What do “Soft Tissue Predictability” mean after orthognathic surgery?

The soft tissue exchange coefficient is a mathematical ratio that indicates the extent to which the facial skin, lips, and cheeks will move in response to surgically moving the jawbone. For example, when the upper jawbone is moved 10 mm forward, the tip of the upper lip is not moved in exactly the same proportion as the bone, but usually in a ratio of $1:0.75 or $1:0.8 (i.e., 7.5 – 8 mm); when the lower jaw is moved forward, the skin of the chin moves with the bone in almost a ratio of $1:1. These coefficients vary depending on the patient’s skin thickness, age breakdown, and tissue elasticity, and allow surgeons to most accurately calculate the “postoperative estimated profile” that they will present to the patient using computer simulation programs before surgery.