There is a buzz in the orthopaedic community regarding how we size and position knee replacement implants, in essence challenging how we perform knee replacement surgery. Although touted by some as a paradigm shift in technique, I believe it represents an evolution of technologies, such as computer assisted navigation, presently used in our clinical practice.
Late last fall, Surgeons at Brantford General Hospital “made Canadian medical history” (-link with video), by becoming the one of the first centres outside the United States to perform a custom-fit knee replacement surgical procedure using the OtisMed Shape Matched Technology (this phrase is a trademark of OtisMed, a company purchased by Stryker Corporation) (-link).
Simplified, this technique as I see it involves the following steps:
A detailed MR scan of the arthritic knee is performed, using a specific protocol.
The MR scan information is sent to Otis Med in California and analyzed by proprietary software that allows computers to reconstruct a 3-dimensional model of what the knee looked like in a pre-arthritic state.
The software then matches implants from the Styker Triathlon system to the surface contours of the pre-arthritic model (the concept of shape matching).
Next, the software calculates where the diseased knee would need to be cut, the size of the implant required, and precisely where the implant should be placed to match the computer generated model of the knee in its pre-arthritic state.
Customized cutting guides (jigs) are then fabricated and shipped to the location where the actual surgery will be performed.
The surgeon then uses the customized guides to perform the knee replacement procedure, replicating the bone cuts and implant placement calculated by the software.
Prior to the availability of Shape-Match™ technology, surgeons have traditionally used cutting guides and techniques to make every replaced knee straight mechanically, aligned and centred on a plumb line running from the centre of the hip joint to the center of the ankle joint (the mechanical axis). To do so often requires balancing of the surrounding soft tissues by releasing contracted ligaments and tightening stretched knee ligaments. This balancing, the exact sizing and the final placement of the implants (especially with respect to rotation), relies largely on the experience and intra-operative judgment of the surgeon.
Next month a group of local surgeons and I will get our first “hands-on” exposure to this system. As we further analyze this radically new way to position knee implants I must say I am intrigued by the OtisMed philosophy. Their concept places critical importance on the balance and range of motion of the knee, with axial alignment secondary and almost an after thought. Traditional techniques stress axial alignment and soft tissue balance above all else.
Some important points to remember:
This technique is more expensive to perform AND requires a MRI scan pre-op
Shape-Match™ technology relies heavily on the software’s ability to predict what the knee looked like in it’s pre-arthritic state.
The actual implants are the same as we currently using here at Guelph General Hospital (Stryker Triathlon -link)
The surgical exposure is the same as we currently use, minus the releases (see above) that are sometimes required for complex knee deformities.
There are very few peer review published articles supporting the Otis Med concept and little in the way of long-term follow-up studies.
“Traditional” alignment techniques have an excellent track record with success rates of 90%, 10 to 20 years after implantation. It will be a long time before we know whether this new technique is able to match or exceed these results.
The debate, which is just heating up within the orthopaedic community, promises to be spirited. I will post after I’ve had further exposure to this system.
K McKenzie MD, FRCS(C)
Orthopaedic Surgeon
