Strain accommodation in the zygomatic arch of the pig: A validation study using digital speckle pattern interferometry and finite element analysis

It has been repeatedly suggested that mammalian cranial sutures act not only to allow growth but also to reduce the levels of strain experienced by the skull during feeding. However, because of the added complexity they introduce, sutures are rarely included in finite element (FE) models, despite their potential to influence strain results. Because sutures present different morphologies and with differing degrees of internal fusion, many different methods of modeling may be necessary to accurately measure strain environments. Alternatively, these variables may exert very little influence on the scale of a whole‐skull model. To validate suture modeling methods, four alternative ways of including a suture in 3D FE models of the pig zygomatic arch were considered and compared with ex vivo experimental data from digital speckle pattern interferometry (DSPI). The use of DSPI rather than traditional strain gauge techniques allows strain gradients around the suture as well as the motions of the two bones to be observed. Results show that the introduction of 3D elements assigned more compliant material properties than the surrounding bone, is the most effective way of modeling both morphologies of suture, both in tension and compression. However, models containing no suture are almost indistinguishable from these compliant suture models, beyond the high strain gradient immediately adjacent to the suture. Conversely, modeling the suture as an open break in the mesh, or with spring elements assigned suture properties, fails to reproduce the experiment. Thus, although a solid but flexible model of sutures is preferred, the similarity between these models and those without sutures tentatively suggests that such extra detail may be unnecessary in pigs if the behavior of the whole skull is of interest. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Biomechanics of the rostrum and the role of facial sutures

The rostrum is a large diameter, thin-walled tubular structure that receives loads from the teeth. The rostrum can be conceptualized both as a rigid structure and as an assemblage of several bones that interface at sutures. Using miniature pigs, we measured in vivo strains in rostral bones and sutures to gain a better understanding of how the rostrum behaves biomechanically. Strains in the premaxillary and nasal bones were low but the adjacent maxillary-premaxillary, internasal, and intermaxillary suture strains were larger by an order of magnitude. While this finding emphasizes the composite nature of the rostrum, we also found evidence in the maxillary and nasal bones for rigid structural behavior. Namely, maxillary strain is consistent with a short beam model under shear deformation from molar loading. Strain in the nasal bones is only partially supported by a long beam model; rather, a complex pattern of dorsal bending of the rostrum from incisor contact and lateral compression is suggested. Torsion of the maxilla is ruled out due to the bilateral occlusion of pigs and the similar working and balancing side strains, although it may be important in mammals with a unilateral bite. Torsional loading does appear important in the premaxillae, which demonstrate working and balancing side changes in strain orientation. These differences are attributed to asymmetrical incisor contact occurring at the end of the power stroke.     

Nasal capsular cartilage is required for rat transpalatal suture morphogenesis

In the cranial vault, suture morphogenesis occurs when the growing cranial bones approximate and overlap or abut one another. Patency of developing sutures is regulated by the underlying dura mater. Once cranial sutures form, bone growth proceeds from the sutures in response to growth signals from the rapidly expanding neurocranium. Facial sutures do not develop in contact with the dura mater. It was therefore hypothesized that facial suture morphogenesis and bone growth from facial sutures are regulated by tissues with an equivalent role to the dura mater. The present study was designed to test this hypothesis by characterizing the morphology and growth factor expression in developing transpalatal (TP) sutures and their surrounding tissues, and then assessing the role of the overlying nasal capsular (NC) cartilages in maintaining suture patency. TP sutures develop as overlapping sutures, similar to cranial coronal sutures, and expression of Tgf-betas in TP sutures was similar to their distribution in cranial coronal sutures. To establish whether NC cartilages play a role in regulating TP suture morphogenesis, fetal rat TP sutures were cultured with associated attached NC cartilages or with NC cartilages removed. Sutures cultured for upward of 5 days with intact NC cartilages remained patent and maintained their cellular and fibrous components. However, in the absence of NC cartilages, the cellular nature of the sutures was not maintained and they became progressively acellular, with bony bridging across the suture. This finding is similar to that for cranial vault sutures cultured in the absence of dura mater, indicating that NC cartilages play an equivalent role to dura mater in maintaining the patency of developing sutures. These studies indicate that tissue interactions likely regulate morphogenesis of all cranial and facial sutures.     

