Biomechanical analysis of pullout strength of the pedicle screws in relation to change bone mineral density

반복 하중 후 골밀도 감소에 따른 척추경 나사못의 고정력(Pullout Strength)감소 형태 분석

Screw loosening and subsequent pullout can be attributed to the reduction in bone mineral density in the vertebrae manifested by osteoporosis in which the decrease in fixation strength between the cancellous bone and screw threads are accelerated by repeated loads exerted by patients own weight and activities following the surgery. In this study, the change in pullout strength of the pedicle screws was investigated before and after repeated loads were imparted. For this purpose. Diapason pedicle screws $(6.7\times40mm)$ were inserted onto fresh porcine spine specimens (T1-L5) after bone mineral density was measured using a DEXA. With an MTS, an axial load was applied at a loading rate of 0.33mm/sec until failure to measure the maximum pullout strength. Flexion moment of 7.5N-m was then imparted at 0.5Hz for 2000 cycles. It was found that the maximum pullout strength was exponentially related to BMD regardless of load types ($107.71\;\times\;\exp^{(1.43{\times}BMD)}r^2=0.93$, P<0.0001 without repeated load; ($107.71\;\times\;\exp^{(2.19{\times}BMD)}r^2=0.78$, P<0.0001 with repeated load). The results suggest that the reduction in pullout strength for pedicle screws is far more prominent in osteoporotic spine than in normal spine especially as number of repeated load was increased. More importantly, it was demonstrated that the level of bone mineral density and the activity level of the patient should be evaluated in more detail for successful implementation of pedicle screw systems in spinal surgery.