Why has knee replacement surgery traditionally been performed using off-the-shelf implants?
Knee replacement surgery has traditionally been performed using off-the-shelf implants because of the limitations of technology. As with most industries, evolution does occur.
Of course, in a perfect world, an implant would always be made precisely to fit a patient. But in the past, technology was not available to make individualized implants.
In the early days of knee replacement, knees came in only a few sizes. As time went on and surgeons began to realize the importance of size, these sizes were expanded to 6 or 7 different sizes. Next, came gender-specific knees, also in different sizes, and also off the shelf.
The downside of this system of using off-the-shelf implants is that it is not an efficient way of delivering implants. More sizes that are required to get closer to fitting more patients also require more inventories of implants and instruments—it gets to be very expensive and complex.
How does the knee anatomy differ from patient to patient?
Knee anatomy differs tremendously from patient. I would say that just as patients appear different on the outside and come in all shapes and sizes, so do their knees. It is definitely not as simple as making a male knee and female knee, or a large knee and a small knee.
What we strive to do as knee surgeons is do our best to recreate the anatomy, but often times this is challenging and hard to anticipate with existing technology.
What are the main reasons why patients are not happy with the results of their knee replacement surgery using off-the-shelf implants?
Studies show anywhere from 15 to 39 percent of patients who undergo traditional total knee surgery are not satisfied with their outcome. Numerous studies have tried to elucidate why.
A small amount of these patients have a complication which is understandable, but the bulk of these unsatisfied patients have a knee surgery that on the surface appears to have gone well. Many of these patients have persistent pain and many are not happy with the way their knee feels and functions.
Pain, we now know, can be related to fit of the implant, positioning of the implant, and stability of the implant (how the implant interacts with the patient’s ligaments, tendons, etc). Studies have shown that an implant being too large by just 3 millimeters can double the risk of having pain.
A lot of patients with painful total knee replacements have some mal-rotation of their implants and some have inappropriate tensioning of their ligaments by the implant leading to a "wobbly" knee.
Some patients have higher expectations for what they want their knee to feel like. As surgeons, we are challenged with managing these expectations while trying to improve our techniques and devices we use to rise to meet these expectations.
Zimmer Holdings, Inc., and DePuy Synthes claim to have introduced personalized implants. Please can you outline how these differ from traditional off-the-shelf implants?
To address the large number of patients that are not satisfied after traditional total knee surgery and to try and meet some of the expectations of more demanding patients, many knee companies are attempting to improve their device offerings to surgeons and patients.
The latest two attempts are the Zimmer Persona knee system and the Depuy/Synthes Attune knee system. These appear to be a further evolution of knee devices, but they remain off-the-shelf implants, rather than true personalization. We are seeing companies continuing to increase the number of sizes offered—these numbers are now in the double digits.
These companies are using their broad experience, databases of images, and other means to create off-the-shelf implants that better fit more patients. Both systems continue to offer patient-specific jigs (cutting guides), but the ultimate implant that is placed is not made for the patient.
ConforMIS has recently introduced the iTotal G2. Please can you describe this implant system and how it differs from others on the market?
The iTotal G2 system from ConforMIS is the first and only true patient-specific total knee system. The company has harnessed technological advances in imaging technology and manufacturing processes to create a knee that is made for each patient.
Patients obtain a CT scan and the images are sent digitally to the company. An implant is then made based on that individual’s anatomy, so it is sized and shaped exactly for that patient.
In addition, all the instruments are "pre-navigated" based on the CT scan and are single use and disposable. The "kit" is packaged in a sterile box and shipped out to the hospital near the patient’s surgical date.
How easy to use is this system?
The system is fairly straight forward to use. All jigs are numbered and used in sequential order. The setup of one box and one reusable tray is very simple compared to the 6 to 9 trays needed for traditional total knee replacements.
The jigs are all pre-navigated based on the CT referencing the bone. It is quite impressive to place the jigs and ultimately the implant and see how everything matches the patient’s anatomy and fits precisely. It leaves little room for error.
Is there a learning curve associated with using this system?
As with all surgical techniques and procedures, there is a learning curve. However, the system is quite intuitive.
The learning curve can be dramatically reduced by taking advantage of the opportunities provided by the company such as cadaver courses, surgeon visitations and the vast print and video resources offered.
If the surgeon goes into their first case properly prepared, they can feel like they have done the case many times before.
Does this system address the drivers of patient dissatisfaction with traditional knee replacement surgery?
The benefits of creating a true patient-specific knee system is that it potentially addresses, what I believe, are the driving factors of patient dissatisfaction after traditional total knee surgery. The implant being too big or two small is no longer an issue, it fits right every time. The pre-navigated guides help to prevent against mal-aligning or mal-rotating the device as it is implanted.
Postoperatively, since the surgical technique does not require drilling holes up the femur and because the implants are covering all the cut bone, I am seeing less blood loss, reduced risk of transfusion, and less swelling of the knees.
Because the shape of the knee is more accurately reproduced, I am seeing patients function better, sooner, and at the end of the recovery process have knees that feel and move more natural. Some early scientific evidence is beginning to emerge to validate these findings.
In what ways could this patient-specific knee implant system be improved?
Patient-specific knees will continue to improve and evolve by being able to address more complex cases and even distorted anatomies. As technology and manufacturing techniques evolve, even better jigs and smarter implants can be made.
How do you think the future of knee replacement surgery will develop?
I believe that patient specific knees are the future of knee replacement, but here now. The major manufacturers have realized this and validated this by creating their systems of "off-the-shelf personalization.”
The interesting thing about going to a “true” patient-specific knee system, like ConforMIS has done, is the way the company can respond and improve the devices in an unprecedented timeframe. Most traditional knee implants require so much inventory of instruments and implants, that any small change is a very large production and expense. With patient-specific technology, implants can continue to be updated and improved very rapidly as new data or evidence comes forward.
Do you think personalization will play a big part in the future of medicine?
Yes. As patients get more demanding, they are expecting better devices. Personalization makes sense to them. Why wouldn't a patient want a device that fits them right?
Where can readers find more information?
Surgeons can find more information at www.PatientSpecific.com. Patients can find more information at http://MyConforMISknee.com.
About Dr Martin
Dr. Gregory M. Martin is a board certified orthopedic surgeon who specializes in hip and knee arthritis and other painful disorders.
He has advanced fellowship training in reconstructive surgery of the hip and knee from Harvard Medical School. His five-year orthopedic surgery residency was completed at Columbia University College of Physicians and Surgeons.
He has learned from many of the pioneers in hip and knee replacement and was educated at several of the most prestigious hospitals in the country for joint replacement surgery (Boston: Brigham and Women’s Hospital and New England Baptist Hospital. New York: Columbia-Presbyterian Medical Center).
He received his Medical Doctorate Summa Cum Laude from the State University of New York and was inducted into Alpha Omega Alpha, the elite medical honor society.
He earned his Bachelors of Science at the University of Miami, where he developed his love of the Florida lifestyle. He currently practices at Preferred Orthopedics of the Palm Beaches in Florida.