Knee replacements are surgical substitutions of a knee joint where the femur and tibia bone meet, with an artificial joint or implant. Although the procedure might seem like a modern one, it actually dates back to the mid-1800s.

Medical device companies refined all manner of artificial joints over the past three decades as doctors discovered more demand for them. An increased number of younger (and more active) potential recipients are wearing down or injuring their joints and asking for permanent or semi-permanent medical solutions.

Doctors recommend knee replacement surgery when patients endure constant pain or when daily life is affected by poor mobility. More than 90 percent of knee replacement patients experience a significant reduction in knee pain, according to the American Academy of Orthopaedic Surgeons.

Recipients can have a partial knee replacement or a total knee replacement. The difference depends on the recipient’s condition and pain level.

When medications, therapies and other treatments don’t work, surgery is the next step to reclaiming mobility. A total knee replacement can get a patient back to work or back to a better quality of life.

The amount of knee replacement surgeries continue to grow in the U.S. and are projected to near 3.5 million procedures each year by 2030. This significant increase, in large part, can be attributed to the growing number of baby boomers (those born between 1946 and 1964) reaching retirement age — when the chance of a worn-out joint is higher — and a rise in obesity and an increased need for follow-up procedures (revision surgery).

Most patients who undergo total knee replacement are between 50 and 80, but recommendations for these procedures are based on a patient’s pain and disability, not age.

The knee joint forms where the femur (thighbone) meets the tibia (shinbone) and fibula (runs alongside the tibia). The patella (kneecap), located on the front of the knee, moves up and down against the femur. These bones are connected by ligaments, muscles and cartilage that help form the joint hinge and give the joint its flexibility.

Although there are four bones around the knee joint, only the femur, tibia and patella are affected by an implant.

While many device companies design and manufacture knee implants, which in turn can be made from a variety of metals, plastics and ceramics, artificial knee joints consist of three components.

These three components are usually cemented in place, but some doctors use a cementless technique that helps the bone grow into the implant to help increase stability.

A cementless technique may be used on patients who are young, healthy and have strong bone structure around the knee. Because bone cement can break away and cause an implant to loosen, cementless knee replacements are less likely to loosen over time.

For this reason, cemented knee replacements are considered better suited for older, less active patients.

There are multiple types of knee implants that doctors and patients can discuss before choosing the best option.

Regardless of type, all knee implants have certain things in common.

All consist of a component that attaches to the thigh bone and one that attaches to the tibia, one of the two lower leg bones. A knee replacement implant may or may not have a patellar component mimicking the kneecap.

Some components, particularly the bearings, may be made of ceramics or a mixture of ceramics and metal. The materials have to be “biocompatible” meaning they cannot trigger the body’s rejection response. Knee implants are relatively lightweight — usually between 15 and 20 ounces.

This is the implant most patients receive. Less mobile and not as strong as other implants, it is best suited for patients who are not overweight and who do not have an active lifestyle. It is usually recommended for elderly patients who will not put serious wear-and-tear on the implant.

Another component is attached to the femur, where it can roll on the polyethylene.

The drawback is that this contact causes wear on the polyethylene. Studies have shown that this stress is a leading cause of fixed bearing long-term implant failures. It wears away the polyethylene component, causing the implant to loosen. Loosening is not only painful, it is also a major cause of implant failure.

Studies have shown that 95 percent of fixed bearing implants continue to function properly 10 years after patients received them.

This design allows the plastic cushion of the tibial component to rotate, giving patients greater flexibility on the medial (inner) and lateral (outer) sides of the knee. Mobile bearing implants work well for young and active patients.

The polyethylene component in mobile bearing implants is designed to fit into the metal tibial tray component in a way that allows limited rotation. The design was introduced to limit the polyethylene wear seen in fixed-bearing designs. It produces less stress between components and in theory should reduce chances of — or at least delay — loosening.

Mobile bearing implants tend to have less stress between the femoral and tibial surfaces, granting the flexibility to benefit a patient’s gait and even deep knee bends. However, their long-term durability is about the same as for fixed bearing implants in older patients.

