Shoulder width reduction, also known as shoulder feminization surgery, is a bone-based surgery not a soft tissue one. There are no methods to reduce the fat or muscle mass of the shoulders, and even if there were, they would not make the shoulders appear more narrow. Effective shoulder width reduction involves reducing the length of the clavicle bone, known as a clavicular reduction osteotomy. (or to be technically correct an ostectomy that involves two osteotomies to do so)
To understand why shortening the clavicle creates shoulder width reduction, one has to look at the anatomy of the bone and what its structural function is. The clavicle is unique in that it is the only long bone in the body that is oriented horizontally. Its functional role is to keep the shoulder blade and upper arm away from the sternum. It is a bone that is partially visible due to the thin soft tissue cover over it particularly along the inner two thirds of its length.
While the clavicle establishes a horizontal distance between the sternum and the shoulder blade, it is not a straight bone. It may appear so when looking at it straight on but it is an S-shaped bone that has two curves. As a result when its horizontal distance is shortened the width of the shoulders will be effectively reduced in a near 1:1 relationship with the amount of bone removed. But they will also roll in or move a bit forward as the horizontal reduction is more like right triangle. While the forward roll of the shoulders is less significant than the horizontal length reduction it does occur and is unavoidable due to the shape of the bone whose length is being reduced.
The clavicle has been described as a long bone that does not have a marrow space. But every clavicle I have cut always has a central marrow space albeit a small one. This is relevant to how the bone will heal once cut and put back together. While clavicle fractures heal slowly due to the type of fracture pattern and the associated surrounding soft tissue injury, clavicle osteotomies heal faster due to better bone contact and less tissue disruption around it. Having an central marrow space helps the bony healing process particularly when it is in good cortical edge alignment. It will still take a full six weeks but this is less than more than several months for most types of clavicle fractures.
The shape of the clavicle also speaks as how to protect it during the healing process. Holding the elbow forward and in front of the body helps to relieve pressure on the osteotomy site particularly when its length is shortened. Due to the curved shape of the bone this bow position is the same shoulder and elbow position that is used in surgery to bring the cut ends of the bone together during plate and screw fixation.
Dr. Barry Eppley