Introduction: External fixators are frequently used for stabilization and treatment of problematic fractures. Pintrack infections cause complications such as osteomyelitis and loss of stability of osteosynthesis. It remains unclear, whether pintrack infection provokes pin loosening, or loss of the pin stability results in pintrack infections. The aim of this study was to investigate the correlation between the mechanical stability of pins, the incidence of pin track infections and the osseus anchorage of pins.
Methods: 27 sheep received an external fixator with 6 Schanz screws (pins). Pin insertion and extraction torque was measured and radiographs were taken postoperatively and postmortem. The daily pin care routine included scoring the pin entries. The animals were sacrificed after 3, 6 and 9 weeks. The pins were analysed microbiologically (incidence of infection: ≥103 colony forming units (CFU)) and histologically. The x-rays and histological slices were scored.
Results: Histomorphometrically, the largest callus area and the highest mineralised bone-fraction was found at 6 weeks. The bone density of the periosteal callus increased over time. The cortical bone density decreased over time.
The relative extraction torque increased over the course of healing (3w: 53.4% ± 7.1%, w: 61.8% ± 20.7%; 9w: 84.0% ± 44.9% of insertion torque).
5% of 108 pins showed clinical signs of infection; the microbiologically detected infection rate was 3 times higher.
Radiologically, 22% of 104 pins showed osteolysis in the cortex (entry or exit), 4% showed medium osteolysis and 2% revealed severe osteolysis along the complete pintrack. An accumulation of pintrack infections (75 %) and the incidence of osteolyses (47% of all osteolyses) were found at 6 weeks. The most severe osteolyses were found at 9 weeks. The histological grading revealed the best results for the earliest time point compared to the latter ones.
Discussion: This study reveals an increasing stability of osseous pin-anchorage over the course of healing. As the cortical bone density decreased over time, the increased anchorage-stability can only be explained satisfactorily by the increase of the periosteal callus bone density. The periosteal callus, biggest at 6 weeks, determines the magnitude of extraction force. In contrast to clinical studies, a very low percentage of pintrack infections was observed in this study. However, the microbiologically affirmed infection rate was three times higher than clinically ascertained. The evidence of osteolyses was twice as high as the microbiological infection-rate. The low infection-rate could not prevent evidence of cortical lyses. No correlation was found between infections, osteolyses and pin loosening. Callus formation seems to be essential in pin fixation and should be supported to prevent pin loosening. Targeting only cortical anchorage may fail to avoid pin loosening in external fixation.
