Orofacial muscles

The functional capacity of the masticatory muscles in adult individuals with DM is weaker than that of healthy subjects of the same sex and age. Adult subjects with DM in western Sweden were found to have a low maximal bite force level, on average less than half of the mean value of matched healthy control subjects, 138 N (standard deviation [SD] 79 N) in 16 females with DM compared with 383 N (SD 130 N) in healthy females; 205 N (SD 59 N) in 8 males with DM compared with 540 N (SD 137 N) in healthy males (p <0.001; Kiliaridis et al., 1989). This has been confirmed by another study in Brazil on 37 subjects (Guimaraes et al., 2007). When looking at masticatory muscle activity, it seems that the disease influences the muscles involved in closing the mandible, as measured by the electromyographic (EMG) activity of the temporal and masseter muscles of adults with DM during maximal clenching and while chewing five peanuts until swallowing when compared to healthy matched subjects (Odman and Kiliaridis, 1996). It was found that the patients with DM had approximately three times less EMG activity in the masticatory muscles during maximal clenching. During chewing, patients with DM had approximately half the activity in the anterior temporal and the masseter muscles. DM thus influences the masticatory muscles, reducing their activity both at a maximal and a functional level.

Ultrasonography has also been applied to measure the thickness and examine the internal structure of the masseter muscle in 16 adult patients with DM compared to healthy individuals matched for age, sex, and number of occluding teeth (Kiliaridis et al., 1995a). The masseter thickness was measured bilaterally both under relaxed conditions and during maximal clenching. The imaging characteristics of the masseter in most DM patients revealed thin muscles with obvious atrophy, increased echo intensity of the intramuscular tissue, and visual loss of the internal structure concerning tendons and fasciae (Figure 13.1). The mean masseter muscle thickness in the DM group was 10.4 mm (SD 2.2 mm) under relaxed conditions and 11.1 mm (SD 2.4 mm) during maximal clenching, compared with 13.3 mm (SD 2.2 mm) and 14.1 mm (SD 2.4 mm), respectively, in the healthy group (p <0.001). Ultrasonography is considered a valuable method for both qualitative and quantitative evaluation of the condition of the masseter muscles. These findings indicate that, in most patients with DM, the masseter muscle is atrophic with obvious signs of degeneration (Kiliaridis et al., 1995a).

In children and adolescents with DM, the characteristics and prevalence of oral motor dysfunction have also been studied and different aspects of oral motor function have been correlated with the type of DM and sex (Sjögreen et al., 2007). In this study, 56 individuals with DM (30 males, 26 females) with a median age of 13 y 2 mo (range 2 y 6 mo–21 y 5 mo) were compared with healthy controls. They were divided into four subgroups: congenital DM (n = 18), mild congenital DM (n = 18), childhood DM (n = 18), and classical DM (n = 2). A speech‐language pathologist assessed different variables of oral motor function, intelligibility, and lip force. The families used a questionnaire to report on eating difficulties and drooling. All individuals with DM had impaired facial expressions. Intelligibility was moderately or severely reduced in 30 patients (60%), excluding 6 patients without speech. Most of the children had moderate or severe impairment of lip motility (76.0%), tongue motility (52%), and lip force (69%), causing deviant production of bilabial and dental consonants. The families reported problems with eating (52%) and drooling (37%). Oral motor dysfunction was most prominent in congenital DM, and males were more affected than females (Sjögreen et al., 2007). In addition to the published findings, most patients were characterized by a downward‐hanging mandibular position and a big free‐way space.

Dentofacial morphology

Occlusal traits and dentofacial morphology have been studied in 24 adult patients with DM and compared with a matched group of healthy individuals (Kiliaridis et al., 1989). A high prevalence of malocclusions was found among the patients with DM, who presented a higher frequency of malocclusions in the sagittal, vertical, and transversal planes, characterized often by Angle Class II malocclusion, anterior open bite, lateral crossbite, and notable space discrepancies (Figure 13.2). This patient group’s craniofacial morphology showed a vertical aberration, characterized by a large angle between the mandibular and palatal planes and a steep mandible (Figure 13.3). These findings seem to be most pronounced in patients with early onset of the disease (Kiliaridis et al., 1989).

Interpretation of these data helps one conclude that the weak masticatory muscles are the likely etiological factor creating the aberration in the dentofacial morphology in adults with DM. To be able to develop and implement possible interceptive measures and to inhibit the development of the detected dentofacial anomalies, knowledge about the functional situation of these patients during childhood and adolescence and the extent and progress of the observed dentofacial aberrations during growth in these individuals is necessary. Therefore, a cohort study of a group of children and adolescents with DM was necessary to reveal their functional and dentofacial characteristics, as well as a longitudinal study to follow up a cohort of young patients with DM and observe the development of their dentofacial anomalies.

