Occurrence of Substance P in Bone Repair under Different Load Comparison of Straight and Angulated Fracture in Rat Tibia

Substance P (SP) has been shown in vitro to stimulate both formation and resorption of bone This seemingly contradictory observation could be explained by in vivo variations m skeletal loading and rate of bone turnover, features which may be explored during different phases of fracture healing In 50 SD rats, the right tibia was fractured and fixed with an intramedullary pin in straight alignment and in anterior angulation resulting in a convex and concave side under different load Fracture repair was assessed by radiography, histology, and semi quantitative immunohistochemistry of SP nerve fiber occurrence at days 7 21, 35, 56 and 84 post fracture During regeneration, days 7-35, abundant SP nerve ingrowth was observed in the fracture callus reaching a side symmetrical peak at day 21 in straight fractures In angulated fractures the SP peak was also observed at day 21 on the concave loaded side, but not until day 35 on the convex unloaded side Each SP peak coincided with cortical bridging During remodeling, days 35-84 a side symmetrical disappearance of SP positive fibers was seen in straight fractures The same pattern was seen on the concave loaded side of angulated fractures However, on the convex unloaded side, where resorption now took place, SP fibers remained until the end of the experiment Our study suggests that neuronal SP during bone regeneration has a stimulatory role on bone formation, while during remodeling increased SP fiber density in unloaded areas may be related to bone resorption (c) 2010 Orthopaedic Research Society Published by Wiley Periodicals Inc J Orthop Res 28 1643-1650 2010