The knee is the largest joint in the body. It is comprised of the lower end of the thighbone (femur), the upper end of the shinbone (tibia), and the kneecap (patella). At the points where the ends of the bone touch, they are covered with a smooth substance that works as a lubricant so the bones glide over each other.
Large bands of connective tissue called ligaments are used like ropes to tie the bones together. This helps keep the joint stable.
The final part of the joint is the meniscus, which acts as a shock absorber, a crucial consideration given the stress and force on the knees when walking, running, jumping, or even standing.
Age, trauma, or disease can disrupt the knee, leading to severe pain and disability. The most common cause of knee pain is arthritis. For the most part, three types affect the knee.
• Osteoarthritis is age-related and damage relates to wear and tear. Although it can occur in younger people, it most often occurs in those over 50. The cartilage wears away, and the bones start to rub against each other, causing pain and stiffness in the joint.
• Rheumatoid arthritis occurs when the membrane surrounding the joint becomes inflamed and thickens. As the inflammation becomes chronic, it damages the cartilage, and its loss results in pain and stiffness.
• Post-traumatic arthritis can follow a serious injury to the knee. Fractures of the bone around the knee or tears of the ligaments damage the bone surface over time.
Considering total knee replacement
When more conservative treatments such as medications and physical therapy no longer work, you may want to consider a total knee replacement (TKR), also called total knee arthoplasty. Damaged bone and cartilage from your thighbone, shinbone, and kneecap are cut away and replaced with an artificial joint made of metal alloys, high-grade plastics, and polymers.
Around 650,000 TKR surgeries are performed every year in the United States. This figure is expected to rise to as many as 3.4 million per year by 2030.
The right knee implant
For most of the last 30 years, manufacturers have mass-produced knee replacement implants. All have a metal cap on the femur, a metal plate covering the tibia, and a plastic piece in between.
“There are many different companies that produce off-the-shelf TKR products,” says Christopher Cannova, MD, a surgeon in private practice at the Washington Joint Institute in Bethesda, Maryland. “All are similar, but all have subtle nuances in their construction. Sort of like cars, where they all look like a car, but there are differences in design that the makers say set them apart from other automobiles.”
Until the last few years, surgeons had to look at a person’s knee and then choose the implant with the best fit. There were usually compromises because everyone’s anatomy is different and not every part of an implant matched up perfectly with a given patient’s knee.
“Over the last 30 years, the surgeons would use their skill and expertise to select and position the right size of implant, trying to get the proper balance for good range of motion in the joint,” said Robert Tait, MD, director of the Joint Replacement Center at the Orthopedic Institute of Henderson, Nevada. “Satisfaction surveys across the country have consistently shown about 20 percent of patients say there is something about the knee they don’t like. Another 6 to 7 percent say they wish they had not had surgery done at all.”
Evolving custom-made knees
In 2011, the Food and Drug Administration (FDA) approved the first patient-specific knee-replacement implant system. The ConforMIS iTotal® Knee Replacement System remains the only one approved for use in the United States. Unlike the options produced in large quantities, this system allows for each implant to be manufactured mirroring the anatomy of one specific knee.
Three-dimensional (3D) printers, combined with specialized computed tomography (CT) scans, are key to patient-specific TKR. Together, this technology allows both the implant and the instruments needed to perform the surgery to conform to the anatomy of the knee being replaced.
The first step of the process is taking a CT scan of the hip, knee, and ankle, with many data points around the knee itself. The surgeon reviews the scan, then it is sent to the ConforMIS plant.
After being fed through a special computer-aided design program, the implant is sent to a 3D printer. Using a specific metal alloy and plastics in a sterilized environment, the resulting implant and tools reflect the size and shape of your original knee.
This level of specificity means that less bone must be removed. If 8 millimeters (mm) of bone has to be removed, it can be replaced by exactly 8 mm of metal in exactly the same position as before. In addition, the surfaces of the joint interface more closely, mimicking how the knee works naturally while correcting for any deformity. Cannova likens this to the difference between buying a suit off of the rack and having one tailor-made.
“The system makes a fibula and tibia that are sized and shaped perfectly to mate to the anatomy of the person having TKR,” said Dr. Tait. “This eliminates the size and shape compromises. That hopefully results in better function of the knee and better satisfaction with the outcome than we see off the shelf.”
Long history with materials
Although the manufacturing process is new, the cobalt chromium molybdenum and ultra-high molecular weight polyethylene or highly cross-linked vitamin-E-infused plastics used to manufacture the implant are the same materials used for off-the-shelf TKR packages. This means that doctors and patients have a long history with the materials and know what to expect. It also eliminates most of the concerns about long-term problems.
“We know the wear characteristics of what goes into making the implants,” said Cannova. “Despite the fact that we are only around six or seven years into this particular technology, we essentially have 25 years of data on what to expect. We should still get to that 25- or 30-year time horizon before replacement.”
Shorter recovery time
Another positive aspect is shorter recovery time. Some patients are released after completing the home health-care part of the physical therapy process. This means that fewer people have to go to outpatient treatment, saving money and returning more quickly to normal daily activities.
“I caution patients seeking this to remember it is still a knee-replacement operation,” said Cannova. “What I mean by that is we still make an incision, place the implant, and, as a result, there is still recovery time. The hope is that the customization will make the implant feel more normal and natural than one that is just taken off the shelf.”
The patient-specific TKR can be used for almost everyone. Those who shouldn’t receive the implants or may not be a candidate for the procedure include:
• those with severe damage to the joint, making a match to their anatomy impossible;
• those who are obese; Tait generally uses a body mass index of 45 or more as the cutoff point; and
• those with a traumatic deformity.
Results so far have been very encouraging. The results of tests used to assess the knee’s range of motion were significantly better for those who underwent patient-specific TKR compared to those with an off-the-shelf model.
Perhaps even more important was the difference in patient-satisfaction scores. The number of people who say they wish they had never had the operation shrank from 6 percent to fewer than 2 percent.
“Over my career, I have done 9,000 operations of this type and the big advantage to patient-specific TKR is that it is not just another gimmick or gizmo,” said Tait. “My job is getting my patients out of pain, and when 5 percent tell me they wish they had never had the operation, that has been emotionally draining. I am thrilled to say that most of that has gone away and I like doing knee replacement again because I can predict that the results will be better.”
Want to learn more about knee replacement surgery? Read “How Long Do Hip and Knee Replacements Last” and “Long-Term Exercise After Knee or Hip Replacement.”