Using Sensitivity Analysis to Develop a Validated Computational Model of Post-operative Calvarial Growth in Sagittal Craniosynostosis

Abstract

Craniosynostosis is the premature fusion of one or more sutures across the calvaria, resulting in morphological and health complications that require invasive corrective surgery. Finite element (FE) method is a powerful tool that can aid with preoperative planning and post-operative predictions of craniosynostosis outcomes. However, input factors can influence the prediction of skull growth and the pressure on the growing brain using this approach. Therefore, the aim of this study was to carry out a series of sensitivity studies to understand the effect of various input parameters on predicting the skull morphology of a sagittal synostosis patient post-operatively. Preoperative CT images of a 4-month old patient were used to develop a 3D model of the skull, in which calvarial bones, sutures, cerebrospinal fluid (CSF), and brain were segmented. Calvarial reconstructive surgery was virtually modeled and two intracranial content scenarios labeled “CSF present” and “CSF absent,” were then developed. FE method was used to predict the calvarial morphology up to 76 months of age with intracranial volume-bone contact parameters being established across the models. Sensitivity tests with regards to the choice of material properties, methods of simulating bone formation and the rate of bone formation across the sutures were undertaken. Results were compared to the in vivo data from the same patient. Sensitivity tests to the choice of various material properties highlighted that the defined elastic modulus for the craniotomies appears to have the greatest influence on the predicted overall skull morphology. The bone formation modeling approach across the sutures/craniotomies had a considerable impact on the level of contact pressure across the brain with minimum impact on the overall predicated morphology of the skull. Including the effect of CSF (based on the approach adopted here) displayed only a slight reduction in brain pressure outcomes. The sensitivity tests performed in this study set the foundation for future comparative studies using FE method to compare outcomes of different reconstruction techniques for the management of craniosynostosis.

Predicting and comparing three corrective techniques for sagittal craniosynostosis

“…This could have been attributed to predicted ICV measurements, particularly at postoperative time points. The simulations were run by increasing the ICV to an ‘average’ value at a specific age based on the literature and our previous studies 34 , 38 . However, when comparing the FE results vs. the average ICV of the patients considered in this study at 9–12 months of age, there was a 20% difference between the two (based on the SAC technique).…”

“…The overall morphology of the skull, spring displacement, the pattern of bone formation across the calvarial, and the level of contact pressure that each technique imposes on the growing brain (here, the intracranial volume) was investigated post-operatively. Note the generic preoperative model used in this study was described and validated in detail elsewhere 34 .…”

“…A previously described model was used for this study 34 . In short, CT data of a pre-operative sagittal synostosis patient at 4 months of age was obtained from the Hôpital Necker – Enfants Malades Craniofacial Surgery Unit (Centre de Référence Maladies Rares Craniosténoses et Malformations Craniofaciales CRANIOST, Paris, France).…”

“…All materials were defined as linear isotropic. Bone, ICV, suture and craniotomy properties were assigned an elastic modulus of 421 MPa, 10 MPa, 30 MPa and 0.3 MPa, respectively 32 , 34 , 36 , 37 . Sensitivity tests were carried out which varied these stiffnesses initially (see: Supplementary Table S1 & S2 ) to achieve the target craniotomy widening (i.e.…”

“…These changes in the elastic modulus of the bone/newly formed bone were estimated based on extrapolation of the bone properties that were measured during the development of normal mouse 37 to human (considering ICV growth). A radius of 0.2 mm for every month of calvarial growth was selected for the coronal, lambdoid and squamosal suture formation based on observations in literature 39 , 40 and prior sensitivity studies 34 to predict the timing of closure. The metopic suture and anterior fontanelle were set to completely form by 24 months of age to represent the in vivo scenario 41 , 42 .…”