By Dr Ananya Mandal, MD
Knee arthroplasty or knee replacement surgery has developed to its present advanced stage over the last few decades. This is mainly due to technical advances and better understanding of knee pathophysiology and functions.
Knee arthroplasty involves removal of the damaged, worn or diseased knee joint with an artificial joint or a prosthetic implant.
Why is knee arthroplasty needed?
The most common reason for performing a knee arthroplasty is joint arthritis.
The number of total knee replacements is on the rise with more than 70,000 knee replacements being carried out in England and Wales each year.
The majority of patients who need and undergo such operations are over 65 years of age.
Once replaced, the new knee will last for at least 15 to 20 years, especially if taken good care of. (1-5)
Types of knee replacement
There are 3 types of knee replacement –
- Unicompartmental knee replacement or Partial knee replacement – This means a replacement of a single compartment of the knee. This can be either the inner part of the knee (medial) or the outer part (lateral).
It is preferred in patients with osteoarthritis who have one part of the knee damaged by disease. It is not suitable for patients with rheumatoid arthritis (RA) which usually affects the whole joint.
- Unconstrained bicompartmental knee replacement or Total knee replacement – This is the commonest method of total knee replacement. The lower end of the thigh bone is replaced by a prosthetic made up of metal (titanium or cobalt/chromium-based alloys) that is similar to the bone it replaces.
The top part of the shin bone that forms the lower end of the knee joint is made of a plastic (polyethylene) cushion and a stem that joins with the shin bone or tibia below. The artificial joint functions like a normal knee joint.
- Constrained bicompartmental knee replacement – This surgery is performed in bone tumors and is rare. Here the artificial joint is fixed and so does not move or bend like a normal joint.
Choice of prosthetic device
There are over a hundred different knee replacement designs the surgeons may choose from. A complete prosthesis now weighs about 15 to 20 ounces.
Some factors that determine choice of the prosthetic device include age and weight of the patient. Desire for a certain level of activity, quality of life, health and cost of the prosthesis are other determinants.
Reasons for knee replacement surgery
Reasons to opt for knee replacement surgery include:
- osteoarthritis (commonest indication)
- rheumatoid arthritis
- haemophilia or bleeding disorder
- other types of arthritis
- bone cancers and tumors
Prior to surgery
Before surgery the knee is examined and imaging studies like X rays, MRI and arthroscopic studies of the knee joint are advised.
The choice of the prosthetic and extent of surgery is based on the results of these studies. In case of a joint infection, blood vessel disease knee arthroscopy cannot be performed.
How is knee replacement surgery performed?
Knee replacement surgery is performed under regional anethesia.
Commonly epidural anesthesia is employed where the anethetic injection is made in the spine over the lower back and this renders the patient numb to pain below the waist.
A cuff like tourniquet is placed over the thighs just before the incision. When inflated this aids in reducing the blood flow to the operative site and reduces the risk of bleeding.
An eight to twelve inch incision is made in the front or side of the knee. The muscles are cut open and the joint cavity is exposed.
The damaged parts of the joint including the lower part of the femur, upper part of the tibia are removed or sawed off.
The new implants are sealed on the sawed off edges with the help of bone cement.
Some implants are uncemented. These can be set over the edges of the sawed off bones in a press fit system.
Prosthetic cartilage is placed between the prosthetic ends of the femur and tibia to aid normal movement of the knee.
Minimally invasive surgery
Minimally invasive surgery as the name suggests involves a smaller incision of three to five inches.
Here the surgeon works the implant through a smaller opening. This leads to less pain and more rapid recovery.
After the surgery the tourniquet is removed or deflated. The incision and the operative site is checked for any bleeding points.
The surrounding muscles and ligaments are sewn back together and these provide stability to the joint and assist it in movement.
A small drainage tube is placed within the incision to drain of blood or excess fluids and the incision is sutured or stapled.
The wound is dressed and the knee is cast in a splint to aid recovery.
Patients are encouraged to walk on the second postoperative day. Early movement and physiotherapy is recommended for rapid recovery. Early discharge is encouraged at between 5 days and 2 weeks.
Complications of knee arthroplasty
Common complications of knee arthroplasty include infection, blood loss, pain, thromboembolism or formation of clots that may travel up to the lungs and brain causing life threatening consequences.
Other complications include nerve damage, damage to blood vessels, fractures, loosening of prosthesis etc.
Edited by April Cashin-Garbutt, BA Hons (Cantab)
When is knee arthroplasty needed?
Risks associated with knee arthroplasty
Recovery from knee arthroplasty
Prevention of knee arthroplasty