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Reaming the medullary cavity leads to an increase in intramedullary pressure, which can cause local necrosis and lung malfunction after intramedullary nailing. This investigation concentrates on the effect of reamer design parameters on intramedullary pressure increase.Pressure measurements were obtained for three newly designed solid reamers and one hollow reamer. The AO/ASIF reamer was used for reference values. All reamers were connected with small flexible shafts. The pressures were measured in Plexiglas tubes filled with a mixture of Vaseline and paraffin oil with flow properties at 20°C equivalent to those of bovine medullary fat at 36°C. The reaming assemblies were inserted into the tubes using a materials testing machine at a constant speed. In addition, pressure measurements were made using five pairs of human femora.The highest pressures occurred with the AO/ASIF reamer head (258 ± 29 mm Hg, p ≤ 0.05). By creating a conical form and enlarging the flutes, pressures were reduced by up to 37% (164 ± 13 mm Hg, p ≤ 0.05) compared with the AO/ASIF reamer, depending on the depth of the flutes. With a newly designed hollow reamer, pressure was reduced by 58% (108 ± 19 mm Hg, p ≤ 0.05) compared with the AO/ASIF reamer.The results show that optimizing the design of the reamer head leads to a significant reduction in pressure increase. These results should be taken into consideration when designing new reaming systems in an attempt to minimize the complication rate for intramedullary nailing.