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Dr. Kurtz had an article online
that had concerns about the head neck rations of hip
resurfacing compared to total hip replacement. We wanted
to provide potential hip resurfacing patients with
opinions of other top resurfacing surgeons to explain
their views.
Hi
Vicky,
Very
nice to hear from you. I'm sorry for the delay in
responding, but I wanted to take time to craft a thoughtful response
on this subject, as I'm sure many people look to you for sound
advice.
I have read over the material at this website many
times. I have concluded, as I'm sure you have, that it was written by
someone who has a very limited experience with resurfacing. In
short, it full of speculation and non-factual conclusions, interspersed
with a few truths. As such, it can appear convincing at first
glance, but when really scrutinized, many of the arguments don't hold
water.
In the section entitled the biomechanics of hip
resurfacing, it is actually true that a resurfacing implant will have a
poorer head neck ratio than a comparable large diameter metal on metal
THR. It is also true that head-neck ratio influences that amount of
motion prior to impingement. However, the question is whether this would
translate to a clinical difference or not. Furthermore, some of the
arguments that he makes are not accurate:
"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."
First off, this argument doesn't account for the
acetabular size at all. As we know, the acetabular and femoral head sizes
are linked together. So the fit of the acetabulum will influence
what head size is used. So, it isn't a rote fact that the neck will be
6-9 mm smaller than the head diameter. The relationship of the
acetabulum and the femoral head size is such that the head neck
ratio doesn't change much from preop to postop. I have performed
research analyses on the typical head ratio following resurfacing. It is
approximately 1.34, as compared to 1.39 preoperative. The reason that
it is slightly lower is to preserve acetabular bone.
"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."
This is a dangerous technique, one that is not
recommended by any experienced resurfacer. By leaving the femoral component
higher, you will increase the risk of fracture.
"Second, the acetabular bone has a particular shape to
the anterior wall that allows more motion before the femoral neck
hits the acetabular rim. "
This doesn't change, even after the resurfacing. If put
in properly, the acetabular component should still be below the
native 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. "
The last statement is ridiculous and unfounded. This is
certainly not the main cause of fracture, otherwise all the fractures
would occur late, and not within the first few months.
In any case, the argument about a poorer head neck ratio
is a theoretical and don't have clinical relevance. That is,
they don't translate to any detriment to patients. It clearly can't
mean that motion is going to be limited, otherwise how do we see
so many success with ROM, such as yourself? It is true that the
components have to be put in perfectly to ensure good ROM, and I
will test this while the patient is on the table, so that I may adjust
it if necessary.
I hope that is helpful.
Regards
Dr. Edwin Su
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