Th typical perception of the outer shape of the jaw angles is that they are a flat surface. This is certainly how they are seen in looking at regular x-rays and on most plastic jaw models. But in looking at many 3D CT scans for jawline implant designs it is apparent that their actual topography is more complex. An improved understanding of their shape provides the opportunity for improved jaw angle/jawline implant designs.
Anatomically the shape of the jaw angles is primarily the result of the functional load applied to it by the masseter muscles and its genetically driven growth and development. The large quadrilateral masseter muscle originates from the back end of the zygomatic bone as well as along the inferior aspect of the zygomatic arch. It extends downward to insert along the perimeter of the jaw angle and the lateral surface of the ramus.(technically this is the superficial head of the muscle) The deeper smaller head inserts into the upper half of the ramus and the coronoid process and does not directly influence the lower jaw angle shape.
The masseter muscle influences the external shape of the jaw angle in two specifics areas, the outer rim of the bone around the angle and the contour of the cortical surface above it. The shape of the jaw angle reflects the fact that the much thicker part of the muscle is above the bony edge of the angle. This can be easily demonstrated on oneself by biting down and feeling where the bulge of the muscle occurs compared to the bottom edge of the bone. As a result this is why the ramus has a recess or concave shape to the bone known as the fossa to accommodate its bulk. The very edge or rim of the bone is higher (more lateral) than the fossa due to the influence (pull) of the tendinous attachments on it.
This common bony jaw angle topography has significance when designing the angle portion of a jawline implant. To take into account the thickness of the masseter muscle and with the goal of a more visible jaw angle shape, most jaw angle designs should have a concave or flared out shape. The rim around the angle should be raised and be the most lateral part of the design. This allows the bulk of the masseter muscle to fall into the fossa above the rim of the angle bone and permit it to be more visible.
While it is true that a more defined jaw angle can be surgically created by an implant, its design/shape must allow more of the bony rim to be seen rather than just enlarging/expanding the bulk of the masseter muscle.
Dr. Barry Eppley
Indianapolis, Indiana