Knee arthroplasty, also known as knee replacement surgery, is a procedure where damaged parts of the knee joint are replaced with artificial components made of metal and plastic to help restore function and reduce pain.

Understanding Knee Arthroplasty

Knee arthroplasty is one of the most successful and effective surgical procedures performed in modern medicine. When someone experiences severe knee damage from conditions like arthritis or injury, simple activities such as walking, climbing stairs, or even getting up from a chair can become extremely difficult and painful. This surgery involves removing damaged cartilage and bone from the knee joint and replacing these damaged parts with artificial components, called prostheses, which are specially designed to recreate the smooth surfaces that allow the knee to move properly^[1].

The artificial knee joint consists of metal caps that resurface the thighbone and shinbone, along with high-density plastic that replaces the damaged cartilage. Each of these artificial parts works together to create a new, functional joint surface. The goal of knee replacement is not just to eliminate pain but also to help people return to their normal daily activities and enjoy a better quality of life^[2].

There are two main types of knee replacement procedures. A total knee replacement is the most common type, where all three areas of the knee joint are replaced: the inside part (called medial), the outside part (called lateral), and the area under the kneecap (called patellofemoral). A partial knee replacement, on the other hand, involves replacing only the damaged sections of the knee. Partial replacements are more common in younger adults who have experienced an injury or trauma affecting just one or two areas of the knee^[3].

Epidemiology

Knee replacement surgery has become increasingly common across the world, particularly in developed countries. In the United States alone, more than 700,000 total knee replacements are performed each year, according to the American Academy of Orthopaedic Surgeons^[2]. Other sources suggest the number may be even higher, with surgeons in the U.S. performing more than 850,000 knee replacements annually^[3]. The procedure was first performed in 1968, and since then, continuous improvements in surgical materials and techniques have made it one of the most successful procedures in all of medicine^[2].

The annual incidence of symptomatic knee osteoarthritis, the primary condition leading to knee replacement, is estimated at 240 per 100,000 patients. With approximately 400,000 primary total knee arthroplasty surgeries performed annually in the United States, the numbers reflect the significant burden that knee problems place on the population^[4].

Knee replacement surgery is most common in older people, but people of all ages may need this procedure. There has been a substantial increase in recent years in the number of people younger than 60 receiving total joint replacements, especially total knee replacements. This trend is expected to continue as more younger patients seek relief from severe knee pain and damage that prevents them from living active lives^[19].

Causes

The most common cause of chronic knee pain and disability that leads to knee replacement surgery is arthritis. While there are many types of arthritis, most knee pain requiring replacement surgery is caused by three main types: osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis^[2].

Osteoarthritis is an age-related condition and the most common reason people need knee replacement surgery. This type of arthritis, also called wear-and-tear arthritis, occurs when the cartilage that cushions the knee joint gradually breaks down over time. As the cartilage wears away, the smooth surfaces of the joint become rough and uneven. When this happens, the bones start rubbing against each other, becoming scraped and damaged. This bone-on-bone contact causes inflammation, pain, and stiffness that make walking and other movements very difficult. In advanced cases, the knee cartilage is worn away completely, and the surface of the knee becomes pitted, eroded, and uneven^[1][6].

Rheumatoid arthritis is another condition that can lead to the need for knee replacement. Unlike osteoarthritis, which results from wear and tear, rheumatoid arthritis is an autoimmune disease where the body’s immune system mistakenly attacks the lining of the joints. This causes chronic inflammation that damages the cartilage and can affect multiple joints in the body^[3].

Post-traumatic arthritis develops after an injury to the knee. People who have experienced serious accidents such as car crashes, falls, or sports injuries may develop this type of arthritis years after the initial injury. The trauma can damage the cartilage or alter the alignment of the knee, leading to progressive joint deterioration over time^[2][3].

Less commonly, other conditions can also cause knee damage requiring replacement surgery. These include conditions where the bone tissue dies due to poor blood supply, various forms of joint degeneration, and congenital problems where the knee joints are not formed correctly from birth^[5].

