# Evaluation of HMDs by QFD for Augmented Reality Applications in the Maxillofacial Surgery Domain
## 2 Materials and Methods
### Desk Research on Maxillofacial Practices
#### 2.1.2 Orthognathic Surgery  
Orthognathic surgery is the subspecialty of plastic and maxillofacial surgery that corrects congenital and acquired deformities of the head, skull, face, neck, jaw, and associated structures. This subspecialty was pioneered by Dr [PERSON], who studied the techniques to correct facial anomalies through a transcranial approach in the second half of the 20 th century. This specialty has grown to become the subspecialty of plastic surgery that restores facial form by addressing both skeletal and soft tissue abnormalities. Orthognathic surgeons have expertise in bones, muscles, nerves, fat, and skin.  
It is said that \"beauty will save the world\" or, at least, may help in physical and psycho-social health; correcting dentofacial deformities solves skeletal malocclusion and improves the aesthetics of the face, reaching a harmonious facial balance [16]. Studies are now focusing on the ability to forecast soft tissue shape after bone repositioning during pre-operatory planning. The zygomatic region is one of the most involved areas, and a deeper knowledge of this region could lead to the development of predictive surgical tools [17; 18].  
Surgical intervention in early childhood obviates many problems: the greater plasticity of the tissue allows there to be significant remodelling and subsequent growth that is not observed in adult tissues [19]. Orthognathic surgery is not just an aesthetic matter; infants and children with syndromes may have more difficult airways and complicated surgical repair with more bleeding. Associated anomalies include facial and airway features that make mask ventilation and intubation difficult [20]. Orthognathic surgery requires a high level of accuracy because the treatment is applied to a sensitive area. Specific complications include venous air embolism, dural tears, extradural or subdural hematoma, blindness, permanent neurological deficits [21].  
To restore the facial contour, as well the functionality of the craniofacial skeleton, an autogenous bone graft (iliac, costal, tibial, and calvari grafts) is used, while alloplastic bone substitutes can be used to alter facial contour. Today, the mix of traditional procedures with new tools to make harvesting easier and faster [22], and the use of titanium miniplates to stabilise bone grafts in craniofacial and orbital osteotomies [23], makes the procedures safer and less taxing for the patient. Orthognathic surgeons are also responsible for the repositioning of the maxilla, mandible, or chin. The Bilateral Sagittal Split Osteotomy (BSSO) is one of the most common types of jaw surgery [24]. A BSSO first-mantible methodology can be summarised as follows: (1) the surgeon performs the first osteotomy, (2) the mandible is set in the correct position using the intermediate splint, (3) the surgeon performs the second osteotomy, (4) the maxilla is placed using the final splint, and (5) upper and lower jaws are screwed to the skull bone with customized titanium plates.  
A visual assistance system could be useful to compare the positioning of maxilla and mandible to the preoperatively planning, acting as a kind of feedback for the surgeon to verify the deviation between the planned model and the surgical outcome.