Morphological Study on Bone Formation Induced by Carbon Dioxide Laser Irradiation in Rat Tibiae

Abstract

Alveolar bone is one of the most important target tissues for dental treatment by means of tissue engineering techniques. Bone metabolism is regulated by a variety of growth factors, including bone morphogenetic proteins and fibroblast growth factors, systemc hormones, such as parathyroid hormone, and locally produced cytokines. In addition to these fluid bone-regulating factors, mechanical forces on bone tissues also influence bone formation and bone resorption. However, the mechanism for the action of mechanical forces on bone remains to be studied. One such mechanical force is the carbon dioxide laser, which has been widely used in medical and dental treatment; and, low level laser treatment (LLLT) is of particular interest due to its stimulatory action in wound healing and tissue regeneration. In the present study, the effects of CO₂ laser irradiation on bone formation in rat tibiae were histologically examined. The left tibia was irradiated daily at the midshaft for 15 min at a power of 1.0 W, and irradiation was continued for six days. Total energy density was 6.6 kj/cm². At 7.14, and 21 days after irradiation was started, the tibiae were removed and subjected to histological and bone morphometric analysis. In the CO₂ laser-irradiated groups, in the periosteum and endosteum, while soft X-ray analysis did not reveal and significant differences between the control and irradiated groups. Consistent with histological analysis, bone morphometric analysis showed that the rates of bone formation and bone mineral density were elevated in the irradiation groups. Alkaline phosphatase-positive osteoblasts lined the surface of the newly formed bone, and expression of type-I collagen and osteopontin was observed on the surface, with these proteins accumulating in the bone matrix. On the other hand, in nonirradiated groups, tartrate-resistant acid phosphatasepositive osteoclasts resided on the surface of cortical and trabecular bones. However, the number of osteoclasts was reduced by CO₂ laser irradiation. Taken together, the present results indicate that the CO₂ laser expands the mass of active osteoblasts in both periostium and endostium regions by stimulating the differentiation of osteogenic cells, and that it reduces the number of bone-resorbing osteoclasts, resulting in increased bone formation. These results may open the way for new applications of the CO₂ laser in dental treatment.