A Computational Framework to Predict Calvarial Growth: Optimising Management of Sagittal Craniosynostosis

Functional adaptation of the infant craniofacial system to mechanical loadings arising from masticatory forces

The morphology and biomechanics of infant crania undergo significant changes between the pre- and post-weaning phases due to increasing loading of the masticatory system. The aims of this study were to characterize the changes in muscle forces, bite forces and the pattern of mechanical strain and stress arising from the aforementioned forces across crania in the first 48 months of life using imaging and finite element methods. A total of 51 head computed tomography scans of normal individuals were collected and analysed from a larger database of 217 individuals. The estimated mean muscle forces of temporalis, masseter and medial pterygoid increase from 30.9 to 87.0 N, 25.6 to 69.6 N and 23.1 to 58.9 N, respectively (0-48 months). Maximum bite force increases from 90.5 to 184.2 N (3-48 months). There is a change in the pattern of strain and stress from the calvaria to the face during postnatal development. Overall, this study highlights the changes in the mechanics of the craniofacial system during normal development. It further raises questions as to how and what level of changes in the mechanical forces during the development can alter the morphology of the craniofacial system.

Understanding the influence of surgical parameters on craniofacial surgery outcomes: a computational study

Sagittal craniosynostosis (SC) is a congenital condition whereby the newborn skull develops abnormally owing to the premature ossification of the sagittal suture. Spring-assisted cranioplasty (SAC) is a minimally invasive surgical technique to treat SC, where metallic distractors are used to reshape the newborn's head. Although safe and effective, SAC outcomes remain uncertain owing to the limited understanding of skull-distractor interaction and the limited information provided by the analysis of single surgical cases. In this work, an SC population-averaged skull model was created and used to simulate spring insertion by means of the finite-element analysis using a previously developed modelling framework. Surgical parameters were varied to assess the effect of osteotomy and spring positioning, as well as distractor combinations, on the final skull dimensions. Simulation trends were compared with retrospective measurements from clinical imaging (X-ray and three-dimensional photogrammetry scans). It was found that the on-table post-implantation head shape change is more sensitive to spring stiffness than to the other surgical parameters. However, the overall end-of-treatment head shape is more sensitive to spring positioning and osteotomy size parameters. The results of this work suggest that SAC surgical planning should be performed in view of long-term results, rather than immediate on-table reshaping outcomes.

Multiscale mechanical characterisation of the craniofacial system under external forces

Premature fusion of craniofacial joints, i.e. sutures, is a major clinical condition. This condition affects children and often requires numerous invasive surgeries to correct. Minimally invasive external loading of the skull has shown some success in achieving therapeutic effects in a mouse model of this condition, promising a new non-invasive treatment approach. However, our fundamental understanding of the level of deformation that such loading has induced across the sutures, leading to the effects observed is severely limited, yet crucial for its scalability. We carried out a series of multiscale characterisations of the loading effects on normal and craniosynostotic mice, in a series of in vivo and ex vivo studies. This involved developing a custom loading setup as well as software for its control and a novel in situ CT strain estimation approach following the principles of digital volume correlation. Our findings highlight that this treatment may disrupt bone formation across the sutures through plastic deformation of the treated suture. The level of permanent deformations observed across the coronal suture after loading corresponded well with the apparent strain that was estimated. This work provides invaluable insight into the level of mechanical forces that may prevent early fusion of cranial joints during the minimally invasive treatment cycle and will help the clinical translation of the treatment approach to humans.

Spring-Assisted Surgery of Unilambdoid Craniosynostosis

Craniosynostosis is traditionally treated with extensive cranial vault reconstructions (CVRs). Although less invasive techniques, such as endoscopic strip craniectomy with postoperative helmet therapy, have been successful, they also present difficulties. An alternative method is distraction osteogenesis using either manually controlled devices or specially designed springs. In this study, the authors provide the first comparison of spring-assisted surgery (SAS) with CVR for the treatment of unilambdoid synostosis (ULS). Fourteen consecutive patients (8 CVR and 6 SAS) treated for ULS at Sahlgrenska University Hospital between 2005 and 2018 were included. Skull shape and deviations were evaluated using previously defined measurement points on 3-dimensional computed tomography scans preoperatively, at spring removal, and at 3 years of age. Posterior and middle cranial fossa (PCF and MCF, respectively), skull-base cant, facial twist, and mastoid bulge (MB) were measured, and clinical data were obtained from chart reviews. The results indicated that at the 3-year follow-up, PCF, MCF, and MB improved in both groups, with no significant difference in outcome observed between methods. In the SAS group, duration of operation [61±27 min (mean±SD)] and perioperative bleeding (3.5±2.8 mL/kg body weight) were both significantly lower relative to the CVR group (P<0.05). These findings showed that both SAS and CVR resulted in similar improvements in treating ULS, although neither produced complete normalization of skull shape. The results suggest that early diagnosis and operation allow less extensive SAS to be performed without adversely affecting the results.

