BIOMECHANICAL, SCANNING ELECTRON-MICROSCOPY, AND MICROHARDNESS ANALYSES OF THE BONE-PIN INTERFACE IN HYDROXYAPATITE-COATED VERSUS UNCOATED PINS

Objective: To evaluate the bone-pin interface in hydroxyapatite coated versus uncoated pins. Design: Eighty-four bicylindrical stainless ste el external fixation pins were implanted in a test group of 14 sheep. One-half of the pins were coated with hydroxyapatite and the rest rema ined uncoated. Intervention: Six coated pins were implanted in the lef t tibia of seven sheep, and six uncoated pins were implanted in the le ft tibia of the other seven sheep. In all sheep, the right tibia was l eft intact. During pin implantation, the final insertion torque was me asured, and a linear external fixator was mounted on the pins. Then th e medial tibial mid-diaphysis was exposed and a 5-mm resection osteoto my was performed. The sheep were killed six weeks after surgery. Main Outcome Measures: The extraction torque was measured on four pins remo ved from each sheep. Radiographic pin tract rarefaction was measured o n all the pins. Two pins from each sheep were used for histologic, sca nning electron microscopy (SEM), and microhardness analysis. Histomorp hometric analysis was carried out on the SEM specimens at x 36 magnifi cation. Results: 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 hy droxyapatite 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). Histomorphometr ic 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 unc oated pins (p < 0.01). Microhardness analysis showed that bone tissue close to the pins was softer than bone tissue far from them. Conclusio n: Hydroxyapatite coating is an effective method of refining the bone- pin interface and may improve the clinical results of the external fix ation technique.