Torque-Free Spinal Screw Locking Mechanism
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Solution Overview
Problem
Existing spinal fixation systems require significant torque to secure screws, which can lead to 'blowout' of the screw through the bone, and are cumbersome to operate, often necessitating additional instrumentation and increasing the risk of trauma and instrument costs.
Innovation Solution
A locking mechanism that secures a polyaxial screw and rod without applying torque, using a radially deformable locking ring and a resilient cap to absorb radial stresses and provide a secure, releasable lock without the need for external countertorque or complex threading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a threaded locking mechanism with set screw is used to secure the rod to the polyaxial screw, then the mechanical advantage provides strong locking force, but significant torque is required which can cause blowout of the screw through the bone
Solution Approach 1:
The patent replaces the traditional threaded locking mechanism with a friction-based compression system. A compression device is inserted through the polyaxial screw into the rod, and when expanded, it creates frictional contact with the rod's inner surface. This friction-based locking eliminates the need for high-torque threaded fasteners that cause blowout, while still providing strong securing force through normal and friction forces.
Solution Approach 2:
The invention changes the locking mechanism from a threaded mechanical advantage system to a friction-based system. By expanding the compression device radially within the rod, the normal force between the compression device and rod increases, thereby increasing friction force. This parameter change (from threading to radial expansion) eliminates torque requirements while maintaining strong locking.
2Strength
If a set screw or rotating locking element is used to tighten the locking mechanism, then the mechanical advantage creates large axial forces, but the requirement of torque introduces substantial risk of blowout and requires additional instrumentation
Solution Approach 1:
The patent eliminates rotating locking elements and threaded mechanisms entirely. Instead, a compression device is inserted axially through the screw and rod, then expanded radially within the rod. The locking force is generated by friction between the expanded compression device and the rod's inner surface, eliminating the need for torque application and complex threading instrumentation.
Solution Approach 2:
The invention extracts and removes the problematic threaded locking mechanism and set screw from the system. The compression device serves multiple functions (insertion, expansion, and locking) without requiring external torque application tools or complex threading instruments, thereby simplifying the overall device and procedure.
3Reliability
If traditional threaded locking mechanisms are used, then secure locking can be achieved, but the operation becomes cumbersome requiring careful balancing of torque with countertorque and additional hands-free operation
Solution Approach 1:
The patent replaces the complex torque-balancing threaded system with a simple radial expansion mechanism. The compression device is inserted through the screw and rod, then expanded radially within the rod using a dilation tool. The friction-based locking provides secure fixation without requiring torque balancing or countertorque instrumentation, making the procedure straightforward and easier to perform.
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 solution minimizes the risk of screw 'blowout' and simplifies the operation by eliminating the need for torque, reducing trauma and instrument complexity, while providing a strong and efficient locking mechanism that can be easily released.
Implementation Method 1
a locking mechanism that can be tightly secured without applying torque... using a radially deformable locking ring
Implementation Method 2
using a radially deformable locking ring and a resilient cap to absorb radial stresses
Data Source
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AI summary
A locking mechanism (70) for locking a rod (40) in a screw (30) and rod fixation assembly includes a cap (72) having a sidewall (74) and a locking element (90) extending from at least a portion of the sidewall. Locking mechanism cooperates with a receiver component (50) for a spinal fixation rod to lock the rod in the receiver without requiring application of torque. A method of locking a spinal fixation member in a receiver of a screw fixation assembly includes the steps of placing the spinal fixation member into a channel (69) of a receiver, axially advancing a locking cap into the receiver toward the spinal fixation member without application of any torque, and securing the locking cap in a locked position above the spinal fixation member without application of any torque. The locking element may be settled in an annular recess (76) formed in the sidewall, and expand into an annular groove (65) formed in the receiver.