Bottom-Loading Bone Anchor Assembly for Polyaxial Spinal Fixation
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Solution Overview
Problem
Large diameter pedicle screws pose challenges due to increased size requirements for receiver heads, which can interfere with bony anatomy and limit polyaxial range of motion, and require larger closure mechanisms, necessitating a non-top-loading polyaxial screw design.
Innovation Solution
A bottom-loading bone anchor assembly with a large diameter shank, featuring a receiver member, a bone-engaging shank with a head that fits within the receiver, a retaining member that accommodates the shank, and a locking member that secures the retaining member within the receiver, allowing for polyaxial movement without the need for a larger closure mechanism.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a larger diameter screw shank is used to carry large loads or engage large pedicles, then the load capacity and engagement capability are improved, but the receiver head size must increase which interferes with bony anatomy and limits polyaxial range of motion
Solution Approach 1:
The screw shank is inserted through the bore from the distal end, and the retaining member is nested within the receiver member to secure the shank. This nested arrangement allows the screw components to be housed within the receiver member's internal volume rather than requiring a proportionally larger receiver head outer dimension, thus maintaining compact overall size while supporting large diameter shanks for high load capacity
Solution Approach 2:
The loading direction is changed from top-loading (proximal end insertion) to bottom-loading (distal end insertion). This dimensional change in the assembly approach allows the screw components to be configured within the receiver member's internal space, enabling large diameter shanks to be accommodated without proportionally increasing the receiver head's external dimensions that would interfere with bony anatomy
2Strength
If a larger diameter screw shank is used, then the load carrying capability is improved, but a larger closure mechanism is required which increases device complexity
Solution Approach 1:
The closure mechanism function is extracted and integrated into the retaining member itself. The retaining member's inner surface is shaped to accommodate the head of the bone-engaging shank and lock it in place, eliminating the need for a separate, larger closure mechanism that would be required in top-loading designs. This integration maintains simplicity while supporting large diameter shanks
Solution Approach 2:
The retaining member serves multiple functions: it retains the bone-engaging shank within the receiver member, provides the locking interface through its interaction with the locking member, and eliminates the need for a separate closure mechanism. This multi-functionality allows the same sized retaining member to be used with large diameter shanks without increasing device complexity
Data Source
AI summary
Bone anchor assemblies having a large diameter for fixing a spinal connection element to bone and methods of assembly are described. The assembly includes a receiver member for receiving the spinal connection element, a bone-engaging shank for engaging bone, a retaining member for retaining the head of the shank within the receiver member and a locking member for locking the retaining member within the receiver member.