Polyaxial Bone Anchor Assembly With Twist-Lock Insert Retention
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Solution Overview
Problem
Existing polyaxial bone screws face challenges with smaller, lighter implants that are difficult to rigidly fix in desired angular positions, prone to slippage under high loading, and require multiple components, complicating handling and assembly during surgery.
Innovation Solution
A polyaxial bone screw assembly featuring a shank with a resilient retaining structure and a receiver design that allows for bottom-loading, enabling secure engagement and fixation through frictional contact between the shank upper portion and the receiver, facilitated by a tool engagement formation for easy assembly and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If smaller, lighter implants are used, then weight and profile are reduced, but rigidity and strength decrease leading to slippage under high loading
Solution Approach 1:
The implant is divided into multiple functional components: a polyaxial bone screw with shank and head, a separate receiver, and a resilient retaining structure. This segmentation allows each component to be optimized independently - the shank can be lightweight while the receiver and retaining structure provide the necessary strength and locking capability.
Solution Approach 2:
The implant utilizes composite construction with a resilient retaining structure made of elastomeric or polymeric material combined with rigid components. This composite approach allows the lightweight polymer portions to reduce overall weight while the rigid elements maintain structural strength and prevent slippage under load.
2Weight of moving object
If smaller, lighter implants are used, then weight and profile are reduced, but the number of components increases complicating handling and assembly
Solution Approach 1:
The retaining structure integrates multiple functions into a single component: it acts as both the locking mechanism and the connection element between the shank and receiver. The resilient retaining structure combines the functions of a lock, a spacer, and a structural connector, reducing the total component count despite maintaining polyaxial capability.
Solution Approach 2:
The receiver serves multiple functions: it provides the polyaxial articulation interface, houses the retaining structure, receives the rod, and provides tool engagement surfaces. This multi-functionality reduces the need for separate components for each function, simplifying the overall assembly.
3Weight of moving object
If smaller, lighter implants are used, then weight and profile are reduced, but ease of operation decreases due to difficult handling and assembly
Solution Approach 1:
The retaining structure is pre-formed with its resilient properties and geometric configuration that enables self-centering and automatic engagement. The polyaxial design allows the shank to be inserted at various angles without requiring precise pre-alignment, and the retaining structure automatically positions itself during assembly, reducing surgical complexity.
Solution Approach 2:
The resilient retaining structure provides self-locking capability through its elastic deformation and recovery. When compressed during assembly, it automatically engages and locks the shank and receiver together without requiring additional fasteners or complex locking procedures. The tool engagement formations also enable self-alignment during insertion.
4Device complexity
If fixed head bone screws are used, then structural simplicity is maintained, but adaptability to different angular configurations is lost
Solution Approach 1:
The bone screw transitions from a fixed configuration to a dynamic polyaxial system where the receiver can rotate about the shank axis. The resilient retaining structure enables this dynamic capability while maintaining a relatively simple overall structure. The system allows angular adjustment during insertion and then locks in the desired position, combining adaptability with structural efficiency.
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 assembly provides a lightweight, low-profile solution that is easy to use, securely fastens to bone, prevents unintentional disassembly, and reduces the number of components, ensuring stable angular orientation and ease of handling during surgery.
Implementation Method 1
The retaining structure is resilient and open, including first and second spaced ends being movable toward and away from one another. The shank upper portion and the retaining structure are sized and shaped to be bottom loadable into the receiver, with the retaining structure being compressed during insertion. Upon expanding to an original form, the retaining structure engages the receiver and captures the shank upper portion within a cavity of the receiver.
Implementation Method 2
The shank upper portion is sized, shaped and positioned to receive a downward force with a rod seated in the channel. In operation, a closure structure operably applies a force through a rod that is transmitted onto the upper portion of the bone screw shank, which in turn frictionally engages both a spherical surface of the retaining structure and a spherical surface of the cavity thereby fixing the bone screw shank body in a selected angular orientation with respect to the receiver.
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver having a lower cavity with a circumferential groove adjacent a bottom opening, an upper first channel with a downward-facing surface formed therein below a closure mating structure, and a substantially cylindrical surface extending below the downward-facing surface with an alignment stop structure. The assembly also includes a shank having a capture portion configured for uploading into the cavity through the bottom opening, and an open ring retainer uploadable into the circumferential groove through the bottom opening after the capture portion so as to retain the capture portion within the cavity. The assembly further includes a pressure insert positionable into the first channel with a second channel of the pressure insert being transverse to the first channel of the receiver, after which the pressure insert is rotatable within the receiver until upwardly-facing surfaces on the insert rotate beneath the downward-facing surface to inhibit upward movement of the pressure insert, and a protruding rotational abutment structure on the insert engages the alignment stop structure to inhibit further rotation of the pressure insert when the second channel is aligned with the first channel.