Introduction: External fixation is used widely in the management of fractures, despite a relatively high incidence of complication, arising from pin loosening and infection. Diamond like carbon (DLC) is a low surface energy coating that can be applied to external fixator pins and may reduce biofilm formation and infection resulting in a lower incidence of pin loosening. Hydroxyapatite (HA) is well established as a coating to enhance fixation of external fixator pins. This study tests the hypothesis that HA and DLC coatings on stainless steel (SS) external fixator pin shafts modify integration of the implant with soft/hard tissues.

Materials and Methods: An Orthofix external fixator was used to stabilise a tibial osteotomy with 6 self-drilling/tapping 6mm pins in 32 skeletally mature Friesland ewes. Animals were divided into four groups; SS, DLC, HA partially coated (threads only) and HA fully coated (threads and pin shaft). Pin insertion torque was measured using a torque wrench and extraction torque similarly obtained at 10 weeks when animals underwent euthanasia. Pin performance indices (PPI) were calculated as a ratio of extraction to insertion torque x100%. Pin site 2 was preserved for hard grade resin histology and subsequent pin tissue integration analysis. Pin site 3 was used for analysis of the soft tissue pin shaft interface using transmission electron microscopy. Pin site 5 was examined for the presence of biofilm formation using scanning electron microscopy. Pin site 6 was swabbed for microbiological analysis.

Results: SS and DLC pins achieved significantly higher insertion torques compared to HA partially coated pins (p=0.001, 0.002). Both groups of HA coated pins demonstrated a significantly higher, extraction torque and therefore PPI for all pin site positions compared to SS and DLC (p< 0.001– 0.025). The epithelium was found not to be in contact with the pin shaft in all cases. No significant differences were found between the different pin groups for epidermal down growth and dermal contact. Both groups of HA coated pins showed a significantly higher percentage of new bone in direct contact with the embedded threads compared to SS and DLC pins (p< 0.001, p=0.004). The proportion of soft tissue in contact and within the thread, of fully coated HA pins was significantly lower compared to stainless steel (p=0.003, p=0.017), DLC (p=0.004, p=0.002) and HA partially coated pins (p=0.006, p=0.02). Biofilms were evident on all pins except those coated with DLC. More bacteria were observed on the fully HA coated pins. DLC had significantly lower number of bacterial colonies in culture compared to SS (p=0.028) and fully coated HA pins (p=0.005).

Discussion: Coatings of DLC and HA do have a significant affect on hard/soft tissue reactions. However coatings do not have a significant effect on epidermal down growth or dermal attachment to the pin shaft surface. DLC coated pins had the cleanest surface with no bio-film present and significantly lower numbers of bacteria present. Fully HA coated pins despite evidence of bio-film formation, bacteria and high microbiological counts had significantly higher PPI. In addition fully coated HA pins demonstrated significantly reduced amounts of soft tissue at the pin bone interface. Therefore soft tissue reactions may affect bone integration.