Sutural growth restriction and modern human facial evolution: an experimental study in a pig model

A shared pattern of midfacial bone modelling in hominids suggests deep evolutionary roots for human facial morphogenesis

Midfacial morphology varies between hominoids, in particular between great apes and humans for which the face is small and retracted. The underlying developmental processes for these morphological differences are still largely unknown. Here, we investigate the cellular mechanism of maxillary development (bone modelling, BM), and how potential changes in this process may have shaped facial evolution. We analysed cross-sectional developmental series of gibbons, orangutans, gorillas, chimpanzees and present-day humans (n = 183). Individuals were organized into five age groups according to their dental development. To visualize each species's BM pattern and corresponding morphology during ontogeny, maps based on microscopic data were mapped onto species-specific age group average shapes obtained using geometric morphometrics. The amount of bone resorption was quantified and compared between species. Great apes share a highly similar BM pattern, whereas gibbons have a distinctive resorption pattern. This suggests a change in cellular activity on the hominid branch. Humans possess most of the great ape pattern, but bone resorption is high in the canine area from birth on, suggesting a key role of canine reduction in facial evolution. We also observed that humans have high levels of bone resorption during childhood, a feature not shared with other apes.

Cyclic loading failed to promote growth in a pig model of midfacial hypoplasia

Yucatan miniature pigs, often used as large animal models in clinical research, are distinguished by a breed-specific midfacial hypoplasia with anterior crossbite. Although this deformity can be corrected by distraction osteogenesis, a less invasive method is desirable. We chose a mechanical cyclic stimulation protocol that has been successful in enhancing sutural growth in small animals and in a pilot study on standard pigs. Yucatan minipigs (n = 14) were obtained in pairs, with one of each pair randomly assigned to sham or loaded groups. All animals had loading implants installed on the right nasal and frontal bones and received labels for cell proliferation and mineral apposition. After a week of healing and under anesthesia, experimental animals received cyclic tensile loads (2.5 Hz, 30 min) delivered to the right nasofrontal suture daily for 5 days. Sutural strains were recorded at the final session for experimental animals. Sham animals received the same treatment except without loading or strain gauge placement. In contrast to pilot results on standard pigs, the treatment did not produce the expected sutural widening and increased growth. Although sutures were not fused and strains were in the normal range, the targeted right nasofrontal suture was narrowed rather than widened, with no statistically significant changes in sutural cell proliferation, mineral apposition, or vascularity. In general, Yucatan minipig sutures were more vascular than those of standard pigs and also tended to have more proliferating cells. In conclusion, either because the sutures themselves are abnormal or because of growth restrictions elsewhere in the skull, this cyclic loading protocol was unable to produce the desired response of sutural widening and growth. This treatment, effective in normal animals, did not improve naturally occurring midfacial hypoplasia in Yucatan minipigs.

Anatomical and immunohistochemical analyses of the fusion of the premaxillary-maxillary suture in human fetuses

PURPOSE

The development of the premaxillary-maxillary suture (PMS) in human fetuses and a possible association between the fusion time of the PMS and maxillary deficiency were investigated. Expression of transforming growth factor beta (TGF-β1 and TGF-β3) and of fibulins (fibulin‑1 and fibulin-5) were also investigated.

METHODS

We analyzed 36 human fetus cadavers (19 males, 17 females; average age 23.97 ± 2.57 gestational weeks [gws], range 11-35 gws). Two cases, diagnosed with Down syndrome (DS), were characterized with maxillary deficiency; 34 fetus cadavers did not show any craniofacial abnormalities. The PMS was analyzed anatomically, followed by semi-quantitative immunohistochemical (IHC)-based expression analyses (i.e., TGF-β1/-β3, fibulin-1/-5). Spearman correlation test was conducted to investigate correlations.

RESULTS

In the fetuses without DS, the labial region of the PMS was open at 11 gws, after which it began to ossify from the middle to the upper and lower ends of the suture, typically fusing completely at 27 gws. Fetuses with DS demonstrated complete fusion of the labial region of PMS with a spongy bone structure at 23 gws and those without DS at 27 gws. IHC revealed similar patterns of TGF-βs and fibulins expression in the PMS during the human fetal period. There were significant positive correlations between the expression of TGF-β1 and TGF-β3 (r = 0.64, p = 0.009), TGF-β1 and fibulin‑1 (r = 0.66, p = 0.008), and TGF-β3 and fibulin‑1 (r = 0.67, p = 0.006).

