The knee is the largest and most complex joint in the body. It is a hinged synovial joint that is located where the tibia and femur meet.
There are 4 main bones that connect and make up the knee joint:
- Femur (Thighbone) – the femur head creates the ball-and-socket joint of the hip (at the acetabulum) and creates the top of the knee at the lower end. The femur is the main bone of the leg and supports the weight of the body on the leg.
- Tibia (Shinbone) – connects with the knee at the upper end and the ankle at the lower end. It bears and distributes weight across the knee and to the ankle
- Fibula (Calf Bone) a slender bone located at outer side of the leg parallel to and slightly behind the tibia. Ligaments connect it to the two ends of the tibia. It helps strengthen the tibia and provides support in the slight rotation of the knee.
- Patella (Kneecap) – a tendon at the top of the patella and a ligament at the bottom hold the nearly heart-shaped bone in place at the center of the knee. It protects your knee joint.
The ends of these bones are covered with articular cartilage, a smooth substance that acts as a shock absorber and helps bones move easily. Articular cartilage is found on the femur, the top of the tibia, and the back of the patella. All remaining surfaces of the knee are covered by a thin lining called the synovial membrane, which releases a fluid that lubricates the cartilage and reduces friction to nearly zero in a healthy knee.
Between the femur and tibia are two C-shaped wedges of cartilage called menisci. These act as shock absorbers that cushion and protect the joint.
Numerous bursae (fluid-filled sacs) help the knee move smoothly.
Large ligaments connect the femur, tibia and kneecap and provide stability. The four main ones are:
- Anterior Cruciate Ligament (ACL) – the ACL controls the tibia’s rotation and forward movement.
- Posterior Cruciate Ligament (PCL) – located in the center of the knee, the PCL controls the tibia’s backward movement.
- Medial Collateral Ligament (MCL) – provides stability to the inner knee.
- Lateral Collateral Ligament (LCL) – provides stability to the outer knee.
Tendons (tough bands of soft tissue that connect muscles to bones) connect the knee bones to the leg muscles that move the knee joint and provide stability. The largest tendon in the knee is the patellar tendon, which covers the kneecap, runs up the thigh, and attaches to the quadriceps.
A knee replacement is a major surgery involving resurfacing the parts of the knee joint damaged by arthritis, injury or disease that can’t be controlled by more conservative treatments. The ends of the bone that form the knee joint and kneecap are resurfaced and capped with metal and plastic parts (implants). According to the American Academy of Orthopaedic Surgeons, Americans have more than 600,000 knee replacements each year, a number that is expected to reach 1.28 million by 2030.
Why do people get knee replacements?
A knee replacement may be indicated for people who have severe arthritis or a severe knee injury. The most common reason for knee replacement surgery is osteoarthritis (OA), a degenerative joint condition in which the articular cartilage and adjacent bones in the knee gradually wear out. OA affects mainly middle-aged and older adults.
A knee replacement may also be recommended for people suffering from other forms of arthritis including:
- Post-traumatic Arthritis – develops after a fracture, meiscus tear, ligament/tendon injury or other trauma that causes accelerated wear and tear on the articular cartilage.
- Reactive Arthritis (Reiter’s Syndrome) – arthritic pain and inflammation as a reaction to an infection, such as a gastrointestinal infection (causes dysenteric reactive arthritis), genitals (genitourinary/urogenital infection) or urinary tract (UTI). Numerous bacteria can cause reactive arthritis, including those caused by sexually transmitted diseases (STDs) – most commonly chlamydia.
- Rheumatoid Arthritis (RA) – an autoimmune disease that causes inflammation of the synovial membrane which, in turn, causes excessive synovial fluid.
- Psoriatic Arthritis – a chronic, degenerative autoimmune disease in which people suffer from both psoriasis and arthritic joint pain, stiffness and inflammation. Psoriasis is a skin condition caused by a faulty immune response which causes skin cells to build up and form patches of itchy, dry skin called plaques. The plaques can then inflame joints, including one or both knee joints, causing the arthritic symptoms.
- Gout – an inflammatory arthritis that can cause a build-up of uric acide crystals (monosodium urate crystals) in the soft tissues of the knee joint, which can lead to excruciating pain, swelling, redness and warmth in the joint.
- Calcium Pyrophosphate Deposition (CPPD), also known as Pseudogout – a build-up of microscopic crystals in a joint that can lead to sudden, severe knee pain, swelling, warmth, and redness.
Types of Knee Replacements
The four main types of knee replacement are:
- Total Knee Replacement (Knee Arthroplasty) – replacement of the surfaces of the thigh bone and shin bone that connects to the knee. This is the most common knee arthroplasty surgery.
- Unicompartmental (Partial) Knee Replacement – replacement of only one side of the knee joint.
- Kneecap Replacement (Patellofemoral Arthroplasty) – replacement of the under-surface of the kneecap.
- Complex (Revision) Knee Replacement – for people with very severe arthritis or who have already had more than one knee replacement surgery.
Knee implants are categorized by the materials used in the knee replacement surgery. These materials are what rub against each other when you flex your knee.
- Metal on Plastic – most common type of implant. A metal femoral component rides on a polyethylene plastic spacer attached to the tibial component.
- Ceramic on Plastic – uses a ceramic femoral component instead of metal (or a metal component with a ceramic coating) that rides on a plastic spacer.
- Ceramic on Ceramic – femoral and tibial components are both made of ceramic.
- Metal on Metal – femoral and tibial components are both made of metal. Used much less often in recent years because of concerns over traces of metal leaking into the bloodstream, these may be considered only for young, active men, because they may last longer than other materials.
Many adults wait too long to get a knee replacement. This is a big mistake because these people will not receive the full benefit of the surgery and recovery time could take quite a bit longer. Others rush into it too quickly. If you have knee pain due to arthritis or a knee injury, try physical therapy first. Physical therapy has been proven to be an effective treatment method for many painful knee conditions and it could help you prevent or at least delay painful knee surgery that can sideline you for months.