Polyaxial Bone Fastener Geometry for Stable Spinal Rod Fixation
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Solution Overview
Problem
Existing fastener devices for spinal fusion procedures are complex and lack the ability to securely and easily mount elongated members to the spine, particularly due to issues with polyaxial rotation and stability.
Innovation Solution
A fastener device with an anchor body and fastener design featuring a convex outer surface and concave inner surface configuration, allowing for polyaxial movement and secure mounting, along with a biasing member to restrict unwanted movement, enhancing stability and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing fastener devices are used for spinal fusion, then the procedure can be performed, but the devices are complex and lack secure mounting capability for elongated members
Solution Approach 1:
The fastener device is divided into distinct functional components: an anchor body for securing to the vertebra, a polyaxial mechanism for angular adjustment, and an elongated member receiving structure. This segmentation allows each component to be optimized independently for its specific function while simplifying the overall device design and assembly.
Solution Approach 2:
The patent extracts the polyaxial rotation capability as a separate mechanical feature within the anchor body, allowing the elongated member to be secured at multiple angles without requiring complex external adjustment mechanisms. This extraction of the essential function simplifies the overall device while maintaining secure mounting capability.
2Adaptability or versatility
If polyaxial rotation capability is added to fastener devices, then flexibility in positioning is improved, but stability and security of mounting deteriorates
Solution Approach 1:
The fastener device incorporates a dynamic polyaxial mechanism that allows rotational movement during the insertion and positioning phase, then transitions to a locked stable configuration once the elongated member is secured. This dynamic design provides both the needed flexibility for positioning and the stability for final fixation.
Solution Approach 2:
The device changes its mechanical parameters from a loose, rotatable state during insertion to a locked, stable state after positioning. The polyaxial mechanism allows angular parameter variation during insertion, then locks to maintain a fixed angular parameter for stable long-term fixation of the spinal rod.
3Ease of operation
If simple fastener design is used, then ease of operation is improved, but ability to restrict unwanted movement deteriorates
Solution Approach 1:
The fastener device incorporates self-aligning and self-locking features that automatically restrict unwanted movement without requiring complex external controls or multiple adjustment steps. The simple design allows easy insertion while the self-service mechanisms provide automatic stabilization and movement restriction once positioned.
Data Source
AI summary
A fastener device includes an anchor body that defines a through hole. The anchor body further includes an inner surface that defines a least a portion of the through hole. The fastener includes a head, a threaded shaft that extends out with respect to the head in a distal direction, and a neck between the head and threaded shaft. The head includes an outer surface configured to articulate along the inner surface when the fastener head is inserted in the through hole. At least a portion of the outer surface is convex and defines a portion of a sphere that defines a first diameter. The neck defines a second diameter and the fastener defines a ratio of the first diameter to the second diameter in a range between about 2 to 1 and about 3 to 1.