CONCLUSION

Premature fusion of the PMS in the labial region during the human fetal period may be associated with maxillary deficiency, which is related to a class III malocclusion. Overall, the similar expression patterns of TGF-β1, TGF-β3 and fibulin‑1 suggested a close relationship between these factors in regulating the development of the PMS.

Surgical management of maxillary trauma in pediatric special needs patient using modified cap splint

Pediatric maxillofacial trauma is a rare entity, which is primarily the reason for an individual surgeon's inexperience in managing such injuries. More so, maxillary injuries are infrequent. Pediatric maxillofacial injuries are usually a result of blunt force trauma such as falls, motor vehicle accidents, bicycle injuries, sports-related injuries, assault, and child abuse. The atypical pattern of facial injuries in the pediatric population necessitates each surgeon to approach individual cases with a unique and innovative technique of management, while still following the basic principles of surgical management of maxillofacial injuries. Since facial trauma and surgical interventions both have the potential to lead to disturbance in growth and development, management should be as conservative as possible. The foundation of any surgical intervention must be developed keeping in perspective, the future growth, and development of dentofacial structures. Pediatric facial trauma management is in itself a disconcerting situation for a maxillofacial surgeon, but when a special needs child is involved it becomes an even more perplex decision. We present a case of maxillary trauma in a pediatric patient with global developmental delay, the treatment dilemma, and a review of current literature.

Premaxillary-maxillary suture development in the first trimester : An ultrasound study

PURPOSE

Our study was designed to investigate premaxillary-maxillary suture growth in fetuses from the first trimester of pregnancy using the B‑ultrasound technique in order to determine the suture fusion time.

METHODS

We selected 169 healthy Han singleton pregnancies as subjects. All subjects received routine pregnancy tests and were divided into three groups based on the gestational age of the fetus: group 1, the 11th gestational week; group 2, the 12th gestational week; and group 3, the 13th gestational week. Fetal biometric measurements were recorded during consecutive prenatal ultrasonographic examinations. These measurements included nuchal translucency thickness, crown-rump length, and premaxillary-maxillary length. Intergroup comparisons were performed using analysis of variance (ANOVA).

RESULTS

The premaxillary-maxillary suture grows gradually and its measured length at the 11th, 12th and 13th week was 0.54 cm, 0.65 cm, and 0.74 cm, respectively. We observed a significant linear correlation between the premaxillary-maxillary length and the week of gestation in the first trimester. The growth rate of the maxilla at the 11th, the 12th and the 13th week are significantly different with a descending order of growth rates being week 12, week 11 and week 13, with the 12th week rate being the most rapid.

CONCLUSION

The premaxillary and maxillary growth at 11 and 12 gestational weeks in the first trimester steadily accelerated, peaking at the 12th week. The rate of growth slows down after week 12 which may be associated with the fusion of the premaxillary-maxillary suture.

Asymmetric Protrusion of the Midface in Young Adults

PURPOSE

This study evaluated midfacial asymmetry using an alternative method that involved comparing bilateral patterns of the zygomaticomaxillary prominence in a young adult population.

MATERIALS AND METHODS

Three-dimensional reconstructed images based on computed tomography scans of 100 Koreans (mean age, 24.7 years) were evaluated with reference to lines spaced at 30° intervals and radiating from the center of an interporion line in a superior view. The surface inclination of the zygomaticomaxillary region was quantified on the same reference lines using a 3-dimensional ruler.

RESULTS

The 30°-interval line (at the level of the zygomaticotemporal suture) was longer on the left side than the right side in both males and females, whereas the left 60°-interval line (at the level of the zygomaticofrontal suture) was longer in females. Comparing the surface protrusion revealed that the zygomaticomaxillary region was more prognathic and inflated on the left side.

