Hydroxyapatite coating enhances fixation of loaded pedicle screws: a mechanical in vivo study in sheep

Loosening of the pedicle screws with subsequent non-union or loss of correc tion is a frequent problem in spinal instrumentation. In a clinical pilot s tudy, coating of pedicle screws with plasma-sprayed hydroxyapatite (HA) res ulted in a significant increase of removal torque. An experimental study wa s performed to investigate the effects of HA coating on the pull-out resist ance. Thirteen sheep were operated on with destabilising laminectomies at t wo levels, L2-L3 and L4-L5. Two instrumentations with four pedicle screws i n each were used for stabilisation. Uncoated screws (stainless steel) or th e same type of screws coated with plasma-sprayed HA were used in either the upper or the lower instrumentation in a randomised fashion. Four sheep wer e examined immediately after the application of the screws, three sheep at 6 weeks and four sheep at 12 weeks. Two sheep were euthanised early due to complications. The pull-out resistance was recorded in two HA-coated and tw o standard screws in each animal. The maximum pull-out resistance was highe r for the HA-coated screws at 0 weeks (P < 0.02) and at 12 weeks (P < 0.01) when compared to the uncoated screws, while there was no significant diffe rence between the groups at 6 weeks. We believe that the higher pull-out re sistance for HA-coated screws at 0 weeks was mainly caused by differences i n surface roughness, while the difference at 12 weeks was due to a favourab le bone reaction around the HA-coated screws. At 12 weeks, the average stif fness was significantly higher for the HA-coated screws, while there was no significant differences in stiffness between the two screw types at 0 and 6 weeks. Energy to failure was significantly higher for coated screws when compared to the uncoated screws at all three time points. HA coating improv es fixation of loaded pedicle screws, with increased pull-out resistance an d reduced risk of loosening.