Biodegradable Spinal Fusion Cage Density Optimization
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Solution Overview
Problem
Biodegradable materials used in reconstruction applications tend to be less stiff than permanent materials and suffer further stiffness reduction over time, leading to potential device failure, especially in load-bearing applications like spinal fusion cages, due to bulk erosion without shape change.
Innovation Solution
A topology optimization method is employed to create a density distribution map for degradable materials, accounting for base material degradation, using time and degraded base stiffness weighting factors to maintain sufficient stiffness through the degradation process, applicable to devices like spinal fusion cages made from poly(propylene fumarate)/beta-tricalcium phosphate, and extendable to other degradable materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If biodegradable materials are used to replace permanent materials in reconstruction devices, then biocompatibility and tissue integration are improved, but stiffness and load-bearing capability deteriorate over time due to bulk erosion
Solution Approach 1:
The patent applies topology optimization to modify the density distribution of the biodegradable material, creating a non-uniform density map that concentrates material in high-stress regions. This changes the physical parameters of the device structure to maintain stiffness while using biodegradable material, resolving the contradiction between biocompatibility and strength retention during degradation
Solution Approach 2:
The patent uses composite biodegradable materials (e.g., poly(propylene fumarate)/beta-tricalcium phosphate) that combine multiple materials with complementary properties. The composite structure provides both biocompatibility and enhanced mechanical strength, allowing the device to maintain load-bearing capability while degrading
2Strength
If the device design is optimized for initial stiffness, then load-bearing capability at implantation is improved, but stiffness reduction during degradation accelerates leading to early device failure
Solution Approach 1:
The patent performs preliminary topology optimization that anticipates future degradation by calculating density distributions at multiple time points during degradation. The optimization process pre-compensates for expected stiffness loss by strategically placing material in regions that will become critical as the device degrades, ensuring long-term performance rather than just initial strength
Solution Approach 2:
The patent creates a dynamic design approach where the density distribution is optimized for multiple stages of degradation (t=0, t=0.5T, t=T). The final device structure adapts to changing mechanical requirements over time, with higher density regions positioned to compensate for degradation in specific areas, allowing the device to maintain adequate stiffness throughout its service life
3Ease of manufacture
If uniform material density is used throughout the device, then manufacturing simplicity is maintained, but stress distribution becomes uneven leading to localized failure during degradation
Solution Approach 1:
The patent applies local quality by creating a non-uniform density distribution where material concentration varies by location based on the stress environment. The topology optimization identifies high-stress regions and increases material density specifically in those areas, while reducing density in low-stress regions. This localized material placement ensures uniform stress distribution and prevents localized failure during degradation, while still being manufacturable through techniques like 3D printing
Data Source
AI summary
A method of manufacturing biodegradable/bioresorbable tissue augmentation/reconstruction devices by defining material density distributions at selected time points during a material degradation lifecycle. These different density distributions are then superposed using general linear and/or nonlinear functions that could include both time and degraded base stiffness weighting factors. The material density distribution may be created using topology optimization, image-based design or computed aided design methods to create a degradable device that retains sufficient physical properties (ie modulus, strength, electrical conductivity, thermal conductivity) through the material degradation lifecycle process. Thus, any bulk degrading material can be designed using this process for any tissue augmentation/reconstruction application.