Polyaxial Bone Fixation Assembly with Interrupted Thread
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Solution Overview
Problem
Polyaxial bone fixation assemblies are recognized as less solid than monoaxial fasteners due to issues like 'cold welding' that make screw extraction difficult, require expensive torque tools, and are prone to screw protrusion and thread deterioration, leading to reduced security and increased operation duration.
Innovation Solution
A two-part polyaxial bone fixation assembly with a plate and screw design featuring complementary spherical surfaces and a single-threaded screw head with interrupted threads, allowing for reliable screwing and compression without the need for torque tools, and enabling multiple assembly and disassembly cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If polyaxial fixation uses threaded engagement without mechanical stop, then screw insertion flexibility is improved, but screw extraction difficulty increases due to cold welding
Solution Approach 1:
The engagement mechanism is segmented into two distinct parts: a mechanical stop component (spherical or conical surface) that provides angular positioning and a threaded portion that provides locking. This segmentation allows the screw to be stopped at any angle via the mechanical stop while the threads provide secure locking, enabling both flexible insertion and easy extraction.
Solution Approach 2:
A mechanical stop (spherical or conical surface) is introduced as an intermediary element between the screw head and the plate hole. This intermediary provides a mechanical stop that prevents cold welding by limiting thread engagement, while still allowing threaded locking for security. The mechanical stop acts as a mediator that enables both angular flexibility and easy extraction.
2Adaptability or versatility
If polyaxial fixation uses intermediate piece with ball joint, then screw angular positioning is improved, but plate thickness increases
Solution Approach 1:
The mechanical stop function and the hole structure are merged into a single integrated design. The plate hole directly incorporates the spherical or conical mechanical stop surface, eliminating the need for a separate intermediate piece or ball joint. This merging reduces plate thickness while maintaining polyaxial angular positioning capability.
Solution Approach 2:
The intermediate piece (ball joint) is extracted from the system and replaced by a direct mechanical stop surface formed in the plate hole. This extraction simplifies the structure, reduces the number of components, and decreases plate thickness while preserving the angular positioning function.
3Device complexity
If polyaxial fixation uses threaded engagement without stop, then assembly simplicity is improved, but screw protrusion risk increases
Solution Approach 1:
The mechanical stop (spherical or conical surface) is built into the plate hole structure in advance, before screw insertion. This preliminary action ensures that when the screw is inserted, the mechanical stop automatically prevents over-insertion and screw protrusion, maintaining both assembly simplicity and reliability.
4Ease of manufacture
If polyaxial fixation uses standard screwdriver, then cost is reduced, but locking torque control is insufficient
Solution Approach 1:
The mechanical stop (spherical or conical surface) enables the screw to self-limit its insertion depth and angular position without requiring external torque control instruments. The screw automatically engages the mechanical stop and threads, providing self-controlled locking torque and eliminating the need for expensive calibrated torque screwdrivers.
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
A polyaxial bone fixation assembly (1) comprising a plate (2) with at least one through-opening (3) and at least one fixation screw (4) for fixing the plate (2) to a bone, the body (6) of the screw (4) being capable of passing through the opening (3) and the head (5) being capable of being held in the opening (3) of the plate (2), by means of the bearing contact of complementary male (7) and female (8) spherical surfaces (7, 8), the female spherical surface (8) provided in the opening (3) of the plate (2) forming a seat inside which the male spherical surface (7) is capable of coming to bear when the screw (4) is inserted into the opening (3). The bore of the opening (3) of the plate (2) comprises, from the bottom surface (22) towards the top surface (21) of the plate (2), an inner flange (9), a cylindrical portion (10) and the female spherical surface (8) forming the seat, and the head (5) of the associated fixing screw (4) comprises, between the male spherical surface (7) and the connection area (11) of same connecting to the body (6) of the screw (4), a threaded portion (12) with a single thread, this threading being interrupted in at least two areas (13) in order to allow, concomitant to the rotational movement of the screw (4) through said opening (3) to a position in which the male (7) and female (8) spherical surfaces are in bearing contact with each other, a machining of the inner flange (9) of the associated opening (3) of the plate (2).