Functional indications for corrective jaw surgery

Corrective jaw surgery is often perceived as cosmetic surgery. Indeed, the cosmetic enhancement is probably the most obvious outcome of the surgery. However, it is primarily a functional correction of a discrepancy in the bite disharmony caused by disproportionate growth of the jaws.

How does a disproportionate growth of the jaw affect function?

1. Inability to chew food properly.

The human jaws is designed or evolved, to have the upper jaw slightly in front of the lower jaw such that the upper teeth overlaps the lower teeth slightly. This is to ensure maximum inter-digitation of the teeth for efficient chewing of food. Each group of teeth has its own unique function. The incisors and canines cut food off into bite sizes and then premolars and molars mash it up before swallowing. If the teeth are occluding onto each other properly, biting efficiency will be compromised. Mastication will take longer and the food bolus in the mouth will be swallowed without completely being mashed up.

2. Inability to pronounce certain words properly

Speech is a complex coordination of muscle movement to create intelligible sounds. While muscular development is not directly involved in jaw deformities, abnormal jaw dimensions affects the function of the muscles. For example, having an open bite makes it difficult for the lips to achieve a proper seal which is needed in the pronunciation of certain words. However, since most patients who seek corrective jaw surgery are already in their teens or adulthood, the speech pattern would have been established and speech therapy may be needed in addition to corrective jaw surgery to achieve proper diction.

3. Temporomandibular joint disorder

The jaw joint is a joint unlike any others in the body. In the initial stages of mouth opening, the joint functions like a hinge joint, rotating around an axis the joins the left and right joints. This rotating movement only works for the initial opening of the mouth up to about 20mm. At the same time, it is also a ball and socket joint, allowing the jaw to move side to side while opening and closing. For wider opening of the mouth, the condyle slides forward, effectively dislocating the condyle (i.e. the “ball”) out of the fossa (“the socket”). This can be done with a side to side movement at the same time. All these movements are facilitated by a disc of soft tissue in the joint that allows the condyle to glide and rotate smoothly. The joint is also able to withstand a lot of force as evident from the biting force that can be generated. Given the complexity of the jaw movement, a jaw deformity will exert uneven and abnormal forces on the joint which may result in dysfunction. For example, in cases where the lower jaw is disproportionately short, excessive forward translation of the condyle is needed to achieve contact between the incisors. This causes repeated and excessive stretching of the disc and may cause it to be displaced. A displaced disc can cause clicking of the joint, pain and limitation of movement.

4. Obstructive sleep apnea

Sleep is an essential for normal physiological functioning of the body. It is a time of rest of the body to regenerate. However, sleep can be disturbed when the breathing is obstructed. This happens when the relaxed muscles in the tongue and soft palate collapse and fall back and block the airway thereby preventing air from entering the windpipe and lungs. This reduced air intake forces the heart to pump harder and faster to send enough oxygen to the body. The reduced oxygen concentration in the blood coupled with the excessive effort by the heart during sleep may lead to long term health issues such as hypertension and heart failure. The jaws form the skeletal framework upon which the tongue and muscles around the throat attach. If the jaws are underdeveloped, there will be excess muscular soft tissue present around the throat thereby narrowing the airway. This problem is made worse during sleep when the muscles are relaxed and results in further narrowing or even complete blockage of the airway. Corrective jaw surgery can correct this problem by repositioning the jaws into a more forward position which will then bring the muscles away from the airway and at the same time increase the tone of the muscles.

5. Premature wear of the teeth and gums
   

In many case of dentofacial deformities, the upper and lower teeth often do not meet completely. This results in some teeth bearing the brunt of the load of chewing while some teeth are unde-utilized. This often happens in open and underbite situations where the front teeth are not functional, leaving the molars to do all the work.

6. Drooling

When there is disproportionate growth of the jaw, the accompanying soft tissue such as the lips do not follow the same pattern of growth. This mismatch of soft tissue development with the bone growth may result in difficulty in closing the lips. This may result in drooling of saliva when speaking or at rest. While most patients habitually contracts the lip muscles to maintain closure, some drooling may happen inadvertently.

7. Teeth biting on opposing gum

In overbite situations where the lower jaw is underdeveloped, the lower incisors may be biting on the gums on the back of the upper incisors, effective stripping the teeth of the supporting bone and gum. Correcting the position of the jaws with surgery brings the teeth into proper occlusion.

8. Cheek or tongue biting

Proper interdigitation of the upper teeth with the lower teeth is needed not just for proper chewing of food but is also important for avoidance of cheek or lip biting. Properly aligned teeth have a slight overlap to avoid getting the cheek or lip caught during the chewing process. Disproportionate jaw development results in either excessive overlap of the upper or lower teeth causing the lip to be trapped. Conversely, the teeth may be meeting on in an edge to edge position without any overlap and that will predispose to cheek biting. A combination of surgical correction of the jaw and orthodontic alignment is needed to solve this problem.

Corrective jaw surgery is primarily a functional treatment with enhancement of facial aesthetics as an additional benefit. Dentofacial deformities causes both functional as well as aesthetic challenges. Orthognathic surgery corrects both function and aesthetics.