Computational modelling for personalised treatment of congenital craniofacial abnormalities

Periodic Reporting for period 3 - CAD4FACE (Computational modelling for personalised treatment of congenital craniofacial abnormalities)

Période du rapport: 2021-03-01 au 2022-08-31

Craniosynostosis is a group of congenital craniofacial abnormalities affecting 1 in 1,700 newborns and consisting in premature fusion of one or more cranial sutures during infancy (Fig. 1). This results in growth restriction perpendicular to the axis of the suture and promotes growth parallel to it, causing physical deformation of the craniofacial skeleton, as well as distortion of the underling brain, with detrimental effects on its function: visual loss, sleep apnoea, feeding and breathing difficulties, and neurodevelopment delay. Conventional management of craniosynostosis involves surgery delivered by excision of the prematurely fused sutures, multiple bone cuts and remodelling of the skull deformities, with the primary goal of improving patient function, while normalising their appearance. Craniofacial remodelling surgical procedures, aided by internal (stainless steel springs, Fig. 2) and external (rigid external distractor, RED frame, Fig. 3) devices at Great Ormond Street Hospital (GOSH), have proven functionally and aesthetically effective in correcting skull deformities, but final results remain unpredictable and often suboptimal because of an incomplete understanding of the biomechanical interaction between the device and the skull.

The overall aim of this grant is to create a validated and robust computational framework that integrates patient information and device design to deliver personalised care in paediatric craniofacial surgery to improve clinical outcomes. A virtual model of the infant skull with craniosynostosis including mechano-biology regulation will be developed to simulate device implantation and performance over time, and will be validated using clinical data from patient populations treated with current devices. Bespoke new devices will be designed allowing for pre-programmed 3D shapes to be delivered with continuous force during the implantation period. Patient specific skull models will be used to virtually test and optimise the personalised devices, and to tailor the surgical approach for each individual case.

PATIENT SPECIFIC COMPUTATIONAL MODELS
Craniosynostosis surgical outcomes and the extent of head reshaping depend on skull bone properties. However, limited information is available on the bone structure and material properties in craniosynostosis children. To help predict final head shape changes during craniofacial procedures, bone samples discarded from surgery were collected from infants who underwent sagittal cranioplasty (Fig. 1). Bone samples were imaged at high resolution and mechanically tested: not only the child skull morphology, but also the bone structure affects outcomes. Patient age was not a determinant of bone properties, indicating the need for tailoring patient-specific treatments to other parameters.
Head growth curves specific for each craniosynostosis disease were defined to account for normal size changes that happen during childhood in the models. A 3D portable surface scanner, less invasive than computed tomography (CT), can provide reliable shape information, allowing close and frequent follow-up in these patients after surgery. Head surface captures allowed building of a statistical shape model to monitor head growth and shape changes due to spring implantation over time.

Since 2008, >400 craniosynostosis (Fig. 1) children were treated with stainless steel springs at GOSH. Retrospective data were analysed to create and validate computational models of spring assisted cranioplasty that can be used for surgical planning and to design new devices.
Sagittal – Analysis of the clinical database resulted in the definition of the spring dynamic behaviour in situ, with springs fully opened after 10 days from insertion. A computational model of sagittal spring cranioplasty was created and validated by replicating patient specific skull anatomies, surgical cuts, spring positioning and population specific bone material properties. By comparison with spring opening measurements gathered during surgery and at x-ray follow ups, the model reliably predicted head reshape.
Unicoronal – The intrinsic asymmetry of this condition makes it complex to correct using springs. A parametric computational study was designed to assess the effect of different osteotomy locations, dimensions and force requirements, and to evaluate the potential application of new nitinol devices, without osteotomies. A significant amount of force is required to reshape the skull without osteotomies; thus nitinol may not provide sufficient expansion to correct the skull deformity.
Lambdoid – Long-term aesthetic surgical outcomes may be suboptimal in this condition due to rapid skull growth at early ages, bone and suture property changes, and limited deformation vectors provided by the springs. Three lambdoid patients were simulated including skull growth and suture ossification modelling over time, showing clinically acceptable surface deviations from postoperative image skull reconstructions.
Posterior vault expansion (Fig. 2) – Similarly to the sagittal population, a computational model was developed for these patients. Spring kinematic was retrieved from follow-up x-rays, showing slower expansion in these cases compared to sagittal procedures. Simulated intracranial volume and craniofacial ratios were comparable to those measured from clinical images.

Since 2005, >100 patients underwent frontofacial advancements with RED (Fig. 3) at GOSH. Shape changes due to this surgery are complicated to model and predict because of the intrinsic complexity of the facial anatomy, the surgical techniques that act on the bony structures and the natural child growth. Thorough quantification of current treatment outcomes is required. Facial shape 3D changes together with bone displacements were measured in a small cohort of cases with available pre and post-operative CT data, using a semi-automatic method developed to consider the wide variability in face shape features, and the changes due to surgery vs. growth in these patients.
In parallel, a computational model of the RED frame and an instrumented prototype are under development to assess the behaviour of the device in the interaction with the patient skull, and measure the forces exerted during frontofacial distraction.

NEW NITINOL DEVICES
The experience gained with stainless steel spring cranioplasty was leveraged to design a new nitinol spring that can exert lower forces on the patient skull, more desirable for head reshaping. Computational models enabled fine-tuning of material properties and geometry. Prototypes were mechanically tested and for sterilisation. The computational models developed for the sagittal stainless steel spring were used as an in-silico clinical trial for the nitinol devices, showing superior outcomes. Implantation of the new nitinol springs in 10 patients requiring sagittal cranioplasty was approved by GOSH Innovation and Development Committee – on hold due to COVID-19 restrictions.
Further research on nitinol devices will first explore springs for posterior vault expansion, and then meshes for unicoronal/lambdoid craniosynostosis.

Structural and mechanical properties of the infant bone with craniosynostosis.
Validated computational model of sagittal synostosis spring assisted cranioplasty.
Development of a computational model for spring assisted posterior vault expansion.
New nitinol springs ready for first-in-child in sagittal craniosynostosis.

Fig. 3 - Rigid external distractor (RED frame, KLS Martin, Germany).

Fig. 1 - Different craniosynostosis types affecting different skull sutures.

Fig. 2 - Stainless steel springs in posterior vault expansion: pre-op (red) and post-op (yellow).

Periodic Reporting for period 3 - CAD4FACE (Computational modelling for personalised treatment of congenital craniofacial abnormalities)

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