Normal Anatomy of the Knee
The knee joint is made up of three bones: the femur (thigh bone), the tibia (shin bone), and the patella (knee cap). There is a fourth bone, the fibula which lies along the side of the tibia, but it has only a small role in the function of the knee joint. These three bones work together to allow for flexing and extending the leg at the knee joint. The lower part of the femur connects to the upper part of the tibia and to the patella with ligaments, tendons, and other connective tissues. Amongst these connective tissues is a 2-part structure called the meniscus. The two parts are the inner (medial) side and the outer (lateral) side. The meniscus helps to reduce friction, balance out the weight distribution in the knee, and acts as a shock absorber. The surfaces of the bones where they meet in the knee joint are covered in a special type of cartilage called articular cartilage. This cartilage is very strong and smooth, and it acts to reduce the friction in the knee joint even more. The patella (knee cap) is vital for knee function. Without the patella the knee joint would have a much more difficult time bending and straightening. You can think of the patella like a fulcrum and the leg bones like levers. You use a lot less energy to lift something using a lever with a fulcrum under it than just trying to move it with a lever alone. Since our knees have to support virtually all of our body weight, and have a lot of stress upon them during daily activities, things can sometimes go wrong.
Injuries and Problems of the Knee Joint
The knee joint is vulnerable to stresses, traumatic injury, overuse, and arthritis. The articular cartilage in the knee can be damaged by disease, injury, or normal wear and tear over time. When the cartilage is damaged, the joint surfaces may no longer be smooth. Moving bones along a rough, damaged joint surface is difficult and causes pain. Damaged cartilage can also lead to arthritis in the joint. Articular cartilage does not heal itself well, so sometimes surgical intervention is required. Also, damage to the shock absorbing meniscus, tendons, and ligaments can occur. This is common in traumatic injuries such as in sports, but also in falls. When the knee is not able to properly support the weight of the body, joint damage and arthritis can occur. Often, when there is pain and dysfunction, you will hear that the knee joint is “bone on bone.” This means that the articular cartilage and the meniscus have become damaged, and there is no more friction reduction or shock absorption in the knee. Additionally, certain conditions such as infections or diseases such as rheumatoid arthritis and avascular necrosis can lead to damage in the knee joint. Conditions like these may require surgical intervention to restore pain-free function of the knee joint.
Types of Surgical Interventions for the Knee
There are several different options when it comes to knee replacement surgery. Your surgeon will discuss with you the best option for your particular needs.
Partial Knee Replacement
This procedure, also called unicompartmental partial knee replacement, involves only one side of the knee joint. As mentioned above, you have an inner (medial) side and an outer (lateral) side of the knee structures. Sometimes the damage or arthritis only affects either the inner or outer portion of the joint. In those cases you may be a candidate for partial knee replacement. This procedure can have a quicker recovery time and may even have better short-term functionality in the joint as there is preservation of more of your own bone and tissues. The incision for a partial knee replacement is also smaller, and certain minimally invasive surgical techniques can be employed to further reduce recovery time. It is important to note that in order to have a partial knee replacement you should have good quality bone structure in the knee, and the ligaments and tendons need to be in good shape too. You should still have essentially a normal range of motion in the knee as well. In a partial knee replacement procedure, a small part of the patella is removed, and the damaged portion of the top surface of the tibia (tibial plateau) is removed to make room for the lower implant. The lower portion of the thigh bone (femoral condyle) is removed to make room for the upper part of the implant. Finally, a portion of the meniscus is removed. The femoral implant component is a smooth, curved piece of metal that mimics the shape of the natural bone. The tibial implant component is a flat shaped piece of metal which will hold a special plastic spacer. The metal components are typically cemented into position on the bone. After the metal components are affixed to the bones, a final polymer plastic spacer is inserted into the tibial component. This plastic spacer will act as a friction reducer and shock absorber, much like the natural meniscus. After range of motion and stability in the joint are assessed, the surgeon will suture the tissues and a splint will be wrapped around the leg to prevent excessive movement. Note that over time, partial knee replacements can wear out faster than total knee replacements, and would require a revision knee replacement surgery.
Total Knee Replacement
This procedure, called a total knee arthroplasty (TKA), involves replacing all of the joint surfaces in the knee. In a total knee replacement, sometimes the under-surface of the patella (knee cap) is resurfaced and polymer plastic implant is attached to it. This is done when the arthritis has damaged the under-surface of the knee cap and there is bone rubbing on bone. Total knee replacement surgery accounts for about 90% of knee replacement surgeries. In a total knee replacement surgery, the lower portion of the thigh bone (femoral condyles) is removed to make a space for the upper implant. The damaged top surface of the tibia (tibial plateau) is removed to make a space for the lower implant. The under-surface of the patella is shaved down to make room for the polymer plastic implant. Any bone spurs would also be removed at this time. Finally, the meniscus is removed. Some surgical techniques involve the use of robotic arm technology. This technology helps the surgeon to get the proper alignments when removing portions of the bone. Note that the robotic arm does not perform the surgery; it just assists the surgeon during the surgical procedure. The femoral implant component is a smooth, curved piece of metal that mimics the shape of the natural bone. The tibial implant component is a flat shaped piece of metal which will hold a special polymer plastic spacer. The patellar component is a rounded polymer plastic piece that is affixed to the underside of the knee cap. The metal components are typically cemented into position on the bone. After the metal components are affixed to the bones, a final polymer plastic spacer is inserted into the tibial component. This plastic spacer will act as a friction reducer and shock absorber, much like the natural meniscus. After all components have been fit into proper position, the knee is assessed for range of motion and stability. When everything looks good, the tissues will be sutured and a splint will be wrapped around the leg to prevent unnecessary movement. Although total knee replacement surgery takes longer, and there is a higher rate of complications and cost, the lifetime of the implants is longer than for partial knee replacement implants. Physical therapy will be a bit more challenging with a total knee replacement as well, but the benefits of greater patient satisfaction, higher functioning, and longer lasting components may make this a better choice for your particular needs.
This procedure is a patella (knee cap) replacement. It involves removing a portion of the front of the lower end of the femur. This is the surface that contacts the underside of the patella. The underside of the patella is then shaved off and is fitted with a polymer plastic implant. This helps to restore pain free movement of the patella on the knee joint. This technique can be used in patients who have arthritis limited to only the patellofemoral region. It can also be performed on patients who have a condition called trochlear dysplasia, which is an abnormally shaped under-surface of the patella. The success of patellofemoral arthroplasty seems to depend more upon patient suitability than other factors. This technique is not performed very often as it is less common to have arthritis limited to only the patellofemoral area, and some surgeons believe that the long term outcomes are better with a total knee replacement.
Complex or Revision Knee Replacement
This procedure is done when the original knee replacement surgery has either failed or has reached the end of its useful life. Sometimes, an artificial joint can become infected necessitating removal and replacement of the implanted components. Another reason for a revision knee replacement is a fracture of the bone in the area of an implant component. A revision knee replacement procedure is more complicated as the knee has previously been surgically altered. Specialized implants need to be used, and there is a lot more planning involved for this procedure. Often these implants have thicker stems that fit deeper into the bones of the femur and tibia.