Biomechanical Analysis for Pull-Out Strength of Pedicle Screws

• Main Authors: Chao-Chiang Chang, 張昭強
• Other Authors: 趙 振 綱
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/12661686729259526024

碩士 === 國立臺灣科技大學 === 機械工程系 === 90 === Pedicle screws have been widely used for internal fixator on the spine. Clinical observation indicates that the pullout and loosening of the screws were both of the principal failure modes of the internal spinal fixator, especially in the porotic spine. Therefore, prevention of the pullout of the screws was the main concern on the design of spinal fixator. The purpose of this study was to compare the experimental results of the pullout strength of the two new designed screws (Formosa Posterior Implants 3 & 4, FPI-3, FPI-4) with other commercial ones (Diapason, CD Horizon, CCD and Moss-Miami systems) inserted into both the polyurethane saw bones and porcine spines separately. Basically, we divide them into two profiles: FPI-4, Diapason and CD Horizon are of conical in shape, while FPI-3, CCD and Moss-Miami are of cylindrical in shape. A 3D model simulating the pedicle screw inserted into the polyurethane saw bone was developed to obtain the linear regressive correlation between contact areas, removed volume and pullout strength. The last part of this study was to compare our experimental result with modified theoretical calculation. The pullout strength of the two new designed screws (FPI-4 & FPI-3) was of little difference with that of other commercial screws inserted in porcine spine. In saw bone, the pullout strength of FPI-4 was greater than that of other screws (p<0.05) while the pullout strength of FPI-3 was nearly the same as that of other screws. Therefore, the screw with conical shape seemed to have better holding power on the screw/bone interface inserted in the cancellous bone. The reasons are possibly due to greater outer diameter at the base, deeper thread depth, and tapering form on the distal tip. Besides, we found that the pullout strength of Diapason dramatically decreased with unscrewing by 180° and the pullout strength of FPI-4 was greater than that of other screws with unscrewing by 360°. On the linear regressive correlation between contact areas, removed volume and pullout strength, we found that the predicted pullout strength by using removed volume was more accurate than that by using contact area, especially on conical shape (R=0.97). By using the middle parameters of bone screw depth and outer diameter and modifying the TSF as , the pullout strength was obtained with better accuracy on modified theoretical calculation. The experimental result and modified theoretical calculation in this study would be helpful to design the bone screws from both engineering and clinical points of view.