Designing Custom Implants More Efficiently

All medical product design processes are complex, but creating custom implants and prosthetics further lengthens the workflow to assure a finished component with perfect fit, comfort and aesthetics. Because custom implants and prostheses serve extreme cases — war wounds, car accidents, oncology cases or necessary revisions for off-the-shelf implants — they often involve irregular, hard-to-define shapes for devices surgeons and their patients need in a hurry. The process of creating implants tuned to a particular patient's morphology requires exceptional speed, and at the same time, the utmost in precision.

Biomet Inc. is a worldwide leader in the design and manufacture of products for joint replacement of the hip, knee, shoulder, elbow and other small joints. Its Patient-Matched Implants (PMI) business unit employs 30 professionals who develop more than 1,000 customized highly durable joint replacement components annually.

Biomet's engineers are continually working to streamline the design process and leverage vendor innovations to better serve patients. For the past few years, Biomet has relied on a design process for PMI implants using STL file types in which SensAble's FreeForm® 3D modeling system is an integral component. Biomet has used FreeForm to deliver a precise fit for a wide range of custom components — from triflange acetabular cups, patellofemoral replacements and rotating hinge knee replacements, all the way through to complete hip replacements.

Unlike traditional CAD systems which use spline-based models, FreeForm is based on voxel geometry (3D pixels), which delivers unparalleled modeling speed and flexibility for creating complex, highly detailed organic shapes, such as those found in custom implants. FreeForm also includes the ability to import and export STL files — a file type commonly used for data exchange in working with scan data, rapid prototyping and milling. Additionally, FreeForm's use of haptics or virtual touch technology provides force feedback to the user, making direct modeling in 3D faster than ever before. The designer uses a PHANTOM® haptic device, instead of a computer mouse, to push, pull, deform and extrude virtual clay or “putty” as he/she creates an implant on the computer screen.

'Digital Putty' For Precise Organic, Irregular Shapes

Designing the 3D model for a proposed implant can take up to one week. By working with FreeForm, Biomet engineers are able to shave at least several days off this process. The following example illustrates the workflow.

An orthopedist sought a patient-matched hip implant for a 52-year-old female suffering from development dysplasia of both hips and progressive radiolucencies in her superior acetabulum, as well as in her femoral stem. The custom implant was a replacement after a previous resection arthroplasty. After careful evaluation, the physician specified a custom-designed triflange (hip joint) cup with porous coating on the back side, which would be matched to an off-the-shelf femoral replacement stem. A precise fit was essential to the success of the procedure.

Biomet's team used FreeForm once CT image data was segmented (Figure 1, below) and reconstructed as a three-dimensional model in the computer. Biomet's engineers then imported the model into FreeForm to create the precise bone geometry of the triflange cup on top of the initial patient scan.

In the past, Biomet literally would have had to place putty onto a physical model created from the digitized CT images in an effort to design the implant to match the patient's specific bone anatomy and contour it to match the bone surface. Now, with FreeForm, Biomet engineers use their sense of touch while working with “digital putty” to quickly model the implant in an all-digital environment (Figure 2, page S23). These medical designers/engineers literally feel the model as they push putty into shapes that perfectly match the desired regions of the implant. As a result, Biomet's engineers can work more efficiently.

Emailed 3D Mark-ups —Going the Distance

Normally at this point in the process, after working with FreeForm, Biomet engineers would create a draft plastic model of the implant using a rapid prototype machine. Physicians like the comfort factor of seeing an actual model to size up the proposed implant or prosthesis and need the ability to approve a final design. Biomet would deliver the model to the surgeon, who then would order any necessary adjustments to best fit the patient.

Yet, as the use of e-mail by surgeons continues to increase, the possibilities abound for an even more streamlined workflow that eliminates waiting for a marked-up plastic implant to arrive — or interpreting edits scrawled on plastic that could interject possible errors.

By using little-known support for STL files in Adobe® Acrobat® 3D, Biomet engineers now are eliminating an additional few days from the approval process — and moving toward an all-digital, e-mail-driven workflow for even greater time-savings.

With the advent of Adobe Acrobat 3D several years ago, CAD/CAM professionals in many vertical markets began implementing digital approval processes where their clients can view and annotate 3D designs online and e-mail back their edits. Adobe Acrobat 3D allows a ready review of files by anyone who downloads the free Acrobat 3D Reader software — avoiding the need for doctors and other casual review team members to download costly CAD tools, although the purchase of Adobe Acrobat Professional is required for annotation. Acrobat 3D readily imports STL files and ensures the physician sees the model exactly as it was created in FreeForm.

Now, once Biomet's team completes its 3D models in FreeForm, design engineers can simply export an STL file that can be used to create a 3D PDF and then e-mail it to the physician (Figure 3, above). From there, the doctor can view the model in 3D, rotate it, annotate it if they have the appropriate software and save his or her edits in the native PDF format. Or, if physicians prefer, they can view the model in 3D on-screen while speaking on the phone with Biomet's patient-matched implant staff — where both parties are viewing the same model.

“It's amazing the efficiency that we can bring to the process with FreeForm and a 3D PDF-based approval process,” says Troy Hershberger, director of the patient-matched implants division at Biomet. “With a recent project, our team started with a CT scan in the morning, designed a triflange implant in a few hours, then e-mailed a 3D PDF copy of the bone and implant to the doctor by the end of the day. Before FreeForm, digital design of one triflange implant took a full day. Since implementing FreeForm, single digital triflange designs take just one- to one-and-a-half hours. This is a tremendous efficiency improvement.”

“While physicians' need for a physical copy isn't going to completely go away,” Hershberger says, “with the right tools in our tool kit, we can see our approval process going increasingly electronic and more streamlined than ever before.”