Polyaxial Screw Fork Head Stability via Axial Pressure Piece Insertion
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Solution Overview
Problem
Existing polyaxial screws have a weakened fork head due to a protruding lateral opening for inserting the pressure piece, which compromises stability and increases installation space requirements.
Innovation Solution
The fork head features a through-opening for the screw anchor and an obliquely inclined pressure piece with a cut-out, allowing insertion from above, eliminating the need for a protruding lateral opening and reducing the need for additional weakening features like radial inner access openings or grooves, enhancing stability and reducing installation space.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a protruding lateral opening is used for inserting the pressure piece, then the pressure piece can be inserted, but the fork head is weakened and stability is compromised
Solution Approach 1:
The pressure piece is inserted from the axial direction (from above) rather than through a lateral opening. The oblique inclination of the pressure piece allows it to pass through the axial space above the fork head and engage with the bearing, eliminating the need for lateral openings that would compromise fork head stability.
Solution Approach 2:
The pressure piece is designed to nest within the space defined by the fork head's arms when viewed from above. The cut-out in the pressure piece allows it to be inserted obliquely through the axial opening while its receiving region engages with the bearing, creating a nested configuration that maintains fork head integrity.
2Ease of operation
If radial inner access openings or grooves are added for mounting, then the pressure piece can be mounted, but the fork head requires additional weakening features
Solution Approach 1:
The axial opening in the fork head serves multiple functions: it allows insertion of the screw anchor, provides access for inserting the obliquely inclined pressure piece from above, and enables mounting operations. This multi-functional opening eliminates the need for separate radial access openings or grooves.
Solution Approach 2:
The mounting function is extracted from the lateral/radial direction and integrated into the axial insertion path. The oblique inclination of the pressure piece allows the mounting operation to occur through the existing axial opening, removing the need for additional radial mounting features.
3Ease of operation
If a through-opening is used for the screw anchor, then the screw anchor can be inserted, but the fork head structure is simplified
Solution Approach 1:
The pressure piece insertion path is changed from lateral to axial with oblique inclination. This allows the pressure piece to be inserted through the same axial opening used for the screw anchor, avoiding the need for additional lateral openings that would compromise fork head strength.
Data Source
AI summary
The invention relates to a polyaxial screw including a screw anchor and a fork head which has two arms, is U-shaped in a lateral view and has a receiving opening for a correction element, and including a pressure piece which can be arranged in the fork head between the head of the screw anchor and the correction element, rests on the head of the screw anchor and can be loaded by the correction element.