A Reinvestigation of Murine Cranial Suture Biology: Microcomputed Tomography versus Histologic Technique

Craniosynostosis and Resynostosis: Models, Imaging, and Dental Implications

Craniosynostosis occurs in approximately 1 in 2,000 children and results from the premature fusion of ≥1 cranial sutures. If left untreated, craniosynostosis can cause numerous complications as related to an increase in intracranial pressure or as a direct result from cranial deformities, or both. More than 100 known mutations may cause syndromic craniosynostosis, but the majority of cases are nonsyndromic, occurring as isolated defects. Most cases of craniosynostosis require complex cranial vault reconstruction that is associated with a high risk of morbidity. While the first operation typically has few complications, bone rapidly regrows in up to 40% of children who undergo it. This resynostosis typically requires additional surgical intervention, which can be associated with a high incidence of life-threatening complications. This article reviews work related to the dental and maxillofacial implications of craniosynostosis and discusses clinically relevant animal models related to craniosynostosis and resynostosis. In addition, information is provided on the imaging modalities used to study cranial defects in animals and humans.

Changes in the Zygomaticomaxillary Suture During Aging

Trans-sutural distraction osteogenesis (TSDO) has been successfully used to correct midfacial hypoplasia in growing patients for years. The effects of TSDO, however, remain difficult to predict in adult patients. The aim of this study was to determine the biologic basis for the age-related increase in difficulty of performing TSDO. A total of 45 male Sprague Dawley (SD) rats were obtained in 3 age groups: 4 weeks old (4W, N = 15), 3 months old (3M, N = 15), and 13 months old (13M, N = 15). The zygomaticomaxillary sutures (ZMS) were dissected, and their morphology was evaluated by histology and micro-computed tomography (micro-CT). Quantitative real-time polymerase chain reaction of the ZMS and blood samples taken from the abdominal aorta were also used to assess the effects of age at the molecular level. Compared with the 4W rats, the number of fibroblasts in the ZMS was decreased, the bone plate adjacent to the ZMS was thicker, and the texture was denser in the 13M rats. Micro-computed tomography analysis showed the density of the ZMS was significantly increased in the 13M group (P < 0.05). The density ratio of the ZMS to the adjacent bone was increased from 0.14:1 in the 4M group to 0.54:1 in the 13M group. The gene expression of osteocalcin (OC) was significantly lower at 13M than at 3M (P < 0.05). The OC and alkaline phosphatase (ALP) serum levels were significantly lower at 13M than at 4W (P < 0.05). During aging, the decreased osteogenesis activity both systemically and locally may be the biologic effect that limits the application of TSDO in older patients.

Reduced Euchromatin histone methyltransferase 1 causes developmental delay, hypotonia, and cranial abnormalities associated with increased bone gene expression in Kleefstra syndrome mice

Haploinsufficiency of Euchromatin histone methyltransferase 1 (EHMT1), a chromatin modifying enzyme, is the cause of Kleefstra syndrome (KS). KS is an intellectual disability (ID) syndrome, with general developmental delay, hypotonia, and craniofacial dysmorphisms as additional core features. Recent studies have been focused on the role of EHMT1 in learning and memory, linked to the ID phenotype of KS patients. In this study we used the Ehmt1(+/-) mouse model, and investigated whether the core features of KS were mimicked in these mice. When comparing Ehmt1(+/-) mice to wildtype littermates we observed delayed postnatal growth, eye opening, ear opening, and upper incisor eruption, indicating a delayed postnatal development. Furthermore, tests for muscular strength and motor coordination showed features of hypotonia in young Ehmt1(+/-) mice. Lastly, we found that Ehmt1(+/-) mice showed brachycephalic crania, a shorter or bent nose, and hypertelorism, reminiscent of the craniofacial dysmorphisms seen in KS. In addition, gene expression analysis revealed a significant upregulation of the mRNA levels of Runx2 and several other bone tissue related genes in P28 Ehmt1(+/-) mice. Runx2 immunostaining also appeared to be increased. The mRNA upregulation was associated with decreased histone H3 lysine 9 dimethylation (H3K9me2) levels, the epigenetic mark deposited by Ehmt1, in the promoter region of these genes. Together, Ehmt1(+/-) mice indeed recapitulate KS core features and can be used as an animal model for Kleefstra syndrome. The increased expression of bone developmental genes in the Ehmt1(+/-) mice likely contributes to their cranial dysmorphisms and might be explained by diminished Ehmt1-induced H3K9 dimethylation.

