Summary:Even though the problems of intramedullary pressure increase and its effects on pulmonary function have not yet been resolved, intramedullary nailing after reaming remains a relevant treatment procedure, especially for pseudarthrosis of the tibial and femoral shaft, because good results have been achieved in these areas. Our study aimed to clarify whether modifications to reamer and shaft design would reduce intramedullary pressure increase. Measurements were taken in plexiglass tubes filled with a vaseline/paraffin oil mixture with viscoelastic properties at 20°C, corresponding to the viscoelastic properties of bovine medullary fat at 36°C. Measurements were also taken at the metaphysis in pairs of human femora. The results clearly show that a reduction in shaft diameter will produce a sustained reduction of intramedullary pressure and that the modification of reamer design combined with thinner shafts will lead to a further reduction. In comparison with the conventional AO reaming system (9.0-mm shaft + 9.5-mm AO reamer), the use of thinner shafts and a modified reamer design reduced intramedullary pressure as follows: (a) 9.0-mm shaft + 9.5-mm hollow reamer by 22%; (b) 7.0-mm shaft + 9.5-mm AO reamer by 47%; (c) 7.0-mm shaft + 9.5-mm hollow reamer by 65%; and (d) 6.0-mm shaft + 9.5-mm hollow reamer by 75%. These results show how efficiently the intramedullary pressure increase during reaming can be reduced by simple modification, which in turn minimizes the risk for the patient.