Spherical Polyaxial Bone Fixation Element for Secure Spinal Rods
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Solution Overview
Problem
Existing fastener devices for spinal fusion procedures lack simplicity and flexibility in securing elongated members to the spine, while ensuring secure mounting and polyaxial rotation.
Innovation Solution
A fastener device comprising an anchor body and a fastener with a convex outer surface and a concave inner surface, allowing for polyaxial rotation and secure mounting, featuring a ratio of outer diameters between 2:1 and 3:1, and a biasing member to restrict relative movement, enhancing strength and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the anchor body is made pivotable with respect to the fastener to provide polyaxial rotation, then flexibility in positioning the elongated members is improved, but device complexity increases
Solution Approach 1:
The fastener head features a spherical outer surface that articulates within a spherical inner surface of the anchor body, enabling polyaxial rotation while maintaining a relatively simple device structure. This spherical articulation mechanism allows the fastener to pivot in multiple directions without requiring complex mechanical components.
Solution Approach 2:
The invention specifies a diameter ratio between the fastener head outer surface and the neck outer surface of at least 2:1, which optimizes the articulation geometry. This parameter change ensures sufficient rotation capability while maintaining structural integrity and simplicity.
2Ease of operation
If the fastener head outer surface articulates along the anchor body inner surface, then ease of insertion and positioning is improved, but stress concentration may increase
Solution Approach 1:
The invention specifies that the diameter of the fastener head outer surface is at least twice the diameter of the fastener neck outer surface. This parameter change distributes stresses more effectively during articulation, reducing stress concentration at the neck while maintaining ease of insertion and positioning.
Solution Approach 2:
The spherical surfaces of the fastener head and anchor body provide smooth articulation during insertion and positioning. The curved geometry distributes contact forces across a larger area, reducing stress concentration compared to sharp-edged or flat interfaces.
3Strength
If the ratio of fastener head diameter to neck diameter is increased to 2:1 or 3:1, then strength and stress distribution are improved, but device complexity increases
Solution Approach 1:
The invention optimizes the diameter ratio of the fastener head to neck at至少 2:1, which improves stress distribution while maintaining a relatively simple geometric form. This parameter optimization achieves enhanced strength without requiring complex multi-component structures.
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.