Rapid re-synostosis following suturectomy in pediatric mice is age and location dependent

Craniosynostosis is the premature fusion of the cranial sutures early in development. If left untreated, craniosynostosis can lead to complications resulting from cranial deformities or increased intracranial pressure. The standard treatment involves calvarial reconstruction, which in many cases undergoes rapid re-synostosis. This requires additional surgical intervention that is associated with a high incidence of life threatening complications. To better understand this rapid healing, a pediatric mouse model of re-synostosis was developed and characterized. Defects (1.5mm by 2.5mm) over the posterior frontal suture were created surgically in weanling (21 days post-natal) and adolescent (50 days post-natal) C57Bl/6J mice. In addition, defects were created in the frontal bone lateral to the posterior frontal suture. The regeneration of bone in the defect was assessed using advanced image processing algorithms on micro-computed tomography scans. The genes associated with defect healing were assessed by real-time PCR of mRNA isolated from the tissue present in the defect. The results showed that the weanling mouse healed in a biphasic process with bone bridging the defect by post-operative (post-op) day 3 followed by an increase in the bone volume on day 14. In adolescent mice, there was a delay in bone bridging across the defect, and no subsequent increase in bone volume. No bridging of the defect by 14 days post-op was seen in identically sized defects placed lateral to the suture in both weanling and adolescent animals. This study demonstrates that bone regeneration in the cranium is both age and location dependent. Rapid and robust bone regeneration only occurred when the defect was created over the posterior frontal suture in immature weanling mice.

The role of the sutures in biomechanical dynamic simulation of a macaque cranial finite element model: implications for the evolution of craniofacial form

The global biomechanical impact of cranial sutures on the face and cranium during dynamic conditions is not well understood. It is hypothesized that sutures act as energy absorbers protecting skulls subjected to dynamic loads. This hypothesis predicts that sutures have a significant impact on global patterns of strain and cranial structural stiffness when analyzed using dynamic simulations; and that this global impact is influenced by suture material properties. In a finite element model developed from a juvenile Rhesus macaque cranium, five different sets of suture material properties for the zygomaticotemporal sutures were tested. The static and dynamic analyses produced similar results in terms of strain patterns and reaction forces, indicating that the zygomaticotemporal sutures have limited impact on global skull mechanics regardless of loading design. Contrary to the functional hypothesis tested in this study, the zygomaticotemporal sutures did not absorb significant amounts of energy during dynamic simulations regardless of loading speed. It is alternatively hypothesized that sutures are mechanically significant only insofar as they are weak points on the cranium that must be shielded from unduly high stresses so as not to disrupt vitally important growth processes. Thus, sutural and overall cranial form in some vertebrates may be optimized to minimize or otherwise modulate sutural stress and strain.

Microanatomical assessment of nasomaxillary suture patency

In addition to acting as a growth site, sutures in the facial skeleton are important for distributing mechanical forces during mastication. In the present study, the extent of fusion of a facial suture is assessed in two samples of adult bushbabies (Galago moholi and Otolemur garnettii). Microanatomical techniques were used to determine the loci of osseous bridges across the nasomaxillary suture (NMS). Histological sections containing sutures with osseous bridging were rated as "fused." One of the specimens was studied using micro-computed tomography before paraffin embedding and serial sectioning. At all ages, O. garnettii shows more advanced fusion of the NMS than G. moholi. The youngest O. garnettii shows multiple foci of fusion of the NMS; however, 13% of the posterior most suture is patent. Throughout the NMS of this animal, sutural fusion is isolated to one or two small osseous bridges, typically of woven bone. These bridges are most often on the external (superficial) surface of the suture, but in numerous sections the site of fusion occurs deep to an external notch. In G. moholi, the youngest adults studied showed little or no fusion across the NMS. However, the nasal and maxillary bones were indirectly tethered at some levels by other bones that were fused to both nasal and maxillary bones. These results indicate that microanatomical evidence is required to fully assess the extent of fusion of facial sutures. These findings also support previous observations of differing magnitude of suture fusion between the two species.

A biomechanical study on the effect of premature fusion of the frontosphenoidal suture on orbit asymmetry in unilateral coronal synostosis

OBJECTIVE

The coronal ring of patients with unilateral coronal synostosis (UCS) presents premature fusion. This study aims to elucidate whether or not the dynamic behavior of the orbit in response to intracranial pressure (ICP) differs between patients in whom the premature fusion exists only in the frontoparietal suture (FPS) and those in whom the premature fusion extends to the frontosphenoidal suture (FSS).

