Spinous process hypertrophy
associated with implanted devices
in the external link model
AUTHORS: Nicole M. Homb, B.S., D.C., Davenport ’09,
and Charles N.R. Henderson, D.C., Ph.D.
The Palmer Center for Chiropractic Research conducts studies
at facilities on each Palmer campus, including the William
and Jo Harris Building on the Davenport Campus.
INTRODUCTION: The recent development of a small animal (rat)
model as an experimental mimic of the chiropractic subluxation
employs the use of linked titanium implants on the spinous
processes of three contiguous lumbar segments. Varying degrees
of spinous process hypertrophy have been noted with this model,
and this appeared to be greater in animals with intervertebral
links than with control rats that were never linked. The purpose
of this study was to examine serial radiographs of implanted rats
for evidence of spinous hypertrophy, and to evaluate possible
correlating factors, such as linking history and exudate formation.
The possible role of spinous hypertrophy as a determining factor
in intervertebral hypomobility was also examined.
METHODS: Serial lateral radiographs of 73 male Sprague Dawley
(400-600 g) rats with surgically implanted spinous attachment
units were reviewed. An initial radiograph (baseline film) was
taken following a six-week surgical recovery period with a
subsequent radiograph taken eight weeks later. The degree of
hypertrophy at the L4, L5, and L6 spinous processes was
determined by measuring spinous width on the radiographs
with a modified micrometer. Bone resorption and exudate
build-up were graded using a four point grading scale, (0-3,
none-severe). Trends in the data were described by crosstabulated
counts, ANOVA, and regression analysis.
RESULTS: Spinous hypertrophy crosstabulation suggested a
difference in hypertrophy prevalence between rats that had been
linked in experimental fixation and never-linked control rats.
However, correlation analysis did not demonstrate a role for
spinous hypertrophy as a predictor of intervertebral mobility.
Similarly, exudate level did not predict the presence or severity
of hypertrophy. As crosstabulation suggested, SAU linking and
vertebral level had a significant interaction, with moderate and
severe hypertrophy occurring more frequently at L4 and L6
in previously linked rats. By contrast, age did not appear to
materially affect spinous hypertrophy.
CONCLUSIONS: Results from these studies suggest that mechanical
stresses produced on the implant-bone interface by links in the
ELM contribute to spinous hypertrophy beyond those associated
with the presence of the SAU on the bone. However, the study
findings also suggest that spinous hypertrophy does not
contribute significantly to intervertebral hypomobility produced
in the ELM. This is important because it supports the argument
that stiffness and hypomobility produced in the ELM is not
materially an artifact of spinous hypertrophy. The analysis upon
which these findings were drawn is a secondary analysis of data
drawn from studies not specifically designed to address these
research questions. Consequently, this study does not validate
the ELM as a research tool; but it does illuminate the path for
future ELM investigations.
This abstract was presented in a poster,
Hypertrophy in the External Link Model,”
at the World Federation
of Chiropractic (WFC) 10th Biennial Congress and the International
Conference on Chiropractic Research World Federation of Chiropractic
and the Foundation for Chiropractic Education and Research (FCER)
Montreal, Que., Canada 2009, p. 179.
Nicole M. Homb, D.C., graduated from Palmer’s Davenport Campus in
2009, at which time she was awarded Research Honors for this project.
She also was awarded 1st place in the student poster competition for
this poster at the WFC/FCER International Conference on Chiropractic
Research in Montreal, Que., Canada, April 30-May 2, 2009.
Charles Henderson, D.C., Ph.D., is an associate professor at the
Palmer Center for Chiropractic Research on the Davenport Campus
and served as a faculty mentor on this project.