A Phylogenetic Study of Late Growth Events in a Mammalian Evolutionary Radiation—The Cranial Sutures of Terrestrial Artiodactyl Mammals

We recorded the relative timing of fusion of 29 ectocranial sutures in 480 skulls belonging to 35 extant and four fossil species from all major clades of terrestrial artiodactyls. The resulting data were studied in a phylogenetic context, using mapping of event-pairing of suture fusion events and Parsimov. As phylogenetic framework we generated a compound phylogeny from several previously published analyses. Overall suture closure per species ranged from five to all 29 sutures in Hexaprotodon. All living non-ruminants (suids, camelids, and hippopotamids) fuse more than 50% of the studied sutures (most over 75%), whereas in almost all ruminants less than 50% of the sutures fuse completely. Phylogenetic regression found a significant correlation between suture closure and body mass. In all species we observed an early fusion of the sutures surrounding the foramen magnum (Exoccipital-Supraoccipital, Exoccipital-Basioccipital), a consistent scheme also among other mammals. Scaling the number of changes to the number of sutures in each of the usually recognized skull modules reveals relatively equal numbers of changes in the cranial vault, the zygomatic-sphenoid region, the orbit, and the anterior oral-nasal region. Only the basicranium shows a much smaller number in terms of absolute and relative amount of suture fusion change. Some species show a unique pattern of suture fusion, such as the early fusion of many sutures in the palatal region in Pecari or that of premaxillary sutures in †Cainotherium, perhaps related to feeding mode. A strategy to strengthen the skull by obliterating the sutures could explain the pattern of increased sutural fusion in ruminant species with large cranial appendages.     

Relationships between growth at the sutures of the rabbit calvarium

The present study was designed to elucidate the relationships between growth increments at the cranial vault sutures in rabbits. Thirteen male New Zealand white rabbits were followed regularly from age 31 to 142 days using a roentgen stereophotogrammetric system. Spherical tantalum markers were implanted into the nasal, frontal, and parietal bones, and implant stability was checked at each stereo examination. Problems with instability were encountered only in the nasal bones. Registered growth rates conformed to our previous investigations. High correlations were observed between the following areas; the coronal suture to the frontonasal suture, the first principal component of the neurocranial suture group to the frontonasal suture, and the principal component of the craniofacial suture group to the coronal suture. Remaining relationships demonstrated dispersion to various extents. The findings indicate that there seems to exist a basic mutual dependence between neural and facial skeletal growth, as well as complex covariations between the various sutures of the rabbit calvarium.     

Strain in the braincase and its sutures during function

The skull is distinguished from other parts of the skeleton by its composite construction. The sutures between bony elements provide for interstitial growth of the cranium, but at the same time they alter the transmission of stress and strain through the skull. Strain gages were bonded to the frontal and parietal bones of miniature pigs and across the interfrontal, interparietal and coronal sutures. Strains were recorded 1) during natural mastication in conjunction with electromyographic activity from the jaw muscles and 2) during stimulation of various cranial muscles in anesthetized animals. Vault sutures exhibited vastly higher strains than did the adjoining bones. Further, bone strain primarily reflected torsion of the braincase set up by asymmetrical muscle contraction; the tensile axis alternated between +45 degrees and -45 degrees depending on which diagonal masseter/temporalis pair was most active. However, suture strains were not related to overall torsion but instead were responses to local muscle actions. Only the coronal suture showed significant strain (tension) during jaw opening; this was caused by the contraction of neck muscles. All sutures showed strain during jaw closing, but polarity depended on the pattern of muscle usage. For example, masseter contraction tensed the coronal suture and the anterior part of the interfrontal suture, whereas the temporalis caused compression in these locations. Peak tensile strains were larger than peak compressive strains. Histology suggested that the skull is bent at the sutures, with the ectocranial surface tensed and the endocranial surface predominantly compressed. Collectively, these results indicate that skulls with patent sutures should be analyzed as complexes of independent parts rather than solid structures.     