Like the fixed-bearing design, mobile bearing knee implants have a 95 percent survival rate ten years after being implanted.

The medial pivot design incorporates a mobile bearing to more effectively replicate the natural function of the knee. It rotates, twists, bends and flexes like a natural knee joint. More complicated than fixed or standard mobile bearing designs, it can be more expensive.

Instead of a “hinge joint” approach used by more conventional knee replacements, the components of a medial pivot implant incorporate a “ball-and-socket” approach. It tends to be more stable than some other implant designs and because it allows for a large contact area between components, it reduces wear-and-tear extending the life-expectancy of the implant. But this design is more dependent on tissue and surrounding ligaments to prevent dislocation. This can result in tissue becoming caught in the implant which can cause pain.

A 2017 study of 325 patients, with a total of 345 medial pivot knee implants, showed good long-term results for the design. Researchers conducted follow ups with patients over an average 15.2 years.

They found that 98.8 percent of medial pivot implants survived to 17 years and 94 percent of patients were able to do age-appropriate activities, with an average knee bend of 120 degrees, in their final follow up sessions.

The posterior cruciate ligament (PCL) runs along the back of the knee, connecting the femur to the tibia. This design keeps the PCL intact, but it depends on several patient factors: good bone quality, few defects in the bones, intact soft tissue around the knee and a functional PCL.

There are two types of PCL implants: retaining and substituting. The difference between the two comes in part from how the PCL is affected by the implant surgery. A surgeon may remove the ligament to implant a PCL substituting implant. But if the ligaments are in good condition, the surgeon may want to preserve it. In that case, he or she will use a PCL retaining implant.

Preserving the ligament may not necessarily result in better post-operative knee function. A simple polyethylene bearing attaches to a metal component implanted in the tibia, and a metal implant in the femur hinges on the polyethylene. It is a minimal design dependent on the PCL to stabilize the implant but a tight PCL may lead to excessive wear on the bearing.

Less bone is removed with a PCL retaining implant. A PCL retaining implant is less likely than a PCL substituting version to result in a condition called “patella clunk” syndrome. This syndrome results from scar tissue forming near the implant and becoming caught in part of the device as the knee is fully extended.

A 2015 study reviewed 15 years of followups of patients who had received PCL retaining implants. It found 98.7 percent of the implants survived 10 years and 83.6 percent survived 17 years.

This design replaces ligament with plastic components. It is recommended primarily for patients with severely damaged knees or weak ligaments. Also used in revision surgery.

It is indicated for patients with severe knee deformities or who have had previous knee replacement surgery or have had a knee cap removed.

The PLC substituting design has a tall post attached to the tibia which fits into a deep box attached to the femur. The two are not connected with any kind of hinge joint. A polyethylene bearing attaches to the tibial component.

A 2016 study of the PLC substituting design found that 88.5 percent of PLC substituting implants used in initial knee replacement surgery survived 10 years. For revision surgery, the 10 year survival rate was 75.8 percent if the revision surgery was due to some cause other than infection, and 54.6 percent if revision was due to infection.

Women undergo more knee replacements than men – accounting for roughly two-thirds of all knee replacement surgeries in the U.S. Surgeons performing knee replacements have long reported anatomical differences between men and women, which make a “one-size-fits-all” approach ineffective in knee-replacement surgery.

“Female anatomy is a little different,” Dr. Anthony Unger, a surgeon who has performed hundreds of knee replacements told NPR in 2007. “The end of the bone called the femur (thigh) is a little different contour.”

Many women may be shorter or have smaller body frames than the overall population. Gender-specific implants, engineered to the various, different dimensions of male and female body frames, are designed to perform better for specific patients.

Knee replacement surgery is recommended when conservative treatments like physical therapy, braces, steroid injections and anti-inflammatory medication fail to alleviate pain.

If a patient requires surgery on both knees, it’s called a bilateral knee replacement. The two procedures can be done simultaneously or doctors may choose to use a staged bilateral approach that schedules the second surgery several days, weeks or months later.

