Clinical, retrospective case-control study on the mechanics of obstacle in mouth opening and malocclusion in patients with maxillofacial fractures

This study aims to identify and distinguish various factors that may influence the clinical symptoms (limited mouth opening and malocclusion) in patients with maxillofacial fractures. From January 2000 to December 2009, 963 patients with maxillofacial fractures were enrolled in this statistical study to aid in evaluating the association between various risk factors and clinical symptoms. Patients with fractured posterior mandibles tended to experience serious limitation in mouth opening. Patients who sustained coronoid fractures have the highest risk of serious limitation in mouth opening (OR = 9.849), followed by arch fractures, maxilla fractures, condylar fractures, zygomatic complex fractures and symphysis fractures. Meanwhile, the combined fracture of zygomatic arch and condylar process results in normal or mild mouth opening. High risks of sustaining malocclusion are preceded by the fracture of nasal bone (OR = 3.067), mandible, condylar neck/base, combined fracture of zygomatic arch and condylar process, mandibular body, bilateral condylar, dental trauma, mandibular ramus, symphysis, mandibular angle and mid-facial. Patients who experienced serious limitation in mouth opening are treated with surgery more frequently (OR = 2.118). No relationship exists between the treatment options and the patients with malocclusion.

The primary goals in treating maxillofacial fractures are to establish and maintain normal occlusion and attain the preinjury mobility and function of the jaws and the preinjury 3-dimensional (3D) facial contours 1,2 . Numerous studies have been conducted on the epidemiology and treatment of maxillofacial fractures. However, works on the basic mechanism of clinical symptoms, such as limited mouth opening and malocclusion, are limited; for instance, why different types of maxillofacial fractures display different symptoms in mouth opening and occlusion. In clinical practice, on the basis of experience alone, the occurrence and type of maxillofacial fractures are difficult to determine. More seriously, maxillofacial fractures are difficult to treat effectively and accurately 3 . Accordingly, postponed or inappropriate intervention may lead to the malformation of osseous callus and soft tissue fibrosis 4 , and even the occurrence of facial deformity (facial depression 5 , temporomandibular joint ankylosis and micrognathia 6 ).
In previous studies, we analysed the mechanisms in the production of mandibular fractures 5,7-10 and evaluated various factors that are correlated with maxillofacial fractures [11][12][13][14][15][16] . In the present study, we attach more importance to the symptoms of patients who sustained maxillofacial fractures. The exploration and analysis of the mechanism of the clinical symptoms of maxillofacial fractures could provide an in-depth understanding of the mechanism of maxillofacial fractures for an accurate and effective assessment of the patients' condition and treatment of maxillofacial fractures, while reducing the individual, family, social and national burden and promoting . Continuous variables were reported as mean ± SD and assessed using independent sample t-tests as necessary. The chi-square test was used to compare the categorical variables. Fisher's exact test was utilized when observation in any cell of the 2 × 2 table was expected to be less than five. Odds ratio (OR) and 95% confidence interval (CI) were used to assess the risk of patients who sustained limited mouth opening or malocclusion. The risk factors of limited mouth opening were further analysed by using ordinal logistic regression. Logistic regression analysis was utilized to control the confounding variables. Probabilities of P < 0.05 were considered significantly different.
Results. The risks of patients who sustained serious limited mouth opening according to various variables are summarized in Table 1 (ordinal logistic regression). Patients who sustained fractures of the posterior mandible tended to be associated with serious limitation in mouth opening (coronoid fracture: OR = 2.989; condylar fracture: OR = 2.370; ramus fracture: OR = 1.592), compared with patients who sustained fractures of the anterior mandible (alveolar fracture: OR = 1.567; symphysis fracture: OR = 1.297; body fracture: OR = 1.225), except angle fractures (OR = 1.249) and condylar head fractures (OR = 1.206).
The relationship between various etiologies and the risk of limited mouth opening is further summarized in Table 2. Patients who sustained coronoid fractures had the highest risk of serious limitation in mouth opening (OR = 9.849), followed by arch fractures (OR = 3.202), maxilla fractures (OR = 2.914), condylar fractures (OR = 2.764), ZCF fractures (OR = 2.701) and symphysis fractures (OR = 2.694). As far as traumatic etiologies, such as assault, bicycle and MVA, are concerned, most of the patients tended to sustain moderate mouth opening (1 cm < opening ≤ 2 cm, OR > 1) and less probability of serious limitation in mouth opening (opening ≤ 0.5 cm, OR < 1). Patients with condylar neck/base fractures were more prone to the occurrence of mild mouth opening (OR > 1) and less to the occurrence of severe or serious limitation in mouth opening (OR < 1).
