Micrognathism is a condition where the jaw is undersized. It is also something called mandibular hypoplasia. It is common in infants but usually self-corrected during growth due to the jaws’ increasing size. It may be because of abnormal tooth alignment and, in severe cases, can hamper feeding. Anatomic fluctuations that vary from individual to individual also influence the length of the mandible.

What Are the Causes of Micrognathia?

Congenital conditions.
Intrauterine acquired conditions.
Chromosomal abnormalities.
Mendelian inherited conditions.
Autosomal dominant conditions.
Autosomal recessive conditions.
X-linked inherited conditions.
Autoimmune conditions.
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What Is True Micrognathia?

True micrognathia can be classified as either congenital or acquired.

1. Congenital:

The etiology of the congenital type is unknown or elusive. Although in many instances, it is associated with underlying congenital abnormalities, including congenital heart disease and the Pierre Robin syndrome. If associated with congenital anomalies, this disease occasionally follows a hereditary pattern.

Micrognathia of the maxilla may also frequently occur in this pattern due to a deficiency in the premaxillary area, and patients with this deformity appear to have the middle third of the face retracted. Medical research has been suggestive of the fact that mouth breathing is a cause of maxillary micrognathia. It is also likely, as research states, that the micrognathia may occur in an individual owing to the associated maldevelopment of the nasal and nasopharyngeal structure.

True mandibular micrognathia of the congenital type is often difficult to diagnose. These patients clinically appear to have a severe retrusion of the chin, but according to the actual measurements, the mandible may be found to be within the normal limits of variation. Such cases may be due to the anatomic variation in the posterior positioning of the mandible with regard to the skull or a steep mandibular angle resulting in an apparent retrusion of the jaw.

Agenesis of the condyles also results in a true mandibular micrognathia.

2. Acquired:

The acquired type of micrognathia is of postnatal origin and usually results from a disturbance in the area of the temporomandibular joint. Ankylosis of the joint, for example, may be caused by trauma or by infection of the mastoid, of the middle ear, or even of the temporomandibular joint itself.

Since the normal growth of the mandible depends to a considerable extent on the developing condyles and its muscle functions, it is evident to physicians and surgeons how condylar ankylosis may indeed result in a jaw deficiency.

How Does Micrognathism Develop?

The hypoplasia or the smallness of the mandible occurs genetically due to disturbance or abnormalities of the first branchial arch. This is caused by a deficiency or insufficient migration of neural crest cells and usually occurs around the 4th week of pregnancy or gestation.

What Are the Radiographic Features of Micrognathism?

This condition can be diagnosed in an antenatal ultrasound that guides the earliest picture of this anomaly. Micrognathia is best diagnosed in the third trimester as a large portion of mandibular growth occurs during this period of pregnancy. A Sagittal facial image shows a receding chin.

Significance of Radiology or Antenatal Ultrasound:

Due to a high association rate with other anomalies, the detection of micrognathia should also prompt the surgeon or physician to investigate or diagnose a careful search for other fetal abnormalities.

What Are the Complications of Micrognathia?

Severe micrognathia can potentially compromise neonatal respiration after birth.

What Is Macrognathia?

Macrognathia refers to the condition of abnormally large jaws. An increase in the size of both jaws is frequently proportional to a generalized increase in the size of the entire skeleton, for example, in pituitary gigantism. More commonly, only the jaws are affected, but macrognathia may be associated with certain other conditions, such as:

Paget’s disease of bone, in which overgrowth of the cranium and maxilla or occasionally the mandible occurs.
Acromegaly, in which there is a progressive enlargement of the mandible owing to hyperpituitarism in the adult.
Leontiasis ossea, a form of fibrous dysplasia in which there is an enlargement of the maxilla.

How Does Macrognathism Develop?

Cases of mandibular protrusion or prognathism are uncomplicated for the surgeon to treat. However, it is not an uncommon clinical occurrence if it is involved with any systemic condition. The etiology of this protrusion, though unknown in some cases, follows a genetic or hereditary pattern. In many instances, the prognathism is due to a disparity in the size of the maxilla or the disparity of the upper jaw in relation to the mandible. It can be vice versa in cases where the mandible is measurably larger than normal.

What Factors Tend to Favor Macrognathia?

General factors that would mainly influence or tend to favor mandibular prognathism or mandible protrusion are also responsible for macrognathia. They are,

Increased height of the ramus.
Increased mandibular body length and gonial angle.
Anteriorly positioned glenoid fossa.
Decreased maxillary length.
Posteriorly positioned maxilla in relation to the cranium.
Prominent chin button.
Varying soft tissue contours.

How Is Facial Asymmetry Affected?

The geometric attributes of the face are affected in anomalies of jaw size or anomalies of jaw-cranial base relationship. Mainly, however, these deformities can affect six different geometric attributes such as size, position, orientation, shape, symmetry, and completeness according to the medical classification of craniofacial anomalies.

Both micrognathia and macrognathia, or in other words, mandibular hypoplasia or hyperplasia, are associated with the size, shape, and symmetry of the jaw. The term "hyperplasia" is synonymous with pathological enlargement, while the term "hypoplasia" is associated with the inability or failure to attain a particular or proportionate size.

Hence, in micrognathia, it is the "mandibular hypoplasia'' that occurs of the jaw, while in macrognathia, it is the case of "mandibular hyperplasia." Thus the geometric or proportional structure generally observed in healthy individuals appears "distorted" in individuals affected by the pathologic enlargement of the mandible or lower jaw either due to genetic or pathologic factors. Due to the physically deranged appearance of the jaw, the facial symmetry of the individual is also affected in these conditions.

How Is Micrognathia or Macrognathia Treated?

Surgical correction is always feasible for both conditions, as suggested by the maxillofacial surgeon. Ostectomy or resection of a portion of the mandible to decrease its length is an established procedure to treat macrognathia.

The overall prognosis for micrognathia, on the other hand, is highly variable that is dependent on the presence of other associated anomalies (like isolated fetal breathing or respiratory difficulties at the time of birth due to a smaller jaw). In selected cases, mandibular distraction osteogenesis with an advanced genioplasty (jaw repositioning or reshaping some for cosmetic purposes) may be a viable long-term alternative in severe micrognathia.

Conclusion:

Both Micrognathia and macrognathia need thorough assessment by the maxillofacial surgeon who can intervene at the earliest possibility to rectify both facial asymmetry and lifetime correction of these facial anomalies.