Reduction of stress shielding beneath a bone plate by use of a polymeric underplate. An experimental study in dogs.

Plating of fractures results in osteopenia beneath the plate because of the stress-shielding effect. This study was performed to determine if a dynamic compression plate fixed to a bone with a low-density polyethylene underplate would loosen over time and with cyclic loading (due to creep and/or accumulated plastic deformation of the underplate) and thus decrease its stress shielding of the bone. Three groups of ten canine femora each were plated and tested in vitro. A nonosteotomized control group (without an underplate) and an experimental group (with an underplate) were cyclically loaded 10(6) times in compression under identical conditions. The third group of bones was osteotomized and plated with underplates and cyclically loaded 10(5) times to ensure that the construct would not fail before early fracture union could occur. The plate screws of the experimental group were significantly looser than those in the control group. This corresponded to a decrease in the underplate thickness that was more than sufficient to account for the looseness of the screws. Strain readings from the plate and bone were highly variable and not conclusive. Therefore, a transfer of load, reflected in the strain magnitude, from the plate to the bone during loosening of the screws could not be documented. None of the osteotomized bones in the third group had fixation failure. This experiment demonstrated that this plate-underplate construct provided strong initial fixation of an osteotomized bone in the laboratory setting without failure of fixation. This construct gradually loosened with time and cyclic loading in vitro and therefore should decrease the stress-shielding effect when applied in vivo.