The earliest reported discussion and analysis of the lesser (lower) jaw began in 1911 when Shukowsky reported two cases, one in a patient he treated in 1902, when he believed that posterior tongue drop was the main cause of dyspnea and was the first to be treated with a labiolingual adhesion in which the tongue and lower lip were sutured together, but no further details of the procedure or follow-up were reported. The second patient, whom he treated in 1903, unfortunately died of asphyxia. The autopsy report was suggestive of a small mandible, cleft palate and uvula, and the first complete case was published in 1923 by Pierre Robin, a Parisian dentist. In his initial article, he described three features: micrognathia, retrognathia and airway obstruction, and 10 years later he proposed cleft palate as another characteristic manifestation of the disease, and over the next 30 years he wrote more than 20 articles and monographs on RS, detailing the embryologic development, anatomy, complications and treatment of the disease. In 1926, Lenstrup reported three cases and advocated treatment by changing the position of the child, but the results were poor. This in turn stimulated clinicians to experiment with various methods of treatment, including clubbing and wire-fastened traction. Over the next 100 years, clinicians and researchers have gradually discovered and developed the clinical features, causes, and treatments for this condition. [Mandibular hypoplasia is a misalignment of the bite and facial deformity due to insufficient forward growth of the mandible. If severe hypoplasia of the mandibular body, mandibular branch and chin are also present, it can be called micro (lower) jaw deformity. The patient’s inherent oropharyngeal cavity volume is reduced and the tongue drops back, resulting in narrowing and obstruction of the upper airway. [The etiology of micrognathia is related to genetic and environmental factors, and many of the micrognathia treated in clinical practice are genetically related. According to the etiology, we can classify micromaxillary deformities into three categories. (1) congenital small jaw deformity; (2) developmental small jaw deformity; and (3) acquired small jaw deformity. Most congenital small jaws are partial manifestations of certain well-known craniofacial syndromes, and there are more than 60 reported syndrome manifestations including small jaw deformities. Examples include: oculo-ocular spondylolisthesis syndrome, unilateral or bilateral first two gill arches syndrome; maxillofacial hypoplasia syndrome (Treacher Collins syndrome); Pierre Robin sequence syndrome, etc. A small number of micro (lower) jaw deformities are non-syndromic or are a clinical manifestation of some unnamed syndromes. Developmental micromaxillary anomalies are mainly manifested by An’s Class II malocclusion combined with the development of an unexplained underdevelopment of the lower jaw. Acquired micrognathia can be caused by radiological injuries, neoplastic defects, traumatic injuries, etc. For example, birth injuries during infancy, infections causing condylar injuries or TMJ ankylosis can lead to severe micrognathia in adulthood. [The congenital (small) mandibular malformation can be observed at birth or shortly after birth with short retracted jaws, narrow pharyngeal cavity, airway obstruction, posterior tongue drop, respiratory distress, feeding difficulties, etc. Some severe cases die from respiratory distress or asphyxia during feeding. After the eruption of teeth, the patient may present with recession and height deficiency of the lower 1/3 of the face, An’s Class II malocclusion of the posterior teeth, and enlargement of the anterior tooth covering and overbite are the typical clinical manifestations of the small (lower) jaw. Many patients with severe mandibular hypoplasia have temporomandibular joint disorder syndrome and may also have obstructive sleep apnea syndrome (OSAS). The chin protrusion in micromaxillary deformity is severely deficient, the chin muscle is tense, and the chin-neck distance is too short while the upper jaw is relatively protruding, forming a typical “bird’s mouth” deformity, especially in patients with severe mandibular hypoplasia secondary to TMJ ankylosis. [Treatment] The specific treatment plan will be determined according to the etiology, clinical manifestations and radiographic projections of the small (lower) jaw. Treatment principles: a multidisciplinary approach is required, early attention should be paid to respiratory and dietary problems; subsequently, obstruction of the linguopharynx due to mandibular hypoplasia should be treated aggressively, and tracheotomy may be performed to maintain a ventilated airway if necessary. Depending on the patient’s