Bottom-Loaded Polyaxial Bone Screw Assembly with Compression Insert
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Solution Overview
Problem
Existing polyaxial bone screws face challenges in securely fixing rods within their receivers, especially during spinal surgery, due to the difficulty in positioning the rod within a fixed head screw.
Innovation Solution
A polyaxial bone screw assembly featuring a shank with a multi-part retainer and a compression insert, allowing for polyaxial motion and secure locking of the rod within the receiver, utilizing spring tabs to maintain the insert's alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a fixed head bone screw is used, then the structure is simple, but the rod cannot be positioned at various angles and insertion is difficult
Solution Approach 1:
The bone screw incorporates a polyaxial mechanism that allows the head to rotate and pivot relative to the shank, transforming the fixed structure into a dynamic one. This enables the head to be positioned at multiple angles (polyaxial motion) before final locking, providing rod positioning flexibility while maintaining a relatively simple overall structure.
Solution Approach 2:
The screw assembly is divided into separable components including the shank, head, retainer, and compression insert. This segmentation allows the head to be independently positioned and locked at desired angles relative to the shank, enabling angular adjustment without significantly increasing the complexity of individual components.
2Adaptability or versatility
If a polyaxial mechanism is added to allow head rotation, then rod insertion flexibility improves, but the assembly becomes more complex
Solution Approach 1:
The retainer and compression insert are nested within the head structure, with the retainer fitting inside the head cavity and the compression insert positioned within the retainer. This nesting arrangement provides polyaxial positioning capability while minimizing the increase in overall structural complexity by utilizing existing space within the head.
Solution Approach 2:
The retainer acts as an intermediary component between the head and shank, providing the polyaxial motion mechanism. The compression insert serves as another intermediary that secures the retainer in place. These intermediary components enable head positioning adaptability without requiring complex direct integration between the head and shank.
3Reliability
If multiple components (retainer, insert) are added for secure locking, then fixation reliability improves, but assembly complexity increases
Solution Approach 1:
The compression insert is pre-positioned within the head cavity before the retainer is installed. This preliminary placement of the compression insert provides a secure base for the retainer and ensures proper alignment, thereby improving fixation reliability while minimizing the complexity of the assembly process.
Solution Approach 2:
The retainer and compression insert work together as a combined locking mechanism within the head. The retainer provides the polyaxial motion and locking function, while the compression insert reinforces the retention and maintains compression forces. This merging of functions into two coordinated components improves reliability without requiring a large number of separate parts.
4Manufacturing precision
If the shank upper portion is bottom-loaded into the receiver, then assembly precision improves, but the retainer must be spaced below the insert
Solution Approach 1:
The retainer is designed with a configuration that extends in multiple dimensions within the head cavity, allowing it to be positioned below the compression insert while still providing adequate spacing. This dimensional arrangement enables precise shank positioning through bottom-loading while accommodating the retainer's functional requirements without excessive complexity.
Data Source
AI summary
A method of assembling a pivotal bone anchor assembly includes positioning an insert and a two-part retainer into a central bore of a receiver having a seating surface proximate a bottom opening, a closure mating structure proximate a top opening, and integral engagement structures protruding inwardly between the closure mating structure and the seating surface. The insert includes an upper surface for engaging a rod and side surfaces engageable with the integral engagement structures to restrict motion of the insert with the central bore. The retainer includes two separable retainer parts together defining outer retainer surfaces engageable with the seating surface and inner retainer surfaces. The method also includes uploading a shank head of a shank through the bottom opening and into the retainer, with the two retainer parts operable to separate and then move back together to capture the shank head, after which a downwardly-directed force is configured to drive the outer retainer surfaces into frictional engagement with the seating surface to lock the angular orientation of the shank with respect to the receiver.