In 2015 160,000 hip and knee replacements were performed in England and Wales, with similar numbers of patients in each group. Knee replacement is routine and successful, and the procedure continues to evolve.
The most common indication for a total knee replacement (TKR) is pain. Some patients also experience knee stiffness but, while TKR can alleviate this, it is harder to improve range of movement with a knee replacement than it is to reduce pain.
The patients who stand to gain the most from TKR are those with significant pain but good preoperative range of movement.
The most common cause of pain is degenerative change in the knee, leading to osteoarthritis (OA). OA is best diagnosed on a plain X-ray. When requesting plain X-rays for patients with knee pain it is useful to request a standing, weight bearing X-ray because the appearance of the knee joint can change dramatically when a patient stands up.
Some surgeons also request ‘skyline views’ or ‘patellofemoral views’ because these may be useful in defining the state of the patellofemoral joint.
There are three compartments in the knee; the medial, the lateral and the patellofemoral joint.
The medial joint
Between 60% and 80% of body weight is carried through the medial part of the joint and this is the most common place for the joint surface to be eroded. The articular cartilage that lines the surface of the joint gradually narrows and ultimately wears down to the underlying subchondral bone.
On plain X-ray a narrowing of the joint space in the medial compartment can be seen, and when the patient holds their leg out straight it is usually possible to see a varus deformity.
The lateral joint
It is possible for the lateral side of the joint to erode first, giving a valgus deformity to the joint. But in practice this is the classical appearance of rheumatoid arthritis. The knee is vulnerable to fractures and meniscal tears sustained in sports and industrial injury.
When a valgus deformity accompanies an osteoarthritic picture there is often a history of injury in earlier life. For example, if a man in his 40s presents with surprisingly well-developed OA there is often clear evidence of an earlier sports injury, and possibly the scars of open meniscal surgery.
A knee that doesn’t fully extend straight is said to have a fixed flexion deformity. This can usually be improved during the knee replacement operation.
In some respects, the exact nature of the deformity doesn’t really matter and can usually be overcome. However, a valgus deformity is associated with risk of injury to the common peroneal nerve and subsequent foot drop. I always warn patients with valgus deformity of this at the time of consent but I have never actually had a patient develop foot drop in these circumstances.
The operation takes between 60 and 90 minutes and is performed through a longitudinal incision down the front of the knee. In my unit the average length of stay after surgery is around three to four days, although of course some patients stay on the ward for much longer than this.
Most operations replace both sides of the joint and use a plastic component over the tibia - referred to as ‘metal on plastic’. Other materials have been tried, including ceramic joints, but they have yet to work their way into mainstream surgery.
Oxinium is a more modern - and more expensive - material that may be associated with a longer lasting knee replacement, although this is controversial. The components are secured using polymethylmethacrylate (PMMA) cement. PMMA cement is also used in most total hip replacements, although there is debate about the merits of cemented and uncemented hips replacements. A small minority of manufacturers have experimented with uncemented TKRs.
The patellofemoral joint
When OA affects the patellofemoral joint patients complain of pain at the front of the knee, and pain that is worse when rising from a chair or climbing stairs. Rising from a squatting position is particularly painful in patients with patellofemoral joint OA.
It is possible to replace the back of the patella with a plastic button that articulates with the trochlea surface of the metal femoral component, but many studies have demonstrated no reduction in postoperative pain when this is performed. I do not routinely replace the back of the patella, even when it is very degenerate. There is a risk of complications from replacing the patella and this can avoided by leaving it alone.
Postoperatively most patients do well. But as a group they are slower to recover than most total hip replacements. While most patients are pleased with postoperative outcomes, a significant minority are disappointed at six week follow up. However many of these patients improve by six months.
A postoperative range of movement of 90 degrees is typical. Some patients achieve more than 110 degrees.
Some younger or more adventurous patients complain that their knees feel artificial. This may be because of the extensive soft tissue damage done to the joint during surgery. For example , we routinely remove the anterior cruciate ligament and many surgeons also remove the posterior cruciate ligament, although this requires a different kind of prosthesis.
Unicompartmental knee replacements
Because many patients present with damage to only the medial compartment, a smaller knee replacement that only replaces the medial compartment has been developed. The best known unicompartmental joint replacement is the Oxford knee replacement, although most manufacturers offer a unicompartment prosthesis as part of their product range.
In expert hands these operations can be successful. Unicompartmental knee replacements may allow a much greater range of movement and provide a more normal feeling to the patient than total knee replacement.
Unicompartmental knee replacements are popular in younger patients who wish to have a higher degree of activity. However, the results of unicompartmental knee replacements are variable and a small proportion of patients soon return asking to have the whole joint replaced instead.
Very specialised knee surgeons may offer replacement of the lateral compartment in isolation, although this is quite rare. Similarly, isolated patellofemoral joint replacement is performed by surgeons with a specific interest in this procedure. But the results are variable and some surgeons would recommend replacement of the whole joint.
Between 1% and 2% of patients having TKR develop an early deep infection, which is perhaps the most feared complication in joint surgery. The risk is higher - closer to 2% - for patients with diabetes. Patients taking prednisolone, such as those with rheumatoid arthritis, are also at higher risk of postoperative infection.
Infection can usually be eliminated with further surgical procedures or irrigation of the joint and antibiotics. However, for some patients the infection is never eliminated and very rarely it might result in amputation above the knee or a fusion of the joint.
Around 1-3 patients die from TKR per 1,000 cases performed. This figure rises with advancing patient age.
About 1% of patients require further surgery each year, and about 90% of patients still have a working knee replacement 10 years postoperatively. Some of the more recent research papers actually suggest a better than 90% survival at 20 years.
Pain and clicking
Some patients complain of ongoing pain or clicking in the knee. Clicking may reflect patella maltracking and in principle it is possible to reoperate to correct this, although in practice any attempt to reopen the joint carries a 2% risk of deep infection and should probably be avoided.
- Mr Steven Cutts is consultant orthopaedic surgeon at James Paget Hospital in Norfolk