To The Editor:
The article "Six-Pin Halo Fixation and the Resulting
Prevalence of Pin-Site Complications" (83-A: 377-82, March 2001),
by Nemeth and Mattingly, contains observations that are in conflict with
the literature1 and with general
clinical experience.
It seems intuitive that six fixation screws equally torqued will
hold an encircling ring to a plastic sphere more firmly than four
will, just as six screws, properly applied, will hold two pieces of
wood together more firmly than four. The measured load to failure
was pure axial cephalad-directed distraction that produced distortion
of the alginate skull model. The end point was the observation of
visible movement of the pin. The clinical failure used for comparison was
any loss of torque resistance of any pin at any time during halo
fixation after its initial insertion to a level below that first
applied. The halo apparatus was in place for a minimum of two weeks
with no average, mean, or maximum time of fixation noted. There
is also no report of follow-up after hospitalization or use of any
records other than hospital records.
This is not similar to the usual clinical situation in many respects.
More than four pins have been frequently used in children and others
with thin skulls, motion disorders, skull fracture involving an
oblique-shaped forehead, traumatic or surgical wounds, and other conditions
that would compromise the usual fixation2.
I have used as many as twelve pins with a custom-made ring. The
pins are inserted clinically in living bone, which has an obligatory
initial creep and then a resorptive response to pressure over time,
well shown in an elegant in vivo study3.
This has also been studied in fresh human cadaver skulls, and significant dimunition
of the force against the skull occurs in the first five minutes
after insertion in multiple models of pin and ring4.
Halo pins, of necessity, are placed percutaneously, just as a
traction pin or external fixation pin is placed in an extremity;
thus, they are similarly associated with a greater risk of chronic, low-grade,
superficial infection. If a halo pin is inserted initially to 8
in-lb (0.90 Nm), it will significantly loosen within eight to twenty-four
hours and, thus, many surgeons retighten pins at this time. Further
loosening occurs inevitably in a manner that is time-dependent due
to the biologic resorptive response of bone to pressure, and it is
affected by other factors, including activity of the patient, muscular strength
of the patient, poor hygiene and/or poor pin-site care,
motion disorders, and osteopenia. When the pins are sufficiently
loose, they become painful and require tightening unless the apparatus
is no longer needed. Rarely will a halo become dislodged unless
it is poorly applied, the skull is unusually shaped, the skin is
not cared for frequently, the torsion of the pins is not checked
and adjusted periodically, or the apparatus is left on for an inordinately
long time. The average length of halo fixation therapeutically is
usually three to four months.
Thus, I feel that this study adds little, if anything, to our
knowledge and that it is misleading to orthopaedists, neurosurgeons,
and orthotists.
J.A. Nemeth and L.G. Mattingly reply:
We welcome the opportunity to respond to the concerns raised
by Dr. Whitesides and to discuss further our methods of halo application.
At the outset, there appears to be some misunderstanding about
the structure of our article: the measured load to failure of axial
distraction was not compared with loss of torque resistance in the patient
records. Our study consisted of two separate parts: first, a force-distraction
study to document the difference in fixation strength between four-pin
and six-pin halo application, and second, a retrospective analysis
of patient records to compare complication rates for six-pin application
with those established for four-pin application. The findings of the
two parts were separate but allowed for comparison and inferences
to be made between the two at the conclusion of each part of the
study.
We agree with the statement that it is intuitive that six-pin
fixation would be stronger than fixation with four pins; however,
we believe that this intuition still requires scientific examination
to verify the hypothesis and to quantify the difference in fixation
strength. We also recognize the limitations of our physical model
in duplicating the dynamic processes inherent in living bone, and
we recognize that axial distraction forces are only one set of forces acting
on a halo. We would like to pursue in further studies the micromotion forces
at the pin-bone interface, as we believe that these may have a greater relationship
to pin loosening and ultimately to rates of infection.
Some clarification concerning the patient records that were analyzed
also appears to be necessary. They were not limited to the hospital
records alone (although those were included) but were the complete
records of the patient’s treatment at the Barrow Neurological
Institute; we stated in our Materials and Methods section that the hospital
charts needed to be complete and to include documentation of follow-up
treatment. Our protocol for follow-up involved office visits at
three-week intervals for the duration of the halo treatment; we
believe this to be one of the most comprehensive follow-up protocols
in the nation. Our methodology for examining patient records was
consistent with that in the literature from which we derived the
complication rates; however, we agree that a discussion of the duration
of halo fixation, including an average (which was approximately
four months), would have enhanced our discussion.
We would challenge on several points Dr. Whitesides’ assertion
that dislodgment is a rare occurrence. First, dislodgment was not
the focus of our comparison of complication rates—our greater
concern was pin loosening and the potential for infection. Second,
in our clinical practice, dislodgment becomes a greater concern
in patients who require a longer-than-average duration of halo fixation.
Also, in the "rare" instances when a halo does become
dislodged, it often has more to do with poor vest application and
poor biomechanical function leading to failure at the pins and rings.
One concern of Dr. Whitesides is that our article is "in
conflict with . . . general clinical experience." We would submit
that this conflict was part of our goal in writing the article,
as we are hoping to effect a change in the current paradigm for
halo application. The retrospective analysis of our patient records
demonstrates the uniqueness of our clinical experience in several
ways. First, pin loosening has not been an inevitable event in our
practice, and our definition of pin loosening as any loss of torque
resistance below 8 in-lb (0.90 Nm) is therefore consistent with
our experience. We attribute this increased strength to our standard
use of six pins in fixation.
In addition, our clinical experience differs from the norm in
that our greater success in halo application has allowed us to utilize
halos more fully as a clinical tool. We have often used halos for periods
longer than six months. The cases that require longer wear of the halos
prompted our initial use of additional pins in clinical practice,
since in such instances, greater fixation strength is not only required
but imperative.
This change to the current paradigm for halo application is the
greatest value of our study, and we believe that it is of great
importance to orthopaedists, neurosurgeons, and orthotists as well
as to patients who may benefit from improved halo application techniques. There
will always be those who disagree with our clinical experience,
but we are grateful to have sparked further discussion.