Sutural expansion osteogenesis for management of the bony-tissue defect in cleft palate repair: experimental studies in dogs

A series of experimental studies on sutural expansion osteogenesis for management of the bony-tissue defect in cleft palate repair was performed between 1995 and 1997. Forty-five young dogs in weaning were used in four experiments that were divided into two parts. Part I probed the possibility of closing the surgically constructed hard palate cleft not only with mucoperiosteum but also with bony tissue by the technique of sutural expansion of lateral palatine sutures. Part II explored the possibility of pushing the palatine bone posteriorly and advancing the maxillary segment anteriorly by transverse palatine suture expansion. In Part I, a ring-shaped suture expander made of nickel-titanium shape memory alloy was used to expand the lateral suture of palatine bones. Expansion forces of 200 G, 360 G, and 480 G were used for the first experiment. A force of 360 G was chosen for two other experiments; this force is equivalent to the distraction rate of 0.5 mm per day of a jackscrew device. The ring-shaped suture expander was opened and its two feet were fixed in the medial sides of residual horizontal plates of the palatine bones immediately after a hard palate cleft was constructed surgically under endotracheal general anesthesia. At the eighth postoperative day, under the traction of 360 G, the two sides of the 8-mm-wide hard palate cleft were brought into contact with each other, and 8 or 9 days later the closed palatal cleft had healed completely with mucosal tissue. This experiment was repeated twice and yielded the same results. Sutural expansion osteogenesis was evaluated physically, fluorescently, histologically, and ultrastructurally to examine the deposition of the regenerated bone in the suture areas. Additionally, the influence of sutural expansion osteogenesis of the palatal bones on other facial bones was also studied cephalometrically. In Part II, a bow-shaped suture expander made of nickel-titanium shape memory alloy was applied to expand either the left or the right side of the transverse palatal suture of each of the experimental dogs. At the postoperative week 4 to 6, the maxillary segment was moved forward 5 to 6 mm on the expanded side, and the palatal bone was pushed backward 5 mm. The changes of bone position were assessed radiographically and cephalometrically. Tissue response of circum-maxillary sutures was examined histologically. These experiments led to the following conclusions: (1) Bony closure of the surgically constructed hard palate cleft with a ring-shaped suture expander made of nickel-titanium shape memory alloy is possible. (2) Anterior advancement of the maxillary segment and posterior lengthening of the hard palate using a bow-shaped suture expander made of nickel-titanium shape memory alloy applied at the palatomaxillary suture (transverse palatal suture) of the hard palate are also possible. Thus, in humans, a new approach for cleft palate repair may be a worthwhile investigation.     

Mechanics of cranial sutures during simulated cyclic loading

Previous computational and experimental analyses revealed that cranial sutures, fibrous joints between the bones, can reduce the strain experienced by the surrounding skull bones during mastication. This damping effect reflects the importance of including sutures in finite element (FE) analyses of the skull. Using the FE method, the behaviour of three suture morphologies of increasing complexity (butt-ended, moderate interdigitated, and complex interdigitated) during static loading was recently investigated, and the sutures were assumed to have linear elastic properties. In the current study, viscoelastic properties, derived from published experimental results of the nasofrontal suture of young pigs (Sus scrofa), are applied to the three idealised bone-suture models. The effects of suture viscoelasticity on the stress, strain, and strain energy in the models were computed for three different frequencies (corresponding to periods of 1, 10, and 100s) and compared to the results of a static, linear elastic analysis. The range of applied frequencies broadly represents different physiological activities, with the highest frequency simulating mastication and the lowest frequency simulating growth and pressure of the surrounding tissues. Comparing across all three suture morphologies, strain energy and strain in the suture decreased with the increase in suture complexity. For each suture model, the magnitude of strain decreased with an increase in frequency, and the magnitudes were similar for both the elastic and 1s frequency analyses. In addition, a viscous response is less apparent in the higher frequency analyses, indicating that viscous properties are less important to the behaviour of the suture during those analyses. The FE results suggest that implementation of viscoelastic properties may not be necessary for computational studies of skull behaviour during masticatory loading but instead might be more relevant for studies examining lower frequency physiological activities.     

