Masticatory muscle reaction in simulated low-velocity rear-end impacts

AIMS: To evaluate the electromyographic (EMG) activity of masseter and temporalis muscles in relation to impact awareness, gender, impact magnitude, and kinematics of head movement in simulated low-velocity rear-end impacts. METHODS: Twenty-nine individuals (17 men and 12 women) were subjected in random order to 3 rear-end impacts: 2 unexpected impacts (chair accelerations of 4.5 m/s2 and 10.1 m/s2) and 1 expected impact (chair acceleration of 10.1 m/s2). The EMG activity of the deep and superficial masseter muscle was recorded bilaterally. EMG activity was also recorded for the left anterior temporalis muscle. Angular acceleration and angular displacement of the head were also recorded. The temporal relationship between onset of the masticatory muscle activity and maximum peak of the kinematics of head movement was determined. RESULTS: The magnitude of normalized masticatory EMG activity ranged from 1.4 to 1.8 times higher (P < .05) for fast unexpected impacts compared to slow unexpected impacts in all masticatory muscles. The magnitude of normalized anterior temporalis EMG peak response ranged from 1.8 to 2.5 times higher (P < .05) in female subjects than in male subjects for all impacts. No significant differences were identified for impact awareness in the magnitude of normalized EMG activity for any masticatory muscle. No significant differences were identified with respect to timing of masticatory muscle response (P > .05). CONCLUSION: EMG activity increased with increased impact magnitude. Temporal and amplitude awareness of a simulated impact did not produce a difference in the masticatory muscle response. Gender differences were identified in the anterior temporalis muscle response. The onset of the masticatory muscle response occurred after peak angular acceleration of the head but prior to peak angular displacement of the head.     

Can mouthguards prevent mandibular bone fractures and concussions? A laboratory study with an artificial skull model

Abstract –  Some sports' accidents are responsible for inflicting traumatic brain injuries and mandibular bone fractures when impacts occur to the chin. It is often thought that mouthguards can prevent many of these injuries. However, such assertions may be insufficient without adequate research. It is therefore necessary to establish a systematic method of investigation to solve this problem. In the present laboratory study, tests were performed using pendulum impact equipment and an artificial skull model connected to strain gages and accelerometers to simulate and measure the surface distortions related to bone deformation or fractures and the acceleration of the head related to concussions. As impacts, direct blows to the mandibular undersurface were applied. As a result, wearing a mouthguard decreased ( P   < 0.01) the distortion to the mandibular bone and the acceleration of the head significantly compared with not wearing a mouthguard (54.7%: to the mandible – measured at a total of three different points, 18.5%: to the head measured at a total of three different points). Within the limits of this study, the following conclusions were drawn: The present measuring system in this study was able to evaluate the distortion to the mandibular and the acceleration of the head from the direct blow to the mandibular undersurface. Mouthguards can reduce distortion to the mandibular and the acceleration of the head from the same blow. So mouthguards might have the possibility to prevent mandibular bone fractures and concussions. However, further well-designed and exhaustive studies are vital to show that mouthguards reduce the incidence of concussions and mandibular bone fractures.     

Cartilage changes link retrognathic mandibular growth to TMJ disc displacement in a rabbit model

Recent experimental research demonstrated that non-reducing temporomandibular joint (TMJ) disc displacement in growing rabbits impaired mandibular growth. TMJ disc displacement is also shown to induce histological changes of the condylar cartilage. The authors hypothesized that the severity of these changes would correlate to the magnitude of mandibular growth. Bilateral non-reducing TMJ disc displacement was surgically created in 10 growing New Zealand White rabbits. Ten additional rabbits constituted a sham operated control group. Aided by tantalum implants, growth was cephalometrically determined for each mandibular side during a period equivalent to childhood and adolescence in man. At the end of the growth period, histologically classified cartilage features were correlated with the assessed ipsilateral mandibular growth. Non-reducing displacement of the TMJ disc during the growth period induced histological reactions of the condylar cartilage in the rabbit model. The severity of cartilage changes was inversely correlated to the magnitude and the direction of mandibular growth, which resulted in a retrognathic growth pattern.     

