Polyaxial Screw With One-Way Locking Mechanism

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Solution Overview

Problem

Screw fixation in orthopedic applications, such as facet joint fixation, often experiences loosening due to physiologic movement and bone remodeling, leading to unintended screw withdrawal and compromised fixation.

Innovation Solution

A low-profile, self-contained, polyaxial screw and washer system that automatically resists screw unthreading through a one-way mechanism, allowing free rotation in one direction and locking in the opposite direction, utilizing a ball and socket joint with a spring-biased mechanism for secure fixation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional screw fixation system is used, then the screw can be easily inserted and positioned, but the screw tends to loosen and back out over time due to physiologic movement and bone remodeling

Engineering Contradiction:

Improvefixation stabilityVSAvoidscrew retention time

Core Design Contradiction:

ReliabilityVSDuration of action of moving object

Solution Approach 1:

The screw system incorporates a dynamic one-way locking mechanism that allows the screw to rotate freely in the insertion direction while automatically locking in the opposite direction. This dynamic behavior adapts to physiologic movements and bone remodeling without compromising fixation stability, resolving the contradiction between ease of positioning and long-term retention.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The anti-backout mechanism is self-activating and requires no additional components or manual intervention. The screw automatically locks itself upon rotation in the tightening direction, providing self-service functionality that maintains fixation stability over time without requiring external assistance.

Inventive Principle:

Principle #25Self-service

2Reliability

If a polyaxial screw system with one-way locking mechanism is implemented, then screw back-out is prevented, but the device complexity increases

Engineering Contradiction:

Improveanti-backout capabilityVSAvoidscrew system structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The anti-backout mechanism is merged directly into the screw head structure, eliminating the need for separate locking components. The spherical bearing surface and ramp features are integrated with the screw head, creating a compact design that prevents back-out without significantly increasing overall device complexity.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The screw system is segmented into distinct functional zones: the spherical head for polyaxial adjustment, the ramp features for one-way locking, and the threaded shaft for bone engagement. This segmentation allows each component to perform its specific function efficiently while maintaining overall system simplicity.

Inventive Principle:

Principle #1Segmentation

3Adaptability or versatility

If the screw allows free rotation for positioning, then adaptability to bone surface topography is improved, but the screw may unintentionally rotate back out

Engineering Contradiction:

Improvepolyaxial adjustment capabilityVSAvoidunintentional screw withdrawal

Core Design Contradiction:

Adaptability or versatilityVSObject-affected harmful factors

Solution Approach 1:

The screw head features asymmetric ramp structures that create different resistance characteristics for rotation in opposite directions. Free rotation is permitted in the insertion direction to accommodate polyaxial positioning, while the asymmetric ramps automatically engage to prevent rotation in the back-out direction, resolving the contradiction between adaptability and prevention of unintentional withdrawal.

Inventive Principle:

Principle #4Asymmetry

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

Prevents unintentional back-out and loosening of the screw, ensuring stable fixation by locking the screw and washer assembly together when rotated in a specific direction, even with bone surface topography variations.

Implementation Method 1

utilizing a ball and socket joint with a spring-biased mechanism for secure fixation

Methodology Applied

Scientific EffectSpring mechanism: Spring

Implementation Method 2

utilizing a ball and socket joint with a spring-biased mechanism for secure fixation

Methodology Applied

Scientific EffectBall and socket joint: Ball

Implementation Method 3

allowing free rotation in one direction and locking in the opposite direction

Methodology Applied

Scientific EffectFriction locking: Friction

Data Source

PatentUS8998966B2Polyaxial facet fixation screw system with fixation augmentation

Publication Date: 2015.04.07 WENZEL SPINE

AI summary

A screw system includes a screw and a washer assembly captive to the screw. The washer assembly is polyaxially pivotable relative to the screw. The screw may be freely rotated in one direction relative to the washer assembly, but frictionally binds with the washer assembly when rotated in a second direction. Various auxiliary fixation features are disclosed.