In patients undergoing bilateral tibial lengthening , we compared those receiving an osteotomy site injection of autologous bone marrow aspirate concentrate ( BMAC ) plus platelet-rich plasma ( PRP ) with those not receiving such an injection in terms of external fixator index ( time in external fixation divided by amount of lengthening ) , full weightbearing index ( time until a patient was permitted to do full weightbearing divided by amount of lengthening ) , four cortical healing indexes ( time until each cortical union divided by amount of lengthening ) , and callus shape and type .