METHODS

A total of 15 UCS patients were included in the present study. Patients in whom premature fusion was seen inside the FPS and those in whom premature fusion extended to the FSS were categorized as FP Only (4.2 +/- 1.4 m/o) and FP + FS groups (4.6 +/- 2.2 m/o), respectively. On the basis of computed tomography (CT) data, computer-aided design models were produced. Pressure of 15 mm Hg was applied to the neurocranium of each skull model to simulate ICP. Using the finite element method, the displacements presented by each model's orbits were calculated. Displacements of the two groups were compared.

RESULTS

The orbit demonstrated greater displacement in the FP Only group than in the FP + FS group, suggesting that premature closure of the FSS disturbs growth of the orbit in response to ICP.

CONCLUSION

In performing surgical treatment for UCS, the degree of fusion that the FSS presents should be evaluated carefully. In cases in which the FSS presents premature fusion, it is recommended to release the fusion at an early stage of cranial growth to improve the appearance of the orbital region.

Semi-automatic classification of skeletal morphology in genetically altered mice using flat-panel volume computed tomography

Rapid progress in exploring the human and mouse genome has resulted in the generation of a multitude of mouse models to study gene functions in their biological context. However, effective screening methods that allow rapid noninvasive phenotyping of transgenic and knockout mice are still lacking. To identify murine models with bone alterations in vivo, we used flat-panel volume computed tomography (fpVCT) for high-resolution 3-D imaging and developed an algorithm with a computational intelligence system. First, we tested the accuracy and reliability of this approach by imaging discoidin domain receptor 2- (DDR2-) deficient mice, which display distinct skull abnormalities as shown by comparative landmark-based analysis. High-contrast fpVCT data of the skull with 200 μm isotropic resolution and 8-s scan time allowed segmentation and computation of significant shape features as well as visualization of morphological differences. The application of a trained artificial neuronal network to these datasets permitted a semi-automatic and highly accurate phenotype classification of DDR2-deficient compared to C57BL/6 wild-type mice. Even heterozygous DDR2 mice with only subtle phenotypic alterations were correctly determined by fpVCT imaging and identified as a new class. In addition, we successfully applied the algorithm to classify knockout mice lacking the DDR1 gene with no apparent skull deformities. Thus, this new method seems to be a potential tool to identify novel mouse phenotypes with skull changes from transgenic and knockout mice on the basis of random mutagenesis as well as from genetic models. However for this purpose, new neuronal networks have to be created and trained. In summary, the combination of fpVCT images with artificial neuronal networks provides a reliable, novel method for rapid, cost-effective, and noninvasive primary screening tool to detect skeletal phenotypes in mice.

Transgenic mice are key models to shed new light on gene function during development and disease. Reliable high-throughput screening tools will facilitate the identification of transgenic mice with distinct phenotypes. In particular, alterations of the skull are difficult to detect by visual inspection due to its very complex morphological structure. Here, we used high-resolution flat-panel volume computed tomography (fpVCT), a novel semi-automatic screening tool to image skull-shape features of mice. The resulting 3-D datasets were combined with artificial neuronal networks and complex nonlinear computational models to permit rapid and automatic interpretation of the images. Compared to the extremely laborious landmark-based analysis, the manual work in our approach was reduced to the control of skull segmentation of images obtained by fpVCT. We applied our approach to genetically altered mice and various mouse strains and showed that it is an accurate and reliable method to successfully identify mice with skeletal phenotypes. We suggest the new methodology will also be a valuable tool for an in vivo, rapid, cost-effective, and reliable primary screen to identify skull abnormalities generated by random mouse mutagenesis experiments.

Validation of x-ray microfocus computed tomography as an imaging tool for porous structures

X-ray microfocus computed tomography (micro-CT) is recently put forward to qualitatively and quantitatively characterize the internal structure of porous materials. However, it is known that artifacts such as the partial volume effect are inherently present in micro-CT images, thus resulting in a visualization error with respect to reality. This study proposes a validation protocol that in the future can be used to quantify this error for porous structures in general by matching micro-CT tomograms to microscopic sections. One of the innovations of the protocol is the opportunity to reconstruct an interpolated micro-CT image under the same angle as the physical cutting angle of the microscopic sections. Also, a novel thresholding method is developed based on matching micro-CT and microscopic images. In this study, titanium porous structures are assessed as proof of principle. It is concluded for these structures that micro-CT visualizes 89% of the total amount of voxels (solid and pore) correctly. However, 8% represents an overestimation of the real structure and 3% are real structural features not visualized by micro-CT. When exclusively focusing on the solid fraction in both the micro-CT and microscopic images, only an overestimation of about 5% is found.