Pedicle Bone Fastener Thread Geometry for Polyaxial Load Sharing
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Solution Overview
Problem
Traditional fastener thread designs fail to provide sufficient fixation and load sharing under multi-axial and off-axis loading conditions in surgical procedures involving bone and other tissues, leading to loosening of fasteners over time.
Innovation Solution
The development of fastening devices with improved thread designs, including a pedicle bone fastener featuring a shaft with helical threads and an integrated polyaxial head, allowing for polyaxial adjustment and secure attachment to spinal stabilization implements, which includes a tulip with locking mechanisms for securing rods, enhancing bone fixation and load sharing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional fastener thread designs are used, then the device complexity remains low, but the bone fixation strength and load sharing capability are insufficient under multi-axial and off-axis loading conditions
Solution Approach 1:
The thread design is segmented into multiple distinct surfaces (first undercut surface, second undercut surface, third surface, fourth surface) with different orientations and functions. Each surface is optimized to resist specific loading directions, allowing the fastener to handle multi-axial and off-axis loads through distributed resistance across multiple thread surfaces rather than relying on a single thread geometry
Solution Approach 2:
Different portions of the thread geometry are given different local properties: the first and second undercut surfaces are configured to resist loads in one direction while the third and fourth surfaces resist loads in opposite directions. This local differentiation of thread surface properties enables the fastener to provide superior fixation strength under varied loading conditions without requiring complex external structures
2Adaptability or versatility
If traditional fixed orientation fasteners are used, then the device complexity is low, but the adaptability to various loading conditions and surgical configurations is limited
Solution Approach 1:
The attachment feature transitions from a fixed orientation to a dynamic, adjustable configuration. The polyaxial head and tulip assembly enable the fastener to be oriented at multiple angles relative to the bone axis, allowing adaptation to various surgical configurations and loading conditions. This dynamic adjustability is achieved through the semi-spherical interface that permits rotation and angular adjustment before final locking
Solution Approach 2:
The attachment system uses a nested structure where the polyaxial head is integrated into the fastener shaft, the tulip is received within the polyaxial head, and the rod passes through the tulip. This nested arrangement allows for polyaxial adjustment while maintaining a compact overall structure, achieving high adaptability without proportionally increasing device complexity
3Reliability
If fasteners with improved thread designs are used, then the bone fixation and load sharing are enhanced, but the risk of bone blowout may increase due to higher insertion forces
Solution Approach 1:
A pilot hole is drilled into the bone before inserting the fastener. This preliminary action creates a prepared pathway that reduces the insertion forces required for the fastener threads to engage the bone, thereby decreasing the risk of bone blowout while still allowing the improved thread design to achieve superior fixation reliability once inserted
Solution Approach 2:
The thread geometry is designed with specific surface orientations and undercut configurations that distribute insertion and loading forces across multiple bone interfaces. The first and second undercut surfaces angle toward one end of the shaft while the third and fourth surfaces angle toward the other end, creating a force distribution pattern that cushions against peak stresses that could cause bone blowout
Data Source
AI summary
A pedicle bone fastener may include a shaft, a helical thread, and an integrated attachment feature. The shaft may include a proximal end, a distal end, and a longitudinal axis. The helical thread may be disposed about the shaft along the longitudinal axis between the proximal and distal ends of the shaft. The helical thread may include a first undercut surface and a second undercut surface. The first undercut surface may be angled toward one of the proximal end and the distal end of the shaft and the second undercut surface may be angled toward the other one of the proximal end and the distal end of the shaft. The integrated attachment feature may be disposed at the proximal end of the shaft and configured to be adjustably secured to a spinal stabilization implement.