CONCLUSION

Functional deviations are considered to be causes of asymmetric craniofacial growth. Postnatal growth allometry across the circummaxillary sutures as elucidated by this study could be useful information in craniofacial surgery.

The biological basis of treating jaw discrepancies: An interplay of mechanical forces and skeletal configuration

Jaw discrepancies and malrelations affect a large proportion of the general population and their treatment is of utmost significance for individuals' health and quality of life. The aim of their therapy is the modification of aberrant jaw development mainly by targeting the growth potential of the mandibular condyle through its cartilage, and the architectural shape of alveolar bone through a suture type of structure, the periodontal ligament. This targeted treatment is achieved via external mechanical force application by using a wide variety of intraoral and extraoral appliances. Condylar cartilage and sutures exhibit a remarkable plasticity due to the mechano-responsiveness of the chondrocytes and the multipotent mesenchymal cells of the sutures. The tissues respond biologically and adapt to mechanical force application by a variety of signaling pathways and a final interplay between the proliferative activity and the differentiation status of the cells involved. These targeted therapeutic functional alterations within temporo-mandibular joint ultimately result in the enhancement or restriction of mandibular growth, while within the periodontal ligament lead to bone remodeling and change of its architectural structure. Depending on the form of the malrelation presented, the above treatment approaches, in conjunction or separately, lead to the total correction of jaw discrepancies and the achievement of facial harmony and function. Overall, the treatment of craniofacial and jaw anomalies can be seen as an interplay of mechanical forces and adaptations occurring within temporo-mandibular joint and alveolar bone. The aim of the present review is to present up-to-date knowledge on the mechano-biology behind jaw growth modification and alveolar bone remodeling. Furthermore, future molecular targeted therapeutic strategies are discussed aiming at the improvement of mechanically-driven chondrogenesis and osteogenesis.

Human feeding biomechanics: performance, variation, and functional constraints

The evolution of the modern human (Homo sapiens) cranium is characterized by a reduction in the size of the feeding system, including reductions in the size of the facial skeleton, postcanine teeth, and the muscles involved in biting and chewing. The conventional view hypothesizes that gracilization of the human feeding system is related to a shift toward eating foods that were less mechanically challenging to consume and/or foods that were processed using tools before being ingested. This hypothesis predicts that human feeding systems should not be well-configured to produce forceful bites and that the cranium should be structurally weak. An alternate hypothesis, based on the observation that humans have mechanically efficient jaw adductors, states that the modern human face is adapted to generate and withstand high biting forces. We used finite element analysis (FEA) to test two opposing mechanical hypotheses: that compared to our closest living relative, chimpanzees (Pan troglodytes), the modern human craniofacial skeleton is (1) less well configured, or (2) better configured to generate and withstand high magnitude bite forces. We considered intraspecific variation in our examination of human feeding biomechanics by examining a sample of geographically diverse crania that differed notably in shape. We found that our biomechanical models of human crania had broadly similar mechanical behavior despite their shape variation and were, on average, less structurally stiff than the crania of chimpanzees during unilateral biting when loaded with physiologically-scaled muscle loads. Our results also show that modern humans are efficient producers of bite force, consistent with previous analyses. However, highly tensile reaction forces were generated at the working (biting) side jaw joint during unilateral molar bites in which the chewing muscles were recruited with bilateral symmetry. In life, such a configuration would have increased the risk of joint dislocation and constrained the maximum recruitment levels of the masticatory muscles on the balancing (non-biting) side of the head. Our results do not necessarily conflict with the hypothesis that anterior tooth (incisors, canines, premolars) biting could have been selectively important in humans, although the reduced size of the premolars in humans has been shown to increase the risk of tooth crown fracture. We interpret our results to suggest that human craniofacial evolution was probably not driven by selection for high magnitude unilateral biting, and that increased masticatory muscle efficiency in humans is likely to be a secondary byproduct of selection for some function unrelated to forceful biting behaviors. These results are consistent with the hypothesis that a shift to softer foods and/or the innovation of pre-oral food processing techniques relaxed selective pressures maintaining craniofacial features that favor forceful biting and chewing behaviors, leading to the characteristically small and gracile faces of modern humans.