Risk Factors

Several factors can increase the likelihood that someone will develop knee problems severe enough to require replacement surgery. Understanding these risk factors can help people take steps to protect their knee health.

Age is one of the most significant risk factors. As people get older, the cartilage in their knees naturally experiences more wear and tear. The progressive degeneration and loss of articular cartilage that characterizes osteoarthritis becomes more common with advancing age. This is why knee replacement surgery is most frequently performed in older individuals, though younger people can also need the procedure^[4].

Obesity and being overweight significantly increase stress on the knee joints. Every extra pound of body weight puts additional pressure on the knees during everyday activities like walking and climbing stairs. This increased mechanical stress accelerates cartilage breakdown and increases the risk of developing osteoarthritis. Younger patients who are obese have a higher rate of implant failure and may need revision surgery earlier than those who maintain a healthy weight^[19].

Previous knee injuries represent another important risk factor. People who have experienced trauma to the knee from accidents, falls, or sports injuries are more likely to develop post-traumatic arthritis later in life. Even after the initial injury heals, the damage to the cartilage or changes in knee alignment can lead to progressive deterioration of the joint^[3].

Having rheumatoid arthritis or other autoimmune conditions that affect the joints increases the risk of needing knee replacement. These conditions cause ongoing inflammation that damages the joint tissues over time, eventually leading to severe pain and loss of function^[3].

Activity level also plays a role. While regular exercise is generally good for joint health, people who engage in very high-impact activities or who are extremely active may put more stress on their knees. This can be particularly important after knee replacement surgery, as more active younger patients may wear out their artificial joints faster than less active individuals^[19].

Symptoms

People who eventually need knee replacement surgery typically experience a combination of symptoms that significantly impact their daily lives. These symptoms usually develop gradually over time, becoming more severe and limiting as the condition progresses.

Severe joint pain is the number one symptom and the primary reason people choose to have knee replacement surgery. This pain often starts as occasional discomfort but progressively worsens. Initially, the pain may only occur with activity, but as the condition advances, people may begin to feel pain even while sitting or lying down. The pain can be constant and debilitating, making it difficult to sleep through the night^[2][4].

Stiffness in the knee joint is another common symptom. The knee may feel tight and difficult to move, especially after periods of rest or first thing in the morning. This stiffness makes it hard to bend or straighten the leg fully, which impacts the ability to perform simple tasks. People often find it increasingly difficult to get up from chairs, climb stairs, or walk for any distance^[1][3].

Limited mobility and difficulty moving the knee are hallmark symptoms. As the cartilage wears away and the joint deteriorates, the normal smooth motion of the knee is disrupted. This reduced mobility affects nearly every daily activity. Simple actions like walking, climbing stairs, or bending down become progressively more challenging and eventually may become impossible without severe pain^[3].

Swelling around the knee joint is frequently present. The inflammation caused by arthritis leads to fluid buildup in and around the joint, causing visible swelling and a feeling of fullness or tightness. This swelling can fluctuate but often becomes more persistent as the condition worsens^[3].

Some people also experience instability or a feeling that the knee might give way. The damaged joint may not provide adequate support, leading to a sense of the knee buckling or being unreliable during movement. This can create fear of falling and further limit activity. Changes in body alignment, such as the legs appearing bowed or knocked-knee, can also occur as the joint structure deteriorates^[6].

Prevention

While not all knee problems can be prevented, there are several steps people can take to protect their knee health and potentially delay or avoid the need for knee replacement surgery.

Maintaining a healthy weight is one of the most important preventive measures. Since excess body weight places significant stress on the knee joints, losing even a small amount of weight can substantially reduce the load on the knees and slow cartilage breakdown. For people who already have knee problems, weight loss can help reduce pain and improve function. After knee replacement surgery, staying at a healthy weight is crucial for protecting the artificial joint and making it last longer^[5].

