Evaluation of an expandable stem total hip replacement system.

The expandable nail concept uses high-pressure saline to inflate the diameter of a cylindrical implant to achieve fixation within the medullary canal of bone. Expandable nails are used clinically in a number of fracture management applications and have been more recently developed as a femoral component for total hip replacement. In this study, the expandable total hip replacement stem design was evaluated, specifically testing to determine if acrylic cement can be used in place of saline for permanent expansion, to observe the amount of stem subsidence after cyclic loading, and to document if stem expansion creates untoward stresses in the femur. The results included that the expandable femoral component could be expanded with cement if careful control of cement viscosity is used with a modified filling technique. Neither untoward hoop strains nor stem subsidence was found in tests with the cadaveric femurs. Bench testing has confirmed the stability of these stems. In addition, the substitution of cement for saline would make the construct permanent, avoiding the risk of deflation and loosening. The concept of an expandable femoral prosthesis is appealing and would have many potential clinical applications. The need for cemented stems or the more difficult and costly "Ling technique" could be avoided with their use in tumor surgery, hip fracture management, and total hip replacements associated with osteoporosis or a patulous femora in both primary and revision settings.