Loosening of sacral screw fixation under in vitro fatigue loading

Sacral screw fixation is frequently used for fusion of the lower lumbar spi ne, but sacral screws appear to offer less secure fixation than lumbar pedi cle screws, and failure due to loosening under fatigue loading is common. T he aim of this study was to examine in vitro the stability of medial and la teral bicortical and unicortical sacral screw fixation under a physiologica lly relevant fatigue-loading pattern. Bone mineral density, screw insertion torque, and screw-fixation stiffness were measured prior to cyclic loading between 40 and 400 N compression at 2 Hz for 20,000 cycles. The screw-fixa tion stiffness was measured every 500 cycles, and the axial pullout strengt h of the screws was recorded following loading. All of the lateral insertio ns loosened under the applied loading, but some of the medial insertions re mained stable. Medial insertions proved stiffer and stronger than lateral i nsertions, and bicortical fixations were stronger than unicortical fixation s. Bone mineral density and insertion torque were correlated with screw sti ffness and pullout strength, although better correlation was found for inse rtion torque than bone mineral density. Bone mineral density is a good preo perative indicator of sacral screw-fixation strength, and insertion torque is a good intraoperative indicator. An insertion torque greater than 1.5 Nm is suggested as an indicative value for a stable medial unicortical insert ion, whereas an insertion torque greater than 2 Nm suggests a stable medial bicortical insertion. It appears that, apart from the choice of technique (screw orientation and depth), minimizing the load on the screws during the initial part of the fusion process is also critical to maintain stability of the fused section and to obtain a solid fusion mass.