Medical Device & Diagnostic Industry

ORTHOPEDICS DESIGN >>

Mimics (Materialise; Leuven, Belgium) or 3D Doctor (Able Software; Lexington, MA), which can take slice data and turn them into an STL file that can be brought into a 3-D modeling package. These products are increasingly supporting the ability to import and export STL files.

Example: Design for a Revision
Hip Implant

A designer and manufacturer of orthopedic medical devices was assigned to provide a custom acetabular (hip) revision component for a 70-year-old female patient. The patient’s existing off-the-shelf hip implant was dislocating into her pelvic cavity. She reported increasing levels of pain to her orthopedic surgeon and was unable to bear weight on her hips without the use of crutches.

The patient had a portion of her hip intact, but the acetabular implant had been revised before, and while the stem was part of her original implant, it showed some sign of bone resorption. The surgeon needed to create an implant that was slightly bigger than the present, damaged implant in place. He concluded that an off-the-shelf implant was unlikely to be effective in a case this extreme and requested a custom part from an orthopedic manufacturer.

The manufacturer had recently partnered with a contract manufacturing company to create portions of the implants for unusual cases. The manufacturer specialized in all-digital work flows for creating custom body parts. The process used by the contract manufacturing company exemplifies how new technologies can be employed.

CT Scan Conversion to 3-D Model. The patient had recently obtained a CT scan as part of the diagnostic process.

The company used a software program to convert the medical imaging files into a file compatible with the FreeForm 3-D modeling system. It also cleaned up the image to remove scatter artifacts present in the scan because of the existing metallic implants in place.

Predesign Surgeon Input. After the scan was put into FreeForm, both the orthopedic firm and the contract manufacturer discussed key factors with the surgeon, including surgical objectives, the patient’s challenges, patient-specific design issues such as age and weight,

A VIDEO OF THE FREEFORM MODELING SYSTEM SHOWS THE DIGITAL CREATION OF AN ACETABULAR IMPLANT devicelink.com/mddi/custom_implant

and bone condition. The team also discussed other requirements for the final implant, such as the surgical approach, associated implants to be used along with this custom implant, expected outcome, and other special tools.

Design Refinement. Extensive effort went into refining the 3-D model to suit the patient’s needs. This typically requires one to three weeks, depending on the review process required by surgeons and other parties. In the hip revision case, the design needed to enlarge the size of the patient-specific implant compared with the patient’s existing implant. This is because the new implant would be replacing slightly more of the patient’s existing bone structure. However, the resulting shape of the implant it defined was asymmetrical, and not typical of the normal hip bone structure. A technician modified the model to ensure the most beneficial implant design.

The team used a sculptural CAD

modeling system because it provided modeling speed and flexibility. The system is based on voxel technology (think of voxels as 3-D pixels) instead of mathematics. Unlike traditional CAD, this approach has no topology restrictions and the order of operation does not matter. Additionally, the system uses haptics or virtual touch technology to mimic physical modeling. The designer held what is called a phantom device instead of a computer mouse to work the model. Force feedback is applied as the designer pushes, pulls, deforms, and extrudes virtual clay or putty to create an implant on the computer screen.

The design work required extreme precision and skill from the technician. As is typical of hip, knee, spine, and even cranial implant design, the work on the computer was to create mesh structures that follow an unusual, irregular shape. Creating them requires a deep understanding of anatomy as well as fluency with the design system.

Online Design Review. The team used e-mail and uploaded images of the 3-D model of the implant, rather than wait for overnight shipments to arrive. In some cases, discussion of the design took place via conference call or Web-based review. This way, the surgeon received expedited delivery and was able to make edits to the digital copy, instead of to a less precise, physical one. As the team discussed changes, the designer in charge rotated the model, made minor adjustments quickly, and let the surgeon immediately see his requests on the model.

Although most people are comfortable with an all-digital work flow, some surgeons feel more comfortable seeing a physical model of the proposed implant before moving forward with manufacturing preparations. By reserv-