Association between shape of the mandibular condylar head and the occurrence of unilateral condylar fracture – A retrospective computed tomographic study

This retrospective study aims to evaluate the correlation between the shape of the mandibular condylar head and the incidence of unilateral condylar fracture using computed tomography.Medical records of patients diagnosed with unilateral condylar fractures from the year 2012–2019 were reviewed. The shape of the condylar head on the non - fractured side was analysed using a Radiant Dicom Viewer. The analysis was done using both visual and analytical methods. In the analytical method, a horizontal line was drawn at the base of the curvature of the condylar head. The highest peak point of the head was marked, and a perpendicular line was drawn connecting the highest point to the horizontal line. The shape was categorized into four types as convex, flat, angled, and round based on these lines.201 CT scans were examined, of which 69 were excluded as they did not meet the inclusion criteria. The remaining 132 were included in our study. On examining the shape, flat-shaped condyle was seen in 57 scans (43.2%), followed by convex in 31 scans (23.4%), angled in 30 (22.7%) and round in 14 scans (10.6%). The relationship between the shape of the mandibular condylar head and the incidence of unilateral condylar fracture was analysed using a chi-square test, which showed high statistical significance (p value 0.0001). The flat-shaped condylar head was more prone to fracture, and the round-shaped condylar head was least prone to fracture.In conclusion, the shape of the mandibular condylar head had a statistically significant association with the incidence of unilateral condylar fracture. The assessment of the shape of the condylar head can be taken as a guide to suspect condylar fractures and other associated mandibular fractures.     

细长型髁突增生病下颌骨及下颌神经管位置CT形态学研究

目的应用CT进行细长型髁突增生病（HE）下颌骨解剖学及下颌神经管形态学研究,为临床治疗提供参考。方法对19例HE患者进行多层CT扫描,采用Mimics 10.0软件进行三维重建,在不同断面重建图像,进行下颌神经管、下颌孔和骨皮质测量,并与无下颌骨病变的对照组进行比较。结果在第一磨牙中心长轴断面的舌侧、第二磨牙中心长轴的颊侧及上缘、磨牙后区中心至下颌角连线断面的颊侧、上缘及下缘、下颌孔下缘下5 mm处水平位断面的舌侧、前缘及后缘,2组下颌神经管外缘距下颌骨表面距离的差异具有统计学意义（P<0.05）;在第一磨牙颊侧和下缘,2组下颌骨骨皮质厚度的差异具有统计学意义（P<0.05）;2组下颌孔至下颌升支前缘及下颌骨下缘距离的差异具有统计学意义（P<0.05）。结论细长型髁突增生病骨皮质厚度从下颌第一磨牙到下颌升支在各个方向均逐渐减小。与正常颌骨相比,下颌神经管在下颌第二磨牙及磨牙后区偏颊侧并靠上方,下颌孔在升支内侧较靠前并偏低。     

Modeling of jaw-head-neck dynamics during whiplash

Clinicians report signs and symptoms of temporomandibular joint (TMJ) disorders in many patients who have experienced automobile rear-end collisions involving neck hyperextension (whiplash). In order for relationships between TMJ disorders and rear-end collisions to be explored, the dynamic response of the jaw to hyperextension of the neck associated with automobile rear-end collisions needs to be quantified. To achieve this goal, we extended an existing head-neck model by adding a movable jaw, and performed preliminary computer simulations of the jaw movements during rear-end collisions at 6.71 ms-1 (15 mph) and 13.41 ms-1 (30 mph). The initial computer simulations produced promising kinematic and dynamic results, such as: relative angle between the head and jaw (jaw-opening angle), predicted TMJ torques, and displacement and linear acceleration of the jaw's center of mass. However, the absolute values and time course of our results must be viewed with caution, since the jaw model has not yet been validated with experimental data. Nevertheless, the modeling approach, detailed in this study, shows good promise for eventually providing quantitative dynamic information about the jaw-head-neck system during impulsive loading.     

