A preliminary study of biomechanical changes produced by orthognathic surgery

A deep dive into the static force transmission of the human masticatory system and its biomechanical effects on the temporomandibular joint

OBJECTIVE

This study aims to investigate the biomechanical behavior and reveal the force transmission patterns of the human masticatory system through advanced three-dimensional finite element (FE) models.

METHODS

The FE model was constructed according to the medical images of a healthy male adult. It contains full skull structures, detailed temporomandibular joints (TMJs) with discs, complete dentitions, masticatory muscles, and related ligaments. Several static bite scenarios were simulated to demonstrate the effects of bite positions and muscle force recruitments on the force transmission patterns.

RESULTS

Molar occlusal surfaces are the primary force transmission region for clenching. Sensitivity analysis demonstrated that the stiffness of the bite substance would not alter the force transmission patterns but could affect the maximum contact stresses on the discs and the occlusal surfaces. During the unilateral clenching tasks, the high-stress region on the discal surfaces shifted ipsilaterally. The presence or absence of the molar cushions would significantly affect the biomechanical response of the masticatory system.

SIGNIFICANCE

FE analysis is an effective way of investigating biomechanical responses involving complicated interactions. Enriching the static analysis of the masticatory system with a detailed model can help understand better how the forces were transmitted and the significance of TMJs during the clenching process.

A Biomechanical Analysis of Muscle Force Changes After Bilateral Sagittal Split Osteotomy

A basic procedure affecting maxillofacial geometry is the bilateral sagittal split osteotomy. During the surgery, the bony segments are placed in a new position that provides the correct occlusion. Changes in the geometry of the mandible will affect the surrounding structures and will have a significant impact on the functioning of the masticatory system. As a result of the displacement of the bone segment, the biomechanical conditions change, i.e., the load and the position of the muscles. The primary aim of this study was to determine the changes in the values of the muscular forces caused by mandible geometry alteration. The study considered the translation and rotation of the distal segment, as well as rotations of the proximal segments in three axes. Calculations were performed for the unilateral, static loading of a model based on rigid body mechanics. Muscles were modeled as spring elements, and a novel approach was used to determine muscle stiffness. In addition, an attempt was made, based on the results obtained for single displacements separately, to determine the changes in muscle forces for geometries with complex displacements. Based on the analysis of the results, it was shown that changes in the geometry of the mandibular bone associated with the bilateral sagittal split osteotomy will have a significant effect on the values of the masticatory muscle forces. Displacement of the distal segment has the greatest effect from -21.69 to 26.11%, while the proximal segment rotations affected muscle force values to a less extent, rarely exceeding 1%. For Yaw and Pitch rotations, the opposite effect of changes within one muscle is noticed. Changes in muscle forces for complex geometry changes can be determined with a high degree of accuracy by the appropriate summation of results obtained for simple cases.

Functional evaluation in orthodontic surgical treatment: long-term stability and predictability

Background

The introduction of electromyographic and kinesiographic technology in orthodontics allows to obtain objective data regarding the functional aspects of the mandibular movements and the masticatory muscular activity. It is then important to be able to correlate the data obtained by instrumental activity with the clinical ones. The aim of this study consists to analyse the post ortodontic surgical stability through instrumental evaluation of the masticatory muscles and mandibular movements.

Method

30 patients undergo electromyographic and kinesiographic evaluations through all the surgical orthodontic iter and were than followed during other 4 years. JMP software was used to analyze and correlate the electromyographic and knesioographic data during treatment and during the follow up.

Results

A linear correlations between some functional objective values collected from the examinations at the beginning and during therapy and the follow up one has been demonstrated.

Conclusion

It is important to submit patients in surgical ortodontic treatment to instrumental analysis which can evidence how masticatory function and mandibular movements are performed. It is also important to highlight some functional values also from the beginning of the treatment because an alteration of such values can be related to a better or worse postsurgical rehabilitation.

Positional changes of the masseter and medial pterygoid muscles after surgical mandibular advancement procedures: an MRI study

This study evaluated whether surgical mandibular advancement procedures induced a change in the direction and the moment arms of the masseter (MAS) and medial pterygoid (MPM) muscles. Sixteen adults participated in this study. The sample was divided in two groups: Group I (n=8) with a mandibular plane angle (mpa) <39° and Group II (n=8) with an mpa >39°. Group I patients were treated with a bilateral sagittal split osteotomy (BSSO). Those in Group II were treated with a BSSO combined with a Le Fort I osteotomy. Pre- and postoperative direction and moment arms of MAS and MPM were compared in these groups. Postsurgically, MAS and MPM in Group II showed a significantly more vertical direction in the sagittal plane. Changes of direction in the frontal plane and changes of moment arms were insignificant in both groups. This study demonstrated that bimaxillary surgery in patients with an mpa >39° leads to a significant change of direction of MAS and MPM in the sagittal plane.

