A Biomechanical Study of Two Kinds of Tapered Pedicle Screws in Osteoporotic Lumber Spine

Jang, Eui Chan; Seo, Jung Hwan; Song, Kwang Sup; Ryu, Ho Sung

Abstract

PURPOSE: To compare the pullout strength and insertion torque of proximally tapered screw (PT) with fully tapered screw (FT) and to investigate the correlation between the pullout strength and bone mineral density, morphology of pedicle, and insertion torque of the screw in osteoporotic lumbar spine.
MATERIALS AND METHODS
Fifteen osteoporotic lumbar vertebrae from four white human cadavers were used. Bone mineral density, pullout strength and insertion torque were measured by dual energy x-ray absorptiometry, MTS and torque screw driver individually.
RESULTS
The FT screw provided greater pullout force and insertion torque than the PT screw in 12 of 15 vertebrae tested (p<0.01). Pullout strength was correlated with insertion torque in PT (r=0.666, P=0.0006) and FT (r=0.464, P=0.19) screws.
CONCLUSIONS
Results of this study indicate that the FT screws provide higher pullout strength and insertion torque than PT screws in osteoporotic lumbar spine and suggest that the development of tapered minor diameter may lead to an improved pedicle screw with high pullout strength and insertion torque. Nevertheless, further study is needed to investigate the effect of tapering the minor diameter on the mechanical bending strength of the screw.