Link http://www.orthoassociates.com/Totalhip1.htm
How long will a hip replacement function before revision surgery is necessary?
This feature appeared in Maine Medical Center’s
research journal, INQUIRY, August, 2000.
In a presentation at the Harvard Medical School sponsored
course, “Total Hip Replacement: The Dawn of a New Era” in
September 1999, Dr. McGrory presented the results of total
hip replacements preformed by Dr. Omar D. Crothers with a
minimum 10 year follow-up. At a median follow-up of 13.2
years, 74 % of the total hip replacements studied had not
been revised, and the majority of these were functioning
well. On follow-up radiographs, there was significant
wearing of the plastic (ultra-high molecular weight
polyethylene) portion of the hip replacement in 13 %.
Radiograph of a hip replacement showing that the femoral
head (ball) has worn into the plastic lining of the
acetabulum (cup). Note the dark area above the cup which
shows bone loss due to a reaction to the small plastic
particles produced.
Of the 26 % that underwent revision surgery, over one-half
had surgery to insert a new plastic component of the hip
replacement.
This study is valuable because it one of the longest
follow-up studies for contemporary type prostheses. Previous
studies are not as useful (even though the follow-up time
may be longer) because they look at hip prostheses that are
no longer in use. The study shows that the major limiting
aspect of contemporary hip replacement surgery is the
prosthesis durability, particularly the plastic liner of the
socket.
Why will a total hip replacement ultimately fail?
In order to clarify the reason for the failures of the hip
replacements discussed above, a retrieval analysis of the
worn plastic inserts removed at surgery was undertaken.
Dr. McGrory of the
Maine Joint Replacement Center, in conjunction with
Massachusetts General Hospital researchers Orhun Muratoglu,
Lamia Mounib, Charles R. Bragdon, Murali Jasty, and William
H. Harris carefully examined the metal backed acetabular
liners retrieved at revision surgery. The components were
analyzed under reflected light for cracks in the plastic,
and 75 % of the specimens were microtomed parallel to the
surface of the extended lip of the liner. A total of 20-30
serial sections of 80 um thickness were collected, and a
portion of each liner was selected for oxidation analysis.
In a poster presentation at the 24th Annual Meeting of the
Society for Biomaterials in San Diego, California, the
authors showed their results in an abstract entitled “
Anisotropic Oxidation and Radial Cracks in Retrieved
Acetabular Components.” They found that all components
exhibited a focal distribution of multiple, grossly visible
radial cracks superiorly and posteriorly in the rim of the
plastic liner. The infra-red analysis of the four quadrants
studied showed on average 50% higher oxidation indices in
the regions with radial cracks.
These findings allow insight into the failure of the
polyethylene-bearing surface of contemporary total hip
replacements. The prosthesis surfaces are at higher risk of
failure if the plastic has oxidized, because oxidized
polyethylene is weaker and more brittle. The location of the
weak portion of the socket liners was also important.
This study was the first to demonstrate that the oxidation
known to weaken plastic liners may be increased by stress,
and is not necessarily uniform throughout the component.
Such information confirms the need to sterilize and store
plastic components in inert or vacuum environments if the
durability of the plastic is to be improved.
Based on information in these and similar studies, it has
become clear that the current limiting factor in the
durability and longevity of total hip replacements is the
“wearing out” of the plastic portion of the articulation of
the ball and cup.
Is there a way to extend the life of contemporary total
hip replacements?
Several new and pending products aim to address this very
question. A ceramic-on-ceramic (refers to the ball and
socket surfaces) type of hip replacement is undergoing
pre-FDA approval studies in the United States, and has been
in use for some time in Europe. In contrast to the
traditional metal on plastic articulation, ceramic surfaces
are said to have better wetability and toughness. Ceramic
components, however, are brittle and can fracture more
easily that other materials. Another alternative, a
metal-on-metal type of hip replacement, was recently
approved by the Food and Drug Administration (FDA). The
first such surgery at Maine Medical Center with this type of
articulation was performed in March of this year. Precisely
machined metal on metal surfaces have shown minimal wear in
European studies, but concerns remain about metal ion
release into the patient’s body. One of the most exciting
potential advances in articulation technology, however, is
crosslinking of the ultra-high molecular weight polyethylene
that has been used since total hip replacement surgery was
introduced to the United States in the late 1960s.
Crosslinking is a process in which polyethylene molecules
are bonded together to result in a stronger material,
substantially improving the material’s wear resistance.
About Minimally Invasive Surgery (MIS) at OAP
Resurfacing Total Hip Replacement Surgery
Total hip replacement (THR) for painful arthritis is one of
the most successful operations in modern surgery. Pain
relief is almost universal and greater mobility and
increased function are expected in the vast majority of
patients. One unresolved issue that is of utmost importance
to those considering this surgery is the durability of the
hip replacement—that is, how long will it last?. Three
general questions must be explored in order to understand
this issue: 1. How long will a hip replacement function
before revision (re-operation) surgery is necessary?; 2. Why
will the hip replacement ultimately fail?; and 3. Is there a
way to extend the life of contemporary total hip
replacements? Research to answer these questions has been
one focus of the adult reconstruction service of Orthopaedic
Associates of Portland.
How long will a hip replacement function before revision
surgery is necessary?