The decision to have a total versus a partial knee replacement is based on a doctor’s recommendation after a full patient evaluation is performed.

A total knee replacement is actually the resurfacing of the bones in all three compartments of the knee joint: The medial compartment, the lateral compartment and the patellofemoral (front) compartment.

The procedure resurfaces the end of the femur, the top of the tibia, and the patella. The ligaments that support the knee can be left in place or removed, depending on their condition and the type of implant chosen. Any existing cartilage between the femur and tibia is replaced with a plastic cushion.

A partial knee replacement is a procedure that resurfaces one (unicompartmental) or two (bicompartmental) sides of the knee. Partial implant components are named for the area of the knee they resurface: Medial unicondylar component, lateral unicondylar component and patellofemoral component.

A patellofemoral surgery replaces the end of the thigh bone with a metal piece and resurfaces the back of the patella. A partial knee replacement can also include a knee osteotomy, in which a surgeon removes or adds a wedge of bone to the tibia or femur to take some of the weight off of the damaged part of the knee.

While knee implants give many recipients a second chance at walking and living a more normal life, complications do occur.

Most problematic is a loosening of the implant, which can be caused by a defective implant or poor positioning at the time of surgery. Implant loosening is a primary cause for revision surgery.

Although rare, metallosis is possible after knee replacement surgery. Metallosis, also known as metal poisoning, can occur when metal implant pieces rub against each other and release tiny metal particles into the knee joint and bloodstream.

This can happen when the plastic cushion separating the femoral and tibial components wears down from high usage. This usually takes a few years to occur, and revision surgery is required.

Joint infection is also rare — occurring in less than 2 percent of patients — but is considered one of the most serious complications following knee replacement surgery. Infections are usually related to sterility issues and the hospital where the surgery takes place.

Many of these common complications go away or significantly improve within a couple of weeks, but rehabilitation after surgery typically lasts several months.

Knee replacements generally last at least 15 years and can last far beyond 20 years, but not all patients experience the same results.

For patients whose artificial joint wears out earlier than expected — or who received a defective implant — revision surgery is required. Any serious complication can also lead to revision surgery. In addition, younger recipients require revision surgery when the first implant wears out.

Revision surgery rates for knee replacements are quickly rising. In 2010, more than 55,000 revision surgeries were performed in the U.S., with 48 percent of the revisions in patients under 65. Total costs for each surgery averaged to be more than $49,000.

By 2030, estimates predict that there will be nearly 268,000 procedures performed each year — a 600 percent increase from 2005, when there were approximately 38,000 knee replacement revisions.

Implants that are used in revision surgeries typically have thicker, longer stems for added stability and to replace bone loss. In addition, surgeons typically implant a constrained (hinged) knee during revision surgery because it provides strength for severely damaged knees and weak ligaments. The vast majority of these procedures result in favorable long-term outcomes. However, as with any surgery, there is a risk of complications.

There are well over a dozen knee replacement manufacturers in the U.S., but most of the market is dominated by four companies, each making several models of implants.

Zimmer and Biomet merged in 2014 and together dominate the global hip and knee replacement market with about a 35-percent share. The company produces roughly 25 percent of all knee implants worldwide. It makes implants for total knee replacement, partial replacement, revision surgery components and other products to preserve or repair damaged knees.

This Johnson & Johnson subsidiary is the second largest hip and knee implant manufacturer in the world. It had an estimated $1.5 billion in sales of knee replacements for 2016.

Stryker Corporation reported nearly $1.5 billion in sales of its knee implants for 2016. That was 34 percent of all implant sales for the company and the most for any joint replacement the company manufactured. The company received FDA approval for its Mako robotic arm, assisted-knee implant technology in 2014 and has reported strong sales of the devices, which help surgeons plan and carry out knee implant surgery.

UK-based Smith & Nephew reported $932 million in revenue from its knee implants in 2016. The company also introduced its first robotic knee implant procedures using its newly acquired NOVIO system that same year. Some Smith & Nephew knee replacements use a proprietary oxidized zirconium bearing the company claims has less wear than plastic ones.