Slimming of the broad face can be done by toxins injection or surgery. It depends on whether it is a muscular problem or square jaw which needed to be surgically treatment. Muscle injection is temporary and permanent treatment involves surgery to the muscles.
Facial jaw recontouring can narrow a broad face (within limits) by reduction of the masseteric muscles and mandibular angles by injection of muscle toxins. Surgery is indicated when repeated injections are not working anymore. Surgical reduction of masseteric muscles is permanent.
Surgical reduction of the mandibular angles and masseteric muscles by surgery may be needed in some cases and often gives better results that toxin injections.
Jaw shapes that may benefit from jaw recontouring procedures: square jaws with enlarged angles
Square jaw reduction surgery is not a major procedure though technically difficult as all the incisions are done inside the mouth. There would be up to a 2 to 3 weeks of discomfort and swelling but most patients are able to function well first 2 weeks after surgery.