Graded Cellular Bone Implant Resolving Stress Shielding
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Solution Overview
Problem
Current orthopaedic implants face challenges such as bone resorption and interface instability due to mechanical incompatibility, leading to revision surgeries, as they are stiffer than surrounding bone and cause stress shielding, which results in bone fracture and implant loosening.
Innovation Solution
A graded cellular implant with a non-homogeneous distribution of material properties, featuring a lattice microstructure with optimized unit cell topology and pore geometry to minimize bone loss and interface failure, designed using multiscale mechanics and multiobjective optimization to match the mechanical properties of the host bone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If uniform density homogenous material is used for implant, then manufacturing is simple and structural integrity is maintained, but stress shielding occurs and bone resorption increases
Solution Approach 1:
The implant uses a graded cellular structure where material density and pore size vary spatially throughout the implant body. Regions closer to the bone interface have lower density and higher porosity to match softer bone and reduce stress shielding, while distal regions have higher density for structural support. This local variation in material properties eliminates the harmful stress shielding effect while maintaining manufacturability through controlled fabrication processes.
Solution Approach 2:
The implant combines multiple material phases including solid metal matrix and porous cellular structures with different density ratios. This composite approach allows the implant to exhibit graded mechanical properties ranging from stiff to compliant regions, enabling simultaneous achievement of structural integrity and bone compatibility without requiring entirely new manufacturing methods.
2Strength
If implant stiffness is increased to maintain structural integrity, then implant strength is improved, but stress shielding increases and bone-implant interface stability deteriorates
Solution Approach 1:
The graded cellular structure creates a spatial gradient in material stiffness, with softer regions at the bone interface that match the mechanical properties of surrounding bone tissue. This local compliance reduces stress shielding and promotes bone remodeling while maintaining overall implant strength through progressively stiffer regions toward the distal end, thereby ensuring both implant strength and interface stability simultaneously.
Solution Approach 2:
The implant utilizes controlled variations in pore size, porosity, and cell wall thickness as key parameters to modulate local mechanical properties. By adjusting these parameters throughout the implant volume, the structure achieves optimal stiffness matching with bone at the interface while maintaining sufficient overall strength, resolving the contradiction between strength and interface stability.
3Object-affected harmful factors
If implant stiffness is reduced to minimize stress shielding, then bone resorption is reduced, but bone-implant interface shear stress increases and interface stability deteriorates
Solution Approach 1:
The graded structure provides locally optimized mechanical properties where softer regions at the interface reduce stress shielding and bone resorption, while the progressive transition to stiffer regions toward the distal end provides gradual stress transfer that minimizes interface shear stresses. This spatially varying stiffness distribution simultaneously addresses both bone preservation and interface stability concerns.
Solution Approach 2:
The graded cellular structure creates a dynamic stress distribution pattern under loading conditions, where stresses are gradually transferred from the compliant proximal regions to the stiffer distal regions. This progressive load transfer mechanism reduces peak shear stresses at the bone interface while maintaining overall structural functionality, resolving the contradiction between reducing bone resorption and maintaining interface stability.
4Object-affected harmful factors
If non-homogeneous material distribution is used to match bone mechanical properties, then stress shielding is reduced and bone loss is minimized, but manufacturing complexity increases
Solution Approach 1:
The implant is designed as an assembly of repeating unit cells with systematically varied parameters. This segmentation approach allows complex graded structures to be fabricated using controlled manufacturing processes that deposit material in layered or modular fashion, where each unit cell layer can be independently controlled. The repetitive modular structure simplifies the manufacturing of what would otherwise be a continuously varying complex geometry.
Solution Approach 2:
The graded cellular structure utilizes controlled parameter variations in pore size, porosity, and cell wall dimensions that can be systematically implemented through manufacturing process controls. By defining these parameters as functions of spatial position rather than continuous variations, the manufacturing complexity is reduced while still achieving the desired graded mechanical properties for minimizing bone loss.
Data Source
AI summary
A methodology integrating multiscale analysis and design optimization to design a novel bone replacement implant made of a functionally graded cellular material that meets fatigue requirements imposed by cyclic loadings. The pore microarchitecture, described by interconnectivity, porosity, pore size as well as pore topology, is optimally designed for tissue regeneration and mechanical strength. A bone implant with a graded cellular microstructure is also provided.