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

Background: Successful bone ingrowth around cementless implants requires adequate initial stability. Hoop stress fractures during stem insertion can potentially hinder prosthesis stability. Hypothesis: We 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 methods: Grit-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. Results: Prosthetic radial head stems that were oversized by 1 mm caused small cracks in the radial neck. Micromotion of oversized stems (42 +/- 7 mu m) was within the threshold conducive for bone ingrowth (<100 mu m) and not significantly different from that for the maximum sized stems (50 +/- 12 mu m) (P >= .4). Discussion: Contrary 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. Conclusion: A small radial neck fracture occurring during insertion of a radial head prosthetic stem oversized by 1 mm does not necessarily compromise initial stability. Level of evidence: Basic Science Study, Biomechanical Study. (C) 2012 Journal of Shoulder and Elbow Surgery Board of Trustees.