In vitamin D-fed chicks 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 were implanted into experimentally-produced fractures of the mid-tibia. The mechanical and biochemical properties of the tibia were evaluated for two weeks, including torsion tests, measurement of alkaline phosphatase activity, 45Ca incorporation, and Ca2+ content. Both dihydroxylated metabolites of vitamin D3 had a direct effect on endochondral bone formation. 24,25(OH)2D3 strengthened the callus, and raised alkaline phosphate activity in the first seven days after fracture. 1,25(OH)2D3 decreased the strength of the callus concomitant with a reduction in 45Ca incorporation. It is suggested that local application of 24,25(OH)2D3 into fractures may accelerate healing and prevent non-union.

The levels of the active metabolites of vitamin D were measured in the callus and in the epiphyseal growth plate of chicks given radioactive cholecalciferol during fracture healing. Those levels were correlated with the histological findings. Three groups of chicks were studied: a control group with no fracture, chicks with fractures fixed by Kirschner wire, and chicks with unfixed fractures. A significant increase in the levels of the active metabolites was found in the callus during the first few days after fracture. The levels of 25-hydroxycholecalciferol [25(OH)D3] and of 24,25-dihydroxycholecalciferol [24,25(OH)2D3] were higher when there was no fixation, while those of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] were higher after fixation. The concentrations of these metabolites in the proximal epiphysis of the tibia were similar to those found in the callus. Based on these findings it is suggested that the active metabolites of vitamin D are directly involved in the process of fracture repair.