Polyaxial Pedicle Screw With Radial Extensions
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Solution Overview
Problem
Existing polyaxial pedicle screws are not capable of adequately supporting high transverse loads when transverse pivoting is blocked, and they are complex to manufacture and assemble, leading to increased costs and the need for specialized instrumentation.
Innovation Solution
A pedicle screw design with additional support areas between the receiving sleeve and screw head that prevent transverse pivoting while allowing mobility about a pivot axis, using extensions projecting beyond the spherical screw head to form supportive surfaces with the receiving sleeve, which can absorb high lateral loads without impeding pivoting in the desired plane.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional polyaxial pedicle screws with flattened heads and inlay/insert are used to block transverse pivoting, then transverse pivoting is blocked, but the screws cannot adequately support high transverse loads and suffer from rapid wear
Solution Approach 1:
The screw head is segmented into a spherical basic shape and separate extensions that project beyond it. These extensions are positioned to form support surfaces with the receiving sleeve, creating distinct functional zones: the spherical portion allows pivoting while the extensions block transverse pivoting and support high transverse loads.
Solution Approach 2:
The extensions project radially beyond the spherical basic shape of the screw head, adding a dimensional element that creates new support surfaces. This radial extension into additional space allows the screw to maintain pivoting capability in one plane while blocking transverse pivoting in another plane, resolving the contradiction between mobility and stability.
2Strength
If extensions projecting beyond the spherical screw head are added to form support surfaces with the receiving sleeve, then lateral stability and load transmission are improved, but the screw head geometry becomes more complex
Solution Approach 1:
The screw head has different local geometries serving different functions: the spherical basic shape provides pivoting capability, while the extensions projecting beyond it provide lateral support. This local differentiation allows the screw to exhibit both mobility and stability without requiring complete geometric redesign.
Solution Approach 2:
The screw head maintains a spherical basic shape which inherently allows pivoting movement. Extensions are added to this spherical form rather than replacing it, preserving the beneficial pivoting geometry while adding the necessary lateral support functionality through the extensions.
3Force
If high lateral forces are transmitted through traditional flattened screw heads, then transverse pivoting is blocked, but the flanges become elastically deformable and cannot adequately support the forces
Solution Approach 1:
The extensions are pre-positioned to abut against the radially inner circumferential side of the receiving sleeve before high transverse loads are applied. This preliminary contact establishes a stable load path that prevents elastic deformation of the screw head flanges during force transmission.
Solution Approach 2:
The extensions project beyond the spherical basic shape to provide excessive lateral support beyond what the spherical head alone could provide. This excessive action ensures that even under high transverse loads, the support surfaces maintain contact and prevent force-induced deformation.
Data Source
AI summary
A polyaxial pedicle screw has a screw shaft and a screw head with a substantially spherical base shape. The polyaxial pedicle screw is mounted in a receiving sleeve or area. An insertion sleeve can be pressed against the screw head to determine a relative pivot position between the receiving sleeve or area and the screw shaft. Two pivot guide or pivot restriction units each have at least one extension projecting radially and in the longitudinal direction of the screw beyond the spherical base shape of the screw head. The extensions form stops on their radial outer surfaces and are positioned to permit relative pivoting of the screw shaft and the area or receiving sleeve only in one pivoting plane, and to abut in a supporting manner against a radially inner circumferential side of the area or receiving sleeve during relative pivoting in another pivoting plane.