Monolithic Variable-Diameter Spinal Rods to Reduce Stress Shielding
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Solution Overview
Problem
Conventional spinal rods with constant cross-sectional areas face challenges such as stress shielding, implant loosening, proximal junction kyphosis or failure (PJK/PJF), and adjacent segment disease due to rigid fixation, which do not accommodate patient-specific anatomy and require manual coupling with couplers or brackets.
Innovation Solution
Development of a spinal rod with variable bending stiffness and smooth transitions between regions of different diameters, formed as a monolithic structure without sharp steps, allowing for customizable placement and screw installation at transitions, reducing stress concentrations and improving flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a rod with constant cross-sectional area is used, then the manufacturing process is simple and the structure is straightforward, but it causes stress shielding, implant loosening, and proximal junction kyphosis due to rigid fixation
Solution Approach 1:
The rod is divided into multiple segments with different cross-sectional areas along its length. Each segment can be optimized for specific spinal regions, allowing gradual transitions in stiffness that reduce stress shielding and prevent implant loosening while maintaining manufacturing feasibility through modular design
Solution Approach 2:
Different portions of the rod are given different cross-sectional areas to match the local mechanical requirements of the spinal column. The rod features gradual transitions between these regions, providing appropriate stiffness locally while avoiding rigid fixation that causes proximal junction kyphosis
2Adaptability or versatility
If multiple different rods with constant cross-sectional areas are coupled together, then each rod can be optimized for its specific spinal segment, but the connection points weaken due to fatigue over time
Solution Approach 1:
Multiple rod segments with different cross-sectional areas are merged into a single monolithic structure without separate couplers or brackets. This integration eliminates connection points that would otherwise be subject to fatigue, while each segment remains optimized for its specific spinal region through varying cross-sectional dimensions
3Reliability
If stepped diameter rods are used, then proximal stiffness can be reduced to allow gradual load transfer, but screw placement becomes constrained and intra-operative variability increases
Solution Approach 1:
The rod features gradual changes in cross-sectional diameter rather than abrupt steps, allowing smooth transitions in stiffness. This parameter variation enables gradual load transfer to reduce stress shielding while providing sufficient screw placement flexibility because the gradual transitions avoid creating hard stops that constrain screw positioning
Data Source
AI summary
A spinal rod may comprise at least two segments having a constant diameter, where a first segment has a first constant diameter and a second segment has a second constant diameter, and where the first constant diameter is different from the second constant diameter. The spinal rod may also comprise a transition region positioned between the at least two segments, where the transition region has a variable diameter that gradually transitions between the first constant diameter and the second constant diameter, where the transition region comprises a tapered shape, and at least one curve extending along the at least two segments.