Hydroxyapatite enhancement of posterior spinal instrumentation fixation

The ability of hydroxyapatite (HA) materials to enhance the fixation strength of posterior spinal instrumentation was examined in 19 adult mongrel dogs.

Sixteen dogs underwent bilateral placement of lumbar transpedicular screws from L1 to L6, sacral alar screws, and posterior iliac rods. The six transpedicular screw test groups included standard and plasma-sprayed HA-coated screws with the recommended 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 initial fit. The sacral alar screws and posterior iliac rods were either uncoated or HA-coated. Six dogs were killed immediately; ten dogs were killed at 6 weeks, and the fixation elements were mechanically tested or histologically examined. Three additional dogs and synthetic bone material were used for additional baseline mechanical testing.

The strength of standard screws with recommended insertion did not change after 6 weeks in vivo. HA-coated screws were initially 13% less resistant to pull out than standard screws, but this difference was not significant at 6 weeks. Screws inserted with a poor initial fit technique were significantly weaker initially; at 6 weeks, pull-out strength was similar to the standard screws properly inserted. The HA-grout material significantly enhanced pull-out strength for both screw types at 6 weeks. Sacral alar screw pull-out strength was not significantly different between standard and HA-coated screws initially or at 6 weeks. HA-coated rods were initially twice as resistant to pull out than standard rods and became stronger after 6 weeks in vivo, whereas standard rods became significantly weaker. Histologically, the quantity and morphology of bone around all implants was similar, with HA-coated rods and screws demonstrating regions of direct attachment to bone. An osteoconductive response and new bone formation was observed within the HA-grout material. Scanning electron microscopic observation of mechanically tested implants revealed a shear failure of surrounding bone (and HA if present) at the screw outer thread margin or at the bone-metal or HA-metal interfaces for the posterior iliac rods.

The strength of poorly inserted transpedicular screws was significantly enhanced in vivo by the resorbable HA-grout material. The lower strength of HA-coated screws was attributed to screw geometry changes resulting from the coating process, and modifications of screw coating are recommended.