INCREASE OF STABILITY IN EXTERNAL FRACTURE FIXATION BY HYDROXYAPATITE-COATED BONE SCREWS

A major problem in fracture treatment by external fixation is screw lo osening, which often results in reduced stability and can lead to prol onged treatment. A load-carrying experiment was conducted to determine whether coating implants with bioactive hyroxyapatite (HA) increases screw stability. Twelve HA-coated ASIF screws with 3 different macropo rosities were inserted in 12 sheep that had already been fitted with a 6-pin external fixator for the treatment of a tibial osteotomy. The s ame number of uncoated polished steel screws served as controls. Altho ugh initial stability was not different for HA-coated screws, average removal torque after a 9-week implantation period increased with incre asing macroporosity of the HA coating (p < .002). Instability of some screws was accompanied by histologic findings of cartilagenous tissue and proliferation of periosteal callus. Near the threads in the tibial cortex and in the shaft area of the screw were seen large numbers of HA particles that had been sheared off during implantation as well as during screw removal because of high contact forces between the HA coa ting and bone. Particulate debris of HA particles as well as the relea se of small bone fragments during explantation is likely to be unavoid able since HA adherence to bone is greater than adherence to steel aft er several weeks of implantation. (C) 1995 John Wiley and Sons, Inc.