Cell morphologic changes and PCNA expression within craniofacial sutures during monkey Class III treatment

OBJECTIVES

To evaluate and compare the cellular morphologic changes and proliferating cell nuclear antigen (PCNA) expression within craniofacial sutures in growing Rhesus monkeys treated with a Class III functional appliance.

MATERIALS AND METHODS

Six Rhesus monkeys in the mixed dentition stage were divided into three groups: a 45-day experimental group, a 90-day experimental group, and a control group. Monkeys in the experimental groups were fitted with a Class III magnetic twin-block appliance. Cellular changes in six craniofacial sutures-the zygomaticomaxillary, zygomaticotemporal, transverse palatine, pterygopalatine, zygomaticofrontal, and frontomaxillary sutures were qualitatively and quantitatively evaluated by means of histomorphologic analysis, TEM, and immunohistochemical test of PCNA.

RESULTS

Obvious and altered bone remodeling combined with bone deposition and resorption was present in craniofacial sutures in the experimental groups. Increased activity of enlarged fibroblasts with abundant organelles was revealed. PCNA expression increased in the 45-day group compared with the control group, followed by the 90-day group. The highest percentage of PCNA-positive cells was found in the pterygopalatine suture in the 45-day group and the zygomaticomaxillary suture in the 90-day group.

CONCLUSIONS

The pterygopalatine and zygomaticomaxillary sutures are more active among the craniofacial sutures in the craniofacial complex remodeling during Class III treatment. The magnetic twin-block appliance effectively promoted suture remodeling by enhancing the activity and proliferation of osteoblasts, osteoclasts, and fibroblasts, especially in the early phase.

Ancillary procedures necessary for translational research in experimental craniomaxillofacial surgery

INTRODUCTION

Swine are often regarded as having analogous facial skeletons to humans and therefore serve as an ideal animal model for translational investigation. However, there is a dearth of literature describing the pertinent ancillary procedures required for craniomaxillofacial research. With this in mind, our objective was to evaluate all necessary procedures required for perioperative management and animal safety related to experimental craniomaxillofacial surgical procedures such as orthotopic, maxillofacial transplantation.

METHODS

Miniature swine (n = 9) were used to investigate perioperative airway management, methods for providing nutrition, and long-dwelling intravenous access. Flap perfusion using near-infrared laser angiography and facial nerve assessment with electromyoneurography were explored.

RESULTS

Bivona tracheostomy was deemed appropriate versus Shiley because soft, wire-reinforced tubing reduced the incidence of tracheal necrosis. Percutaneous endoscopic gastrostomy tube, as opposed to esophagostomy, provided a reliable route for postoperative feeding. Femoral venous access with dorsal tunneling proved to be an ideal option being far from pertinent neck vessels. Laser angiography was beneficial for real-time evaluation of graft perfusion. Facial electromyoneurography techniques for tracing capture were found most optimal using percutaneous leads near the oral commissure.Experience shows that ancillary procedures are critical, and malpositioning of devices may lead to irreversible sequelae with premature animal death.

CONCLUSIONS

Face-jaw-teeth transplantation in swine is a complicated procedure that demands special attention to airway, feeding, and intravascular access. It is critical that each ancillary procedure be performed by a dedicated team familiar with relevant anatomy and protocol. Emphasis should be placed on secure skin-level fixation for all tube/lines to minimize risk for dislodgement. A reliable veterinarian team is invaluable and critical for long-term success.

The effects of altered maxillary growth on patterns of mandibular rotation in a pig model

OBJECTIVES

A thorough understanding of influence of maxillary growth on patterns of mandibular rotation during development is important with regard to the treatment of skeletal discrepancies. In the present study, we examined whether experimentally altered maxillary position has a significant influence on patterns of mandibular rotation in a pig model.

DESIGN

Maxillary growth was altered in a sample of n=10 domestic pigs via surgical fixation of the circummaxillary sutures. We compared the experimental group to control and surgical sham samples and assessed the effects of altered maxillary growth on mandibular form using geometric morphometric techniques. We tested for significant differences in mandibular shape between our samples and examined axes of morphological variation. Additionally, we examined whether altered mandibular shape resulting from altered maxillary position was predictably associated with morphological changes to the condylar region.

