Polyaxial Pedicle Screw Assembly for Low-Profile Rigid Fixation
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Solution Overview
Problem
Existing spinal fixation systems, particularly pedicle screw and rod constructs, are not cost-effective and lack sufficient rigidity for large deformity corrections, often requiring larger stainless steel rods that increase the construct profile and necessitate material incompatibility.
Innovation Solution
A polyaxial pedicle screw system with a bone screw member and housing that allows for selective angular positioning up to 80 degrees, featuring a head with cylindrically and spherically shaped portions for secure engagement, and an anvil with a compound curve cavity to accommodate various rod diameters, along with a compression ring to maintain alignment and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If larger stainless steel rods are used to provide sufficient rigidity for large deformity corrections, then the rigidity and strength of the construct is improved, but the construct profile increases and material incompatibility issues arise
Solution Approach 1:
The system divides the fixation construct into modular components: a titanium alloy pedicle screw with a polyaxial head, a separate rod, and a set screw for securing. This segmentation allows optimization of each component's properties - the screw provides anchorage while the rod provides rigidity, avoiding the need for a monolithic oversized construct
Solution Approach 2:
The system employs composite material construction by combining titanium alloy (for the pedicle screw) with stainless steel (for the rod). This allows each material to be used where it provides optimal performance - titanium for corrosion resistance and biocompatibility in the bone interface, stainless steel for rigidity in the rod - while maintaining overall construct strength without increasing profile
2Adaptability or versatility
If polyaxial pedicle screws with angular positioning capability are used to enable multi-angle rod attachment, then the adaptability and versatility are improved, but the device complexity increases
Solution Approach 1:
The pedicle screw features a dynamic polyaxial head that can rotate and tilt within a defined angular range (up to 80 degrees) relative to the screw shaft. This dynamic capability allows the rod to be attached at multiple angles without requiring multiple pre-drilled holes or complex alignment procedures, simplifying the overall implantation process despite the added mechanical complexity of the polyaxial mechanism
Solution Approach 2:
The polyaxial head incorporates a spherical or curved geometry that enables rotational movement within the housing. This spheroidal design provides smooth angular adjustment capability while maintaining a compact form factor, allowing multi-angle attachment without proportionally increasing the device's overall complexity or size
3Reliability
If the bone screw member head is designed with dual-shaped portions for secure engagement, then the reliability of attachment is improved, but the manufacturing precision requirements increase
Solution Approach 1:
The screw head is segmented into distinct functional portions: a cylindrical section that interfaces with the polyaxial housing and a spherical section that provides rotational engagement. This segmentation allows each portion to be manufactured with optimized tolerances for its specific function, reducing overall manufacturing complexity while maintaining reliable attachment
Solution Approach 2:
The incorporation of a spherical portion in the screw head provides inherent self-centering and rotational capability. The spherical geometry naturally accommodates angular variations and provides smooth engagement with the polyaxial mechanism, reducing the need for extremely tight manufacturing tolerances while maintaining reliable multi-angle attachment
Data Source
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. A rod reducer may be secured to the polyaxial pedicle screw to secure a spinal rod within the housing.