Introduction
A hip resurfacing involves replacing the just
the top of the femoral head and the socket of
the pelvis bone with metal parts that then act
as a new hip joint. The acetabular component is
"press fit" to your pelvis and the femoral
component is cemented to your femur with bone cement.
Today's parts are made of cobalt chrome.
Multiple orthopedic companies have hip
resurfacing implants ((Birmingham / Smith &
Nephew), (Cormet / Stryker), (Conserve / Wright),
(ASR / Depuy), (Durom /
Zimmer), (Recap / Biomet)), but currently only
the Birmingham and Cormet are approved by the FDA.
Hip resurfacing
has received a lot of press recently with
Floyd
Landis, a famous biker, and
Smith & Nephew, an orthopedics company,
leading the charge. Before the reader gets
to involved in this information, I need to state
a disclaimer. I feel a vast majority of
patients would be better suited with a
traditional total hip replacement for reasons discussed
below. Second, I have serious concerns
regarding the biomechanics of hip resurfacing
and the fact that it was tried in the 70's and
80's and was generally regarded as a failure and
abandoned. As a surgeon, I am an advocate
for the preservation of as much of the patient's
normal anatomy as is safely possible, but hip
resurfacing definitely requires a considerably
larger soft tissue dissection, and sacrifices acetabular bone in order to preserve some of the
proximal femoral bone.
The purpose of the following information is to
explain to patients the major aspects of hip
resurfacing surgery,
inform them of its major risks, and hopefully
help them make a well informed decision about
their joint disease. If after reading this
discussion, you would like more information or
would like to be evaluated for a hip resurfacing
surgery,
please a schedule an appointment.
History of
Hip Resurfacing
Hip resurfacing was developed in the late 1970s.
Many articles in the 1980's showed poor outcomes
with hip resurfacing and most surgeons abandoned
the procedure. The first generation hip resurfacing procedures
failed largely because of the large metal
femoral head rubbed on the plastic
(polyethylene) socket and worn it out. The
components loosened and femoral neck fractures
occurred. Advocates of today's hip
resurfacing state that these problems are fixed
with improved metal on metal surfaces, but only
time will tell.
Biomechanics of Hip Resurfacing
My major concern with hip resurfacing is my
feeling that hip resurfacing has a serious
biomechanical disadvantage, namely a small
head-neck ratio. Orthopedic surgeons have
realized for years that increasing the head to
neck ratio improves the range of motion (ROM) of
a total hip replacement. Not as much
attention has been given to the head neck ratio
in hip resurfacing though. The typical
response from a hip resurfacing advocate is that
the resurfacing just replaces exactly what it
takes off, but for reasons discussed below, I do
not think that is accurate.
Unfortunately, understanding my concern with the
head-neck ratio in hip resurfacing requires a
little background information and a technical
discussion. The typical head size in a
total hip replacement is anywhere from a 28 to a
36 mm for traditional metal on poly and can be
as big as 50 mm with large metal on metal heads.
The neck size for most femoral implants is
between 12 and 14 mm. The head-neck ratio is
therefore between 2 (28/14) and 4 (48/12) with a
hip replacement. All hip resurfacing
femoral components on the market have a
difference between the head diameter and neck
opening diameter of 6 to 9 mm. The larger sizes
typically have a 9 mm difference and the smaller
sizes have a 6 mm difference. If a small
lady has a 46 mm head diameter, her hip
resurfacing would likely have a 46 mm head
diameter, a 40 mm neck diameter, and a 1.15
(46/40) head-neck ratio. If a large man
has a 58 head diameter, his resurfacing head
would likely have a 58 mm head diameter, a 49 mm
neck diameter, and a 1.18 head-neck ratio.
If the surgeon selects a femoral component
slightly larger than the native femoral head,
then the neck opening diameter would be larger
than with the actual femoral neck diameter.
