Polyaxial Pedicle Screw Segmented Locking Mechanism
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Solution Overview
Problem
Existing polyaxial pedicle screws face issues with locking mechanisms that disrupt polyaxial movement, lead to misalignment, or result in unintended detachment due to complex designs or insufficient locking, causing instability in spinal fixation.
Innovation Solution
A detachable polyaxial pedicle screw design featuring a tulip head assembly with a locking ball and collet, secured by pins, allowing polyaxial movement before locking, and utilizing an expandable aperture with internal threads and locking tabs to securely engage with the bone screw, preventing counter-rotation and disassembly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a locking mechanism with threads or features on the spherical portion is used, then the head can be secured to the bone screw, but the polyaxial movement is disrupted
Solution Approach 1:
The locking mechanism is segmented into two distinct functional zones: the spherical portion maintains a smooth surface for unrestricted polyaxial rotation, while the cylindrical portion contains the threading and locking features. This segmentation allows the head to rotate freely in all axial directions before locking, and then secures firmly in the desired position without disrupting the rotation capability.
Solution Approach 2:
The cylindrical portion acts as an intermediary element between the spherical rotation interface and the bone screw threading interface. It mediates the transition from free polyaxial movement to secure locked positioning, enabling both functions to coexist without interference.
2Reliability
If a complex locking mechanism is used, then the head can be securely locked, but the device complexity increases leading to potential failure
Solution Approach 1:
The complex multi-component locking mechanisms found in prior art are extracted and replaced with a simple two-part design: a spherical portion for rotation and a cylindrical portion with internal threading for locking. This extraction of unnecessary complexity reduces the number of parts, minimizes potential failure points, and simplifies the overall device while maintaining secure locking capability.
Solution Approach 2:
Instead of using a complex mechanism to enable movement and then lock, the design inverts the approach by having a inherently free-rotating spherical portion that requires no mechanism for movement, and a simple cylindrical threading system that provides locking through basic screw-thread engagement rather than complex interlocking components.
3Ease of manufacture
If an overly simplistic locking mechanism is used, then the device is simple to manufacture, but unintended movement or detachment may occur
Solution Approach 1:
The head assembly uses asymmetric geometry with a spherical portion for rotation and a cylindrical portion with specific threading for locking. This asymmetric design provides inherent mechanical advantage where the spherical part enables easy polyaxial movement during implantation, while the cylindrical threaded part provides progressive, self-centering engagement that prevents unintended movement or detachment once locked.
Solution Approach 2:
The spherical portion of the head provides curved surface geometry that naturally guides and accommodates polyaxial rotation during implantation. This spheroidal shape simplifies the manufacturing of the rotation interface compared to flat or complex curved surfaces, while the transition to the cylindrical threaded portion provides a reliable locking interface that prevents backout or detachment.
4Reliability
If threads are engaged on the spherical head, then the head can be secured, but the head can no longer rotate polyaxially
Solution Approach 1:
The head is segmented into two distinct functional zones: the spherical portion maintains a smooth surface for unrestricted polyaxial rotation, while the cylindrical portion contains the threading and locking features. This segmentation allows the head to rotate freely in all axial directions before locking, and then secures firmly in the desired position without disrupting the rotation capability.
Data Source
AI summary
The present invention is directed at a polyaxial pedicle screw comprising a detachable polyaxial head and bone screw. The detachable polyaxial head may comprise a tulip head, a collet with one or more connectors (e.g., pins) to secure it, and a spherical locking member which may lock onto the bone screw.