Head and neck surgeons of all types, like otolaryngologists and oral and maxillofacial surgeons, as well as dentists and dental specialists, like orthodontists, will commonly describe the positions of the maxilla and mandible in three dimensions. The first is in the anterior-posterior plane, often referred to as the AP plane. This view is seen when looking at the patient from a side profile, or radiographically, a sagittal plane. The second is the transverse plane, or looking directly at a patient in frontal view, radiographically synonymous with the coronal plane. The third is vertical, this is appreciated like the AP plane, and best understood when looking laterally at a patient.
Retrognathia is a term used to describe an unusual position of the mandible. Retro implies that there is deficient growth and 'gnathia' means about the jaws (particularly mandible). To the specialists above, describing a patient as retrognathic has the specifically means the patient is deficient in the AP plane of growth. That is, when viewed from the side, the mandible is posterior to and behind where it should be. There is often a vertical component in growth deficiency and sometimes a transverse issue, but they are less critical in thinking about retrognathia unless one is planning a surgical correction. To summarize, when you describe a patient as retrognathic, it implies the mandible is deficient in growth, particularly in the AP plane.
Why does retrognathia matter as a diagnosis? First, to rule out associated syndromes with far-reaching health effects, to recognize and be able to appropriately refer for orthodontic evaluation with possible need for surgery surgical correction, to consider if you are a health care professional who would ever deal with or manage an airway, and to consider the implications in the setting of obstructive sleep apnea.
The mandible develops in the same area as the first pharyngeal arch and is associated with Meckel’s cartilage, though Meckel’s cartilage does not become the mandible. The mandible begins as a condensation of mesenchymal tissue lateral to Meckel’s cartilage and proceeds through development via intramembranous bone formation. At six weeks in utero, it starts to ossify. The condyle of the mandible, which articulates with the skull as part of the temporomandibular joint, develops as independent secondary cartilage, at approximately ten weeks, and undergoes endochondral ossification.
Simply put, the mandible grows down and forward, or anteriorly and inferiorly. It does this through a combination of processes. These include concurrent resorption of bone from the anterior surface of the ramus with apposition of bone along the posterior surface of the mandible. This process, in conjunction with growth at the condyle, creates the anterior and inferior vector of growth.
In growth and development, there are standard charts commonly used by health care professionals detailing the spectrum of height, weight, age, and growth. It is worth considering there can be significant variation between individuals and that there are both chronologic and biologic ages (considering menarche/puberty). In general, facial height is the last dimension to finish growth and in females will be complete around ages 17-18, and in males in the early twenties. In a retrognathic patient or patients who have hypoplastic mandibles, this growth can be done much earlier.
Why does this matter? The timing of this growth and development is crucial when considering skeletal surgery to address retrognathia and is carefully ascertained for each patient.
There are no extensive studies that have looked at the epidemiology of retrognathia. There are some, smaller reported case studies that have evaluated isolated populations, but these generally examine retrognathia in the context of a craniofacial syndrome. 
On physical exam, a good place to start is with an intraoral examination. Dr. Edward Angle, widely known as the father of orthodontics, created a classification system for the occlusion of teeth and how they relate to each other. It is based on the relationship between certain parts of the first molars and canines. Suffice to say for those who do not specialize in teeth; Class 1 occlusion means there is an appropriate relationship between the maxillary teeth and mandibular teeth. Class 2 means that the mandibular teeth are posterior as in patients with retrognathia. Finally, Class 3 indicates that the mandibular teeth are anterior to the where they should be (think of a bulldog with the lower jaw sticking out).
A Class 2 occlusion can often be associated with retrognathia. However, one must keep in mind that the teeth can hide this as our bodies will often dentally compensate for skeletal discrepancies (more in-depth discussion on this point is the purview of dental specialists).
Often just looking at the patient extraorally can help you determine if they are retrognathic. When viewed from the lateral, you will notice an exaggerated convex facial profile. On grown adults, as part of a standard airway examination, examining the thyromental distance can be a predictor of a difficult airway. This implies retrognathia if the patient has a mandible that has not developed, the thyromental distance will be less than the standard three finger’s breadth, or approximately 7mm.
Radiology plays a significant role in determining retrognathia. The standard lateral cephalometric radiograph and associated analysis are part of the determination. There are hundreds of different interpretations and, suffice to say, that there are many points and combinations of angles that measure the relationship between the cranial vault and the facial skeleton to determine growth and are used in treatment planning for orthodontics and orthognathic surgery.
The most common diagnosis in the practice of orthodontics and oral and maxillofacial surgery associated with retrognathia is mandibular AP hypoplasia, a common diagnosis that supports a surgical correction of their deficiency, most commonly through a bilateral sagittal split osteotomy of the mandible.
Depending on the patient age and especially with infants and children, an astute practitioner must keep in mind some associated syndromes and conditions that are often associated with a hypoplastic mandible. This includes Pierre-Robin Sequence, Hemifacial Microsomia, DiGeorge Syndrome, Nager Syndrome, Treacher Collins, Goldenhar Syndrome, and Mobius Syndrome. (These specific entities are discussed in more detail under Treatment and Management).
Prior surgery to the head and neck or history of trauma can also produce retrognathia. For example, this practitioner recently had a male in his twenties with a history of a fall off his bicycle in the second grade. This caused trauma to one of his condyles and a unilateral aplasia in that location, resulting in a crooked face and retrognathic appearance, warranting surgical correction. Because of the condylar portion of the growth of the mandible, temporomandibular trauma can restrict growth. Any blow to the chin or lower face, if severe enough, can cause this type of growth pattern and result in retrognathia and is worth asking about during a history.