RESULTS

There was a statistically significant difference in mandibular shape between the experimental and control/sham groups. As a result of vertical displacement of the snout, mandibles in the experimental sample resulted in greater anterior rotation when compared to the control/sham pigs. Variation in rotation was correlated with morphological changes in the condyle including the shape of the articular surface and condylar orientation indicative of greater anterior mandibular rotation.

CONCLUSIONS

Vertical displacement of the maxilla had a significant effect on mandibular shape by encouraging anterior mandibular rotation. This result has important implications for understanding the effects of altered mandibular posture on condylar remodeling the treatment of skeletal discrepancies such as the correction of hyperdivegent mandibular growth.

Geographic Variation in Zygomaxillary Suture Morphology and its Use in Ancestry Estimation

Angled/curved zygomaxillary suture coding is widely employed in cranial assessments of ancestry. However, the efficacy of this method has not been extensively evaluated across diverse populations. In this study, zygomaxillary suture morphology was assessed on a total of 411 human crania from six populations (European, Native American, African, Asian, Arctic Circle, and Aboriginal Australian) using a novel 3D coordinate landmark method. Our results indicate a predominance of angled sutures among native peoples of the Arctic and North America (85-86%), a prevalence of curved sutures among Africans and Aboriginal Australians (77-81%), and essentially equal proportions of both configurations in Asians and Europeans (50-56%). Statistically, angled/curved coding generally discriminates poorly between groups, except when populations with antithetically high frequencies of the two configurations (e.g., African vs. Native American) are compared. Moreover, comparisons across previous studies reveal conflicting frequencies for many populations, further suggesting limited utility of this trait in ancestry estimation.

Postnatal changes in the growth dynamics of the human face revealed from bone modelling patterns

Human skull morphology results from complex processes that involve the coordinated growth and interaction of its skeletal components to keep a functional and structural balance. Previous histological works have studied the growth of different craniofacial regions and their relationship to functional spaces in humans up to 14 years old. Nevertheless, how the growth dynamics of the facial skeleton and the mandible are related and how this relationship changes through the late ontogeny remain poorly understood. To approach these two questions, we have compared the bone modelling activities of the craniofacial skeleton from a sample of subadult and adult humans. In this study, we have established for the first time the bone modelling pattern of the face and the mandible from adult humans. Our analyses reveal a patchy distribution of the bone modelling fields (overemphasized by the presence of surface islands with no histological information) reflecting the complex growth dynamics associated to the individual morphology. Subadult and adult specimens show important differences in the bone modelling patterns of the anterior region of the facial skeleton and the posterior region of the mandible. These differences indicate developmental changes in the growth directions of the whole craniofacial complex, from a predominantly downward growth in subadults that turns to a forward growth observed in the adult craniofacial skeleton. We hypothesize that these ontogenetic changes would respond to the physiological and physical requirements to enlarge the oral and nasal cavities once maturation of the brain and the closure of the cranial sutures have taken place during craniofacial development.

Nasal septal and craniofacial form in European- and African-derived populations

As a component of the chondrocranium, the nasal septum influences the anteroposterior dimensions of the facial skeleton. The role of the septum as a facial growth center, however, has been studied primarily in long-snouted mammals, and its precise influence on human facial growth is not as well understood. Whereas the nasal septum may be important in the anterior growth of the human facial skeleton early in ontogeny, the high incidence of nasal septal deviation in humans suggests the septum's influence on human facial length is limited to the early phases of facial growth. Nevertheless, the nasal septum follows a growth trajectory similar to the facial skeleton and, as such, its prolonged period of growth may influence other aspects of facial development. Using computed tomography scans of living human subjects (n = 70), the goal of the present study is to assess the morphological relationship between the nasal septum and facial skeleton in European- and African-derived populations, which have been shown to exhibit early developmental differences in the nasal septal-premaxillary complex. First we assessed whether there is population variation in the size of the nasal septum in European- and African-derived samples. This included an evaluation of septal deviation and the spatial constraints that influence variation in this condition. Next, we assessed the relationship between nasal septal size and craniofacial shape using multivariate regression techniques. Our results indicate that there is significant population variation in septal size and magnitude of septal deviation, both of which are greater in the European-derived sample. While septal deviation suggests a disjunction between the nasal septum and other components of the facial skeleton, we nevertheless found a significant relationship between the size of the nasal septum and craniofacial shape, which appears to largely be a response to the need to accommodate variation in nasal septal size.

