A Poly Head-Based Osseointegration Strategy to Reduce Pedicle Screw Loosening : A Finite Element Study

다국어 초록 (Multilingual Abstract)

Introduction: Pedicle screw fixation systems are commonly used in lumbar interbody fusion surgery to provide postoperative stability and promote fusion progression. However, insufficient fixation stability and screw loosening under repetitive physiological loading conditions remain clinically significant complications in pedicle screw systems. Recently, advances in additive manufacturing technologies have enabled the application of porous bone-mimetic structures to promote osseointegration and improve long-term fixation stability at the bone–implant interface. Nevertheless, direct application of porous structures to the pedicle screw shaft may still present biomechanical and practical limitations related to fatigue resistance, structural reliability, and revision surgery. Therefore, this study evaluated the biomechanical effect of localized osseointegration at the inferior surface of the U-shaped poly head on pedicle screw fixation stability using finite element （FE） analysis.

Materials and methods: A validated three-dimensional FE model of the lumbar spine （L1–L5） was developed to construct one-level （L4–5）, two-level （L3–5）, and three- level （L2–5） fixation models. TLIF with pedicle screw fixation and fixation-only without TLIF were modeled for each fixation length. Three osseointegration conditions （0%, 50%, and 100%） were assigned at the inferior surface of the U-shaped poly head to represent nonunion, partial union, and complete union at the poly head– bone interface. Physiological loading conditions consisting of a 400 N follower load and 7.5 Nm pure moments were applied under flexion-extension, lateral bending, and axial rotation. Peak von Mises stress （PVMS） distributions in the vertebrae, TLIF cage when present, and pedicle screw system were evaluated. Screw loosening was quantified based on the relative displacement at four representative screw–bone interface locations （A–D）.

Results: Across all models, PVMS values in the vertebrae, pedicle screw system, and TLIF cage when present remained below the corresponding material yield strengths, indicating structurally stable conditions under all loading conditions. TLIF presence did not significantly affect screw loosening behavior, whereas increasing osseointegration significantly reduced screw loosening in all fixation constructs （p 〈 0.05）. In contrast, the difference between the 50% and 100% osseointegration conditions was relatively small compared with the reduction from the 0% condition. Compared with the 0% osseointegration condition, overall mean screw loosening in the one-level construct decreased by 53.6% and 59.8% in the TLIF group and by 55.2% and 61.2% in the fixation-only group at 50% and 100% osseointegration, respectively. In the two-level construct, the corresponding reductions were 61.0% and 63.4% in the TLIF group and 62.2% and 64.9% in the fixation-only group. In the three-level construct, the reductions were 66.0% and 70.0% in the TLIF group and 63.6% and 65.9% in the fixation-only group. The reduction in screw loosening was most pronounced at measurement locations C and D and during axial rotation and lateral bending.

Discussion: Progressive osseointegration at the inferior surface of the U-shaped poly head consistently reduced pedicle screw loosening across one-level, two-level, and three-level fixation constructs. In contrast, TLIF presence itself had a limited influence on screw loosening behavior, whereas the osseointegration level was a dominant determinant of construct stability. The reduction tendency was most evident at the distal screw measurement locations and under axial rotation and lateral bending conditions, indicating that fixation enhancement at the poly head–bone interface may effectively suppress local micromotion at the pedicle screw–bone interface. These findings suggest that the proposed osseointegration strategy maintained structural stability while reducing screw loosening without increasing biomechanical failure risk in the surrounding vertebral structures or implant system.

Conclusion: Increasing osseointegration at the inferior surface of the U-shaped poly head reduced pedicle screw loosening across one-level, two-level, and three-level fixation constructs, with the most pronounced benefit observed under axial rotation and lateral bending. These findings suggest that shifting fixation enhancement away from the pedicle screw shaft and toward the inferior surface of the U-shaped poly head may provide a more favorable balance between stability improvement and practical implant management than strategies that rely on direct thread-level osseointegration.

목차 (Table of Contents)

• I. Introduction 1
• 1. Background 1
• 2. Objective of the Study 4
• II. Materials and methods 5
• 1. Finite element model of the native lumbar spine and validation 5
• 2. Finite element models with lumbar fixation system 8
• 3. Control of osseointegration on bone and U-shaped poly head 12
• 4. Physiological loading and boundary conditions for finite element solving 14
• 5. Finite element (FE) solving and data analysis 15
• III. Results 18
• 1. Structural safety evaluated by peak von Mises stress 18
• 2. Screw loosening in the one-level fixation construct 20
• 3. Screw loosening in the two-level fixation construct 24
• 4. Screw loosening in the three-level fixation construct 27
• IV. Discussion 30
• V. Conclusion 35
• VI. References 36