The intramedullary hydraulic pressure tolerance of the human femur.

Abstract

A new range of clinically used, biodegradable, bone-bonding copolymers has hydrogel properties. With intramedullary press fit, these polymers generate considerable pressure through swelling caused by water absorption. In total hip arthroplasty, high intramedullary pressures can be attained by insertion of the prosthesis into femora by using methylmethacrylate cement and a cement plug. Because the effects of such intramedullary pressures are not fully known, 10-cm-long segments of the proximal diaphysis of fresh femoral bones from 22 human cadavers were tested for their intramedullary pressure resistance. The intramedullary pressure resistance, calculated burst forces, and stress on the bone were correlated to age, gender, bone mineral content per unit length, and bone mineral density. In most bone segments, a burst fracture occurred just opposite the linea aspera. The hydraulic bursting pressures ranged from 2.3 to 13.4 MPa (age adjusted gender effect [male-female], 2.2 MPa [p = 0.05]). The forces at fracture were calculated to be between 265 and 1361 N (regression coefficient: 247/unit bone mineral content per unit length). The tangential stress appeared to be between 7.54 and 27.54 MPa (regression coefficient: 11.6/unit bone mineral density; bone mineral density adjusted gender difference [male-female], 4.5 MPa [p < 0.05]). Biochemical examination of the bones did not show any abnormalities. The results of this study show that the human femora can resist mechanically an intramedullary pressure of at least 2 MPa. From a mechanical point of view, intramedullary pressures generated by swelling polymers should remain below 2 MPa for safe use.