Vector analysis of porosity evidences bone loss at the epiphysis in the BTX rat model of disuse osteoporosis

Abstract

IntroductionBotulinum toxin (BTX) injected in a muscle causes paralysis with a subsequent bone loss. It represents a model of disuse osteoporosis. Although bone loss has been regularly evaluated at the metaphysis of long bones, little is known concerning the bone changes occurring in the epiphysis. Material and methodsTen Copenhagen male rats received a single BTX injection in the Mus quadriceps femoris on the right side and unilateral paralysis developed in the following days. Animals were euthanized after 28 days; femur and tibia were harvested and analyzed by microCT. Vector analysis of porosity was applied to the 2D sections and produced a frontal image with mapping in pseudo-colors. This allows quantitative analysis at the epiphysis and metaphysis. “Hot spot” were evidenced and indicated bone loss. Quantitative analysis of these images was done by decomposition of the R, G and B planes and deriving the ratio of R+G pixels on the whole pixel number. ResultsAt the metaphysis, this ratio was correlated with measurement of the bone volume obtained by microCT. At the epiphysis, which has a complex shape in 3D, the method easily identified the bone loss. DiscussionParalysis of a unilateral quadriceps induces bone loss at the metaphysis of the long bones. However, the epiphysis, having a reduced bone remodeling is also concerned by disuse. MicroCT analysis of this part of the bones is difficult due to its complex shape in 3D. Vector analysis is a new and robust method to quantify bone loss in such complex areas.