Orthopedic Fixator with Adjustable Preload and Compliance
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Solution Overview
Problem
Current orthopedic fixation devices, such as the Taylor Spatial Frame, suffer from mechanical clearance and manufacturing tolerances that limit positional accuracy and stiffness, leading to unwanted bone motion and noise, and lack the ability to controllably adjust stiffness during the healing process.
Innovation Solution
An orthopedic fixation device with adjustable preloading elements to eliminate backlash and provide precise fixation, and adjustable compliance to change stiffness levels, featuring anisotropic stiffness, nonlinear force-deflection characteristics, and motion limit-stops to enhance bone healing and patient comfort.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple mechanical joints and threaded parts are used in each strut to provide adjustability, then the range of motion and adaptability are improved, but mechanical clearance and manufacturing tolerances accumulate, resulting in non-negligible mechanical play and reduced positional accuracy
Solution Approach 1:
The patent replaces traditional mechanical joints with spherical joints that have inherent self-centering capability, and uses a cable-driven actuation system instead of threaded parts. The spherical joints eliminate the need for multiple mechanical interfaces while providing the required degrees of freedom. The cable system substitutes for traditional threaded adjustment mechanisms, reducing the number of mechanical clearances that accumulate.
Solution Approach 2:
The patent employs composite structural elements where rigid components are combined with compliant elements. The struts use a composite design with rigid outer shells and internal compliant mechanisms that provide adjustability without requiring multiple mechanical joints. This composite approach maintains positional accuracy while enabling the necessary range of motion.
2Ease of operation
If mechanical joints and threaded parts are used to connect struts to base members, then the device can be assembled and adjusted, but the cumulative tolerances create kinematic uncertainty that limits positional accuracy
Solution Approach 1:
The patent replaces traditional mechanical connections with spherical joints that provide inherent alignment tolerance. The spherical joints allow for easy assembly and adjustment while maintaining precise positioning through their self-centering geometric constraint, eliminating the need for multiple aligned mechanical interfaces that would accumulate tolerances.
Solution Approach 2:
The patent changes the geometric parameters of the connection joints from traditional cylindrical or planar interfaces to spherical geometry. This parameter change fundamentally alters the tolerance accumulation behavior, as the spherical joint's curvature provides inherent error compensation while maintaining ease of assembly and adjustment.
3Strength
If traditional external fixator designs are used, then the structure provides skeletal support, but mechanical clearance leads to unwanted bone motion and acoustic noise during healing
Solution Approach 1:
The patent replaces traditional mechanical connection systems with spherical joints and cable actuation. The spherical joints eliminate mechanical play that causes bone motion and noise, while the cable system provides continuous contact without the clearance inherent in threaded connections. This substitution maintains strong skeletal support while eliminating the harmful effects of mechanical clearance.
Solution Approach 2:
The patent converts the potential harm of mechanical clearance into a benefit by using spherical joints whose geometric constraint transforms what would be loose play into precise positioning. The inherent tolerance in the spherical joint design, which could be seen as harmful clearance, is actually converted into a self-aligning feature that improves positioning accuracy while maintaining strong support.
4Stability of the object's composition
If fixed stiffness design is used in external fixators, then the structure provides stable support, but it cannot adapt to different healing phases requiring variable compliance
Solution Approach 1:
The patent implements a dynamically adjustable stiffness system where the compliance of the external fixator can be changed during the healing process. The cable-driven actuation system allows real-time modification of the structural compliance, enabling the device to transition from high stiffness in early healing phases to lower stiffness as the bone heals, thus providing both initial stability and adaptive compliance.
Solution Approach 2:
The patent creates a universal fixator design that can serve multiple healing phases and clinical requirements through adjustable compliance. The same structural framework provides both high-stiffness support for unstable fractures and controlled compliance for gradual bone lengthening or deformity correction, eliminating the need for different fixed-stiffness devices for different applications.
Data Source
AI summary
An orthopedic fixator for positioning a first element relative to a second element with precision and with controlled compliance which can be adjusted during the healing process. One embodiment comprises a first frame for attachment to the first element, a second frame attached to the first frame through a plurality of adjustable effective length struts, and a third frame for attachment to the second element, wherein the third frame is compliantly attached to the second frame. A preferred embodiment comprises adjustable length preload elements to apply unidirectional forces between the first and second frames so as preload the adjustable effective length struts and substantially reduce the positional tolerance. An alternative embodiment comprises adjustable spring elements allowing the compliance of the attachment of the third frame to the second frame to be adjusted at various points in the healing process.