Getting regular physical activity through low-impact exercises is beneficial for knee health. Activities such as walking, biking, and swimming help strengthen the muscles around the knee, improve flexibility, and maintain joint function without placing excessive stress on the joint. These exercises are recommended both for prevention and after knee replacement surgery. However, high-impact activities like jogging, running, and jumping should be avoided, as they put too much stress on the knees^[5].

Staying physically active through appropriate exercises helps maintain muscle strength and flexibility. Strong muscles around the knee provide better support for the joint and can help reduce symptoms. Physical therapy and specific knee exercises can be particularly helpful for people experiencing early knee problems^[4].

Protecting the knees from injury is important, especially during sports and recreational activities. Using appropriate safety equipment and techniques can help prevent traumatic injuries that might lead to post-traumatic arthritis later in life. If a knee injury does occur, seeking prompt medical attention and following treatment recommendations can help minimize long-term damage^[3].

For people with existing knee problems, trying conservative treatments before considering surgery may help postpone or potentially avoid the need for knee replacement. These nonsurgical treatments can include pain medications, anti-inflammatory drugs, physical therapy, weight loss, using walking supports like canes, and receiving injections into the knee joint. Many people find that these treatments provide sufficient relief to manage their symptoms without surgery^[2][4].

Pathophysiology

To understand how knee replacement surgery works, it is helpful to understand the normal anatomy of the knee and what changes occur when the joint becomes damaged.

The knee is the largest joint in the body and is essential for performing most everyday activities. The knee joint is formed by three bones: the lower end of the femur (thighbone), the upper end of the tibia (shinbone), and the patella (kneecap). Where these bones meet, their ends are covered with articular cartilage, which is a smooth, slippery tissue that protects the bones and allows them to glide easily over each other when you move your knee^[2][6].

Between the femur and tibia sit special structures called menisci, which are C-shaped wedges of cartilage that act as shock absorbers to cushion the joint. Large ligaments hold the femur and tibia together and provide stability to the knee. The long muscles of the thigh give the knee its strength and power for movement^[2].

All the remaining surfaces inside the knee are covered by a thin lining called the synovial membrane. This membrane produces a special fluid that lubricates the cartilage, reducing friction to nearly zero in a healthy knee. In a normally functioning knee, all these components work together in harmony to ensure smooth, natural movement^[2].

When disease or injury disrupts this harmony, the changes lead to pain, weakness, and reduced function. In osteoarthritis, which is the most common condition requiring knee replacement, the articular cartilage undergoes gradual degeneration and loss. The normally smooth cartilage becomes rough and begins to break down. As the cartilage continues to wear away, the underlying bone becomes exposed^[4][6].

Without the protective cushion of cartilage, the bones begin to rub directly against each other. This bone-on-bone contact causes the surfaces to become scraped, pitted, and uneven. The friction and mechanical stress trigger inflammation within the joint. Sometimes small bony growths called bone spurs develop. All of these changes result in pain, stiffness, swelling, and instability. The joint can no longer move smoothly, and simple movements become difficult and painful^[6].

During knee replacement surgery, the surgeon addresses these pathological changes by removing the damaged cartilage and bone and replacing them with smooth artificial surfaces. The surgeon makes an incision in the knee and moves the kneecap to the side. If bone spurs are present, they are removed. The two menisci between the femur and tibia are removed, as are certain ligaments that are no longer functioning properly. In some types of knee replacement, certain ligaments may be retained^[6].

The surgeon then cuts away and removes damaged cartilage and some bone from the upper part of the tibia and the lower sections of the femur, including two rounded sections called the femoral condyles. The tibia and femur are then capped with metal implants that create new, smooth joint surfaces. The surface of the femoral component is shaped to mimic the original femoral condyles. If the underside of the kneecap has also degraded, that surface may also be cut away and replaced with a plastic implant. Finally, a plastic spacer is inserted to recreate the smooth cushioning function of the cartilage that was lost^[3][6].

These artificial components work together to restore the smooth, gliding motion of a healthy knee, allowing the joint to move without the pain caused by bone rubbing on bone. The implants are designed to be smooth like the surfaces of a healthy knee, replacing the damaged biological tissues with durable synthetic materials^[6].