancy" for showing. Are there still problems with a treeless and swayback? We are using a Supracor dressage pad with the treeless, and it seems to work well--at least he is not pinning his ears anymore. I'm wondering why you did not mention that as an option, or are there problems with treeless that the treeless people don't tell you about?
Answer: Horses with a swayed back can go very well in a treeless saddle. The article you are referring to was about a gaited show horse, and there are no treeless saddles designed to go in the show ring for them. Western horses have an excellent choice in treeless saddles that look just like a regular western saddle, the Bob Marshall saddle. For other sports there are many styles available, and for short-backed horses, swayback horses and general comfort treeless saddles can be an excellent choice.
As far as fitting a treeless saddle, the part no one talks about is the relationship of the rider's leg to the horse's back shape. If the rider has a thin thigh and the horse has a relatively narrow back, or at least is narrow through the rider's leg area, both horse and rider will be very comfortable. The problem comes when the rider's thigh is thicker and the horse has a thicker or wider back. Here the rider's hip joints are spread so far apart that his/her pelvis has to roll back in order to open the hips just like a small child on a big horse bareback. The problem here is that the rider then lacks stability in the lower leg. The rider is forced to sit on her buttocks, with the lower legs loose. In this position, if the horse spooks, the rider will likely feel insecure or fall off.
When the rider has a saddle with a seat (and some treeless saddles are beginning to figure out how to make a seat--for example, the Sensation Treeless) the rider can now get her legs under her and be stable. She can stay in the saddle even if there are unplanned moves. So the real key to determining how safe and comfortable a treeless saddle is to pay attention to the horse and rider shapes.
Then for the horse's side, horses with backbones that stick up need a pad with a center gullet or space for the spine. Horses that collect rainwater down their spine (fat or well muscled) can use the saddle without much padding because they provide their own. Horses with very high withers may never do well with a treeless because they need to have nothing touching their withers but air space. They do not need more padding across the withers as I have seen some brands provide. Any white hairs that show up are signs of pressure points or rubs. Many treeless saddles will create white hairs along the stirrup straps or girth straps. So be on the lookout. Treeless saddles do not place much pressure from other parts of the saddle--the rider's thighs usually take most of the weight.
If you need a pad under the saddle to protect the spine from pressure there are a number of pad types that can work. Wool felt often works well if the spine is fairly high as it will not compress down when you sit on it. Open cell foam can compress down especially if it is thin. Closed cell foam can be bouncy if it is thick. Experiment with what feels comfortable to you while it protects the horse's spine.
Treeless saddles can be a useful way to fit difficult horses, to have a spare saddle for when you horse is too fat or too thin for his regular saddle and to have fun riding where you can really feel his back move, all if you pay attention to how your leg fits your horse. If you feel unsafe, believe it and look for something with more of a seat to be secure in.