Spinal Closure Thread Angles to Prevent Receiver Arm Splay
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Solution Overview
Problem
Existing bone screw closures for spinal fixation, such as v-thread and square thread forms, suffer from outward splay of receiver arms due to unbalanced loading, leading to deformation and instability under torque.
Innovation Solution
A closure structure with balanced mating guide and advancement flange forms on both the closure and bone anchor, featuring a dual-start helically wound design, includes a compression insert to control splay by applying axial loading and clearance, ensuring the receiver arms remain aligned during tightening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If v-thread closure forms are used, then the closure can be easily manufactured and installed, but the receiver arms experience outward splay and deformation under torque
Solution Approach 1:
The patent changes the geometric parameters of the thread form from conventional v-thread to a modified form with specific load flank and stab flank angles. The load flank angle is optimized to direct forces axially rather than radially, preventing receiver arm splay while maintaining manufacturability through standard threading processes.
Solution Approach 2:
The thread form introduces asymmetry between the load flank and stab flank surfaces. The load flank is angled to provide axial force direction during tightening, while the stab flank provides clearance and guidance. This asymmetric design prevents radial splay forces that occur with symmetric v-thread forms.
2Force
If square thread forms are used, then radial reactions are minimized, but splay still occurs under moderate to heavy loads
Solution Approach 1:
The patent modifies the thread geometry parameters by introducing specific flank angles (load flank angle and stab flank angle) that optimize force distribution. The load flank angle is designed to maintain axial force direction under various load conditions, preventing the splay that occurs with square threads under moderate to heavy loads.
3Power
If higher torque is applied during closure tightening, then secure fixation is achieved, but receiver arms deform and splay outward
Solution Approach 1:
The optimized flank angles allow higher tightening torque to be applied without causing receiver arm deformation. The load flank angle directs the torque-induced forces axially through the receiver arms rather than radially outward, enabling secure fixation while maintaining structural integrity.
Solution Approach 2:
The thread form geometry is designed to preemptively counteract the splay tendency before it occurs. The load flank angle is configured to redirect forces away from radial paths that would cause splay, providing preliminary protection against deformation even before full tightening is achieved.
Data Source
AI summary
A method for securing an elongate rod to a spinal fixation structure having a receiver and a threaded closure. The method includes providing the elongate rod, the receiver, and the closure. The closure includes a thread form having a stab flank, a loading flank, an outer crest surface with a crest height, and a root surface. The thread form also includes a base height that is greater than the crest height.