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Fall 2010


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.
William and Jo Harris building

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, “Quantifying Spinous 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.  

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