The knee joint is formed by the lower leg bone (shin bone), called the tibia, and the thigh bone, called the femur. The ends of the bones are covered with a smooth layer of cartilage. The joint is enclosed by a fibrous tissue envelope or capsule with a smooth tissue lining called the synovium. The synovium produces fluid that reduces friction and wear in the joint. Normal joints allow nearly frictionless and pain-free movement. When the cartilage is damaged or diseased by arthritis, joints become stiff and painful.
Osteoarthritis is the result of wear and tear on the joint. The normal cartilage lining is gradually worn away and the underlying bone exposed. It may be many years before the damage to the joint becomes noticeable. Pain and stiffness are the usual problems associated with osteoarthritis. The medial (inside) part of the knee is most commonly affected by osteoarthritis. Previous injury to the knee ligaments or cartilages (menisci) may predispose to this kind of arthritis. Being overweight may also increase the likelihood of arthritis, or make it worse.
Inflammation of the joints can be caused by a variety of conditions, the commonest of which is rheumatoid arthritis. Inflammation causes damage to the cartilage lining of the joints. This causes arthritis which is often more widespread than osteoarthritis, and typically affects several joints. This kind of arthritis can affect people when they are relatively young and may cause severe disability.
Treatment of arthritis
If the arthritis is inflammatory in nature, then a specific diagnosis should be sought. Blood tests and other tests can help to determine the type of arthritis involved. Anti-inflammatory and other drugs may be helpful.
For patients with osteoarthritis, it is important to remain flexible and to maintain as much movement as possible. Analgesics (pain killers) can help patients to maintain activity. Many modern pain killers also have an anti-inflammatory effect, which may be beneficial in arthritis.
If arthritis is severe and significantly affects activity, then surgery may be appropriate.
Knee Replacement Surgery
In knee replacement surgery, the damaged bone and cartilage are replaced with metal and plastic surfaces that are shaped to restore knee movement and function. The new artificial knee is called a prosthesis. The prosthesis is generally composed of two metal pieces fitted onto the ends of the tibia (shin bone) and the femur (thigh bone) and a plastic piece inserted between them to act as a bearing. Stainless steel, cobalt or chrome alloys or titanium may be used for these components. Durable, wear resistant polyethylene (plastic) is used for the bearing. A plastic bone cement may be used to anchor the prosthesis into the bone. Some joint replacements also can be implanted without cement when the prosthesis is designed to fit and lock onto the bone directly. See Total Knee Replacement Surgery RealPlayer Video.
Unicompartmental knee replacement surgery
When only one part of the knee joint is arthritic, it may be possible to replace just this part of the joint. The procedure is similar to a total knee replacement, but only one side of the joint is resurfaced. A metal component is fit onto the femur (thigh bone) and a plastic bearing is inserted either directly onto the tibia (shin bone) or onto a metal tray which has been fit onto the tibia. Recovery time is generally slightly shorter following this kind of surgery. See Shockwave Animation.
High Tibial Osteotomy
Sometimes, if predominantly one side of the knee is arthritic, an operation may be performed to realign the knee joint in order to take pressure off the affected side and redistribute weight bearing more onto the other side of the knee. This operation, called a high tibial osteotomy, involves making a cut in the tibia (shin bone) and removing a wedge of bone to change the angle of the knee joint. A staple or plate and screws are used to hold the bone in place until it heals. Although this procedure is not suitable for all patients, it may produce significant relief and delay the need for knee replacement surgery.
Anesthesia for knee replacement surgery
Knee replacement surgery can be performed under general, spinal or epidural anesthesia. A combination of techniques is often used.
The patient is asleep throughout the procedure. A breathing machine (ventilator) may be used to assist the patient's breathing during anesthesia. Many patients feel drowsy or groggy after a general anesthetic. The use of newer anesthetic drugs has significantly decreased the occurrence of post-operative nausea and vomiting.
Spinal block or epidural block
These techniques use local anesthetic to block the passage of nerve impulses, including pain, in the spinal cord. The block can be positioned at various spinal levels, but the patient generally has no feeling from the waist down. The patient is usually sedated during the procedure, but a ventilator is usually not required for this kind of anesthetic. It thereby can lower some of the risks of general anesthetic. Depending on the medications used for the block, the period of pain relief after surgery can be prolonged for several hours, reducing the need for pain medication after surgery. Because these blocks provide temporary loss of sensation below the waist, a urinary catheter (tube into the bladder to drain urine) is often inserted.
Possible Complications of total knee replacement surgery
Any surgical procedure carries various risks and potential complications. Although uncommon in joint replacement surgery, the following complications sometimes occur:
Blood Loss Requiring a Blood Transfusion
In any joint replacement surgery, some blood loss will occur. Occasionally a blood transfusion may be required. Precautions will be taken during the operation to minimize blood loss. Some patients may be given the option to donate their own blood pre-operatively.
Any surgery performed via an incision in the skin carries the risk of infection. Many precautions are taken during surgery to minimize the risk of infection. Intravenous antibiotics are usually given during surgery to help prevent infection. The operating room is also equipped with special air flow devices that minimize bacteria in the air.
Blood Clotting in the Legs (Deep Venous Thrombosis [DVT])
The circulation to the legs may be decreased during knee replacement surgery due to immobilization of the legs. Decreased movement of blood through the veins (venous stasis) can cause the blood to clot. Blood thinning medication is used to help minimize this risk, but a small percentage of patients still develop blood clots in the leg.
Blood Clot Moving to the Lungs (Pulmonary Embolism [P.E.])
Occasionally, blood clots that form in the legs may become detached and travel to the circulation in the lungs. This complication, although extremely rare, can be life threatening. Symptoms include chest pain and shortness of breath.
Any incision can result in damage to the sensory nerves in the area of the incision. Significant nerve damage, which may cause loss of muscle function, can occur after knee replacement.
Activity after a Knee Replacement
Very few restrictions to activity are required following knee replacement surgery. It is extremely important to move and exercise the knee early in the postoperative period in order to regain the best range of movement possible. Physiotherapy will usually be commenced on the first postoperative day, or on the day of the operation. If the joint is not moved early on, it will become stiff and have a decreased range of movement, which will limit the utility of the new knee.
Recovery time after a Knee Replacement
By the time patients are discharged, they should be able to get in and out of bed and walk to the bathroom. Most patients are around 80% recovered by six weeks after surgery. They will continue to improve more gradually over the next three to six months. Recovery varies from person to person depending on their level of pain and activity before their surgery.