Pivotal Bone Anchor With Expansion-Only Retainer Ring
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Solution Overview
Problem
Existing modular polyaxial bone screw designs are prone to pull-out forces during spinal reduction procedures, and they often require contractile locking engagements that can be weak and difficult to manage.
Innovation Solution
The design incorporates a split retainer ring with a collet-like upper portion that does not participate in the locking engagement, using an expansion-only retainer ring base for secure fixation, providing resistance to larger pull-out forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a contractile locking engagement is used in modular polyaxial bone screw designs, then the screw assembly can be assembled and disassembled, but the locking engagement becomes weak and difficult to manage, leading to pull-out forces during spinal reduction procedures
Solution Approach 1:
The patent inverts the conventional contractile locking mechanism by using an expansion-only retainer ring. Instead of the retainer contracting to grip the shank, the retainer expands outwardly against the shank to create friction fit and locking engagement. This inversion eliminates the weak contractile engagement and provides superior resistance to pull-out forces while maintaining modular assembly capability.
Solution Approach 2:
The patent changes the mechanical parameter of the retainer ring from contractile to expansion-only. By modifying the retainer's operational mode, the friction fit and locking engagement are significantly improved. The expansion-only mechanism creates more reliable engagement with the shank, preventing pull-out forces during spinal reduction procedures while preserving the modular design benefits.
2Reliability
If an expansion-only retainer ring base is used, then resistance to pull-out forces is enhanced, but the device complexity increases due to the split retainer ring structure
Solution Approach 1:
The retainer ring is segmented into multiple sections that can be compressed together and locked in place. This segmentation allows the retainer to be inserted as a compressed unit and then expand to engage the shank. The split structure, when properly designed and locked, provides enhanced pull-out resistance while the modular nature helps manage device complexity through standardized components.
3Device complexity
If the collet-like upper portion of the retainer does not participate in locking engagement, then the retainer structure is simplified, but the locking engagement relies solely on the retainer ring base which may reduce overall locking surface area
Solution Approach 1:
The patent extracts the locking function from the collet-like upper portion and concentrates it in the retainer ring base. By removing the locking engagement requirement from the upper portion, the structure is simplified and the retainer ring base can be optimized for expansion and friction fit. This extraction maintains adequate locking surface area through the expanded base structure while reducing overall complexity.
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 security and resistance to pull-out forces compared to prior art, ensuring a stronger and more stable connection between the bone screw shank and the receiver assembly.
Implementation Method 1
a friction fit resilient expansion-only split retainer for capturing the shank head in the receiver lower cavity
Implementation Method 2
The retainer 12 is made from a resilient material, such as a stainless steel or titanium alloy, so that the retainer 12 body 116 may be expanded
Implementation Method 3
The shank is finally locked into a fixed position relative to the receiver by frictional engagement between the insert and a lower split ring-like portion of the retainer base
Implementation Method 4
an insert 14 that has locked the shank 4 in a desired angular position with respect to the receiver 10, by, for example, compression from the rod 21 and closure top 18
Implementation Method 5
the insert 14 is wedged into engagement with the receiver 10 at the inner surface 84
Data Source
AI summary
A pivotal bone anchor assembly includes a receiver having a channel for receiving a rod and an internal chamber with a bottom opening, a lower region adjacent the bottom opening, and an upper region above the lower region. The assembly also includes a bone anchor with a capture portion and an anchor portion. The assembly further includes a resilient open retainer ring that is positioned and restrained in the upper region of the internal chamber prior uploading to the capture portion through the bottom opening, at which point the open retainer ring is configured to expand about and capture the capture portion of the bone anchor and then for downward displacement toward the lower region until an uppermost surface of the open retainer ring becomes engaged with a downwardly-facing surface located in the upper region to inhibit the open retainer ring from moving back up within the receiver.