developmental status, surgery to maintain and restore the shape of the facial contour should follow a certain sequence of treatment. (1) Treatment of small mandibular deformities in neonates and infants: The main focus is to address dysfunction such as respiratory and feeding problems, and later to consider the reconstruction of the profile or mandibular structure. We can perform postural treatment, labiolingual adhesions, tracheotomy and other methods. (1) Tongue-lip adhesion (TLA): Shukowsky first briefly described this procedure in 1911, and the surgical technique and philosophy have changed dramatically since then. However, the basic principles and goals of the procedure have not changed, which is to anteriorly relocate the tongue, especially the base of the tongue, to relieve the tongue deformity and relieve the impact on the pharyngeal cavity. Some reports suggest that TLA can cause certain complications and sequelae such as tongue fixation. However, a considerable number of clinical articles still suggest that this procedure has the advantages of reducing airway obstruction in infants, resolving feeding difficulties, shortening hospital stay, reducing the workload of families and caregivers, simplicity of operation, and few complications [23], and thus lip and tongue adhesions are more widely used in clinical practice than jaw lengthening and tracheotomy. Although controversies regarding TLA persist, labiolingual adhesions have long been used as the primary surgical approach for the treatment of posterior tongue drop. The exact timing of tongue liberation after labiolingual adhesions is not clearly supported by theory and is much debated, although most surgeons endorse the liberation of the tongue at the time of or before the cleft palate. (2) Tracheotomy: to release the upper airway obstruction, but long-term tube with many inconvenience and related complications, currently only used for palliative and emergency. For other surgeries such as tonsil and adenoidectomy, uvulopalatopharyngoplasty (UPPP), partial tongue resection, partial epiglottotomy and lateral and retropharyngeal extension, etc., they only solve the problem of soft tissue obstruction around the upper airway. They have little effect on upper airway stenosis caused by bony malformations and play only a relatively supportive role. (2) Consider combined surgical orthodontic treatment for small (lower) jaw deformity in post-adolescent patients: 1) Preoperative orthodontics: small (lower) jaw deformity mostly has dental substitution and Spee curve abnormalities, which need to be corrected by orthodontics before surgery. If the lower teeth are crowded and the lower anterior teeth have serious labial tilt, orthodontic extraction is considered to align the teeth and decompensate. By depressing and inwardly retracting the lower anterior teeth and the labially inclined upper anterior teeth, the overjet in the anterior region is reduced, the mandibular Spee curve is flattened and the maxillary arch is widened. After the orthodontic treatment is completed, the occlusal guide is made and set aside during the surgery. 2）Orthognathic surgery: ①Anterior mandibular migration includes bilateral mandibular ascending sagittal split ramus osteotomy (SSRO), bilateral mandibular ascending inverted L-shaped or C-shaped osteotomy. (2) Anterior chin migration orthoplasty. 3) Postoperative orthodontics: Generally, the orthodontist becomes involved 2-3 days after surgery. For patients with a tendency of recurrence, Class II intermaxillary traction is used. For patients with open dentition, the orthodontic treatment can be supplemented with vertical traction in the anterior region in a timely manner. 4) Mandibular distraction osteogenesis (DO): DO technique is gradually being used to treat severe mandibular underdevelopment and defective deformities, and to effectively relieve the associated OSAS, which is a technique to lengthen the bone by cutting open the long bone and applying a traction device to slowly pull the bone to form new bone in the interstitial space. Since 1992, when McCarthy et al. successfully used extra-oral traction to treat mandibular deformities caused by facial hypoplasia, DO has gradually become an important tool in the treatment of certain dental and maxillofacial deformities caused by insufficient bone mass in the jaw. The small mandibular deformity caused by TMJ ankylosis is often accompanied by posterior tongue drop and posterior inferior displacement of the hyoid bone, resulting in a narrow pharyngeal cavity. Therefore, anterior displacement or lengthening of the mandible along with TMJ reconstruction is the key to treat OSAS. Previous procedures have greatly reduced the therapeutic effect due to partial resorption of the implanted bone, insufficient amount of local soft tissue and strong tension of the descending mandibular muscle group. 