Effect of hoop stress fracture on micromotion of textured ingrowth stems for radial head replacement

BackgroundSuccessful bone ingrowth around cementless implants requires adequate initial stability. Hoop stress fractures during stem insertion can potentially hinder prosthesis stability.HypothesisWe hypothesized that an oversized radial head prosthetic stem (1 mm “too large” and causing a hoop stress fracture during insertion) would result in an unacceptable amount of micromotion.Materials and methodsGrit-blasted radial head prosthetic stems were implanted into cadaveric radii. Rasp and stem insertion energies were measured, along with micromotion at the stem tip. The sizes were increased until a fracture developed in the radial neck.ResultsProsthetic radial head stems that were oversized by 1 mm caused small cracks in the radial neck. Micromotion of oversized stems (42 ± 7 μm) was within the threshold conducive for bone ingrowth (<100 μm) and not significantly different from that for the maximum sized stems (50 ± 12 μm) (P ≥ .4).DiscussionContrary to our hypothesis, hoop stress fractures caused by implantation of a stem oversized by 1 mm did not result in loss of stability. Stem micromotion remained within the range for bone ingrowth and was not significantly diminished after the fracture. This suggests that if a crack occurs during the final stages of stem insertion, it may be acceptable to leave the stem in place without adding a cerclage wire.ConclusionA small radial neck fracture occurring during insertion of a radial head prosthetic stem oversized by 1 mm does not necessarily compromise initial stability.