Multi-Component Bone Anchor Bottom Loading
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Solution Overview
Problem
Existing spinal fixation devices with large diameter bone screws face limitations in angulation and stability due to increased profile and risk of breakage, especially with favored-angle bone anchor assemblies that are subjected to tensional forces.
Innovation Solution
A multi-component bone anchor assembly with a polyaxial seat and expandable inner and outer rings, allowing bottom-loading of the bone anchor and providing secure fixation, includes a compression cap to lock the shank in a fixed angular orientation relative to the receiver member, enhancing stability and angulation capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a large diameter bone screw is used, then bending force resistance is improved, but the profile of the bone anchor assembly increases
Solution Approach 1:
The bone anchor assembly is divided into separate components: a bone screw with head, a receiver member, and a closure member. This segmentation allows the bone screw to have a large diameter for strength while the receiver member can be designed with a low profile, resolving the contradiction between bending force resistance and assembly profile.
Solution Approach 2:
The closure member is received within the receiver member, and the bone screw head is positioned within the receiver member. This nesting arrangement allows compact packaging of components, maintaining a low overall profile while accommodating the large diameter bone screw through bottom-loading configuration.
2Adaptability or versatility
If bottom-loading configuration is used with large diameter bone screws, then shank diameter independence from receiver opening size is achieved, but angulation capability and stability deteriorate
Solution Approach 1:
The closure member is designed to allow dynamic adjustment of the bone screw head position and angulation relative to the receiver member before final locking. This dynamic capability enables proper angulation during implantation while maintaining the bottom-loading configuration that provides shank diameter independence.
Solution Approach 2:
The system allows adjustment of angular parameters and positional parameters of the bone screw relative to the receiver member through the closure mechanism. This parameter adjustment capability resolves the contradiction by enabling both bottom-loading independence and proper angulation capability.
3Stability of the object's composition
If favored-angle bone anchor assembly is used, then directional stability is improved, but resistance to tensional forces and risk of breakage worsen
Solution Approach 1:
The closure member is designed to lock the bone screw head to the receiver member after proper positioning and angulation are achieved, creating a predetermined favorable angle. This preliminary action of establishing the correct angle before loading ensures both directional stability and resistance to tensional forces by properly orienting the load path.
Solution Approach 2:
The bone anchor assembly uses composite construction with the bone screw, receiver member, and closure member made from materials optimized for their specific functions. This composite approach allows the assembly to withstand tensional forces while maintaining directional stability through the favored-angle configuration.
4Device complexity
If monoaxial or unidirectional assembly is used, then structural simplicity is improved, but adaptability to different vertebra shapes and sizes deteriorates
Solution Approach 1:
The polyaxial closure member allows the bone screw to be positioned at multiple angles relative to the receiver member, providing adaptability to different vertebra shapes and sizes. This dynamic angular adjustment capability maintains reasonable structural simplicity while significantly improving versatility compared to fixed monoaxial designs.
Data Source
AI summary
Bone anchor assemblies and methods are provided having a multi-component bone anchor that is configured to allow the shank of the bone anchor to be bottom loaded into a receiver member. In one embodiment, a bone anchor assembly is provided having a shank with a distal threaded portion and a proximal head portion, a receiver member having an aperture formed in a distal end thereof through which the head portion of the shank can be received, the receiver member defining a polyaxial seat, a non-expandable outer ring configured to be polyaxially disposed within the polyaxial seat of the receiver member and defining a central opening through which the head portion of the shank can be received, and an expandable inner ring having an inner surface configured to mate with the head portion of the shank and an outer surface configured to mate with an inner surface of the outer ring to thereby lock the outer ring in a fixed position relative to the shank.