Jaw surgery (orthognathic surgery) is intended to correct skeletal issues that cannot be fixed with braces alone. These include:
1) Correction of upper/lower or both jaw size proportion
Some patients may have a size mismatch between the upper and lower jaws, leading to protruding upper front teeth or underbites. Crossbites, especially those occurring on both sides of the jaw may have a skeletal component.
2) Correction of skeletal canting (slanting)
Canted jaws cannot be completely fixed with braces alone. Especially those in the lower jaw that also cause slanting of the jawline and chin.
3) Vertical maxillary excess
Excessive vertical growth of the upper jawbone (maxilla) can cause the appearance of a gummy smile or maxillary protrusion. Camouflage is normally done with gum surgery (gingivectomy or crown lengthening) but if the bony excess is severe, camouflage results are usually unsatisfactory.
Braces alone with the usage of additional appliances like temporary anchorage devices (TADs) can be used to help the braces to mask certain issues like canting.
If the jaw size proportion is not surgically corrected, then other camouflaging methods such as extractions may be required. These methods cannot completely fix issues such as a weak/receding chin.
