Total knee replacement has not been in
widespread use quite as long as hip replacement,
but it has a long track record and is now quite
common. The first designs that resembled the modern
knee replacements used today appeared in the 1970's
and rapidly went through a number of evolutionary
changes. It has now reached the status of a mature
technology in the past couple of decades, and it is
widely accepted in the orthopaedic literature that most
patients undergoing total knee replacement can have an
expectation of at least 95% success rate at 10 year
follow-up or longer.
The basic concept of a total knee replacement (also
known as total knee arthroplasty) is to replace the
rough, irregular surfaces of the ends of the bones (the
femur and tibia) with new surfaces. This eliminates the
"bone on bone" changes from severe arthritis and
allows the ends to glide smoothly over one another,
with artificial surfaces that have no nerves in them.
The undersurface of the patella (knee cap) may or may
not be replaced also with a plastic button.
These new surfaces resemble a metallic cap that is
affixed to the ends of the bone (most often with
cement, although press fit cementless prostheses are
sometimes used). For this reason, although "total knee
replacement" has been the term used for several
decades, "knee resurfacing" would probably be a more
accurate description since it is usually half an inch or
less that is actually removed from the ends of the bones
and replaced. It is not unlike a dental procedure in
which a bad tooth is capped. After the joint is
replaced, there is no longer any arthritis in the joint,
because the joint surface is entirely artificial.
Partial knee replacements also exist, most often as a
unicondylar knee replacement, which replaces one side
of the knee only. These are less invasive procedures
and typically have a quicker recovery, with the
advantage of retaining more "factory original" parts.
However, only some patients are candidates for a
partial knee replacement. It will only help the portion
of the knee it replaces in most cases, and if both sides
of the knee joint are worn out, it is often better to
consider a total knee replacement. Some patients also
have significant deformity or angulation, making it
difficult or impossible to correct alignment and
biomechanics without a total knee replacement.
At the time of surgery, the ends of the thigh bone
(femur) and upper leg (tibia) are typically quite worn
out. Frequently, the ends of the joint look very similar
to two heads of cauliflower in a very worn out knee,
covered with lumpy and bumpy osteophytes (spurs)
and areas of exposed bone, grinding against each other.
Nuts & Bolts: Total Knee Replacement
Procedure
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| The arthritic surfaces of the bone are cut away (left) and the new knee replacement
components fit securely over the ends of the bone (right). Note that the knee replacement on the right
is black and specular; this is an Oxinium(TM) implant made from oxidized zirconium. |
Regardless of the surgical approach used, the same
general steps have to be performed during the surgery.
Some surgeons use a tourniquet for the procedure, and
others prefer to identify transected blood vessels and
ligate them at the time of surgery (rather than have
them bleed into the joint after surgery when the
tourniquet is released). Tourniquets can also be a
source of soreness and circulation problems after
surgery, and for that reason we typically do not use
them in routine knee replacement surgeries in our
practice.
After exposing the knee joint - usually with a
vertical incision in the front of the knee - the irregular,
arthritic ends of the femur and tibia are resected.
These cuts are made in a way to keep the mechanical
axis of the knee properly aligned, which usually requires
keeping the perpendicular cut at about 5 to 7 degrees
off of the vertical axis. Because the end of the femur is
rounded (i.e., shaped like a cam mechanism), it is also
necessary to make chamfer cuts. These are usually
made in such a way that the new "cap" fits very tightly
over the chamfer cuts. Remnants of the menisci and
anterior cruciate ligament, if they are still present, are
removed.
Next the upper end of the tibia is resected. It is
important for the surgeon to cut and prepare this
surface at the proper angles also; if the cut is tilted too
much side to side, the knee will either be excessively
bowed or knock-kneed. Similarly, if it is angled too far
up or down when viewed from the side, knee flexion
and extension may be adversely affected. It is also
important for patients with severely bowed legs to
understand that full correction of the deformity may
not be possible at the time of surgery.