下颌骨髁突仿生复合支架的制备与表征

目的 构建下颌骨髁突一体化壳聚糖（CS）-聚己酸内酯（PCL）-羟磷灰石（HA）（HA/PCL-CS）仿生复合支架,并探讨其在髁突组织工程中应用的可行性。方法 应用快速成形技术形成下颌骨髁突模具,利用溶液浇铸-冰沥方法制备下颌骨髁突一体化仿生复合支架模型,PCL︰CS按4︰1的比例混合,分别加入质量比为40%、50%、 60%、70%的HA,分为 a、b、c、d组,观察支架的微观形貌、孔隙率、红外光谱、 X线衍射、力学性能等特性。结果 支架与下颌骨髁突形状相吻合,外观黄白色,坚硬,分上层及下层两部分。扫描电子显微镜显示该复合支架具有三维网络空间结构,孔隙率70%~85%,孔径大小10~200 μm。红外光谱显示随着HA的含量减少,其波峰强度降低。X线衍射结果显示随着HA含量的增加,其衍射峰强度相对降低。HA含量为50%时支架具有适宜的抗拉伸强度及抗压、抗弯强度。结论 溶液浇铸-冰沥方法制得的支架具有良好的综合材料性能,有望成为一种髁突自体组织工程用支架材料。     

Influence of forward head posture on condylar position

There are several reports suggesting that forward head posture is associated with temporomandibular disorders and restraint of mandibular growth, possibly due to mandibular displacement posteriorly. However, there have been few reports in which the condylar position was examined in forward head posture. The purpose of this study was to test the hypothesis that the condyle moves posteriorly in the forward head posture. The condylar position and electromyography from the masseter, temporal and digastric muscles were recorded on 15 healthy male adults at mandibular rest position in the natural head posture and deliberate forward head posture. The condylar position in the deliberate forward head posture was significantly more posterior than that in the natural head posture. The activity of the masseter and digastric muscles in the deliberate forward head posture was slightly increased. These results suggest that the condyle moves posteriorly in subjects with forward head posture.     

Rotational head motion concurrent to rhythmical mandibular opening movements

We have previously demonstrated the existence of a functional-rhythmical coupling between the head and the mandible using maxillary and mandibular incisal tracking points. However, that data did not provide information neither on the movement of the head as a whole nor on the location of its instantaneous centre of rotation. Thus, the objective of the present study was to determine whether the head undergoes a rotational motion during mouth opening and to locate its putative instantaneous centre of rotation. The same 6 d.f. (degree of freedom) measuring device employed in our previous studies was used again to analyse data from five male adults (age range: 26-29 years old) chosen as subjects. Concomitant head and mandibular movements were assessed in the sagittal plane by allocating several reference points in the head (upper incisor, cranial base, occipital and parietal points) and a mandibular incisor point during maximal mouth open-close movements. Then, the magnitude and inclination of the vectors of motion in each reference point during the opening phase were calculated. The instantaneous centre of rotation was defined as the point showing the least amount of motion in a determined area around each head reference point. The mandibular incisal point and the maxillary incisal point showed concomitant movements; that is, during opening the mandibular point moved downwards and the maxillary incisor point upwards. Making a large jaw opening movement caused an inferior-anterior displacement in the O point, a posterior-inferior displacement of the P point, and an anterior-superior displacement in the C point in all subjects. During jaw closing all points followed a trajectory opposite to that described above. In other words, during opening the head moved clockwise and counter-clockwise during closing, at least in the sagittal plane of the subjects' left side. These results suggest that the head undergoes a rotation-like sagittal movement during mouth opening whose rotation centre seems to be located above the cranial base point, which was set close to the centre of mass of the head. However, its location varies according to the magnitude of mouth opening.     

Head movement properties during voluntary rapid jaw movement in humans

The purpose of this study was to determine whether the start of the synchronized head movement during mandibular movement is evoked by the peripheral reflexes following mandibular movement (i.e. stretch or trigemino-neck reflexes), or, alternatively, is started by pre-programmed central command. Head movement accompanying voluntary rapid jaw opening movement was studied using accelerometers fixed to the upper and lower incisors, as well as electromyographs (EMGs) of the neck muscles. The direction of head acceleration at the upper incisor was towards head extension at the beginning of jaw opening movement in 89.2% of all trials, opposite to the direction of lower jaw acceleration. The onset of head acceleration was later than that of the lower jaw acceleration by averages of 6.2-10.7 ms, and the onset of electromyographic activities of the sternocleidomastoid (SCM) muscle preceded that of head acceleration by an average of 12.5-24.3 ms. These findings suggest that head movement during mandibular movement is not started by peripheral reflexes but by pre-programmed central commands. This may be relevant to muscular discomfort in the neck and shoulder regions of patients with stomatognathic disorders.     