[Diagnosis and therapy of adult patients with facial asymmetry]

The goal of orthodontic treatment is to improve the patient's life by enhancing dental and jaw functions and dentofacial esthetics [Graber TM, et al., Orthodontics current principles and techniques. 4(e) ed. St Louis: Elsevier, 2005.]. Harmonious occlusion is achieved following improvements of malocclusion via orthodontic treatment [Ehmer U and Broll P, Int J Adult Orthod Orthognath Surg 1992;7:153-159. Throckmorton GS, et al., J Prosthet Dent 1984;51:252-261.]. Perfect facial symmetry is extremely rare, and normal faces have a degree of asymmetry. Patients with dentofacial deformity more frequently have asymmetry of the face and jaws. There was a relationship between the type of malocclusion and the prevalence of asymmetry; 28% of the Class III group, but 40% to 42% of the Class I, Class II and long face groups respectively, were asymmetric [Severt TR and Proffit WR, Int J Adult Orthod Orthogn Surg 1997;12:171-176.]; therefore, facial asymmetry is a common complaint among orthodontic patients. Treatment of severe facial asymmetry in adults consists mainly of surgically repositioning the maxilla or the mandible [Bardinet E, et al., Orthod Fr 2002;73:243-315. Guyuron B, Clin Plast Surg 1989;16:795-801. Proffit WR, et al., Contemporary treatment of dentofacial deformity. 2003. St Louis: Mosby, 2003:574-644.], however, new methods, i.e. orthodontic tooth movement with implant anchorage, have recently been introduced [Costa A, et al., Int J Adult Orthod Orthognath Surg 1998;3:201-209. Creekmore TD and Eklund MK, J Clin Orthod 1983;17:266-269. Miyawaki S,et al., Am J Orthod Dentofacial Orthop 2003;124:373-378. Park HS, et al., J Clin Orthod 2001;35:417-422. Roberts WE, et al., Angle Orthod 1989;59:247-256.], and various treatment options can be chosen in patients with facial asymmetry. In this article, we describe the diagnosis and treatment of adult patients with facial asymmetry.

Place of sagittal split osteotomy in mandibular surgery

With the development of craniofacial surgery, today sagittal split mandibular osteotomies are frequently used for the treatment of mandibular deformities with modifications and different fixation techniques. In clinics, 57 patients are evaluated in this study. Of these patients, 35 were male, 22 were female, and their ages were between 17 and 47 years (on average, it was 23 years). Thirty-seven of these patients were treated with bilateral sagittal split mandibular osteotomy (SSMO), 12 of them with unilateral SSMO, 6 of them with bilateral SSMO + Le Fort I osteotomy, and 2 of them with bilateral SSMO + Le Fort II osteotomy. Orthodontic treatment modalities were performed for 8 patients preoperatively and to all of the patients postoperatively. All patients were examined with cephalometric analysis preoperatively and postoperatively. Internal and external distractors were applied for 14 patients. Rigid fixation with plates and screws was done for 21 patients. Fixation with screws for 5 patients, fixation with wires for 3 patients, and intermaxillary fixation for 8 patients were performed. Patients were followed up on average 13 months (6-22 months). In 1 patient, infection on the osteotomy line and recurrency developed. Sensory area of the alveolar nerve became normal in 10 to 16 weeks usually, but in 3 patients, sensory loss developed at this region. Complaints of temporomandibular joint dysfunction developed in 1 patient at the third postoperative month. When the clinical use was determined, we observed that SSMO could give satisfactory results in selected patients with mandibular deformities. Dental structures are important in relation to the postoperative occlusion and masticatory functions, for this reason, we think that postoperative orthodontic treatment will be useful for these patients.

Positional changes and stability of bone segments during simultaneous bilateral mandibular lengthening and widening by distraction

The purpose of this study was to analyse the skeletal changes and stability of the distracted segments during and after simultaneous widening and bilateral lengthening of the mandible in baboons with a miniaturized intraoral bone-borne osteodistractor. Distraction appliances were activated 5 days after vertical posterior body and midsymphyseal osteotomies at a rate of 0.9 mm/day for 10 days. The appliances were then stabilized for a period of 8 weeks, after which the animals were killed. The distraction gaps and gingival tissues were studied clinically and on standardized radiographs. The proportional movement of the distracted segments that we found supports the clinical use of the miniaturized intraoral bone-borne distraction appliance to widen and lengthen the mandible selectively. It also supports the concept of positioning the mandibular osteodistractors parallel to the common vector of distraction, which should parallel the corrected maxillary occlusal plane.

Effect of mandibular setback surgery on occlusal force

PURPOSE

This study investigated the effect of mandibular setback surgery on occlusal force and evaluated the extent to which postsurgical changes in such force can be explained by the type of operation, the duration of maxillo-mandibular fixation (MMF), and the changes in the mechanical advantage of the jaw musculature.