Restricted growth at the frontonasal suture: alterations in craniofacial growth in rabbits

Apposition of bone at the sutural margin is generally thought to be a compensatory adjustment to growing soft-tissue organs such as the brain or eyes within the skull. The frontonasal suture which is located at the interface between the cranial and facial skeletons is a site of extremely active growth in the young rabbit. Recently, we showed that premature closure of a cranial suture, the coronal suture, can alter the growth not only at the adjacent frontonasal suture but also of the basicranium and midface. This study examines the effects of restricted growth at the frontonasal suture on both growth at adjacent cranial sutures and linear growth of the basicranium and midface. Thirty newborn New Zealand White rabbits were subdivided into experimental and sham-treated groups of equal size and distribution for sex and birth weight. At 9 days of age, the frontonasal suture of each experimental animal was immobilized by bilateral application of methyl-cyanoacrylate adhesive across the frontonasal suture. Growth and morphometric changes were monitored by radiocephalometric methods through 120 days of age by bilateral implantation of radiopague markers on each side of frontonasal, coronal, and anterior lambdoid sutures. Results indicate that restricted growth at the frontonasal suture results not only in a significant shortening of the midface but also in significant decreases in growth at the coronal and internasal sutures. Growth at the interfrontal and sagittal sutures is increased. Furthermore, growth at the anterior portion of the nasal bones is significantly increased, thereby offsetting a portion of the decreased nasal bone length resulting from frontonasal restriction.     

A genomic survey of the fish parasite Spironucleus salmonicida indicates genomic plastiCity among diplomonads and significant lateral gene transfer in eukaryote genome evolution

Background

Craniosynostosis, the premature fusion of calvarial sutures, is a common craniofacial abnormality. Causative mutations in more than 10 genes have been identified, involving fibroblast growth factor, transforming growth factor beta, and Eph/ephrin signalling pathways. Mutations affect each human calvarial suture (coronal, sagittal, metopic, and lambdoid) differently, suggesting different gene expression patterns exist in each human suture. To better understand the molecular control of human suture morphogenesis we used microarray analysis to identify genes differentially expressed during suture fusion in children with craniosynostosis. Expression differences were also analysed between each unfused suture type, between sutures from syndromic and non-syndromic craniosynostosis patients, and between unfused sutures from individuals with and without craniosynostosis.

Results

We identified genes with increased expression in unfused sutures compared to fusing/fused sutures that may be pivotal to the maintenance of suture patency or in controlling early osteoblast differentiation (i.e. RBP4, GPC3, C1QTNF3, IL11RA, PTN, POSTN). In addition, we have identified genes with increased expression in fusing/fused suture tissue that we suggest could have a role in premature suture fusion (i.e. WIF1, ANXA3, CYFIP2). Proteins of two of these genes, glypican 3 and retinol binding protein 4, were investigated by immunohistochemistry and localised to the suture mesenchyme and osteogenic fronts of developing human calvaria, respectively, suggesting novel roles for these proteins in the maintenance of suture patency or in controlling early osteoblast differentiation. We show that there is limited difference in whole genome expression between sutures isolated from patients with syndromic and non-syndromic craniosynostosis and confirmed this by quantitative RT-PCR. Furthermore, distinct expression profiles for each unfused suture type were noted, with the metopic suture being most disparate. Finally, although calvarial bones are generally thought to grow without a cartilage precursor, we show histologically and by identification of cartilage-specific gene expression that cartilage may be involved in the morphogenesis of lambdoid and posterior sagittal sutures.