In other words, there would be a step off where
the femoral component stopped and the femoral
neck started, and the smaller femoral bone
diameter would improve the head neck ratio. Our femoral bone has to have a larger neck
diameter to support our body weight than a metal
femoral replacement does. Because the
femoral bone is larger, it gets in the way of
hip motion.
But what does this mean for the range of motion
before the components and bones start to hit one
another? The equation for determining the
functional range of motion of the hip joint is
easy to determine for known geometrical shapes
like in hip resurfacing and hip replacement.
Dr. Yoshimine published this equation in
Journal of Biomechanics in 2002.
The A variable
is the hemispherical nature of the opening which
is 180° for a
hemispherical acetabular component in hip
replacement. Hip resurfacing components
are less than a hemisphere and the A variable is
simply 360°
minus the acetabular sector angle. The
acetabular sector angle is 159° for the
Birmingham, 165° for the ASR and Durom and 168°
for the conserve. The A variable for hip
resurfacing is 192° to 201°
for resurfacing components. The
important understanding of this equation is that
as the head-neck ratio approaches one
(n=r), then the right side of the equations
approaches 180°
and the functional range of motion,
θ,
approaches zero for a hemispherical cup and 12-21°
for a
resurfacing cup.
 |
| Functional ROM in total hip
replacement/resurfacing |
Using the above formulas (anyone can put the
numbers into a calculator), the functional ROM
for any given head and neck sizes are listed
below.
| Components |
Head Size |
Neck Size |
Head-Neck Ratio |
A variable |
Functional ROM |
| hip resurfacing |
50 mm |
44 mm |
1.14 |
192° |
69° |
| typical total hip |
28 mm |
12 mm |
2.33 |
180° |
128° |
| large MoM total
hip |
50 mm |
12 mm |
4.16 |
192° |
164° |
A picture is typically more informative than
numbers and words.
 |
 |
 |
|
Large Metal on Metal |
Traditional THA with
28 head |
Hip Resurfacing |
This decreased motion before impingement is more
evident when the picture includes the femoral
bone.
|
 |
 |
 |
| Degrees of
flexion/extension in hip resurfacing |
Degrees of
abduction/adduction in hip resurfacing |
Degrees of internal
and external Rotation in hip resurfacing |
One additional factor influences the head neck
ratio in hip resurfacing, the femoral component
height. Sometimes, a surgeon will attempt
to lengthen a patient's leg during hip
resurfacing by removing less bone off the top of
the femur and placing the femoral component
higher or more proud. When the surgeon
raises the femoral component, he/she
inadvertently raises the femoral head/neck
junction. The femoral neck diameter
increases as you move up the femoral neck until
it fads into the femoral head. Therefore,
if a surgeon raises the femoral component,
he/she is ensuring an increased neck diameter
and a sub-optimal head/neck ratio.
|
 |
 |
|
Raised femoral
component with a larger neck diameter
and decreased head/neck ratio |
Lowered femoral
component with a smaller neck diameter
and increased head/neck ratio |
The above information originally seemed to
exaggerated to believe. After all, is not
a hip resurfacing just replacing what is already
there? Yes and no. To answer this
question, you have to understand what happens in
normal hip flexion. The normal hip does
have a poor head-neck ratio, but there are two
things in its favor. First, the normal hip
has excellent stability; the large femoral head
and the conformity of the socket prevents
dislocation. Therefore, when the anterior
femoral neck hits the acetabular rim with hip
flexion, the hip does not dislocate, but instead
the pelvis flexes and allows the leg to continue
to flex. This femoral neck impinging on
the acetabular rim occurs at ~100°
in normal hips and ~90°in
femoral acetabular impingement. Obviously,
additional leg flexion is possible when the
pelvis also flexes. In other words,
impingement happens all the time in the native
hip, but the hip joint is well accustomed to
handle the impingement (i.e. the soft labrum
cushions the blow) Second, the acetabular
bone has a particular shape to the anterior wall
that allows more motion before the femoral neck
hits the acetabular rim.
So, is a hip resurfacing just like a native hip?