MATERIALS AND METHODS

Maximal molar bite force was measured before surgery and at MMF removal, and 3, 6, and 12 months thereafter in 26 patients with mandibular prognothism. To correlate bite force and skeletal change, the cephalometric tracings were measured, tabulated, and statistically analyzed.

RESULTS

Mean bite force was 13.7 kg before surgery, 7.6 kg at MMF removal, 14.2 kg at 3 months, 19.7 kg at 6 months, and 26.1 kg at 12 months post-surgery. The bite force was positively correlated with the surgical change in mandibular plane angle and mandibular body length. The recovery of bite force was significantly affected by the type of operation and duration of MMF.

CONCLUSIONS

To hasten recovery and increase bite force after orthognathic surgery, long periods of MMF and injury to the masticatory muscles should be avoided.

Functional and morphologic alterations after anterior or inferior repositioning of the maxilla

PURPOSE

The purpose of this investigation was 1) to compare functional and morphologic measurements between controls and patients scheduled for anterior or inferior repositioning of the maxilla, and 2) to examine how these patients' oral function adapted after surgery.

PATIENTS AND METHODS

Nine male patients undergoing anterior and/or inferior repositioning of the maxilla were compared with 26 male controls preoperatively and up to 3 years after surgery. Measures of skeletal morphology, mandibular range of motion, maximum voluntary bite force, and levels of electromyographic (EMG) activity in the anterior and posterior temporalis and masseter muscles during isometric bites were made on all subjects over time. One-way ANOVA was used to compare the controls, the patients before surgery, and the patients after surgery.

RESULTS

Before surgery, most of the patients had morphologic characteristics of mandibular prognathism and maxillary retrognathism. Surgery made the patients' skeletal morphology similar to controls except for mandibular length, upper facial height, and palatal plane angle, which were significantly greater than those of controls. There were no significant differences in jaw muscle mechanical advantage between controls and patients either before or after surgery. Hypomobility of the jaw was apparent at 6 weeks and 6 months after surgery, but returned to normal values within 1 to 2 years. Before surgery, the patients had maximum voluntary bite forces significantly less than those of controls. Bite forces in patients steadily increased after surgery, approaching significantly higher values than those of controls. Before surgery, patients' muscle activity levels per unit of bite force were not significantly different from those of controls. Most of the patients' muscle activity levels per unit of bite forces at all bite positions showed no significant change after surgery.

CONCLUSIONS

The results of this study suggest that anterior or inferior repositioning of maxilla produces some significant functional benefits in patients.

Functional and morphologic changes after combined maxillary intrusion and mandibular advancement surgery

PURPOSE

The purposes of this investigation were 1) to compare the morphology and function of patients with combined vertical maxillary excess (VME) and mandibular retrognathia with that of controls, and 2) to examine how these parameters change after combined maxillary intrusion and mandibular advancement surgery.

PATIENTS AND METHODS

Fifteen female VME/retrognathic patients were compared with 26 female controls before and for up to 3 years after orthognathic surgery. Facial skeletal morphology, mandibular range of motion, maximum isometric bite force, and levels of electromyographic activity (EMG) in selected muscles of mastication were measured on all subjects. Where appropriate, one-way analysis of variance (ANOVA) or t-tests were used to compare the patients with controls. Univariate repeated-measures ANOVA was used to study longitudinal changes.

RESULTS

Preoperatively, patients' morphologic measurements were characteristic of VME compounded by mandibular retrognathia. At surgery, the maxilla was elevated an average of 2.8 mm, and the mandible was lengthened by an average of 7.1 mm. All of the postoperative morphologic measurements were closer to normal values. The patients' masseter mechanical advantage was significantly lower than that of controls both before and after surgery. Surgically induced changes in mechanical advantage were very small. The patients' maximum range of motion and excursion during mastication were all lower than those of controls before surgery. All measurements of mobility decreased immediately after surgery, with a gradual return to preoperative values. However, even 3 years after surgery, all of the motion measurements remained smaller than those of the controls. Before surgery, the patients had maximum isometric bite forces significantly lower than those of controls. Bite forces increased significantly after surgery, approaching normal values within 2 years. The activity levels in the muscles of mastication during isometric bites were not significantly altered by surgery.

CONCLUSIONS

This study confirms that VME/retrognathia patients suffer from substantial deficiencies in their oromotor function. Surgical correction of this particular type of dentofacial deformity improves both the morphologic and functional deficits. Although some changes were not statistically significant, all were toward normalization of the presurgical values.

Functional and morphologic alterations secondary to superior repositioning of the maxilla

PURPOSE

The purpose of this investigation was to 1) compare morphological characteristics and functional performance of a sample of patients with vertical maxillary excess (VME) with controls, and to 2) examine how the patients' oral motor function adapts to surgery.