Conclusion

This study has provided further insight into the complex signalling network which controls human calvarial suture morphogenesis and craniosynostosis. Identified genes are candidates for targeted therapeutic development and to screen for craniosynostosis-causing mutations.     

Strain patterns in the horncores, cranial bones and sutures of goats (Capra hircus) during impact loading

The design of bovid horns and skulls for the function of head-to-head fighting has been well studied using mechanical and kinematic analyses, but actual strain measurements from the horncores, cranium and sutures during impact loading have never been made. During in vitro impact loading of one or both horns on the heads of female domestic goats, principal strains were measured from the surfaces of the horncore bases, frontal and parietal bones, and interfrontal and frontoparietal sutures. Impact produced a bending moment at the bases of the horncores, with compressive strains on the posterior surfaces more than doubling the anterior tensile strains. These ratios of compression to tension exceed our predictions (and those in the literature) based on curved beam theory. Principal strains in the cranial bones dropped by as much as 50% crossing the sutures, which acted like springs or hinges to allow cranial bone movement. Sutures also experienced very high strain magnitudes, being more than 10 times greater than those of the surrounding cranial bones. Sutural strains during impact loading of the horns also greatly exceeded the strain levels recorded during mastication in other studies. The decrease in strain between adjacent cranial bones across a suture and the large strains at the sutures indicate that the sutures function as shock absorbers during impact.     

Functional implications of squamosal suture size in paranthropus boisei

It has been hypothesized that the extensively overlapping temporal and parietal bones of the squamosal sutures in Paranthropus boisei are adaptations for withstanding loads associated with feeding. Finite element analysis (FEA) was used to investigate the biomechanical effects of suture size (i.e., the area of overlap between the temporal and parietal bones) on stress, strain energy, and strain ratio in the squamosal sutures of Pan troglodytes and P. boisei (specimen OH 5) during biting. Finite element models (FEMs) of OH 5 and a P. troglodytes cranium were constructed from CT scans. These models contain sutures that approximate the actual suture sizes preserved in both crania. The FEM of Pan was then modified to create two additional FEMs with squamosal sutures that are 50% smaller and 25% larger than those in the original model. Comparisons among the models test the effect of suture size on the structural integrity of the squamosal suture as the temporal squama and parietal bone move relative to each other during simulated premolar biting. Results indicate that with increasing suture size there is a decreased risk of suture failure, and that maximum stress values in the OH 5 suture were favorable compared to values in the Pan model with the normal suture size. Strain ratios suggest that shear is an important strain regime in the squamosal suture. This study is consistent with the hypothesis that larger sutures help reduce the likelihood of suture failure under high biting loads. Am J Phys Anthropol 153:260–268, 2014. © 2013 Wiley Periodicals, Inc.     

Cranial suture complexity in white-tailed deer (Odocoileus virginianus)

Neurocranial expansion and mastication are commonly implicated as the two major biomechanical factors affecting suture morphology. In deer the antlers provide an additional source of biomechanical stress acting on the skull. Equivalent stresses are not found in females, who lack antlers. We analyzed the complexity and interdigitation of the interfrontal and coronal sutures that surround the antler-bearing frontal bones of (n = 67) white-tailed deer (Odocoileus virginianus) to 1) evaluate changes in suture morphology throughout ontogeny, and 2) test the hypothesis that male deer have more complex sutures than females. Two methods were used to quantify suture morphology: fractal analysis and length-ratios (actual suture length divided by direct straightline length). Both techniques produced similar results, although the two methods cannot be considered equivalent. Suture complexity increases markedly throughout ontogeny, but appears to level off after animals have reached adulthood. Cranial size in males, but not females, continues to increase in adults. No significant increase in suture complexity with age in the adult cohort was detected. While deer are highly dimorphic in size and the presence of antlers, no significant differences existed between males and females for any measure of suture complexity. No consistent patterns emerged between suture complexity and skull size or antler characteristics. The presence of antlers appears to have a minimal effect on suture complexity in white-tailed deer. Factors that may contribute to the lack of dimorphism in suture complexity are discussed.     

