Orthodontic correction of severe Class II malocclusion in a patient with Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a complex disorder that affects multiple systems and may cause craniofacial and dentofacial abnormalities. However, there is still a lack of evidence in the literature regarding the progress of orthodontic treatment in patients with PWS. This case report describes the successful orthodontic treatment of a patient with PWS. A girl, 9 years 0 months of age, who had been diagnosed with PWS had protruding maxillary incisors and a convex profile. Her malocclusion was due to the posteriorly positioned mandible. Screening tests for sleep apnea syndrome showed that she had sleep-disordered breathing, including obstructive sleep apnea and bruxism. We also observed an excessive overjet of 10.0 mm, a deep overbite of 6.8 mm, and the congenital absence of the mandibular second premolars. The patient was diagnosed with an Angle Class II malocclusion and a skeletal Class II jaw-base relationship with a deep overbite. Functional appliance therapy with mandibular advancement, which can enlarge the upper airway and increase the upper airspace, was performed to prevent further deterioration of the patient’s obstructive sleep apnea. An acceptable occlusion with a proper facial profile and functional excursion were achieved without interference after comprehensive 2-stage treatment that incorporated orthodontic therapy for the patient’s excessive overjet and deep overbite. The resulting occlusion was stable, and the occlusal force and the contact area gradually increased over a 2-year retention period. These results suggest that orthodontic treatment offers the opportunity to greatly improve the health and quality of life of people with PWS.


  • We report the successful orthodontic treatment of a patient with a severe skeletal Class II malocclusion and Prader-Willi syndrome.

  • Orthodontic treatment can contribute to improving the health and quality of life of patients with Prader-Willi syndrome.

Prader-Willi syndrome (PWS; OMIM 176270) is a complex genetic disorder that arises from paternally inherited imprinted genes on the chromosome 15q11-q13 region. It develops when the paternal alleles are defective, missing, or silenced. The prevalence of PWS is 1 in 10,000 to 30,000 live births, and 350,000 to 400,000 people in the world are affected by the syndrome. In Japan, the estimated prevalence rate has been reported to be in this range. Most persons with PWS have neonatal and infantile hypotonia, obesity, hypogonadism, childhood-onset hyperphagia, a high pain threshold, small hands and feet, and learning and behavioral problems. With regard to the craniofacial features, PWS is characterized by short stature—associated with growth hormone (GH) deficiency—almond-shaped eyes with upslanted palpebral fissures, bitemporal narrowing, and strabismus. Sleep abnormalities, including narcolepsy and cataplexy, are also common features of PWS that may be related to narrowing of the upper airway. Thus, there is a high prevalence of obstructive sleep apnea (OSA) among PWS patients. The oral manifestations of PWS have been reported to include soft tooth enamel, thick saliva, high caries activity, periodontal disease, and extreme tooth wear caused by clenching and bruxism.

These features indicate that special precautions should be taken when orthodontists treat patients with PWS; however, there is still little information in the literature regarding the progress of orthodontic treatment in these patients. We report the successful orthodontic treatment of a patient with PWS who had a skeletal Class II malocclusion with a deep overbite and congenitally missing mandibular second premolars. This case provides further evidence of the benefits of orthodontic treatment in patients with PWS and highlights some functional insights.

Diagnosis and etiology

A girl, 9 years 0 months of age, visited the outpatient dental clinic of Okayama University Hospital in Okayama, Japan, with chief complaints of protruding maxillary incisors and a convex profile ( Figs 1 and 2 ). She had been clinically diagnosed with PWS by a pediatrician. The characteristic facial features of PWS, short stature, small hands and feet, and hyperphagia were observed ( Figs 1 and 3 ). She had circumoral musculature strain on lip closure. Her facial photographs showed a symmetrical face, a convex profile, an acute nasolabial angle, and protruded and incompetent lips. An excessive overjet of 10.0 mm with Angle Class II molar relationships on both sides and a deep overbite of 6.8 mm were observed. The canine relationship was also Class II, whereas the incisor relationship was Class II Division 1. The maxillary dental midline almost coincided with the facial midline; however, the mandibular dental midline was shifted 3.0 mm toward the left. A dental panoramic tomogram confirmed the absence of the mandibular second premolars and showed no pathologic problems in the root structure or the periodontal condition ( Fig 1 , D ). The patient showed no significant symptoms of temporomandibular disorder. The interincisal distance on maximal opening without pain was 39 mm. The occlusal force and the occlusal contact area were calculated to be 564 N and 9.8 mm 2 , respectively, using an occlusal-force recording system (Dental Prescale & Occluzer; Fujifilm, Tokyo, Japan) ( Table I ). A screening sleep study with a polysomnography recorder (Apnomonitor III; Chest M.I., Tokyo, Japan) confirmed the airway collapse with decreased oxygen level and habitual snoring, with arousal from sleep indicating obstructive sleep apnea (OSA). The apnea-hypopnea index is used to evaluate the severity of OSA, and an apnea-hypopnea index score of more than 5 events per hour of sleep is considered to be diagnostic of OSA. According to the American Academy of Sleep Medicine, it is categorized as mild (5-15 events), moderate (15-30 events), or severe (>30 events). The patient’s apnea-hypopnea index score was 9.3 events per hour, classified as mild OSA.

