Rapid manufacturing methods are gaining ground in the production of custom-fit medical implants. “3D printing is becoming an increasingly popular method for design and direct digital manufacturing (DDM) where customization and quick turnaround are important,” says Jon Cobb, vice president and general manager for Dimension 3D Printing Group, a business unit of Stratasys, Eden Prairie, Minn. (dimensionprinting .com). “This is especially true in the medical-device field, where 3D printing is increasingly used in custom medical devices such as jigs and implants.”

Our company is one of the few that uses CT scan data in the machining of patient-specific knee replacements and the additive manufacturing of the associated jigs and fixtures. Of our three brands of knee-implant packages, two include parts 3D-printed by several in-house Dimension (business unit of Stratasys) Elite machines. Our iUni system, for instance, is an early intervention, minimally invasive partial knee replacement intended to relieve arthritic pain. Included in the package are the implant and a surgical drill guide or jig, both manufactured from a CT scan for a particular patient.

How it works

A patient is first sent for a CT scan. This is conducted to our recommendations, which suggest capturing the full knee and some portion of the hip and ankle. The scanning center uploads the CT images to a secure Web server. Images are then imported into our proprietary software that derives the outer contour of the bone and generates its surfaces.

With the bone's surface model showing the spatial orientation of the knee, the software determines interactively defines the extent of misalignment present in the knee. The profile of the patient's tibia defines the geometry of the tibial implant. Tibial components are designed for minimal bone cut and provide a smooth articulating surface for the femoral bone.

Because the implant is designed for a particular patient, it provides complete cortical-rim coverage, a result not consistently possible with off-the-shelf implants. The placement of the fixation features for a patient is based on design principles for unicondylar implants.

Finally, iFit uses the same scan data in the DDM of the disposable instrumentation and cutting jigs, built with the Dimension machines. Because the cutting jig, called iJig, matches the patient's anatomy, it significantly reduces the number of steps required to size and place cutting guides and the number of cuts. The iJig ensures that bone preparations also exactly match the position, size, and shape of the implants. As a result, the iJig provides a graphical way to provide pre-operativeprecision.

The upshot is a patient-specific femoral component in the 3D CAD system. Here, a minimal posterior bone cut of about 3 to 4 mm is incorporated into the design to facilitate implant placement based on the patient's posterior condylar geometry and orientation. We 3D-print jigs and fixtures while a contract manufacturer machines the implants.

Femoral and distal regions

Femoral components are machined from wrought alloy cobalt chrome conforming to ASTM 1537, a nonstandard material for knee femoral components. Typically, such components are manufactured from cast cobalt chrome. We specify the wrought alloy for its better machinability and higher strength. Polishing characteristics for the wrought alloy are equivalent to those of the cast material.

The fit is exact because the sagittal geometry of the femoral component was derived from the patient's CT scan. In the distal region, the component sits on the subchondral bone and the remaining cartilage must be removed from this area. The only bone removal on the femoral side is a small posterior cut not more than 5-mm thick.

The iJig guides a surgeon when drilling holes for two fixation lugs which are angled forward about 3° from the posterior cut. This angle feature aids with the insertion and provides compression to the cement in the posterior cement pocket of the component.

Tibial component

The tibial implant is a modular design, with a geometry also defined by the patient's bone, to allow for load bearing on the cortical rim. Fully assembled tibial components are 9 or 11-mm thick, including both the metal-backed tray and poly insert.

The tibial tray is made from the same wrought alloy as the femoral implant. The inset pocket is highly polished to reduce the insert's backside wear. The insert snap has a posterior slide and an anterior elastically deformable snap. The surgeon slides the insert into the posterior portion of the tray and then presses down in the anterior region to engage the snap. The snap is not loaded by normal knee forces.

The 7 or 9-mm high tibial inserts come in compression-molded GUR 1020 ultra-high molecular weight polyurethane. Total tibial component height is a result of the poly insert height combined with the 2 mm high tibial tray base. The joint geometry is generally flat, which lets the femoral component seek its own place for bearing. This provides a larger “landing zone” for the femoral component. Vaporized hydrogen peroxide provides sterilization. It avoids the material-property issues associated with gamma irradiation. Many studies have shown the adverse effects from gamma irradiation.

DDM FOR JUST-IN-TIME

ConforMIS says Dimension machines help it support its business model of just-in-time manufacturing. It does so by helping eliminate a large amount of inventory. Once the company packages and sterilizes the implants and jigs, they are shipped to hospitals. Total turn-around time is six to eight weeks.

Other applications for RP in medical include:

• Medical equipment such as blood-pressure devices, air and fluid devices,molecular diagnostics instruments, and surgical simulator systems.

• Thermoforming for packaging design and for plastic pharmaceutical packaging.

• Prototypes for orthopedic devices.

• Models of bones and organs for pre-surgery review.