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Do You Have This Implant in My Size?

Fri, 02/15/2008 - 6:25am

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A breakthrough technology uses CAD functionality to convert CT and MRI scans into individualized, minimally invasive articular replacement systems. The end result, customizable orthopedic implants, could become much more commonplace for patients who require this type of treatment. This article examines the proprietary technology that is at the heart of enabling this to be possible.

By John Slamin and Dr. Daniel Steines
Personalized medicine is one of the fastest growing trends in the healthcare industry. While this trend has mainly been seen in the drug sector, medical device manufacturers have also recognized the benefits of individualizing their products to meet the needs of different patient groups. The orthopedic implant manufacturers have recently launched implants optimized for different genders or geographies. While these are a step in the right direction, technological advances now provide an opportunity to create implants optimized for an individual.Traditional orthopedic implants, designed for mass manufacture, have a limited range of sizes and shapes. Fitting the implant to the patient required extensive bone resection and shaving to alter the joint to fit the shape of the implant. The surgical technique for a total knee replacement, for example, is invasive and requires sacrificing healthy bone stock in addition to the diseased area. In addition, the hospital had to carry a large range of surgical cutting blocks, spacers, and instruments that would need to be stored and sterilized after each procedure.

Despite the challenges, total knee replacement is one of the most common surgical procedures. It is estimated that 19 million Americans seek medical attention for knee pain according to the American Association of Orthopedic Surgeons and of those, about 500,000 undergo knee replacement surgery.

Customization

This image shows the iFit technology mapping the surface topography of a patient's virtual knee.

iFit Technology, a proprietary technology developed by ConforMIS, converts MRI and CT scans into individualized, minimally invasive articular replacement systems capable of re-establishing normal articular shape and function in patients with osteoarthritis. By starting with imaging data, the approach results in implants that conform to healthy bone or cartilage, and reduces the need for invasive tissue resection. The implant is made to fit the patient rather than the reverse. By designing devices that conform to a patient's unique anatomy, the implants allow the surgeon to resurface rather than replace the joint, providing for far more tissue preservation, a reduction in surgical trauma, and a radically simplified technique.The "image-to-implant" process begins with the patient having a CT or MRI scan, which can be performed with commonly available machines. ConforMIS provides a standardized protocol which ensures the data needed to design an implant is captured properly. ConforMIS then takes the pre-operative CT or MRI data and uses a computer-aided design (CAD) program with the iFit programming to generate a patient specific 3D model of the patient's knee. This process provides the design and manufacturing team with a 3D image of the knee that guides the creation of a precise implant that will conform to that particular patient's anatomy (Figure 1). The design file is used to fabricate the patient-specific implant and custom instrumentation in approximately four to six weeks.

The Proprietary Programming

ConforMIS developed the iFit Technology utilizing advanced medical image processing and surface modeling algorithms developed out of Stanford University and by the company's computer scientists. The algorithms combine pre-operative, multi-planar CT or MRI images into a 3D topographical map of the articular surface to be treated. Based on the extracted patient anatomy, the software then derives the appropriate implant shape and size that restores a patient's healthy anatomy.Design expertise and decision rules gathered from orthopedic surgeons and multi-year cadaveric and patient studies have been embedded in the software. The expert system provides a set of automated processes that guide the design and manufacturing in creating a precise implant that conforms to the individual's knee joint.

How iFit Produces a Knee Implant

The following example provides an illustration of the technology at work for an implant designed to address just the medial or lateral compartment of the knee (a uni-condylar implant). When it is determined that a particular patient is indicated for a uni-compartmental knee replacement, the patient is sent for a CT scan. The CT scan is conducted according to a protocol provided by ConforMIS which prescribes that the full knee and some portion of adjoining anatomy be captured.The imaging center uploads the CT images to a secure web server, which is then imported into the proprietary software. Within the software, the first step is to derive the outer contour of the bone, which is done via the proprietary algorithms referenced earlier. An IGES surface is imported into a standard 3D CAD software package.

This image shows the CAD image of the patient specific iUni femoral component on the distal femur.

With a surface model now residing in the CAD program showing the correct spatial orientation of the knee, the company then determines the type and extent of deformity present in the patient. The iFit process interactively defines the extent of misalignment present in the knee and builds it into the implant and instrumentation, so correction to alignment is designed into the implant.

This image shows the CAD image of the patient specific iUni tibial implant.

Using the patient-specific bone geometry to drive the implant geometry, the process results in a patient-specific uni-compartmental femoral component (Figure 2) in the CAD system. The patient's bone defines the sagittal geometry of the femoral component. A minimal posterior bone cut for the implant is incorporated into the design based on the patient's specific posterior condylar geometry and orientation.

On the tibial plateau, the patient's specific bone profile of their tibia (Figure 3) defines the geometry of the tibial implant. The modular tibial plateau and tibial inserts are designed on a minimal bone cut and provide a smooth articulating surface for the femoral component. Importantly, because the implant is designed for this particular patient, the implant always provides for complete cortical rim coverage, a result that can not be achieved consistently with off-the-shelf implants. The placement of the fixation features for that patient is based on design principles for uni condylar implants.1

This image shows the CAD image of the patient specific femoral iJig used to place the femoral component in the correct orientation.

Finally, the iFit technology also allows for the creation of disposable, patient-specific instrumentation and cutting jigs from the same scan data. The iJig is exactly matched to the implant and to the patient anatomy, radically reducing the number of steps required to size and place the cutting guides, as well as the actual number of cuts (Figure 4). The iJig ensures bone preparations are exactly matched to the position, size, and shape of the implants determined digitally using the scan data. As a result, the iJig provides precision comparable to expensive surgical navigation systems or surgical robots via pre-operative navigation.

Conclusion

Off-the-shelf orthopedic solutions will continue to be a part of implantable medicine for some time; however, as customizable alternatives become more commonplace and affordable, they could surpass traditional treatment options. Technologies such as the iFit system will certainly play a key role in enabling this change to occur.

References

1 Scott, R.D. Total Knee Arthroplasty, Elsevier Inc., 2006.

Online

For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or ConforMIS at www.conformis.com.

John Slamin is the VP of knee implant engineering for ConforMIS. He can be reached at john.slamin@conformis.com. Daniel Steines, MD, MS is the senior VP of research and development for ConforMIS. He can be reached at daniel.steines@conformis.com.

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