To the Editor:
The success of total hip replacement (THR) in the 20th century has been
tremendous with improvements in the durability of new designs, bearing
materials and fixation techniques. However, the young and active patients have
historically had high revision rates compared with older, more sedentary
patients, notably when the etiology of the disease is osteonecrosis. Despite
great improvements in cementless stem fixation, hip resurfacing arthroplasty
(HRA) has the advantages of replicating leg length and offset, and maintaining
proximal bone unlike THR. Moreover, dislocation in THR remains a problem when
small femoral heads are used. Resurfacing patients also do not report thigh
pain as it sometimes happens after THR.
When it comes time for revision surgery, as should be expected for most
young and active patients, whether treated with a resurfacing or a primary THR,
the preserved bone stock with hip resurfacing provides more favorable
conditions for a successful surgery and the technical difficulty of the
conversion is comparable to that of a primary THR. This enables patients
seeking to restore their previous lifestyle to be more active than with a THR,
and numerous authors have reported high levels of physical activity in patients
after hip resurfacing.
Preserving bone and replacing only the affected articular surfaces has
always been and remains a worthy treatment goal, but the first generation of
hip resurfacing suffered from the use of ultra-high molecular weight
polyethylene sterilized in air, a material unsuitable for a large bearing size
in young and active patients. Unfortunately, and because of the poor results of
the materials used during that era, many become detractors of the concept of
The current generation of resurfacing devices uses a metal-on-metal
(MoM) bearing because the volumetric wear is low, even with large diameter
components, and the material permits manufacturing of thin one-piece acetabular
components with porous ingrowth for cementless fixation.
Recent long-term data show that certain currently available hip
resurfacing devices can reach up to 99.7% survivorship at 10 years in patients
with good bone quality and implanted with large component sizes. The importance
of patient selection for hip resurfacing and the key role of component size
have been highlighted by the reports of several large centers and the findings
of hip registries. However, improvements in surgical technique have
considerably reduced the rate of aseptic femoral failures (femoral neck
fractures and femoral component loosening) associated with the learning curve
of the surgeons who pioneered the procedure, even with patients who have risk
In the past 3 years, the report of numerous adverse local tissue
reactions (ALTR) has raised doubts in the orthopedic community over the
benefits of MoM implants used for HRA and even more so for THR, probably
because of the corrosion products generated at the ball-stem tapered interface.
Often misunderstood, the hypersensitivity or allergic reactions to metal are a
very rare occurrence. In nearly every case, ALTR are associated with increased
wear from poorly positioned or poorly designed components or both. The recall
of a particular MoM resurfacing device resulted in intense medico-legal
solicitations accompanied by grossly unbalanced reporting in a media blitz and
has brought intense scrutiny to all MoM devices. Anxiety has grown among
patients and surgeons despite the fact that the vast most of MoM devices are
functioning well with up to 20 years of follow-up. This was only aggravated by
two reports of extremely rare cases of cobaltism in association with very high
wear of a metal-on-metal bearing. Naysayers abound, but few have researched the
literature where the answers to the problem can be found (i.e., proper
acetabular component design and orientation).
The acetabular components with the largest femoral head coverage have
been associated with low production of metal ions and a virtual absence of
ALTR. The hypothesis tested in several centers is that a reduced coverage of
the head coming from a combination of increased abduction and/or anteversion
leads to more instances of edge loading, and in the case of the recalled
device, a possible double edge loading, increasing the volumetric wear of the
The current focus of most debates about MoM bearings has so far been the
wear properties of the material. I believe that the problems ALTR associated
with MoM devices, unlike the first generation of resurfacing with polyethylene
bearings, are not a bearing material issue per se, but one of device design and
technique and can be prevented by proper component socket design and optimized
socket orientation in both the coronal and sagittal planes.
Because the need for proper socket orientation is so crucial, emphasis
must be placed on using systems in which the instrumentation provides the
surgeon with the tools to precisely implant the socket. Since all currently
available designs have a reduced coverage in the smaller sizes, safe zones for
socket orientation should be determined for all systems using an accurate
measurement method for both cup abduction and anteversion (e.g.