In a presentation at the Harvard Medical School sponsored
course, “Total Hip Replacement: The Dawn of a New Era” in
September 1999, Dr. McGrory presented the results of total
hip replacements preformed by Dr. Omar D. Crothers with a
minimum 10 year follow-up. At a median follow-up of 13.2
years, 74 % of the total hip replacements studied had not
been revised, and the majority of these were functioning
well. On follow-up radiographs, there was significant
wearing of the plastic (ultra-high molecular weight
polyethylene) portion of the hip replacement in 13 %.
Radiograph of a hip replacement showing that the femoral
head (ball) has worn into the plastic lining of the
acetabulum (cup). Note the dark area above the cup which
shows bone loss due to a reaction to the small plastic
particles produced.
Of the 26 % that underwent revision surgery, over one-half
had surgery to insert a new plastic component of the hip
replacement.
This study is valuable because it one of the longest
follow-up studies for contemporary type prostheses. Previous
studies are not as useful (even though the follow-up time
may be longer) because they look at hip prostheses that are
no longer in use. The study shows that the major limiting
aspect of contemporary hip replacement surgery is the
prosthesis durability, particularly the plastic liner of the
socket.
Why will a total hip replacement ultimately fail?
In order to clarify the reason for the failures of the hip
replacements discussed above, a retrieval analysis of the
worn plastic inserts removed at surgery was undertaken.
Dr. McGrory of the Maine Joint Replacement Center, in
conjunction with Massachusetts General Hospital researchers
Orhun Muratoglu, Lamia Mounib, Charles R. Bragdon, Murali
Jasty, and William H. Harris carefully examined the metal
backed acetabular liners retrieved at revision surgery. The
components were analyzed under reflected light for cracks in
the plastic, and 75 % of the specimens were microtomed
parallel to the surface of the extended lip of the liner. A
total of 20-30 serial sections of 80 um thickness were
collected, and a portion of each liner was selected for
oxidation analysis.
In a poster presentation at the 24th Annual Meeting of the
Society for Biomaterials in San Diego, California, the
authors showed their results in an abstract entitled “
Anisotropic Oxidation and Radial Cracks in Retrieved
Acetabular Components.” They found that all components
exhibited a focal distribution of multiple, grossly visible
radial cracks superiorly and posteriorly in the rim of the
plastic liner. The infra-red analysis of the four quadrants
studied showed on average 50% higher oxidation indices in
the regions with radial cracks.
These findings allow insight into the failure of the
polyethylene-bearing surface of contemporary total hip
replacements. The prosthesis surfaces are at higher risk of
failure if the plastic has oxidized, because oxidized
polyethylene is weaker and more brittle. The location of the
weak portion of the socket liners was also important.
This study was the first to demonstrate that the oxidation
known to weaken plastic liners may be increased by stress,
and is not necessarily uniform throughout the component.
Such information confirms the need to sterilize and store
plastic components in inert or vacuum environments if the
durability of the plastic is to be improved.
Based on information in these and similar studies, it has
become clear that the current limiting factor in the
durability and longevity of total hip replacements is the
“wearing out” of the plastic portion of the articulation of
the ball and cup.
Is there a way to extend the life of contemporary total hip
replacements?
Several new and pending products aim to address this very
question. A ceramic-on-ceramic (refers to the ball and
socket surfaces) type of hip replacement is undergoing
pre-FDA approval studies in the United States, and has been
in use for some time in Europe. In contrast to the
traditional metal on plastic articulation, ceramic surfaces
are said to have better wetability and toughness. Ceramic
components, however, are brittle and can fracture more
easily that other materials. Another alternative, a
metal-on-metal type of hip replacement, was recently
approved by the Food and Drug Administration (FDA). The
first such surgery at Maine Medical Center with this type of
articulation was performed in March of this year. Precisely
machined metal on metal surfaces have shown minimal wear in
European studies, but concerns remain about metal ion
release into the patient’s body. One of the most exciting
potential advances in articulation technology, however, is
crosslinking of the ultra-high molecular weight polyethylene
that has been used since total hip replacement surgery was
introduced to the United States in the late 1960s.
Crosslinking is a process in which polyethylene molecules
are bonded together to result in a stronger material,
substantially improving the material’s wear resistance.
Schematic drawing showing how radiation is used to
“crosslink” the polyethylene chains of the plastic lining of
the acetabulum (cup) resulting in a more durable product.
It has been reported that the amount of crosslinking of
polyethylene is directly related to its wear performance,
and significant reductions in polyethylene wear have been
shown in joint simulation studies with highly crosslinked
polyethylene components.
Such a material should lead to much better resistance to
wear, fewer polyethylene particles, less osteolysis (bone
loss around the hip because of the body’s reaction to the
particles) and less frequent implant loosening. The best
method to crosslink the polyethylene, and to what degree,
are unknown. Further, relevant long-term clinical studies
are lacking for this new re-engineered product.
One such study is currently underway at Maine Medical
Center. Dr. McGrory is a co-investigator in a multi-center,
blinded, prospective, randomized study comparing one type of
highly crosslinked polyethylene (the “melt-irradiated” type)
with conventional polyethylene. Patients who enroll in the
study will be followed carefully for a decade or more, and
radiographs will be analyzed with three-dimensional digital
technology to compare wear rates. The Maine Medical Center
Internal Review Board has approved the study, which hopes to
prove or disprove the superiority of this type of
polyethylene.
Conclusion
Two recent projects supported by the
Joint Replacement Center at Orthopaedic Associates of
Portland add to our knowledge of the expected longevity
of total hip replacements and the primary cause of their
failure. A third, ongoing study will prospectively assess
crosslinking of the plastic portion of the artificial hip.
This is a recent innovation that has the potential of
significantly extending the life of hip replacements.