1994, Moore et al. first reported the success of applying the DO technique to lengthen the mandible to treat Treacher-Collins syndrome and improve the upper airway stenosis. Subsequent decades of continuous clinical practice have demonstrated that the DO technique is not only effective in correcting small mandibular deformities, but also in relieving secondary obstructive sleep apnea, and that its effectiveness in relieving upper airway obstruction and natural recovery of shape are superior to those of traditional surgery. During the traction process, the mandibular body osteotomy is slowly tractioned to extend the distal end of the mandible forward, and new bone grows in the gap formed by the traction, and this new bone has a rich blood supply, thus avoiding the problems of bone grafting and recurrence of traditional surgery, which is increasingly preferred by clinicians. Patients with bilateral temporomandibular joint ankylosis need to undergo joint ankylosis correction and temporomandibular joint reconstruction. Further small (lower) jaws caused by bone defects following neoplastic surgical resection require mandibular reconstruction or traction techniques, depending on the situation. Small (lower) jaw deformity caused by radiation or scarring should preferably be prioritized to correct soft tissue contracture deformity and restore normal soft tissue coverage on the jaw surface, and then orthognathic surgery or traction technique should be considered. Craniomaxillofacial syndromes associated with congenital micromaxillary deformity Congenital micromaxillary deformity is mostly a part of some well-known craniofacial syndromes, and there are more than 60 reported syndromes that include micromaxillary deformity. This article only lists some of the syndromes commonly seen in clinical practice: (a) Pierre Robin sequence: This syndrome was named by Pierre Robin. This syndrome is based on the triad of micrognathia, cleft palate and retrognathia, and is most common in cleft palate with micrognathia. The typical clinical manifestations are micrognathia or retrognathia, with more than 90% of children suffering from micrognathia, showing a mandibular arch 10-12 mm posterior to the maxillary arch, a short jaw body, a rounded chin angle, and a posteriorly positioned condyle; 70%-85% of patients have a retrognathia; 14-91% of patients may or may not have a cleft palate. Cleft palate, which can be hard, soft, uvula or cryptic, usually 80% of cleft palates are “U” shaped rather than “V” shaped like a normal cleft palate; respiratory obstruction, with severe cases of obstructive sleep distress syndrome; feeding difficulties, malnutrition The main cause of respiratory distress is the small jaw. The main cause of respiratory distress is a small jaw or receding jaw rather than an oversized tongue, and only in isolated cases is there a giant tongue sign, tongue fixation or a small tongue deformity. Some patients have ear deformities, otitis media, conductive deafness. There are malformations of the nasal roots, malformations of the teeth and human middle, and laryngeal cartilage chondromalacia. Cleft palate voice disorder is more severe than in patients with cleft palate alone. (ii) Stickler syndrome: also known as Marshall-Stickler syndrome, Wagner-Stickler syndrome, hereditary arthrogryposis. It is a very common disorder of connective tissue disorders inherited in an autosomal dominant manner. The syndrome often has four typical features. ①1. Eye diseases: congenital high myopia (>-8 diopters), retinal detachment, retinal choroidal pigmentation, retinal splitting sign, retinal fissure, cataract, astigmatism, strabismus, glaucoma and eventually blindness. ②Joint changes: Changes in joints vary with age and show a variety of manifestations, sometimes so mild that they can only be detected on X-ray examination. ③Maxillofacial features: cleft palate, cleft palate with submucosa and abnormal palatal motility; underdeveloped midface flatness, short upper jaw, small lower jaw, reduced chin, low flat nasal dorsum, protruding eyes, inner canthus, short nose, etc. Abnormal tooth eruption and underdeveloped enamel. ④ Deafness: with severe progressive deafness. Occasionally, mental retardation is seen. Other deformities include invasive arthropathy, mitral valve prolapse, and osteogenesis imperfecta. (iii) Velo-cardio-facial syndrome (palate a heart side syndrome): also known as the most common of the human polydactyly syndromes. Most of the cases are new. From the initial large number of reported cases, and after more than sixty years of tireless efforts by clinicians, pediatricians, geneticists and others, it has now been