Effects of the Insertion Type and Depth on the Pedicle Screw Pullout Strength: A Finite Element Study

Abstract

Purpose The pedicle screw is a surgical device that has become widely used in spinal fixation and stabilization. Postsurgical complications such as screw loosening due to fatigue loading and screw breakage still need investigations. Clinical parameters such as the screw insertion type and depth, the bone density, and the patient degree of mobility greatly affect the mechanisms of the implant's failure/success. Methods The current finite element study focused on the prediction of the pedicle screw pullout strength under various conditions such as insertion type, insertion depth, bone quality, and loading mode. Results As depicted in this study, the preservation of the pedicle cortex as in the N1 insertion technique greatly enhances the pullout resistance. In addition, the higher the screw-anchoring depth, permitting to gear a maximum number of threads, the better the protection against premature breakouts of pedicle screws. Conclusions In agreement with experimental data, the type of insertion in which the first screw thread is placed immediately after the preserved pedicle cortex showed the best pullout resistance for both normal and osteoporotic bone.

Effects of pedicle screw number and insertion depth on radiographic and functional outcomes in lumbar vertebral fracture

“…Patients inserted with 4 short screws gave lower VAS and ODI compared with others. This may show that radiographic difference in LS and SS and 4S and 6S may not affect with age [8]. Anchorage of pedicle screws strongly depend on their implantation depth and thus loosening the screw is easily done on partially embedded screws [6].…”

“…Several observed cases showed that the inserted screws had variable depths of penetration. Due to excessive mechanical stress, some of these screws had undergone failure in vivo [ 8 ]. To evaluate the effect of the inserted screw number and depth on spinal global balance, we analyzed the following key parameters: pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and extent of spinal curvatures, especially lumbar lordosis (LL), segment lordosis (SL), and fractured vertebral lordosis (FL).…”

“…Jendoubi et al confirmed the significance of bone density in spine fixation procedures. He found that low-density bone characterizing the elderly is found to have most of its pullout strength (~ 65%) lost with age [ 8 ]. Anchorage of pedicle screws strongly depend on their implantation depth and thus loosening the screw is easily done on partially embedded screws [ 6 ].…”

Optimization of pedicle screw position using finite element method and neural networks

“…This force was defined for being close to the force of failure in a typical pullout strength test [10]. The von Mises stresses were evaluated in the cortical portion of the bone surrounding the screw [12,23].…”

“…The load at failure is influenced by several factors, including but not limited to the studied vertebra, bone density, insertion angle, insertion technique and screw geometry. The force at failure is usually in the 200 N to 800 N range [3,4,[9][10][11][12].…”