HYDROXYAPATITE ENHANCEMENT OF POSTERIOR SPINAL INSTRUMENTATION FIXATION

Study Design. The ability of hydroxyapatite (HA) materials to enhance the fixation strength of posterior spinal instrumentation was examined in 19 adult mongrel dogs. Methods. Sixteen dogs underwent bilateral p lacement of lumbar transpedicular screws from L1 to L6, sacral alar sc rews, and posterior iliac rods. The six transpedicular screw test grou ps included standard and plasma-sprayed HA-coated screws with the reco mmended insertion technique, standard and HA-coated screws with a poor initial fit insertion technique using an oversized pilot hole, and HA -grout augmentation of standard and HA-coated screws with a poor initi al fit. The sacral alar screws and posterior iliac rods were either un coated or HA-coated. Six dogs were killed immediately; ten dogs were k illed at 6 weeks, and the fixation elements were mechanically tested o r histologically examined. Three additional dogs and synthetic bone ma terial were used for additional baseline mechanical testing. Results. The strength of standard screws with recommended insertion did not cha nge after 6 weeks in vivo. HA-coated screws were initially 13% less re sistant to pull out than standard screws, but this difference was not significant at 6 weeks. Screws inserted with a poor initial fit techni que were significantly weaker initially; at 6 weeks, pull-out strength was similar to the standard screws properly inserted. The HA-grout ma terial significantly enhanced pull-out strength for both screw types a t 6 weeks. Sacral alar screw pull-out strength was not significantly d ifferent between standard and HA-coated screws initially ar at 6 weeks . HA-coated rods were initially twice as resistant to pull out than st andard rods and became stronger after 6 weeks in vivo, whereas standar d rods became significantly weaker. Histologically, the quantity and m orphology of bone around all implants was similar, with HA-coated rods and screws demonstrating regions of direct attachment to bone. An ost eoconductive response and new bone formation was observed with in the HA-grout material. Scanning electron microscopic observation of mechan ically tested implants revealed a shear failure of surrounding bone (a nd HA if present) at the screw outer thread margin or at the bone-meta l or HA-metal interfaces for the posterior iliac rods. Conclusions. Th e strength of poorly inserted transpedicular screws was significantly enhanced in vivo by the resorbable HA-grout material. The lower streng th of HA-coated screws was attributed to screw geometry changes result ing from the coating process, and modifications of screw coating are r ecommended.