Evaluation of fixation of various screws in the lumbar vertebrae of dogs

Abstract

Various screws for posterior fixation of the lumbar spine were evaluated for their firmness of fixation in mongrel dogs. Four types of screws: stainless steel, titanium base alloy, porous-coated, and hydroxyapatite-coated were inserted into vertebrae in dogs and examined for firmness of fixation in bone. Dogs were killed immediately and 2, 4, 6, and 8 weeks after insertion, and firmness of fixation was assessed by measuring the twisting force (i.e., the torque) required to loosen the screws. Also, the interface between screws and bone was histologically investigated. The hydroxyapatite-coated screws required the highest torque, compared with other screws, 2 weeks after insertion. At 8 weeks, the torque was 235% of that of the titanium alloy screws, indicating that the fixation of the hydroxyapatite-coated screws was highly stable. With the hydroxyapatite-coated screw, bone formed a direct bond with the hydroxyapatite, the connection to bone apparently being augmented with time. The results of these experiments suggest that the hydroxyapatite-coated screw has the advantage of firmer fixation in vertebrae over other conventional screws.