An Enlarged Parietal Foramen in the Late Archaic Xujiayao 11 Neurocranium from Northern China,and Rare Anomalies among Pleistocene Homo

We report here a neurocranial abnormality previously undescribed in Pleistocene human fossils, an enlarged parietal foramen (EPF) in the early Late Pleistocene Xujiayao 11 parietal bones from the Xujiayao (Houjiayao) site, northern China. Xujiayao 11 is a pair of partial posteromedial parietal bones from an adult. It exhibits thick cranial vault bones, arachnoid granulations, a deviated posterior sagittal suture, and a unilateral (right) parietal lacuna with a posteriorly-directed and enlarged endocranial vascular sulcus. Differential diagnosis indicates that the perforation is a congenital defect, an enlarged parietal foramen, commonly associated with cerebral venous and cranial vault anomalies. It was not lethal given the individual’s age-at-death, but it may have been associated with secondary neurological deficiencies. The fossil constitutes the oldest evidence in human evolution of this very rare condition (a single enlarged parietal foramen). In combination with developmental and degenerative abnormalities in other Pleistocene human remains, it suggests demographic and survival patterns among Pleistocene Homo that led to an elevated frequency of conditions unknown or rare among recent humans.     

Rhinoplasty: a simplified, three-stitch, open tip suture technique. Part I: primary rhinoplasty

Tip suture techniques offer a reliable and dramatic method of tip modification without needing to interrupt the alar rim strip or add tip grafts. The present simplified three-stitch technique consists of the following: (1) a strut suture to fix the columella strut between the crura, (2) bilateral domal creation sutures to create tip definition, and (3) a domal equalization suture to narrow and align the domes. If required, columella septal sutures can be added; either a dorsal rotational suture or a transfixion projection suture can be used. This simplified method represents a refinement based on more than 13 years of experience with tip suture techniques. It does not require a complex operative sequence or specialized sutures. Primary indications are moderate tip deformities of inadequate definition and excessive width and certain specific tip deformities, including the parenthesis tip and nostril/tip disproportion. The primary contraindications are for patients with minor tip deformities that are best done through a closed approach and those with severe tip deformities requiring an open structure graft. The technique is simple, efficacious, and easily learned.     

The correlation between craniofacial and long bone growth: an experimental investigation in normal rabbits

The present study was undertaken to elucidate the relationships between craniofacial and long-bone growth. Nine male New Zealand white rabbits received spherical tantalum bone markers in the tibial epiphyses and in the nasal, frontal, and parietal bones. The animals were followed from 30 to 143 days of age. Growth changes were calculated with a roentgen stereometric system, and the results statistically evaluated. Except for the final interval when all variables varied at random, high correlations between tibial and frontonasal or coronal sutural growth were demonstrated; and the respective linear regression lines were homogeneously assembled. The relationship between the tibia and the sagittal suture displayed great variations between individual animals as well as between the suture's parts, although growth at the interfrontal suture was clearly correlated to tibial growth upon exclusion of the time factor. The first principal component of the three neurocranial sutures was calculated and seemed accurately correlated to long-bone growth. The present study concluded that growth at the frontonasal and coronal sutures normally seems to parallel general somatic development, while growth at the sagittal suture appears individually displaced in time. Nevertheless, when the principal component of the combination of the coronal suture and the neurocranial section of the sagittal suture was computed, this was highly correlated to body growth.     