Einzel-Bild-Rontgen-Analyse; Innsbruck, Austria) in a large series with
long-term results. After analysis of our 1350 Conserve Plus Hips (Wright
Medical Technology), a safe zone was produced for all sizes suggesting 42°
± 10° of abduction angle and a 15· ± 10· of
anteversion angle, which I believe represents a target well within reach for an
Although the study of navigation systems applied to hip resurfacing has
so far essentially focused on the placement of the femoral component, a case
could certainly be made for the use of navigation systems, which have shown
efficacy in positioning accurately the acetabular component in THA, especially
for inexperienced surgeons. Just as in THR, exposure of the acetabular cavity
is essential for an accurate placement of the cup and, despite short-term
reports of hip resurfacing performed with small incisions, the minimally
invasive surgery technique can hardly be recommended for this procedure.
Unfortunately, the critical importance of these factors has not been
well-understood until recently, the common belief being that a large diameter
head solves the problem of dislocation in THR just as it does with HRA, while
component orientation was less important because of the increased jump
Every bearing material used in joint replacement has had its specific
downsides, and this remains true today despite all the technological advances.
For example, the necessity of proper acetabular component orientation for
ceramic-on-ceramic bearings to prevent dislocation, chipping, runaway wear and
squeaking was identified after clinical experience and resolved after in vitro
testing. Similar to the ceramic-on-ceramic bearing wear, the MoM wear issues of
design and component orientation became apparent after clinical experience, and
there is tangible evidence that the problem is solvable. Clearly, this is
highly desirable since MoM is the only proven and highly successful material
for resurfacing at the present time permitting high levels of activity. The
well-oriented and fixed components continue to perform at up to 15 years, with
no increase in blood-ion concentration.
The porous fixation of the one-piece acetabular components used in the
current designs of hip resurfacing is usually a cobalt chromium aggregate of
beads with or without hydroxy apatite coating although several hip resurfacing
systems now have titanium plasma spray porous sockets. This type of fixation,
although reliable, may not match the performance of recently developed porous
tantalum or titanium material mimicking trabecular bone. This new technology
has already been adapted to hip resurfacing, although its availability is
While most surgeons understand the increased difficulty of performing
resurfacing compared with THR, there has been insufficient education for
surgeons to gain supervised experience. Fellowships and resident training
programs to learn the hip resurfacing techniques hopefully can produce more
experienced surgeons ready to use with competence the full spectrum of
reconstructive devices available today and in the future.
Improving the socket designs of existing devices to provide larger
femoral head coverage by the socket will give the surgeon more flexibility of
implantation and lessen the likelihood of abnormal wear. Modifications to
socket designs are already in progress to address the weaknesses identified
with certain devices. The manufacturing quality of MoM bearings is excellent
nowadays in most designs, particularly the control of clearance and roundness
of the components, and is certainly sufficient to produce safe and successful
hip arthroplasty devices as long as conservative guidelines for cup
implantation are followed. However, there will undoubtedly be further
improvements and reduction in the wear properties of MoM bearings in the near
MoM hip resurfacing is not a new technology any more, and the devices
that have been approved by the FDA after sufficient clinical trials have been
available for more than a decade. It is important to avoid making statements
about a procedure based on the poor results of subsequent designs which were
still in their experimental (non-FDA approved) phase.
Many have written off MoM HRA, and this is unfortunate because superb
results have been achieved with several devices in several centers for the
young and active patient. The MoM bearing, unlike polyethylene of first
generation devices, is not the problem. It is a treatment option for arthritis
that is worth pursuing and improving. It just makes sense to save the head and
neck, and adhere to a fundamental tenet of orthopedics espoused by our
forebears save bone.
It is my fervent desire to improve surgeon and patient education and
emphasize that there are techniques to solve the problems associated with MoM
hip resurfacing. Charles Kettering said, The world hates change, yet it
is the only thing that has brought progress.
Harlan C. Amstutz MD
Disclosure: Amstutz has received research support from St.
Vincents Medical Center, and research and royalities from Wright Medical
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