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To evaluate the bone-pin interface in hydroxyapatite coated versus uncoated pins.Eighty-four bicylindrical stainless steel external fixation pins were implanted in a test group of 14 sheep. One-half of the pins were coated with hydroxyapatite and the rest remained uncoated.Six coated pins were implanted in the left tibia of seven sheep, and six uncoated pins were implanted in the left tibia of the other seven sheep. In all sheep, the right tibia was left intact. During pin implantation, the final insertion torque was measured, and a linear external fixator was mounted on the pins. Then the medial tibial mid-diaphysis was exposed and a 5-mm resection osteotomy was performed. The sheep were killed six weeks after surgery.The extraction torque was measured on four pins removed from each sheep. Radiographic pin tract rarefaction was measured on all the pins. Two pins from each sheep were used for histologic, scanning electron microscopy (SEM), and microhardness analysis. Histomorphometric analysis was carried out on the SEM specimens at × 36 magnification.Radiographic pin tract rarefaction was significantly lower in the hydroxyapatite coated pins than in the uncoated pins (p < 0.001). Group average insertion torque was 960 ± 959 N/mm in the hydroxyapatite coated pins, and 709 ± 585 N/mm in the uncoated pins (p = not significant). Group average extraction torque was 1485 ± 1308 N/mm and 298 ± 373 N/mm, respectively (p = 0.0001). Histomorphometric analysis showed that the group average bone-pin contact was 50.7 ± 16.9% in the hydroxyapatite coated pins and 27.6 ± 7.1% in the uncoated pins (p < 0.01). Microhardness analysis showed that bone tissue close to the pins was softer than bone tissue far from them.Hydroxyapatite coating is an effective method of refining the bone-pin interface and may improve the clinical results of the external fixation technique.