Kinematic and kinetic observations on ballistic depression and elevation of the human mandible

To study mandibular motions with respect to time (kinematics) and the forces causing and resulting from these motions (kinetics), four subjects generated rapid depression and elevation of the mandible (displacement of 0.224 m; peak velocity of 0.237 m s(-1) during depression and 0.269 m s(-1) during elevation). The motion of depression (duration of 0.195 s; kinetic energy of 2.072 x 10(-3) J) could be divided into a phase of acceleration (2.742 m s(-2); +/- 0.28 gn) and a phase of deceleration (2.264 m s(-2); - 0.23 gn), and the terminal excess kinetic energy of depression was absorbed and dissipated by, primarily, the temporomandibular joint. Similarly, the ensuing motion of elevation (duration of 0.182 s; kinetic energy of 2.948 x 10(-3) J) could be divided into a phase of acceleration (3.498 m s(-2); + 0.36 gn) and a phase of deceleration (2.931 m s(-2); -0.30 gn), and the terminal excess kinetic energy of elevation was absorbed and dissipated by, primarily, the dentitions and, secondarily, by the temporomandibular joint. Rapid depression of the mandible appeared to be under the central control of a preprogrammed motor command, and ensuing rapid elevation of the mandible appeared to be under the peripheral control of a segmental and/or transcortical reflex. During rapid depression and elevation of the mandible, the anterior suprahyoid, anterior temporalis, and sternocleidomastoid muscles were myoelectrically active 56%, 73%, and 71% of the time, respectively, and myomechanically active 42%, 59%, and 57% of the time, respectively. Over a follow-up period of 12 months, the studied mandibular motions did not cause injury to the dentitions and temporomandibular joint.     

Implant-supported mandibular splinting affects temporomandibular joint biomechanics

Objectives: Mandibular functional movements lead to complex deformations of bony structures. The aim of this study was to test whether mandibular splinting influences condylar kinematics and temporomandibular joint (TMJ) loading patterns. Materials and methods: Six subjects were analyzed by means of dynamic stereometry during jaw opening–closing with mandibles unconstrained as well as splinted transversally by a cast metal bar fixed bilaterally to two implant pairs in the (pre)molar region. Statistical analysis was performed by means of ANOVAs for repeated measurements (significance level α=0.05). Results: Transversal splinting reduced mandibular deformation during jaw opening–closing as measured between two implants in the (pre)molar region on each side of the mandible significantly by 54%. Furthermore, splinting significantly reduced the distance between lateral condylar poles (average displacement vector magnitude of each pole: 0.84±0.36 mm; average mediolateral displacement component: 45±28% of the magnitude) and led to a medial displacement of their trajectories as well as a mediolateral displacement of stress‐field paths. Conclusions: During jaw opening–closing, splinting of the mandible leads to a significant reduction of mandibular deformation and intercondylar distance and to altered stress‐field paths, resulting in changed loading patterns of the TMJ structures. To cite this article:
Zaugg B, Hämmerle CHF, Palla S, Gallo LM. Implant‐supported mandibular splinting affects temporomandibular joint biomechanics.
Clin. Oral Impl. Res. 23 , 2012; 897–901
doi: 10.1111/j.1600‐0501.2011.02241.x     

The interrelationship between bolus breakdown, mandibular first molar displacement and jaw movement during mastication

The purpose of this study was to clarify the interrelationship between food bolus breakdown, mandibular first molar displacement and jaw movement during mastication. Finite element models were constructed of the maxillary first molar crown, the mandibular first molar consisting of crown, root, periodontal ligament and alveolar bone, as well as the food bolus were constructed. Based on the actual measurement of the jaw movement pattern and the characteristics of food bolus, the patterns of mandibular first molar displacement and bolus breakdown on time course in the progress of mastication were simulated, to investigate the biomechanical significance of tooth displacement and jaw movement during mastication, using finite element non-linear dynamic analysis. The results showed that the patterns of tooth displacement and jaw movement and characteristics of food bolus changed with an interrelationship to each other as mastication progressed. Particularly at the initial phase, it was suggested that the patterns of mandibular first molar displacement and jaw movement worked inter-dependently to accomplish an efficient hard-bolus breakdown.     

