Analysis of the external fixator pin-bone interface.

External fixator pins were inserted into tibiae of dogs under four in vivo loading conditions to examine the mechanism of pin loosening. Pins were quantitatively measured for pin torque resistance, and the pin tracts were studied radiographically and histologically. The pins holding an unstable fracture had more gross pin loosening. Pins also may become loose under static loads. Radiographic lucency of 1 mm or more in the cortical bone around a pin was evidence of gross pin loosening. Histologic examination showed that tight pin tracts were characterized by a lack of bone remodeling. Loose pin tracts were characterized by extensive bone resorption and inflammatory infiltrates. Pin loosening can be detected radiographically. Pin insertion technique is important to improve the initial pin torque resistance to minimize pin loosening. Sixty-nine percent of pins with an initial torque resistance of less than 68 Ncm became grossly loose compared with only 9% of pins with an initial torque resistance greater than 68 Ncm, regardless of the experimental group. Unstable external fracture fixation is another important factor in producing pin loosening. Pins loaded under unstable fracture fixation had the highest incidence of gross loosening. When applying an external fixator, the fracture rigidity should be critically evaluated and, if necessary, protected weight bearing must be introduced initially to minimize pin loosening.