Morphological integration between the cranial base and the face in children and adults

The primary aim of the present study was to assess morphological covariation between the face and the basicranium (midline and lateral), and to evaluate patterns of integration at two specific developmental stages. A group of 71 children (6-10 years) was compared with a group of 71 adults (20-35 years). Lateral cephalometric radiographs were digitized and a total of 28 landmarks were placed on three areas; the midline cranial base, the lateral cranial base and the face. Geometric morphometric methods were applied and partial least squares analysis was used to evaluate correlation between the three shape blocks. Morphological integration was tested both with and without removing the effect of allometry. In children, mainly the midline and, to a lesser extent, the lateral cranial base were moderately correlated to the face. In adults, the correlation between the face and the midline cranial base, which ceases development earlier than the lateral base, was reduced. However, the lateral cranial base retained and even strengthened its correlation to the face. This suggests that the duration of common developmental timing is an important factor that influences integration between craniofacial structures. However, despite the apparent switch of primary roles between the cranial bases during development, the patterns of integration remained stable, thereby supporting the role of genetics over function in the establishment and development of craniofacial shape.

Nasal septal and premaxillary developmental integration: implications for facial reduction in Homo

The influence of the chondrocranium in craniofacial development and its role in the reduction of facial size and projection in the genus Homo is incompletely understood. As one component of the chondrocranium, the nasal septum has been argued to play a significant role in human midfacial growth, particularly with respect to its interaction with the premaxilla during prenatal and early postnatal development. Thus, understanding the precise role of nasal septal growth on the facial skeleton is potentially informative with respect to the evolutionary change in craniofacial form. In this study, we assessed the integrative effects of the nasal septum and premaxilla by experimentally reducing facial length in Sus scrofa via circummaxillary suture fixation. Following from the nasal septal-traction model, we tested the following hypotheses: (1) facial growth restriction produces no change in nasal septum length; and (2) restriction of facial length produces compensatory premaxillary growth due to continued nasal septal growth. With respect to hypothesis 1, we found no significant differences in septum length (using the vomer as a proxy) in our experimental (n = 10), control (n = 9) and surgical sham (n = 9) trial groups. With respect to hypothesis 2, the experimental group exhibited a significant increase in premaxilla length. Our hypotheses were further supported by multivariate geometric morphometric analysis and support an integrative relationship between the nasal septum and premaxilla. Thus, continued assessment of the growth and integration of the nasal septum and premaxilla is potentially informative regarding the complex developmental mechanisms that underlie facial reduction in genus Homo evolution.

The global impact of sutures assessed in a finite element model of a macaque cranium

The biomechanical significance of cranial sutures in primates is an open question because their global impact is unclear, and their material properties are difficult to measure. In this study, eight suture-bone functional units representing eight facial sutures were created in a finite element model of a monkey cranium. All the sutures were assumed to have identical isotropic linear elastic material behavior that varied in different modeling experiments, representing either fused or unfused sutures. The values of elastic moduli employed in these trials ranged over several orders of magnitude. Each model was evaluated under incisor, premolar, and molar biting conditions. Results demonstrate that skulls with unfused sutures permitted more deformations and experienced higher total strain energy. However, strain patterns remained relatively unaffected away from the suture sites, and bite reaction force was likewise barely affected. These findings suggest that suture elasticity does not substantially alter load paths through the macaque skull or its underlying rigid body kinematics. An implication is that, for the purposes of finite element analysis, omitting or fusing sutures is a reasonable modeling approximation for skulls with small suture volume fraction if the research objective is to observe general patterns of craniofacial biomechanics under static loading conditions. The manner in which suture morphology and ossification affect the mechanical integrity of skulls and their ontogeny and evolution awaits further investigation, and their viscoelastic properties call for dynamic simulations.