Axial and tangential fixation strength of pedicle screws versus hooks in the thoracic spine in relation to bone mineral density

Study Design. In vitro biomechanical testing was performed to analyze the axial and tangential fixation strength of pedicle screws versus pedicle and laminar hooks in the human thoracic spine. Bone mineral density of each tested vertebra was determined. Objective. To study the fixation strength of pedicle screws and hooks in the thoracic spine in relation to bone mineral density. Summary of Background Data, Biomechanical properties of pedicle screws have proven to be superior in the lumbar spine, but there is a paucity of data concerning the thoracic spine in particular with regard to bone mineral density. Methods. In 72 human thoracic vertebrae standard pedicle hooks between T4 and T8 and supralaminar hooks between T9 and T12 were tested against pedicle screws in pairs in one vertebra each. The bone mineral density of each vertebra was determined by means of quantitative computed tomography. Screws and hooks were loaded either axially strictly longitudinal to failure or in a coronal plane strictly perpendicular to the longitudinal axis nondestructively. A total of 78 pullout tests and 66 tangential loadings were performed. Results. The correlation of bone mineral density and fixation strength was significant for pullout strength of pedicle screws (P < 0.001) and pedicle hooks (P < 0.001). The average pullout strength of the pedicle screws was significantly higher than that of pedicle hooks (P = 0.003) and supralaminar hooks (P = 0.02). The difference was not significant if bone mineral density was <100 mg hydroxylapatite/mL. The average tangential elastic and permanent displacement of pedicle screws was significantly lower than that of pedicle (P < 0.001) and laminar hooks (P = 0.002). Conclusion. Pedicle screws may be beneficial for instrumentation of the thoracic spine because they are significantly more resistant to axial and tangential loadings than pedicle and laminar hooks on the condition that bone mineral density is >100 mg/mL hydroxylapatite.