A direct in vivo measurement of the three‐dimensional orientation of the occlusal plane and of the sagittal discrepancy of the jaws

The aim of the present investigation was to three dimensionally assess craniofacial relationships in vivo. Specifically, by using a non‐invasive direct technique, the following measurements were made: 1) natural head position relative to the ground; 2) orientation of the occlusal plane relative to the subject's intrinsic facial planes; and 3) anteroposterior discrepancy of the dental bases, taking into consideration all the facial hard‐ and soft‐tissue structures. Several dental and soft‐tissue facial landmarks were directly digitized from 24 adult healthy volunteers with Angle Class I occlusions by means of an electromagnetic three‐dimensional computerized digitizer. In natural head position, the three‐dimensional orientation of Camper's, occlusal, and mandibular planes were measured along with the anteroposterior maxillo‐mandibular discrepancies. In the frontal plane projection, all the measured planes appeared about horizontal. In the lateral plane projection, on average, Camper's plane deviated from the true horizontal by approximately 18° (in a ‘head flexed’ direction). The occlusal plane deviated from the same horizontal by about 14°, while the mandibular plane had a steeper inclination (about 30°); both planes were significantly correlated to Camper's plane. The measurements of anteroposterior jaw discrepancy revealed a wide range of sagittal relationships in the analyzed subjects. The method was found to be repeatable and fast. This direct three‐dimensional in vivo assessment of the orientation of occlusal plane relative to the other facial planes could allow for a more comprehensive analysis of maxillo‐mandibular sagittal discrepancies.     

Head motion may help mouth opening in children

Concomitant head and mandibular movement during jaw function is well known in adults; however, its importance in children has not been studied. The brain attains 85–90% of its adult weight at 5 years of age, though the maximum rate of condylar growth is attained at approximately 14 years of age. These findings suggest that the coordination of the head and mandible may differ between children and adults. This study investigated head and mandibular movements of 19 children with complete primary dentition (average age: 5 years 5 months) and compared their functional integration of jaw and head movements to those of 16 female adults (average age: 20 years 3 months) with permanent dentition. Although the mandibular opening distance was significantly greater in the adults, the magnitude of concomitant head motion was greater in children. The results suggest that head extension in children helps increase the magnitude of mouth opening more than in adult women.     

人下颌恒切牙管间峡区解剖的显微CT研究

目的研究人离体下颌恒切牙管间峡区的解剖学特点。方法选择离体下颌恒切牙拍摄近远中向X线片,选出双根管牙33颗,对距根尖6 mm以内的牙根组织进行显微CT扫描。扫描层厚20 μm,每颗牙齿获得扫描截面300个。通过三维重建观察牙根管间峡区的解剖形态。记录距根尖1、2、3、4、5、6 mm处各截面根管数目及管间峡区的出现情况,并测量根管壁的最小厚度。结果所有双根管下颌恒切牙都存在管间峡区,完全峡区占49.7%,部分峡区占4.5%。距根尖6 mm内各截面管间峡区的出现率为10.0%~85.5%,卡方检验显示距根尖1~6 mm处管间峡区出现率的差异有统计学意义（P=0.001）。距根尖3~6 mm处管间峡区出现率较高,以5 mm处最高,为85.5%。距根尖6 mm以内的根管壁最小厚度均小于0.5 mm。结论下颌恒切牙管间峡区的出现率较高,根管壁最小厚度位于峡部,在临床进行根管治疗和牙髓外科手术时应引起重视。     

Evaluation of differential growth and orthodontic treatment outcome by regional cephalometric superpositions.

Cephalometric superimposition on cranial base is the accepted method for evaluating mandibular displacement during orthodontic treatment and/or growth. However, assessing mandibular position relative to the maxillary base may yield different information. The aim of this study was to evaluate the effects of regional superpositions (cranial versus maxillary) on interpreting mandibular displacement. Both methods were applied to pre- and posttreatment cephalograms of 22 growing children (12 female, 10 male) treated for Class II Division 1 malocclusion. Differences in linear and angular measurements of three mandibular landmarks (pogonion, gnathion, menton) between cranial and maxillary superpositions were statistically significant (p = 0.0001). Vertical displacement of these landmarks contributed significantly to the differences (p = 0.0001). The contribution of horizontal displacement was not statistically significant. The results support the proposition that, in growing children, posttreatment displacement of mandibular skeletal and dental components should be assessed by both maxillary and cranial base superimpositions. The maxilla is subject to rotational and translational changes during growth that may affect the position of the mandible relative to the maxilla in a way inconsistent with the mandibular displacement perceived upon cranial superposition. Since occlusion is directly associated with the positions of the maxillary and mandibular basal bones, the positions of these bones relative to each other is critical in assessing occlusal changes in individual patients.     