determined that the disease is caused by the loss of a small variant on human chromosome 22 (22q11.2). The disease is named differently in different countries and by different physicians, such as SEDLACKOVA syndrome (Eastern Europe), DiGeorge syndrome and Shprintzen syndrome (USA), among others. The clinical manifestations and types of the syndrome are so diverse that different doctors have different findings and there is a lot of crossover between many of the names. To quote Dr. Shprintzen, “the understanding of VCFS is like the blind man touching the elephant in a fable”. The main clinical findings include: cardiac disease; heritage changes; dental and craniofacial malformations; psychological, learning and language deficits; ocular malformations; impaired intellectual development; vascular system malformations; brain malformations; and pharyngeal muscle malformations. Their facial deformities include excessive vertical height of the upper jaw, flattened cheekbones, receding lower jaw, protruding nose, wide nasal root, hypoplastic nasal wings, long human middle, thin upper lip, and often maintained open mouth. Cleft palate deformity. (iv) Treacher Collins Syndrome (TCS): also known as Franceschetti syndrome, Franceschetti-Zwahlen-Klein (F-Z-K) syndrome or mandibulo-facial dysotosis, MFD (mandibulo-facial dysplasia). It involves mainly the temporo-cranial, mid-facial and lower parts of the body, including bony and soft tissue deformities. TCS is characterized by craniofacial hypoplasia (especially zygomatic and mandibular), bilateral exophthalmos, macroglossia, facial fistulae, and external ear deformity, etc. The pathogenesis of TCS is mainly a composite craniofacial cleft deformity, which is considered by most to be a composite cleft of Tessier’s craniofacial cleft types 6, 7, and 8. It is also thought to be a developmental malformation of the first and second gill arches. It has been shown to be an autosomal dominant trait, and TCOF1 is the gene that is now generally recognized to be associated with TCS. Mutations in the TCOF1 gene can be identified in about 90-95% of patients. Its chromosomal location is in the range 5q31.3-q33.3 on the long arm of chromosome 5. The incidence in neonates is reported to be variable, ranging from 1/25,000 to 1/50,000 1/500,000, and is an autosomal dominant disorder, with 60% of cases being disseminated. There are no racial or gender differences in incidence. It was first reported by Thomson and Toynbee in 1846-1847, and in 1889, Berry reported two cases (mother and daughter) with typical eyelid deformities and mentioned the role of genetic factors in the disease. The most basic feature of this syndrome was first described by the English surgeon and ophthalmologist Edward Treacher Collins in 1900: a defect in the zygomatic bone and lower eyelid, which was named Treacher-Collins syndrome. In continental Europe, physicians are more comfortable calling it Franceschetti-Zwahlen-Klein syndrome. It is mainly due to the fact that in 1949, the Swiss ophthalmologists Franceschetti, Zwahlen and geneticist Klein described in detail all the features of the whole syndrome and collected more records of mandibulofacial hypoplasia from 1944 to 1949, suggesting that the disease is a separate malformation and naming it mandibulofacial hypoplasia. The typical clinical manifestations of the disease are: anticanthalmia with a downward fissure of the external eye, hypoplasia or absence of the external lower eyelid and lid, absence of eyelashes, poor attachment of the external brow, shortening of the eyelid fissure, cut marks on the upper eyelid margin and eyebrows; hypoplasia and absence of the zygomatic arch of the zygomatic bone, flattening of the zygomatic area; narrow and hypertelorism of the maxilla, high and narrow palatal arch; hypoplasia of the ascending part of the mandible with a beak deformity, low jaw plane in front and high in the back, Class III malocclusion with anterior teeth, open jaw, and receding chin. The nasal bone is protruding and wide, and the frontal nasal angle is flat or hawk nose. Other deformities of the external ear, absence of the external auditory canal, middle ear abnormalities, hearing loss, and megalodontia may be present. (E) Nager syndrome: also known as pre-axial limb-to-facial dysplasia. It is a syndrome of oromandibular dysplasia with limb developmental malformations. Although some of the symptoms are the same as the dominant symptoms of Treacher Collins syndrome. It is still considered a separate disorder. The main symptoms include downward sloping lid fissures, underdeveloped cheekbones, high nasal bridge, external auditory canal atresia, cleft palate, small mandibular deformities, thumb and radial developmental deformities, and short humerus.