There are several gender-specific differences in jawline anatomy with men overall having wider, larger and more square-shaped jaws than females from the front. Conversely, female jawlines ideally have a more V-shaped appearance in the frontal view with a smaller, narrower and slightly less projected chin. The facial muscles generally also tend to be thicker and larger in males than females. The angle of the jaw (gonial angle) viewed from the side is also different, tending to be more acute in males and more obtuse in females. Other important measurements are the bizygomatic width (the linear distance between the outermost part of the cheekbones viewed from the front), and the bigonial width (the linear distance between the right and left external angles of the jaw viewed from the front). In males, the bigonial width is ideally slightly less than the bizygomatic width with the jaws being almost as wide as the cheekbones from the front. This gives the typical square shaped jawline and facial frame of the male. In females, the bigonial width is significantly less than bizygomatic width when compared to males, evident as a narrower V-shaped jawline. These differences are not just of academic interest but provide important gender-specific references and principles for the way a jawline augmentation is performed. 

In both men and women, there is gradual and progressive reabsorption of bone during the ageing process. This results in a decrease in the height of the jawbone and a reduction in chin projection, which may change the balance between the lower third and upper two-thirds of the face.

A naturally weak chin and jaw (again, relative to ideal proportions), however, is not only a condition of advancing age but is quite often seen in young to middle-aged patients due to their genetically determined facial features. Rarely, an underlying birth or developmental abnormality may cause a range of facial deformities including those of the jawbone, chin and teeth alignment.

If the structural support provided by the jawbone and chin is reduced, the surrounding soft tissues become more prone to the effects of gravity, leading to facial sagging. These changes are compounded over time by increased skin damage, skin thinning, fat volume loss, fat redistribution and downward displacement, as well as both skin and soft tissue laxity (including weakening of the ligaments that hold facial fat compartments and salivary glands in place).

The overall result is a relative excess of skin and soft tissue in the lower face with the formation of jowls and loss of definition of the jawline and neck areas. This may be particularly noticeable on the front half of the jawline and neck in the area under the chin, which can become less defined and loose with blunting of the angle between the jaw and neck. A combination of a weak chin bone, excess neck fat, droopy salivary glands, sagging neck (platysmal) muscle and loose skin can give rise to a classic “double chin” appearance. Loose skin and a sagging platysma neck muscle with fat loss can lead to a “turkey neck”, where the neck displays multiple loose folds and wrinkles.

Treatment of the lower face with specific types of volumising fillers provides a localised enhancement of the jawline to improve contours. By providing structural support to the surrounding facial soft tissues, HA fillers counter gravitational changes, restore fullness and offer a lift with gentle skin expansion and stretch, helping to reduce facial sagging. They also provide a “forward push” to the soft tissues of the jaw, including the superficial neck muscle, such that neck contours usually become better defined, particularly if combined with a non-surgical chin augmentation.

These treatments are performed using denser forms of HA fillers that are first injected just onto the jawbone for structural support of the overlying soft tissue. They can also but be layered on top to provide an even greater definition. Depending on how the filler is injected, the vertical or horizontal lengths of the bone at the angle of the jaw can be increased to change the apparent angle. The outer edge of the jaw can also be augmented to increase the apparent jaw width. 

The masseters are a pair of large muscles, with one on each side of the face, that run from outer cheekbone to the back of the lower jaw, and are muscles used for chewing and grinding food. Some female patients, particularly from certain races, develop overgrown (hypertrophic) masseter jaw muscles, which can excessively widen their jawline, appearing more square-shaped when viewed from the front. This masculinised jawline can be an unattractive feature in female patients where the aesthetic ideal is a more V-shaped profile that is typically found in the younger female face. Male patients can sometimes also develop quite a significant masseter muscle enlargement that can excessively widen their jawlines. In both genders, the ideal proportions of the face become disturbed as a result of the lower face becoming wider than the mid-face leading to a less aesthetically pleasing appearance.

In both female and male patients, treating masseter muscle enlargement with injections of a temporary-acting prescription-only muscle relaxant can help reduce the thickness of the muscle such that the jawline appears slimmer when viewed from the front. This treatment can be quite transformative in certain patients and help to significantly rebalance the lower face with the mid-face to a more aesthetically ideal and attractive proportion, giving a slimmer facial appearance. The doses used do not noticeably affect an individuals ability to chew food but can help reduce undesirable teeth grinding.

Some patients with masseter muscle hypertrophy may also complain of grinding or clenching of the teeth and bracing or thrusting of their jawbone (a condition termed “bruxism”). This is a complex condition with different subtypes and characteristics and the cause of which is likely to be multifactorial, including temporomandibular joint disorders, for instance.

Many patients with bruxism find that treating masseter muscle hypertrophy with muscle relaxant can improve their symptoms, and this observation is currently the subject of clinical research.