EXPOSURE TO LOW-INTENSITY ULTRASOUND INCREASES AGGRECAN GENE-EXPRESSION IN A RAT FEMUR FRACTURE MODEL

The effects of ultrasound stimulation on various parameters of bone re pair after diaphyseal injury were assessed in a standard rat femur fra cture model. Bilateral closed femoral fractures were made in 79 skelet ally mature male Long-Evans rats. An ultrasound signal consisting of a 200 microsecond burst sine wave of 0.5 MHz repeating at 1 kHz, with a n intensity of 50 or 100 mW/cm(2) spatial and temporal average, was ap plied to one fracture in each animal. The contralateral fracture was n ot exposed to ultrasound and served as a control. Mechanical testing o f the healing fracture was performed 3 weeks after injury. In fracture s treated with a 50 mW/cm(2) ultrasound signal, the average maximum to rque (223.5+/-50.5 Nmm compared with 172.6+/-54.9 Nmm, p=0.022, paired t test) and average torsional stiffness (13.0+/-3.4 Nmm/(o) compared with 9.5+/-2.9 Nmm/(o), p=0.017) were significantly greater in treated than in control fractures. In animals treated with a 100 mW/cm(2) ult rasound signal, the average maximum torque and torsional stiffness wer e greater in treated than in control fractures, but this trend did not reach statistical significance. Biochemical analysis of callus in ult rasound-treated and control fractures failed to demonstrate significan ce. Biochemical analysis of callus in ultrasound-treated and control f ractures failed to demonstrate significant differences in cell number, collagen content, or calcium content. Evaluation of gene expression i n fractures treated with 50 mW/cm(2) ultrasound demonstrated a shift i n the expression of genes associated with cartilage formation; aggreca n gene expression was significantly higher on day after fracture and s ignificantly lower on day 21 (p=0.003 and 0.035, respectively). alpha 1(II) procollagen gene expression was similarly modified, but this tre nd did not reach statistical significance. Expression of genes coding for bone-related proteins, including alpha 1(I) procollagen, bone gamm a-carboxyglutamic acid protein, alkaline phosphatase, and transforming growth factor-beta 1, did not differ between ultrasound-treated and c ontrol fractures. These data suggest that ultrasound stimulation incre ased the mechanical properties of the healing fracture callus by stimu lating earlier synthesis of extracellular matrix proteins in cartilage , possibly altering chondrocyte maturation and endochondral bone forma tion.