The surgeon checks that the knee is "ligamentously
balanced" at this point, which is the most difficult part
of the biomechanics to restore. For example, the knee
might be too tight when flexed but loose and unstable
when fully extended, or vice-versa. Although there are
a wide variety of knee replacement designs used to
address different problems, the majority of knee
replacements are designed to keep most of the patient's
own ligaments, which keep the knee stable when
moving back and forth and from side to side.
Many combinations of biomechanical challenges
need to be resolved at this point to make sure the knee
moves in as natural a way as possible, which is one
reason why knee replacements are arguably more
technically demanding than hip replacements.
A polyethylene (special plastic) spacer is also
selected to fit between the two metal components in
most knee designs. This has a very low friction surface
that allows the new knee replacement parts to smoothly
glide over one another. Some knee replacement
designs utilize an "all-polyethylene" tibial component,
which is all plastic without a metal backing.
These spacers have many different geometries and
sizes, depending on the biomechanical needs identified
by the surgeon. Some are designed to replace the
function of the posterior cruciate ligament, and others
are designed to work with an intact posterior cruciate
ligament. Some are designed to allow more range of
motion and others favor more stability. As with most
things, there typically are some trade-offs made in
order to find the best replacement for each patient.
Trial components are usually used before cementing in
the final components, which allows the surgeon to
check the range of motion, knee tracking, and
ligamentous stability before implantation of the final
(real) components.
The undersurface of the patella (knee cap) may or
may not be resurfaced. There are some surgeons who
always resurface the patella, and some who never
resurface it. Many surgeons decide at the time of
surgery whether it is warranted. Replacing the
undersurface of the patella with a small plastic button
replaces one more arthritic surface, but it does have the
potential to introduce mechanical problems with
tracking of the knee cap and also is another cement
interface that will eventually wear out over the years.
For this reason, many surgeons will not replace it
unless they feel it is warranted.
The artificial parts are typically cemented into place
with polymethylmethacrylate bone cement. While
there has been a definite trend in hip replacements to
utilize cementless components that are porous and
allow bone growth into the prosthesis, most surgeons
have departed from cementless knee replacements.
Early results in the 1990's showed a higher incidence of
early failure and loosening in the knee when cement
was not used, and this has been thought to be the result
of the the different biomechanics and forces seen in the
knee as opposed to the hip. However, some
researchers have begun advocating cementless knee
replacements again with newer data, and this trend may
reappear in the future.
Antibiotics are sometimes mixed into the cement if
there is concern about the patient having an increased
vulnerability to infection. These antibiotics typically are
gradually released out of the cement into the joint and
offer a protective effect for several months.
After the new knee is solidly fixed in place and
tested with the final parts cemented into place, the knee
joint is then typically closed in multiple layers with
various sutures. There are several ways to close the
skin, ranging from staples to traditional sutures to
absorbable sutures with special surgical glue. In our
practice, we tend to use absorbable sutures with glue
for most patients since we have found this heals quite
nicely and does not require removal of any sutures or
staples. A drain may or may not be placed depending
on surgeon preference and the degree of bleeding
noted during the case. Some surgeons may also inject
the knee at the end of the case with various pain
medications. We typically inject the knee with
morphine and a long-acting anesthetic (marcaine) as we
finish the closure.
Patellofemoral Knee Arthroplasty
(Partial Knee Replacement)
Another form of partial knee replacement is to
replace only the undersurface of the patella and the
groove that it rides in along the front of the femur.
The replacement part in the groove is a smooth piece
of metal that is inlaid flush with the surrounding
bone/cartilage. Patellofemoral knee arthroplasty is not
often used, as it is uncommon for a patient to have a
problem with only the knee cap and its groove that is
severe enough to warrant surgery without also having
worn out the rest of the joint (usually to the point of
needing a total knee replacement). However, it may
have a role in very select cases.
Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.