MATERIALS AND METHODS

Fifteen female VME patients were compared with 26 female controls before and up to 3 years after maxillary intrusion surgery. Measures of skeletal morphology, mandibular range of motion, maximum isometric bite force, and levels of electromyogram (EMG) activity in some of the muscles of mastication were made on all subjects over time. One-way analysis of variance (ANOVA) was used to compare the controls with the patients before and after surgery. Univariate repeated measures ANOVA was used to study longitudinal changes in the patients.

RESULTS

Preoperatively, the patients possessed morphological measurements characteristic of vertical maxillary excess. Superior repositioning of the maxilla averaged 3.3 mm. Concurrently, most skeletal measures were brought closer to normal values. Masseter muscle mechanical advantage was significantly lower in the patients than in controls both before and after surgery (P < or = .05). There was no significant difference between patients and controls for other biomechanical measurements. Mandibular hypomobility was apparent at 6 weeks after surgery, but returned to control values within 6 to 12 months. Before surgery, the patients had maximum isometric bite forces significantly less than those of controls. Bite forces steadily increased after surgery, approaching normal values within 2 years. Before surgery the patients' muscle activity levels per unit of bite forces were equivalent to those of controls or somewhat lower. After surgery some of the patients' muscles had significantly lower levels of muscle activity per unit of bite force than did controls.

CONCLUSIONS

The results of this study suggest that correction of vertical maxillary excess with maxillary intrusion surgery improves some characteristic functional deficits.

Effects of local anesthesia on bite force generation and electromyographic activity

Maximum voluntary bite force has been used to evaluate functional changes following orthognathic surgery. It has been proposed by others that maximum voluntary bite force may depend, in part, on sensory input from the dentition. However, results from previous studies have shown contradictory effects of local anesthesia on bite force following anesthetization of the dentition. The purpose of this study was to investigate the effects of drug-induced local anesthesia on the generation of first molar bite force and electromyographic (EMG) activity in adults. Twenty normal adults (3 women, 17 men) were evaluated. Electromyographic activity was monitored from four muscles of mastication bilaterally, and bite force was concurrently recorded at the right and left first molars. Maximal and submaximal bite forces were then measured after sequential unilateral anesthetization of the right mandible and maxilla with 2% lidocaine containing 1:100,000 epinephrine. No statistically significant differences in bite force or integrated EMG levels were observed between the unanesthetized and anesthetized sides, nor on the anesthetized side at different levels of anesthesia.

Muscle activity patterns and control of temporomandibular joint loads

The strategy used by the neuromuscular system to distribute reaction forces to the temporomandibular joints on the working and balancing sides has not been fully defined. Theoretical studies and experimental measurements suggest that the two joints are unevenly loaded during unilateral biting or closures for posterior teeth. However, previous electromyographic studies suggest that muscle activity patterns may attempt to balance the distribution of the two joint forces. This study measured bilateral activity in the anterior temporal, posterior temporal, and superficial masseter muscles during isometric bites or closures and chewing at five different positions along the teeth. The resulting ratios of muscle activity on the working/balancing sides were compared with ratios required to maintain equal joint forces. The values of the muscle activity were also used to estimate the ratio of joint forces on the working/balancing side at each tooth position. Results indicate that the muscle activity patterns do not maintain equal joint forces, nor are the muscles responding to joint forces exceeding critical limits. These results suggest that patterns of muscle activity are designed to control the position and magnitude of occlusal forces rather than temporomandibular joint forces. If these same patterns of activity are maintained following repositioning of dental and skeletal elements, adverse temporomandibular joint forces could result.

Occlusal force after mandibular advancement in adult rhesus monkeys

Maxillofacial surgery not only alters facial appearance and occlusion but also affects the morphology, physiology, and biomechanics of the craniofacial skeleton and the muscles of mastication. This study was performed to evaluate one aspect of craniofacial function--maximum stimulated bite force (BF)--after mandibular advancement. BF in the molar region was measured with a transducer during muscle stimulation in 63 rhesus monkeys that had been divided into two groups: a control group of various ages and sexes (n = 50); and an experimental sample of 13 monkeys that had undergone mandibular advancement via C-osteotomy at least one year prior to BF measurement. Biomechanical analysis of lateral cephalograms was performed to estimate the amount of the change in BF in the experimental group that could be attributed to alteration of craniofacial form. Regression analysis demonstrated a significant relation between molar bite force and weight in control animals (r = 0.94). In the monkeys that underwent surgery, BF values were an average of 16.7% less than those found for the control animals relative to body weight. However, the biomechanical analyses suggested a smaller and less variable BF loss in the operated monkeys (11.6%). These results indicate a greater and more variable change in BF following mandibular advancement than would be predicted on the basis of biomechanical considerations.