I think there are many similarities and some
differences. First, both the native hip
and the hip resurfacing have excellent
stability, so when impingement occurs with hip
resurfacing, the pelvis flexes and the leg
continues to flex. However, a main
difference between the native hip and a hip
resurfacing is this impingement. In the
native hip, a soft labrum cushions the femoral
neck as it impinges of the relatively soft
acetabular rim. In a hip resurfacing, the
femoral neck impinges of the metal rim of the
acetabular component. I feel that the
repetitive hard impact is the main cause of femoral
neck fractures. Currently, most
surgeons feel that "notching" the femoral neck
weakens the femoral bone and causes of femoral
neck fracture. On the other hand,
"notching" the femoral neck effectively
diminishes the difference between the head and
neck diameter, lowers the already poor head-neck
ratio further, and increases the occurrence of
this impingement. Retained bone cement at
the head neck junction can also effective
diminish the head/neck ratio and cause
impingement and possibly femoral component
loosening.
Interestingly, patients
with a native hip joint and femoral acetabular impingement (i.e. alpha
angle of Notzli > 50°)
had a head-neck ratio of <1.15 in their diseased
femoral bone. (Beaule
PE, et al. JBJS 2007 Jan, 89B(1)) In other
words, hip resurfacing has a similar head-neck
ratio as femoral acetabular impingement.
For these reasons, I feel that along the
anterior femoral neck, the surgeon must ensure
that there is some additional room or decrease
in neck diameter where the femoral component
stops and the neck starts; in other words, the
surgeon must oversize the femoral component to
improve the head neck ratio as much as
possible.
The other main difference between the native hip
and a hip resurfacing is the shape of the
acetabular rim. The hip resurfacing does
not have the anterior rim contour of the native
acetabular rim and therefore impingement occurs
sooner with flexion.
Acetabular component position is important in
hip replacement for instability reasons;
however, acetabular component position is
important in hip resurfacing for impingement
reasons. Unfortunately, it is extremely
difficult to tell the acetabular component
position on a x-ray in a hip resurfacing because
of the overlap of the large femoral head.
In short, impingement naturally occurs in the
both the native hip and hip resurfacing because
the femoral neck bone must be strong and big
enough to support our body weight and therefore
the head-neck ratio suffers. Both the native hip
and hip resurfacing have excellent stability, so
that when impingement occurs, the pelvis flexes
instead of the hip dislocating. The
recurrent impingement in hip resurfacing,
especially with a poorly positioned acetabular
or femoral component, might cause femoral neck fractures
and/or femoral component loosening.
It should be noted that many studies have shown
good outcomes with hip resurfacing, and the
clinical implications of the above biomechanical
concerns is uncertain. It is possible that
a patient's pelvis may flex and extend enough to
allow the hip to move in all of the necessary
directions and that the poor head neck ratio is
unimportant.
Pros of hip resurfacing
-
Proximal
femoral bone preserved
-
Later
femoral revision easier
-
Large metal
head offers excellent hip stability
-
Patients
with large femoral offset can keep their femoral
offset
-
Proprioceptive
benefits
Cons of hip resurfacing
-
Large soft
tissue dissection - more muscles cut
-
More
acetabular bone removed because larger
acetabular component used
-
More
difficult to adjust leg length or offset
-
Increased
impingement as described above
-
Decreased
ROM before impingement
Patient Selection
So who is a good candidate for a hip
resurfacing? The best candidate is a large
male with large femoral offset and health bone.
The Surface Arthroplasty index was designed by
Beaule to identify the patients that had the
greatest risk of failure. Patients who should
consider avoiding hip resurfacing are people
smaller than 82 kg (they have a smaller femoral
head), people with previous hip surgery,
patients with femoral cyst, and people
with activity levels greater 6 on the UCLA
activity scale. Obviously, these criteria
are conservative, and many patients outside of
this criteria are receiving hip resurfacing
procedures. Only time will tell is this is
a safe procedure or a mistake.
