Hi chicago60602-ga, and thanks for your question. I'm sorry to hear
of your client's misfortune.
Perhaps the best study that's pertinent to your question is from the
journal Spine, published by a group from the neurosurgery department
at the Medical College of Wisconsin:
Pintar FA, Yoganandan N, Voo L. Effect of age and loading rate on
human cervical spine injury threshold. Spine. 1998 Sep
15;23(18):1957-62.
Unfortunately, this article is not freely available online. You can
request a free reprint from Dr. Pinter:
fpintar@post.its.mcw.edu.
This article actually goes somewhat further than your question asks,
and addresses the effect of subject age and the rate at which the load
is loaded onto the spine. As the authors state, there are many
variables to consider when trying to determine what axial forces are
necessary to injure the cervical spine:
"To determine preventive interventions for cervical spinal injury, the
human threshold of injury must be defined as biomechanical variables.
It has long been known that the biomechanical properties of the spine
are dependent on many factors. It has been shown that patient range of
motion decreases with increase in age,[3,5] and women have greater
range of motion than men.[5] An inverse correlation between age and
vertebral body strength has been identified,[15,18,19] and there is
also a direct correlation between rate of loading and bone
strength.[2] These previous studies provide a basis of understanding
for hypothesizing what the intact human cervical spine injury
tolerance may depend on. Therefore, the hypotheses to be tested in the
current study are: Cervical spine compressive strength decreases with
an increase in subject age; the male population has more strength than
the female population; and with an increase in the loading rate,
strength increases."
To better nail down these issues, the authors looked at cadavers of various ages:
"Compressive loading tests were conducted on 30 human cadaver
head-neck complexes (age, 29-95 years; 12 women/18 men). The
biomechanical results have been published.[11,13,14] In the current
study, the results of these three investigations were merged to apply
statistical analysis models and test the stated hypotheses. Specimens
were loaded under quasistatic load application at 0.25 cm/sec up to
dynamic (high-velocity) loading rates of 800 cm/sec. The methods of
specimen preparation and mounting have been described for these
quasistatic [13] and dynamic [11] studies. Briefly, human cadaver
head-neck complexes were mounted in fixative at T1-T2 inferiorly. The
cranium was left intact superiorly. The inferior of the preparation
was mounted to a load cell and firmly fixed to the platform of an
electrohydraulic piston apparatus (MTS Systems Corp., Minneapolis,
MN). The cervical spine was prealigned to remove the lordosis of the
column by including 15° to 30° of head flexion. The head was held in
place using pulleys, and impact was imposed at the vertex using the
piston apparatus."
"All preparations were tested to failure. Twenty-five specimens were
selected for inclusion in the study because of the alignment of the
spinal column at or close to the stiffest axis (±0.5 cm). This was
determined by pretest radiograph. This subgroup consisted of specimens
aged from 38 to 95, 15 men and 10 women. Results of a previous study
had demonstrated that this alignment "window" was a necessary
condition to produce midcolumn compression fractures of the cervical
spine.[14] The axial load to failure parameter was chosen as a measure
of cervical spine strength. Deformation of the preparations were
derived from vertebral marker kinematic measurements recorded by
high-speed film cameras and linear variable differential transformer
displacement from the electrohydraulic piston. All specimens were
tested to failure with piston contact displacements of as much as 40
mm. The pathologic extent of injury was determined by analysis of
plain radiographs, computed tomograms, and cryomicrotomy sections
before and after the test."
In Figure 1, the authors summarize their results:
http://img81.imageshack.us/img81/1788/cspinefailure8zv.jpg
Some useful conversions:
1 kiloNewton (kN) = 224.8 pounds
This figure is a little bit counterintuitive in some ways, and I found
that Figure 4, in which the authors calculate the probabilities of
failure for various (quasi-static) conditions, was somewhat easier to
make sense of:
http://img81.imageshack.us/img81/7797/kmcspine0ji.jpg
From this figure, for a load of 100 pounds (0.44 kiloNewtons), we see
that the probability of failure is nearly zero. Looking at other
loading rates, the probability of failure is also nearly zero. This
of course, doesn't take into account any pre-existing conditions or
predilections to failure that your client might have had.
The overall range of forces required for failures is summarized in this excerpt:
"The compressive tolerance then varied from 7 kN in the young (third
decade) to 2 kN in the very old (ninth decade). Considering rate of
loading but not age and gender, a second-order polynomial regression
equation was the best fit to the data with a correlation coefficient
of 0.72. The force at failure values ranged from 2 kN at quasistatic
loading rates to 5 kN at dynamic loading rates (800 cm/sec)."
So, it takes 2 - 5 kN (450 - 1,124 pounds) for various rates of
loading the axial force to cause failure.
_______________
To get some feel for how accurate the data from this study is, I also
took a look at this study, which looked at simulated cervical spine
forces generated in NFL tackles that typically resulted in
concussions:
Viano, David C. Dr. med., Ph.D.; Pellman, Elliot J. M.D. Concussion in
Professional Football: Biomechanics of the Striking Player-Part 8.
Neurosurgery. 56(2):266-280, February 2005.
This article is also not freely available. You can request a free
reprint from Dr. Viano at the Mild Traumatic Brain Injury Committee at
the National Football League:
dviano@comcast.net
Here's what they found:
"In this study, neck compressive forces were recorded above the
current tolerance for serious neck injury. Using the neck injury
criterion of the NHTSA, the Nij averaged 1.25 ± 0.16 in eight cases
above the tolerance limit of Nij = 1.0. The average neck compression
force was 6631 ± 977 N (range, 5210?8194 N) for the nine cases above
the tolerance limit of 4000 N neck compression force. The neck
tolerance criterion represents a risk of serious injury in 30- to
35-year-old men."
The tolerance limit is stated as 4,000 Newtons (4 kN), or about 899
pounds. From the above excerpt, one would expect more neck injuries
in NFL players who are getting concussions, since they are
experiencing forces in the 5210-8194 Newton range. Here's how the
authors explain this:
"...[T]he NFL data are from a population of players with superior
physical conditioning and training. Strengthening exercises for neck
muscles give these players far greater tolerance to neck compression,
and the players have an ability to maintain an axial alignment of
their cervical spine during head-down tackle.
There is another reason why the NFL players are not experiencing neck
injuries in concussive impacts. NFL players are typically bigger than
the 50th percentile dummy used in the game reconstructions. It is
known that the larger the player, the greater the tolerance to impact
force. By using established scaling procedures, the NHTSA has found
that the tolerable neck compression force for the 95th percentile man
is 5440 N, compared with 4500 N for the 50th percentile man (8).
However, even if the 95th percentile level is used, 8 of 27 NFL
reconstructions exceeded the higher tolerance limit. Obviously, NFL
players have a unique ability to sustain impact forces. Nonetheless,
the NFL experience reported here provides new tolerance information
relevant to a wide range of safety assessments."
___________
I hope this information is useful. Feel free to request clarification
prior to rating.
-welte-ga |
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