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KMID : 1189320210150050566
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Asian Spine Journal
2021 Volume.15 No. 5 p.566 ~ p.574
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Impact of Screw Diameter and Length on Pedicle Screw Fixation Strength in Osteoporotic Vertebrae: A Finite Element Analysis
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Matsukawa Keitaro
Yato Yoshiyuki
Imabayashi Hideaki
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Abstract
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Study Design: Biomechanical study.
Purpose: To quantitatively investigate the effect of screw size on screw fixation in osteoporotic vertebrae with finite element analysis (FEA)
Overview of Literature: Osteoporosis poses a challenge in spinal instrumentation; however, the selection of screw size is directly related to fixation and is closely dependent on each surgeon’s experience and preference.
Methods: Total 1,200 nonlinear FEA with various screw diameters (4.5?7.5 mm) and lengths (30?50 mm) were performed on 25 patients (seven men and 18 women; mean age, 75.2±10.8 years) with osteoporosis. The axial pullout strength, and the vertebral fixation strength of a paired-screw construct against flexion, extension, lateral bending, and axial rotation were examined. Thereafter, we calculated the equivalent stress of the bone-screw interface during nondestructive loading. Then, using diameter parameters (screw diameter or screw fitness in the pedicle [%fill]), and length parameters (screw length or screw depth in the vertebral body [%length]), multiple regression analyses were performed in order to evaluate the factors affecting various fixations.
Results: Larger diameter and longer screws significantly increased the pullout strength and vertebral fixation strength; further, they decreased the equivalent stress around the screws. Multiple regression analyses showed that the actual screw diameter and %length were factors that had a stronger effect on the fixation strength than %fill and the actual screw length. Screw diameter had a greater effect on the resistance to screw pullout and flexion and extension loading (β=0.38?0.43, p<0.01); while the %length had a greater effect on resistance to lateral bending and axial rotation loading (β=0.25?0.36, p<0.01) as well as mechanical stress of the bone-screw interface (β=?0.42, p<0.01).
Conclusions: The screw size should be determined based on the biomechanical behavior of the screws, type of mechanical force applied on the corresponding vertebra, and anatomical limitations.
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KEYWORD
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Finite element analysis, Pedicle screws, Osteoporosis, Screw size, Fixation strength
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