Pivotal Bone Anchor Assembly with Vertical Locking Mechanism
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Solution Overview
Problem
Existing polyaxial bone screws in spinal surgery often have a floppy head that can be difficult to position correctly, and they typically require the screw head and receiver to be assembled as a single unit, which can be cumbersome during surgical procedures.
Innovation Solution
A polyaxial bone screw assembly featuring a separate shank with an integral head, a receiver with a compression insert and a resilient expansion locking split retainer, allowing for independent assembly and locking of the shank head within the receiver, providing a non-floppy yet movable connection that can be locked into a desired configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the screw head and receiver are assembled as a single unit, then the structural integrity is maintained, but the ease of operation deteriorates due to cumbersome assembly during surgery
Solution Approach 1:
The bone screw assembly is divided into separate modular components: a shank with an integral head and a separate receiver. This segmentation allows the shank to be inserted into bone independently, then the receiver is separately attached to the shank head, providing ease of operation while maintaining structural integrity through secure connection mechanisms.
Solution Approach 2:
The receiver incorporates a polyaxial mechanism with a retainer that allows dynamic adjustment of the receiver's orientation relative to the shank. The retainer can be rotated and locked into various positions, enabling the surgeon to optimize the angle and positioning during the procedure, then secure it in place.
2Ease of operation
If the head is made floppy for ease of positioning, then the ease of operation improves, but the stability deteriorates during fixation
Solution Approach 1:
The polyaxial mechanism provides dynamic flexibility during positioning, allowing the receiver to be rotated and adjusted to the desired angle. Once positioned, the retainer is locked into place, transforming the system from dynamic to static, thereby ensuring stability during fixation while maintaining positioning flexibility when needed.
Solution Approach 2:
The mechanical locking system replaces the need for a permanently fixed rigid connection. The retainer mechanism substitutes for traditional fixed-geometry connections, allowing angular adjustment through mechanical means while maintaining stability through secure locking, thus resolving the contradiction between flexibility and stability.
3Manufacturing precision
If the shank head is made movable for precise positioning, then the manufacturing precision improves, but the device complexity increases
Solution Approach 1:
By separating the shank and receiver into independent components, each can be manufactured with high precision independently. The shank can be precisely engineered for bone insertion, while the receiver can be precisely designed for angular adjustment. This segmentation reduces the overall complexity compared to manufacturing a single complex integrated unit with precise positioning capabilities.
Solution Approach 2:
The dynamic polyaxial mechanism with the retainer provides precise positioning capability through controlled movement and locking. The mechanism allows the receiver to be positioned at various angles relative to the shank, achieving manufacturing precision in positioning while managing device complexity through a well-defined mechanical system with clear functional components.
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
This design enhances the ease of use and precision in spinal surgery by allowing for independent assembly and secure locking of the screw components, reducing the complexity of inserting the screw into the bone and improving the stability of the spinal fixation.
Implementation Method 1
a resilient, tiered, expansion locking split retainer for capturing the shank head in the receiver lower cavity
Implementation Method 2
a locking insert having a lower compression friction fit collet, the shank head being frictionally engaged with, but still movable
Data Source
AI summary
A pivotal bone anchor assembly includes a first member having a central bore, a lower portion with a bottom opening, and a pair of upright arms defining an open channel and including upper tool engagement structures configured for engagement and holding by tooling. The assembly also includes a shank comprising an anchor portion for fixation to the bone of a patient and a head portion for uploading into the lower portion of the first member through the bottom opening. The assembly further includes a second member cooperating with the first member and having a center aperture alignable with the central bore, lower tool engagement structures, and an upwardly-facing surface engageable with a rod. The second member is downwardly displaceable relative to the first member by the tooling from a first position to a second position, so as to lock an angular position of the shank relative to the first member in a locked configuration prior to securing the rod in the open channel with a closure.