MANAGED CARE June 2009. ©MediMedia USA
Unicompartmental joint operation doesn’t require storing various joint fittings to accommodate each patient’s anatomy
Many of us remember a Dennis the Menace movie where Dennis breaks the two front teeth off of a set of Mr. Wilson’s dentures and replaces them with two pieces of Chiclets gum that were readily available but did not quite fit.
Although funny as a story line, none of us would like a similar approach for replacement of a major joint. However, that is exactly what is currently done in orthopedic practice for joint replacement, which is technically termed an arthroplasty.
Using knee replacement as an example, the surgeon assembles off-the-shelf pieces of metal and plastic (the femoral component and the matching metal-polyethylene tibial component). The pieces don’t quite fit correctly, so the surgeon uses the corresponding bone cutting templates (also termed instruments). He cuts the bone in the femur to match the profile of the metal piece, attaching the two pieces. The process is repeated on the tibia — thus resurfacing the bones of the knee. But as in the case of Mr. Wilson’s dentures, there are issues.
There are many variables considered when evaluating a patient for a knee replacement, including the extent and location of the damage, the size of the patient, the angles of force, the size and shape of the knee joint, and the strength of the bone that will support the weight of the patient once the hardware is installed. All of these variables result in a need for a large selection of off-the-shelf components in order to have the correct one available to the surgeon in the operating room at the time of surgery.
The orthopedic industry has faced this challenge by creating a large array of joint replacement parts, but even that is somewhat of a work-around as the number of variables in human anatomy dwarfs the number of available joints, even when accounting for the different manufacturers. Also, since the actual replacement part is usually not chosen until the time of surgery, the hospital must stock a large variety of hardware pieces and the matching surgical template instruments used during surgery.
In addition, since a total knee replacement results in the loss of a huge amount of bone, the patient often faces a dilemma. Revision surgery, years later when the artificial surface inevitably wears out, will be quite a challenge due to lack of available bone to physically support the second implant. Surgeons have long recommended that young people with damaged knees avoid an arthroplasty as long as possible because they may not be able to have one later in life when the first one wears out. Given the epidemic in knee arthritis in ever younger patients due to the obesity epidemic, this results in many years of relative disability for those who develop arthritis early in life.
One work-around that can result in less bone being removed is the recent development of partial or unicompartmental knee replacements. This allows for the replacement of just the inner or outer part of the knee joint, sparing the undamaged area. This is relatively common, as the knee joint does not always wear out evenly on both sides.
The advent of a unicompartmental replacement device has addressed this issue with some success but is also hindered by the rather limited number of sizes and shapes of hardware.
A better approach would be the custom manufacture of a replacement, as done by dentists to crown a fractured or badly damaged tooth. This results in a resurfaced tooth that matches the anatomy of the surrounding teeth and matches the opposing teeth in a way that spreads the considerable forces evenly, just as nature intended.
For those of you who have not had the pleasure of experiencing a crown, I will explain. The dentist removes the top of the tooth and grinds the remaining portion into a shape that allows the replacement to be attached. The dentist them makes an impression of the new shape of the tooth as well as of the surrounding and opposing teeth. A temporary crown made from plastic is attached with a temporary adhesive. The temporary crown is not made of material hard enough to withstand years of chewing.
The impression is then sent to a dental lab where a permanent, durable porcelain crown is manufactured, which is installed a few weeks later. The end result is a tooth that can withstand the high forces of chewing and matches the exact shape needed to prevent damage to the opposing teeth.
Recent developments do away with the lab’s part of the process. The dentist manufactures the permanent crown and affixes it in the same session that began with preparation.
And a related approach is being utilized with knee replacements, since no one would suggest a temporary knee replacement with a second surgery a few weeks later!
A small, privately held company, Conformis, has solved this problem. By obtaining an MRI or CT scan, a detailed three dimensional image can be created and fed into a computer-aided design application, and a direct digital machining process can create a custom unicompartmental joint replacement. This is made from a cobalt-chromium-molybdenum metal blank designed to replace just those areas of the knee that need to be replaced.
This approach may be applicable to 20 percent to 45 percent of patients who otherwise would face a more extensive surgery.
The image can be obtained from just about any of the MRI or CT machines being used in the United States. The process takes about six weeks from the submission of the digital images. The company also provides the customized cutting template instruments and placement guides needed by the surgeon to assist in planning the bone cuts, thus eliminating the need for the operating room to have all of the extra instrument trays available.
The end result is a personalized custom-machined resurfacing of the knee that requires less surgery time, less bone loss, and is anatomically matched to the individual. This process also eliminates the need for a huge stockpile of inventory and the resultant re-sterilization costs associated with the traditional approach.
As is the norm for joint replacement components, a 510(k) premarket notification process that uses a substantially equivalent determination was used by Conformis to obtain FDA approval for this new technology.
Conformis holds 250 pending or approved patents covering the image processing, imaging software, implant design, surgical techniques and instrumentation, which should rapidly result in other joint resurfacing products being submitted to the FDA.
Although the cost of the new technology is about the same as the older technology, the total costs to plans may increase. The Conformis approach is more attractive to younger patients because it is less invasive. Since less bone is taken, a second surgery decades later in life is now more possible. For those of us in the “boomer” generation, this advancement could not have come at a better time, again proving that Tomorrow’s Medicine is here today!