Beaulé PE, Dorey FJ, LeDuff MJ, Gruen T, Amstutz
HC. Risk factors affecting outcome of metal on
metal surface arthroplasty of the hip. Clin
Orthop 2004;418:87-93.
Surgical Approach
Hip resurfacing is performed through a lateral
(trans-gluteal) or posterior (Kocher) approach,
although other approaches are possible.
Posterior Approach
The posterior approach
(Kocher-Langenbock) has the advantage of not
injuring the abductor muscles and the dissection
can be extended in case more access to the femur
or pelvis is necessary. The posterior approach
is probably the most popular approach for a
total hip replacement used today. The
disadvantage of the posterior approach is that
the posterior capsule and muscles are cut during
the approach. They are typically repaired at
the end of the case which helps prevent
dislocations, but the posterior approach does
have a higher dislocation rate than the other
approaches. Most surgeons limit the patient’s
motion after surgery with a posterior approach
to prevent any compromising leg positions that
might cause a hip dislocation. Because the
abductor muscles are spared, most patients have
historically had the lowest rate of limp with
the posterior approach.
With
regards to hip resurfacing, some physicians are
concerned about damaging the blood supply to the
remaining femoral head with the posterior
approach because the major blood vessels
(lateral epiphyseal branch of the medial femoral
circumflex artery) to the femoral head are cut
during a posterior approach. Some
physicians feel that the femoral component
loosening is a result of the decreased blood
supply. In addition, both the anterior and
posterior capsule is often cut and removed
during a hip resurfacing in order to get the
femur out of the way to fix the socket.
The gluteus maximum tendon is usually cut during
this approach as well. The typical
skin incision is typically 2-3 times larger for
a hip resurfacing (> 9 inches) than for a hip
replacement (3 inches).
Bearing
Surface options:
The bearing surfaces
for hip resurfacing are only available in metal-on-metal.
Metal
Polished metal bearing
surfaces have the advantage of good wear
properties and the ability to make thin
implants. Metal bearing surfaces therefore come
in the largest head sizes. The use of larger
head sizes increases the stability.
Metal bearing surfaces increase the blood levels
of metal ions and have been reported to cause a
metal hypersensitivity reaction in select
patients. Currently, women who are
contemplating child birth and patients with
renal failure should probably not have metal
bearing surfaces. The increased metal ions in
the blood are excreted in the urine and have not
been shown to increase cancer risks over the
past 30 years.
Risks
of a hip resurfacing
For reasons discussed above, hip resurfacing
replacements have a risk of a femoral neck
fracture. This could occur because
blood supply to the femoral bone is injured
during surgery or because of the repetitive
impact of the femoral neck bone against the
metal acetabular component. If a
femoral neck fracture occurred, then the
femoral component would have to be replaced
to a total hip femoral component. The
published rates of femoral neck fracture are
1 to 4% over 5 years.
Component Loosening:
Occasional the bone will not grow into the
implanted components. The components may
loosen and change position. The motion of
the loose component may cause pain and
require another surgery to revise the
components. Earlier hip resurfacing
implants had a relatively high incidence of
acetabular and femoral loosen likely from the femoral
component impacting the acetabular component
and loosening it. The acetabular
component can not receive any screws, so the
initial press fit fixation is imperative.
Blood Clots:
Blood clots in your leg veins are possible
after any surgery on the lower extremities.
The occurrence of blood clots can be
minimized with blood thinners, foot pumps,
and early mobilization. The main danger of
blood clots is if they dislodge and travel
to your veins in your lungs. This
phenomenon is called a pulmonary embolus and
can result in respiratory difficulty, chest
pain, or even death. Blood clots may or may
not hurt or cause swelling in your leg and
can occur anywhere in either leg. If you
have unexplained pain or swelling in your
legs, let Dr. Kurtz know as he may order a
duplex ultrasound to look for a blood clot.
If you feel chest pain or breathing
difficulties, you should call 911 and then
call Dr. Kurtz. The risk of these clots
causing death has been drastically reduced,
and is less than 0.1%. The treatment for a
proven blood clot is additional blood
thinners, and occasionally a filter in your
veins.
