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 correction is a frequent problem in spinal instrumentation. In a clinical pilot study, coating of pedicle screws with plasma-sprayed hydroxyapatite (HA) resulted in a significant increase of removal torque. An experimental study was performed to investigate the effects of HA coating on the pull-out resistance. Thirteen sheep were operated on with destabilising laminectomies at two levels, L2-L3 and L4-L5. Two instrumentations with four pedicle screws in each were used for stabilisation. Uncoated screws (stainless steel) or the 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 were 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 two standard screws in each animal. The maximum pull-out resistance was higher 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 difference between the groups at 6 weeks. We believe that the higher pull-out resistance for HA-coated screws at 0 weeks was mainly caused by differences in surface roughness, while the difference at 12 weeks was due to a favourable bone reaction around the HA-coated screws. At 12 weeks, the average stiffness 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 improves fixation of loaded pedicle screws, with increased pull-out resistance and reduced risk of loosening.