Interestingly, fracture of the zygomatic arch or condylar process resulted in the high occurrence of serious limitation in mouth opening (OR > 1), whereas the combined fracture of zygomatic arch and condylar process were more prone to normal or mild mouth opening (OR by ordinal logistic regression: 0.558; OR by logistic regression analysis: OR minor mouth opening = 2.956, OR normal mouth opening = 2.020).
Accordingly, patients who sustained serious limitation in mouth opening were treated by surgery more frequently (OR = 2.118), whereas patients with normal mouth opening were frequently treated nonsurgically. The risk of sustaining malocclusion in associated with different factors, which are summarized in Table 3. We found that nearly all traumatic factors had a high risk of occurrence of malocclusion (OR > 1), except for patients who sustained fractures of the condylar head/mandibular alveolar/zygomatic arch/coronoid process/maxilla alveolar/single mandible/single maxilla. The high risks of sustaining malocclusion in descending order are as follows: fracture of nasal bone (OR = 3.067), mandible (OR = 2.721), condylar neck/base (OR = 2.173), combined fracture of zygomatic arch and condylar process (OR = 1.846), mandibular body (OR = 1.731), bilateral condylar (OR = 1.595), dental trauma (OR = 1.564), mandibular ramus (OR = 1.432), symphysis (OR = 1.338), mandibular angle (OR = 1.330) and mid-facial (OR = 1.249). Interestingly, body injuries (including traumatic head and ocular injuries) resulted in the high occurrence of malocclusion (OR > 1). Heavy impact damages (motor vehicle accidents or motorcycle accidents) also resulted in the high occurrence of malocclusion (OR > 1). Interestingly, fractures of the zygomatic arch resulted in the low occurrence of malocclusion (OR = 0.452).
No relationship exists between the treatment delay times and the clinical symptoms of limited mouth opening or malocclusion, regardless of the length of time that passed after the fracture of the upper and lower jaws (Tables 1 to 3, OR ≈ 1). No relationship exists between the treatment options and the patients with malocclusion (Table 3, OR = 1.011).
Discussion. The movement of the mandible (mouth opening and closing) is controlled by several muscles that are attached to it. The masticatory muscles (from the zygomatic arch, inserted to the lateral aspect of ramus), temporalis (from the temporal lines of the parietal bone, inserted to the coronoid process and anterosuperior border of ramus) and medial pterygoid (from the pterygoid process and tuberosity, inserted to the medial aspect of ramus) control the elevation of mandible, while the medial and lateral pterygoids (from the pterygoid process, pyramidal process of palatine bone and tuberosity, inserted to the condyle and anteromedial part of the disk) control the protrusion and laterotrusion of the mandible. As the antagonism mechanisms, the temporalis controls the mandible retrusion, while the suprahyoid musculature controls the mandible decline 22 . However, the occurrence of maxillary or mandible fracture (or injury) leads to broken muscle balance and the change in the movement mechanism of the mandible. A comprehensive understanding of the various factors that influence the movement of mandible after maxillofacial trauma is important in providing clinical and research data for the effective management of these injuries.
The fracture of condylar process results in high risk of serious limitation in mouth opening. However, further study revealed that the condylar head leads to the high occurrence of serious or severe limitation in mouth opening, whereas condylar neck/base fracture does not affect mouth opening. These phenomena may be attributed to several reasons. The articular disk is attached to the medial and lateral poles of the condylar process, ensuring the synchronized movements of the condyle and disk during gliding movements 22 ; however, in patients with condylar head fractures, the continued traction of the lateral pterygoid muscle results in the anteromedial displacement of the fragment in condylar head fractures 23 , which may impede the movement of the condylar head 8 . Condylar neck/base fractures destroy the integrity of the mandible, and the muscle tension of the articular disk and lateral pterygoid cannot perform the function of mandibular movement. Consequently, the gliding movements, protrusion and laterotrusion movement of the mandible are weakened. However, the descending movement of the mandible that is controlled by the suprahyoid musculature remains, and mouth opening is recovered with the weakening of edema and muscle spasms. Interestingly, the present study revealed that patients with bilateral condylar fractures are more prone to the occurrence of mild mouth opening. We propose that mandible movement that is centred by the mandibular foramen (a fulcrum, controlled by sphenomandibular ligament) is retained despite of the disappearance of the temporomandibular joint activity.
