Polyaxial Bone Anchor Assembly With Expansion Locking Against Pull-Out

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Solution Overview

Problem

Existing polyaxial bone screws suffer from floppy rotation of the receiver relative to the shank, making surgical procedures difficult, and the prior art's contractile locking mechanisms are weak against pull-out forces.

Innovation Solution

A polyaxial bone screw design featuring a split retainer ring with a radiused surface and expansion-only locking engagement, which provides a secure, non-floppy connection between the shank and receiver, using a friction fit and spring tabs to stabilize the assembly.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If a contractile locking mechanism is used to connect the shank and receiver, then the assembly can be locked in position, but the locking mechanism is weak against pull-out forces

Engineering Contradiction:

Improveresistance to pull-out forcesVSAvoidlocking mechanism complexity

Core Design Contradiction:

StrengthVSDevice complexity

Solution Approach 1:

The locking mechanism is divided into two independent functions: the split retainer ring provides expansion-only locking engagement to prevent pull-out forces, while spring tabs provide rotational stabilization. This segmentation allows each component to specialize in one function, improving overall strength without excessive complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Instead of using a contractile (compressive) locking mechanism that collapses inward, the patent uses an expansion-only mechanism where the split retainer ring expands outward to lock against the shank. This inversion provides superior resistance to pull-out forces because the locking force acts in the same direction as the retained object.

Inventive Principle:

Principle #13The other way round (Inversion)

2Ease of operation

If the receiver is allowed to rotate freely relative to the shank, then assembly is easier, but the connection becomes floppy and difficult to manipulate surgically

Engineering Contradiction:

Improvesurgical manipulation easeVSAvoidconnection stability

Core Design Contradiction:

Ease of operationVSStability of the object's composition

Solution Approach 1:

The spring tabs are designed to be flexible during assembly, allowing the receiver to rotate freely relative to the shank for easy positioning. Once locked in the desired position, the tabs engage with the shank to provide rotational stability. This dynamic behavior transitions from mobile during assembly to stable during use.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The spring tabs are pre-positioned in a relaxed state that allows free rotation during assembly. After the receiver is positioned correctly and locked by the split retainer ring, the tabs are then engaged to prevent rotation. This preliminary flexible state followed by subsequent stabilization allows easy assembly then stable operation.

Inventive Principle:

Principle #10Preliminary action

3Strength

If an expansion-only split retainer ring is used for locking, then resistance to pull-out forces is enhanced, but the manufacturing precision requirements increase

Engineering Contradiction:

Improveresistance to pull-out forcesVSAvoidradiused surface precision

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

The split retainer ring features a radiused (curved) surface that contacts the shank, replacing a flat or sharp surface. This curvature distributes contact forces more evenly, reduces stress concentrations, and provides a more forgiving tolerance profile during manufacturing while maintaining superior pull-out resistance.

Inventive Principle:

Principle #14Spheroidality (Curvature)

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

The design offers enhanced resistance to pull-out forces and allows for secure, stable fixation of the shank relative to the receiver, enabling easier surgical manipulation and improved spinal correction techniques.

Implementation Method 1

spring tabs to stabilize the assembly

Methodology Applied

Scientific EffectElasticity: Elasticity

Implementation Method 2

friction fit and spring tabs to stabilize the assembly

Methodology Applied

Scientific EffectFriction: Friction

Data Source

PatentUS20260026846A1Method of assembling a pivotable bone anchor assembly with independent locking by insert side compressing member

Publication Date: 2026.01.29 JACKSON CORP

AI summary

A method of assembling a pivotal bone anchor assembly includes positioning a capture portion of a bone anchor and an insert within a central bore of a receiver, with the receiver having upstanding arms defining an open channel for receiving a rod, a cavity for receiving the bone anchor, and with at least one upstanding arm configured to slidably receive an insert compressing member in a direction parallel with a longitudinal axis of the receiver. The method also includes pressing continuously downward on the insert with the insert compressing member until the insert contacts the capture portion of the bone anchor to establish a pressable and releasable locking engagement of the bone anchor with respect to the receiver that is independent of a closure top. The method further includes positioning the elongate rod within the open channel of the receiver, followed by securing the closure top to a guide and advancement structure of the receiver above the elongate rod to establish a final locked configuration of the elongate rod and the pivotal bone anchor assembly, followed by slidably removing the insert compressing member from the receiver.