Polyaxial Bone Anchor Assembly With Twist-In Pressure Insert
Find Innovative SolutionsGenerate Solutions
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 surgical handling and assembly.
Innovation Solution
A polyaxial bone screw assembly with a resilient retaining structure and tool engagement formation, allowing for secure, low-profile fixation to bone and receiver, using a collar-like retaining ring that expands to capture the shank upper portion within the receiver, and a closure structure that applies force to frictionally engage the shank and receiver, ensuring stable angular orientation.
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 making them difficult to fix securely
Solution Approach 1:
The implant is divided into separate functional components: a bone screw with threading for bone anchorage, a receiver for rod placement, and a closure top with compression mechanism. This segmentation allows each component to be optimized independently - the closure top can apply concentrated compressive forces to secure the rod without requiring the entire implant to be bulky.
Solution Approach 2:
The closure top is designed to be inserted and compressed into the receiver before final assembly, pre-compressing the rod against the bone screw. This preliminary compression action secures the rod in position before the implant is fully assembled and implanted, ensuring stable fixation without requiring excessive material bulk.
2Volume of moving object
If smaller, lighter implants are used, then profile is reduced, but handling difficulty increases due to small size
Solution Approach 1:
The implant components are segmented with distinct geometric features - the bone screw has external threading and a head portion, the receiver has a cylindrical cavity with opening, and the closure top has a compressed configuration. These segmented features provide adequate gripping surfaces for surgical instruments while maintaining a compact overall profile.
Solution Approach 2:
The closure top transitions from an uncompressed state to a compressed state upon insertion, dynamically changing its configuration. This dynamic compression allows the closure top to fit within the receiver cavity while providing sufficient external surface area for tool engagement during implantation.
3Volume of moving object
If smaller, lighter implants are used, then profile is reduced, but tool engagement surfaces become insufficient
Solution Approach 1:
Tool engagement surfaces are segmented and distributed across different components: the bone screw has external threading for driver engagement, the receiver has an opening for instrument access, and the closure top provides compression force. This segmentation ensures adequate tool engagement surfaces are available despite the compact overall size.
Solution Approach 2:
The closure top is pre-formed with a compressed configuration that provides adequate engagement surfaces before insertion. The compression mechanism is designed to engage with the rod and receiver in a predetermined manner, ensuring reliable tool interaction during the assembly process.
4Reliability
If multiple components are used to rigidly fix the implant, then fixation reliability is improved, but device complexity increases
Solution Approach 1:
The implant is segmented into three main components: bone screw, receiver, and closure top. Each component performs a specific function - the bone screw provides bone anchorage, the receiver holds the rod, and the closure top secures the rod through compression. This segmentation achieves reliable fixation with a manageable number of components.
Solution Approach 2:
The closure top combines multiple functions into a single component: it provides compression force to secure the rod, defines the final assembly configuration, and provides a closure for the receiver opening. This merging reduces the total number of components needed while maintaining fixation reliability.
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 polyaxial bone screw that is easily assembled, securely fastened, and resistant to unintentional disassembly, with reduced components, enhancing surgical ease and stability.
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
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 method of assembling a pivotal bone anchor assembly includes positioning a capture portion of a shank, a retainer, and an insert into a receiver. The receiver includes an internal cavity with a circumferential groove adjacent a bottom opening, a first channel with a discontinuous downward-facing surface below a guide and advancement structure, and an internal cylindrical surface with a fixed rotation abutment surface. The shank with a capture portion is uploadable through the bottom opening. The retainer is uploadable into the circumferential groove to engage and hold the capture portion within the cavity. The insert includes a cylindrical body with a second channel between upstanding arms and is positionable into the first channel with the second channel transverse to the first channel. The method further includes rotating the insert within the receiver until the second channel of the insert is aligned with the first channel of the receiver.