Hydroxyapatite enhancement of posterior spinal instrumentation fixation

Study Design. The ability of hydruxvapatits (HA) meterials to enhance the fixation strength of posterior spinal instrumenlation was examined In 19 adult mongrel dOflS. Methods Sixteen aogs underwent bilateral placemcnl of lumbar transpedicular screws from L, lo L0, secrer alflr acr&ws, and posterior iliac rods. The six transpedicular screw test groups included standard and plasma-sprayed HA-coeted screws with the recommended insertion technique, standard and HA-uaatad 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 fir. The swfal alar screws and posterior iliac rods were either uncoated or HA coatcd. Six digs wore killed Imnuadiataly: ten dogs ware killed at 6 weeks, am the fixation elements were mechanically tested gr histologically examined. Three additional dogs and synthetic bone male rial were used for additional baseline me chanical testing. Results, The strength of standard screws with recommended insertion did not change after & weeks in vM k HA-coated screws were initially 13% less resistant to pull out than standard screws, but this difference was not significant at 6 Wesks, Screws inserted with 9 poor initial tit technique were significantly weaker initially; at 6 watks, pdf-out strength was similar to the standard screws properly inserted. The HA-fjrout material s:gnificantly enhanced pull-out sirangth for bath screw types at 6 weeks, Secraalar acrew pufl-out strength was not significantly different between standard and HA-coated screws initially or at 6 weeks, HAcoaled rods were initially twice as resistant to pull out than standard rads and became etronger after 6 weeks in vivo, whaneas standard rods became significantly weaker. Histologically, the quantity and morphology of bone around all Impliments was similar, with HA-coated rods and screws dcmonelrating rations of dlract attachment to bone, An oat&oconductive response and new bone formation was observed within the HA-grout material, Scanning electron microscopic observation of mechanically lasted 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-melHl interfaces for the posterior iliac rods. Conclusions. The strength of poorly inserted transpedleular screws was significantly enhanced in vivo by the resorbable HA-grout material. The lower strongw of HA-coated screws was attributed to scraw geometry chgntjee resulting from the coating process, and modifications of screw coating are recommended.