The craniofacial morphology of 26 young individuals with congenital or childhood‐onset DM was studied on lateral cephalograms in a cross‐sectional manner and compared with normal standards obtained from healthy individuals from the Michigan Growth Study, according to their age and sex (Fontinha et al., 2018). Subsequently, the growth changes that occurred in the facial morphology of the children with DM over five years were recorded longitudinally and compared to the growth changes observed in healthy children (Fontinha et al., 2021). On examination of initial radiographic measurements, patients with DM showed a more retruded mandible sagitally, with larger ANB and smaller SNPg angles. Analysis of the vertical plane showed the mandibular plane angle and the intermaxillary angle to be larger. It was interesting to notice that during the five‐year follow‐up period, the intermaxillary angle remained the same in the children with DM, whereas this angle decreased in healthy individuals. Thus, it is important to stress that the young patients with DM already had a more retrognathic profile and hyperdivergent skeletal aberration with a steep mandibular plane and large intermaxillary angle at a young age, compared with healthy individuals. Their facial morphology seems to be established early and does not deteriorate during growth. With regard to different growth patterns, in children with DM the intermaxillary angle did not seem to decrease during the observation period, contrary to what was observed in healthy individuals (Fontinha et al., 2018).

The malocclusions of 36 children and adolescents with DM were studied on dental casts and compared with a control group of 50 healthy growing individuals (Fontinha et al., 2021). The changes that had occurred in the sagittal, vertical, and transversal planes of occlusion of 26 of these patients over a median time of nine years during their growth were then traced longitudinally and compared to the changes observed in healthy growing individuals. It was found that the children and adolescents with DM had a higher prevalence of anterior open bite and posterior crossbite. When compared to controls, patients with DM presented smaller upper and lower intermolar as well as intercanine widths. In both groups, the individuals revealed longitudinal changes with a decrease in both upper and lower arch lengths and an increase in palatal vault height. During the follow‐up period, the prevalence of malocclusions remained almost the same, only significantly differing with regard to the maxillary intermolar width, which decreased in individuals with DM. Similar to what was found in the craniofacial morphology of children and adolescents with DM, it has been shown that a higher prevalence of both anterior open bite and posterior crossbite is found already at an early age, compared to healthy control individuals. These occlusal traits do not change with time except for the maxillary narrow intermolar width, which further decrease over time (Fontinha et al., 2021).

Thus, concerning the dentofacial characteristics of patients with DM, it is important to stress that contrary to what one would expect, young patients with DM already have a more retrognathic profile and hyperdivergent skeletal aberration with a higher prevalence of anterior open bite and posterior crossbite from an early age, compared with healthy individuals. It seems that these characteristics have been established early and do not dramatically deteriorate during growth.

Orthodontic treatment

Due to the severity of the malocclusion and dentofacial deformity present, as well as possible functional limitations resulting in impairments in daily living, orthognathic surgery may be the best option to be considered in some patients with DM (Bezak et al., 2016). Orthognathic surgery can play an important role in managing deformities of the facial skeleton due to DM (Manzon and Philbert, 2007), especially in cases where, due to the magnitude of the discrepancy, the aesthetic and functional goals of treatment are not achievable with conventional orthodontics alone.

However, patients with DM present certain particularities that may complicate the task of deciding whether orthognathic surgery will be proposed as a viable treatment option. One must consider both the benefits and risks of such a procedure. In this population, the potential risks of elective surgery and general anesthesia may outweigh such procedures’ benefits (Manzon and Philbert, 2007). Moreover, the indication to propose orthodontic treatment and orthognathic surgery to improve esthetics and masticatory function is difficult to determine, since the progressive nature of MD implies that the prognosis with regard to the development of dentofacial abnormalities and the outcome and stability after a particular treatment remain unpredictable (Balasubramaniam et al., 2008).

Despite this, the psychosocial effect related to esthetics and facial appearance may also influence the treatment decision process (Bezak et al., 2016). The importance of esthetics cannot be overlooked for teenagers and young adults, who are often defined and limited by the physical manifestations of their disability (Phillips and Beal, 2009; Marik and Hoag, 2012). Considering that the treatment can result in a significant improvement in facial esthetics, jaw function, self‐esteem, and quality of life, these situations may lower the threshold in favor of orthognathic surgery in selected patients (Manzon and Philbert, 2007; Bezak et al., 2016). A treatment plan involving orthognathic surgery, coupled with multidisciplinary management, in patients with DM and associated dentofacial deformities can surely offer satisfactory esthetic and functional outcomes, but the question of whether this satisfactory result will remain stable in the long term is a question that is central to the treatment planning and decision‐making process.