Fig 1
Pretreatment records: A, facial photographs; B, intraoral photographs; C, lateral cephalogram; D, panoramic radiograph.

Fig 2
Pretreatment dental casts.

Fig 3
Characteristic features of PWS in our patient included A-C , small hands and feet, and D and E , longitudinal changes in height and body weight. Bar: 50 mm.

Table I
Longitudinal change in occlusal function during orthodontic treatment
Pretreatment Pre-edgewise Posttreatment Postretention (1 mo) Postretention (3 mo) Postretention (6 mo) Postretention (2 y)
Occlusal force (N) 564 467 435 426.7 553.1 603.6 657.7
Occlusal contact area (mm 2 ) 9.8 8.3 6.8 7.3 9.8 11.1 12.5
Maximum opening (mm) 39 40.5 39

Compared with Japanese norms, the patient’s cephalometric analysis showed a skeletal Class II jaw relationship due to the relatively posterior position of the mandible (ANB, 9.2°; SNA, 76.9°; SNB, 67.7°) and a steep mandibular plane angle (FMA, 34.5°) ( Fig 1 , C ; Table II ). The maxillofacial features included reduction of the maxilla, the ramus of the mandible, the mandibular body, and the anterior and posterior facial heights. The maxillary and mandibular incisor angles were within the normal ranges (103.3° and 99.8°, respectively). The patient’s lower facial height ratio was slightly small (N-Me, 118.4 mm; Me/PP, 60.2 mm), and the upper and lower lips protruded against the esthetic E-line (upper, 5.0 mm; lower, 5.0 mm). An optoelectronic jaw-tracking system (gnathohexagraph system, version 1.31; Ono Sokki, Kanagawa, Japan), which can record jaw movements with 6 degrees of freedom, indicated that the movement of the mandible and both sides of the condylar heads were not stable during the maximum opening and closing of the jaw. Irregular patterns of movement were also observed in a chewing test using a hard gummy jelly ( Fig 4 ).

Table II
Summary of cephalometric findings
Variable Japanese norms (woman) SD Pretreatment Preedgewise Posttreatment Postretention
Angular (°)
ANB 2.8 2.44 9.2 6.5 8.0 8.0
SNA 80.8 3.61 76.9 75.5 76.0 76.0
SNB 77.9 4.54 67.7 69.0 68.0 68.0
FMA 30.5 3.60 34.5 33.9 33.7 33.7
UI-FH 112.3 8.26 103.3 103.1 90.6 91.5
LI-Mp 93.4 6.77 99.8 101.9 96.0 96.5
Interincisal angle 123.6 10.64 122.3 121.1 139.7 138.0
Occlusal plane to SN 18.0 3.50 29.8 28.6 30.0 30.0
Gonial angle 122.2 5.29 121.5 120.9 117.2 117.2
Linear (mm)
S-N 67.9 3.65 68.5 71.7 71.8 71.8
N-Me 125.8 5.04 118.4 122.9 127.2 127.2
Me/PP 68.6 3.71 60.2 62.5 66.5 66.5
Go-Me 71.4 4.14 59.4 65.1 67.6 67.6
Ar-Me 106.6 5.74 87.6 93.4 95.9 95.9
Ar-Go 47.3 3.33 38.7 39.7 41.9 41.9
Overjet 3.1 1.07 10.0 6.0 1.5 3.0
Overbite 3.3 1.89 6.8 5.5 2.0 3.5
UI/PP 31.0 2.34 28.7 29.5 28.0 30.0
U6/PP 24.6 2.00 17.4 19.1 20.9 20.9
LI/Mp 44.2 2.68 44.2 44.7 44.5 44.5
L6/Mp 32.9 2.50 32.7 33.9 37.8 37.8
E-line to upper lip -2.5 1.9 5.0 2.0 0 0
E-line to lower lip 0.9 1.9 5.0 2.0 -1.5 -1.5
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Dec 8, 2018 | Posted by in Orthodontics | Comments Off on Orthodontic correction of severe Class II malocclusion in a patient with Prader-Willi syndrome
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