Effects of rasp mismatch on plasma spray radial head stems

Background: Radial head prosthetic stems designed for bone ingrowth are available with both plasma spray and grit blasted surfaces. A recent study comparing micromotion between the 2 demonstrated greater micromotion in the plasma spray than grit blasted stems, even though the latter had lower surface roughness. This raised the question that perhaps the size mismatch for grit-blasted radial head stems (0.5 mm) might be inadequate for plasma spray stems. Hypothesis: A tighter initial press-fit with plasma spray radial head stems may be gained by preparation with an undersized rasp. Methods: Paired cadaveric radii were implanted with plasma spray stems. The surgical control was prepared with a rasp designated for its corresponding stem size (" size-matched"), while the experimental group was prepared with a rasp 0.5 mm smaller than designated (" undersized"). Results: The micromotion for the undersized rasp group (46 ± 12 μm) was not significantly different than for the size-matched rasp group (21 ± 12 μm) (P = .1). Discussion: Contrary to our hypothesis, no reduction in micromotion was observed when using an undersized rasp with a plasma spray stem. The micromotion results were not different from those observed when using a size-matched rasp, and actually approached significance in the opposite direction. This may be due to the rough stem surface chipping away bone fragments, rather than the bone being cut away precisely as is done with a rasp. Conclusion: The use of an undersized rasp prior to implantation of a plasma spray radial head prosthesis does not confer any added benefit in terms of initial stability.