Bundles of Spider Silk,Braided into Sutures,Resist Basic Cyclic Tests: Potential Use for Flexor Tendon Repair

Repair success for injuries to the flexor tendon in the hand is often limited by the in vivo behaviour of the suture used for repair. Common problems associated with the choice of suture material include increased risk of infection, foreign body reactions, and inappropriate mechanical responses, particularly decreases in mechanical properties over time. Improved suture materials are therefore needed. As high-performance materials with excellent tensile strength, spider silk fibres are an extremely promising candidate for use in surgical sutures. However, the mechanical behaviour of sutures comprised of individual silk fibres braided together has not been thoroughly investigated. In the present study, we characterise the maximum tensile strength, stress, strain, elastic modulus, and fatigue response of silk sutures produced using different braiding methods to investigate the influence of braiding on the tensile properties of the sutures. The mechanical properties of conventional surgical sutures are also characterised to assess whether silk offers any advantages over conventional suture materials. The results demonstrate that braiding single spider silk fibres together produces strong sutures with excellent fatigue behaviour; the braided silk sutures exhibited tensile strengths comparable to those of conventional sutures and no loss of strength over 1000 fatigue cycles. In addition, the braiding technique had a significant influence on the tensile properties of the braided silk sutures. These results suggest that braided spider silk could be suitable for use as sutures in flexor tendon repair, providing similar tensile behaviour and improved fatigue properties compared with conventional suture materials.     

The Parietal Problem: How to Cut This Gordian Knot?

Although no one has disputed that the piscine progenitors of the tetrapods have a homologue of the human parietal bone, opinions differ as to where in the skull roof this homologue is located. One view holds it to be either of two interorbital bones that together surround the foramen neuroepiphysium (and the so-called pineal plates); another, that it is each of the two mesial bones which comes next in order, behind the orbital cavities. Both of these views are untenable because neither of the proposed bones has proved to be amenable to conversion to the parietal bone of man. In seeking a solution to this issue thoughts turn to the tentorium cerebelli, whose topographic relationships and comparative morphology place it in the key position as a plausible derivative of the posterior half of the cranium of the tetrapod forerunners. Following this line of reasoning, it can be suggested that the tentorial exoskeleton, here called the pluteal bones, originally was situated in the dermis superjacent to the synotic tectum. In early therian phylogeny, these skull bones were covered by the backwardly expanding cerebral hemispheres which, concomitantly, became overgrown and thus protected by epiotic exoskeleton. It follows that the likely homologues of the human parietal bones are those parts of the skull roof of the piscine progenitors of the tetrapods that lie dorsolateral to the otic capsules.     

Long-term effects of tissue expansion on cranial and skeletal bone development in neonatal miniature swine: clinical findings and histomorphometric correlates

Progressive tissue expansion induces significant gross, histologic, and bony changes in skulls and long bones of neonatal miniature swine. These bony changes consist of erosion underlying tissue expanders, with bony lipping and bone deposition at the periphery of the expander. Cranial suture lines underneath expanders appear effaced and convoluted. Serial CT scans reveal decreased bone thickness and volume (p less than 0.02) but identical bone density (p = 0.60) beneath expanders. Increased bone volume and thickness occur at the periphery of expanders (p less than 0.02). Bone density (CT number) is unaffected by tissue expansion in both cranial and long bones. These findings have histomorphometric correlates: Osteoclastic bone resorption occurs underneath expanders with periosteal reaction at the periphery of expanders. Cranial sutures are similarly affected, but no cranial synostosis results. No changes to the inner table of the skull or stigmata of increased intracranial pressure were observed either in CT scans or in behavioral changes in long-term animals. The pathophysiology of bony changes is a remodeling effect, not one of simple pressure deformation. Increased bone resorption and complete inhibition of bone formation occur until the pressure is removed. Cranial bone is significantly more affected than long bone. After removal of the expanders, reparative bone remodeling begins within 5 days and nearly complete healing of the cranial defects occurs within 2 months (p less than 0.02). No plagiocephaly results despite early coronal suture changes. On the basis of this study, we conclude that tissue expansion causes significant but reversible effects, readily monitored by high-resolution CT scans, on neonatal and infant cranial and long bones.