Spinal Screw Head Assembly With Plane-Specific Angulation Limits

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Solution Overview

Problem

Existing spinal surgery techniques using polyaxial pedicle screws lack effective mechanisms to limit angulation in specific planes, leading to potential misalignment and instability during surgical procedures.

Innovation Solution

A spinal screw apparatus with a movable head that allows more angulation in one plane than in a perpendicular plane, featuring a collet and movable head with internal and external surfaces designed to limit motion in specific directions, and sub-motion-limiters to define the range of angulation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If polyaxial pedicle screws allow angulation in various degrees of freedom, then adaptability is improved, but stability deteriorates due to potential misalignment

Engineering Contradiction:

Improveangulation freedomVSAvoidalignment stability

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The movable head is segmented into multiple components: a spherical interface allowing rotation, a collet mechanism for radial constraint, and planar surfaces for angular limitation. This segmentation enables independent control of different motion degrees, providing adaptability in one direction while maintaining stability in perpendicular directions.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the movable head have different constraint characteristics: the spherical interface provides freedom of rotation, the collet provides radial constraint, and the planar surfaces provide angular limitation. This local differentiation of constraint quality allows the screw to be adaptive in needed directions while stable in directions requiring precision.

Inventive Principle:

Principle #3Local quality

2Ease of operation

If the movable head allows more angulation in one plane, then ease of operation is improved, but manufacturing precision requirements worsen

Engineering Contradiction:

Improveangulation adjustmentVSAvoidplanar surface alignment

Core Design Contradiction:

Ease of operationVSManufacturing precision

Solution Approach 1:

The spherical interface between the screw head and movable head provides inherent rotational freedom without requiring precise manufacturing tolerances for the rotation axis. The curvature of the spherical surfaces naturally guides the rotation motion, reducing sensitivity to manufacturing variations while enabling easy angulation adjustment.

Inventive Principle:

Principle #14Spheroidality (Curvature)

Solution Approach 2:

The movable head employs asymmetric planar surfaces oriented at specific angles to create different constraint characteristics in different planes. This asymmetry allows greater angulation freedom in one plane while maintaining precise control in the perpendicular plane, balancing ease of operation with manufacturing feasibility.

Inventive Principle:

Principle #4Asymmetry

Data Source

PatentUS12471963B2Apparatus and method for limiting a range of angular positions of a screw

Publication Date: 2025.11.18 SEASPINE ORTHOPEDICS CORP

AI summary

A spinal screw assembly is provided. The assembly may be provided with a screw and a movable head in which angulation of the screw relative to the movable head is constrained so that angulation within limits is allowed in one plane but lesser or no angulation is allowed in another plane. Methods to assemble the disclosed apparatus are also disclosed.