Eccentric Transpedicular Polyaxial Screw Spinal Fixation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing transpedicular fixation methods in spine surgery often result in irreversible damage to facet joints due to overhang of the screw head beyond the vertebral body, necessitating excessive resection and leading to postoperative complications.
Innovation Solution
A transpedicular polyaxial screw with an eccentrically positioned threaded shaft, featuring a head with a slot for the fixation rod, an upper thrust element fixed within the head, and a lower thrust element with a hinge connection, allowing the spherical end of the threaded shaft to be pressed against the head's wall, thereby reducing the risk of facet joint damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the screw head is positioned symmetrically with the spherical end of the shaft at the center, then the polyaxial screw provides good adaptability and ease of operation, but the screw head overhangs the centre of the spherical end by at least 6 millimeters causing irreversible damage to the facet joints
Solution Approach 1:
The patent applies asymmetry by positioning the spherical end of the threaded shaft eccentrically within the cylindrical head, specifically 2-4 millimeters from the center. This asymmetric positioning allows the screw head to remain outside the facet joint while maintaining the polyaxial movement capability, thus preventing facet joint damage while preserving ease of operation.
Solution Approach 2:
The patent introduces a new dimensional parameter - the radial offset distance (2-4 millimeters) of the spherical end from the center of the cylindrical head. This creates an additional degree of freedom in the design space, allowing simultaneous optimization of both polyaxial movement and facet joint protection by controlling the offset distance within this specific range.
2Ease of manufacture
If the spherical end of the shaft is placed symmetrically in the head, then the design is simple and manufacturing is easier, but the head of the screw overhangs the vertebral body requiring excessive resection of the outer rim of the facet joint
Solution Approach 1:
The patent deliberately introduces asymmetry in the positioning of the spherical end within the cylindrical head, placing it 2-4 millimeters from the center. This asymmetric design reduces the required resection of the facet joint outer rim while maintaining manufacturability through precise positioning features and tolerances specified in the patent.
3Object-affected harmful factors
If the head of the screw is positioned to minimize overhang, then facet joint damage is reduced, but the polyaxial movement and rod placement comfort may be compromised
Solution Approach 1:
The patent resolves this contradiction by implementing asymmetric positioning of the spherical end 2-4 millimeters from the center of the cylindrical head. This specific offset range maintains sufficient space for polyaxial movement and comfortable rod placement while ensuring the screw head remains outside the facet joint, thus protecting against facet joint damage.
Solution Approach 2:
The patent changes the critical parameter of spherical end positioning from centered (0 millimeter offset) to eccentric (2-4 millimeter offset). This parameter change optimizes both facet joint protection and operational comfort by finding the optimal balance point that satisfies both requirements simultaneously.
Data Source
AI summary
A transpedicular polyaxial screw, especially a transpedicular polyaxial screw with eccentrically mounted threaded shaft, with the head (1) of the screw provided with a slot (7) from the above in which the fixation rod (3) is placed. The fixation rod (3) is arrested with the upper thrust element (2) also fixed in the head (1) of the screw. The bottom of the slot (7) is connected with the outer side (8) of the head (1) of the screw through an opening (9) that leads out eccentrically at the transition of the base and the outer side (8) of the head (1) of the screw. In the opening, the lower thrust element (4) and the spherical end (5) of the threaded shaft (6) are placed. The upper surface of the lower thrust element (4) is in contact with the fixation rod (2) while the lower surface of the lower thrust element (4) is in contact with the spherical end (5) of the threaded shaft (6), thus pressing it to the wall of the opening (9) in the head (1) of the screw.