Polyaxial Pedicle Screw Segmented Head Design
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Solution Overview
Problem
Current spinal fixation systems, particularly pedicle screw and rod constructs, face challenges in providing a cost-effective and rigid solution for large, stiff deformity corrections, where larger rods require larger screw heads, increasing the construct's profile and material incompatibility issues.
Innovation Solution
A polyaxial pedicle screw design featuring a bone screw member with a head having a threaded shaft and a housing that allows selective angular positioning and securement, utilizing a combination of cylindrical and spherical portions for secure insertion and a compression ring to prevent reorientation, along with an anvil and set screw for rod securement, enabling multi-planar fixation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If larger diameter stainless steel rods are used to improve strength for large deformity corrections, then the strength of the screw and rod construct is improved, but the profile of the construct increases and material incompatibility issues arise
Solution Approach 1:
The screw head is segmented into two distinct portions: a first portion (cylindrical) that fits through the housing opening, and a second portion (spherical or enlarged) that remains outside the housing to provide a large bearing surface for the rod. This segmentation allows the rod to contact the second head portion directly, eliminating the need for a large screw head profile while maintaining construct strength
Solution Approach 2:
The housing acts as an intermediary component that mediates between the screw and rod. The housing contains the rod and transmits loads between the rod and the screw, allowing the screw head to have a reduced profile while the housing provides the necessary structural interface for strong rod-screw connection
2Strength
If larger diameter stainless steel rods are used to improve strength for large deformity corrections, then the strength of the screw and rod construct is improved, but material incompatibility issues arise
Solution Approach 1:
The housing serves multiple functions: it contains the rod, provides a bearing surface for the rod, transmits loads between the rod and screw, and enables compatibility between titanium screws and stainless steel rods. This multi-functional design allows different material combinations without compromising construct strength
Solution Approach 2:
The housing acts as an intermediary that decouples the material requirements of the screw and rod. By introducing this intermediate component, the system can use titanium screws (biocompatible, corrosion-resistant) with stainless steel rods (high strength) without direct material incompatibility issues, as the housing mediates the interface between different materials
3Reliability
If the screw head is made larger to contain larger rods, then the rod containment is improved, but the ease of insertion and surgical profile are worsened
Solution Approach 1:
The screw head is divided into two portions with different functions: the first portion (smaller, cylindrical) that fits through the housing opening for easy insertion, and the second portion (larger, spherical) that provides rod containment. This segmentation allows the screw to be inserted through a small opening while the larger second portion remains outside to contain the rod
Solution Approach 2:
The solution moves the rod containment function to a different spatial dimension. Instead of increasing the head size in all dimensions, the second head portion extends in a specific direction (outside the housing) to provide rod containment, while the insertion dimension (through the housing opening) remains small
Data Source
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AI summary
A polyaxial pedicle screw includes a housing, a bone screw member, and an anvil. The bone screw member includes a head and a threaded shaft extending from the head. The head is selectively securable within the housing. The anvil is positionable within the housing adjacent to the head of the bone screw member when the anvil and the head of the bone screw member are positioned within the housing. The anvil may define one or more grooves in an outer surface of the anvil. The groove defines a flap that is flexibly attached to the anvil to enable the anvil to flex an amount sufficient to maintain the head of the bone screw in constant contact with the anvil when the bone screw member is moved relative to the anvil.