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Knee Arthroplasty
Replacement with prosthesis

Over 200,000 patients in the USA undergo knee replacement surgery each year, and the prime reason is painful arthritis. The three main forms of arthritis causing knee pain are osteoarthritis, traumatic and rheumatoid arthritis. When severe pain, stiffness and swelling is no longer treatable with medication, the sufferer may need replacement of the knee with a metal prosthesis.

Anatomy and Physiology

  • The knee is a complex mechanism formed by the joining of bones, ligaments, muscles and protective cartilage (Figure 1)
  • Bones
    1. Femur. The femur or thighbone extends from the hip to the knee and forms the upper aspect of the knee
    2. Tibia. The tibia is the major bone in the lower leg, sometimes called the shinbone. The tibial plateau is the portion of the tibia that forms the lower aspect of the knee
    3. Fibula. The fibula is a slightly shorter bone that runs parallel and along the outside of the tibia
    4. Patella. The small bone in front of the knee: the kneecap
  • Major ligaments
    1. Medial collateral ligament---limits inward bending of the knee. This ligament extends from the lower end of the femur on the medial side (inside) of the knee to the tibia
    2. Lateral collateral ligament---limits outward bending of the knee. This ligament extends from the lower end of the femur on the lateral side (outside) of the knee to the fibula
    3. Anterior cruciate ligament--- This is a rope like ligament that runs from the center of the femur to the anterior aspect (front) of the tibia. It is a major stabilizer of the knee that prevents the femur from rotating and sliding back on the tibia (Figure 2)
    4. Posterior cruciate ligament--- This is a rope like ligament that runs from the center of the femur to the to the posterior aspect (back ) of the tibia. It works to stabilize the knee by preventing the femur from rotating and sliding forward on the tibia
  • Muscles
    1. The quadriceps muscle ends in the patellar tendon that covers the patella
    2. The hamstring muscles strengthen the back of the knee
  • Cartilage
    1. Smooth white cartilages cushion the knee, line the surface of the femur, tibia and patella within the joint
    2. Menisci are C-shaped cartilages sandwiched between the femur and tibia, one on the inside (medial meniscus) and one on the outside side of the knee (lateral meniscus)
  • Synovium is a special tissue that lines all motion joints and produces a lubricating fluid
  • Knee function
    1. The knee is not just a simple hinged joint that bends backward (flexion)
    2. It also has a rotary motion that locks the femoral condyles into the tibial plateau on straightening (extension) the leg
    3. On extension of the knee, the ligaments become tight and convert the knee into a rigid locked structure
    4. The knee unlocks on flexion allowing the increased range of motion seen as the lower leg swings backward
Figure 1 - Anatomy of the knee joint

Pathology

  • Osteoarthritis is the most common form of arthritis that leads to knee replacement surgery
    1. In this form or arthritis, the cartilage and synovium gradually wear down to expose the underlying bone so that the femoral and tibial bones grind directly on each other (Figure 2)
    2. Osteoarthritis is usually seen over the age of 60 years
  • Traumatic arthritis occurs following injury to the knee causing damage to the joint particularly the joint cartilage
  • Rheumatoid arthritis may attack any joint in the body
    1. The body's immune system attacks the synovium and cartilage of the joints causing damage to the joints
    2. This results in painful swelling and deformity with loss of joint motion
    3. Women are more often affected than men

History and Examination

  • Osteoarthritis
    1. Pain and swelling of the joint develops slowly over time
    2. Older population, usually greater than 60
    3. Obese patients
    4. Individuals that have repeated minor injuries (e.g. football player)
    5. Familial history of osteoarthritis
    6. Major joint inflammation is usually not seen
  • Rheumatoid arthritis
    1. Usually progressive joint involvement
    2. Many joints involved at the same time
    3. Marked stiffness of the joints after resting
    4. Marked tenderness in all the inflamed joints
    5. Women predominate
    6. Major joint inflammation is commonly seen
    7. Associated joint deformity
    8. Nodules beneath the skin at sites of chronic irritation
  • Traumatic arthritis
    1. History of significant injury to the knee
    2. Usually only one joint involved
  • Examination of the knee (regardless of underlying disease)
    1. The knee may be limited when bent (flexion) or straightened (extension)
    2. The knee may be abnormally angled out (bowlegged) or in (knock-kneed)
    3. There may be muscle weakness about the knee
    4. There may be a looseness of the cruciate ligaments
    5. If the knee is swollen, fluid may be taken to check for infection, gout or rheumatoid arthritis

Tests

  • Plain X-rays of the knee in both the anterior-posterior (front to back) and lateral (side to side) views are taken to determine if there is a fracture or degenerative changes in the joint, such as a loss of joint space or irregularity of the joint surface. (Figure 2) An additional anterior-posterior X-ray with the patient standing may show additional problems with the joint space not seen when stress is not applied
  • CT (computerized tomography) may be helpful to pinpoint a questionable fracture
  • MRI (magnetic resonance imaging) is now the method of choice for evaluating the non-bony (soft) tissues of the knee. This is particularly useful in determining injuries to the menisci, the presence of loose bodies of cartilage, and irregularities of the joint surface
  • Laboratory tests abnormal with rheumatoid arthritis such as
    1. Anemia
    2. Sedimentation rate of red blood cells increased
    3. Positive rheumatoid factors in the blood
    4. Knee joint fluid may have 3000 to 50,000 white blood cells
Figure 2 - X-ray of the right knee as seen from the front. Note the femur and tibia lie against each other (bone on bone) and there is a hardening (sclerosis) of the bone. Courtesy S. Kassab, MD

Non-operative therapy

Replacement knee surgery should not be considered unless medical and physical therapy have been exhausted.