Patients who sustained coronoid or zygomatic arch fractures are the most prone to serious limitation in mouth opening. Coronoid fractures seem highly co-related to zygomatic arch fractures. Our previous study revealed that majority of the patients who sustained coronoid fractures also had fractured zygomatic archs (20 of 25 patients, 80%). Furthermore, nearly all patients (23 of 25 patients, 92%) with coronoid fractures showed limited mouth opening 5 . We propose that the fracture of the zygomatic arch or coronoid processes is usually attributed to the muscle compression or mechanical barrier in the process of mouth opening and closing, thus leading to the serious limitation in mouth opening. The masticatory muscles and temporalis are squeezed by the fracture of the zygomatic arch or coronoid process, resulting in reduced or dysfunctional muscle in the traction and shrinking process. In our experience, coronoid fracture removal is usually conducted when limited mouth opening cannot be resolved after the open reduction of maxillofacial fractures 5 .
The fracture of mandible seems more prone to the limited mouth opening than mid-facial fractures, except for the fracture of the zygomatic arch. Additionally, patients with fractured posterior mandibles are prone to serious limited mouth opening compared with the patients with fractured anterior mandibles. This is understandable because most of the muscles (masticatory muscles, temporalis, medial pterygoid and lateral pterygoid) control the movement of the mandible attached to the posterior mandible. Interestingly, for patients who sustained serious limited mouth opening are concerned, the possibility of surgical treatment is high (OR = 2.118). The risk of surgical treatment dropped to 0.703-fold for patients with normal mouth opening. This phenomenon explains the main purpose of surgical treatment is to recover normal mouth opening. Thus, in the present study, surgical treatment is considered for patients with fractured mandibles compared to patients with mid-facial fractures, especially for patients with fractured posterior mandibles.
Malocclusion common in patients with fractured mandibles, except those with fractured coronoid process/condylar head/mandibular alveolar. Patients with mid-facial fractures have no obvious relationship with malocclusion. This is not surprising because the porous structure of the mid-face lowers the risk of malocclusion in patients with sustained mid-facial fractures (OR zygomatic arch = 0.452, OR ZCF = 1.051, OR maxilla = 1.086, OR orbital = 1.037 and OR Lefort = 1.253). In clinical practice, we rarely find situations where the dental arch is SCiEnTifiC REPORTS | (2018) 8:7724 | DOI:10.1038/s41598-018-25519-0 divided into several sections in the upper jaws of patients with mid-facial fractures. Patients who sustained heavy impact damage (from motor vehicle accidents or motorcycle accidents) are more prone to malocclusion; however, no relationship exists between the heavy impact damage and the limitation in mouth opening. Interestingly, patients with other body fractures/injuries (including traumatic head injuries or ocular injuries) are more prone to both malocclusion and serious limited mouth opening.
Patients with different fracture levels of the mandibular condylar process display different clinical symptoms. Patients with condylar base/neck fractures are more prone to malocclusion than to limited mouth opening. In contrast, patients with condylar head fractures show high occurrence of limited mouth opening than of malocclusion. In the present study, we found that patients who sustained serious limited mouth opening were treated by surgery more frequently (OR > 1), whereas patients with normal mouth opening were more frequently treated by nonsurgical approaches. Accordingly, the surgical treatment of condylar head fractures are considered more frequently than before. In fact, increasing studies are reporting good results and advantages of the surgical treatment    self-recovery or automatic healing. A surgical treatment plan should be considered for patients who sustained serious limited mouth opening. For instance, patients with fractured coronoid process/zygomatic arch/maxilla/ condyle/ZCF/symphysis/Lefort/combined body injuries (including traumatic head injuries or ocular injuries) ( Table 2) and simultaneously associated with serious limited mouth opening should consider surgical treatment first. Patients who sustained only malocclusion could consider a non-surgical treatment procedure (intermaxillary traction and/or fixation of loose teeth). We acknowledge some flaws in our study. As a retrospective clinical case-control study, the incomplete information collected results in part loss of sample subjects. Information were collected based on case histories, and thus the reliability of information is dependent on how accurately the patients provided the information and the standard uniform recording by different physicians. Additionally, we should point out that some kinds of malocclusion are present in populations without facial fractures, we also don't know if they had these malocclusion before the fractures occurred. However, a large sample size (963 patients) over a long period (10 years) may partially compensate for the above shortcomings. Regardless of this study's results, a prospective, multicentre/ multilevel and large sample study should be conducted in the future.
The present study revealed that patients associated with serious limited mouth opening should prefer surgical procedure. Patients who sustained only malocclusion or acceptable/normal mouth opening may consider a non-surgical treatment procedure; nonetheless, the patient's appearance (3-dimensional facial contours) is also an important factor that needs to be considered. The accurate assessment of patients' condition are helpful in the timely, correct and effective treatment of the their disease, while reducing the individual, family, social and national burden. Such a retrospective study can be used to guide the future funding of public health programs that are geared towards the prevention and treatment of such injuries.