Bone Anchor Receiver Recess Geometry for Secure Polyaxial Locking
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Solution Overview
Problem
Existing polyaxial bone screws suffer from floppy connections between the receiver and shank, making surgical procedures difficult and prone to pull-out forces, and prior designs with contractile locking engagements are weak against such forces.
Innovation Solution
A polyaxial bone screw assembly with a split retainer ring having a smaller radius than the shank head, providing a frictional engagement that resists pull-out forces, and a compression insert that locks the shank into a fixed position using expansion-only locking engagement, ensuring secure attachment without disassembly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a contractile locking engagement is used to connect the receiver and shank, then the connection allows for polyaxial movement, but the connection becomes weak and prone to pull-out forces
Solution Approach 1:
The locking mechanism is divided into two independent systems: an expansion locking system that provides strength against pull-out forces, and a contractile locking system that enables polyaxial movement. The expansion lock engages radially outward against the shank head while the contractile lock engages axially to allow movement, separating the functions of strength and adaptability into distinct structural components.
Solution Approach 2:
The patent combines both expansion locking and contractile locking mechanisms within a single receiver assembly. The expansion lock provides radial engagement for strength, while the contractile lock provides axial engagement for polyaxial movement capability. Both locking systems operate simultaneously on the same shank head, merging two different locking approaches to achieve both strength and adaptability.
2Ease of manufacture
If the retainer ring has the same radius as the shank head, then the engagement is simple, but the connection allows excessive movement and is weak against pull-out forces
Solution Approach 1:
The retainer ring is designed with non-uniform radial engagement: the expansion lock engages radially outward at a specific location with a smaller radius than the shank head, creating a localized frictional engagement point. This localized engagement provides strong resistance to pull-out forces while the rest of the retainer ring maintains a simpler geometry for ease of manufacture.
3Strength
If a frictional engagement with smaller radius is used, then pull-out forces are resisted, but the manufacturing precision requirements increase
Solution Approach 1:
The expansion lock is designed to dynamically engage and lock into place during the assembly process. As the retainer ring is inserted, the expansion lock radially expands to engage the shank head, automatically establishing the frictional contact. This dynamic engagement reduces the need for pre-machined precision radius tolerances, as the locking action itself creates the necessary engagement geometry.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design offers a secure, non-floppy connection that resists pull-out forces and allows for easy manipulation during surgical procedures, functioning like a fixed monoaxial screw with independent locking features.
Implementation Method 1
providing a frictional engagement that resists pull-out forces
Implementation Method 2
locks the shank into a fixed position using expansion-only locking engagement
Data Source
AI summary
A receiver of a bone anchor assembly includes a receiver body having a base portion and a pair of upright arms defining a channel for receiving a rod, the upright arms having top surfaces, opposed interior surfaces, and exterior side surfaces extending downward across the upright arms and base portion to a bottom of the receiver body. The receiver also includes a horizontally-elongate upper tool-engaging groove formed into the exterior side surface of each upright arm proximal to and below the top surface of the upright arm, extending horizontally to at least a front face or a back face of the upright arm, and including a downwardly-facing upper surface, an upwardly-facing lower surface, and an outwardly-facing inner surface. The receiver further includes a tool-engaging recess extending across the upper tool-engaging groove of each upright arm, with the tool-engaging recess extending upwardly through the downwardly-facing upper surface and downwardly through the upwardly-facing lower surface and including an outwardly-facing planar surface opposite and parallel to an outwardly-facing planar surface of the tool-engaging recess of the other upright arm.