Most people assume because their horse has big shoulders or the horse is getting rubbed shoulders the answer it to put shoulder gussets in to their summer sheets and under rugs. Nothing could be further from the truth. Shoulder gussets in lightweight rugs such as cottons and woollens cause a whole new set of problems. These fabrics do not have enough structure to hold the gusset shape like a canvas rug can. Below is a lovely fitting neckline of this cotton sheet with shoulder gussets, it has just been put on and horse has not moved.
Unfortunately it does not stay that way. Once the horse moves around or you add a top rug and the sheet settles, the gusset will open up and stay open, allowing the neckline to enlarge. This causes the front strap to slip up into the gullet and the top of the rug to slip back onto the wither causing a sore wither, even to the point of ulcerations.
The summer Sheet with gussets, sitting nicely in front of the wither
After adding top rugs or the horse moving around
The result pressure on the chest and wither, often causing sores