The body of the maxillofacial and orthodontic literature is relatively light with regard to orthodontic and orthognathic surgical management of patients with DM. A limited number of cases have been described in the literature over the past 40 years. To our knowledge, one case with camouflage orthodontic treatment (Thind and Turbill, 2015) and ten cases with combined orthodontic and orthognathic surgical treatment (Müller and Punt‐van Manen, 1982; Kaufman et al., 1983; Manzon and Philbert, 2007; Bezak et al., 2016; Cacucci et al., 2017; Antonarakis et al., 2019) have been reported. These are presented in Table 13.1. However, among these cases only four refer to postsurgical follow‐up (12 and 18 months in two cases, both of which refer to follow‐up post surgery and not post orthodontics, while two cases present postorthodontic follow‐up, one of 7 months and one of 13.5 years). Moreover, all of these cases were treated at a relatively young age, either in late adolescence or in early adulthood.

In cases where postsurgical follow‐up is reported (Manzon and Philbert, 2007; Kaufman et al., 1983), it is not clear when orthodontic treatment was terminated, since the authors refer to postoperative and not posttreatment follow‐up and it could well be that the postoperative orthodontic phase lasted for the greatest part of this postoperative follow‐up period. In addition, records shown in these cases are incomplete without a lateral cephalometric radiograph immediately postoperatively and after the 12–18‐month follow‐up to assess skeletal and dental stability. In the two cases where postorthodontic follow‐up is discussed (Cacucci et al., 2017; Antonarakis et al., 2019), both discuss relapse and the difficulties in maintaining long‐term stability. The case showing the longest follow‐up (13.5 years) presents a case with significant relapse of the anterior open bite within these years of follow‐up (Antonarakis et al., 2019), which brings to light the difficulties in maintaining stability given the muscular weakness (Figure 13.4). One major contributing factor to the relapse of the open bite was the overeruption of posterior teeth, shown in the cephalometric superimpositions (Figure 13.5), which may be due to muscular weakness (Antonarakis et al., 2019).

With this in mind, the complexity of the disease, the perioperative morbidity that can accompany DM, and the unpredictability of long‐term treatment results dictate the need for a thorough evaluation from the multidisciplinary team when planning a combined orthodontic and orthognathic surgical intervention (Manzon and Philbert, 2007). The orthodontist and maxillofacial surgeon involved must be knowledgeable about the disease process, the physical characteristics, and the concomitant systemic anomalies, which helps facilitate proper planning, timely interventions, and efficient postoperative care.

When carrying out orthognathic surgery in these individuals, rigid fixation must be used and maxillomandibular fixation avoided to maintain patency of the upper airway and promote early return to function (Manzon and Philbert, 2007). Orthognathic surgical movements must be designed to favor long‐term skeletal stability because of the high relapse potential due to progressive weakness of the masticatory muscles (Manzon and Philbert, 2007). Weakness of the masticatory muscles may result in immediate relapse of surgical movements to correct the anterior open bite, or may present as a late complication due to the overeruption of both the maxillary and mandibular posterior dentition (Manzon and Philbert, 2007; Kaufman et al., 1983; Antonarakis et al., 2019; Kiliaridis et al., 2019). The abnormal patterns of vertical facial growth (Fontinha et al., 2018; Staley et al., 1992), linked with altered neuromuscular function, may present difficulties for orthodontists and maxillofacial surgeons treating these patients.

The high potential for instability following orthodontic and orthognathic correction of an anterior open bite in patients with DM raises the issue of long‐term retention. There is a critical need for long‐term retention in these patients, and the orthodontist must evaluate the most appropriate way to provide retention to avoid the reappearance of an open bite. This may be with a posterior bite block, through its positive effect on the maxillary and mandibular molars, or elastics attached to skeletal anchorage plates that act on the maxillary and mandibular molars. This was attempted by the authors of one case that asked the patient to wear elastics from their removable retainers to skeletal anchorage plates, and subsequently elastics directly on the anterior teeth to decrease the relapse of the open bite (Cacucci et al., 2017).

As with any long‐term retention protocol, patient compliance may be challenging. This highlights the importance of informed consent, which must be properly obtained from the individual in question. Suppose patients with DM choose to correct their dentofacial anomaly. In that case, they must be aware that the result may only be stable for a limited time due to the high relapse potential, perhaps even despite any long‐term lifelong retention protocols. One must always keep in mind that not undergoing any treatment also remains an option.

Regardless of which retention protocol is chosen to favor long‐term stability, chewing and mandibular exercises may also be of benefit. The adaptability of the healthy masticatory muscles to training, such as chewing exercises, has been demonstrated in healthy individuals (Kiliaridis et al., 1995b; Ingervall and Bitsanis, 1987; Georgiakaki and Kiliaridis, 1998; He et al., 2013). Whether this is the case in individuals with DM and whether it would help in maintaining stability is, however, unknown. No studies have been carried out on patients with DM, but some data exist on patients with other neuromuscular disorders where masticatory exercises are found to improve masticatory function and performance (Kawazoe et al., 1982; Nozaki et al., 2010; van Bruggen et al., 2015). Nevertheless, it is not known whether in individuals with DM, and if so in which types of DM, chewing exercises might help prevent severe molar overeruption and thus diminish the chance of anterior open bite development.