Does immobilization influence the systemic acceleratory phenomenon that accompanies local bone repair?

The rate of remodeling in the region of a bone defect exceeds normal tissue activity. It was Frost who described this reaction as the regional acceleratory phenomenon (RAP). We previously showed that restoration of a local bone defect in the rat leads not only to RAP but also leads to a systemic acceleration of osteogenesis (systemic acceleratory phenomenon, SAP) in distant sites of the skeleton. In this study we investigated the impact of immobilization of the defect-bearing extremity on the development of SAP. A hole 1.2 mm in diameter was drilled in the diaphysis of the left tibia of female rats. In the experimental group (n = 15), a knee tenotomy was performed in the defect-bearing left hind leg. We examined both femora, both tibiae, and the fourth lumbar vertebra by computed x-ray densitometry on day 7 postoperatively. Immobilization of the defect-bearing limb led to a decrease in x-ray density not only of the immobilized (p < 0.0001) but also of the contralateral tibia (p < 0.0001). Both femora (p < 0.001) and the fourth lumbar vertebra (p < 0.025) of the experimental group also showed a significant decrease in x-ray density. We previously showed that SAP leads to an increase in x-ray density of both femora. This increase is no longer detectable in animals after immobilization of the defect-bearing limb. Thus we conclude that immobilization interferes with SAP. This suggests the possible dependence of SAP on mechanical load. Furthermore, these data suggest a possible impact of local immobilization on the rest of the skeleton.