In one example of the software’s use, several times each month, as many as 30 surgeons and medical experts gather in Hershey, Pa., at Penn State’s College of Medicine (pennstatehershey.org) for Surgical Innovation Group (SIG) meetings. The purpose of the meetings is to generate a vibrant discussion about challenges in day-to-day surgical and medical care. The discussions are a process that generates ideas, projects and eventually devices that have been vetted for their value and likelihood to reach the marketplace and benefit patients. Participating in the discussions is Thermoplastic Products Corporation (TPC) of Hummelstown, Pa., whose principal is Barry Fell. When the topic of craniosynostosis, or grossly misshapen infant heads, arose recently, Fell had an elegant answer that typifies the way volumetric CAD is advancing medical treatment.
In craniosynostosis, the baby’s cranial bones are prematurely fused and, fueled by the underlying brain?growth, the? skull becomes misshapen. The premature fusion of the skull bones can also lead to increased intracranial pressure on the brain, swelling of the optic disc and vision loss, and potential long-term developmental problems. The only treatment option is surgery. The skull bones are cut to release the fused structure, and then they are reshaped and placed back on the patient much like large puzzle pieces. The surgeon accounts for the increased space needed for the brain to grow by careful bone placement and fixation.
In the past, surgeons would plan craniosynostosis surgeries by reviewing infants’ CT scans and formulating a generic game plan. However this approach would offer only an approximate guide to the infant’s unique anatomy, and more importantly, it would defer determining much of the surgical plan until the patient’s cranium was open and exposed during surgery.
Since most craniosynostosis patients receive a CT scan of the head preoperatively, Fell decided to leverage volumetric CAD and FEA technology to help both surgeon and patient.
A surgeon working on a craniosynostosis case delivered a conventional CT scan from an infant patient to TPC, which converted it into an STL format. Engineers imported the STL file into Freeform and began by cleaning up “noise,” or artifacts, from the original CT scan to more clearly identify the cranial fissure lines. Engineers then used Freeform’s AutoSurfacer to create a surface mesh — a needed step to ensure the evolving design would accurately capture the varying thicknesses of the skull.
In past attempts, Fell needed between two to four hours to create a mesh file of a curved, organic shape with varying thickness in a traditional CAD package. Other software could not surface the parts because of their complexity. Therefore, the attempt would go on for a long time and then fail.
But by using Freeform, Fell’s team cleaned up the STL file, performed the autosurfacing, and completed a final design in about 30 minutes. Freeform’s AutoSurfacer feature worked well on even the most unusual shapes that TPC needed.
For the corresponding surgical guide, TPC’s team took the Freeform file and used it as their source for an FEA projection of the baby’s head shape if it was allowed to grow several months without surgery. From FEA predictions of how the bones would be shaped in the future, surgeons had a highly accurate, digital model from which to plan specific surgical bone cuts to allow the optimal reassembled cranium.
Working together over a computer, the surgeon and TPC engineer then “fixed” the skull by performing surgical cuts on the bone — using Freeform — and placed the bone back into the anticipated post-surgical position. TPC then adjusted the design file, repeated the FEA, and confirmed that the new shape would have uniform stress predictions throughout the skull so that it would maintain the desired shape as the baby’s brain grows.
Once engineers determined the final skull shape, Freeform helped TPC design a surgical template for the surgeon to guide the bone cuts for the ideal skull shape. The surgeon actually lays a patient-specific template on top of the bone to guide the cuts.
After surgeon review and approval of the surgical guide, TPC confirmed final placement of the patient-specific template on the skull model and predicted the future shape. The team then used a rapid-prototyping machine to build the disposable template out of an FDA-approved biocompatible resin for the actual surgery.