PAPER 21: A COMPARISON OF STANDARD AND PARTIALLY HYDROXYAPATITE-COATED APEX PINS IN A LOADED SHEEP MODEL

Abstract

Introduction: Pin loosening is a common complication associated with external fixation. Various attempts such as coating the pins with calcium phosphates, have been made to modify the pin surface in order to reduce pin loosening and provide good fixation. Animal and clinical studies showed that fixation using tapered external fixation pins coated with hydroxyapatite is superior to standard pins. However, there is no data on cylindrical pins either fully or partially coated with HA. A partial coating could be a solution to optimize pin fixation with the advantage of easier removal compared to fully coated pins. Our purpose was to compare standard and partially coated cylindrical Apex pins implanted in a sheep model at 2 and 6 weeks. As controls we included fully coated tapered pins.

Materials/Methods: Five groups of pins were studied. Group A included standard cylindrical Apex pins implanted in sheep which were euthanized 2 weeks after surgery; Group B included partially coated Apex pins implanted in sheep which were euthanized 2 weeks after surgery; Group C included tapered pins fully coated with HA implanted in sheep which were euthanized 2 weeks after surgery; Group D included Standard Apex pins implanted in sheep which were euthanized 6 weeks after surgery; Group E included partially coated Apex pins implanted in sheep which were euthanized 6 weeks after surgery. With the tapered pins, full contact between the coated surface and both cortices was obtained. After pin implantation, a unilateral external fixator was mounted onto the pins (Stryker carbon blue monotube Ø 20/250mm). A 5mm long removal osteotomy was performed in the mid-part of the tibial diaphysis, so that 3 pins were situated above the gap and 3 below to ensure load transfer through the bone-pin interface. Extraction torque and tibial torque resistance and histological analyses were obtained after pin removal.

Results: At 2 weeks, mean insertion torque was significantly higher for group C compared to group A (p = 0.03). Mean extraction torque was statistically higher for group B compared to group A (p = 0.001). A statistically significant difference was found in the Pettine index (p = 0.03) between groups A and B. At 6 weeks, no differences in extraction torque were seen.

Discussion: We believe that the partial application of the coating reduced the potential for osteointegration and the ultimate fixation of the coated Apex pins.