Stress on facial skin of class III subjects during maxillary protraction: a finite element analysis

The results of the von Mises yield criterion are shown in Figs. 3 and 4 and Table 1. For both FM models, greater tensions were recorded at the level of the chin cup with higher stresses and deformations observed for the Petit FM (29 KPa and 44 KPa, for Delaire and Petit FM, respectively (Table 1). When Delaire FM was tested, maximum stresses were observed on the upper border of the chin cup corresponding to the inferior lip and to marginal gingiva of lower incisors (Fig. 3A). Moreover, maximum stresses tended to decrease constantly as moving away from this area. Similarly, tensile strength distribution analysis showed maximum stresses in correspondence of chin cup area after Petit FM application (Fig. 4A). In particular, tensile tensions were more extensively distributed and localized in correspondence of both upper border and central area of the chin. Within all tests performed, lower stresses were observed in the forehead area respect to the chin area (Figs. 3B,4B). Heavier tensions developed on the forehead were at least three times lower than the maximum stresses affecting the chin (7 KPa and 3 KPa, induced by Delaire and Petit FM, respectively).

FM therapy represents the gold standard for correcting skeletal Class III malocclusion in growing subjects [1‐5, 16‐19]. Several factors play a substantial role in terms of efficacy and effectiveness of orthopedic Class III treatment, including individual skeletal pattern, protraction device adaptation, patients’ compliance, and device wear-time [20‐24]. Stocker et al. [23] and Ozkalayci and Cicek [24] analyzed wearing time and patient compliance with FM reporting a significantly lower wearing time than the prescribed instructions. The not customized design of both FM models can cause poor device adaptation to the patient’s face. The less adhesion of the device could determine skin irritations and mild swelling, with negative effects on patient compliance [1‐5]. Stresses and load distribution developed on skeletal structures during orthopedic protraction treatment were widely analyzed [6, 7, 25]. More recently, in a previous investigation stresses developed during maxillary protraction and their effects on FM structure were evaluated by means of FEM analysis [13]. The absence of permanent plastic deformations and efficacy persistence of FM components highlighted the importance of a careful management of the device during treatment in order to grant its best performance. Although adverse effects on facial skin are known and very frequent in daily practice, however no data are available in literature regarding distributions and effects of tensile forces on facial skin. To our knowledge, both Petit and Delaire FM can lead to skin irritations caused by the plastic forehead and chin pads. Plastic supports of FM should to be adjusted to fit patient’s face maximizing the contact surface with the skin for a homogeneous distribution of the loads applied and avoiding skin wounds. Overall results of the present study highlighted higher tensile tensions at the level of the chin cup after the application of the Petit FM when compared with the Delaire FM (44 kPa and 29 kPa, respectively) (Table 1). More extended chin support of Delaire design reduced the intensity of the residual tensions and stresses developed on the patient skin face, while smaller chin support of Petit FM tended to increase the tensile tensions on the face exposing the patient to more skin irritation. Lower tensile tensions decrease the risk of skin irritations, but also maximize orthopedic forces transmission to the bone structure [2]. Moreover, tensile tensions were localized on the whole chin cup area with Petit FM, while more extended chin cup pads of Delaire FM determined maximum stresses in correspondence of its upper border. In both FM models’ analysis, tensions registered on forehead region resulted particularly low and thus, clinically irrelevant, especially when Petit FM was tested. Maximum stresses ranged from 3 kPa (Petit FM) to 7 kPa (Delaire FM) and were mostly localized on the lower area, near to the eyebrows (Table 1). Color uniformity observed on the forehead support indicates the absence of significative stresses induced by applying Petit FM (Fig. 4B). Forehead caps can be considered negligible in terms of discomfort and fit if compared with the chin supports. Tensile strength analysis confirmed good adaptation for both FM designs. It can be supposed that all tensions and stresses developed during orthopedic protraction are maximized and focused on the chin area near the center of load application, especially in the Petit structure. The results of the present FEA investigation suggested that Petit FM appears clinically less comfortable than the Delaire model with a higher risk of skin irritation and mild swelling on the chin. On the other hand, a prolonged stress applied by the border of the chin cap could interfere with the periodontal health of the lower incisors [26]. An external compression on the labial mental grove exerted by the chin pad could transmit a compression on the lower incisors’ marginal gingiva with resulting processes of gingival retraction. According to the findings of the present study, FM designs with more extended chin supports should be preferred to achieve wearing comfort. As previously suggested [8] a clinician-made customization of the chin cup using a poly-vinyl siloxane could be used to uniform the contact surfaces of the orthopedic device structure improving its performance and control of the load distribution.

Limitations of the current study were due to the 3D facial model used to perform FEM analysis of forces induced by different FM models. Standardization of 3D model did not provide data about how forces distribution could change on different facial biotype. Further clinical studies will be necessary to analyzed stresses and loads distribution of orthopedic forces induced by maxillary protraction considering different facial patterns and biotypes in order to grant optimum management and customized fit of the device.

Highest tensions were observed at the level of chin cup area than at the level of forehead cups for both Delaire and Petit FM. Stresses following Delaire FM application were mostly observed on the upper border of the chin cup at the level of the inferior lip and lower incisor marginal gingiva. Petit FM produced higher tensile tensions than the Delaire design on the chin cup with stresses more extensively distributed to the central area of the chin.