  • Nonsteroidal anti-inflammatory drugs (NSAIDs, e.g. ibuprofin), aspirin and COX-2 inhibitors (e.g. celecoxib, rofecoxib) may give symptomatic relief of pain
  • Drugs used specially for rheumatoid arthritis (e.g. gold compounds, methotrexate)
  • Local heat or cold for temporary pain relief
  • Bracing of the knee to protect against stress
  • Weight reduction program to reduce stress on the knee
  • Exercise program to improve knee motion and muscle tone and strength
  • Injection of steroids into the knee joint

Indications and Contraindications

  • Surgery is resorted to only if conservative measures fail
  • Contraindications
    1. Inadequate bony support for the prosthesis in the femur or tibia
    2. Previous joint infection
    3. Degeneration of the nerves to the joint
    4. Conditions that increase the load on the implanted prosthesis and reduces the chance of a satisfactory long term result (e.g. marked obesity)
    5. Skeletally immature bone structure
    6. Sometimes if there are inadequate collateral ligaments

Surgical Procedure

  • The object is to remove the painful knee joint distorted by disease and replace it with metal prostheses covering both the femoral and tibial ends
  • A strong, smooth, low-wearing plastic insert anchored to the tibia is placed between the two metal prostheses and creates a smooth surface for the moving metal parts
  • The following description is abbreviated in that the measurements that the orthopedic surgeon uses in making the various bone cuts are omitted
  • An incision is made over the front of the knee (Figure 3A), and the tibia and femur separated from the surrounding tissues
  • A resection guide is attached to the end of the tibia. The direction of the saw cuts in the bone is determined by passing the saw blade through the slots in the guide. This is similar to a carpenter using a miter box to cut wood at a given angle. The upper end of the tibia that forms the knee joint is removed (figure 3B)
Figure 3a - Steps in the surgical procedure. See the text for explanation Figure 3b - Steps in the surgical procedure. See the text for explanation
  • A reamer is passed through a hole near the center of the joint surface of the lower end of the femur and into the femur shaft, thus determining the direction of the long axis of the femur, which is necessary to apply the prosthesis properly (Fgure 3C)
  • A resection guide is attached to the lower end of the femur and the portion forming the joint is removed (Figure 3D)
Figure 3c - Steps in the surgical procedure. See the text for explanation Figure 3d - Steps in the surgical procedure. See the text for explanation
  • Another resection guide is anchored to the end of the femur (Figure 3E) Pieces of the femur are cut off the front and back as directed by the miter slots in the guide. Then cuts are made to bevel the end of the femur to fit the prosthesis
  • Finally the prostheses are cemented over the ends of the femur and tibia. (Figure F) and the plastic insert is secured to the tibial prosthesis (Figures 4A and B).
  • The surgical wound is the closed
Figure 3e - Steps in the surgical procedure. See the text for explanation Figure 3f - Steps in the surgical procedure. See the text for explanation
Figure 4a - Postoperative X-rays (A and B) of the same knee showing the prosthesis. Courtesy S. Kassab, MD Figure 4b

Complications

  • Increased wear of the plastic surfaces of the prosthesis
  • Loosening of the prosthesis due to resorption (softening) of the bone
  • Bending, cracking or fracture of the bone
  • Dislocation, excessive rotation or loss of motion of the prosthesis
  • Lengthening or shortening of the leg
  • Infection
  • Hematoma in the tissues
  • Deep venous thrombosis and pulmonary embolus
  • Foreign body reaction to the prosthesis or cement
  • Delayed skin healing or sloughing of skin
  • Sensitivity reaction to the metal
  • Loosening or migration of the prosthesis due to injury or loss of fixation
  • Damage to adjacent blood vessels
  • Damage to surrounding nerves causing pain, numbness, or weakness
  • Angular deformity of the joint
  • Calcification about the joint with loss of motion

Postoperative Care

  • There may be a drain in the wound attached to a suction device which is usually removed a day or two after surgery
  • Intravenous fluids may be given as well as antibiotics
  • To get the knee moving faster, the leg may be placed in a Continuous Passive Motion machine
  • The knee remains straight and elevated unless being exercised
  • Physical therapy aimed at getting the patient in and out of bed, sitting, standing and walking is started the following day
  • Standing and walking is aided with crutches or a walker
  • Discharge from hospital occurs usually in three to five days

After Care

  • Pain medication is given as necessary
  • Contact the surgeon if
    1. There is an elevated temperature
    2. Bleeding
    3. Numbness
    4. Drainage from the wound
    5. Increased swelling, redness or pain
    6. Shortness of breath
  • Antibiotics should be ordered for future surgery or dental work Exercise the muscles around the knee. Occasionally physical therapy may be needed