Prosthetic radial head stem pull-out as a mode of failure: A biomechanical study

Purpose: Press-fit cementless radial head implant longevity relies on adequate bone ingrowth. Failed implant osseointegration remains a clinical concern and has been shown to lead to prosthetic failure. The purpose of this study was to test the hypothesis that implants with sufficient initial press-fit stability would be less likely to fail due to implant pull-out, as demonstrated by an increasing amount of energy required to remove the prosthesis from the canal. Methods: Ten cadaveric radii were implanted with five sizes (6-10 mm in 1-mm increments) of grit-blasted, cementless radial head stems. A customised slap hammer was used to measure the energy required to remove each stem. Stem-bone micromotion was also measured. Results: The suboptimally sized stem (Max - 1) (i.e. 1 mm undersized) required less energy (0.5 ± 0 J) to pull out than the optimally sized stem (Max) (1.7 ± 0.3 J) (p = 0.008). The optimally sized stem demonstrated greater initial stability (45 ± 7 μm) than the suboptimally sized stem (79 ± 12 μm) (p = 0.004). Conclusions: This investigation demonstrates the importance of obtaining adequate press-fit stability for the prevention of radial head stem pull-out failure. These data add to the relatively scant knowledge in the literature regarding radial head biomechanics. The energy required to remove a prosthetic radial head ingrowth stem decreases in conjunction with diameter. The use of an inadequately sized stem increases the stem's micromotion as well as the risk of prosthetic loosening due to pull-out.