Temporal coordination between mandibular and head-neck movements during jaw opening-closing tasks in man

Previous finding of concomitant mandibular and head movements during jaw function suggest a functional relation between the human jaw and neck regions. This study examined the temporal coordination between mandibular and head-neck movements during maximal jaw opening-closing tasks, at fast and slow speed. Twenty-four healthy individuals, median age 25 years, participated in the study. They were seated with firm back support but without head-neck support. Mandibular and head movements were simultaneously monitored by a wireless optoelectronic system for three-dimensional movement recording. The timing of head movement in relation to mandibular movement was estimated at defined time-points (start, peak, end and maximum velocity of movement), and during the entire course of the jaw-opening and jaw-closing phases. The results showed that the head in general started to move simultaneously with or before the mandible, reached the peak position simultaneously with, before or after the mandible, and reached the end position after the mandible. A higher degree of temporal coordination was found for fast speed at the start and the peak positions. The head most often attained maximum velocity after the mandible, and mostly lagged behind the mandible during the entire jaw-opening and -closing phases. These findings support the notion of a functional linkage between the human temporomandibular and craniocervical regions. They suggest that "functional jaw movements" comprise concomitant mandibular and head-neck movements which involve the temporomandibular, the atlanto-occipital and the cervical spine joints, and are caused by jointly activated jaw and neck muscles. It is proposed that these jaw and neck muscle actions, particularly at fast speed, are elicited and synchronized by preprogrammed neural command(s) common to both the jaw and the neck motor systems. From the present results and previous observations of concurrent jaw and head movement during fetal yawning, we suggest that these motor programmes are innate.     

跨下牙槽神经种植相关解剖结构的影像学测量及分析研究

目的 通过锥形束CT影像数据测量下颌第二磨牙处下颌神经管位置,分析跨下牙槽神经种植术的理论植入范围,为临床上使用该方法解决下颌后牙区种植骨量不足问题提供理论依据。方法 选取80例下颌第二磨牙缺失且缺牙区垂直骨高度<9 mm的患者CBCT图像,测量该处下颌神经管到颊侧骨皮质、舌侧骨皮质、牙槽嵴顶距离,并模拟跨下牙槽神经种植,测量种植体颊舌向倾斜的角度范围。结果 下颌第二磨牙处下颌神经管到颊侧骨皮质、舌侧骨皮质、牙槽嵴顶的距离分别是(6.913±1.222)、(2.859±0.891)、(7.991±0.783)mm,下颌神经管到颊侧骨皮质距离明显大于到舌侧骨皮质距离。75%的患者可行跨下牙槽神经种植术,模拟植入种植体颊舌向倾斜最小角度为19.360°±7.086°,最大角度为39.462°±6.924°。结论 下颌第二磨牙处下颌神经管明显偏向舌侧,保障了颊侧足够的骨量,多数下颌第二磨牙处无法垂直植入短种植体的患者仍可通过跨下牙槽神经种植术植入常规长度种植体。     

Adaptive condylar growth and mandibular remodelling changes with bionator therapy--an implant study

The purpose of this study was to describe condylar growth and mandibular remodelling changes associated with bionator therapy. Twenty-five patients (15 males and 10 females) between 6.9 and 11.2 years of age with Class II division 1 malocclusions were randomly allocated to either control (n = 11) or treatment (n = 14; bionator only) groups and followed longitudinally for approximately 1 year. Treatment consisted of a bionator only, constructed to clear the buccal dentition by 2 mm and to position the mandible into an edge-to-edge incisor relationship. Using metallic implants for superimposition, mandibular growth, displacement, and true rotation were evaluated cephalometrically. The results showed significant changes in the direction (more posterior) but not in the overall amount of condylar growth. The bionator appliance produced greater than expected posterior drift of landmarks in the condylar and gonial regions. Cranial base superimposition showed greater than expected anterior mandibular displacement, but little or no true mandibular forward rotation with bionator therapy. The bionator appliance alone produced changes in condylar growth direction and remodelling changes associated with mandibular rotation and displacement.