Dislocation:
The ball of the new hip joint may become
dislodged from the socket. The risk of
this occurring can be less with hip
resurfacing but has occurred. If your hip
resurfacing dislocates, Dr. Kurtz would
manipulate your leg with sedation to place
the ball back in the socket. An unstable hip resurfacing
could need to be revised
to correct this condition if it keeps
occurring.
Infection:
Antibiotics are given before and after
surgery to decrease the risk of infection,
but an infection still can occur immediately
or even years after the surgery. It is
usually treated with another surgery to
remove infected tissue and often the
prosthesis as well. If the components are
removed, a revision hip prosthesis can
sometimes be inserted months later if the
infection clears, but sometimes the patient
is left without a hip joint. Although
patients rarely have life threatening
problems from their joint infection, an
infection is a devastating complication.
Unequal Leg Lengths:
The length of your legs usually is within 1
centimeter after surgery, but it is
difficult to adjust the leg length with a
hip resurfacing. If the unequal
lengths are bothersome, a lift can be built
or inserted into the shoe of your shorter
leg.
Nerve Injury:
Although extremely rare, nerves to your leg
and feet, or the nerve to your thigh can be
damaged by the surgery. These nerves may or
may not recover by themselves. If they do
not, you may need a brace for your ankle or
for your knee to walk, and your walking
ability could be limited.
Bleeding:
Rarely, the blood vessels around the hip are
damaged by the surgery and excessive
bleeding occurs after or during the surgery,
requiring either surgery or special
procedures under X-ray control to correct
the problem. Occasionally, blood gathers in
the wound even if no major blood vessel is
damaged and further surgery (or observation)
is required to correct the problem.
Limp:
The limp that most people have before the
surgery usually persists until the muscles
become stronger after surgery. It sometimes
never goes away, and sometimes the surgery
creates a new limp. Most people, however,
note that the way they walk is greatly
improved by the surgery.
Fracture:
The femur or pelvis can crack when preparing
the bone for insertion of the components,
actually inserting the components, or even
years after the surgery. Fractures usually
are treated with metal cables or a plate,
and usually heal.
Bearing surface:
Each bearing surface has its unique risk.
Polyethylene bearings carry the risk of
wearing and causing osteolysis. Ceramic
bearings can squeak and break. Metal
bearing surfaces can cause a
hyper-sensitivity allergic reaction and
release metal ions into the blood and
surrounding tissue.
Need for Further Surgery:
Though uncommon, hip replacements
occasionally fail sooner than expected. Some
other problems can also make further surgery
necessary, including: bone forming where it
should not, breaking of the bone around the
prosthesis (during or after surgery), and
irritation of the soft tissues by wire or
sutures.
Death:
Though very rarely, patients have died
following hip replacements. This can be due
to underlying medical or heart problems that
arise or worsen after the surgery. It can
also be due to blood clots traveling to the
lungs as mentioned above, or from the stress
placed on the body by more than the usual
amount of bleeding.
Other Problems:
This list is meant to cover only the major
problems most frequently encountered. Just
as everyone is unique, so are many problems.
It
is important to remind patients that though
numerous complications have been reported in
the literature, most are minor and rare.
Measures that
you can take to help prevent complications
include:
-
Telling your doctor immediately of any
possible infection anywhere on your body.
Also let my office know.
-
Receiving antibiotics before any dental,
urinary, or rectal procedure for two years.
You will require pre-procedure antibiotics
for a longer period if you have a disease
that compromises your immune system. (Call
my office if there are any questions.)
-
Always mention to any doctor performing an
invasive procedure on you that you have a
hip replacement.
Revision Hip Surgery
Replacing a previously
inserted prosthesis is more difficult and less
predictable than the first surgery. Each case
has its own unique problems and risks. In all
cases, the risks are much greater than the risks
with first-time surgery. The recovery is often
longer, and the results are less certain. The
outcomes following revision surgery have greatly
improved over the years.
