Cortex distraction forces caused by the insertion of external fixator pins.

Abstract

Insertion of a bicortical threaded external fixator pin can cause bone damage. An unexpected crack can propagate along the bone when the pin touches the far cortex. The objective of this study was to investigate whether drilling and inserting bicortical pins into the shafts of long bones can cause large distraction forces to be generated between the cortices. Two flat samples of bone or bone substitute (Tufnol) were mounted parallel at each end of a specially designed force measurement column. Three common pin designs were inserted into the samples: tapered, self-threading and self-drilling, and self-threading pins. The axial thrust and the cortex distraction force between the two samples were measured as the pins were inserted. High distraction forces were measured between the cortices for all pin designs, typically 700 to 1,000 Newtons. The clinical significance of these findings is that the insertion of bicortical pins of the designs tested may generate a large distraction force between the cortices. This may drive a fracture apart or propagate an undisplaced fracture. Therefore, care must be taken to ensure that bicortical pins are inserted in locations free of local defects.