I want to measure standard deviation of two groups?

I am trying to do a meta-analysis about an issue related to osteogenesis. I have the osseous defect size at the baseline and at the end of the procedure. But I need the amount of defect size reduction, which means I should reduce the last defect size out of the first one. But I have this amount in the form of mean+SD and I need mean and SD of defect size reduction, I have sample size and I know how to measure net mean. But I don't know how to measure SD for this parameter.