Physical exam, as described above, is the best way to analyze patients for retrognathia. A common adjunctive radiological review is the lateral cephalometric x-ray. This is typically traced and measured with a full cranio-cephalometic analysis that is beyond the scope of this article. There are a series of measurements that look at the position of the mandible to skull base to diagnosis mandibular hypoplasia (for example, the Sella-Nasion-B point angle (SNB) is generally 80 degrees in a Caucasian population, so an SNB angle of 72 degrees is indicative of retrognathia).
The treatment and management will depend on what your concerns are and what a patient or their parents desire. Most commonly retrognathia will be identified in children between the ages of four to ten years old and if the child has a dental home, will likely be referred to an orthodontist for evaluation ideally around the ages of seven to eight. Generally, this is a benign condition without any other effects. (In this practitioners’ experience, one could walk a busy street and identify an individual every minute who has retrognathia to some degree.) This will allow the orthodontist to begin tracking the patient with serial cephalometric radiographs and potentially intervene early with non-surgical therapy like headgear or newer intra-oral techniques that have a similar effect in repositioning the mandible. If the child does not have a dental home or has not seen an orthodontist, a referral is recommended.
If one is concerned about airway management on a patient with retrognathia, this should be a strong indicator of a potentially difficult airway. Underdeveloped structures, anatomic distortion, potential malpositioning of the epiglottis, and tighter constriction of the muscles and surrounding structures all need to be considered. One is much more likely to have a Cormack and Lehane Grade III or IV view, and that needs to be planned for.
If the infant or child presents with retrognathia, or even if they are older and have associated findings that lead you to suspect a syndrome or myriad of issues, consider the following conditions:
The classic triad of micrognathia (colloquially synonymous with retrognathia), glossoptosis, and cleft palate. Airway obstruction is likely, though the disease can present with a range from mild to severe. The patient will need a referral to a cleft and craniofacial team for evaluation and treatment.
Part of the spectrum of Oculo-Auriculo-Vertebral (OAV) Syndromes, this is another entity with variable penetrance. It arises due to pathologic dysregulation of the first and second pharyngeal arches and can be unilateral or bilateral. It results in hypoplastic growth of not only the mandible and lower face but can affect the midface, temporomandibular joint, orbits, ears, and soft tissue. A common classification system used is the Kaban Classification (Types I, IIa, IIb, and III), which help guide surgical correction. If discovered as an infant, the patient will need a referral to a cleft and craniofacial team for evaluation and treatment. In this author’s experience, a surprisingly high number of adults have a mild expression which needs to be taken into close account during facial and orthognathic surgical planning.
Part of the spectrum of Oculo-Auriculo-Vertebral (OAV) Syndromes, the etiology behind Goldenhar Syndrome is unclear, with multiple chromosomes potentially affected and only theories offered as to a cause. This entity combines features of Hemifacial Microsomia with further facial clefting, colobomas of the eyes, vertebral anomalies, and other findings. The patient will need a referral to a cleft and craniofacial team for evaluation and treatment as well as ophthalmology.
Treacher Collins Syndrome
This syndrome is an autosomal dominant condition of a mutation of the TCOF1 gene with variable expression. Patients will have a unique facial appearance with extreme retrognathia leading to a very convex facial profile when viewed from the side, antimongoloid slanting eyes (that is the lateral corners of the eyes are turned down), colobomas, significant facial hypoplasia of the midface of the hard and soft tissues, and auricular deformities. Once born, these infants will need immediate and intense multidisciplinary therapy.
A very rare entity with etiology unclear, this presents with a characteristic “masklike face” as a component is paralysis of some or all of cranial nerves III, IV, V, VI, VII, IX, X, and XII. Retrognathia, ocular deformities, and other skeletal and limb abnormalities. The patient will need a referral to a cleft and craniofacial team for evaluation and treatment, as well as any other specialties warranted.
A patient who has retrognathia from mandibular AP hypoplasia will enter the treatment planning phase of their case during the first visit with the orthodontist. This author has immense respect for orthodontists because they plan treatment years into the future and make movement and teeth extraction decisions based on skeletal growth and facial characteristics. Once the patient is ready for surgical correction, they will be referred to an oral and maxillofacial surgeon. A full series of photos and dental models are generally taken along with radiographs and a full workup completed and discussed with the treatment team, orthodontist, and patient. At this point, the surgical options are presented to the patient, and a surgical course decided upon. The standard workup and planning involving all proposed surgeries be completed on dental models and precise movements documented. Acrylic splints are then fabricated for use in stabilization of the teeth and bones for rigid internal fixation during surgery. More common over the last decade is virtual surgical planning, where a CT scan and patient models are digitally merged, and the operation is virtually planned within 0.01mm; final surgical splints are custom printed on a 3D printer.. 
The prognosis is benign, unless the retrognathia contributes to another more morbid condition like obstructive sleep apnea, temporomandibular joint dysfunction, or is associated with a more severe syndrome.
Retrognathia can produce or lead to some undesirable effects. The malocclusion often associated with retrognathia, and Angle Class II malocclusion most commonly prevents patients from masticating properly. There are many associated psychosocial stigmas associated with having a dentofacial deformity, especially in adolescents. Obstructive sleep apnea is likely more common in retrognathia patients as the airway is more closed and constricted. Airway management can be significantly more difficult in this patient population.
Refer to general dentist or orthodontist for evaluation
Early intervention with referrals to general dentistry, orthodontist, and if warranted, cleft/craniofacial centers is critical. Many of these children may also have other congenital disabilities, and it is vital to get the appropriate specialist involved early on. Because of the cosmetic deficit, depression and social isolation are not uncommon. Hence, a mental health expert should also be part of the an interprofessional team. The outcomes in most children are fair. A complete reversal of the deformity is not possible, but most children do have a decent cosmetic result and an improvement in their orofacial function. (Level V)
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