Machine Design

SL models guide surgeons during intricate operation

The team of surgeons that recently separated Egyptian conjoined twins, Ahmed and Mohamad Ibrahim, relied on a series of 3D stereolithography (SL) models of the twin's anatomies to prepare for virtually every aspect of the 34-hr surgery.

Anatomical models of the vascular structures of the twins' brains built from selectively colorable RenShape SL Y-C 9300 photopolymer let doctors see the veins' relation to surrounding bones before the delicate surgery. MRI and CT scans supplied data for the models.
Conjoined twins, Ahmed and Mohamed Ibrahim, have lived at the Medical City Dallas Hospital since June 22, 2002 while surgeons planned the complex separation surgery which was completed successfully at the Children's Medical Center Dallas on October 12, 2003.

Numerous models were built from a selectively colorable, medical-grade SL material, RenShape SL Y-C 9300, by Medical Modeling LLC, Golden, Colo. ( The company used MRI and CT scan images to generate intricate models including clear skull sections and red-tinted areas that mapped the maze of blood vessels shared by the twins.

According to Andy Christensen, president of Medical Modeling, "Several dozen models were made of the twins' bones, skin, brains, and vasculature." The photopolymer from Huntsman Advanced Materials, RenShape Solutions Tooling Group, East Lansing, Mich. (, conforms to FDA-approved USP 23 Class VI biocompatibility tests. It also is sterilizable and durable enough to withstand ongoing handling by the medical team.

Nonmedical applications for the photopolymer include prototypes featuring different-colored subassemblies and stress-analysis models incorporating shaded sections. The material has tensile and modulus strengths of 6.5 and 191 ksi, respectively, a 75 Shore D hardness, a glass-transition temperature of 126°F, and a density of 1.18 gm/cm3. The material works with SLA 3500, 5000, 7000, and Viper Si2 equipment.

-- Jean M. Hoffman

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