Normal human craniofacial growth and development from 0 to 4 years

Knowledge of human craniofacial growth (increase in size) and development (change in shape) is important in the clinical treatment of a range of conditions that affects it. This study uses an extensive collection of clinical CT scans to investigate craniofacial growth and development over the first 48 months of life, detail how the cranium changes in form (size and shape) in each sex and how these changes are associated with the growth and development of various soft tissues such as the brain, eyes and tongue and the expansion of the nasal cavity. This is achieved through multivariate analyses of cranial form based on 3D landmarks and semi-landmarks and by analyses of linear dimensions, and cranial volumes. The results highlight accelerations and decelerations in cranial form changes throughout early childhood. They show that from 0 to 12 months, the cranium undergoes greater changes in form than from 12 to 48 months. However, in terms of the development of overall cranial shape, there is no significant sexual dimorphism in the age range considered in this study. In consequence a single model of human craniofacial growth and development is presented for future studies to examine the physio-mechanical interactions of the craniofacial growth.

Biomechanical evaluation of a novel intervertebral disc repair technique for large box-shaped ruptures

Objective: The purpose of this study was to analyze the feasibility of repairing a ruptured intervertebral disc using a patch secured to the inner surface of the annulus fibrosus (AF). Different material properties and geometries for the patch were evaluated. Methods: Using finite element analysis, this study created a large box-shaped rupture in the posterior-lateral region of the AF and then repaired it with a circular and square inner patch. The elastic modulus of the patches ranged from 1 to 50 MPa to determine the effect on the nucleus pulposus (NP) pressure, vertical displacement, disc bulge, AF stress, segmental range of motion (ROM), patch stress, and suture stress. The results were compared against the intact spine to determine the most suitable shape and properties for the repair patch. Results: The intervertebral height and ROM of the repaired lumbar spine was similar to the intact spine and was independent of the patch material properties and geometry. The patches with a modulus of 2-3 MPa resulted in an NP pressure and AF stresses closest to the healthy disc, and produced minimal contact pressure on the cleft surfaces and minimal stress on the suture and patch of all models. Circular patches caused lower NP pressure, AF stress and patch stress than the square patch, but also caused greater stress on the suture. Conclusion: A circular patch with an elastic modulus of 2-3 MPa secured to the inner region of the ruptured annulus fibrosus was able to immediately close the rupture and maintain an NP pressure and AF stress similar to the intact intervertebral disc. This patch had the lowest risk of complications and produced the greatest restorative effect of all patches simulated in this study.

A preliminary analysis of replicating the biomechanics of helmet therapy for sagittal craniosynostosis

PURPOSE

The aim of this study was to investigate the biomechanics of endoscopically assisted strip craniectomy treatment for the management of sagittal craniosynostosis while undergoing three different durations of postoperative helmet therapy using a computational approach.

METHODS

A previously developed 3D model of a 4-month-old sagittal craniosynostosis patient was used. The strip craniectomy incisions were replicated across the segmented parietal bones. Areas across the calvarial were selected and constrained to represent the helmet placement after surgery. Skull growth was modelled and three variations of helmet therapy were investigated, where the timings of helmet removal alternated between 2, 5, and 8 months after surgery.

RESULTS

The predicted outcomes suggest that the prolonging of helmet placement has perhaps a beneficial impact on the postoperative long-term morphology of the skull. No considerable difference was found on the pattern of contact pressure at the interface of growing intracranial volume and the skull between the considered helmeting durations.

CONCLUSION

Although the validation of these simulations could not be performed, these simulations showed that the duration of helmet therapy after endoscopically assisted strip craniectomy influenced the cephalic index at 36 months. Further studies require to validate these preliminary findings yet this study can lay the foundations for further studies to advance our fundamental understanding of mechanics of helmet therapy.

Multisuture craniosynostosis: a case report of unusual presentation of chromosome 14q32 deletion

Multisuture craniosynostosis is associated with a number of syndromes and underlying gene mutations. It is rarely caused by chromosome disorders. For the management, multisuture craniosynostosis raises concerns about abnormal head shape and risks of increased intracranial pressure in affected patients. Calvarial reconstruction to reshape the skull shape and expand the intracranial volume plays an essential role in correcting particular problems. Here, we report a 2-month-old female infant presenting with low birth weight, abnormal head shape, dysmorphic facies and pinnae, hypotonia, and feeding difficulty. Three-dimensional computed tomographic scans revealed left unicoronal and sagittal synostoses. Chromosome microarray analysis revealed de novo chromosome 14q32.12-q32.31 deletion. Among the deleted genes, YY1 and BCL11B are the most likely candidate genes causing craniosynostosis. Some clinical features of the patient are similar to Temple syndrome indicating that the deleted region is paternal in origin. In summary, this is a rare case of chromosome 14q32 deletion with multisuture craniosynostosis. We also report the multidisciplinary management and clinical outcomes after early cranial vault remodelling procedures.