Structural Designs and Biomechanical Analyses of the Orthopaedic Screws

• Main Authors: Ching-Chi Hsu, 徐慶琪
• Other Authors: Ching-Kong Chao
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/11761799661059412257

博士 === 國立臺灣科技大學 === 機械工程系 === 94 === Interlocking nail and internal spinal fixator have been extensively used to treat tibial shaft fracture and spinal diseases respectively. In clinical application, tibial locking screws and spinal pedicle screws are the weakest part of the implants. Screw failure may cause loss of fracture fixation, non-union, and delayed union. Screw fracture and screw loosening are two main clinical failure modes. The purpose of this dissertation was to evaluate bone holding power and bending strength of tibial locking screws and spinal pedicle screws by biomechanical tests and three-dimensional finite element analyses. In biomechanical analyses of tibial locking screws, bone holding power and bending strength were assessed by bone holding power tests, yielding tests, and fatigue tests. Three-dimensional finite element models were established to simulate the results of biomechanical tests. The parametric study was done by Taguchi robust design method. In addition, the optimum design of tibial locking screws was obtained by using genetic algorithm and geometric constraints. The analytical results of tibial locking screw models were closely related to those of biomechanical tests with high correlation coefficient (>0.90). In the parametric study, outer diameter, pitch, and half angle were the main factors for bone holding power and inner diameter and root radius were the important factors for bending strength. Titanium tibial locking screw with larger root radius could significantly increase its fatigue strength. In optimum study, tibial locking screw with higher bone holding power and bending strength could be found by genetic algorithm and the unreasonable designs could be eliminated by geometric constraints. The design of commercial tibial locking screws could be improved according to the results of the optimum analysis. In biomechanical analyses of spinal pedicle screws, bone holding power and bending strength were evaluated by pullout tests, stripping tests, yielding tests, and fatigue tests. Three-dimensional finite element models were established to simulate the results of biomechanical tests. Bone holding power and bending strength of pedicle screws were discussed by changing their geometry and dimension. The analytical results of spinal pedicle screw models were closely related to those of biomechanical tests with high correlation coefficient (>0.85). The bone compaction effect could enhance bone holding power of pedicle screws. Pedicle screws with longer conical length and larger inner diameter at the hub had higher bending strength. In the parametric study, inner diameter was the main factor for bone holding power and conical angle was the important factor for bending strength. Conical pedicle screw could produce higher bone holding power and bending strength simultaneously. The results of this dissertation could help surgeons in selecting suitable devices for their patients and assist design engineers in developing new orthopeadic implants.