Evaluation of hydroxyapatite-coated screws for lumbar spine fixation in dogs

Abstract

To evaluate the firmness of fixation of hydroxy-apatite(HA)-coated screws used for spinal fixation, research has been done to measure the force required to pull out such screws. However, despite its importance, no research has been done to evaluate the twisting force necessary to loosen the screw. With regard to the twisting force, we evaluated the effects of HA coating on securing titanium alloy screws to canine lumbar vertebrae. Bonding of HA-coated (HA) screws and titanium alloy (Diapason; D) screws to the bone was examined serially over a period of 24 weeks by measuring the torque required to loosen the screws after a period of fixation and by histological examination of the screw-bone interface. Twelve screws (six screws of each type) were used in each of seven mongrel dogs, on each side of the lumbar vertebrae (L1-L6). The torque required to loosen D screws 6 weeks after insertion was three times higher than that used at time 0. With the HA screw, the torque at 2 and 6 weeks was four and six times higher, respectively, than that at time 0. Eight weeks after insertion, the torque for the HA screw was 2.3 times greater than that for the D screw (P < 0.01). Histological examination showed direct binding of HA to the bone early after insertion. Our results indicate that HA screws are firmly fixed in bone within a short period after insertion into the vertebra. In the clinical setting, the use of HA screws for spinal fixation may produce stable spine fixation within a short period after operation.